Drosophila has six receptor protein tyrosine phosphatases (RPTPs), five of which are expressed primarily in neurons. Mutations in all five affect axon guidance, either alone or in combination. Highly penetrant central nervous system (CNS) and motor axon guidance alterations are usually observed only when specific combinations of two or more RPTPs are removed. This study examined the sixth RPTP, Ptp4E, which is broadly expressed. Ptp4E and Ptp10D are closely related type III RPTPs. Non-drosophilid insect species have only one type III RPTP, which is closest to Ptp10D. This study found that Ptp4E mutants are viable and fertile. Ptp4E Ptp10D double mutants were found to die before the larval stage and have a mild CNS phenotype in which the outer longitudinal 1D4 bundle is frayed. Ptp10D Ptp69D double mutants have a strong CNS phenotype in which 1D4 axons abnormally cross the midline and the outer and middle longitudinal bundles are fused to the inner bundle. To examine if Ptp4E also exhibits synthetic phenotypes in combination with Ptp69D, Ptp4E Ptp69D double mutants and Ptp4E Ptp10D Ptp69D triple mutants were made. No phenotype was observed in the double mutant. The triple mutant phenotype differs from the Ptp10D Ptp69D phenotype in two ways. (1) The longitudinal tracts appear more normal than in the double mutant; two or three bundles are observed, although they are disorganized and fused. (2) Axons labelled by the SemaIIB-τ Myc marker often cross in the wrong commissure. Motor axon guidance was examined, and no phenotypes were observed in any Ptp4E double mutant combination. However, triple mutants in which Ptp4E Ptp10D was combined with Ptp69Dor Ptp52F exhibited stronger phenotypes than the corresponding Ptp10D double mutants. It is concluded that type III RPTPs are required for viability in Drosophila, since Ptp4E Ptp10D double mutants die before the larval stage. Unlike Ptp10D, Ptp4E appears to be a relatively minor player in the control of axon guidance. Strong phenotypes are only observed in triple mutants in which both type III RPTPs are eliminated together with Ptp69D or Ptp52F. These results allow the construction of a complete genetic interaction matrix for all six of the RPTPs (Jeon, 2008).

Signalling via tyrosine phosphorylation is essential for axon guidance in many systems. Target proteins involved in signal transduction and cytoskeletal dynamics in growth cones are phosphorylated by tyrosine kinases (TKs) and dephosphorylated by tyrosine phosphatases (PTPs). In a simplified view of phosphotyrosine pathways controlling cell growth and differentiation, signaling is triggered by engagement of receptor tyrosine kinases (RTKs) by ligands. Ligand binding induces receptor dimerization and phosphorylation of downstream targets. RTK signalling is downregulated by dephosphorylation of autophosphorylated RTKs and other signalling molecules by cytoplasmic PTPs. In this scenario, the PTPs are passive modulators of a process in which the 'informational' event that initiates signalling is ligand binding to the RTK (Jeon, 2008).

In contrast, phosphotyrosine signalling pathways involved in growth cone guidance in the Drosophila embryonic central nervous system (CNS) involve receptor tyrosine phosphatases (RPTPs) and cytoplasmic TKs. Like RTKs, RPTPs are modular signalling receptors. They have cell adhesion molecule-like extracellular (XC) domains, linked via a single transmembrane region to one or two cytoplasmic PTP domains. Five of the six fly Rptp genes are selectively expressed in CNS neurons, and all of these genes have loss-of-function phenotypes that affect axon guidance (Jeon, 2008).

The TK that is central to many growth cone guidance events in the Drosophila >embryo is Abl, a cytoplasmic kinase. Drosophila has many RTKs, but no functional RTK has been implicated in embryonic axon guidance (the kinase-related axon guidance receptors Derailed and Off-track are thought to lack enzymatic activity). These facts suggest that phosphotyrosine signalling in growth cones could be controlled in a manner opposite to that used in RTK pathways. In this scheme, the growth cone would use a cytoplasmic TK to constitutively phosphorylate targets, and the 'information' that alters signalling strength would be transmitted via engagement of RPTPs by ligands located on the surfaces over which the growth cone travels (Jeon, 2008).

Of course, this is a greatly oversimplified picture, because there are many other receptors that can influence phosphotyrosine signalling in embryonic growth cones. For example, the Roundabout 1 (Robo1) receptor is an essential regulator of axon guidance across the midline. Phosphorylation of Robo1 by Abl may be regulated by Robo1's engagement of its ligand Slit, and in this case the 'information' that triggers signalling would be delivered via Slit binding to Robo1. Also, it is unlikely that phosphorylation by Abl is an unregulated, constitutive process. Nevertheless, it is striking that the receptors are kinases and the cytoplasmic modulators are phosphatases in pathways that regulate cell growth, while the reverse seems to be true for pathways that control neuronal growth cone guidance (Jeon, 2008).

RPTP pathways are poorly understood relative to RTK pathways, partially because in vivo ligands that regulate axon guidance and synaptogenesis have been identified only for the Drosophila Lar RPTP. These are the heparan sulfate proteoglycans Syndecan and Dallylike. However, Lar also has non-heparan sulfate proteoglycan ligands, and ligands for the other five fly RPTPs have not yet been defined. It has also been difficult to identify substrates that are important for RPTP signalling in vivo (Jeon, 2008).

Five Drosophila RPTPs have been genetically characterized in published papers. Four of these (Ptp10D, Lar, Ptp69D, Ptp99A) are expressed only on CNS axons in late embryos, and the fifth, Ptp52F, is CNS-specific but is expressed on both axons and cell bodies. All of the zygotic phenotypes for these genes are alterations in axon guidance, suggesting that this is the major function of this gene family in Drosophila. In contrast, many mammalian RPTPs are expressed in non-neural tissues and have functions unrelated to axon guidance (Jeon, 2008).

The RPTPs regulate both CNS and motor axon guidance. There is extensive redundancy among the five genes, so that highly penetrant guidance phenotypes are usually observed only when two or more RPTPs are genetically removed. Studies of motor axon guidance indicate that each guidance decision made by motoneuron growth cones requires a specific subset of the RPTPs. For example, axons in the ISNb nerve are unable to defasciculate from the common ISN pathway in Lar Ptp69D Ptp99A triple mutants. The later decision by ISNb axons to enter their target muscle field fails in Lar single mutants, so that the axons bypass the muscle field, but the bypass phenotype is suppressed and muscle field entry restored in Lar Ptp99A double mutants. This example illustrates that RPTPs can exhibit either functional redundancy, in which the absence of one RPTP is compensated for by another RPTP, or competition, in which removal of a second RPTP suppresses the guidance errors caused by the absence of the first RPTP. Similar genetic interactions among RPTPs may also occur in vertebrates, as two recent papers (Stepanek, 2005; Uetani, 2006) show that double mutant combinations and RNA interference perturbations involving the vertebrate Lar and type III (Ptp10D-like) RPTP subfamilies produce complex alterations in motor axon guidance (Jeon, 2008).

This paper examines the functions of the sixth and last Drosophila RPTP, Ptp4E. This protein is closely related to Ptp10D, and is the product of a recent gene duplication. Unlike the other RPTPs, Ptp4E is widely expressed in late embryos. When these studies started, it was thought that Ptp4E mutations might have phenotypes affecting many non-neural tissues, since loss of Ptp4E could not be compensated for by neural-specific RPTPs. However, the findings show that Ptp4E single mutants have no detectable phenotypes, because Ptp4E is redundant with the closely related Ptp10D. Double mutant embryos lacking both of these RPTPs die at hatching, but they have specific phenotypes affecting only CNS axons and tracheal cells (Jeon, 2008).

This study describes the axon guidance phenotypes produced by Ptp4E mutant combinations. Tracheal phenotypes will be described elsewhere. The data in this paper, together with those in earlier papers (Desai, 1997; Schindelholz, 2001; Sun, 2001), allow construction of complete pairwise interaction matrices that define how all six Drosophila RPTPs regulate CNS and motor axon guidance (Jeon, 2008).

Ptp10D and Ptp4E are clearly the result of a gene duplication that occurred much more recently than the split between the other Drosophila Rptp genes. The amino acid sequences of their catalytic PTP domains share 89% identity, versus 36%-40% identity for pairwise comparisons of Ptp4E with other Drosophila RPTPs. Their XC domains have a very similar organization, containing chains of 11 FN3 repeats in Ptp4E and 12 FN3 repeats in Ptp10D, and are 58% identical in amino acid sequence. The Ptp4E gene encodes two predicted preproteins, of 1,767 and 1,607 amino acids, while the Ptp10D gene encodes preproteins of 1,931 and 1,631 amino acids. The sequences that differ between the alternative gene products are at the carboxyl terminus in both cases, but there is no sequence similarity between the Ptp4E and Ptp10D proteins within this region. Both genes reside on the X chromosome, and the nine Ptp4E introns within conserved coding sequence all correspond exactly in position to Ptp10D introns. Ptp10D has one additional intron not found in Ptp4E (Jeon, 2008).

The Caenorhabditis elegans gene dep-1 is the ortholog of both Ptp10D and Ptp4E. Humans and mice have five genes encoding type III RPTPs, defined as proteins with XC domains composed of long chains of FN3 domains and a single PTP domain. Among these, the product of the PTPRB gene (PTPβ, not to be confused with RPTPβ, which is a different protein also known as PTPζ) has a somewhat higher alignment score to Ptp10D and Ptp4E than the other four mammalian type III proteins. These are: DEP-1/CD148, encoded by the Ptprj gene; PTPRO; SAP-1, encoded by the Ptprh gene; and PTPRQ. Since the radiation into the five mammalian genes seen today occurred after the split between arthropods and mammals, one cannot define any of the type III mammalian genes as an ortholog of one of the fly genes. Schindelholz (2001) presents more complete description of the relationships among all the Drosophila, C. elegans, and mammalian RPTPs (Jeon, 2008).

The recent availability of genome sequences from twelve different Drosophila species, three mosquito species, two hymenopterans, a beetle, and the silkmoth allowed tracing of the evolution of the Ptp10D/Ptp4E gene pair within insect lineages. Surprisingly, it was found that the Ptp4E gene is found only in drosophilid species. Mosquitoes, which are also dipterans, and all other sequenced insects have only a single Rptp gene corresponding to this gene pair. This gene is always much more closely related to Ptp10D than to Ptp4E. In addition, the Ptp4E sequence exhibits more sequence diversity among the drosophilid species than does the Ptp10D sequence. These data indicate that Ptp10D is the ancestral gene and that its sequence has been constrained more by evolution than the Ptp4E sequence since the time of the duplication. Ptp4E has evolved much more rapidly, suggesting that it may have acquired new function(s) since its emergence or was less essential for fitness than Ptp10D (Jeon, 2008).

The Ptp10D ortholog found in all insect species always contains the Ptp10D-specific intron, and all Ptp4E orthologs in drosophilids lack this intron. This suggests that the intron may have been lost at the time of the duplication from the copy that evolved into Ptp4E. This would have been between 235 million years ago (the estimated time at which the mosquito and fly lineages diverged from each other) and 40 million years ago (the estimated time at which the radiation among the 12 sequenced drosophilid species occurred) (Jeon, 2008).

Attempted were made to trace the history of the duplication by examining the genes adjacent to Ptp10D and Ptp4E, but the organizations of the Ptp10D and Ptp4E regions in D. melanogaster were found to have arisen long after the Ptp10D and Ptp4E genes diverged from each other. Ptp10D is flanked by the Rst(1)JH and bifocal genes. Rst(1)JH is found upstream of the Ptp10D ortholog in both the obscura and melanogaster groups, but is separated from it in D. willistoni and all other drosophilids. bifocal orthologs are adjacent to the Ptp10D gene only in the melanogaster group. Similarly, the two genes flanking Ptp4E, SIP3 and CG4068, are located next to the Ptp4E ortholog only within the obscura and melanogaster groups. There are no significant sequence similarities between the genes that flank Ptp10D and Ptp4E (Jeon, 2008).

The published in situ hybridization data suggest that Ptp4E mRNA is ubiquitously expressed in late embryos, although there are some level differences between tissues (Oon, 1993). To further analyze expression, and to ensure that the observed pattern was not affected by cross-hybridization between the closely related Ptp4E and Ptp10D phosphatase domain sequences, this analysis was repeated using a probe from the first four Ptp4E exons, which are not closely related to Ptp10D (Jeon, 2008).

In gastrulating embryos, Ptp4E mRNA is enriched in the invaginating mesoderm. In germ band extended embryos (stage 10-11), the strongest expression is observed in the posterior midgut primordium. There is also an interesting 'scalloped' pattern of expression observed at the ectodermal border. This has an intriguing correspondence to visceral mesoderm dpERK staining, suggesting that Ptp4E may be enriched at sites of RTK activation. Interestingly, thisbe, which encodes a ligand for the fibroblast growth factor receptor Heartless, is expressed in a similar pattern. Examination of a germ-band extended embryo expressing a UAS-linked Ptp4E-green fluorescent protein (GFP) fusion from the engrailed-GAL4 driver shows the expected striped pattern of expression, confirming that the probe recognizes Ptp4E and allowing an estimate of the relative levels of driven versus endogenous Ptp4E mRNA (Jeon, 2008).

At stage 14, Ptp4E is widely expressed, with highest levels observed in the midgut. Expression in the CNS can also be seen. Finally, at stage 17 expression is much higher in the gut than elsewhere, with particularly strong expression observed in the hindgut and at the anterior end of the midgut (Jeon, 2008).

To examine Ptp4E protein expression and localization, a variety of mouse monoclonal and polyclonal antibodies were generated against Ptp4E. Although the ubiquitous staining observed with the antibody is consistent with the in situ hybridization data, it is uncertain whether that antibody staining in wild-type embryos is due to Ptp4E protein, because it is not significantly reduced in Ptp4E mutant embryos (Ptp4E¹ or Df(1)ovo4). This finding could be explained in two ways. First, Ptp4E¹ mutants might continue to make an abnormal Ptp4E protein(s) due to initiation of translation at methionine residues encoded in exons not removed by the excision mutation (the second methionine residue in Ptp4E is at amino acid 377, within an undeleted exon). This protein, if it exists, would lack a signal sequence and may be nonfunctional, because the CNS phenotypes of Ptp4E¹ Ptp10D¹ and Df(1)ovo4 Ptp10D¹/Y embryos are identical. Df(1)ovo4 deletes the entire Ptp4E gene. The presence of antibody staining in Df(1)ovo4 mutants could be due to persistence of Ptp4E protein synthesized from maternal mRNA, since early embryos contain large amounts of Ptp4E mRNA. Second, it is possible that the Ptp4E antibodies cross-react with another ubiquitously expressed protein. They do not cross-react with Ptp10D, because the signal does not decrease in Ptp10D¹ mutant embryos (Jeon, 2008).

Although the antibody could not be used to define where Ptp4E is expressed in the embryonic CNS in wild-type embryos, its ability to recognize overexpressed Ptp4E protein allowed analysis of whether Ptp4E can localize to axons. To do this, Ptp4E-GFP wis drived with the pan-neuronal elav-GAL4 driver. In these embryos, bright staining of both CNS and peripheral nervous system (PNS) axons is observed with anti-Ptp4E and anti-GFP antibodies, and the two patterns are superimposable. Interestingly, Ptp4E-GFP also appears to localize to neuronal cell bodies in the PNS and CNS. In contrast, Ptp10D, Ptp69D, Lar, and Ptp99A, which are restricted to axons in wild-type embryos, are also axon-specific when overexpressed (Jeon, 2008).

Expression of Ptp10D protein was detected only in the nervous system in published work. It is selectively expressed on embryonic CNS axons (Tian, 1991), and in the neuropil of the larval and adult brain (Qian, 2007). Recent data, however, show that Ptp10D is also expressed by embryonic tracheal cells. These findings suggest that the embryonic/larval lethality of Ptp4E¹ Ptp10D¹ animals might be due to either nervous system or tracheal phenotypes. In fact, it was found that these embryos have severe tracheal defects. Their nervous system defects, however, are relatively mild, and would not be expected to produce early lethality. Consistent with this, it was found that GAL4-driven pan-neural expression of a UAS-Ptp4E-GFP fusion, which is capable of rescuing the tracheal phenotype when driven in tracheal cells by breathless-GAL4, does not rescue lethality in the Ptp4E¹ Ptp10D¹ background (Jeon, 2008).

Driving Ptp10D in tracheae with breathless-GAL4 in a Ptp4E¹ Ptp10D^EP1172 background (the EP1172 line is a UAS-containing P element insertion upstream of the gene, so it allows rescue by crossing in GAL4 drivers) rescues lethality, allowing some adults to emerge. These data confirm that lethality in the double mutant is rescuable by Ptp10D expression in tracheae (or in other cells that express breathless-GAL4). Attempts were made to rescue lethality by ubiquitous expression of Ptp4E, but pancellular overexpression of Ptp4E-GFP driven by tubulin-GAL4 was found to be lethal (Jeon, 2008).

In Drosophila, five of the six RPTPs have been reported to be neural-specific in late embryos, and all the zygotic Rptp phenotypes that have been published are axon guidance alterations. In contrast, many of the 17 mammalian RPTPs are expressed in non-neural cell types and have a variety of functions unrelated to axon guidance. Since Ptp4E is the only widely expressed Rptp gene, it is speculated that studying its mutant phenotype might reveal new functions for Drosophila RPTPs outside the nervous system, and that these might provide information about functions of mammalian non-neural RPTPs. One might have expected that Ptp4E mutations would cause lethality and produce strong phenotypes, since no other RPTPs would be able to compensate for the loss of Ptp4E in non-neural cells. This, however, is not the case. Ptp4E mutants are viable, fertile, and apparently healthy, and have no detectable phenotypes in the nervous system or elsewhere. Furthermore, evolutionary analysis indicates that Ptp4E is a relatively recent invention; it is present in drosophilids but not in mosquitoes or non-dipteran arthropods. Within the drosophilids, its sequence also changes more rapidly than that of Ptp10D, suggesting that it has been less constrained by evolution. All of these considerations indicate that Ptp4E is not essential for development of non-neural cell types in Drosophila (Jeon, 2008).

Perhaps in Drosophila the functions executed by mammalian RPTPs in non-neural cell types are carried out by one or more of the eight nonreceptor PTPs. Some of these are ubiquitously expressed. Only three have been genetically characterized. Csw and PTP-ER are involved in cell fate determination. Mutations in ptpmeg produce axonal defects in the adult brain. Ptpmeg, however, does not act in the neurons that exhibit the axonal phenotypes, but is required in surrounding cells. Thus, it is unlikely to participate in growth cone signal transduction in the same manner as the RPTPs (Jeon, 2008).

Ptp10D and Ptp4E are the only Drosophila RPTPs that are members of the same subfamily; the other four are each the sole fly representative of their subfamily. Mutations in three of the other four Rptp genes (Lar, Ptp52F, Ptp69D) cause lethality. This suggests that the viability of Ptp10D and Ptp4E single mutants might be due to compensation by the other member of the subfamily, and that a Ptp4E Ptp10D double mutation would cause lethality. This is in fact observed; the double mutant dies at hatching. However, it does not have generalized defects. Rather, the defects are all within the nervous system and the tracheal network. Ptp10D is also selectively expressed in tracheal cells. It is suggested that Ptp4E Ptp10D double mutant phenotypes are observed only where Ptp10D is expressed (Jeon, 2008).

The analysis described in this paper, together with that in several other papers allowed assembly of a complete genetic interaction matrices for pairwise combinations of mutations in all six of the Rptp genes. A matrix is presented depicting the functions of the RPTPs in regulation of longitudinal axon guidance in the CNS, as assayed by 1D4 staining. The lines represent different types of genetic interactions. Synthetic phenotypes, where neither of the single mutants exhibits a detectable phenotype but the double mutant has a phenotype, are seen for Ptp10D Ptp69D and Ptp4E Ptp10D. Enhancement of a single mutant phenotype by removal of a second RPTP are observed only for Ptp52F, since this is the only single mutant that has a CNS phenotype detectable with 1D4. Finally, suppression, where removal of a second RPTP suppresses the single mutant phenotype, is observed for Lar Ptp52F mutants, and may indicate that these two RPTPs function in a competitive manner (in a formal genetic sense) to regulate a CNS signalling pathway (Jeon, 2008).

The interaction matrix for motor axon guidance is different from the CNS interaction matrix, it can be concluded that the relationships between the RPTP signalling pathways differ in some cases between motor neurons and CNS interneurons. However, Ptp10D Ptp69D double mutants have a synthetic SNa phenotype, so these two RPTPs interact strongly in regulating both CNS and motor axon guidance. Loss of Ptp10D also enhances both the CNS and motor axon defects of Ptp52F mutants (Jeon, 2008).

As in the CNS, there are competitive relationships between RPTPs, but they are seen for a different RPTP pair. In motor axons, removal of Ptp99A completely suppresses the Lar ISNb parallel bypass phenotype. Lar mutations enhance the Ptp52F motor axon phenotypes rather than suppressing them as they do in the CNS (Jeon, 2008).

This paper has defined the phenotypes associated with simultaneous elimination of the functions of two RPTP subfamilies, by examining triple mutants removing both Ptp4E and Ptp10D together with each of the other three RPTPs whose absence produces lethality. This analysis shows that the Ptp10D/Ptp4E subfamily is redundant with Ptp69D in controlling guidance decisions made by three neuronal types, but Ptp4E mutants have relatively minor effects relative to Ptp10D mutants. For guidance of 1D4 and SemaIIB axons within the CNS, removal of both members of the Ptp10D/Ptp4E subfamily together with Ptp69D modulates the phenotype observed in Ptp10D Ptp69D mutants. For SNa axons, the triple mutant has an enhanced phenotype, in that 10% of SNa nerves now fail to extend altogether; this is almost never observed in double mutants. Enhancement of a Ptp10D Ptp52F ISN truncation phenotype was seen by removal of Ptp4E, but no strong interactions between Ptp4E Ptp10D and Lar are observed in the CNS or neuromuscular system (Jeon, 2008).

These results suggest that there is a special relationship between the Ptp10D/Ptp4E subfamily and Ptp69D. Perhaps these two types of RPTPs have similar substrates in both CNS interneurons and motor neurons. In CNS neurons, some critical substrate(s) dephosphorylated by Ptp69D might also be dephosphorylated by either Ptp10D or Ptp4E, so that certain phenotypes, such as crossing of all the SemaIIB axons in the wrong commissure, are observed only when all three RPTPs are eliminated. However, in CNS neurons such as the neuroblast 2-5 lineage, whose axons ectopically cross the midline in Ptp10D Ptp69D double mutants, Ptp4E cannot compensate for the loss of Ptp10D. Perhaps in these cells the relevant Ptp69D substrate(s) can be dephosphorylated by Ptp10D but not by Ptp4E; however, this seems unlikely given that Ptp4E and Ptp10D have PTP domains that are much more similar to each other than are those of Ptp69D and Ptp10D. Alternatively, perhaps the Ptp4E concentration is too low in these neurons for efficient dephosphorylation to occur. Another possibility is that growth cones of these neurons contact Ptp10D ligands, but not Ptp4E ligands, and that ligand contact is required for signalling. An understanding of the biochemical origins of these genetic interactions will require identification and characterization of RPTP ligands, substrates and downstream signalling proteins, as well as localization of these proteins to specific neuronal types (Jeon, 2008).

Receptor tyrosine phosphatases (RPTPs) are required for axon guidance during embryonic development in Drosophila. This study examined the roles of four RPTPs during development of the larval mushroom body (MB). MB neurons extend axons into parallel tracts known as the peduncle and lobes. The temporal order of neuronal birth is reflected in the organization of axons within these tracts. Axons of the youngest neurons, known as core fibers, extend within a single bundle at the center, while those of older neurons fill the outer layers. RPTPs are selectively expressed on the core fibers of the MB. Ptp10D and Ptp69D regulate segregation of the young axons into a single core bundle. Ptp69D signaling is required for axonal extension beyond the peduncle. Lar and Ptp69D are necessary for the axonal branching decisions that create the lobes. Avoidance of the brain midline by extending medial lobe axons involves signaling through Lar (Kurusu, 2008).

The mushroom bodies (MBs) are highly conserved paired structures in the insect brain that are essential for olfactory learning and other higher-order functions. MBs vary greatly in size between insect species, but their overall organization is very similar in all insects. In Drosophila, an adult MB contains about 2500 principal neurons, known as Kenyon cells. All Kenyon cells are generated from four neuroblasts (NBs) in each brain hemisphere. Each NB produces three types of Kenyon cells (γ, α′/β′, and α/β) in a strict temporal order, and the four lineages are indistinguishable. An MB is thus a fourfold-symmetric structure. γ neurons are generated by the mid-third instar stage, α′/β′ neurons between mid-third instar and puparium formation, and α/β neurons after puparium formation (Kurusu, 2008).

Kenyon cell dendrites extend into a glomerular structure called the calyx, which receives olfactory input from the projection neurons of the antennal lobe (AL). The axons of Kenyon cells from each lineage group fasciculate into a bundle, and the four bundles merge to form the peduncle, a massive parallel tract that extends ventrally and then splits into two branches, each composed of intertwined lobes (Kurusu, 2008).

The three types of Kenyon cells differ with respect to their dendritic and axonal projection patterns in the calyx and lobes. These anatomical subdivisions are likely to be important for the analysis of complex sensory inputs, because γ, α′/β′, and α/β neurons receive projections from different sets of AL glomeruli (Kurusu, 2008).

In larvae, the axonal structures formed by the splitting of the peduncle are called the dorsal and medial lobes. Every Kenyon cell axon bifurcates during outgrowth and sends a branch into each lobe. The γ axons extend first and may serve as pathways to guide the axons of later-born α′/β′ neurons in the larva. α/β neurons follow similar trajectories during the pupal phase. The distal portions of the γ axons degenerate during pupal development, and their medial branches later regrow to form the adult γ lobe. The adult MB has a dorsal branch composed of the intertwined α and α′ lobes, and a medial branch containing the β, β′, and γ lobes. The lobes within the medial branch extend toward the corresponding lobes of the MB in the other hemisphere, but stop at the edge of a midline region that is devoid of MB axons (Kurusu, 2008).

The temporal order of Kenyon cell birth is reflected in the organization of axons in the peduncle and lobes, with the axons of the youngest neurons (the core fibers) in the center and those of older neurons arranged as a series of concentric rings around the core. The core fibers and the layers formed by older axons can be distinguished using antibody markers, GAL4 drivers, and staining for polymerized actin using phalloidin. The actin-rich core fibers are a transient population, because the ingrowth of new axons from sequentially generated later-born neurons occurs successively at the center and displaces the old core fibers outward into the innermost ring. Axons lose bright phalloidin staining as they move outward within the peduncle. This organization is consistent with the idea that the core fibers form a pioneer pathway used to guide extension of the axons of later-born neurons, which then in turn become the new core and are used as pathways by still later axons. To understand axon guidance and lobe morphogenesis during MB development, it is thus valuable to define and genetically characterize receptors and adhesion molecules expressed on these core fibers (Kurusu, 2008).

This study shows that four receptor tyrosine phosphatases (RPTPs) are selectively expressed on core fibers within the peduncle and lobes, and that RPTP signaling regulates segregation of core fibers into a single bundle. RPTPs have cellular adhesion molecule (CAM)-like extracellular regions containing immunoglobulin (Ig) domains and/or fibronectin type III (FN3) repeats. The ligands that interact with these extracellular domains are largely unknown. However, vertebrate and Drosophila Lar RPTPs bind to heparan sulfate proteoglycans (HSPGs), and the cell-surface HSPG Syndecan (Sdc) contributes to Lar's functions during axon guidance and synaptogenesis (Kurusu, 2008).

The four RPTPs localized to core fibers are Lar, Ptp10D, Ptp69D, and Ptp99A. They are all selectively expressed on central nervous system (CNS) axons in the embryo. There are a total of six RPTPs encoded in the Drosophila genome. The other two RPTPs, Ptp52F and Ptp4E, were not studied because Ptp52F mutants die as embryos and Ptp4E mutants have not been characterized. Also, good antibodies against these two proteins are not available (Kurusu, 2008).

RPTPs have been extensively analyzed as regulators of motor axon guidance in the embryonic neuromuscular system. Each guidance decision made by motor axons can be defined by a requirement for a specific subset of the five RPTPs that have been examined. There is substantial redundancy among RPTPs, so that high-penetrance alterations in a guidance decision are usually observed only when two or more RPTPs are removed. For example, axons of the ISNb nerve fail to leave the common ISN pathway and thus remain fasciculated to ISN axons ('fusion bypass' phenotype) when Lar, Ptp69D, and Ptp99A are all absent. When only Lar is missing, ISNb axons leave the ISN but then sometimes fail to enter their target muscle field (Kurusu, 2008).

RPTPs also regulate CNS axon guidance in a redundant manner. A subset of longitudinal axons abnormally cross the midline in Ptp10D Ptp69D double mutants (Sun, 2000), while all longitudinal axons are rerouted into midline-crossing commissural pathways in Lar Ptp10D Ptp69D Ptp99A quadruple mutants (Sun, 2001). All four RPTPs thus participate in midline crossing decisions in the embryo. This paper shows that Lar regulates avoidance of the brain midline by MB axons in the larval brain, and that high-penetrance ectopic crossing phenotypes are observed when only Lar is absent (Kurusu, 2008).

RPTP function has also been characterized during later development and in adulthood. Lar mutants have reduced numbers of boutons at neuromuscular junctions (NMJs) in larvae, and Lar and Ptp69D mutants display alterations in photoreceptor axon guidance and synaptic maintenance in the optic lobes. Finally, hypomorphic Ptp10D mutants are defective in long-term memory formation, and this phenotype can be rescued by restoration of Ptp10D expression in the MB or by acute induction of Ptp10D in adults (Qian, 2007). This result indicates that RPTPs are likely to regulate synaptic function in mature animals. Consistent with this, this study shows that all four RPTPs are expressed in specific patterns within the neuropil of the adult brain (Kurusu, 2008).

This paper demonstrates that RPTPs are selectively expressed on the core fibers of the MB. At any one time during larval development, the core fiber bundle is composed of the youngest axons within the peduncle and lobes. Core fibers are likely to serve as pathways for guidance of axons of later-born neurons. These later axons then become the new core fiber bundle and displace the old core fibers outward into concentric rings (Kurusu, 2008).

Two of the RPTPs, Ptp69D and Ptp10D, are expressed only on the innermost core fibers, while Lar and Ptp99A are expressed within an expanded core region including somewhat older axons. The data indicate that Ptp10D and Ptp69D are involved in segregation of core fibers. Split-core phenotypes, in which two or more phalloidin-rich, OK107::mCD8-GFP-low bundles are observed within the peduncle, are observed in MBs lacking Ptp69D function. They were also observed in larvae hemizygous for a complete deletion of Ptp10D and the adjacent bif gene, Δ59. The split-core phenotype occurs only when both Ptp10D and Bif are absent. These two genes have been shown to genetically interact in other contexts (Kurusu, 2008).

Ptp69D is also required for outgrowth of axons from the peduncle into the lobes, because regions near the junctions of the peduncle and the lobes are expanded at the expense of the lobes in MBs lacking Ptp69D function. Another distinctive phenotype seen in these experiments is abnormal extension of medial lobe axons across the brain midline. This is seen in Lar mutant larvae and in larval and adult Lar mutant NB clones (Kurusu, 2008).

Prior to this work, only two genes had been identified as having mutant phenotypes affecting core fiber segregation. Both of these encode adhesion molecules. mRNA from the Dscam gene, which encodes homophilic Ig domain CAMs, is alternatively spliced to generate up to 38,000 different protein isoforms. Like the RPTPs, Dscam is selectively expressed on young axons that travel within the core fiber bundle. Dscam mutant MBs and NB clones exhibited multiple core fiber bundles within the peduncle. The similarities between Dscam, Ptp69D, and Ptp10D phenotypes and expression patterns suggest that these RPTPs could be involved in Dscam signaling during outgrowth of young axons within the peduncle. Alternatively, the RPTPs could regulate adhesion via other pathways that also affect core fiber segregation and are partially redundant with Dscam (Kurusu, 2008).

Core fiber segregation defects were also observed in ~25% of NB clones bearing null mutations eliminating expression of the homophilic Ig-domain CAM FasII. FasII, unlike Dscam and the RPTPs, is expressed only on older axons outside the core region, and engagement between FasII molecules on different cell surfaces is thought to trigger adhesion rather than repulsion. This suggests that core fiber segregation defects in FasII mutants arise by a different mechanism. Perhaps older axons lacking FasII fail to adhere sufficiently to each other and thus open passageways that allow young axons to pioneer multiple pathways within the peduncle (Kurusu, 2008).

Mutants lacking expression of Lar exhibit phenotypes in which one or both of the medial lobes of the bilaterally symmetric MBs extend across the midline, so that they appear fused to one another. It was also found that Lar mutant NB clones extended axons across the midline into the territory occupied by the other MB. These phenotypes suggest that a repulsive signal emanates from the brain midline region, and that Lar participates in reception of this signal by growing MB axons within the medial lobe (Kurusu, 2008).

This is formally similar to repulsion from the CNS midline in the embryo, where binding of midline Slit to Roundabout (Robo) receptors on neuronal growth cones causes them to navigate away from the midline. Axons abnormally cross the midline in Ptp10D Ptp69D double mutant embryos, and genetic interaction studies showed that these RPTPs are positive regulators of Robo signaling in the embryo (Sun, 2000). Lar is also implicated in the decisions of axons to cross the embryonic midline (Kurusu, 2008).

The MB midline crossing phenotypes seen in Lar mutants are probably not mediated through alterations in Slit-Robo signaling. Robo1, 2, and 3 mutant phenotypes in the larval MB have been described, and they do not include fusion of medial lobes across the midline. The Lar ligand Sdc is not selectively expressed at the brain midline, so binding of Lar to Sdc probably does not trigger repulsion of MB axons. Perhaps other, as yet unidentified, Lar ligands are expressed in the midline region, and interactions between these ligands and the RPTP facilitates repulsion. Studies showed that Lar binds to at least one non-HSPG ligand in the embryo (Kurusu, 2008).

A number of other genes that regulate repulsion of MB axons from the midline have been identified in other studies. Mutations in derailed (linotte), which encodes a protein related to Ryk receptor tyrosine kinases, cause overgrowth of medial lobes across the midline. The Derailed receptor responds to a Wnt5 signal, and mediates exclusion of axons from posterior commissural pathways that cross the embryonic midline. Medial lobe fusion across the midline is also observed in fmr1 mutants and in FMR1 overexpression animals. FMR1 is an RNA-binding protein that is orthologous to the human gene affected in Fragile X mental retardation syndrome (Kurusu, 2008).

Other genes potentially involved in repulsion from the brain midline were identified in a microarray screen for MB-expressed genes. ML fusion was observed in some larvae when expression of these genes was inhibited using RNAi methods. The gene with a known function for which RNAi produced ML fusion with the highest penetrance is CG6083, encoding an aldehyde reductase. RNAi for a gene encoding a cGMP phosphodiesterase also produced ML fusion. cGMP signaling has been implicated in growth cone repulsion in vertebrate neuronal cultures (Kurusu, 2008).

The work described in this paper shows that four RPTPs are localized to growing MB axons and are important for the creation of the distinctive architecture of the MB's axonal network. Examination of Rptp mutant phenotypes shows that these CAM-like signaling molecules control several different axon guidance decisions that occur during outgrowth. Ptp10D and Ptp69D regulate segregation of the growing axons into a single core bundle within the peduncle. Ptp69D and Lar are necessary for the later axonal extension and branching events that create the dorsal and medial lobes. Medial lobe axons cease outgrowth before they reach the brain midline, and their decision to stop involves Lar (Kurusu, 2008).

Tyrosine phosphorylation mediates multiple signal transduction pathways that play key roles in developmental processes and behavioral plasticity. The level of tyrosine phosphorylation is regulated by protein tyrosine kinases and protein tyrosine phosphatases (PTPs). Extensive studies have investigated the roles of tyrosine kinases in memory formation. However, there were few studies on PTPs. To date, learning has been shown to be defective only for mouse knock-outs of PTPα, leukocyte common antigen-related, or PTPδ. A major limitation of these studies arises from their inability to distinguish an acute (biochemical) impairment of memory formation from a more chronic abnormality in neurodevelopment. From a behavioral screen for defective long-term memory, chi mutants were found to disrupt expression of the PTP10D protein tyrosine phosphatase gene. chi mutants are normal for learning, early memory, and anesthesia-resistant memory, whereas long-term memory specifically is abolished. Significantly, induction of a heat shock-PTP10D⁺ transgene before training fully rescues the memory defect of chi mutants, thereby demonstrating an acute role for PTP10D in behavioral plasticity. PTP10D was found to be widely expressed in the embryonic CNS and in the adult brain. Transgenic expression of upstream activating sequence-PTP10D⁺ in mushroom bodies is sufficient to rescue the memory defect of chi mutants. These data clearly demonstrate that signaling through PTP10D in mushroom bodies is critical for the formation of long-term memory (Qian, 2007).

A P element insertional mutagenesis to screen was performed for X-linked mutants that disrupt 1 d memory after spaced training. Plasmid rescue of genomic DNA flanking the P element (PlacW) insertion in one identified mutant, which was named chi ('chi' is a Chinese word that can be translated as 'foolish'), revealed the transposon to be inserted in the first intron of PTP10D, 345 bp downstream of the splice donor site of exon 1 and 943 bp upstream of the start codon in exon 2. RT-PCR analyses revealed that expression of the PTP10D transcript is greatly reduced in chi mutants. Western blot analysis identified a single protein band of 190 kDa in control flies, which is consistent with a previous observation (Tian, 1991), and this band was undetected in chi/chi mutants. These molecular results suggest that PTP10D is disrupted in chi mutants (Qian, 2007).

These PTP10D mutants were found to be defective in 1 d memory after spaced training. Both chi/chi and EP(X)1172/EP(X)1172 homozygous mutants showed significantly lower 1 d memory after spaced training than control flies. More importantly, two mutant alleles failed to complement; 1 d memory after spaced training in chi/EP(X)1172 heteroallelic mutants was significantly lower than control flies. This finding demonstrates that loss of function of the PTP10D gene per se is responsible for the defect in 1 d memory after spaced training (Qian, 2007).

Therefore, from a behavioral screen for long-term memory mutants identified chi, which carries a molecular lesion of the PTP10D gene. Homozygotes of another, independently isolated mutation in PTP10D, EP(X)1172, also are defective for long-term memory. Assays for different memory phases reveal the memory defect of chi mutants to be specific for long-term memory; STM, MTM, and ARM all were statistically indistinguishable from controls, as were basic sensorimotor responses to odors and footshock. These observations confirm that mutations in the chi locus lead to specific LTM defects (Qian, 2007).

This defect is not resulted from developmental abnormality for the LTM phenotype of chi mutants and can be rescued by acutely induced expression of an hs-PTP10D⁺ transgene in the adult fly, a conclusion also supported by the lack of abnormalities in gross brain morphology in chi mutants. More specifically, the Chi protein PTP10D plays an essential acute role in formation of LTM but is not required for maintenance of LTM. This rescue occurred only when training took place within 3 h of heat shock when induced expression of PTP10D was abundant. However, the heat shock induction failed to rescue LTM when training was conducted 27 h later at which induced expression of PTP10D was becoming undetectable. Moreover, at testing of memory (27 h after heat shock), although induced expression of PTP10D was undetectable, LTM remained to be rescued if training happened within an interval during which induced expression of PTP10D was abundant. Such observations provide a strong demonstration of an acute role for PTP10D during LTM formation (Qian, 2007).

Finally, function of PTP10D for memory formation is localized in the MB. Transgenic expression of a UAS-PTP10D⁺ transgene only in MBs is sufficient to rescue the LTM defect of chi mutants, a result consistent with many other studies that support a role for MBs during olfactory memory formation. In particular, it has been shown that the absence of the vertical lobes of the mushroom body disrupts LTM, and mutations in Crammer peptides that are expressed in the glial cells around the mushroom body also lead to defective LTM (Comas, 2004). This study presents for the first time that the function of a protein in the mushroom body is required for formation of LTM. All of these lines of evidence lead to the conclusion that this receptor-like tyrosine phosphatase plays a critical role in biochemical cascades that mediate long-term memory formation in Drosophila (Qian, 2007).

Several PTPs have been examined in Drosophila. PTP69D plays a role in axon guidance, whereas Drosophila leukocyte common antigen-related Dlar appears to be required for maturation of growth cones in synapses. The data show that PTP10D is expressed widely in embryonic and adult CNS. Thus, one might expect PTP10D also to play a role in neurodevelopment. To that end, the axon guidance defect of PTP69D mutants is exacerbated in PTP69D;PTP10D double mutants, but PTP10D single mutants display no observable defects in development of the CNS (Desai, 1996; Sun, 2000). The data support these observations and suggest, instead, a more important role for PTP10D in the biochemistry of memory formation (Qian, 2007).

The study provides the following insights. Primarily, this is a novel report in invertebrates that a receptor-like tyrosine phosphatase is involved in memory formation. In vertebrates, there are a number of reports that suggest an involvement of receptor-like tyrosine phosphatases in learning and memory, although it remains to be determined whether such effects are specific to long-term memory formation (Uetani, 2000; Skelton, 2003; Kolkman, 2004). In any case, these observations indicate that signaling via receptor-like tyrosine phosphatases may be a conserved biochemical pathway for learning and memory, as for the cAMP pathway. Also, the extracellular structure of PTP10D is similar to other cell adhesion molecules, suggesting that cell-cell interaction might be a critical signal transduction mechanism during memory formation. Consistent with this notion, mutants for Drosophila integrin and Fas II have been reported to disrupt STM. More generally, cell adhesion molecules have been implicated widely for synaptic plasticity in vertebrates and invertebrates (Qian, 2007).

Receptor-linked protein tyrosine phosphatases (RPTPs) regulate axon guidance and synaptogenesis in Drosophila embryos and larvae. DPTP52F, the sixth RPTP to be discovered in Drosophila, is described. Genomic analysis indicates that there are likely to be no additional RPTPs encoded in the fly genome. Five of the six Drosophila RPTPs have C. elegans counterparts, and three of the six are also orthologous to human RPTP subfamilies. DPTP52F, however, has no clear orthologs in other organisms. The DPTP52F extracellular domain contains five fibronectin type III repeats and it has a single phosphatase domain. DPTP52F is selectively expressed in the CNS of late embryos, as are DPTP10D, DLAR, DPTP69D and DPTP99A. To define developmental roles of DPTP52F, RNA interference (RNAi)-induced phenotypes were examined as a guide to identify Ptp52F alleles among a collection of EMS-induced lethal mutations. Ptp52F single mutant embryos have axon guidance phenotypes that affect CNS longitudinal tracts. This phenotype is suppressed in Dlar Ptp52F double mutants, indicating that DPTP52F and DLAR interact competitively in regulating CNS axon guidance decisions. Ptp52F single mutations also cause motor axon phenotypes that selectively affect the SNa nerve. DPTP52F, DPTP10D and DPTP69D have partially redundant roles in regulation of guidance decisions made by axons within the ISN and ISNb motor nerves (Schindelholz, 2001).

Ptp52F mutants display a variety of SNa guidance defects. The most common defect, as in Ptp52F RNAi embryos, is a failure to bifurcate. In other hemisegments, the SNa has extra branches, or stalls near the bifurcation point. The penetrances of such SNa phenotypes in Ptp52F^18.3 homozygotes or Ptp52F^18.3/Df(2R)JP4 transheterozygotes are 37% and 41%, respectively. The two other Ptp52F alleles and the transheterozygous combinations of the three Ptp52F alleles with Df(2R)JP8 have a lower penetrance of SNa defects (22%-28%) (Schindelholz, 2001).

Single mutants that lack any of the other four neural RPTPs do not display SNa phenotypes. However, combinations of Rptp mutations do affect the SNa. To evaluate how removal of other RPTPs might affect Ptp52F SNa phenotypes, double mutants lacking both DPTP52F and each of the other RPTPs were made. The absence of DPTP10D, DPTP69D or DLAR increases the penetrance of the Ptp52F^18.3 defects, particularly those in which the SNa stalls near the bifurcation point. No new phenotypes are observed in double mutants, however. Removal of DPTP99A does not affect the overall penetrance of SNa phenotypes, but does decrease the frequency of ectopic branches (Schindelholz, 2001).

The ISNb motor nerve innervates the VLMs and contains the axons of the identified RP1, RP3, RP4 and RP5 motoneurons. RP growth cones leave the common ISN pathway at the exit junction, enter the VLM field, and then navigate among the muscle fibers. Synapses begin to form at stereotyped positions by late stage 16. Ptp52F mutations produce any detectable ISNb phenotypes only at low frequencies (Schindelholz, 2001).

Ptp10D mutations produce no ISNb phenotypes. Removal of both DPTP10D and DPTP52F, however, generates a strong phenotype in which the ISNb stalls within the VLMs, often at the proximal edge of muscle 13. This stall phenotype is observed in Ptp52F single mutants, but its frequency can be dramatically increased in double mutants for addition of a Ptp10D mutation to the hypomorphic mutation Ptp52F^8.10.3;. Removal of DPTP69D also greatly enhances the Ptp52F stall phenotype (Schindelholz, 2001).

Dlar Ptp52F double mutants have parallel bypass phenotypes identical to those of Dlar single mutants. Ptp99A mutations cause no ISNb phenotypes on their own or in combination with Ptp52F (Schindelholz, 2001).

The ISN passes its first (FB) and second (SB) lateral branchpoints before reaching the position of its terminal arbor at the proximal edge of muscle 1. In Ptp52F mutants, most ISNs are normal. Dlar mutations produce SB phenotypes with a similar penetrance (19% for null alleles). When Dlar and Ptp52F mutations are combined, the frequency of the SB termination phenotype is similar to that of the single mutants. Ptp99A mutations have no effects on ISN on their own, and also cause no enhancement of the Ptp52F phenotype (Schindelholz, 2001).

Ptp10D and Ptp69D single and double mutants have no ISN phenotypes. However, removal of either of these RPTPs from a Ptp52F mutant background enhances the penetrances of the Ptp52F ISN phenotypes. Ptp10D Ptp52F double mutants have a reduced terminal arbor (T) phenotype that is less frequently observed in Ptp52F single mutants. Removal of DPTP69D does not affect the T phenotype, but produces an increase in the SB phenotype. In summary, these results indicate that DPTP52F, DPTP10D and DPTP69D have partially redundant functions in regulation of ISN outgrowth. It is interesting that Ptp52F mutations do not produce synergistic phenotypes when combined with Dlar mutations, which are the only other Rptp mutations that generate strong ISN phenotypes on their own. Perhaps there are two separate 'functions' needed for normal ISN outgrowth, one of which involves DLAR and the other DPTP52F (Schindelholz, 2001).

DPTP52F is the only RPTP whose removal produces clear phenotypes in the 1D4-positive longitudinal bundles of the CNS. The 1D4 pathways are usually indistinguishable from wild type in single mutants lacking each of the other four RPTPs. Removal of DPTP10D or DPTP69D from a Ptp52F background strengthens the Ptp52F CNS phenotype. The longitudinal 1D4-positive bundles become more irregular, and frequent breaks and discontinuities in the middle bundle are observed. No new synergistic phenotype like that produced by removal of DPTP10D and DPTP69D together is observed. Removal of DPTP99A does not affect the Ptp52F CNS phenotype (Schindelholz, 2001).

In contrast to these results, when a Dlar mutation is introduced into a Ptp52F mutant background, the morphology of the 1D4-positive bundles reverts to wild type. In a few segments of Dlar Ptp52F double mutants, breaks in the outer 1D4-positive bundle are still seen, but defasciculation and irregularities in the inner two bundles are not observed. The suppression is specific to the CNS phenotypes detected at late stage 16, because the introduction of Dlar mutations into a Ptp52F mutant background does not correct the failure of the pCC growth cone to extend at the appropriate time. DLAR also participates in another competitive interaction: the Dlar ISNb parallel bypass phenotype is absent in Dlar Ptp99A double mutants. Here, however, it is a Dlar phenotype that is suppressed by removal of another RPTP, rather than the reverse. Ptp52F mutations do not affect Dlar parallel bypass phenotypes. Determination of the mechanisms that underlie these genetic interactions will require biochemical analysis of DPTP5F and of the signaling pathways in which it participates (Schindelholz, 2001).

Neural receptor-linked protein tyrosine phosphatases (RPTPs) are required for guidance of motoneuron and photoreceptor growth cones in Drosophila. These phosphatases have not been implicated in growth cone responses to specific guidance cues, however, so it is unknown which aspects of axonal pathfinding are controlled by their activities. Three RPTPs, known as DLAR, DPTP69D, and DPTP99A, have been genetically characterized thus far. The isolation of mutations in the fourth neural RPTP, DPTP10D, is reported. The analysis of double mutant phenotypes shows that DPTP10D and DPTP69D are necessary for repulsion of growth cones from the midline of the embryonic central nervous system. Repulsion is thought to be triggered by binding of the secreted protein Slit, which is expressed by midline glia, to Roundabout (Robo) receptors on growth cones. Robo repulsion is downregulated by the Commissureless (Comm) protein, allowing axons to cross the midline. The Rptp mutations genetically interact with robo, slit and comm. The nature of these interactions suggests that DPTP10D and DPTP69D are positive regulators of Slit/Roundabout repulsive signaling. Elimination of all four neural RPTPs converts most noncrossing longitudinal pathways into commissures that cross the midline, indicating that tyrosine phosphorylation controls the manner in which growth cones respond to midline signals (Sun, 2000).

To visualize individual axons and growth cones that are affected in Ptp10D;Ptp69D double mutants, lineage tracing experiments were performed in which the fluorescent dye DiI was used to label all of the progeny of single neuroblasts (NBs) in vivo. Individual neuroectodermal cells were randomly labeled at stage 8, and the embryos allowed to develop until stage 17, after which DiI-labeled NBs arising from the injected cells were identified based on their positions, and the axons and cell bodies of the NB progeny were visualized by confocal microscopy. Analysis of a large number of NB lineages in the double mutants revealed that many CNS axonal pathways are altered in complex ways by the absence of DPTP10D and DPTP69D. The projection patterns of three sets of neurons, the progeny of NBs 3-1, 4-2, and 2-5, are described that illustrate essential aspects of the phenotype. No alterations in numbers or positions of cell bodies are observed for these lineages in Ptp10D;Ptp69D embryos. The NB 2-5 lineage generates 15-22 cells by stage 17, of which 8-16 are intersegmental interneurons. Some of these (4 to 8 neurons) extend axons across the midline in the anterior commissure; these axons then turn anteriorly in the contralateral longitudinal tract and grow all the way to the brain (up to 10 segments). The remaining intersegmental interneurons (4 to 8 neurons) extend axons anteriorly (in the ipsilateral longitudinal tract) that stop after projecting about half as far. These contralateral and ipsilateral axons form the most substantial fibers in the longitudinal connectives. There is also a single motoneuron that extends an axon in the ipsilateral ISNd pathway and innervates muscles 15-17. In Ptp10D;Ptp69D mutants, the contralaterally projecting interneuronal axons cross the midline and turn anteriorly in a normal manner, but then double back across the midline after about two segments and grow posteriorly in the ipsilateral longitudinal tract. The axons of the ipsilateral intersegmental neurons grow anteriorly for a short distance and stop. The ISNd motoneuron extends an axon toward the midline that stalls and never enters the ISN root. This lineage illustrates that interneuronal axons abnormally cross the midline in the Rptp double mutant, and that a motor axon is deflected toward the midline (Sun, 2000).

The NB 4-2 lineage produces about 22 cells, including the well-characterized RP2 motoneuron that extends its axon along the ISN pathway and innervates the dorsal muscle 2. The NB 4-2 also generates the CoR motoneurons, whose axons constitute all of the SNc motor nerve. All of the interneurons are local; two or three of them extend axons across the anterior commissure that bifurcate in the contralateral connective. In Ptp10D;Ptp69D double mutants, the RP2 axon stalls before reaching its target, and the CoR axons do not branch onto all of their target muscles. An ipsilateral longitudinal projection is formed that extends anteriorly from the clone and crosses the segment border; this is never observed in wild type. Finally, the local interneuronal projection splits after crossing the midline, so that two pathways form instead of one; this was observed in all lineages examined. In summary, this lineage illustrates that abnormal longitudinal pathways are formed in mutant embryos and that pathway selection in the commissures is altered (Sun, 2000).

NB 3-1 produces the RP1, RP3, RP4 and RP5 motoneurons, which extend axons across the anterior commissure and into the ISNb nerve, eventually innervating the ventrolateral muscles. It also generates a variable number of interneurons, which cross the midline and project both posteriorly (intersegmental interneurons) and anteriorly (local interneurons) in the contralateral connective. In Ptp10D;Ptp69D mutants, the RP neurons extend axons normally across the commissure and into the ISNb nerve, although they do not form normal synapses. The interneuronal projections, however, are radically altered. They still cross the midline, but do not form defined anterior and posterior projections in the contralateral connective. Instead, they grow anteriorly in a circular path around the neuropil, contacting the midline at the end of their trajectory. Like the other lineages, 3-1 illustrates that longitudinal pathways cannot form normally. Both the anterior and posterior interneuronal projections are missing, and are replaced by a swirl of axons that grow to the midline. These kinds of pathway alterations could give rise to the connective breaks that are observed in mutant embryos (Sun, 2000).

The fact that longitudinal axons can be changed into commissural axons by elimination of RPTP activity suggests that tyrosine phosphorylation controls the manner in which growth cones respond to midline repulsive signals. This is consistent with the observation that pharmacological inhibition of tyrosine kinase activity in grasshopper embryos causes a robo-like phenotype in which the longitudinal axon of the pCC neuron crosses the midline and circles back to the ipsilateral side. Further evidence that the effects of the inhibitor may actually be due to blockage of Robo signaling is provided by the recent observation that the Drosophila pCC axon in robo embryos has a unique branched morphology that is identical in appearance to that of the grasshopper pCC in inhibitor-treated embryos (Sun, 2000 and references therein).

The repulsive response to midline signals is encoded within the Robo cytoplasmic domain. The cytoplasmic domains of fly, nematode and mammalian Robo family proteins (Robos) contain conserved tyrosine-containing PYATT sequence motifs, suggesting that these domains could be direct targets for tyrosine kinases. Phosphorylated tyrosine motifs usually function by binding to SH2 and PTB-domain adapter proteins that mediate downstream signaling events. Robo also contains two proline-rich sequences that could interact with SH3-domain adapters. Robo2 has the tyrosine-containing motif, but lacks the proline-rich sequences (Sun, 2000).

How are Robo signaling pathways regulated by RPTPs? There is no evidence at present that the RPTPs directly alter signaling by the Robo protein. It is possible that the RPTPs and Robo feed into separate pathways that only intersect after several signaling steps. There is, however, a known mechanism for RPTP-mediated positive regulation of tyrosine kinase pathways that suggests how DPTP10D and DPTP69D could facilitate Robo signaling. During T cell receptor (TCR) signal transduction, the RPTP CD45 removes an inhibitory C-terminal phosphate group from the Src-family tyrosine kinase Lck, thereby activating it and allowing it to phosphorylate the z chain of the TCR. The phosphorylated z chain in turn binds to an SH2-domain containing tyrosine kinase (ZAP-70), which mediates downstream signaling events. CD45 is required for TCR signaling because in its absence Lck is not activated and thus cannot efficiently phosphorylate the z chain. (Interestingly, CD45 may also be involved in the termination of the TCR signaling response, since it can dephosphorylate the z chain and prevent it from binding to ZAP-70) Another mammalian receptor phosphatase, RPTPa, also dephosphorylates and activates Src-family kinases. Fibroblasts derived from RPTPa knockout mice have reduced Src and Fyn activities, suggesting that RPTPa is an in vivo regulator of Src family kinase function (Sun, 2000 and references therein).

By analogy to these pathways, DPTP10D and DPTP69D might regulate growth cone repulsion by activating Src-family tyrosine kinase(s) that phosphorylate Robos. This could explain the genetic data, since the loss of RPTP function would be expected to cause a decrease in the extent of Robo phosphorylation. One might also propose that positive regulation of repulsion by the RPTPs occurs through direct dephosphorylation of Robos, and that dephosphorylated Robos are more active in signaling. This would be unusual, however, since normally it is the phosphorylated form of a signaling motif that binds to downstream adapters. A variant of the direct interaction model proposes that Robos become phosphorylated on tyrosines after engagement of Slit, and that DPTP10D or DPTP69D are recruited into a Robo/Slit signaling complex by their interactions with the phosphotyrosine motifs. RPTPs might remain bound to these sites for a significant time period, because they often hydrolyze phosphate-tyrosine bonds quite slowly. The RPTPs could then function as adapters themselves, binding to downstream signaling proteins and recruiting them into Robo/Slit receptor complexes. Determining which, if any, of these models is correct will require biochemical or genetic identification of in vivo substrates for RPTPs (Sun, 2000).