BIOLOGICAL OVERVIEW

Extracellular proteins have a unique role in development; they provide a matrix for the attachment and migration of cells. They also serve as ligands for cell receptors, insuring proper communication between cells. Slit is a complex extracellular protein containing at least four different motifs shared with other differentiation factors and receptors, including the vertebrate epidermal growth factor, the Drosophila receptor Toll, and the matrix protein laminin. Slit protein is made by midline glial cells. It provides a matrix for the migration of ventral nerve cord axons and is therefore likely to serve as an axon guidance protein. The actions of Slit are not confined to the nervous system; the roles of slit in gut and heart differentiation await exploration.

Slit has been identified as the midline repellent for the Roundabout (Robo) receptor. Robo has been shown to be a repulsive guidance receptor on growth cones that binds to an unknown midline ligand. In the original large-scale mutant screen for genes controlling midline axon guidance, 8 alleles were recovered of robo, 2 alleles of commissureless, and 13 alleles of slit. At the time, because slit had such a similar axon phenotype to single minded, which controls midline cell fate and survival, and because of the lack of good midline markers, there was some uncertainty as to whether slit like sim might also control midline cell fate and survival. As a result, initial attention was placed on robo and comm, two genes that clearly control midline axon guidance. Nevertheless, there was always the lingering possibility that Slit might directly control axon guidance. Slit is a large extracellular matrix protein expressed almost exclusively by midline cells; some Slit protein is found on axons, and the slit mutant displays a striking axon pathway phenotype. In slit mutants, growth cones enter the midline but never leave it. With the advent of better markers for midline cells it was shown that midline cell fate and differentiation are relatively normal in slit mutant embryos, thus suggesting that Slit might indeed control axon guidance. The key result that led to the insight that Slit is likely to be the Robo ligand came from a further analysis of Comm. Overexpression of Comm produces a robo-like phenotype in which axons freely cross and recross the midline. If the copy number of the comm transgene is increased, a more severe phenotype results in which axons enter the midline but fail to leave it, leading to a midline collapse of the CNS axon scaffold. The strongest comm gain-of-function phenotype is highly reminiscent of the slit loss-of-function phenotype and led to an evaluation of Slit as a candidate Robo ligand (Kidd, 1999 and references).

Dosage-sensitive genetic interactions between slit and robo are a good indicator that the two gene products are functionally related. The CNS was examined in embryos transheterozygous for slit and robo, that is, embryos carrying one mutant and one wild-type copy of each gene. Would Fas II positive fascicles (those stained with the 1D4 mAb) abnormally cross the midline, particularly the most medial pCC pathway? In either slit or robo heterozygotes, few guidance defects were observed in these pathways. However, depending upon the combination of alleles used, 26%-39% of the segments examined in embryos transheterozygous for slit and robo had Fas II-positive axons inappropriately crossing the midline. Such a dosage-dependent, transheterozygous phenotype is a strong indication that Slit and Robo function in the same pathway. Double mutants for slit and robo were prepared. The genetic distance between the two loci predicted recovery of the double mutant chromosomes at a frequency of 1 in 8: when null alleles of both slit and robo are used instead, the recovery rate is 1 in 35, indicating that removal of one copy of each locus decreases viability (Kidd, 1999).

In a late stage wild-type embryo, the cell bodies of the RP neurons are readily visible between the two commissures. In robo mutants, typically one or both RP cell bodies are obscured by the increased number of axons abnormally crossing in the commissures. However, the longitudinal part of the scaffold always remains outside (lateral to) the RP cell bodies. In slit mutants, this is not the case. The effect of removing one copy of slit on the robo phenotype was tested. When the spacing of the longitudinal axons was examined, slit was found to dominantly enhance the robo phenotype, as judged by the presence of segments displaying greater medial constrictions than are ever seen in robo mutants alone. In some instances, an RP cell body could be seen lateral to the axon scaffold. If Slit is the Robo ligand, then the double robo;slit mutant phenotype would be predicted to resemble that of a slit mutant alone (due to slit having the more severe phenotype). Embryos homozygous for a recombinant chromosome carrying null alleles of both slit and robo resemble the slit null phenotype (Kidd, 1999).

The commissureless phenotype produced by high-level overexpression of Robo suggests that Robo responds to a repulsive cue at the CNS midline. Slit is a large extracellular matrix protein secreted by the midline glia. Slit was reported to be transferred to axons (albeit at a low level). The mAb used for Slit detection displays only a very low level of axon staining, making an analysis of putative transfer in robo mutant embryos inconclusive. Robo is primarily localized to growth cones of the longitudinal portion of the axon scaffold. These expression patterns are consistent with Slit being the repulsive ligand for Robo because Robo-positive axons avoid areas of high Slit expression. slit embryos were stained with anti-Robo mAb 13C9 and it was found that Robo-positive growth cones were then present at the midline. Staining of the mature CNS in slit mutants reveals that Robo protein levels are unaffected (unlike in comm gain-of-function embryos), and thus Robo is expressed at high levels along the midline. In wild-type embryos, Slit and Robo both localize to the muscle attachment sites in complementary dorsoventral gradients, further suggesting the possibility of a functional relationship (Kidd, 1999).

The effect of high-level overexpression of slit in all postmitotic neurons was examined. The resulting phenotype resembles the robo loss-of-function phenotype. However, when individual axon fascicles are examined, the slit overexpression phenotype appears stronger than the robo loss-of-function phenotype. In addition to aberrant midline crossing by axons in the innermost pCC pathway as seen in robo mutants, the medial and lateral pathways are also disrupted, sometimes crossing the midline. These results suggest that when Slit is panneurally expressed throughout the CNS, growth cones are impaired in their ability to respond to Slit at the midline. A similar effect is seen when Netrins are expressed panneurally: the panneural overexpression phenotype resembles the loss-of-function phenotype. In both cases (Slit and Netrins), these results support the notion that the localized distribution of the guidance signal is of crucial importance and that approximating an even distribution throughout the CNS is equivalent to no expression at all (Kidd, 1999 and references).

Slit was ectopically expressed on muscles; the guidance and connectivity of motor axons was then examined. The ISNb motor axons normally innervate muscles 6, 7, 12, and 13. When their muscle targets abnormally express Slit, their innervation is greatly perturbed. Most of these motor growth cones stall in the vicinity of these muscles and fail to innervate them. This lack of innervation is reminiscent of what is observed when the chemorepellent Semaphorin II is ectopically expressed by the same muscles. The morphology of muscles 6, 7, 12, and 13 ectopically expressing Slit was examined and they are normal in attachment sites, size, and position relative to one another and to the epidermis. The motor axon phenotype is not suppressed by removal of robo activity, providing further evidence that there is more than one Slit receptor. Robo2 is a potential candidate for mediating the motor axon response to ectopic expression of Slit (Kidd, 1999).

After gastrulation in Drosophila, many myoblasts migrate laterally at least five to six cell body diameters away from the ventral midline. This migration occurs over the dorsal surface of the neuroepithelium. Later, some ventral body wall muscles extend back toward the midline ventrally under the developing CNS, normally attaching to the epidermis underneath the CNS at some distance from the midline. In contrast, in slit mutant embryos many developing muscles are found near and at the midline, stretching across the midline dorsally over the CNS. This defect is not seen in robo embryos, although very rarely a single muscle can be seen extending inappropriately dorsally across the CNS, suggesting that Robo participates in this process in conjunction with at least one other receptor (possibly Robo2). However, in robo mutant embryos the ventral muscles are frequently found attached closer to the midline than in wild type, suggesting that Robo may in part prevent muscles from extending too close to the midline. When slit mutant embryos are rescued by slit-GAL4 driving UAS-slit, the ventral muscle pattern is restored to near wild type, confirming that Slit expression at the midline is required for migration of muscle precursors away from the midline (Kidd, 1999).

The axon guidance defects seen in robo mutant embryos in Drosophila suggest that the primary function of Slit in controlling Robo-mediated midline guidance is as a short-range repellent. Growth cones that express high levels of Robo do not extend away from the midline, but rather they avoid entering and crossing the midline. For example, the pCC growth cone expresses high levels of Robo, and it extends anteriorly near the edge of the midline. In a robo mutant, the pCC growth cone freely crosses and recrosses the midline; in a slit mutant, the pCC growth cone enters the midline and does not leave it. Although it is possible that Slit might also function as a long-range chemorepellent during axon guidance in Drosophila, causing some growth cones to extend some distance away from the midline, at present the strongest genetic evidence in Drosophila is for a short-range function. This is in contrast to its function during mesoderm migration and muscle formation. After gastrulation in Drosophila, many myoblasts migrate laterally away from the ventral midline. The ventral body wall muscles normally attach to the epidermis underneath the CNS but stay some distance from and do not cross the midline. In contrast, in slit mutant embryos, many developing muscles are found near the midline, stretching across the midline dorsally over the CNS. The slit mutant muscle defects are nearly identical to those seen in single minded mutant embryos in which the midline cells are missing (Lewis, 1994). In contrast, in slit mutants, the midline cells are present but do not secrete Slit into the extracellular environment (Kidd, 1999).

Genetic analysis of sim (Lewis, 1994) shows that after gastrulation the midline cells are required for the migration of muscle precursor cells away from the midline. Many of these mesodermal cells normally migrate at least five to six cell body diameters away from the midline. In the sim mutant, the precursors do not migrate away from the midline, presumably due to the absence of a midline-derived long-range chemorepellent. Moreover, in the sim mutant the muscle precursors that extend ventrally toward the midline are not prevented from crossing the midline, presumably due to the absence of a midline-derived short-range repellent. Rather, when these misplaced muscle precursor cells undergo myogenesis, they form abnormal contacts with each other that freely extend across the dorsal midline of the CNS. slit mutant embryos display the exact same midline mesoderm phenotypes as do sim mutant embryos. This suggests that Slit is both the long-range chemorepellent controlling mesoderm migration away from the midline and the short-range repellent preventing muscles from crossing the midline. The Robo receptor appears to play only a minor role in the ability of Slit to direct the long-range migration of muscle precursors away from the midline. Either Robo2 or some other Slit receptor must function as the major muscle receptor for Slit-mediated long-range chemorepulsion (Kidd, 1999 and references).

If commissural growth cones are so attracted to Netrin, if the highest concentration of Netrin is at the midline, and if when growth cones arrive at the midline they meet their homologs from the other side for which they have a high affinity, why do these growth cones ever leave the midline? Although the mechanism is not fully understood, the answer to this question has something to do with the qualitatively different ways in which growth cones respond to Slit. For growth cones near the midline that do not cross it, Slit forms a strong repulsive barrier. But for growth cones that do cross the midline, Slit cannot be such a strong repellent, rather functioning in a more subtle fashion, somehow preventing them from lingering at the midline and driving them across. In the absence of Slit, growth cones enter the midline but do not leave it, extending in a single fused longitudinal tract at the midline. Thus, Slit must be part of the anti-linger mechanism. One thing is certain: the ability of Slit to form a repulsive barrier requires the Robo receptor. Any growth cone that expresses high levels of Robo cannot cross the midline. So in a robo mutant, growth cones freely cross and recross the midline, but they do not stay at the midline. Two inferences follow from these observations: (1) there must be at least one additional Slit receptor that controls midline guidance, and at present Robo2 is the best candidate; (2) because Slit appears to have two different functions (one as a midline repulsive barrier and the second as a midline anti-linger signal), it follows that either Robo2 signals differently from Robo, or alternatively, that the low levels of Robo2 alone (or Robo2 and Robo together) on growth cones crossing the midline give rise to a qualitatively different response as compared to high levels of Robo. Whether these are two qualitatively different negative responses, or alternatively, quantitative differences in a common repulsive mechanism, is not yet clear. Teasing this mystery apart in the future should shed some light on how growth cones make stereotyped and divergent decisions at complex choice points (Kidd, 1999).

Attractive and repulsive functions of Slit are mediated by different receptors in the Drosophila trachea

Oxygen delivery in many animals is enabled by the formation of unicellular capillary tubes that penetrate target tissues to facilitate gas exchange. The tortuous outgrowth of tracheal unicellular branches towards their target tissues is controlled by complex local interactions with target cells. Slit, a phylogenetically conserved axonal guidance signal, is expressed in several tracheal targets and is required both for attraction and repulsion of tracheal branches. Robo and Robo2 are expressed in different branches, and are both necessary for the correct orientation of branch outgrowth. At the CNS midline, Slit functions as a repellent for tracheal branches and this function is mediated primarily by Robo. Robo2 is necessary for the tracheal response to the attractive Slit signal and its function is antagonized by Robo. It is proposed that the attractive and repulsive tracheal responses to Slit are mediated by different combinations of Robo and Robo2 receptors on the cell surface (Englund, 2002).

The tracheal system develops from 20 clusters of ectodermal cells, each containing about 80 cells. After invagination and without further cell division, each epithelial cluster sequentially extends primary, secondary, fusion and terminal branches to generate the tubular network that facilitates larval respiration. The regular outgrowth pattern of the primary branches is determined by the localized expression of signaling factors in the surrounding tissues. Among these signals, Branchless (Bnl), a member of the Fibroblast Growth Factor family, first directs the outgrowth of multicellular branches to its site of expression, and it then induces the activation of a set of terminal branching genes in the leading cells of the primary branches. Single terminal cells then form a unicellular branch, migrate over substantial distances and finally stretch and bind to distinct parts of the target tissue to facilitate respiration. A single terminal cell of each ganglionic branch (GB), for example, targets each hemisegment of the embryonic ventral nerve cord (VNC). A cluster of bnl-expressing cells just outside the CNS attracts the GB toward the CNS. The GB cells migrate ventrally along the intersegmental nerve (ISN), but just before reaching the entry point into the CNS, they break their contact with ISN and turn posteriorly to associate with the segmental nerve (SN). This substrate switch is promoted by the expression of adrift (aft), a bnl-induced gene required in the trachea for efficient entry into the CNS. Inside the CNS, the GB1 cell extends over a distance of about 50 µm, from the entry point into the CNS via four different neural and glial substrata to its target on the dorsal side of the neuropil. During the first 20 µm of its journey inside the CNS, the GB1 cell moves its cell body and nucleus along the exit glia, the SN and ventral longitudinal glia towards the midline. The rest of the path is covered by a long cytoplasmic projection that turns dorsally at the midline and reaches the dorsal part of the neuropil by the end of embryogenesis. The signals that guide GB1 migration inside the CNS are not known but the substrata that the GB contacts along its path could potentially provide important guidance cues (Englund, 2002).

The importance of glial substrata in guiding the GB1 inside the CNS was investigated. By genetic ablation experiments, it has been shown that different glial cells provide distinct positional cues to the trachea. Longitudinal glia are first required for GB1 migration towards the midline, whereas midline and channel glia are necessary for inhibiting it from crossing the midline and to make it migrate dorsally through the neuropil. Slit signaling plays a major role in the migration of the GB1 cell. Slit is produced by midline cells and prevents GBs from crossing the midline of the VNC. Slit is also required as an attractant for the outgrowth of the primary, dorsal and visceral branches. The Slit receptors Roundabout (Robo) and Roundabout 2 (Robo2) are both required in the trachea independently of their function in axonal migration. The analysis of the tracheal robo and robo2 mutant phenotypes suggests that they may mediate different responses to the Slit signal. These results provide a first insight into the signaling mechanisms that guide the GB in the CNS, and identify an in vivo system for the study of the bi-functional role of Slit in epithelial cell guidance at the level of single cells (Englund, 2002).

A major determinant of axonal pathways inside the CNS is the repellent signal Slit. Midline cells express Slit, a large extracellular matrix protein that functions both as a short- and long-range repellent, controlling axon crossing at the midline and mesodermal cell migration away from the midline. In axon guidance, the Slit repulsive signal is mediated by the Roundabout (Robo) receptors. Different axons express different combinations of the three receptors, which determine the distance of their projections from the midline along the longitudinal fascicles. The midline crossing phenotypes of GBs in embryos expressing Ricin A in the midline glia (thus ablating these cells) suggests that Slit signaling may also guide GB1 in its turn away from the midline. Embryos expressing GFP under the control of the pan-tracheal btl-GAL4 driver, which drives expression of GAL4 in all tracheal cells from stage11, were double stained with antibodies against GFP and Slit or its receptors, and their expression was analyzed by confocal microscopy. The GB1 cell comes close to the midline source of Slit at early stage 16 but it then turns dorsally and posteriorly at the midline. Slit is also expressed in several other tissues close to the migrating tracheal branches. At early stage 14 in the dorsal side of the embryo, two rows of migrating mesodermal cells that will form the larval heart express Slit. These cardioblasts are in close proximity to the two leading cells of the tracheal dorsal branches (DBs), which also migrate towards the dorsal midline and give rise to the dorsal anastomosis (DB2) and the dorsal terminal branch (DB1). Slit expression is also detected from stage 13 on the surface of the midgut, at the sites of contact of the growing tracheal visceral branches (VBs). Finally, Slit is detected in lateral stripes of epidermal cells adjacent to the growing dorsal trunk (DT) and dorsal branches from stage 13. Are the Slit receptors expressed at this time in the trachea? Robo staining can be detected in all tracheal cells as they invaginate from the epidermis already at stage 11. Its tracheal expression is decreased by stage 13, when it is only weakly expressed in the dorsal trunk. No convincing expression of Robo was detected in the trachea after stage 14, even when serial optical sections of the GB1 cell were analyzed along its path in the CNS. Robo2 is also expressed in all tracheal cells from stage 11 and it then becomes restricted to the dorsal trunk and dorsal and visceral branches by stage 13. In contrast to Robo, which becomes undetectable in the trachea by stage 14, Robo2 expression is stronger and is maintained as late as at stage 16 in the DB1 and DB2 cells at the dorsal midline. Robo3 expression could not be detected in the trachea. The expression of Slit in tissues surrounding the developing trachea and the dynamic expression of its two receptors in different tracheal branches suggests a role for Slit signaling in tracheal branch outgrowth towards their target tissues (Englund, 2002).

The morphology of GB1 allows the separation of its tour in the CNS in two parts. In the first part, starting at the entry point into the CNS, GB1 extends broad filopodial projections and moves its cell body and nucleus ~20 µm towards the ventral longitudinal glia. In the second part, the position of the nucleus remains fixed and the tracheal cell sends a 30 µm long extension that navigates first towards the midline and then turns dorsally through a channel towards the dorsal longitudinal glia. GB1 contacts different groups of glial cells during its migration through the ventral nerve cord. The results from genetic ablation of different glial landmarks provide evidence for an instructive role of these substrates in steering GB1 migration and extension. In particular, the GB1 midline crossing phenotype observed after the ablation of midline glia argues for a repulsive signaling mechanism that redirects the cell from its route towards the midline (Englund, 2002).

The elegant analysis of axonal guidance at the midline of the fly CNS establishes the Slit repellent signal as a major determinant of axonal pathways. A gradient of Slit emanating from the midline prevents axons from crossing the midline through the activation of Robo receptors but it also functions as a long range repellent to position axons in distinct lateral fascicles. This later function is mediated by the expression of different combinations of Robo, Robo2 and Robo3 on axons that take distinct positions along the longitudinal tracts (Englund, 2002).

Mammalian Slit can also function as a positive regulator of axonal elongation and branching of sensory axons from the rat dorsal root ganglia and Slit plays an attractive role for muscles during their extension to muscle attachment sites on the Drosophila epidermis. The molecular mechanism behind the different responses to Slit remains unknown. Repulsion versus attraction could reflect a difference in receptor subunit composition or variations in the cytoplasmic signal transduction machinery of the responsive cells. The complex expression pattern of Slit on several tissues close to the growing tracheal branches, together with the tracheal migration defects in slit mutants, indicates that Slit plays an important role in epithelial cell guidance. Lack of Slit affects the oriented outgrowth of the dorsal, visceral and ganglionic primary branches, the cells of these branches either stall their migration towards the Slit expressing target or they become misrouted. Overexpression of Slit with a mesodermal GAL4 driver is sufficient to attract new branches towards the gut and overexpression of Slit on epidermal stripes running along the dorsoventral axis of the embryo redirects the anteroposterior migration of the dorsal trunk branches along the new sites of Slit expression. This re-orientation phenotype becomes stronger in slit mutants indicating that endogenous slit provides a migration cue for these branches. The analysis of loss-of-function and overexpression phenotypes indicates that Slit is a chemoattractant for the outgrowth of several primary tracheal branches towards their targets (Englund, 2002).

The analysis of GB1 phenotypes in slit mutants argues for a repellent function at the midline. In the absence of functional Slit from the CNS midline, 37% of the GB1 cells cross the midline barrier and ectopic expression of slit on the longitudinal glia causes GB1 to stall or turn prematurely when it approaches the longitudinal tracts. Thus, Slit functions as a bi-functional guidance signal in the trachea. The tracheal phenotypes of slit in primary and secondary branches are not fully penetrant, emphasizing the importance of other signals in guiding the tracheal branches to their targets (Englund, 2002).

In CNS and muscle development Slit function is mediated by the Robo receptors. robo and robo2 are expressed in the trachea; the tracheal phenotypes of robo; robo2 double mutant embryos are very similar to the phenotypes of slit mutants, indicating that the tracheal responses to Slit are mediated by Robo and Robo2. Robo and Robo2 receptors can form homo- and hetero-dimers in vitro and the differences in their expression patterns suggests that they might mediate different responses to Slit. Indeed, the comparison of the phenotypes between the mutants for either of the two receptor genes reveals some intriguing differences. In robo embryos, the GBs erroneously cross the midline, suggesting that slit signaling via robo mediates repulsion away from the midline. In contrast, in robo2 mutants GBs fail to enter the CNS, suggesting that Robo2 may mediate an attractive response to Slit. In addition, the stalls in the migration of the dorsal branches detected in slit embryos were only found in robo2 mutants; no stalling phenotypes were detected in the tracheal branches that did not target the CNS in robo mutants. There is also a difference between the phenotypes generated by overexpression of robo and robo2. Overexpression of Robo in GB1 causes most of the branches to turn away from the midline prematurely. This phenotype is much weaker in embryos overexpressing Robo2, indicating that Robo is a more potent repulsive receptor in the GB. In addition, tracheal overexpression of Robo2 cannot rescue the robo mutant GB phenotype, even though this is possible via the tracheal expression of Robo. This result further indicates that Robo and Robo2 are not identical in their output and they cannot simply substitute for one another (Englund, 2002).

To further investigate whether different receptor complexes may mediate different responses to Slit, advantage was taken of the phenotypes caused by overexpression of Slit in the gut. In wild-type embryos, ectopic Slit can attract new visceral branches to its site of expression. This attractive function of Slit requires Robo2, as evidenced by the observation that overexpression of Slit with the same driver does not induce branch outgrowth in robo2 mutants. Robo alone cannot mediate the attractive response to Slit in the visceral branches -- instead it appears to function as an antagonist of the attractive signal mediated by Slit and Robo 2 in the visceral branches, because the number of new branches induced by Slit in robo mutants is three times higher than the number of branches induced under the same conditions in wild-type embryos (Englund, 2002).

Taken together these results suggest that there are qualitative differences between the cellular responses to Robo and Robo2 activation and that each receptor plays a unique role in the control of tracheal cell migration (Englund, 2002).

Mechanosensilla in the adult abdomen of Drosophila: engrailed and slit help to corral the peripheral sensory axons into segmental bundles

The abdomen of adult Drosophila bears mechanosensory bristles with axons that connect directly to the CNS, each hemisegment contributing a separate nerve bundle. In this study the amount of Engrailed protein was altered and the Hedgehog signalling pathway was manipulated in clones of cells to study their effects on nerve pathfinding within the peripheral nervous system. It was found that high levels of Engrailed make the epidermal cells inhospitable to bristle neurons; sensory axons that are too near these cells are either deflected or fail to extend properly or at all. Attempts were made to find the engrailed-dependent agent responsible for these repellent properties. slit was found to be expressed in the P compartment and, using genetic mosaics, evidence is presented that Slit is the responsible molecule. Blocking the activity of the three Robo genes (putative receptors for Slit) with RNAi supported this hypothesis. It is concluded that, during normal development, gradients of Slit protein repel axons away from compartment boundaries - in consequence, the bristles from each segment send their nerves to the CNS in separated sets (Fabre, 2010).

The peripheral sensory system of arthropods is segmented: neurons originate in sensilla in segmental groups in the epidermis and axons project from each of them to the corresponding segmental ganglion in the CNS. To achieve this, the nerves coming from each epidermal segment or compartment must not mix with nerves from neighbouring compartments. The epidermis of the fly and other arthropods is subdivided into a chain of anterior (A) and posterior (P) compartments, the P/A compartment boundary being the true segmental boundary. This segment boundary is recognised by neurons as they build the embryonic nervous system and is not crossed by peripheral sensory neurons in later stages. However, little is known of the molecular mechanisms responsible for this process (Fabre, 2010).

In the adult abdomen of Drosophila, the mechanoreceptive bristles are confined to a region of each A compartment; they develop de novo as sensory organ precursor cells that derive from the epidermal cells or `histoblasts' that proliferate during the pupal stage. Sensory organ precursor cells divide asymmetrically to generate a bristle and their associated neurons and supporting cells; the neurons then extend axons towards the CNS in an orderly manner (Fabre, 2008). These axons remain within their compartments of origin because they are oriented with respect to the body axes: within each A compartment, the more anteriorly situated bristle axons grow backwards, while the posteriorly situated bristle axons grow forwards, and thus both sets of axons meet to form a segmental nerve bundle in the middle of the A compartment (Fabre, 2010).

A and P compartments differ fundamentally: all the P cells but not the A cells, except for a6, express engrailed (en). The en gene encodes a homeodomain-containing transcription factor that induces hedgehog (hh) expression in P cells. Hh is a secreted morphogen that spreads into the A compartment, forming a U-shaped gradient that patterns cell fate and determines cell affinity. Only the epidermal cells of the A compartment produce Patched (Ptc) and Smoothened (Smo), proteins that act as receptors for Hh. Although the mechanosensory neurons are related to epidermal cells by lineage, it is not clear whether they retain all the compartmental properties of their origin (Fabre, 2010).

This study asked how En- and Hh-dependent information positions the neuronal cell bodies, affects the dendrites and influences the pathways followed by axons. To investigate this, cell identities wer altered by manipulating the relevant genes (en, hh, ptc and smo) within clones of cells, and effects on the neurons were examined. Strikingly, cells with P identity, but located within an A compartment, repel nearby neurons. This neuronal repulsion is not directly mediated by En or Hh, but indirectly by activating the expression of slit, a molecule previously implicated in neuronal pathfinding. Also, the response to Slit appears to be mediated by one or more of the Robo proteins. It is proposed that, during normal development, the secretion of Slit from P cells creates a Slit gradient in each A compartment that helps position neurons and orient axon outgrowth and thereby ensures segmental bundling of axons (Fabre, 2010).

In the wild-type fly (and perhaps therefore also in many other invertebrates), it was found that Sli is normally made in the P compartments, spreading forwards and backwards to repel neurons at the back and the front of the A compartments. As a consequence, the axons meet in the middle of the A compartments. Thus, En regulates sli expression to form a Sli gradient, the axons growing away from the source of Sli and down that gradient. Sli may also drive oriented nucleokinesis of the mechanosensory cell bodies away from the compartmental boundaries (Fabre, 2010).

Gradients of morphogens, such as Wingless (Wg), Hh and Decapentaplegic, can act at short or long range to specify cell identity and have also been implicated in axon pathfinding. Numerous studies have concluded that Hh can act as an axonal repellent or attractant, and that axons can respond directly to the gradient of Hh. Surprisingly, in the abdomen evidence is presented that Hh does not guide the mechanosensory neurons. No dependence on the Hh receptors Ptc or Smo is seen. This raises the possibility that some of the previously described effects of Hh might also be indirect. Indeed, in the zebrafish forebrain (Barresi, 2005), Hh acts to guide commissural and retinal axons indirectly by regulating sli expression (Fabre, 2010).

In vertebrates, En affects axon routing. In invertebrates, En modifies axon morphology via the expression of cell adhesion molecules such as Connectin and Neuroglian or the cell adhesion receptor Frazzled. In the cockroach cercus, En is essential for axonal pathfinding, perhaps acting directly on genes needed for guidance and synaptic recognition. There is a hypothesis that En acts directly: En protein has structural domains that could regulate nuclear export, secretion and cell-internalisation, processes also needed for axon pathfinding and target recognition. However, the current experiments in the fly abdomen point to a different conclusion. When smo^- en^- and ptc^- en^- clones were produced in the P compartment, mechanosensory axons traversed anterior cells of the P compartment and a6 cells in which en is expressed. Thus, it is unlikely that En itself repels axons in the abdomen of Drosophila. Evidence suggests instead that En drives the expression of sli autonomously, the effects of En on pathfinding being due to local gradients of Sli concentration. The behaviour of axons emanating from smo^- en^- and ptc^- en^- clones in the P compartment can be understood in this context: the clones are small and even though they do not themselves secrete Sli (because they are transformed into A cells), they nevertheless find themselves in a Sli gradient, high behind and lower in front. Axons leaving such clones behave as expected and grow down that gradient (Fabre, 2010).

The mode of action of Sli in neuronal and axonal repulsion has been studied in numerous systems. In vertebrates, a gradient of one or more of the three Slit genes can induce the arrest of growth cones, similar to that observed here with ptc^- and en-expressing clones, which are ectopic sources of Sli (Fabre, 2010).

In other systems, Sli is received by one or more Robo receptors acting with Dock. In Drosophila, the three Robo genes are typically expressed in distinct but overlapping regions, but only Robo binds to Dock. This study presents evidence that Robo and the co-receptor Dock are expressed in the mechanosensory neurons and also that robo3 is expressed in the multidendritic neurons. Results with RNAi suggest that all three Robo genes are required for the normal fasciculation of the mechanosensory axons; the strongest effect was found with RNAi for robo2. Note that knockdown of any one of the Robo genes is unlikely to produce a very clear phenotype as they can partially substitute for each other (Fabre, 2010).

Flies carrying sli.lacZ suggest that sli is normally strongly expressed only at the back of the P compartments, raising the question of how its expression is controlled in the wild type. In the adult tergites, wg is expressed at the rear of each A compartment and Wg protein is thought to cross over the A/P border to form a gradient that patterns the P compartment. If so, and if a high concentration of Wg were to inhibit sli expression, then sli expression might be blocked in the anterior part of P (p3), but allowed in the posterior part of P. There are two other arguments supporting this hypothesis. First, wg is not (or is weakly) expressed in the most lateral tergite, which could explain why the band of sli expression is broader laterally and fills, or almost fills, the P compartment there. Second, wg is not expressed in the pleura, where sli.lacZ expression is ubiquitous. By contrast, in the sternites, wg is expressed and there sli.lacZ is confined to the P compartments. It could therefore be that ptc^- as well as the en-expressing clones that are transformed towards P identity would not express wg themselves. Thus, when located far from the endogenous source of Wg they should escape repression and transform into p1, which is of extreme posterior P identity, and become sources of Sli, as observed (Fabre, 2010).

Sli might work with other guidance cues in the fly abdomen. In the Drosophila eye, disruption of the Sli/Robo mechanism disturbs the boundary between the lamina and the distal cell neurons. It has been suggested that the Fasciclin adhesion molecules also support the boundary: Fas3 is expressed in the region where distal cell neurons are found, and Fas2 is expressed by the photoreceptor axons that carry Hh to the lamina. It is suspected that Fas2 and Fas3 might contribute to corralling neurons inside of the A compartment by promoting axonal bundling to the APN (Fabre, 2010).

An individual axon might be pushed from behind by a chemorepellent, pulled from afar by a chemoattractant, and hemmed in by attractive and repulsive local cues. These signals constitute what Ramón y Cajal proposed to be an 'intelligent force' guiding axons. It is not easy to dissect out these various signals, this paper has documented one repulsive signal, Sli, that hems in neurons and helps bundle segmental sets of sensory neurons in an arthropod (Fabre, 2010).

Slit and Receptor tyrosine phosphatase 69D confer spatial specificity to axon branching via Dscam1

Axonal branching contributes substantially to neuronal circuit complexity. Studies in Drosophila have shown that loss of Dscam1 receptor diversity can fully block axon branching in mechanosensory neurons. This paper reports that cell-autonomous loss of the receptor tyrosine phosphatase 69D (RPTP69D) and loss of midline-localized Slit inhibit formation of specific axon collaterals through modulation of Dscam1 activity. Genetic and biochemical data support a model in which direct binding of Slit to Dscam1 enhances the interaction of Dscam1 with RPTP69D, stimulating Dscam1 dephosphorylation. Single-growth-cone imaging reveals that Slit/RPTP69D are not required for general branch initiation but instead promote the extension of specific axon collaterals. Hence, although regulation of intrinsic Dscam1-Dscam1 isoform interactions is essential for formation of all mechanosensory-axon branches, the local ligand-induced alterations of Dscam1 phosphorylation in distinct growth-cone compartments enable the spatial specificity of axon collateral formation (Dascenco, 2015).

This study reports on a molecular mechanism regulating Dscam1 activity in growth cones and provides insight in the regulation and spatial specificity of axon collateral formation. Biochemical and genetic results are consistent with the molecular model that the specificity of mechanosensory (ms)-axon branching arises from a spatially restricted change of Dscam1 phosphorylation in growth cone (Dascenco, 2015).

Previous studies on the function of Dscam1 have established the model that isoform-specific homophilic Dscam1-Dscam1 interactions trigger repulsion between sister dendrites. This controls for regular spacing of sister dendrites in a process termed neurite self-avoidance. In addition, cell-intrinsic and isoform-specific interactions have also been shown to be important in sensory axons for growth-cone sprouting and branching. Importantly, for both of these functions, it is thought that Dscam1 signaling is primarily dependent on and initiated by homophilic binding of matching isoforms present on sister neurites. The results reported in this study provide evidence that Dscam1-Dscam1 interactions in axonal growth cones are subject to branch-specific modulation by extrinsic cues. Binding of the ligand Slit to Dscam1 can locally enhance cis-interactions with the receptor tyrosine phosphatase RPTP69D as well as the dephosphorylation of Dscam1. Although homophilic Dscam1 interactions can be considered to play an initial permissive role in all neurite-neurite interactions in a sprouting growth cone, the spatial restriction of an extrinsic Dscam1 ligand likely initiates functional disparity of Dscam1 signaling across different growth-cone compartments (Dascenco, 2015).

The biochemical data support the notion that RPTP69D directly dephosphorylates Dscam1 at specific cytoplasmic tyrosines. Three candidate tyrosines were identified for the regulation of Dscam1 phosphorylation: Y1857, Y1890, and Y1981. Two of the tyrosine residues, Y1857 and Y1890, are part of consensus SH2-binding sites and therefore are likely involved in regulating recruitment of SH2-domain-containing adaptor molecules. Given that these mutations diminish the Dscam1 GOF effects, it seems reasonable to speculate that they are required for downstream signaling and/or receptor turn-over or trafficking. Surprisingly, the single Y1981F mutation causes strong dominant interference with axon branching where the phenotypic effects are qualitatively indistinguishable from a loss of Dscam1 isoform diversity, which is thought to increase the probability of matching isoform interactions (i.e., GOF activity). The primary amino acid sequence surrounding Y1981 does not reveal any distinct signaling motif. However, in silico 3D protein modeling based on structural predictions suggests that phosphorylation of Y1981 could directly result in structural changes of the Dscam1 cytoplasmic domain and thereby influence Dscam1 activity (Dascenco, 2015).

Biochemical results suggest that Slit can enhance Dscam1-RPTP69D complex formation and Dscam1 dephosphorylation. Furthermore, Slit-N can directly bind to the N-terminal Ig domains of Dscam1 (Ig1-4) with an affinity comparable to that of other guidance cue/receptor interactions, suggesting that Slit-N can function as a bona fide Dscam1 ligand. Numerous studies have shown that the repellent as well as the branch-promoting function of vertebrate Slit require the function of Robo receptors. The current results show that for the formation of specific axon collaterals of Drosophila ms-neurons, Slit functions via Dscam1 in a Robo1-3-independent pathway (Dascenco, 2015).

Slit is one of the best characterized 'axon-repellent' cues and also contributes to axon branching. Imaging single ms-axons and growth-cone branching, this study found that in Slit mutant animals, only filopodia or micropodia with a midline-directed growth direction are reduced, consistent with a positive role of Slit in promoting the extension of specific branches. In contrast, branch-point initiation in ms-neurons is likely independent of Slit or RPTP69D (Dascenco, 2015).

Given that high Slit protein concentrations are likely only encountered by filopodia- or micropodia-like extensions that reach the midline proximity, the Slit-Dscam1-RPTP69D interactions are likely only occurring in a spatially restricted sub-compartment of the branching growth cone. It is envisioned that the Dscam1-RPTP69D interactions in ms-axons constitute a molecular selection process, which depends on Dscam1-RPTP69D complex formation in a subset of axonal processes that encounter sufficient Slit protein. As a result, Dscam1 dephosphorylation by RPTP69D is increased locally and triggers a response by either promoting axon-branch extension or blocking repulsion (Dascenco, 2015).

The loss of only a subset of axon branches in RPTP69D/Slit mutants suggests that there are multiple molecular control pathways accounting for the selection of different axon collaterals or the extension of the main axon shaft. Although this study has focused on RPTP69D and Slit, it is most likely that other co-receptors and extracellular cues control the activity of Dscam1 in growth cones (Dascenco, 2015).

GENE STRUCTURE

cDNA clone length - 4.4 kb

Exons - nine

PROTEIN STRUCTURE

Amino Acids

There are two splice variants, one with1469 amino acids and a second with 1480. The alternating segment is near the C-terminal end.

Structural Domains

The N terminal contains a signal peptide. This is followed by a four fold duplicated region each one consisting of an amino flanking region, four leucine rich repeats and a carboxyl flanking region (flank-LRR-flank domain). Slit also has seven copies of the EGF motif that participates in extracellular protein-protein interactions. The last EGF repeat is subject to alternative splicing (Rothberg, 1990). The C terminal contains a cysteine rich domain found in other secreted proteins (Rothberg, 1992). An additional laminin related sequence is found separating the sixth and seventh EGF-like domains (Patthy, 1992). The laminin related sequence between the sixth and seventh EGF repeats of Slit are also found in Drosophila proteins Crumbs and Fat (Patthy, 1992).

A slit cDNA was cloned encoding the complete open reading frame (ORF) from the LD 0-22 hr embryonic library. The ORF was sequenced and an additional leucine-rich repeat (LRR) was identified that is absent from the cDNA previously published (Rothberg, 1990). This additional LRR is between the second and third repeats in the first set of tandem LRR arrays. This LRR is present in vertebrate homologs of slit (Brose, 1999). In addition to the extra LRR, eight amino acid differences were identified. All of the substitutions are in LRR regions, but none occur in highly conserved residues of the motifs (Kidd, 1999).

date revised: 19 June 2024

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