karst

DEVELOPMENTAL BIOLOGY

Embryonic

Karst transcript is not maternally loaded, but the protein (ß_H spectrin) is present in eggs. The karst gene shows no detectable transcript accumulation until stage 10 (fully extended germband) of embryogenesis, after which transcript accumulates through the remainder of embryogenesis. Karst transcripts are always confined to a specific subset of cells and tissues. The first transcripts accumulate at stage 10 in the primordia of the foregut and hindgut, and then in numerous mesodermal cells that appear in a segmentally repeated pattern. These labeled mesodermal cells subsequently increase in number and are probably somatic muscle precursor cells. Concomitant with this, Karst transcript also accumulates deeper in the embryo, in the splanchnopleura, which gives rise to the visceral mesoderm. Karst is thus expressed in much or all of the muscular tissue of the developing embryo at a time comparable to the transcription factors nautilus and Nk1/S59, which are the earliest known markers of muscle tissue. As the germband starts to retract, expression is also seen in the abdominal and thoracic epidermis, and, after dorsal closure, expression is seen in the trachea (Thomas, 1994).

Maternal Karst protein is closely associated with the membrane of the egg during the syncytial stages of embryogenesis, and shows its first dynamic involvement with the developmental process when it becomes associated with the transient mitotic furrows during nuclear cycles 11, 12 and 13. During cellularization, Karst protein is seen in two locations: some Karst remains apically localized in the cell as a circumferential band of staining on the lateral membrane (apicolateral), but staining is also seen to descend with the invaginating cell membranes (basal), becoming concentrated in a region adjacent to the acto-myosin ring. The basal staining disappears during the latter part of cellularization (towards the end of the fast phase) and prior to gastrulation, suggesting that it plays a specific role during the cellularization process itself. The basal distribution of Karst appears to be lost first from the ventral cells that invaginate 5-10 minutes ahead of the dorsal cells. Redistribution of Karst to an apical cap or plate in the cell anticipates cell sheet morphogenesis. Whereas most of the Karst protein at the cellular blastoderm stage is seen to be apicolaterally localized in all cells except the pole cells, certain regions that are undergoing contractile processes to produce morphogenetic movements (ventral and cephalic furrows, and the posterior midgut invagination), have an additional cap of Karst across the apical ends of the cells. In the case of posterior midgut invagination, for example, the redistribution of Karst to a cap, or plate, at the apical surface of the cell clearly anticipates any gross visible movement of the cell sheet. Dynamic redistribution of Karst throughout these early developmental stages is therefore correlated with changes in cell shape that are all probably occurring due to contractile forces generated by nonmuscle myosin. The source of Karst for the new apical domain could be either from a cytoplasmic pool or from a pre-existing membrane domain (Thomas, 1994).

Anti-Karst staining in embryos at later stages (stage 10), after zygotic transcription has initiated, recapitulates the transcript accumulation seen with the whole embryo hybridizations. There is, in addition, visible but diminishing staining in all cells, which can be attributed to the perdurance of the maternally loaded protein (Thomas, 1994).

Karst is closely associated with the plasma membrane of the somatic and visceral musculature: in stage 10-12 embryos, bright Karst staining is seen in the somatic and visceral muscle precursors. The somatic muscle precursors are visible in the ventral mesoderm just above the developing central nervous system (CNS), and appear in a segmentally repeated pattern. As the germband retracts these cells move to more lateral positions, and staining can ultimately be seen in the maturing somatic musculature. The visceral musculature originates in segmentally repeated clusters of cells that join together along the anterior-posterior axis and form the characteristic palisade cell structure of the splanchnopleura. Concomitant with the appearance of Karst in the somatic muscle precursors, the protein is also seen in the developing splanchnopleura, and can eventually be seen staining the mature visceral musculature around the midgut epithelium. The earliest sites of karst transcription are associated with the morphogenesis of both the foregut and the hindgut at the stomodeal and proctodeal invaginations, respectively. The former ultimately results in positive staining for Karst protein in both the pharyngeal musculature and esophagus. The latter early transcription site is found in a layer of cells on the inner surface of the proctodeum, which goes on to give rise to the visceral mesoderm of the hindgut. These cells stain with Karst antibodies and form a layer that stretches along most of the dorsoventral extent of this invagination. These cells are still evident at later stages of development and move in an anterior direction along the hindgut. In contrast to the other tissues, there is no obvious polarization of the cortex in muscle tissue that is reflected in the Karst distribution (Thomas, 1994).

Karst is apically localized in the trachea: as with other invaginations during embryogenesis, each of the 20 bilaterally symmetric tracheal placodes reorganize their membrane skeleton such that Karst is seen across the apices of the cells as they move into the embryo during stage 10. At this stage karst transcription is not detectable in the epidermis, so it is assumed that the protein being redistributed is maternal in origin. As the trachea mature, the sections of the longitudinal trunk connect, the transverse and ventral branches grow out, and transcription starts in this tissue. Although the staining is greatest at the abdominal spiracles, the protein can be detected at the lumenal surface of all the cells in the network, including the branches that penetrate the CNS (Thomas, 1994).

Karst is a prominent component of the cells in the epidermis and amnioserosa: Karst transcript accumulation begins in the epidermis as the germband retracts: this tissue stains intensely for Karst protein. With the exception of sites of invagination the protein is seen exclusively in an apicolateral distribution that was originally established with maternal Karst protein. The amnioserosa also stains very intensely for Karst after (but not before) germband retraction, and transcript accumulation has not been detected here using digoxigenin-labeled whole-embryo hybridization methods. Two possibilities most easily account for the increase in protein staining in the apparent absence of transcription. Either Karst is assembled into the membrane skeleton from a cytoplasmic Karst pool that is not visualized, or transcription is occurring but is not detected. The amnioserosa is a particularly delicate and exposed structure: failure to detect transcription may be due to excessive damage to this tissue during the proteolysis step of the hybridization procedure. It is unlikely that this is due to cross reaction with another protein (Thomas, 1994).

Karst is also localized in an apical cap or plate at later sites of invagination: staining at the apical ends of the cells at the salivary gland placodes is evident as soon as the organ invaginates, and continues to be present at a high level on the lumenal surface of the cells. It is not known exactly when transcription of the karst gene starts in the salivary gland, since whole-mount hybridization is less efficacious after dorsal closure and the start of cuticle deposition, and prior to that, the staining intensity of the epidermal layer is very high, making weak internal signals hard to visualize. The bright line of Karst staining along the ventral midline, which forms as the ventral furrow invaginates and cells on either side come into contact, persists for some time after the ventral furrow disappears and accumulation of Karst transcript starts in the ectoderm. This line of staining lies between the two rows of mesectodermal cells, which will later become part of the CNS at stage 12-13, approximately when the Karst staining disappears from the midline. Since this line appears prior to detectable transcript accumulation in the ectoderm, it is likely to be established with maternally supplied protein (Thomas, 1994).

The presence of Karst staining at sites where contraction is generated by cytoplasmic myosin raises the possibility that these two proteins colocalize in the cell. The basal staining for Karst during cellularization is very close to the region that contains the actomyosin contractile ring at the base of the forming cells. Cytoplasmic myosin has also been observed at the apices of the cells that form the midgut invagination. Double staining for both proteins simultaneously reveals that they are in distinct domains at cellularization but colocalize in the posterior midgut invagination. Thus, while Karst could be involved in the contractile process at the latter location it is unlikely that this is the case at cellularization (Thomas, 1994).

Larval

To investigate the origins of the karst phenotype, the distribution of ß_H protein was examined in both the eye/antennal and wing imaginal disc epithelia. ß_H2-spectrin is present in all cells of both discs where it exhibits an apicolateral localization. Double staining with Shotgun (DE-cadherin) indicates that this localization coincides with the adherens junctions. In the eye disc, there is also a greater concentration of ß_H at the adherens junctions in the photoreceptor preclusters that coincides with more prominent staining for Shotgun. These regions correspond to the type III adherens junctions observed by Takahashi (1996). ß_H also colocalizes with alpha-spectrin, however, the alpha-spectrin staining extends into the basolateral region of the cells, demonstrating that the ß_H distribution is apically restricted within the membrane skeleton. Conventional ß-spectrin presumably partners alpha-spectrin in the basolateral regions (Thomas, 1998).

The alpha-spectrin mutation disrupts the microvillar brush border lining on the apical surfaces of the cuprophillic cells of the larval midgut (Lee, 1993). This mutation presumably compromises the function of both alpha2-/ß2- and alpha2-/ß_H2-spectrins. The midgut brush border was therefore examined for the presence of ß_H in wild-type flies and for any discernible effects of the karst mutation to see if the specific absence of alpha2/ßH2 spectrin would compromise this structure. ßH is found beneath the brush border in both adults and larvae. Since there is some concerned about the perdurance of maternal product, the adult midgut was examined for defects in karst mutants where maternal contribution is not a problem. Here the brush border is well ordered without any consistent irregularities of the cell surface or variations in microvillar length. During the development of a brush border, the microvilli emerge before a terminal web is visible, so it may not be surprising that ßH is not necessary for the assembly of microvilli (Thomas, 1998)

karst mutant flies are very infertile, precluding the maintenance of viable homozygous karst stocks. A few mutant offspring from karst- crosses have been produced, suggesting that, in exceptional cases, individuals may develop in the complete absence of ß_H. alpha-spectrin has been shown to be important for oogenesis both in the follicle cells and in the germline. Staining of ovaries for ß_H reveals that it is primarily concentrated at the apical ends of the somatic follicle cells. Fixed karst mutant egg chambers were stained with rhodamine phalloidin, but this did not reveal any conspicuous defects in the actin cytoskeleton (Thomas, 1998).

The simple cellular composition and array of distally pointing hairs has made the Drosophila wing a favored system for studying planar polarity and the coordination of cellular and tissue level morphogenesis. A gene expression screen was carried out to identify candidate genes that functioned in wing and wing hair morphogenesis. Pupal wing RNA was isolated from tissue prior to, during and after hair growth and used to probe Affymetrix Drosophila gene chips. 435 genes were identified whose expression changed at least 5 fold during this period and 1335 whose expression changed at least 2 fold. As a functional validation, 10 genes were chosen where genetic reagents existed but where there was little or no evidence for a wing phenotype. New phenotypes were found for 9 of these genes providing functional validation for the collection of identified genes. Among the phenotypes seen were a delay in hair initiation, defects in hair maturation, defects in cuticle formation and pigmentation and abnormal wing hair polarity. The collection of identified genes should be a valuable data set for future studies on hair and bristle morphogenesis, cuticle synthesis and planar polarity (Ren, 2005).

The expression karst gene, which encodes betaHeavy-spectrin, increased 5.5 fold from 32 to 40 hrs. Spectrin typically contains 4 chains, 2 alpha and 2 beta; these chains are known to link the actin cytoskeleton to the plasma membrane. Somewhat surprisingly, kst mutants are viable (at reduced levels) and female sterile due to defects in the follicular epithelium. Adult kst mutants have rough eyes and their wings often are cupped downward. kst wings were examined and an additional mutant phenotype was found that is nicely correlated with its expression profile. kst wing cells produce normal looking hairs but the hairs are often found on a small pedestal. The wing cell surface (that is not hair) is rough and at times remnants of cell outlines are visible. This phenotype can also be seen in mosaic clones. The clones can be recognized under the stereo microscope because they are often associated with a dimpling of the wing surface (Ren, 2005).

Thin, a Trim32 ortholog, is essential for myofibril stability and is required for the integrity of the costamere in Drosophila

Myofibril stability is required for normal muscle function and maintenance. Mutations that disrupt myofibril stability result in individuals who develop progressive muscle wasting, or muscular dystrophy, and premature mortality. This study presents investigations of the Drosophila l(2)thin [l(2)tn] mutant. The "thin" phenotype exhibits features of the human muscular disease phenotype in that tn mutant larvae show progressive muscular degeneration. Loss-of-function and rescue experiments determined that l(2)tn is allelic to the tn locus [previously annotated as both CG15105 and another b-box affiliate (abba)]. tn encodes a TRIM (tripartite motif) containing protein highly expressed in skeletal muscle and is orthologous to the human limb-girdle muscular dystrophy type 2H disease gene Trim32. Thin protein is localized at the Z-disk in muscle, but l(2)tn mutants showed no genetic interaction with mutants affecting the Z-line-associated protein muscle LIM protein 84B. l(2)tn, along with loss-of-function mutants generated for tn, showed no relative mislocalization of the Z-disk proteins alpha-Actinin and muscle LIM protein 84B. In contrast, tn mutants had significant disorganization of the costameric orthologs beta-integrin, Spectrin, Talin, and Vinculin, and the initial description for the costamere, a key muscle stability complex, in Drosophila is presented. These studies demonstrate that myofibrils progressively unbundle in flies that lack Thin function through progressive costamere breakdown. Due to the high conservation of these structures in animals, this study demonstrates a previously unknown role for TRIM32 proteins in myofibril stability (LaBeau-DiMenna, 2012).

Effects of Mutation or Deletion

Mutations have been isolated in the gene encoding ß_Heavy-spectrin in Drosophila: this essential locus has been named karst. karst mutant individuals have a pleiotropic phenotype characterized by extensive larval lethality and, in adult escapers, rough eyes, bent wings, tracheal defects and infertility. Within karst mutant eyes, a significant number of ommatidia specifically lack photoreceptor R7 alongside more complex morphological defects. In descending order of penetrance karst mutant flies possess rough eyes, wings that typically curve downwards in an 'inverted spoon' shape and visible extrusion of material from the abdominal spiracles. These melanized protrusions appear most commonly in (but are not restricted to) abdominal segment 3: usually several spiracles can be seen to have material within them. It is presumed that this is coagulated hemolymph and reflects a loss of tracheal integrity. karst mutant escapers are very infertile and produce low numbers of eggs. It is unlikely that the karst mutation has a large effect on cell proliferation or viability in imaginal tissues, since neither the wings nor the eyes appear reduced in size. Immunoblots of karst mutant larvae indicate that, by the third instar, it is no longer possible to detect maternally contributed ß_H, so these phenotypes represent metamorphosis in the absence of detectable wild-type Karst protein (Thomas, 1998).

Immunolocalization of ß_Heavy-spectrin in wild-type eye-antennal and wing imaginal discs reveals that ß_Heavy-spectrin is present in a restricted subdomain of the membrane skeleton that colocalizes with Shotgun. A model is proposed where normal levels of Sevenless signaling are dependent on tight cell-cell adhesion facilitated by the ß_Heavy-spectrin membrane skeleton (Thomas, 1998).

Immunolocalization of ß_Heavy-spectrin in the adult and larval midgut indicates that it is a terminal web protein: no gross morphological defects are seen in the adult apical brush border in karst mutant flies. Similarly, rhodamine phalloidin staining of karst mutant ovaries reveals no conspicuous defect in the actin cytoskeleton or cellular morphology in egg chambers. This is in contrast to mutations in alpha-spectrin, the molecular partner of ß_Heavy-spectrin, which affect cellular structure in both the larval gut and adult ovaries (Thomas, 1998).

Changes in cell shape and position drive morphogenesis in epithelia and depend on the polarized nature of its constituent cells. The spectrin-based membrane skeleton is thought to be a key player in the establishment and/or maintenance of cell shape and polarity. Apical ß_Heavy-spectrin (ß_H), a terminal web protein that is also associated with the zonula adherens, is essential for normal epithelial morphogenesis of the Drosophila follicle cell epithelium during oogenesis. Elimination of ß_H by the karst mutation prevents apical constriction of the follicle cells during mid-oogenesis, and is accompanied by a gross breakup of the zonula adherens. The integrity of the migratory border cell cluster, a group of anterior follicle cells that delaminates from the follicle epithelium, is disrupted. Elimination of ß_H prevents the stable recruitment of alpha-spectrin to the apical domain, but does not result in a loss of apicobasal polarity, as would be predicted from current models describing the role of spectrin in the establishment of cell polarity. These results demonstrate a direct role for apical (alphaß_H)₂-spectrin in epithelial morphogenesis driven by apical contraction, and suggest that apical and basolateral spectrin do not play identical roles in the generation of apicobasal polarity (Zarnescu, 1999).

The distribution of ß_H was examined in developing wild-type ovarioles. ß_H expression in the germline is low to undetectable in the most anterior region of the germarium, and first appears in zone 2, at a time when 16 cell cysts become surrounded by a pool of follicle cells produced by asymmetric division of two to three somatic stem cells. In zone 3, fully formed stage 1 egg chambers emerge, ß_H is found uniformly along the nurse cell and oocyte membranes. Later, in mid-oogenesis, ß_H is slightly enriched on the outer edge of the ring canals. In the soma, ß_H is strongly expressed at the very anterior tip of the germarium in the terminal filament and the cap cells that contact the germline stem cells. In close proximity to the 16 cell cysts in region 2, high levels of ß_H expression are detected in the vicinity of the somatic stem cells and in their progeny, the follicle cells. As individual cysts become enveloped in follicle cells, ß_H is slightly enriched in the cells that move in to segregate adjacent cysts. ß_H continues to be strongly expressed here because these become the stalk cells that separate successive egg chambers along the ovariole. ß_H is apically polarized in the follicle cells. ß_H is downregulated in the migrating border cells at stage 9, but is again expressed on the apical surface of these cells when they begin to secrete the micropyle after stage 10. At stage 10, ß_H is part of a prominent terminal web-like structure at the apical ends of the follicle cells that are secreting egg components. This structure appears to be anchored in the zonula adherens (ZA) by fine fibers of staining around its edge. ß_H is also expressed on the apical surfaces of the follicle cells secreting the dorsal appendages and the chorion. ß_H colocalizes with alpha-spectrin in the germline and soma at all these locations; however, the latter is more widespread, presumably through its association with the conventional ß isoform (Zarnescu, 1999).

In mid-stage 9 egg chambers, the follicle cell monolayer and the border cells migrate in a concerted fashion. However, in 73% of karst egg chambers the border cells migrate ahead of the follicle cell monolayer. The degree to which the border cells migrate ahead of the follicle cells during stage 9 is quite variable in keeping with the variable expressivity of the karst mutation. Occasionally egg chambers are found where the border cells are retarded relative to the follicle cells; however, this extreme situation probably arises from a combination of weak expression of the follicle cell phenotype combined with strong expression of the border cell phenotype. The lack of coordination in cell migration makes it difficult to assess the progression of each chamber through stage 9. A morphometric approach was therefore taken. The following defects are seen in karst mutant egg chamber morphogenesis. (1) Most karst border cell clusters are migrating ahead of the follicle cell monolayer during stage 9. This could arise either due to faster border cell migration or slower follicle cell migration. (2) Most karst mutant oocytes occupy a larger portion of the egg chamber than in the wild-type during follicle cell migration, while the oocyte exhibits no significant overgrowth at the completion of migration. This suggests that karst follicle cells are delayed in their migration relative to growth of the oocyte, or may respond more slowly to oocyte growth. (3) Similarly, most karst mutant oocytes occupy a larger portion of the egg chamber than in wild-type during border cell migration. This effect is not as strong as for the follicle cells, consistent with the observation that the border cells generally migrate ahead of the follicle cells in karst mutant egg chambers, but it does suggest that there is a slight delay in border cell migration. The most parsimonious model accounting for these data suggests that the karst mutation causes a significant disruption of follicle migration onto the oocyte membrane and a slight delay in border cell delamination or migration through the nurse cells (Zarnescu, 1999).

Consistent with the hypothesis that the primary morphogenetic defect lies in follicle cell migration, some follicle cells in karst mutant egg chambers often remain in contact with the nurse cell membranes at stage 10A. These follicle cells still attempt to make the appropriate adhesive contacts with the oocyte membrane, pulling the oocyte membrane towards them and grossly distorting the nurse cells/oocyte interface. In most cases, the subsequent inward migration of the follicle cell layer at stage 10B proceeds along the nurse cell/oocyte interface in a relatively normal fashion. However, in rare, extreme cases, the centripetally migrating cells penetrate between nearby nurse cell membranes and cause one or more nurse cells to become included within the egg, along with the oocyte (Zarnescu, 1999).

The failure of karst follicle cells to complete their migration onto the oocyte by the onset of stage 10B implies that the total apical surface area of the epithelium is greater than that of the oocyte membrane. Moreover, karst mutant follicle cells often appear to have a more cuboidal shape than in the wild-type. Since there is no over-proliferation in the mutant monolayer, this cannot arise due to an increase in cell number. However, an inability of karst follicle cells to properly change their cell shape or constrain their apical surface area at the appropriate size would explain this observation. The apical surface area of wild-type and mutant follicle cells has therefore been compared during monolayer migration. This analysis reveals a sharp decrease in the apical surface area of the wild-type follicle cells as they approach and migrate onto the oocyte. In contrast, the majority of karst mutant follicle cells fail to apically constrict. The mean apical surface area of the mutant follicle cells is almost twice that of the wild-type. Moreover, comparison of the apical surface areas of mutant follicle cells in chambers during migration with those where migration has been completed reveals a slight increase. This suggests that, in addition to the constriction defect, the monolayer cannot withstand the forces exerted by the growing oocyte (Zarnescu, 1999).

Examination of the follicle cell apices stained for Drosophila E-cadherin (Shotgun) also reveals conspicuous disruptions in the staining pattern of Shotgun in karst mutant egg chambers. In the mildest cases, this staining is missing at three- or four-cell vertices, but large breaks are also seen in the normally continuous belt of staining in more extreme cases. These observations are consistent with the hypothesis that the absence of ß_H weakens the ZA, and that it breaks up as the apices attempt to constrict or accommodate the growth of the oocyte. However, the apicolateral polarization of the ZA is largely unaffected. The border cell cluster delaminates from the follicle cell epithelium and migrates between the nurse cells to the anterior of the oocyte during stage 9. In ~10% of karst mutant egg chambers, migratory cells are observed that are well separated from, or trail behind, the main border cell cluster. The trailing cells upregulate alpha-spectrin and Shotgun in a manner that resembles wild-type clusters, suggesting that they are true border cells. These data suggest that the normal number of border cells is specified in karst mutants, but that the cluster is unable to remain together as a unit (Zarnescu, 1999).

ß_H was no longer detectable at the apical domain of the follicle cells in any allelic combination of karst alleles examined. The localization of ß_H to the apical domain is dependent on alpha-spectrin. To see if alpha-spectrin is dependent on ß_H for its localization to the apical domain, and to confirm that no apical spectrin function remains in karst mutants, the distribution of alpha-spectrin was examined in karst follicle cells. While the lateral alpha-spectrin distribution is unaffected by this mutation, apical alpha-spectrin is no longer detectable by immunofluorescence. This indicates that the stable recruitment of alpha-spectrin to the apical domain is dependent on ß_H, and that there is thus a mutual interdependence between alpha-spectrin and ß_H. This further suggests that alphaß_H-spectrin is recruited to the apical domain as a heterodimer or tetramer, or that following separate recruitment only the dimers or tetramers remain stably associated with the apical domain (Zarnescu, 1999).

Spectrin associated with Shotgun has been implicated in the apicobasal cell polarization pathway, and fly alpha-spectrin mutations cause a breakdown in monolayer polarity including the loss of apical ß_H. However, karst mutants form a follicle epithelium that appears to have a well polarized morphology. Given the significant similarity between ß_H and ß-spectrin, it is also possible that polarity is retained in karst mutant epithelia because of functional redundancy between these two proteins. Moreover, the karst mutant phenotype exhibits variable expressivity despite genetic evidence that the alleles are null, and this variable expressivity might also result from functional redundancy. Such redundancy would predict that conventional ß-spectrin would be recruited to the apical domain in the absence of ß_H. However, this is not the case, confirming that ß_H is unnecessary for apicobasal polarity and indicating that the source of variability in the karst phenotype must be sought elsewhere (Zarnescu, 1999).

Current models for the origins of epithelial polarity suggest that spectrin plays a key role in establishing and/or maintaining the apicobasal axis. This model is largely based on the behavior of molecules involved in establishing the basolateral domain. The combined observations on the alpha-spectrin and karst mutant phenotypes suggests that the basolateral membrane skeleton may be playing such a key role; however, these results indicate that apical spectrin [i.e., (alphaß_H)₂] does not. It remains unclear what the precise mechanism is by which apical spectrin acts during morphogenetic events. ß_H exhibits a close colocalization with the ZA, and its levels at this location are regulated in concert with Shotgun. Furthermore, mild disruptions of the ZA are observed in eye/antennal imaginal discs. None of the results to date reveal how closely (alphaß_H)₂ is associated with the ZA; however, the contrasting behavior of the apical and basolateral spectrin-based membrane skeleton during the emergence of apicobasal polarity, combined with the phenotypic data presented in this paper, strongly suggest that these two cytoskeletal structures have somewhat distinct rather than identical roles in their respective domains, at least when it comes to the generation and/or maintenance of apicobasal polarity (Zarnescu, 1999).

The results presented in this paper add to a growing body of evidence that apical spectrin is essential for epithelial morphogenesis. Moreover, it has been shown that an apical SBMS is not required for establishing or maintaining apicobasal polarity, as seems to be the case for the basolateral SBMS. It is unknown at present whether or not ß_H or any other ß-spectrin plays a similar role in morphogenesis in vertebrates. The observations that ß_H is evolutionarily old, that there is a homolog in C. elegans encoded by the sma1 gene, and that the chicken brush border protein TW260 has a similar size and contour length as does ß_H (350 kD), all strongly suggest that there is a vertebrate homologue of ß_H that has yet to be cloned. The karst phenotype bears close resemblance to the phenotype exhibited by C. elegans embryos mutant at the sma1 locus that encodes the worm homolog of ß_H. In sma1 mutants, the worms fail to elongate at the wild-type rate during embryogenesis, resulting in a smaller embryo. Moreover, this phenotype seems to arise from a defective contraction of the embryonic epithelium. Thus, while the geometry of this developmental event is quite different from follicle cell migration in the fly, ß_H appears to be playing a similar role in the two organisms. In vertebrates, the apical accumulation of spectrin has been correlated with the initiation of neurulation in mice, and, in the sea urchin embryo, an apical accumulation of alpha-spectrin is associated with the involution of tissues during embryogenesis. This suggests that apical spectrin has a general role in the morphogenesis of epithelia mediated by apical contraction (Zarnescu, 1999).

Genetic screen in Drosophila melanogaster uncovers a novel set of genes required for embryonic epithelial repair

The wound healing response is an essential mechanism to maintain the integrity of epithelia and protect all organisms from the surrounding milieu. In the 'purse-string' mechanism of wound closure, an injured epithelial sheet cinches its hole closed via an intercellular contractile actomyosin cable. This process is conserved across species and utilized by both embryonic as well as adult tissues, but remains poorly understood at the cellular level. In an effort to identify new players involved in purse-string wound closure a wounding strategy suitable for screening large numbers of Drosophila embryos was developed. Using this methodology, wound healing defects were observed in Jun-related antigen (encoding DJUN) and scab (encoding Drosophila alphaPS3 integrin) mutants and a forward genetics screen was performed on the basis of insertional mutagenesis by transposons that led to the identification of 30 lethal insertional mutants with defects in embryonic epithelia repair. One of the mutants identified is an insertion in the karst locus, which encodes Drosophila beta_Heavy-spectrin. beta_Heavy-spectrin (beta_H) localizes to the wound edges where it presumably exerts an essential function to bring the wound to normal closure (Campos, 2010).

Using previously described DC or wound healing mutants a pilot screen was performed to validate the embryonic wounding strategy. The fact that a member of the DJNK pathway (Jra/DJun) was identified in the assay is in accordance with other reports that implicate this pathway in wound healing. Specifically, two mutations in components of the DJNK pathway, bsk/DJNK and kay/DFos, were previously shown to have defects in fly larval and adult wound closure, respectively. In addition, a reporter construct has been describes that requires consensus binding sites for the JUN/FOS complex to be activated upon wounding. Interestingly, treporter activation was still observed in Jra mutants, which suggests that additional signaling pathways are involved in wound closure (Campos, 2010).

An apparent discrepancy arose when the assay revealed a phenotype with Jra but not with puc mutants, another component of the same signaling pathway. This result might be explained by the fact that Jra and puc function in opposite directions in the DJNK signaling pathway. Puc functions as a pathway repressor, so in a puc mutant the JNK pathway should be less repressed and an opposite effect to a Jra mutation could be expected. In addition, activation of a puc-lacZ reporter has been shown to occur in larvae, wing imaginal discs, and adult wounds that take 18-24 hr to close, but it is only robustly detectable 4-6 hr postpuncture. Embryonic wounds are faster to heal, and even after inflicting a large laser wound on stage 14/15 embryos, no activation of the puc-lacZ reporter (assessed in open wounds 3 hr postwounding by immunofluorescence; data not shown) was detected. This observation suggests that, in rapidly healing epithelial wounds, the JNK pathway is not activated to high enough levels to trigger auto-inhibition (Campos, 2010).

The α-integrin scab was never before implicated in embryonic wound healing, but this mutant's phenotype comes as no great surprise. The first scab mutation was isolated due to its abnormal larval cuticle patterning. The scab gene encodes for Drosophila α-PS3 integrin, which is zygotically expressed in embryonic tissues undergoing invagination, tissue movement, and morphogenesis. Integrin proteins are involved in cell-matrix interactions and α-PS3 integrin regulation, in particular, mediates zipping of opposing epithelial sheets during DC. Similarly, the observation of a wound defect in scb^5J38 mutants is consistent with a role for α-PS3 integrin in zipping of opposing epithelial cells during the healing process (Campos, 2010).

A previous study using confocal video microscopy has shown that Rho1^1B mutants take twice as long to close an epithelial wound when compared to wild type. Rho1 was confirmed in the assay to be important for wound healing, although with a weaker phenotype (22% of embryos had unclosed holes). This result shows nonetheless that the assay can be sensitive enough to pick up a 'weak' wound healing mutant such as Rho1^1B, which is still able to heal wounds albeit slower than wild type (Campos, 2010).

The genes identified in the screen represent a variety of functions indicating that wound healing is a complex mechanism that requires the participation of many cellular processes. A large class of the candidate mutants are involved in several aspects of gene expression, including factors that regulate chromatin remodeling (dUtx and Pc), elongation (dEaf), splicing (Glo and CG3294), and translation (CG33123). These factors are likely needed during wound healing for the induction of a repair transcriptome. Interestingly, JNK signaling-dependent Pc group (PcG) gene downregulation has been observed during imaginal disc regeneration. In addition, a recent study revealed that PcG methylases are downregulated during wound healing, while counteracting demethylases, Utx and Jmjd3, are upregulated. The results for the Pc and Utx mutants are consistent with these studies and highlight the importance of epigenetic reprogramming in the repair process (Campos, 2010).

Some of the genes such as arc-p20 and karst probably have a more direct role in the cell shape changes that drive the tissue morphogenetic movements during epithelial repair. The gene product of arc-p20 is a component of Arp2/3, a complex that controls the formation of actin filaments, and karst encodes a component of the spectrin membrane cytoskeleton. Also related to morphogenesis, CG12913 encodes an enzyme involved in the synthesis of chondroitin sulfate, which is usually found attached to proteins as part of a proteoglycan, suggesting a predictable contribution of the extracellular matrix in the tissue movements necessary for wound healing (Campos, 2010).

The epithelium is the first line of defense of the organism against pathogens and tissue integrity. It would thus seem plausible that genes involved in innate immunity could be identified with the screening protocol. Indeed, two of the genes (Ser12 and CG5198) seem to point to the involvement of the immune response in the healing of the laser-induced wounds. Ser12 is a member of the serine protease family, a class of proteins that has been shown to play a role in innate immunity. The CG5198 gene has no described function in Drosophila so far, but its homolog, CD2-binding protein 2, is involved in T lymphocyte activation and pre-RNA splicing. Another candidate that might represent a link to immunity is Atg2, a gene important for the regulation of autophagy, a process by which cells degrade cytoplasmic components in response to starvation. In Drosophila, autophagy has been linked to the control of cell growth, cell death, and, recently, to the innate immune response mechanism against vesicular stomatitis virus and listeria infection (Campos, 2010).

Isolation of an insertion in the stam gene points to the involvement of the JAK-STAT signaling cascade in this regenerative process. Interestingly, stam has been shown to be involved in Drosophila tracheal cell migration and is upregulated following Drosophila larvae infection by Pseudomonas entomophila (Campos, 2010).

One candidate could be involved in the uptake or export of some important wound signal (CG7627) as this gene encodes for a multidrug resistant protein (MRP), part of the ABC transporter superfamily, involved in drug exclusion properties of the Drosophila blood-brain barrier (Campos, 2010).

The kinase encoded by grapes is the Drosophila homolog of human Check1 (Chk1) involved in the DNA damage and mitotic spindle checkpoints. All the Chk1 literature has focused on its role during the cell cycle. However, the Drosophila late embryonic epithelium is a quiescent tissue, even after wounding. Understanding Grapes function in this context is a challenging task that could lead to new paradigms. One hypothesis is that Grapes is involved in tension sensing, as it is in the spindle checkpoint, or may uncover a cellular repair process that could help damaged cells 'decide' to either die by apoptosis or participate in the repair process (Campos, 2010).

The remaining genes with a putative function represent a wide range of general metabolic processes (aralar1, gs1-like, CG4389, CG9249, CG11089, and CG16833), suggesting that healing the epithelium is a highly demanding process (Campos, 2010).

Finally, a significant number of genes that have not yet been studied and do not contain identifiable protein domains (CG2813, CG31805, CG6005, CG6750, CG10217, CG15170, and CG30010) were selected. At the moment it is not possible to predict the role that these genes may play, but further study may help to identify novel wound healing regulatory mechanisms (Campos, 2010).

One of the mutants identified in the transposon screen was kst^d11183, an insertion in the β_H-spectrin locus. This mutation is likely producing a truncated protein terminating three amino acids into the P-element insertion. Other mutations identified in nearby segments 14 (kst^14.1, kst²) and 16 (kst¹) lead to the production of a detectable truncated protein so it is likely that karst^d11183 mutation also gives rise to a truncated protein. These mutant forms of β_H lack approximately half of the wild-type protein, including a COOH-terminal PH domain region, which is involved in targeting the protein to the membrane, thus producing a potential dominant negative form of β_H. However, the karst^d11183 mutant should still have maternally loaded wild-type protein, as previous studies describe a complete absence of maternal protein only by the third instar larval stage. This maternal contribution is likely the main reason that this mutant, as well as the other mutants isolated in the screen, does not have a fully penetrant wound healing phenotype (Campos, 2010).

β_H-spectrin was shown to localize to the actomyosin purse string, a supracellular contractile cable that forms rapidly upon wound induction. Live imaging has demonstrated that actin and myosin can accumulate in this cable structure within minutes after wounding. Unfortunately, due to the size of the β_H gene (>13 kb) cloning and tagging it for live imaging is not possible using standard methods, but the experiments in fixed tissue reveal that β_H can accumulate very rapidly in this cable structure. β_H accumulation was observed at the earliest time point technically feasible, 15 min postwounding. These observations are consistent with previous studies, also in fixed tissue, demonstrating rapid changes in β_H localization during the process of cellularization in Drosophila embryos. Taken together, it is clear that at least the β_H component of the membrane skeleton is not just a static structural scaffold as the name implies, but rather a dynamic protein capable of responding to or directing changes in cellular dynamics. The studies suggest that polarized redistribution of β_H exerts an essential function to facilitate actin-based cellular responses, such as cable accumulation/maintenance and wound edge filopodia dynamics, which are necessary to properly close a wound (Campos, 2010).

β_H has been previously observed in association with actin 'rings' during development of Drosophila and C. elegans. Arguably, C. elegans provides an example of actin ring function most analogous to the Drosophila wound edge purse string. During the final stages of C. elegans development, cortical arrays of actin in the outer epithelial cells, the hypodermis, dramatically reorganize to form parallel apically localized bundles of circumferencial supracellular actin rings. In this system, sma1, the C. elegans ortholog of β_H, also localizes apically to these actin rings. In sma1 mutants the rings fail to productively contract and begin to disorganize, losing connection to the cell membranes. An additional phenotype observed in these mutants is the inability of cells to change their shape, a process normally 'directed' by these contractile rings, the end result being a short worm, a phenotype seen as functionally analogous to an unclosed wound in the Drosophila system (Campos, 2010).

In Drosophila, β_H has been implicated in modulating cell shape changes during apical constriction of follicle cells (a process also involving actin rings) and has been proposed to function as a link between cross-linked actin networks/rings and the cell membrane. Further studies revealed that the C-terminal domain of β_H has the ability to directly modulate the apical membrane area by regulating endocytosis, adding one more tantalizing piece of evidence pointing to the fact that β_H could be a major player in cell shape changes, not only as a structural link but also by directly modulating the membrane area in response to cytoskeletal clues (or vice versa) (Campos, 2010).

Although it is known from previous studies that the actin cable is not absolutely required for wound closure, the process takes much longer without one. In Rho1 mutant embryos, cells lacking a cable are able to pull the wound closed using filopodia. The filopodial defect observed in karst mutants, adds another line of evidence to the absolute requirement of these structures for wound closure. In addition to the reduced actin cable accumulation and filopodial dynamics in karst mutants (which would lack the C-terminal domain responsible for membrane modulation), a lack of cell shape change is seen in the wound edge cells. Taken together, these data and the published work, introduce the intriguing possibility that β_H could be serving as a link between wound edge dynamics and the coordinated cell shape changes usually observed in wild-type wound edge cells. The combination of the proposed ability of β_H to modulate the apical membrane area as well as cross-link actin and act as an apical membranewide scaffold for other interactions, makes β_H a good candidate to provide the physical link that would coordinate tissuewide actions, such as supracellular actin cable contraction, with the individual cellular responses, such as cell shape change and polarized filopodia activity (Campos, 2010).

Increased levels of the cytoplasmic domain of Crumbs repolarise developing Drosophila photoreceptors

Photoreceptor morphogenesis in Drosophila requires remodelling of apico-basal polarity and adherens junctions (AJs), and includes cell shape changes, as well as differentiation and expansion of the apical membrane. The evolutionarily conserved transmembrane protein Crumbs (Crb) organises an apical membrane-associated protein complex that controls photoreceptor morphogenesis. Expression of the small cytoplasmic domain of Crb in crb mutant photoreceptor cells (PRCs) rescues the crb mutant phenotype to the same extent as the full-length protein. This study shows that overexpression of the membrane-tethered cytoplasmic domain of Crb in otherwise wild-type photoreceptor cells has major effects on polarity and morphogenesis. Whereas early expression causes severe abnormalities in apico-basal polarity and ommatidial integrity, expression at later stages affects the shape and positioning of AJs. This result supports the importance of Crb for junctional remodelling during morphogenetic changes. The most pronounced phenotype observed upon early expression is the formation of ectopic apical membrane domains, which often develop into a complete second apical pole, including ectopic AJs. Induction of this phenotype requires members of the Par protein network. These data point to a close integration of the Crb complex and Par proteins during photoreceptor morphogenesis and underscore the role of Crb as an apical determinant (Muschalik, 2011).

Strikingly, Crb_FLAGintra can only affect photoreceptor cell (PRC) shape and adhesion when expressed during late larval and early pupal development. During this period, PRCs undergo substantial morphogenetic changes to adopt their final shape. It is noteworthy that the epithelial cells of the imaginal disc are already well polarised, with an elaborated ZA encircling the apices of the cells. Therefore, the transition from a larval epithelial cell into the highly modified PRC does not require establishment of polarity, but rather mechanisms that control remodelling of polarity and AJs. This study shows that early expression of Crb_FLAGintra interferes with this process. Similar conclusions were drawn from studies in the Malpighian tubules, where proper Crb levels are essential for maintenance of polarity and epithelial integrity only during the process of tube elongation, which depends on major cell rearrangements. Once most of the morphogenetic changes and remodelling of the ZA have been completed, PRCs are less susceptible to elevated Crb_FLAGintra levels. This is reflected by the observation that cells in which the intracellular domain of Crb is expressed during late pupal development and in the adult, exhibit a normal polarised shape, although junctional and polarity proteins are severely mislocalised in these cells. Two explanations might account for this difference. First, the apical and basolateral membrane domain, as well as the ZA, might be more stable at later stages, so that ectopic apical and junctional components recruited by Crb_FLAGintra are unable to affect apico-basal polarity and AJs. Second, some of the downstream factors required for ectopic apical pole formation might no longer be available at later stages. In fact, Baz is removed from the ZA at ~60% of pupal development and becomes enriched in the rhabdomere, similar to aPKC. Furthermore, Par-6 can be found at the basolateral membrane in adult PRCs. Although the polarised shape is unaffected, PRCs overexpressing Crb_FLAGintra during later stages display defects in ZA positioning and show an increase in stalk membrane length, the development of which is regulated by crb (Muschalik, 2011).

Loss-of-function studies show that crb is not required for the development of an apical pole, yet, as shown in this study, overexpression of its cytoplasmic tail is sufficient to induce formation of ectopic apical membranes. This raises the question of how ectopic apical poles develop under these conditions. The results, from localisation studies and genetic interactions, indicate that, once initiated, development of an ectopic apical membrane domain relies on the same events and requires identical components to those required for formation of the original apical domain. It is suggested that Crb_FLAGintra assembles a new Crb-dependent membrane-associated protein platform at the basolateral membrane domain, enabling the recruitment of effector proteins essential to develop apical features. One of these is β_H-spec, which might stabilise the Crb_FLAGintra complex by linking it to the underlying spectrin-based membrane skeleton. In fact, removal of one copy of kst strongly suppresses the overexpression phenotype and F-actin accumulates at Crb_FLAGintra-positive membranes. In addition, the actin-based cytoskeleton is likely to be directly involved in the formation of ectopic rhabdomeres, as rhabdomeres are composed of microvilli and the terminal web, both of which are actin-rich structures (Muschalik, 2011).

In addition to β_H-spec, Par-6 and aPKC are also recruited into the Crb_FLAGintra complex and both are required to mediate the Crb_FLAGintra-induced overexpression phenotype, as demonstrated by genetic interactions. Furthermore, by using different hypomorphic alleles of aPKC, the function of aPKC in this process could be shown to depend on its ability to bind Par-6 and the presence of an intact kinase domain. In the embryonic epidermis, aPKC ensures apical identity by phosphorylation of the tumour suppressor Lgl, thereby excluding it from the apical domain and restricting its activity to the basolateral side of the cells. Lgl, in contrast, prevents Baz from promoting apical membrane characteristics basolaterally. It is proposed that, upon overexpression of Crb_FLAGintra, Lgl is removed from Crb_FLAGintra-positive sites through phosphorylation by aPKC, which weakens basolateral membrane identity. The observation that other basolateral markers are absent from ectopic rhabdomeres and diminished at membranes surrounding ectopic rhabdomeres supports this assumption. Furthermore, removal of Lgl from the basolateral membrane upon overexpression of Crb_FLAGintra would be consistent with the finding that the lgl loss-of-function phenotype of PRCs mimics the Crb_FLAGintra overexpression phenotype. This is similar to the situation in Drosophila embryonic epithelia, and suggests that there is a conserved mechanism for both cell types. Moreover, it might explain why lowering the dose of lgl does not cause an enhancement of the overexpression phenotype. By contrast, an enhancement was found with yrt, which negatively regulates Crb activity, demonstrating that the experimental approach is suitable for the identification of enhancers. Besides Lgl, aPKC also phosphorylates Baz, as shown in the Drosophila follicle epithelium, the embryonic epidermis and PRCs. Phosphorylation of Baz is required to exclude it from the apical membrane, thereby restricting AJs to more basal positions. Apical exclusion of Baz also requires Crb, which prevents binding of Baz to Par-6. It is suggested that the following scenario occurs upon Crb_FLAGintra overexpression. First, removal of Lgl from the basolateral membrane enables Baz to spread basolaterally. However, under these conditions, Baz becomes immediately excluded from Crb_FLAGintra-positives sites by the same mechanisms occurring at the original apical domain. Delocalisation of Baz, in turn, affects AJs and alters the adhesive properties of the cells, as Baz localisation defines the position of the ZA. The model is consistent with observations from genetic interactions, which have shown that simultaneous expression of Crb_FLAGintra and a non-phosphorylatable version of Baz (GFP-Baz-S980A) strongly suppressed the Crb_FLAGintra overexpression phenotype. This suppression could be the result of Baz S980A either binding to aPKC-Par-6, or to Sdt, therefore preventing aPKC-Par-6 or Sdt from binding to Crb_FLAGintra. Alterations in PRC adhesion might also explain the disruption of the basal lamina and the elimination of PRCs. As no obvious decrease in cell number was noticed at 45-55% of pupal development, elimination is likely to occur during late pupal development (Muschalik, 2011).

Formation of distinct membrane domains also requires polarised protein trafficking. The ectopic localisation of Rh1 and Spam (Eyes shut) upon overexpression of Crb_FLAGintra during late larval and pupal development suggests that the apical secretory machinery becomes reorganised under these conditions. In Drosophila PRCs, delivery of various apical proteins, including Rh1, depends on the small GTPase Rab11 and the exocyst component Sec6. A redistribution of these proteins upon overexpression of Crb_FLAGintra in developing PRCs might account for the delivery of apical transport vesicles to Crb_FLAGintra-positive membranes, which facilitates the formation of a second apical pole. In case of cells with reversed apico-basal polarity the majority of apical vesicles might be targeted to the ectopic apical pole so that the original apical membrane domain receives only minor amounts of apical proteins, with it eventually adopting basolateral membrane identity (Muschalik, 2011).

Another crucial component in polarised vesicle delivery and targeting are phosphoinositides. In developing Drosophila PRCs, PtdIns(3,4,5)P₃ is enriched at the apical membrane, whereas PtdIns(4,5)P₂ predominantly localises at the ZA. Studies in MDCK (Madin-Darby canine kidney) cells have shown that ectopic localisation of either of the above two phosphoinositides is sufficient to cause a switch from one membrane identity to the other. Strikingly, Baz recruits the lipid phosphatase PTEN (phosphatase and tensin homolog) to the AJs of PRCs and embryonic epidermal cells, and Baz is delocalised upon Crb_FLAGintra expression in pupal PRCs. Mutations in, or overexpression of, PTEN cause severe morphogenetic defects, including loss of PRCs and absence or splitting of rhabdomeres, phenotypes that are also observed upon overexpression of Crb_FLAGintra. Given these data, it is tempting to speculate that ectopic Crb_FLAGintra and its associated proteins cause a modification in the lipid composition of the basolateral membrane domain, thereby remodelling the polarity of PRCs (Muschalik, 2011).

The Spectrin cytoskeleton regulates the Hippo signalling pathway

The Spectrin cytoskeleton is known to be polarised in epithelial cells, yet its role remains poorly understood. This study shows that the Spectrin cytoskeleton controls Hippo signalling. In the developing Drosophila wing and eye, loss of apical Spectrins (alpha/beta-heavy dimers) produced tissue overgrowth and mis-regulation of Hippo target genes, similar to loss of Crumbs (Crb) or the FERM-domain protein Expanded (Ex). Apical beta-heavy Spectrin bound to Ex and co-localised with it at the apical membrane to antagonise Yki activity. Interestingly, in both the ovarian follicular epithelium and intestinal epithelium of Drosophila, apical Spectrins and Crb were dispensable for repression of Yki, while basolateral Spectrins (alpha/beta dimers) were essential. Finally, the Spectrin cytoskeleton was required to regulate the localisation of the Hippo pathway effector YAP in response to cell density human epithelial cells. These findings identify both apical and basolateral Spectrins as regulators of Hippo signalling and suggest Spectrins as potential mechanosensors (Fletcher, 2015).

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date revised: 15 July 2017

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