BIOLOGICAL OVERVIEW

Src42A is one of the two Src homologs in Drosophila. Src42A protein accumulates at sites of cell-cell or cell-matrix adhesion. Anti-Engrailed antibody staining of Src42A protein-null mutant embryos indicated that Src42A is essential for proper cell-cell matching during dorsal closure. Src42A, which is functionally redundant to Src64, was found to interact genetically with shotgun, a gene encoding E-cadherin, and armadillo, a Drosophila ß-catenin. Immunoprecipitation and a pull-down assay indicated that Src42A forms a ternary complex with E-cadherin and Armadillo, and that Src42A binds to Armadillo repeats via a 14 amino acid region, which contains the major autophosphorylation site. The leading edge of Src mutant embryos exhibiting the dorsal open phenotype is frequently kinked and associated with significant reduction in E-cadherin, Armadillo and F-actin accumulation. This phenotype suggests that not only Src signaling but also Src-dependent adherens-junction stabilization are essential for normal dorsal closure. Src42A and Src64 are required for Armadillo tyrosine residue phosphorylation but Src activity may not be directly involved in Armadillo tyrosine residue phosphorylation at the adherens junction (Takahashi, 2005).

The vertebrate Src family of non-receptor tyrosine kinases is comprised of nine members, three of which, Src, Yes and Fyn, are widely expressed in a variety of cells. These Src kinases are considered to have crucial roles in modulation of the actin cytoskeleton, a determinant of cell-shape change and cell migration. Transformation of fibroblasts with activated Src kinases gives rise not only to actin-cytoskeleton disruption but also increased tyrosine phosphorylation of many cytoskeleton-associated proteins involved in cell-substratum and cell-cell interactions. The importance of Src kinases as regulators of cell migration and cell-shape change is also underscored by studies using fibroblasts derived from mice deficient in Src, Yes and Fyn (Takahashi, 2005).

The major autophosphorylation site in focal adhesion kinase (FAK: see Drosophila FAK) serves as a binding site for Src homology 2 (SH2)-containing proteins. The FAK-Src complex mediates the phosphorylation of paxillin and p130-Crk-associated substrate, both of which are major scaffolding proteins capable of recruiting other molecules for integrin-based cell-substratum adhesions and which regulate cytoskeleton organization. The absence of FAK gives rise to increase in the number and extent of cell-substratum adhesions. A quantitative assay was conducted of the rate of incorporation of proteins into cell-substratum adhesion; departure of these proteins from this adhesion was measured (Webb, 2004). Src and FAK are crucial for adhesion turnover at the cell front. Thus, the rates of formation, disassembly and/or maturation of cell-substratum-adhesion appear controlled by FAK-Src activity (Takahashi, 2005 and references therein).

Homophilic cadherin interaction is essential for cell-cell adhesion in vertebrates. The loss of E-cadherin (E-cad) expression has been shown to be related to invasive and metastatic cancers. ß-Catenin binds to alpha-catenin and the cytoplasmic domain of E-cad and is essential for linking E-cad to the actin cytoskeleton. Tyrosine-phosphorylation of ß-catenin or other adherens-junction-associated proteins is one means by which cadherin-mediated cell-cell adhesions may be altered. Enhanced tyrosine-phosphorylation of ß-catenin causes weakening of cadherin-actin interaction with consequent loss of cell adhesiveness. Src may be one of the tyrosine kinases responsible for this tyrosine-phosphorylation, because in cells transformed with Src, loss of epithelial cell differentiation and gain in invasiveness and cadherin-mediated adhesion detachment are all correlated with tyrosine-phosphorylation of the E-cad/ß-catenin complex (Takahashi, 2005 and references therein).

Nonetheless, precise determination of the functional roles of individual Src family kinases in vertebrates may be frought with considerable difficulty because compensatory interactions may occur among nine vertebrate Src kinase members. By contrast, Drosophila possesses only two Src kinases, Src64 and Src42A and accordingly, may provide a better and simpler system for clarifying Src functions in development (Takahashi, 2005).

Mutations in Src64 led to a reduction in female fertility, which is associated with nurse cell fusion and ring canal defects. Src64-mutant ring canals fail to undergo extensive tyrosine phosphorylation which normally occurs. Tec29 dominantly enhances the Src64 ring canal phenotype and loss of Tec29 results in a phenotype strikingly similar to that noted following loss of Src64 function. Tec29 kinase is localized in the ring canal, and this subcellular localization requires Src64 function, indicating that Tec29 is a downstream target of Src64 (Takahashi, 2005 and references therein).

Src42A is the closest relative of vertebrate Src in Drosophila. By localized expression of gain-of-function and dominant-negative forms of Src42A, it has been demonstrated that Src42A may be involved in the regulation of cytoskeleton organization and cell-cell contacts in developing ommatidia and that both dominant-negative and gain-of-function mutations of Src42A cause formation of supernumerary R7-type neurons, which is suppressible by one-dose reduction of various components involved in the Ras/MAPK pathway (Takahashi, 1996). A Src42A mutant has been isolated as an extragenic suppressor of Raf. With this and other mild Src42A mutants, it was found that Src42A may serve as a negative regulator of receptor tyrosine kinases in a Ras1-independent manner (Lu, 1999). Lu's genetic data for Src functions in ommatidium formation appear somewhat at variance with those of Takahashi (1996) using gain-of-function and dominant-negative types of Src42A transgenes (Takahashi, 2005).

As with Src64, Src42A may function in a synergistic manner with Tec29. A Tec29 mutation was noted to enhance the lethality of Src42A mutants dominantly (Tateno, 2000). Although no dorsal open phenotype was found in Src42A or Tec29 mutants, the double mutant embryos exhibit the dorsal open phenotype. Src42A has been shown to be functionally redundant to Src64 at least in the dorsal closure (Tateno, 2000). Both dorsal closure of the embryonic epidermis and thorax closure of the pupal epidermis require the Jun amino-terminal kinase (JNK) homolog Basket (Bsk). The severity of the epidermal closure defect in Src42A mutants depends on the degree of Bsk activity; Bsk activity depends on that of Src42A (Tateno, 2000), thus indicating that JNK-pathway activation is required downstream of Src42A (Takahashi, 2005).

Dynamic changes in cellular and subcellular localization of Src42A have been found and phenotypes of a Src42A protein-null and Scr42A Src64 mutants have been described. Genetic and biochemical analyses indicate that E-cad and Armadillo (Arm) form a complex with Src in the membrane and the resultant putative adherens junction complex is required for proper regulation of F-actin accumulation and actin cytoskeleton dynamics in leading edge cells during dorsal closure (Takahashi, 2005).

The results of this study clearly demonstrate the redundant function of Src42A and Src64 to be indispensable in numerous aspects of Drosophila development. Though Src42A is distributed over the entire plasma membrane of all cells, its signal distribution is not uniform. Two major types of Src42A deposition in the membrane could be clearly recognized (Takahashi, 2005).

In ectodermal cells, strong Src42A signals in apical or apicolateral regions were always associated with strong E-cad signals. E-cad is a core component of the adherens junction that is responsible for cell-cell adhesion and, hence, most, if not all, E-cad-associated membranous Src42A is probably related to adherens junction-dependent cell-cell adhesion (Takahashi, 2005).

A considerable fraction of ectodermal cells were also found associated with the second type of basal Src42A free of E-cad. E-cad-free Src42A is localized on the ectoderm/mesoderm interface and eliminated from ectodermal cells that have evaginated or invaginated without mesoderm association. The extracellular matrix (ECM) comprises several groups of secreted proteins such as integrin ligands. During embryogenesis, different cell layers become properly connected, most probably via cell adhesion to ECM. E-cad-free Src42A may thus be related to integrin-mediated cell-matrix adhesion. Cell-ECM adhesion may not be restricted to the interface between ectodermal and mesodermal cell layers. Strong Src42A signals have actually been found present on the interface between mesodermal and endodermal cell layers (Takahashi, 2005).

The current study shows that, as with JNK signaling genes, Src is required not only for thick F-actin accumulation at the leading edge but proper cell-cell matching along the midline seam as well. JNK signaling, which includes hemipterous (hep) and basket (bsk), is essential for dorsal closure of the embryonic epidermis in Drosophila. Based on examination of Tec29 Src42A mutant phenotypes, it has been suggested (Tateno, 2000) that Src42A acts upstream of bsk (Takahashi, 2005).

In vertebrates, JNK is considered to be situated downstream of Src in integrin signaling (Oktay, 1999; Schlaepfer, 1999). Genetic experiments indicate that interactions between Src and arm/shg, genes encoding the core components of the adherens junction are essential for JNK signaling regulation required for dorsal closure. A pull-down assay also shows that Src protein is capable of directly binding to Arm. Both putative adherens-junction Src and integrin-associated Src thus would appear involved in the regulation of JNK signaling (Takahashi, 2005).

The adherens junction is necessary for cell-cell adhesion and thick F-actin accumulation occurs at the level of the adherens junction at the leading edge. Since E-cad and Arm signals along with actin signals are reduced significantly at the leading edge in Src42A^26-1;Src64^P1/+ embryos and the leading edge of the mutants is significantly kinked, the absence of Src protein from the adherens junction may possibly result in destruction of structural integrity, implying that the adherens junction is also involved in dorsal closure regulation in a structural way (Takahashi, 2005).

Dorsal closure and CNS defects similar to those in Src mutants have been observed in abl mutants. In vertebrates, Abl is tyrosine-phosphorylated with Src (Plattner, 1999) and is capable of interacting with delta-catenin, an E-cad-binding protein. Abl may thus function as well downstream of Src signaling in Drosophila. Germ-band retraction and possibly too, head involution, both of which require Src activity, may be regulated by the two above distinct Src functions. alpha1,2-laminin and alphaPS3ßPS integrin have clearly been shown to be essential for spreading a small group of amnioserosa epithelium cells over the tail end of the germ band during germ-band retraction. shg activity has also been shown to be essential for normal germ-band retraction and head involution (Takahashi, 2005).

Src-dependent dynamical regulation of E-cad-dependent cell-cell adhesion may also be necessary for visual system formation. E-cad overexpression or elimination of EGFR activity have been shown to render optic placode cells incapable of invaginating and prevent the separation of Bolwig's organ precursors from the optic lobe. Virtually identical phenotypes were induced by loss of Src activity, suggesting involvement of at least the adherens junction Src in larval visual system formation and that Src should function either upstream or downstream of EGFR signaling (Takahashi, 2005).

Duox, Flotillin-2, and Src42A are required to activate or delimit the spread of the transcriptional response to epidermal wounds in Drosophila

The epidermis is the largest organ of the body for most animals, and the first line of defense against invading pathogens. A breach in the epidermal cell layer triggers a variety of localized responses that in favorable circumstances result in the repair of the wound. Many cellular and genetic responses must be limited to epidermal cells that are close to wounds, but how this is regulated is still poorly understood. The order and hierarchy of epidermal wound signaling factors are also still obscure. The Drosophila embryonic epidermis provides an excellent system to study genes that regulate wound healing processes. A variety of fluorescent reporters were developed that provide a visible readout of wound-dependent transcriptional activation near epidermal wound sites. A large screen for mutants that alter the activity of these wound reporters has identified seven new genes required to activate or delimit wound-induced transcriptional responses to a narrow zone of cells surrounding wound sites. Among the genes required to delimit the spread of wound responses are Drosophila Flotillin-2 and Src42A, both of which are transcriptionally activated around wound sites. Flotillin-2 and constitutively active Src42A are also sufficient, when overexpressed at high levels, to inhibit wound-induced transcription in epidermal cells. One gene required to activate epidermal wound reporters encodes Dual oxidase, an enzyme that produces hydrogen peroxide. Four biochemical treatments (a serine protease, a Src kinase inhibitor, methyl-β-cyclodextrin, and hydrogen peroxide) were found to be sufficient to globally activate epidermal wound response genes in Drosophila embryos. The epistatic relationships among the factors that induce or delimit the spread of epidermal wound signals were examined. The results define new genetic functions that interact to instruct only a limited number of cells around puncture wounds to mount a transcriptional response, mediating local repair and regeneration (Juarez, 2011).

Drosophila wound healing is an example of a regenerative process, which requires localized epidermal cytoskeletal changes, and localized wound-induced changes in epidermal transcriptional activity. This genetic screen with wound-dependent reporters has allowed identification of novel components that regulate the localized transcriptional response to wounding in epidermal cells. This research identifies seven genes that are required to either activate (Duox and ghost/stenosis) or localize (Flo-2, Src42A, wurst, varicose, and Drosophila homolog of yeast Mak3) the expression patterns of epidermal wound reporters. The number of new functions involved in the delimitation of epidermal wound response near wound sites was unexpected, but indicates that considerable genetic effort is devoted to localizing the activity of transcriptional wound responses during regeneration (Juarez, 2011).

One of the genes that limits the spread of epidermal wound reporters after clean epidermal punctures is Flo-2, as mutants of this gene show a broad expansion of epidermal wound gene activation. Drosophila Flo-2 is itself transcriptionally activated around epidermal wound sites, consistent with an evolutionarily conserved role in regeneration after wounding. In vertebrates, reggie-1/Flo-2 gene expression is activated in wounded fish optic neurons, and reggie-1/Flo-2 and reggie-2/Flo-1 morpholino knockdowns in wounded zebrafish retinal explants reduced axon outgrowth compared to controls. Flo-2 transcriptional activation around Drosophila epidermal wound sites is dependent on the grh genetic function, which is required to activate at least a few other epidermal wound response genes. Flo-2 thus appears to act in the same pathway as grh, although it may act both downstream and upstream of grh, since overexpression of Flo-2 can inhibit the activation of other grh-dependent wound response genes. In this respect, Flo-2 resembles stit receptor tyrosine kinase gene (Wang, 2009), which is both transcriptionally activated by Grh, as well as required for grh-dependent activation of other downstream wound genes. Amazingly, overexpression of Flo-2 can even inhibit the global activation of the Ddc and ple-WE1 wound reporters that are induced by the serine protease trypsin, or by hydrogen peroxide. The inhibitory function of overexpression of Flo-2 on wound induced transcription is cell non-autonomous, at least over the range of a few cell diameters, as shown by the ability of striped overexpression of Flo-2 to silence puncture or trypsin-induced gene activation throughout the epidermis (Juarez, 2011).

The only animal where Flo-2 null mutants have so far been characterized is Drosophila, where Flo-2 has been shown to regulate the spread of Wingless (Wg) and Hedgehog (Hh) signals in the wing imaginal discs. In the wing discs, both the secretion rate and the diffusion rate of these two lipid-modified morphogens were increased when Flo-2 was overexpressed, and decreased when Flo-2 and Flo-1 proteins were not expressed. Despite the reduced spread of Wg and Hh morphogen proteins in Flo-2 mutant imaginal discs, adult morphology of mutants was normal, presumably because of compensatory mechanisms that occur later in development. Whereas a reduced range of activation of wg and hh long range transcription target genes was observed in Flo-2 mutant imaginal discs, a greatly increased range of wound-induced gene activation was observed in Flo-2 mutant embryos. This apparent discrepancy could be explained if one invokes of a long-range wound-induced inhibitory signal that in wild type embryos diffuses faster and farther than a wound activating signal, and thereby functions to limit the wound response to nearby epidermal cells, and that in Flo-2 mutants this potential inhibitory signal has reduced secretion, concentration, and/or diffusion range. This notion is consistent with the cell non-autonomous effect of overexpressed Flo-2 on inhibiting wound- or trypsin-induced gene activation. A similar scheme of controlling signal spreading has been seen in the way that Mmp2 acts cell non-autonomously to limit FGF signaling during Drosophila tracheal development and branch morphogenesis. It's also possible that Flo-2 normally is required to set a global threshold that wound-induced signals must overcome in order to activate wound transcription, for example via Flo-2-dependent endocytosis/degradation of a diffusible wound signal and its receptor (perhaps the Stit RTK), and that signal strength normally surpasses the Flo-2 threshold only in the vicinity of a wound. In this model, loss of Flo-2 would result in all epidermal cells being able to exceed the wound signal threshold, and overexpression of Flo-2 would prevent any cells from exceeding the wound signal threshold. The cell non-autonomous effects of Flo-2 overexpression under this model might be explained by an increase in Flo-2-dependent endocytosis/degradation that rapidly depletes an activating signal from the extracellular space (Juarez, 2011).

Many previous studies have documented biochemical, molecular biological, and cell biological interactions between Src family kinases and Flotillins. In Drosophila, lack of Src42A and Flo-2 leads to expanded spread of wound gene activation, and overexpression of Flo-2 or activated Src42A can inhibit wound gene activation, which is consistent with an interaction between the two functions during the process of wound gene regulation. In cultured mammalian cells, Flo-2 can be phosphorylated by Src family kinases in an extracellular signal-dependent fashion. This phosphorylation is associated with changes in the normal intracellular trafficking of Flotillin-containing membrane microdomains and vesicles. Since overexpressed Flo-2 in Drosophila can act in a cell non-autonomous fashion to inhibit wound gene activation, and overexpressed Src42A acts in a cell autonomous fashion to inhibit wound gene activation, one interpretation is that Flo-2 lies genetically upstream of Src42A in the epidermal wound response. This hypothesis appears to be inconsistent with the vertebrate biochemical data indicating that Src kinases phosphorylate Flotillins to activate their diverse functions. However, an observation that is consistent with Src42A activating Flo-2 protein function, is that even when Flo-2 is overexpressed, addition of chemical inhibitors of Src family kinases to wounded embryos, results in widespread Ddc .47 or ple-WE1 wound reporter activation. One interpretation of this suggests Flo-2 protein, no matter the level of expression, is inactive in the absence of Src42A function. Complex feedback loops involving signaling proteins being regulated by a transcription factor, while the activity of the same transcription factors is regulated by the same signaling pathway, have been observed in the control of Drosophila epidermal wound gene expression and reepithelialization, so there may be similar dynamic cross-regulatory interactions between Flo-2 and Src42A in the localization of the epidermal wound response, interactions not easily captured in linear genetic pathway diagrams (Juarez, 2011).

The inhibitory effect of Src42A on wound gene activation suggests that it might antagonize a signaling cascade that leads to the epidermal wound response. A good candidate for such a signaling cascade is the RTK pathway involving the Stit kinase. Stit is a RET-family RTK that is required for robust activation of the Ddc and stit wound reporter genes in wounded embryos (Wang, 2009). Other evidence consistent with RTK pathway importance in wound gene activation is that phosphotyrosine accumulates persistently around wound sites, and that ERK kinase function is required for robust activation of the Ddc wound reporter gene. Interestingly, Src42A has been shown to act as an inhibitor of some Drosophila RTK proteins (those encoded by the torso, Egfr, and sevenless genes) in a few different tissues during Drosophila development. The Flo-2 and Src42A functions in epidermal wound localization after clean wounding are reminiscent of the role of Drosophila WntD during infectious wounding. WntD mutants show higher levels of some antimicrobial peptide genes after septic injury of adults (Juarez, 2011).

Previous evidence suggested that H₂O₂ and Duox could provide wound-induced inflammatory signals and antimicrobial activities. The current studies show that Duox is required to activate wound reporter genes after epidermal wounding, and that injected exogenous H₂O₂ is sufficient to activate widespread epidermal wound gene expression. Overexpression of either Flo-2 or Src42A.CA can inhibit the H₂O₂ -dependent wound reporter expression, suggesting that all of these components are in a common pathway controlling the activation of epidermal wound reporters. However, the ability of trypsin injection to activate the Ddc .47 and ple-WE1 wound reporters in Duox mutants suggests that a serine protease might act downstream of, or in parallel to, H₂O₂-dependent wound signals. A recent report showed that in cultured mammalian cells, a Src kinase phosphorylates and inhibits a Flo-2-associated enzyme, peroxiredoxin-1, which results in increased stability of H₂O₂. This is consistent with the results placing Flo-2, Src42A, and H₂O₂ in a common wound signaling pathway (Juarez, 2011).

Like H₂O₂, the injection of methyl-β-cyclodextrin (MβCD) into wounded embryos triggers a global wound response in the epidermis. MβCD strongly depletes cholesterol and other sterols from membranes and disrupt lipid rafts, but was also shown to remove sphingolipid-associated proteins such as Src-Family Kinases. The effects of MβCD, in combination with the effects of loss of Flo-2, suggests that the integrity of lipid rafts and associated proteins are required to inhibit epidermal wound signals. In cultured cells, MβCD treatments trigger a release of EGF receptors from membrane microdomains, which increases EGFR, and perhaps other RTK, signaling in a ligand-independent manner. Interestingly, in cultured keratinocytes, MβCD treatment can induce the expression of involucrin, which encodes a protein, analogous to Drosophila Ple/tyrosine hydroxylase, which is required for the formation of an epidermal barrier. Similarly, MβCD injections into Drosophila embryos might also cause an increase the levels of a wound signal produced or released from cells adjacent to the wound site, allowing more widespread transcriptional activation of wound reporter genes. The observations that overexpression of Src42A or Flo-2 can inhibit the MβCD -triggered activation of epidermal wound reporter genes suggest that high levels of these proteins might overcome lipid raft-inhibitory effects on wound signaling pathways (Juarez, 2011).

Other genes (wurst and varicose) identified in the screen have phenotypes similar to Flo-2 and Src42A mutants. wurst encodes an evolutionarily conserved trans-membrane protein, containing a heat shock cognate protein 70 binding domain and a clathrin binding motif. wurst is ubiquitously expressed in embryonic epithelial cells, strongly up-regulated during endocytosis-dependent luminal clearance, and mislocalized in mutants with endocytosis defects. wurst mutant embryos have tortuous tracheal tubes, due to a failure to properly endocytose matrix material from the tracheal lumen. varicose encodes an evolutionarily-conserved septate junction scaffolding protein, in the Membrane Associated GUanylate Kinase (MAGUK) family. varicose is expressed in epidermally-derived cells (including the hindgut and trachea) and co-localizes with the septate junction proteins, Coracle and Neurexin4. varicose mutant embryos develop permeable tracheal tubes and paracellular barrier defects in epithelia. Like wurst mutants, varicose mutants also have abnormal matrix composition in the tracheal lumen, and may also have abnormal extracellular matrix composition produced by other epidermal cells (Juarez, 2011).

Another gene (ghost), also known as stenosis) identified in this screen is required for wound reporter activation like Duox or grh. ghost encodes the Drosophila Sec24CD homolog, a coat protein of COPII vesicles in the ER/Golgi trafficking pathway. Transport of cargo from the ER to the Golgi via COPII vesicles is required to achieve normal amounts of secretion of extracellular matrix proteins into the developing Drosophila tracheae and normal apical-basal localization of membrane proteins. Presumably, similar secretion and membrane localization defects occur in non-tracheal epidermal cells, which account for the severe cuticle deposition defects in ghost (Sec24CD) mutants. It is fascinating to note that the finding that ghost (Sec24CD) is required for transcriptional activation of epidermal wound reporter genes is consistent with the finding that RNAi knockdowns of Sec24C in a planaria (Schmidtea mediterranea) interfered with normal regeneration after amputation wounds. It is possible that the ghost mutants do not secrete enough wound signals, or the protein matrix necessary for the propagation of a wound signal (Juarez, 2011).

Another gene required for the activation of wound reporters is shroud (sro). It is believed sro to be an allele in the Drosophila Fos-D isoform, and it was hypothesized that one of the Drosophila kayak/Fos transcription factors was required for the activation of some epidermal wound gene reporters. However, has been recently discovered that sro[1] and other sro point mutant alleles do not map in the kayak/Fos gene, but in an immediately adjacent transcription unit (Nm-g/sro) that encodes an enzyme in the sterol metabolic pathway that is necessary for production of ecdysone hormone. At first glance, the requirement of sro to activate some wound reporters suggested that these reporters rely on ecdysone signaling. This is possible, although deletions were tested that eliminate zygotic functions of the ecdysone receptor gene, as well as of the phantom gene (which encodes another enzyme in the ecdysone synthesis pathway), and embryos that are zygotic mutants in either gene show normal activation of the ple-WE1 wound reporter after puncture wounding (Juarez, 2011).

In summary, though this large unbiased screen, several genes were identified that add to the understanding of the complex pathways that control the signals that activate wound response transcription near puncture wounds. At the cellular level, there appears to be a correlation between genetic functions required to localize wound-induced gene activation, and cellular functions required for endocytosis and/or apical-basal polarity. For example, one function of Flo-2 is in signal-dependent endocytosis, although Flo-2 also plays other roles in vesicular trafficking. There have been many studies showing that endocytosis can regulate extracellular signaling strength and duration. For example, one study found that tagged-FGF8 showed increased accumulation, spread, and target gene activation when Rab-5-mediated endocytosis was reduced in zebrafish embryos. It is believed that further studies on wound response signaling may provide new insights into how membrane microdomains, endocytosis of membrane receptors, and the composition and organization of the extracellular matrix, regulates the transmission of wound signals (Juarez, 2011).

Anisotropic Crb accumulation, modulated by Src42A, is coupled to polarised epithelial tube growth in Drosophila

Tube size control and how tubular anisotropy is translated at the cellular level are still not fully understood. This study investigated these mechanisms using the Drosophila tracheal system. The apical polarity protein Crumbs transiently accumulates anisotropically at longitudinal cell junctions during tube elongation. Evidence is provided indicating that the accumulation of Crumbs in specific apical domains correlates with apical surface expansion, suggesting a link between the anisotropic accumulation of Crumbs at the cellular level and membrane expansion. This study finds that Src42A is required for the anisotropic accumulation of Crumbs, thereby identifying the first polarised cell behaviour downstream of Src42A. The results indicate that Src42A regulates a mechanism that increases the fraction of Crb protein at longitudinal junctions, and genetic interaction experiments are consistent with Crb acting downstream of Src42A in controlling tube size. Collectively, these results suggest a model in which Src42A would sense the inherent anisotropic mechanical tension of the tube and translate it into a polarised Crumbs accumulation, which may promote a bias towards longitudinal membrane expansion, orienting cell elongation and, as a consequence, longitudinal growth at the tissue level. This work provides new insights into the key question of how organ growth is controlled and polarised and unveils the function of two conserved proteins, Crumbs and Src42A, with important roles in development and homeostasis as well as in disease, in this biological process (Olivares-Castineira, 2018).

Since Crb has been proposed to regulate tube length by promoting apical membrane growth, Crb accumulation was examined in the Dorsal Trunk (DT, the main tracheal trunk connecting to the exterior through the spiracles). Crb can localise to different subdomains of the apical membrane during tracheal development: the SubApical Region (SAR) and the Apical Free Region (AFR). The AFR corresponds to the most apical domain, free of contact with other epithelial cells and in direct contact with the lumen in the case of tubular organs like the trachea, while the SAR corresponds to the most apicolateral membrane domain of contact between neighboring epithelial cells. Previous work has shown that during the stages of higher longitudinal DT growth, stage 15 onwards, Crb accumulated strongly in the SAR, displaying a mesh-like pattern that identifies the apical junctional domain. Strikingly, Crb was anisotropically (not uniformly) distributed in the SAR of cell junctions. Cell junctions are classified as longitudinal cell junctions (LCJs), mainly parallel to the longitudinal axis of the tube, and transverse cell junctions (TCJs), perpendicular to the longitudinal axis. Crb accumulation was found to be more visible at LCJs than at TCJs; several examples were observed where accumulation of Crb at TCJs was almost absent. The accumulation of Crb (total fluorescence intensity/junctional length) was quantified at LCJs and TCJs; accumulation was found to be biased to LCJs, where levels were around 30% higher than at TCJs (average % of difference of Crb accumulation at LCJs and TCJs). To compare different embryos the LCJ/TCJ ratio was calculated of Crb accumulation, which showed an average of 1,5 (n = 15 embryos), indicating that Crb is anisotropically distributed, i.e. polarised. In contrast to Crb, DE-Cadherin (DE-cad), a core component of the Adherens Junctions (AJs), was equally distributed among all cell junctions. The ratio of accumulation in LCJ/TCJ was close to 1, indicating that the anisotropic distribution is not a general feature of all junctional proteins. These results indicated that a larger proportion of LCJs accumulate higher levels of Crb than TCJs (Olivares-Castineira, 2018).

To further investigate this observation time-lapse imaging was carried out in embryos carrying the viable and functional CrbGFP allele as the only source of functional Crb protein. Enrichments were observed of Crb protein at LCJs, and less conspicuous accumulations were found at TCJs, from late stage 15 and during stage 16 over a period of 1,30-2 hours. This correlated with an increase in tube length of around a 30% and a moderate increase in tube diameter of an 11% (Olivares-Castineira, 2018).

Altogether these results point to a polarised accumulation of Crb that correlates with an anisotropic growth along the longitudinal axis of the DT during stage 16. It is worth pointing out that anisotropies of Crb, like the one described in this study, or of other apical determinants, have important implications in morphogenesis (Olivares-Castineira, 2018).

Different molecular mechanisms could underlie the preferential accumulation of Crb at LCJs, such as specific Crb degradation at TCJs, specific stabilisation at LCJs, targeted intracellular trafficking, differential protein recycling, among others. To investigate the possible mechanism behind the anisotropic pattern of Crb accumulation FRAP analysis was performed at either LCJs or TCJs of embryos carrying the CrbGFP allele. The amount of fluorescent protein, relative to the pre-bleach value, mobilized during the experimental time (mobile fraction, Mf) was significantly higher at LCJs compared to TCJs, indicating a higher recovery of CrbGFP protein at LCJs. To assess the recovery kinetics the half-time (t1/2, time to reach half of the Mf) was calculated. The half-time was not significantly different at LCJs and TCJs, suggesting that the recovery rate is comparable at the differently oriented junctions. Kymographs of the bleached regions suggested that the recovery was not due to lateral diffusion. Altogether the results indicated a higher mobility of Crb protein at LCJs but a constant rate of incorporation in all junctions (Olivares-Castineira, 2018).

Hence, on the one hand higher levels of Crb accumulate at LCJs, and on the other, FRAP experiments show that Crb protein is more mobile at LCJs. These results could suggest the existence of two molecularly defined different pools of Crb in the junctions with different mobility: a basal level-pool with lower mobility and an enrichment-pool with higher mobility. The basal level-pool would be present in all junctions, while a mechanism acting specifically at LCJs would ensure also the presence of the enrichment-pool there. The increased mobility/instability of the enrichment-pool of Crb at LCJs would contribute to increase the total Crb mobility at LCJs. Further experiments will be required to test this possibility and to understand how the molecular mechanism underlying the increased accumulation of Crb at LCJs relates to the differential mobility of Crb protein that was documented (Olivares-Castineira, 2018).

It was next asked how the anisotropic distribution of Crb is regulated. To investigate this question attention was turned to Src42A, as it triggers one of the mechanisms regulating tube elongation, orienting membrane growth on the longitudinal axis. In conditions of Src42A loss of function, LCJs do not expand and tubes become shorter. Crb accumulation was analyzed in loss of function conditions for Src42A. In one case the Src42AF80 allele was used that lacks the distinct accumulation of phosphorylated Src42A (pSrc42A) at the apical junctional region but does not affect the stability or membrane localisation of the protein. This mutation renders a kinase non-activatable protein that was previously shown to strongly affect tracheal tube elongation. In contrast, a kinase-dead dominant negative form of Src42A (Src42DN) was expressed in the trachea, that was also previously shown to affect tube elongation. In both cases a more uniform distribution was observed of Crb at LCJs and TCJs. Quantification of Crb levels indicated that the differences between the accumulation of Crb at LCJs compared to TCJs were reduced. Analysis of the LCJ/TCJ ratio of Crb clearly showed a significant decrease when compared to the control, indicating a more uniform accumulation in Src42A loss of function conditions. DE-cad LCJ/TCJ ratio in Src42A loss of function conditions remained close to 1, indicating a homogeneous distribution. Altogether these results show that a decrease in Src42A activity leads to a decrease of the anisotropic accumulation of Crb (Olivares-Castineira, 2018).

To investigate whether Src42A promotes an increased accumulation of Crb protein at LCJs or a depletion at TCJs the total levels of protein accumulation were quantified at LCJs and TCJs and control (i.e. heterozygotes) and Src42A mutants (i.e. Src42AF80 homozygotes) were compared from the same experiment. While variability was observed within each genotypic group, different independent experiments indicated that in control embryos there is an increased accumulation of Crb protein at LCJs that is lost in Src42AF80 mutants. In Src42A mutant conditions the levels of Crb accumulation at LCJs and TCJs were similar to those of TCJs of control embryos, indicating that Src42A regulates a mechanism that increases the fraction of Crb protein at LCJs (Olivares-Castineira, 2018).

Consistent with a role for Src42A in regulating directly or indirectly Crb accumulation partial co-localisation was found of Crb and Src42A protein, and with pSrc42A at the SAR. However, no polarised accumulation was detected of the active pSrc42A fraction during tube elongation, as previously documented. While there may be transient anisotropies of pSrc42A accumulation that cannot be detected with the available antibodies, this result suggests that other factors (e.g. mechanical or chemical) modulate the activity of pSrc42A in the different junctions to regulate the anisotropic accumulation of Crb (Olivares-Castineira, 2018).

To further explore Src42A requirement FRAP experiments were performed in CrbGFP embryos in which Src42A was downregulated. Clear differences were found with respect to control: while in the control the Mf and recovery curves of LCJs and TCJs were clearly different, in Src42DN conditions the Mf and recovery curves of LCJs and TCJs were comparable. The Mf at the LCJs of Src42DN was significantly lower than the Mf at the LCJs in control embryos, and was similar to the Mf at TCJs in control and mutant embryos. The halftime recovery, t1/2, was comparable to that of control embryos, indicating a recovery rate similar in all cases (Olivares-Castineira, 2018).

Altogether these results indicate that Src42A contributes to Crb preferential enrichment at LCJs and that it increases Crb mobility there. The fact that Crb levels and Crb recovery are affected particularly at LCJs when Src42A is downregulated strongly suggests that Src42A is (more) active precisely at LCJs. The results are consistent with the proposed model in which a mechanism acting specifically at LCJs, that is now proposed to be mediated by Src42A, would ensure the accumulation of an enrichment-pool of Crb at LCJs with high protein mobility. In the absence of this Src42A-mediated activity, only the basal level-pool of Crb would be present at LCJs and TCJs, leading to comparable levels and mobility of Crb in all junctions. Src42A would not regulate the basal level-pool of Crb, and would instead be required to top up Crb at LCJs with an enrichment-pool of Crb. Future experiments addressing the molecular mechanism by which Src42A regulates Crb accumulation and its mobility will help to fully understand how it regulates the anisotropic accumulation of Crb at LCJs. Src42A-independent accumulation of Crb in tracheal cells together with other Src42A-independent mechanisms of apical membrane growth may be responsible for tube growth in the absence of Src42A (Olivares-Castineira, 2018).

Src42A was found to be required for the anisotropic accumulation of Crb at LCJs. It was then asked whether the overelongation of tubes observed in Src42A overactivation conditions (either overexpression of a wild type form of Src42A or expression of a constitutively active protein) was due to an increased accumulation of Crb at LCJs. The results did not support this expectation. Crb was found to be strongly decreased in the SAR of DT cells both in conditions of overexpression (UASSrc42A) or constitutive activation (UASSrc42ACA). In conditions of mild overexpression of wild type Src42A, rare cases (around 5-8% of embryos) were found where it was possible to detect some levels of Crb in the SAR, which accumulated preferentially at LCJs, as expected. These results suggested that the tube length defects produced by Src42A overexpression/overactivation were caused by a mechanism different than the one operating in physiological conditions. To investigate this possible mechanism the levels and distribution of the total Src42A protein and the pSrc42A active fraction were analyzed in both Src42A overexpression and overactivation conditions. Levels of Src42A protein were found to be increased but still enriched in the membrane region. Interestingly, pSrc42A was not restricted any more to the junctional apical region as in the wild type and instead it was expanded along the whole apicobasal membrane. The increase and expansion of pSrc42A accumulation observed in overexpression and in overactivation conditions indicate that Src42A activity is overactivated in both cases and may explain the similarity of phenotypes. Further analysis also indicated that Src42A overexpression/overactivation leads to a general loss of cell organisation and membrane polarity, as evidenced by the miss-localisation of markers of membrane polarity, like the Septate Junction protein Megatrachea. These results indicate that an unregulated accumulation of active pSrc42A leads to a generalised miss-organisation of the cell and prevents proper Crb accumulation (Olivares-Castineira, 2018).

To investigate the cause of tube overelongation found in Src42A overexpression /overactivation conditions other known tube length regulators were analyzed. One of them is Serp, which regulates the aECM organisation. In Src42A and Src42ACA overexpression conditions Serp was found to be lost from the luminal compartment, although, as in wild type, tracheal cells accumulate Serp at early stages. This result provides explanation for the tube elongation defects observed under these conditions, as Serp absence leads to tube overelongation. Interestingly, defects in Serp accumulation could not be detected in Src42 mutants or in Src42ADN conditions, as previously reported. These results suggested again that Src42A overactivation use a different mechanism than the one used in physiological conditions to drive tube elongation. Hence, the analysis of Src42A overactivation provides new results that allow to revisit and reinterpret previously published work (Olivares-Castineira, 2018).

Altogether the results indicate that an unregulated accumulation of active pSrc42A leads to a generalised miss-organisation of the cell and prevents proper accumulation of Crb and Serp. In addition, it was also observed that DE-cad was not properly localised either. Interestingly, it has been shown that the tracheal accumulation of these proteins depends on their recycling. Thus, the results could suggest a role of Src42A in protein trafficking. In this context, the loss of cell organisation and membrane polarity produced by mislocalisation of pSrc42A could interfere with protein trafficking. Roles for Src42A in protein trafficking have been proposed in different contexts. Src42A could regulate protein trafficking directly, or indirectly through the regulation of the actin cytoskeleton. The actin cytoskeleton plays a capital role in protein trafficking and Src42A acts as a regulator of the actin cytoskeleton. A disruption of actin organisation in Src42A overactivation could lead to defects in the sorting of different cargoes as well as defects in endosomal maturation. Further experiments will be required to investigate a possible involvement of Src42A in protein trafficking during tracheal development (Olivares-Castineira, 2018).

After identifying an anisotropic accumulation of Crb regulated by Src42A, it was asked how this mechanism relates to tube elongation. Crb has been proposed to promote apical membrane growth independently of its role in apicobasal polarity at late stages of epithelial differentiation. In the trachea Crb was proposed to mediate tube elongation by promoting apical membrane growth. Interestingly, the results show an enrichment of Crb in the SAR of LCJ during tube elongation. This observation raises the hypothesis that it is precisely this accumulation of Crb in the SAR of LCJs what favours or facilitates apical membrane expansion (either by membrane growth or membrane transformation leading to cell shape changes), orienting cell elongation and as a consequence the longitudinal growth of the tube. Crb recycling during tracheal development could favour the mobilisation of cellular and/or membrane components facilitating membrane growth or membrane transformation. To investigate this possibility Crb accumulation in the SAR was analyzed in different experimental conditions in which apicobasal polarity was unaffected (Olivares-Castineira, 2018).

Altogether the results confirm a role of EGFR in regulating the accumulation of Crb in the SAR or AFR, at least in tubular organs. EGFR regulates the trafficking of different cargoes, in particular Crb and Serp, raising the possibility that the regulation of the apical surface area depends on targets different than Crb. However, the fact that Serp is not present in the SGs and that Crb has already been proposed to promote apical membrane growth, strongly suggest that Crb is at least one of the targets downstream of EGFR regulating apical expansion. On the other hand, the results correlate apical cell expansion with Crb subcellular localisation in the SAR. It is suggested that Crb accumulation in the SAR of LCJs could promote their expansion facilitating the elongation of the cell along the longitudinal axis, in agreement with the proposed role of Crb promoting apical membrane expansion. Previous observations such as the expansion of the photoreceptor stalk membrane upon Crb overexpression support this hypothesis, indicating that this can be a general mechanism (Olivares-Castineira, 2018).

Crb was proposed to regulate tube size by promoting apical membrane growth. Accordingly, it was found that a weak overexpression of Crb in an otherwise wild type background caused a mild increase in DT dimensions (a significant 12% enlargement of DT and a non-significant 9% diameter expansion) without perturbing the epithelial integrity and polarity. Src42A was shown to control tube elongation through interactions with dDaam and the remodelling of AJs. This study is now showing that Src42A regulates Crb levels, suggesting that Src42A may control tube elongation at least in part through regulation of Crb. Thus, it was asked whether increased levels of Crb can bypass or compensate the requirement of Src42A in tube growth. To evaluate this possibility genetic interaction experiments were performed to test the ability of a weak Crb overexpression in a Src42A loss of function background. Interestingly, Crb overexpression was found to produced a partial but significant rescue of the short-DT phenotype of Src42A loss of function. This result indicates that Crb acts downstream or in parallel of Src42A. Because it was also observed that Src42A is required for Crb preferential enrichment at LCJs, the hypothesis is favored that Crb acts downstream of Src42A contributing to its function in tube elongation (Olivares-Castineira, 2018).

Remarkably, besides a rescue in DT length, an increase was also detected in the diameter of the tube when Crb was overexpressed in a Src42A loss of function background (a 25% expansion with respect to Src42ADN mutants). Under these conditions, the DT diameter was not perfectly smooth and often showed dilations that were not detected in Src42ADN or Crb overexpression conditions on their own. This isometric expansion of the DT along the diametrical and longitudinal directions is interpreted to be the result of an isotropic excess of Crb. Because in the absence of Src42A activity Crb accumulation is not properly polarised, this may promote a non-polarised increase of tube growth. To find support for this interpretation Crb accumulation was analyzed in conditions of weak Crb overexpression. High levels of Crb were detected in the whole apical domain and in vesicles that precluded a proper analysis of Crb localisation and a systematic quantification of Crb accumulation. However, it was possible to observe in examples in which a distinct accumulation of Crb could be detected that the overexpression of Crb in a wild type background leads to high enrichments of the protein particularly at LCJs. This result suggests that the activity of Src42A biases the increased accumulation of Crb to the LCJs, correlating with a preferential growth mainly along the longitudinal axis. In contrast, a more generalised pattern of Crb overexpression could be detected in a Src42A loss of function mutant background, consistent with the isometric tube growth observed. In summary, although it was not possible to directly test whether an anisotropic accumulation of Crb can exclusively compensate tube elongation in Src42A loss of function conditions, the results are consistent with the hypothesis that it is the anisotropic accumulation of Crb, regulated by Src42A, that mediates or promotes oriented tube growth along the longitudinal axis. Future experiments involving the generation of new tools designed to specifically localise Crb protein at desired subcellular domains will be needed to prove the current model and to confirm an instructive and causal role of the anisotropic accumulation of Crb in cell elongation and polarised tracheal tube growth (Olivares-Castineira, 2018).

To summarise, this study finds that Crb is transiently enriched in the SAR of DT cells in a polarised/anisotropic manner. This polarised distribution correlates with different dynamics or turnover of Crb protein, which appears to be more mobile and accumulate more at longitudinal junctions than at transverse ones. This polarised distribution also correlates with the anisotropic expansion of the apical membrane, axially-biased, that drives the longitudinal enlargement of the tracheal tubes. Interestingly it was also found that Src42A is required for this anisotropic accumulation of Crb. Src42A was already known to regulate tube growth along the longitudinal axis, and this study now proposes that it performs this activity at least in part by promoting a Crb anisotropic enrichment. Src42A was also proposed to act as a mechanical sensor, translating the polarised cylindrical mechanical tension (an inherent property of cylindrical structures) into polarised cell behaviour. Hence, it is proposed that Src42A would sense differential longitudinal/transverse tension stimuli and translate them into the cell by polarising Crb accumulation. It is likely that this Crb anisotropic accumulation in the SAR of LCJs mediates apical membrane expansion in the longitudinal direction, which would help to orient cell elongation and as a consequence longitudinal tube growth. A causal role for this Crb anisotropic accumulation in orienting cell elongation awaits definitive confirmation (Olivares-Castineira, 2018).

In light of the current results, the following model is now proposed. Different mechanisms operate to regulate tube growth. On the one hand secretion drives apical membrane growth along the transverse axis independently of Src42A. In addition, a basal level-pool of Crb accumulation independent of Src42A may promote or contribute to isotropic apical expansion. On the other hand the presence of a properly organised luminal aECM also controls tube growth by restricting tube elongation. A Src42A-dependent mechanism acts in coordination with these other mechanisms. Src42A would contribute to tube elongation through interactions with dDaam, the remodelling of AJs and topping up Crb accumulation at LCJs with an enrichment-pool of Crb. This increased accumulation of Crb at LCJs would bias the growth of the tube along the longitudinal axis, counteracting the restrictive activity of the aECM on tube elongation. In the absence of Src42A activity, the Src42A independent mechanism/s of membrane growth would still operate, and would favour a compensatory growth along the transverse axis as observed, as diametrical growth is not restricted by the aECM (Olivares-Castineira, 2018).

The regulation of size and shape of tubular organs is important for organ function, as evidenced by the fact that loss of regulation can lead to pathological conditions such as polycystic kidney disease (PKD), cerebral cavernous malformation (CCM) or hereditary hemorrhagic telangiectasia (HHT). Src proteins have been implicated in malformations like PKD, highlighting the importance of investigating the mechanisms underlying their activities. While Src42A was proposed to regulate polarised cell shape changes during tracheal tube elongation through interactions with dDaam and the remodelling of AJs, no polarised downstream effectors have been identified up to date. Hence, identifying that Crb anisotropy is one of the downstream effects of Src42A activity adds an important piece to the puzzle. Src42A and Crb are conserved proteins with important roles in development and homeostasis and are involved in different pathologies. This work provides an ideal model where to investigate the molecular mechanisms underlying their activities, their interactions, and their roles in morphogenesis (Olivares-Castineira, 2018).

Extracellular matrix stiffness cues junctional remodeling for 3D tissue elongation

Organs are sculpted by extracellular as well as cell-intrinsic forces, but how collective cell dynamics are orchestrated in response to environmental cues is poorly understood. This study applied advanced image analysis to reveal extracellular matrix-responsive cell behaviors that drive elongation of the Drosophila follicle, a model system in which basement membrane stiffness instructs three-dimensional tissue morphogenesis. Through in toto morphometric analyses of wild type and round egg mutants, this study found that neither changes in average cell shape nor oriented cell division are required for appropriate organ shape. Instead, a major element is the reorientation of elongated cells at the follicle anterior. Polarized reorientation is regulated by mechanical cues from the basement membrane, which are transduced by the Src tyrosine kinase to alter junctional E-cadherin trafficking. This mechanosensitive cellular behavior represents a conserved mechanism that can elongate edgeless tubular epithelia in a process distinct from those that elongate bounded, planar epithelia (Chen, 2019).

Follicles have an architecture that is typical of a number of animal organs, with several components that associate to form a 3D acinar epithelium surrounding a lumen. At the same time, the simplicity and highly regular development of the follicle lend themselves to comprehensive analyses. The follicle exhibits straightforward and symmetric geometry for much of its development, while its cells originate from only two stem cell populations and show limited differential fates. Follicles can be genetically manipulated using the powerful Drosophila toolkit, and are well-suited for imaging either in fixed preparations or when cultured live ex vivo (Chen, 2019).

Development of the follicle involves several conserved morphogenetic behaviors including initial primordial assembly, epithelial diversification, and collective cell migration. A major focus for mechanistic studies has been follicle elongation, during which the initially spherical organ transforms into a more tube-like ellipsoid shape. ~2-fold elongation is seen in ~40 h between follicle budding at stage 3 to the end of stage 8; eventually there is ~2.5-fold overall elongation when the egg is laid ~25 h later. This degree of elongation is similar to that in paradigmatic morphogenetic systems such as the amphibian neural plate and mesoderm, or the Drosophila germband. In the latter tissues, the main cellular behavior that drives elongation is convergent extension, as cells intercalate mediolaterally toward a specific landmark that is defined anatomically and/or molecularly. However, these tissues have defined borders, which create boundary conditions to instruct and orient cell behaviors. No such boundary is evident along the edgeless epithelium of the Drosophila follicle, and the cellular changes that drive elongation of this acinar organ are not known (Chen, 2019).

Recent work has shown that mechanical heterogeneity patterned not within the cells of the follicle, but instead within its underlying basement membrane (BM), instructs organ shape (Crest, 2017). Specifically, a gradient of matrix stiffness that is low at the poles and peaks in the organ center provides differential resistance to luminal expansion, leading to tissue elongation. Construction of this pattern relies in part on a collective migration of cells around the follicle equatorial axis, leading to global tissue rotation. But how the cells of the epithelium respond to stiffness cues and engage in the dynamics that actually elongate the organ along the anterior-posterior (A-P) axis remains unexplored (Chen, 2019).

This study has identified an unexpected cell behavior that drives follicle elongation and demonstrates its control by a regulatory axis that responds to BM stiffness cues, thus connecting extracellular mechanical properties to intracellular signaling that drives intercellular morphogenesis in vivo (Chen, 2019).

All three morphogenetic behaviors that are engines for tissue elongation in other systems (cell shape changes, cell division, and cell rearrangement) are seen during follicle elongation. Average cellular elongation changes little during follicle elongation, and an initial cell shape anisotropy-lengthening around the organ circumferential axis-is not perturbed in a round egg mutant. Additionally, tissue rotation alone is insufficient to direct elongation-driving cell dynamics. The manipulations carried out in this study show that oriented cell division is not essential for elongation, and its absence can be compensated by other polarized cell behaviors, as also seen in e.g., the pupal thorax. Moreover, addition of cells to the elongating axis of the follicle continues at stage 7, after all mitoses have ceased. Instead, the data suggest that the major tissue-elongating behavior involves changes in cell orientation. In particular, cells in the follicle anterior shift the direction of their long axis towards the A-P axis. Without changing average cellular elongation, this reorientation of anisotropically-shaped cells changes neighbor relationships; it is suggested that this results in net cellular intercalation along the A-P axis (Chen, 2019).

The number of frank intercalations that occur during follicle elongation appears relatively limited. For instance, of the ~850 cells in a post-mitotic stage 7 follicle, only ~3 are added to the meridional arc during ~8 h prior to stage 8, inducing a ~10% change in aspect ratio. This relative paucity of intercalation events contrasts not only with the rapidly developing Drosophila germband, but also with other elongating tissues such as the Drosophila pupal wing and thorax and the vertebrate neural tube and mesoderm. Three major differences bear consideration here. First, significant growth (~9-fold) of both epithelial cells and the follicle overall occur during the 24-34 h that span stages 4 to 8, while the other systems largely rearrange a constant tissue volume. Second, the other systems have defined boundaries to impose vectorial orientation of cell behaviors, while the topologically continuous follicle epithelium lacks a boundary in the equatorial axis. Third, follicle elongation does not require conventional PCP morphogenetic signaling, nor is there evidence of PCP Myosin localization that remodels cell junctions. Instead, the instructive force seems to be patterned anisotropic resistance to growth, wherein softer BM at the poles triggers regional changes in cell behavior. Indeed, the largest changes in follicle cell orientation are seen at the anterior pole, and genetic manipulation in this region alone can prevent elongation. It is important to note that the data do not identify BM stiffness differences between the anterior and the posterior prior to cell orientation changes in the former, nor differences in pSrc levels, but do identify differences in Ecad dynamics in the latter. It is speculated that A-P patterned fates in the follicle epithelium prevent elongation behaviors in the posterior, consistent with the observation that follicles lacking posterior fate specification elongate at both poles. Overall, these differences with bounded epithelia undergoing elongation via convergent extension emphasize the new perspectives required for analysis of elongating tubular organs (Chen, 2019).

How do cells sense BM stiffness to change their orientations? The elongation-defective phenotype of Rack1-depleted follicles points to one mechanism involving Src. Rack1 is a Src-inhibiting protein, and its loss, with associated increases in cellular Src activity, perturbs follicle elongation. In Rack1-depleted follicles, initial cell orientation is not greatly altered, but anterior follicle cells remain static and are unable to shift their orientation during stages 6-8. This suggests that proper regulation of Src is required for the dynamic reapportionment of cell shape that is reflected in this switch. One familiar regulatory target of Src is integrins and their associated proteins, but no defects in integrin-dependent cell migration are seen when Rack1 is depleted from follicle cells. However, Src has also been implicated in directly regulating AJ remodeling through effects on Ecad trafficking, and FRAP analysis reveals compromised Ecad dynamics when follicles cells lack Rack1. Src is considered a mechanotransducer, and pSrc levels anticorrelate with BM stiffness in wild type, mutant and manipulated follicles. It is therefore proposed that Src mediates the instructive cue provided by BM stiffness, inducing AJ remodeling to drive morphogenesis. Whether the AJ-regulating apical Src seen in follicles is directly phosphorylated by basal integrin activation, or results from an indirect intracellular signaling cascade, remains to be investigated (Chen, 2019).

The defective tissue topology and reduced Ecad mobility seen in Rack1-depleted follicles suggests that Src is required for both elongation-driving and tissue disorder-minimizing cell rearrangements. This role of Src in follicle elongation raises interesting parallels with a second edgeless epithelium: the Drosophila trachea. In this established tubulogenesis model, gain as well as loss of Src42A activity results in shortened but broader tubules, which result from inappropriately oriented cells. Interestingly, depletion of Src42A from the follicle, like its activation, also induced elongation defects, while Src controls tracheal Ecad dynamics, perhaps in response to an ECM, albeit apically localized. Thus, in both organs Src42A could mediate stiffness-cued AJ dynamics that change the orientation of cell eccentricity. Since mammalian kidney tubule development also shows Src-dependence, these results suggest that ECM-mediated control of cell junctions via Src may be a general mechanism for morphogenesis of edgeless epithelia (Chen, 2019).

The data described in this work stem from a comprehensive analysis of follicle morphogenesis, which was enabled by ImSAnE software. ImSAnE allowed identification of critical cell dynamics near the follicle poles, which are most subject to distortion from conventional analyses, and distinguished the cellular basis underlying overtly similar elongation phenotypes. For instance, it revealed that under fat2 depletion, cells throughout the follicle are defective in planar polarized cell orientation from early stages, whereas in Rack1 follicles defective orientation is limited to the anterior and occurs only later, when elongation behaviors initiate. Furthermore, some of the ImSAnE-based morphometric findings are concordant with those recently reported based on conventional imaging. This work focuses on stages 4 through 8, and does not address cell dynamics that elongate the follicle prior to and following these stages. However, it does provide a framework for true in toto analysis of this simple organ, at multiple developmental stages and genotypes. As genetic screens and biophysical studies in the follicle are extended, the quantitative imaging platform reported here will provide a bridge towards mathematical and mechanical modeling of this flourishing system for in toto organ morphogenesis (Chen, 2019).

GENE STRUCTURE

cDNA clone length - 2588bp

Bases in 5' UTR - 538

Exons - 10 (Src42A-RA)

Bases in 3' UTR - 496

PROTEIN STRUCTURE

Amino Acids - 517

Structural Domains

See NCBI Conserved Domain Summary for information on Src42A structure.

EVOLUTIONARY HOMOLOGS

See Src oncogene at 64B for information on Src family evolutionary homologs.

date revised:23 June 2023

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