InteractiveFly: GeneBrief

Protein tyrosine phosphatase 52F: Biological Overview | References

The rapid removal of larval midgut is a critical developmental process directed by molting hormone ecdysone during Drosophila metamorphosis. To date, it remains unclear how the stepwise events can link the onset of ecdysone signaling to the destruction of larval midgut. This study investigated whether ecdysone-induced expression of receptor protein tyrosine phosphatase PTP52F regulates this process. The mutation of the Ptp52F gene caused significant delay in larval midgut degradation. Transitional endoplasmic reticulum ATPase (TER94), a regulator of ubiquitin proteasome system, was identified as a substrate and downstream effector of PTP52F in the ecdysone signaling. The inducible expression of PTP52F at the puparium formation stage resulted in dephosphorylation of TER94 on its Y800 residue, ensuring the rapid degradation of ubiquitylated proteins. One of the proteins targeted by dephosphorylated TER94 was found to be Drosophila inhibitor of apoptosis 1 (DIAP1), which was rapidly proteolyzed in cells with significant expression of PTP52F. Importantly, the reduced level of DIAP1 in response to inducible PTP52F was essential not only for the onset of apoptosis but also for the initiation of autophagy. This study demonstrates a novel function of PTP52F in regulating ecdysone-directed metamorphosis via enhancement of autophagic and apoptotic cell death in doomed Drosophila midguts (Santhanam, 2014).

This study shows that ecdysone-induced expression of PTP52F and the subsequent tyrosine dephosphorylation of TER94 coordinate to construct upstream signaling determinants for a precise time-dependent degradation of larval midgut. The transient expression of Ptp52F gene at the PF stage is regulated by the functional EcR. Immediately after the level of endogenous PTP52F protein is detectable in larval midgut, TER94 becomes dephosphorylated on its Y800 residue. This modification may be critical to the rapid degradation of ubiquitylated proteins through a TER94-dependent regulation of ubiquitin proteasome system (UPS). Although the exact mechanism remains elusive, recent studies have suggested that only the tyrosine-dephosphorylated form and not the tyrosine-phosphorylated form of VCP interacts with cofactors for processing ubiquitylated substrates of UPS. Because VCP and TER94 share some evolutionarily conserved features, it is proposed that the same phosphorylation- and dephosphorylation-dependent mechanism may be adopted by TER94. Ubiquitylated DIAP1, a potential substrate of UPS, was found to be targeted by the Y800 dephosphorylated form of TER94. DIAP1 was rapidly degraded in cells in which levels of PTP52F were increased, as illustrated by in vivo observations in Drosophila midgut during metamorphosis. Consequently, the proteolysis of DIAP1 in response to inducible expression of PTP52F terminates the inhibitory effect on autophagy, allowing the initiation of autophagic cell death accompanied by apoptotic cell death for the destruction of the larval midgut tissues. Since the regulatory role of all Drosophila homologs of caspases have been ruled out in the process of larval midgut histolysis, it is likely that DIAP1 degradation-induced autophagic signaling may activate a yet-unknown pathway leading to the onset of apoptotic cell death in dying midgut. Additional experiments are needed to identify downstream effectors of PTP52F that modulate the cross talk between autophagy and apoptosis in the context of midgut maturation (Santhanam, 2014).

Identification of TER94 as a substrate dephosphorylated by PTP52F in larval midgut is interesting and important. From the t and its vertebrate ortholog VCP have been characterized as key mediators involved in ER-associated degradation (ERAD), a major quality control process in the protein secretary pathway. Additional investigations have demonstrated degradation of proteins with no obvious relationship to ERAD by a VCP-mediated process, suggesting that TER94 and VCP may perform general functions in the proteolysis of ubiquitylated proteins. However, it remains unknown how this process is regulated under physiological conditions. The current study presents evidence that TER94-dependent degradation of ubiquitylated proteins is enhanced by PTP52F-mediated dephosphorylation of the penultimate Y800 residue. It has been suggested that the penultimate tyrosine (Y805 in VCP and Y800 in TER94) must be in a dephosphorylated form in order to interact with substrate-processing cofactors, such as the peptides N-glycanase (PNGase) and Ufd3, during UPS-mediated proteolysis. In addition, tyrosine phosphorylation levels of VCP/TER94 determine how fast ubiquitylated proteins are degraded by the USP pathway. Clearly, the finding that PTP52F is responsible for dephosphorylation of the penultimate tyrosine residue is critical for uncovering the functional role of VCP/TER94 in the regulation of protein degradation under physiologically relevant conditions (Santhanam, 2014).

This study has demonstrated that the timely degradation of DIAP1 in doomed larval midgut of developing flies is regulated by ecdysone-induced PTP52F. DIAP1 was identified to ubiquitylate proapoptotic proteins in living cells, thereby suppressing cell death signaling. Interestingly, DIAP1 can be ubiquitylated for degradation itself. The proteolytic process of ubiquitylated DIAP1 remained unclear until a recent report suggesting that TER94-mediated UPS pathway is involved in this process. This study has further shown that it is the dephosphorylated form of TER94 that is responsible for rapid DIAP1 degradation. In addition, although a previous study suggested that DIAP1 might suppress Atg1-mediated PCD, it was not known whether degradation of ubiquitylated DIAP1 could promote autophagy in vivo. This study has explored the underlying mechanism through which autophagic cell death is initiated by degradation of DIAP1. The data show that the constitutively tyrosine-phosphorylated form of TER94 acts as a gatekeeper ensuring the death signaling downstream of DIAP1 in “switch-off” mode. Developmental stage-dependent dephosphorylation of TER94 by inducible expression of PTP52F converts the autophagic death signaling into “switch-on” mode through degradation of DIAP1. These findings thus explain, at least in part, how the massive destruction of larval midgut is precisely controlled by autophagic cell death. In conclusion, this study shows a novel function of PTP52F involved in the onset of autophagy and apoptosis essential for the removal of obsolete midgut tissues. Reversible tyrosine phosphorylation signaling controlled by PTP52F plays an indispensable role in the process of cell death-directed midgut maturation. Therefore, these findings open a new avenue for understanding the previously unexplored function of R-PTPs linked to regulation of autophagic and apoptotic cell death (Santhanam, 2014).

Midgut-enriched receptor protein tyrosine phosphatase PTP52F is required for Drosophila development during larva-pupa transition

This study investigated the potential role of Drosophila R-PTPs in developmental processes outside the neuronal system and beyond the embryogenesis stage. Through systematic data mining of available microarray databases, it was found the mRNA level of PTP52F to be highly enriched in the midgut of flies at the larva-pupa transition. This finding was confirmed by gut tissue staining with a specific antibody. The unique spatiotemporal expression of PTP52F suggests that it is possibly involved in regulating metamorphosis during the transformation from larva to pupa. To test this hypothesis, RNA interference was used to examine the defects of transgenic flies. Ablation of endogenous PTP52F was found to lead to high lethality characterized by the pharate adult phenotype, occurring due to post pupal eclosion failure. These results show that PTP52F plays an indispensable role during the larva-pupa transition. It was also found that PTP52F could be reclassified as a member of the subtype R3 PTPs instead of as an unclassified R-PTP without a human ortholog, as suggested previously. Together, these findings suggest that Drosophila R-PTPs may control metamorphosis and other biological processes beyond our current knowledge (Santhanam, 2013).

A recent study showed that Tartan, a transmembrane protein, is a candidate substrate of PTP52F (Bugga, 2009). Genetic experiments further illustrate that PTP52F and Tartan act synergistically in pathways of motor axon guidance in embryos. Tartan was also detected in tracheal tissue. However, nothing is known about the distribution or potential functions of Tartan in the midgut of larva and pupa. Therefore, it is difficult to predict whether the phenotype of pharate adult lethality shown in the PTP52F knockdown flies was influenced by the deregulation of Tartan or other yet-to-be-identified substrates of PTP52F. Further investigations are required to delineate the molecular basis for PTP52F-mediated regulation of signaling pathways in the midgut during the larva-pupa transition. Nonetheless, it is interesting to point out that the pharate adult phenotype revealed by the ablation of PTP52F has also been reported in some mutants of ecdysone response genes, including ecdysone receptor (EcR), a key mediator of metamorphosis during the transformation from larva to pupa. The similarity of the phenotype suggests a possibility that PTP52F may participate in ecdysone-mediated signal transduction. Indeed preliminary data support a genetic interaction between EcR and PTP52F. Using the eye phenotype as a readout, it was found that overexpression of EcR B1 form driven by GMR-Gal4 led to the severe loss of hexagonal ommatidia, whereas only mild disruption of the ommatidia arrangement was observed in PTP52F RNAi lines. Interestingly, the eye phenotype revealed in EcR overexpressed flies could be rescued through the knockdown of PTP52F, suggesting a possible synergistic interaction between these gene products in a signaling pathway. Based on these observations, it is hypothesized that Ptp52F might be a downstream gene transcriptionally regulated by EcR. As a typical steroid hormone receptor, EcR recognizes specific consensus motifs on the promoter region of effector genes. The 2 kb region upstream of the first exon of the Ptp52F gene was therefore analyzed to search for potential ecdysone response elements using the software nubiscan-2.0, which is an in silico tool that helps predict nuclear receptor binding sites. Based on the analysis, two possible ecdysone response elements with high scores were identified. Importantly, the sequences of these elements are similar to the consensus motif shown in the known EcR-regulated genes. Together these results suggest that Ptp52F may be a downstream gene of ecdysone action (Santhanam, 2013).

The latest version of computational and bioinformatics analysis defined PTP52F as an unclassified member in the PTP super family without a clear ortholog in humans. However, based on the sequence of full-length PTP52F clones and in an earlier report by Zinn's group, it is proposed that the classification of Drosophila R-PTPs be revised. Apparently PTP52F contains only one catalytic domain in the intracellular region instead of two tandem putative phosphatase domains, as suggested previously. The overall layout of PTP52F architecture composed of multiple fibronectin repeats, a single transmembrane segment plus a single phosphatase domain reclassifies this phosphatase as a member belonging to the subtype R3, together with PTP4E and PTP10D in the Drosophila PTP family. Thus, the earlier study, which defined PTP52F as an unclassified phosphatase, should be modified. There are three Drosophila R-PTPs and six human R-PTPs in the subtype R3. Obviously, it is difficult to classify the ortholog pair merely based on the sequence alignment. Other important criteria such as the regulatory role in evolutionarily conserved signaling pathways or the tissue-specific expression profile of R-PTPs across species must be taken into consideration. Recently, PTP10D and PTP4E were regarded as the functional orthologs of human density-enriched PTP-1 due to their similar expression in epithelial cells and their shared ability to downregulate receptor tyrosine kinase mediated signaling. Following the same principle of consideration, it is proposed that PTP52F might be the functional ortholog of human and mouse stomach-associated PTP-1 (SAP-1). Accumulated data clearly show that mammalian SAP-1 is exclusively expressed in gastrointestinal epithelial cells, similar to the midgut-enriched expression of PTP52F in flies. Such a specific expression profile of both PTP52F and SAP-1 suggests that they may regulate evolutionarily conserved signaling pathways in the gut tissue (Santhanam, 2013).

In conclusion, data shown in the this study suggest that endogenous Drosophila R-PTPs act in developmental control outside the nervous and tracheal systems and also beyond the early embryonic stage and thus potentially play an indispensable role in the regulation of metamorphosis. Although additional experiments are needed to support this hypothesis, this study has opened a new avenue for understanding the role of Drosophila R-PTPs that may mediate signal transduction during development and other biological processes in areas beyond our current knowledge (Santhanam, 2013).

The cell surface receptor Tartan is a potential in vivo substrate for the receptor tyrosine phosphatase Ptp52F

Receptor-linked protein-tyrosine phosphatases (RPTPs) are essential regulators of axon guidance and synaptogenesis in Drosophila, but the signaling pathways in which they function are poorly defined. This study identified the cell surface receptor Tartan (Trn) as a candidate substrate for the neuronal RPTP Ptp52F by using a modified two-hybrid screen with a substrate-trapping mutant of Ptp52F as 'bait.' Trn can bind to the Ptp52F substrate-trapping mutant in transfected Drosophila S2 cells if v-Src kinase, which phosphorylates Trn, is also expressed. Coexpression of wild-type Ptp52F causes dephosphorylation of v-Src-phosphorylated Trn. To examine the specificity of the interaction in vitro, Ptp52F-glutathione S-transferase (GST) fusion proteins were incubated with pervanadate-treated S2 cell lysates. Wild-type Ptp52F dephosphorylated Trn, as well as most other bands in the lysate. GST 'pulldown' experiments demonstrated that the Ptp52F substrate-trapping mutant binds exclusively to phospho-Trn. Wild-type Ptp52F pulled down dephosphorylated Trn, suggesting that it forms a stable Ptp52F-Trn complex that persists after substrate dephosphorylation. To evaluate whether Trn and Ptp52F are part of the same pathway in vivo, motor axon guidance was examined in mutant embryos. trn and Ptp52F mutations produce identical phenotypes affecting the SNa motor nerve. The genes also display dosage-dependent interactions, suggesting that Ptp52F regulates Trn signaling in SNa motor neurons (Bugga, 2009).

This study presents evidence that the cell surface receptor Trn is a substrate for Ptp52F. Trn was identified in a yeast screen for phosphoproteins that bind selectively to a Ptp52F substrate-trapping mutant. Trn can be phosphorylated on tyrosine in S2 cells, and phosphorylated Trn binds selectively to the substrate-trapping mutant of Ptp52F when it is coexpressed with Trn and the v-Src kinase. Wild-type Ptp52F causes dephosphorylation of v-Src-phosphorylated Trn in S2 cells. A purified Ptp52F-wild-type-GST fusion protein can dephosphorylate Trn in a pervanadate-treated S2 cell lysate, but it also dephosphorylates many other proteins in the lysate. However, a Ptp52F-trap-GST fusion protein binds to only one phosphoprotein in pervanadate-treated S2 lysates, and this protein was identified as Trn. Ptp52F-wild type-GST forms a complex with Trn that persists after dephosphorylation. These data demonstrate that the Ptp52F substrate-trapping mutant has a strong specificity for Trn binding. The results fulfill all three of the criteria listed as necessary for the rigorous definition of a protein as a PTP substrate: (i) direct interaction with the PTP substrate-trapping mutant in transfected cells, (ii) modulation of cellular substrate tyrosine phosphorylation by the PTP in transfected cells, and (iii) in vitro dephosphorylation of substrate by the PTP (Bugga, 2009).

The genetic results are consistent with a model in which Trn signaling in SNa motor neurons is necessary for correct axon pathfinding at the SNa bifurcation point, and Ptp52F regulates Trn via dephosphorylation. In trn and Ptp52F mutants, SNa axons destined for muscles 22 to 24 sometimes fail to separate from those destined for muscles 5 and 8. This results in a phenotype in which the anterior branch of the SNa is missing. In SNa growth cones, signaling through Trn and dephosphorylation of Trn by Ptp52F might be regulated by the interactions of these two receptors with unknown ligands on cells near the SNa bifurcation point. Activation of Ptp52F might involve a secreted protein called Folded gastrulation (Fog), which is expressed in this vicinity. Fog is a positive regulator of Ptp52F function in SNa neurons (Bugga, 2009).

Trn is also important for ISNb axon guidance and is involved in many other developmental processes, including tracheal development and the sorting of cells within imaginal discs and developing appendages. Trn signaling in ISNb neurons and tracheae may be independent of Ptp52F,/ since trn and Ptp52F do not share ISNb or tracheal phenotypes (Bugga, 2009).

How does Ptp52F regulate Trn signaling in neurons? One might have expected that Ptp52F would be a negative regulator of Trn, because dephosphorylation of Trn would prevent it from binding to SH2-domain downstream signaling proteins. Several Y residues in the Trn cytoplasmic domain are in sequence contexts that suggest that they could bind to SH2 domains if they were phosphorylated. In this model, however, mutation of Ptp52F should lead to an increase rather than a decrease in Trn signaling. It is not known what consequences might result from increasing Trn signaling, since overexpression of Trn in neurons does not produce phenotypes. However, the fact that the trn and Ptp52F loss-of-function phenotypes are the same and that the two genes interact in a dosage-dependent manner suggests that Trn signaling is reduced in the absence of Ptp52F and thus that Ptp52F is actually a positive regulator of Trn (Bugga, 2009).

Interestingly, these relationships between the Trn receptor, the unknown TK that phosphorylates Trn in vivo, and the RPTP resemble those described for the Robo receptor, which controls axon guidance across the CNS midline. Robo signaling is antagonized by the Abl TK, which can phosphorylate its cytoplasmic domain, and is facilitated by the Ptp10D and Ptp69D RPTPs. There are several models that could explain this apparent positive regulation by dephosphorylation. Tyrosine-phosphorylated Trn or Robo might be downregulated by endocytosis. It is known that Robo can be regulated in this manner: the transmembrane protein Commissureless, which downregulates Robo function in vivo, can remove it from the cell surface by diverting it into an endocytic pathway. In another model, an SH2-domain protein might bind to a phosphorylated tyrosine residue in Trn or Robo and occlude binding of a positive regulator that normally binds to an adjacent site in a phosphorylation-independent manner (Bugga, 2009).

The secreted cell signal Folded Gastrulation regulates glial morphogenesis and axon guidance in Drosophila

During gastrulation in Drosophila, ventral cells change shape, undergoing synchronous apical constriction, to create the ventral furrow (VF). This process is affected in mutant embryos lacking zygotic function of the folded gastrulation (fog) gene, which encodes a putative secreted protein. Fog is an essential autocrine signal that induces cytoskeletal changes in invaginating VF cells. This study shows that Fog is also required for nervous system development. Fog is expressed by longitudinal glia in the central nervous system (CNS), and reducing its expression in glia causes defects in process extension and axon ensheathment. Glial Fog overexpression produces a disorganized glial lattice. Fog has a distinct set of functions in CNS neurons. The data show that reduction or overexpression of Fog in these neurons produces axon guidance phenotypes. Interestingly, these phenotypes closely resemble those seen in embryos with altered expression of the receptor tyrosine phosphatase PTP52F. Epistasis experiments were conducted to define the genetic relationships between Fog and PTP52F, and the results suggest that PTP52F is a downstream component of the Fog signaling pathway in CNS neurons. Ptp52F mutants were found to have early VF phenotypes like those seen in fog mutants (Ratnaparkhi, 2007).

Reduction of Fog in neurons produces subtle axon guidance phenotypes affecting both motor neurons and CNS interneurons. Overexpression of Fog in neurons produces strong CNS phenotypes in which longitudinal axons abnormally cross the midline. The same phenotypes can be produced by overexpressing Fog in CNS longitudinal glia, which are in apposition to the axons. This results suggests that glial Fog causes cytoskeletal changes that alter axon guidance in neurons, implicating Fog as an exocrine as well as an autocrine signal during nervous system development. (Ratnaparkhi, 2007).

Studies of Fog signaling during gastrulation have indicated that the cytoskeletal changes produced by autocrine Fog involve maternal Cta and RhoGEF2, and nonmuscle myosin II. This study tested whether these components participate in Fog signaling in the nervous system by examining the zygotic phenotypes of cta and RhoGEF2 mutants (germline clones do not develop to this stage). Cta may also be involved in Fog signaling during nervous system development, because it was found that cta zygotic mutant embryos display the same defects in the CNS and neuromuscular system as do fog embryos. RhoGEF2 mutants, however, have no visible nervous system defects. Myosin II (zipper) mutant embryos have a variety of generalized defects that preclude analysis of specific axon guidance phenotypes. (Ratnaparkhi, 2007).

Most of the cells in the CNS of late embryos that express fog mRNA at high levels are Repo-positive longitudinal glia. These glia are required for normal morphogenesis of the CNS axon tracts; but no CNS axon phenotypes were observed when Fog expression was reduced in glia. To evaluate Fog's functions in glia, glial morphology was examined directly using a membrane-associated GFP marker. When Fog expression is reduced in glia, glial processes fail to extend normally and ensheath CNS axons. There are gaps in the regular array of glia, glial surface areas are smaller than in wild-type, and the glia have a rounded appearance. These changes in cell shape could involve nonmuscle myosin. (Ratnaparkhi, 2007).

Overexpression of Fog in glia confers lethality during early larval phases. Glial morphogenesis is affected by overexpression, but the phenotypes are different from those seen when Fog is reduced. Glia appear to have normal shapes, but the glial lattice is quite disorganized. In wild-type embryos, lines of glia define the positions of the longitudinal tracts, commissural tracts, and peripheral nerves; these regular arrays are not observed in Fog glial overexpression embryos. Thus, both reduction and elevation of glial Fog produces a disorganized glial lattice, suggesting that a precise level of the Fog signal is necessary for normal glial development (Ratnaparkhi, 2007).

The Fog receptor has not been identified, although it is speculated to be a GPCR because of the requirement of the G protein alpha subunit Cta for Fog signaling. However, existing genetic data do not show that Fog directly activates a GPCR; they are also consistent with models in which Fog regulates signaling through a GPCR-Cta pathway in an indirect manner by interacting with a non-GPCR receptor (Ratnaparkhi, 2007).

PTP52F, like most RPTPs, is an 'orphan receptor'. The motivation to conduct the experiments described in this paper arose from observations that fog and Ptp52F embryos display similar VF phenotypes, and that PTP52F is expressed in ventral furrow cells during the gastrulation phase (Schindelholz, 2001). Based on these results, it was asked whether PTP52F could be the elusive Fog receptor (Ratnaparkhi, 2007).

PTP52F is required for axon guidance in the embryonic CNS and neuromuscular system. Thus, to examine whether Fog and PTP52F might be part of the same signaling pathway, fog axon guidance phenotypes were examined, and genetic interactions between the two molecules were studied. The data show that fog and Ptp52F have similar LOF and GOF phenotypes in the CNS. In the neuromuscular system, LOF mutations in both genes cause SNa bifurcation phenotypes. The definition of a fog GOF CNS phenotype allowed performance an epistasis experiment, and this showed that PTP52F is required for signaling downstream of Fog, at least in the context of this phenotype (Ratnaparkhi, 2007).

The genetic results that were obtained indicate that PTP52F is involved in reception of the Fog signal by neurons, but do not prove that PTP52F is a Fog receptor. The results could also be explained if PTP52F positively regulates signaling through a Fog-GPCR-Cta pathway. For example, GPCRs are phosphorylated (on serine or threonine residues) and internalized; the activities of the relevant kinases and/or the proteins involved in internalization could be modulated by tyrosine phosphorylation. Tyrosine phosphorylation could also regulate effectors downstream of the G protein Cta. (Ratnaparkhi, 2007).

Direct biochemical interaction tests between the PTP52F extracellular domain and several versions of the Fog protein have not yielded positive results. However, Fog might be processed in vivo to create a functional ligand, and this processing does not occur in heterologous systems. In these experiments, it was found that Fog tagged at its N-terminus is secreted from insect cells when expressed using the baculovirus system, but the protein is degraded to produce a ladder of bands ranging in size from ~100 kD (the predicted size of glycosylated full-length Fog) to <20 kD. Fog tagged at its C terminus cannot be detected at all. Fog fused near its C terminus to human placental alkaline phosphatase (Fog-AP) can be expressed as a mixture of apparently full-length and degraded forms, but none of these proteins bound detectably to the tagged PTP52F extracellular domain. Taken together, these data suggest that the C terminal region of Fog is subject to degradation, and that full-length Fog is unstable. There are several dibasic sequences in Fog which could represent proteolytic cleavage sites, and it has been proposed that Fog could be processed in vivo to generate an active fragment that binds to the receptor. In the CNS, such a fragment might derive from the middle region of Fog, because it was observed that antisera against full-length Fog stain late embryos, while antisera against the first 300 amino acids of Fog do not (Ratnaparkhi, 2007).

Regulation of CNS and motor axon guidance in Drosophila by the receptor tyrosine phosphatase DPTP52F

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).

Search PubMed for articles about Drosophila Ptp52F

Bugga, L., Ratnaparkhi, A. and Zinn, K. (2009). The cell surface receptor Tartan is a potential in vivo substrate for the receptor tyrosine phosphatase Ptp52F. Mol Cell Biol 29: 3390-3400. PubMed ID: 19332563

Ratnaparkhi, A. and Zinn, K. (2007). The secreted cell signal Folded Gastrulation regulates glial morphogenesis and axon guidance in Drosophila. Dev. Biol. 308(1): 158-68. PubMed Citation: 17560973

Santhanam, A., Liang, S. Y., Chen, D. Y., Chen, G. C. and Meng, T. C. (2013). Midgut-enriched receptor protein tyrosine phosphatase PTP52F is required for Drosophila development during larva-pupa transition. FEBS J 280: 476-488. PubMed ID: 22780963

Santhanam, A., Peng, W. H., Yu, Y. T., Sang, T. K., Chen, G. C. and Meng, T. C. (2014). Ecdysone-induced receptor tyrosine phosphatase PTP52F regulates Drosophila midgut histolysis by enhancement of autophagy and apoptosis. Mol Cell Biol 34: 1594-1606. PubMed ID: 24550005

Schindelholz, B., et al. (2001). Regulation of CNS and motor axon guidance in Drosophila by the receptor tyrosine phosphatase DPTP52F. Development 128: 4371-4382. 11684671

date revised: 26 December 2014

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