Protein tyrosine phosphatase Meg

InteractiveFly: GeneBrief

Gene name - Protein tyrosine phosphatase Meg

Synonyms -

Cytological map position-61C1-61C1

Function - signaling

Keywords - axon projection, brain

Symbol - Ptpmeg

FlyBase ID: FBgn0261985

Genetic map position - 3L

Classification - FERM, PDZ and PTP domains

Cellular location - cytoplasmic

NCBI link: EntrezGene

Ptpmeg orthologs: Biolitmine

BIOLOGICAL OVERVIEW

Ptpmeg is a cytoplasmic tyrosine phosphatase containing FERM and PDZ domains. Drosophila Ptpmeg and its vertebrate homologs PTPN3 and PTPN4 are expressed in the nervous system, but their developmental functions have been unknown. This study found that ptpmeg is involved in neuronal circuit formation in the Drosophila central brain, regulating both the establishment and the stabilization of axonal projection patterns. In ptpmeg mutants, mushroom body (MB) axon branches are elaborated normally, but the projection patterns in many hemispheres become progressively abnormal as the animals reach adulthood. The two branches of MB α/ß neurons are affected by ptpmeg in different ways; ptpmeg activity inhibits α lobe branch retraction while preventing ß lobe branch overextension. The phosphatase activity of Ptpmeg is essential for both α and ß lobe formation, but the FERM domain is required only for preventing α lobe retraction, suggesting that Ptpmeg has distinct roles in regulating the formation of α and ß lobes. ptpmeg is also important for the formation of the ellipsoid body (EB), where it influences the pathfinding of EB axons. ptpmeg function in neurons is sufficient to support normal wiring of both the EB and MB. However, ptpmeg does not act in either MB or EB neurons, implicating ptpmeg in the regulation of cell-cell signaling events that control the behavior of these axons (Whited, 2007).

Neuronal wiring patterns are crucial determinants of brain function. During development, axons navigate to reach their appropriate targets in response to guidance information in their environment. Once established, axonal projection patterns must be appropriately refined and maintained as the nervous system matures and ages. The maintenance of axonal projections is an active process important for nervous system development, with the selective retention of axonal input sculpting patterns of neuronal connectivity. Disruptions in the maintenance of axonal projections are also implicated in human disease, and axonal atrophy is observed in several common neurological disorders, including Alzheimer's, Parkinson's, and Huntington's diseases. Understanding the molecular mechanisms that control the establishment and maintenance of neuronal connectivity patterns is therefore critical for understanding how the brain's wiring pattern arises during development and how it is maintained in healthy adults (Whited, 2007).

Cell-cell communication is critical for establishing and maintaining neuronal wiring patterns. The initial pathfinding of axons is modulated by extracellular guidance cues that bind guidance receptors on the axon surface and act to repel or attract the growth cone at the axon tip. During the maturation of the nervous system, patterns of axon branch retention and pruning are also strongly influenced by environmental signals. Some signals that control the maintenance of neuronal wiring act systemically, as in Drosophila where the hormone ecdysone modulates neuronal remodeling throughout the nervous system. Other signals that affect the maintenance of neuronal wiring act more locally, as in the mammalian forebrain where semaphorin proteins trigger the pruning of axon branches (Whited, 2007).

Like pruning, the long-term retention of axon branches is an active process involving cell-cell communication. In the Drosophila mushroom bodies, the maintenance of axon branches requires the inhibition of axon branch retraction by RhoGAP, a negative regulator of Rho and related pathways are proposed to act in mice, where focal adhesion kinase negatively regulates axon branch stabilization via Rho GTPases. Despite the importance of axon branch maintenance to the function of neural circuits, little is known about the molecular mechanisms of long-term axon branch maintenance (Whited, 2007).

Tyrosine phosphatases have important roles in the establishment of neuronal connectivity. In Drosophila, the neuronally expressed receptor tyrosine phosphatases LAR, PTP10D, PTP52F, PTP69D, and PTP99A contribute to axon guidance decisions, and LAR regulates synaptogenesis at the neuromuscular junction. In vertebrates, LAR also regulates the formation and maintenance of synapses. Drosophila ptpmeg encodes an evolutionarily conserved cytoplasmic protein tyrosine phosphatase that is characterized by the presence of an N-terminal FERM domain followed by a single PDZ domain. FERM domains are multi-functional protein and lipid binding domains commonly found in membrane-associated signaling and cytoskeletal proteins. PDZ domains are protein-binding motifs often found in scaffolding proteins (Whited, 2007).

Orthologs of Ptpmeg are present in animals from flies to humans. There are two mammalian homologs of Ptpmeg: PTPN3, which acts as a colon cancer tumor suppressor gene in humans (Wang, 2004), and PTPN4. Both PTPN3 and PTPN4, as well as the C. elegans ortholog PTP-1/PTP-FERM, are neuronally expressed (Hironaka, 2000; Sahin, 1995; Takeuchi, 1994; Uchida, 2002), but the developmental functions of these proteins have not been examined. PTPN4 has been detected in post-synaptic density fractions and physically associates with two predominantly post-synaptic proteins, NMDAR2B [also known as glutamate receptor, ionotropic, N-methyl D-aspartate 2B (GRIN2B); gene ID: 14812], a glutamate receptor subunit, and GRID2 (glutamate receptor, ionotropic, delta 2 MG1:95813), a glutamate-receptor related protein (Hironaka, 2000). Both NMDAR2B and GRID2 are important for brain development and function, although the contribution of PTPN4 to their activities is unknown. Among other functions, both NMDAR2B and GRID2 regulate the branching of axons, acting in target cells to control the behavior of innervating axons (Whited, 2007).

This study characterized the function of ptpmeg in Drosophila by analyzing the effects of ptpmeg mutations on neuronal development. ptpmeg was found to be required to stabilize patterns of mushroom body (MB) axon branching as animals reach adulthood and for axon pathfinding in the developing ellipsoid body (EB). Structure-function studies indicate Ptpmeg phosphatase activity is required for normal MB and EB axon patterning, whereas the FERM domain appears specifically required for stabilizing a particular subset of MB axon branches. Together these data substantiate a role for Ptpmeg in the establishment and maintenance of neuronal wiring patterns (Whited, 2007).

This study found that the evolutionarily conserved cytoplasmic tyrosine phosphatase Ptpmeg contributes to the establishment and the maintenance of axonal projections in the Drosophila central brain. ptpmeg is required for the proper establishment of axon projections in the ellipsoid body (EB), where formation of the EB axon ring is not completed in the absence of ptpmeg. ptpmeg is also required for the formation of normal patterns of axonal projections in the adult mushroom body (MB), but in this case ptpmeg is required to stabilize MB axon projection patterns that have already formed. In the MB, ptpmeg promotes the retention of the dorsally directed α and α' axon branches and inhibits the overgrowth of the medially-directed ß and ß' axon branches. The FERM domain of Ptpmeg is required for MB dorsal branch retention, but is dispensable for preventing medial branch overgrowth, suggesting ptpmeg functions via distinct molecular pathways in dorsal and medial MB axon branch stabilization. Members of the Ptpmeg family of tyrosine phosphatase are neuronally expressed in animals from worms to flies to mice. The present work provides the first evidence that a member of the Ptpmeg family is important for neuronal connectivity (Whited, 2007).

The loss of ptpmeg function has different effects on dorsal and medial MB axon branches. In one model, ptpmeg would primarily affect one set of MB axon branches, with the other set of branches affected secondarily. Alternatively, ptpmeg could affect dorsal and medial branches separately. Both structure-function and phenotypic analyses suggest ptpmeg affects dorsal and medial branches separately. The FERM domain of Ptpmeg is not required to stabilize the medial ß lobes, but is essential for stabilizing the dorsal α lobes. Furthermore, in ptpmeg¹ animals with fused ß lobes, ~25% of α lobes appeared normal. Similarly, in ptpmeg¹ hemispheres with reduced α lobes, ~15% of ß lobes appeared normal. Therefore, ß lobe overextension does not always accompany α lobe reduction in ptpmeg mutants and vice versa. Taken together, these data suggest Ptpmeg affects α and ß lobes separately, acting to inhibit α lobe retraction and ß lobe overextension. Since ptpmeg is not required within MB neurons, this suggests that Ptpmeg acts in cells that communicate to dorsal branches and in cells that communicate to medial branches (Whited, 2007).

The ability of ptpmeg to promote the retention of α and α' axon branches could reflect the inhibition of either axon degeneration or retraction by ptpmeg. Degenerating and retracting axons often exhibit distinct morphologies. For example, in degenerating axons, such as the axons of MB γ neurons that degenerate during Drosophila metamorphosis, the entire axon branch often appears to degenerate simultaneously. However, retracting axons often exhibit preferential reductions in thickness at the distal end of the axon branch, with small dots of axonal material left behind. The withdrawal of α lobe axons in ptpmeg mutants does not resemble previously characterized axon branch degeneration, but rather resembles axon retraction, as α lobe reduction appears to proceed in a distal to proximal fashion. In addition, the distal tip of the withdrawing branch is often pointed and small dots of axonal material often lie nearby. Similar morphologies are also associated with branch retraction in other systems, and so it is proposed that Ptpmeg inhibits axon retraction pathways in the dorsal lobes (Whited, 2007).

Previous evidence indicates that the persistent inhibition of axon retraction pathways is important for long-term maintenance of α and α' dorsal lobes. Reductions in the expression of Drosophila RhoGAP, which is proposed to act by inhibiting a Rho-dependent axon retraction pathway, cause dorsal lobe retraction resembling that in ptpmeg mutants. However, there are significant differences between ptpmeg and RhoGAP mutant phenotypes. RhoGAP inhibition causes medial lobe retraction whereas ptpmeg mutation cause medial lobe overextension. Furthermore, defects are detected earlier in RhoGAP than in ptpmeg mutants, with ~50% of RhoGAP RNAi hemispheres exhibiting dorsal lobe reduction by 18 hours PPF, increasing to ~95% by 36 hours PPF. Finally, RhoGAP is required in the MB neurons, but ptpmeg is not. Therefore, it is suggested that Ptpmeg participates in additional mechanisms that maintain mushroom body axon branches (Whited, 2007).

In contrast to the retraction of dorsal MB lobes, there is limited precedent for mechanisms that underlie overextension of medial lobe MB axons across the midline. Although several mutants with MB midline crossing defects have been described, a detailed time-course that could distinguish pathfinding defects from later onset defects has been reported only for fmr1 mutants, defective in the Drosophila homolog of the fragile X mental retardation gene. In fmr1 mutants, α/ß axons extend branches across the midline by 24 hours PPF and medial lobe fusion appears complete by 48 hours PPF, consistent with a defect in initial outgrowth. By contrast, ptpmeg¹ mutants exhibit no midline crossing defects at 48 hours PPF, suggesting most ß lobe axons initially terminate extension, but reinitiate growth at later stages to cross the midline. Alternatively, midline crossing could be restricted to just the subset of ß axon branches that arrive after 48 hours PPF and might reflect the failure of these axons to stop their initial extension. However, the severity of MB fusion observed in many ptpmeg adults suggests a large proportion of ß lobe axons contribute to the phenotype, consistent with the former explanation (Whited, 2007).

How Ptpmeg might influence 'maintenance' of axon projections after initial extension remains to be determined. MB neurons show no evidence of degeneration in ptpmeg mutants; both their cell body and dendritic regions appear normal. One possible source of MB defects is that Ptpmeg could act in synaptic partners of MB neurons and affect axon target recognition or synaptogenesis. A potentially similar scenario has been observed in the cerebellum of mice mutant for GRID2, a PTPN4-interacting protein (Takeuchi, 2005). Alternatively, Ptpmeg could control the production of structures or signals that influence MB axon behavior more indirectly. Identifying the critical cell populations and molecular pathways through which Ptpmeg modulates MB axon behavior will help determine the basis of these defects. Interestingly, ectopic expression of Ptpmeg in the eye and wing antagonizes the effects of insulin receptor signaling; however, such interactions have been observed only in the context of misexpression (Whited, 2007).

ptpmeg is critical for formation of the EB, a higher order brain region implicated in the control of locomotion. The EB contains axons that travel to the midline and extend ventrally to form a complete ring. In ptpmeg mutants, the EB axons fail to fully extend ventrally, leaving a ventral opening in the EB. These defects appear to result from a defect in EB axon pathfinding rather than axon maintenance. In contrast to the MB, which formed normally but became increasingly abnormal with time, the EB axon ring never completely formed and the defect did not become more severe with time. Similar defects in EB formation have been observed in other central complex mutants, including ciboulot, which encodes a regulator of actin dynamics. In ciboulot mutants, the defect in EB ring closure was proposed to result from a failure of EB axon extension caused by a defect in actin assembly in the EB axon. Since ptpmeg is not required in the EB neurons, ptpmeg likely affects the production of a structure or signal that influences the ventral extension of EB axons, rather than interacting with ciboulot directly. Interestingly, Ptpmeg is expressed on fibers that cross the midline near the developing central complex, which could potentially affect EB axon pathfinding (Whited, 2007).

The ventral region of the EB lies adjacent to the ß and ß' lobes of the MBs, raising the possibility that EB and MB defects are interrelated. This is thought unlikely since expression of a wild-type Ptpmeg cDNA in a ptpmeg mutant created many animals in which the EB ring was complete, but the medial lobes remained defective. Thus, restoration of the EB ring did not eliminate medial lobe defects and the presence of medial lobe defects were not always accompanied by EB ring defects, suggesting these defects can arise separately during development (Whited, 2007).

In addition to its phosphatase domains, Ptpmeg also contains FERM and PDZ domains, protein interaction motifs that could facilitate the assembly of Ptpmeg into signaling complexes and the binding of substrates. This analysis indicates that the ability of Ptpmeg to bind and dephosphorylate substrates is essential for the function of Ptpmeg, and that the FERM and PDZ domains also contribute to Ptpmeg function. Complete elimination of the FERM domain disrupts the ability of Ptpmeg to prevent α lobe retraction, while other activities supported by Ptpmeg appear largely normal. In the case of the PDZ domain, mutation of conserved residues in the GLGF motif partially reduced the ability of Ptpmeg to support MB formation, but had not effect on EB development. Given the partial effects of the FERM and PDZ mutations on Ptpmeg function, it will be of interest to perform further mutational analyses of Ptpmeg to determine whether the FERM and PDZ domains might have redundant roles or whether the phosphatase domain can perform many of the major functions of Ptpmeg by itself (Whited, 2007).

The presence of PDZ and FERM domains in Ptpmeg raised the possibility that Ptpmeg could act as a scaffolding protein. In the mouse brain, the Ptpmeg homolog PTPN4 binds the glutamate receptor subunit NMDAR2B and the glutamate-receptor related protein GRID2 (GluRΔ2) (Hironaka, 2000), indicating Ptpmeg family members can interact with synaptic receptors. Several PDZ domain containing proteins are important modulators of receptor complex localization and activity at the growth cone tip and synapse, while other PDZ domain proteins regulate neurite morphogenesis by acting more proximal to the cell body through the control of receptor trafficking. Ptpmeg is strongly expressed on fibers in the developing and adult brain, but that synapse-rich neuropil regions of the central brain are largely devoid of Ptpmeg. When examined specifically within EB neurons, Ptpmeg expression is restricted to the cell body and the regions of the neurite proximal to the cell body and is not present on axons. Such localization of Ptpmeg to axonal regions near the cell body and its absence from synaptic regions suggest Ptpmeg could act in cell body-proximal regions to influence neurite behavior (Whited, 2007).

These studies demonstrate a role for Ptpmeg in the stabilization of neuronal connectivity patterns in the fly mushroom body. As the mushroom bodies are critical for olfactory learning and memory, molecular pathways that can elicit structural changes in mushroom body axons, such as the pathways in which Ptpmeg participates, are interesting candidates for mediating structural plasticity in this region. More generally, this work shows that Ptpmeg activity is necessary to prevent a progressive loss of the fly's normal wiring pattern as it matures, inhibiting distal-to-proximal retraction of dorsal lobe MB axon branches and inhibiting delayed overextension of medial lobe MB axon branches. Progressive distal-to-proximal disruptions in axonal branching are commonly observed in CNS neurodegenerative diseases such as Alzheimer's and Parkinson's as well as neuropathies associated with diabetes, alcoholism and AIDS. Understanding the kinds of genetic lesions that can destabilize axon branches and the mechanisms that modulate axon branch maintenance could provide useful insights into the mechanisms that contribute to neurological disorders in humans (Whited, 2007).

: REGULATION
Protein tyrosine phosphatase PTPN3 inhibits lung cancer cell proliferation and migration by promoting EGFR endocytic degradation

: DEVELOPMENTAL BIOLOGY
Ptpmeg protein expression was examined and it was found that Ptpmeg was enriched along fiber tracts in the brain at all stages examined from third instar into adulthood, including the periods when MB axons begin to exhibit defects. As expected, ptpmeg¹/Df(3L)ED201 mutants exhibited little or no Ptpmeg expression. Consistent with mosaic analyses indicating that ptpmeg did not act in the MB neurons, Ptpmeg expression was not detected on MB axons, However, Ptpmeg was expressed by many neurons in the central brain and the developing visual system. In the embryonic CNS, Ptpmeg expression was also detected in midline glial cells, indicating that expression of Ptpmeg was not entirely restricted to neurons (Whited, 2007).
Although Ptpmeg was expressed on neuronal processes, it was largely excluded from synapse-rich neuropil regions in the central brain. The subcellular localization of Ptpmeg in the central brain was examined in greater detail using a set of highly polarized neurons in the ellipsoid body (EB) that strongly express Ptpmeg. EB neurons are located in two clusters, one in each brain hemisphere. Each EB neuron extends a neurite which branches to form a dendritic tuft and an axon ring. Ptpmeg was concentrated on the cell bodies and proximal neurites of EB neurons. Ptpmeg expression was also detected in EB dendritic regions, but it did not extend into the axon terminals. Even when overexpressed using an EB-specific promoter (EB1-Gal4), Ptpmeg could not be detected in the axon ring. Rather, Ptpmeg accumulated to increased levels in the EB cell bodies (particularly near the cell surface) and on the proximal neurites and the dendrites. These data demonstrate that Ptpmeg can localize to discrete regions within a neuron (Whited, 2007).

EFFECTS OF MUTATION

To study the function of ptpmeg, a four base-pair insertion was introduced into the locus using homologous recombination-mediated gene replacement, creating ptpmeg¹. The 4 bp insertion in ptpmeg¹ is predicted to introduce a translational frameshift, truncating Ptpmeg within the PDZ domain. As predicted, ptpmeg¹ mutants did not express full-length Ptpmeg protein. In addition to ptpmeg¹, two additional disruptions of the ptpmeg locus were obtained from publicly available collections. ptpmeg² (pGATB-NP4498) contains a transposable element insertion upstream of the Ptpmeg open reading frame and expresses reduced levels of Ptpmeg protein. Genetic data below suggests ptpmeg² is a weak loss-of-function allele. In addition, Df(3L)ED201, contains an ~224 kb chromosomal deletion that disrupts the ptpmeg locus and is predicted to delete ~43 additional protein coding genes (Whited, 2007).

Homozygous ptpmeg¹ adults were viable and fertile, but often became trapped alive in their food when cultured under normal conditions. This phenotype was rescued by expression of a Ptpmeg cDNA in the nervous system using elav-Gal4, raising the possibility of nervous system disruptions in ptpmeg mutants.Connectivity patterns in the adult brain were examined and significant disruptions of mushroom body (MB) axon projections were observed(Whited, 2007).

The MBs of the adult fly are a higher order brain structure involved in multiple behaviors including olfactory memory and sleep. MB neurons each extend an axon that bifurcates to send one branch dorsally and one branch medially. Each α'/ß' neuron extends one axon branch dorsally, into the α' lobe, and one branch medially, into the ß' lobe. Similarly, each α/ß neuron extends one axon branch dorsally, into the α lobe, and one branch medially, into the ß lobe. In ptpmeg mutant adults, the dorsally projecting MB lobes were often reduced in thickness and/or length. Meanwhile, the medially projecting MB lobes were often overextended, with the medial lobes of one hemisphere reaching the midline and sometimes fusing with the medial lobes from the contralateral hemisphere. By contrast, the cell body and dendritic regions of MB neurons appeared normal in ptpmeg mutants. Thus, ptpmeg is important for MB axon branch development (Whited, 2007). The analysis of ptpmeg function focused on the axons of α/ß neurons, selectively visualizing in the adult using antibodies directed against the cell adhesion molecule Fasciclin II. In wild-type animals, the α and ß lobes had a highly regular morphology. By contrast, ptpmeg mutant α lobes were frequently reduced, and were often short, thin or absent. In some instances, the tip of the α lobe lost its knob-like appearance, creating a 'thin tip'. Expression of a wild-type ptpmeg cDNA in neurons rescued α lobe reduction in ptpmeg¹ mutants and ptpmeg¹/Df(3L)ED201 animals. Animals homozygous mutant for the partial reduction of expression allele ptpmeg² did not exhibit MB defects. However, ~20% of α lobes were defective in ptpmeg²/Df(3L)ED201 animals. Together these data suggest ptpmeg acts in neurons to regulate α lobe patterning (Whited, 2007).

In addition to exhibiting α lobe defects, ptpmeg mutants also had disrupted ß lobes. In wild-type animals and in ptpmeg¹ heterozygotes, the ß lobe terminated before reaching the midline of the brain. In ptpmeg¹ mutants, the ß lobes often touched the midline and in some cases completely fused with the contralateral ß lobe. Similar to the α lobe defects, the ß lobe defects were rescued by the expression of wild-type ptpmeg in neurons. Thus, ptpmeg regulates both α and ß lobe patterning. Analysis of ß lobe development in Df(3L)ED201 animals was not included in this analysis of ptpmeg function, as Df(3L)ED201 caused dominant ß lobe defects that were not rescued by ptpmeg expression, suggesting that disruption of genes in addition to ptpmeg contributed to Df(3L)ED201-derived ß lobe defects (Whited, 2007).

To determine whether ptpmeg acts in the MB neurons, marked clones of homozygous mutant ptpmeg¹ neurons were generated in otherwise heterozygous animals using the MARCM system. Mutant clones of varying sizes were generated, including small clones containing one to 10 mutant α/ß cells and medium clones containing ~10 to 50 mutant α/ß cells, but in no cases were MB axon defects observed. Larger MB-restricted clones were generated in which nearly all α/ß neurons along with some α'/ß' and γ neurons were mutant, but MB axon branches still appeared normal. Clones containing substantial amounts of mutant brain tissue outside the MBs did confer phenotypes, but did not permit the identification of the critical cell populations in which ptpmeg was required. These data suggest that Ptpmeg acts in neurons to control MB axon patterning, but does not act in the MB neurons themselves (Whited, 2007).

The MB axon defects observed in ptpmeg mutant adults could arise in at least two different ways. In the first scenario, ptpmeg mutants could be defective in the initial pathfinding or elaboration of MB axon branches. Alternatively, ptpmeg MB axons might initially pathfind and elaborate normally, but become progressively abnormal at later times. To distinguish these possibilities, MB axon development was examined in ptpmeg mutants, initially focusing on the α/ß neurons, which are born early during pupariation. By 18 hours post-pupal formation (PPF), α/ß dorsal and medial axon branches can be detected and by 48 hours PPF their branching is well established. In ptpmeg¹ animals, dorsal branches appeared normal at both 18 hours PPF and 48 hours PPF, whereas medial lobe branches were normal in all hemispheres at 18 hours and in 33 of 36 hemispheres at 48 hours. The large increase in MB defects observed between 48 hours PPF and adult - from 0% to ~55% of dorsal lobes defective and from ~10% to ~80% of medial lobes defective - indicates that α/ß axon branching defects are detected only after the α/ß axon projections are well-established. This suggests that ptpmeg is not essential for branching or pathfinding of α/ß axons, but is rather required for these branches to be maintained into the adult (Whited, 2007).

The onset of MB axon projection defects were followed in the dorsal lobes of ptpmeg¹/Df(3L)ED201 animals, using a marker that labels all subsets of MB neurons throughout development. In early third instar larvae, the MB lobes of wild type and ptpmeg mutants were indistinguishable. As larval MB lobes are composed of largely of γ axons with some α'/ß' axons, the initial extension of these axons thus appeared normal. Between third instar and 18 hours PPF in wild-type animals, branches from additional α'/ß' neurons and from α/ß neurons enter the dorsal MB region. However, the overall innervation of dorsal MB regions temporarily decreases due to the pruning of the dorsal branches of γ neurons. Since the dorsal lobes of 18 hours PPF wild-type and ptpmeg mutants were indistinguishable, this stage of development also appears to proceed normally in ptpmeg mutants. The dorsally projecting MB lobes thicken during pupation as additional α/ß neurons send branches into this region. At 24 hours PPF, the dorsal lobes of ptpmeg mutant MBs remained essentially indistinguishable from wild type, as only 1 of 24 hemispheres exhibited defects. At later times, however, MB defects became common: ~15%-20% of dorsal MB lobes exhibited defects at 36 hours and 48 hours PPF (4 of 24 and 5 of 27 hemispheres defective, respectively), increasing to nearly 50% by the first day of adulthood (Whited, 2007).

The morphology of the dorsal lobes in ptpmeg mutants was also informative. Not only did the loss of ptpmeg cause a preferential reduction in the distal region of dorsal lobes, dots of axonal material were frequently observed near the regions where dorsal lobes were reduced. Together these data are consistent with the loss of ptpmeg causing axon retraction in the dorsal lobe (Whited, 2007).

Taken together, the ptpmeg¹ and ptpmeg¹/Df(3L)ED201 time-course data provide a consistent picture in which MB axon elaboration is initially normal, but becomes aberrant over time. As the majority of defects are detected only after the initial elaboration of MB axons is completed, these data suggest that ptpmeg is required for a later stage in MB development. The finding that the onset of dorsal lobe defects is slightly earlier in ptpmeg¹/Df(3L)ED201 animals than in ptpmeg¹/ptpmeg¹ animals raises the possibility that ptpmeg¹ might not be a null allele. Such residual ptpmeg activity could also explain the partial penetrance of MB defects observed here. Alternatively, partial penetrance could reflect the ability of Ptpmeg-independent pathways to maintain apparently normal patterns of MB axon branches in some hemispheres (Whited, 2007).

Having demonstrated a requirement for ptpmeg in the maintenance of MB axon branches, it was asked whether ptpmeg was exclusively required for later stages of development or whether ptpmeg might be needed for the initial pathfinding of other axons in the brain. This question was addressed by examining the role of ptpmeg in the formation of the ellipsoid body (EB). EB axons normally grow to reach the midline and then extend ventrally to form a closed ring, which is completed by 48 hours PPF. In ptpmeg¹ mutants, EB axons reached the midline, but their extension toward ventral regions halted prematurely, leaving an omega-shaped EB open along its ventral aspect. EB axon defects persisted into the adult; ptpmeg¹ mutant adults displayed a ventral cleft in the EB ring. ptpmeg¹/Df(3L)ED201 adults showed similar defects. In contrast to the MB, which was established normally in ptpmeg mutants but became increasingly aberrant over time, the EB appeared never to form normally and the axonal projections defects in the ptpmeg mutant EB did not become more severe at later time points. Taken together these data suggest that ptpmeg is critical for the initial pathfinding of EB axons. The EB projection defects appeared restricted to axons; the dendritic tufts and cell bodies of EB neurons appeared normal in ptpmeg mutants (Whited, 2007).

The identity of the cells in which ptpmeg acts to control EB axon patterning was examined through tissue-specific rescue and genetic mosaic experiments. Expression of a wild-type Ptpmeg cDNA in neurons using Elav-GAL4 rescued the EB defects of ptpmeg mutants, indicating that ptpmeg was required in neurons to correctly pattern the EB axonal ring. To determine if ptpmeg was required within the EB neurons, marked clones of homozygous mutant ptpmeg¹ EB neurons were examined in otherwise heterozygous animals using the MARCM system. Animals containing ptpmeg¹ mutant EB neurons were analyzed, including animals in which essentially all EB neurons were mutant. In no case were defects observed in EB axon projections. Furthermore, EB-specific expression of a wild-type Ptpmeg cDNA using EB1-Gal4 failed to rescue the EB defect. Thus, whereas EB neurons express Ptpmeg, they do not require ptpmeg to control the trajectories of their axons, suggesting that ptpmeg acts in other neurons to control EB axonal projections (Whited, 2007).

The Ptpmeg subfamily of tyrosine phosphatases is characterized by the presence of FERM, PDZ and PTP domains, and the requirements for these domains in brain development were examined. The role of the FERM domain on ptpmeg function was examined using a naturally occurring splice variant that encodes a Ptpmeg without the FERM domain. When expressed in neurons, the ΔFERM variant of Ptpmeg strongly rescued the EB defect. In the MBs, the ΔFERM variant rescued the ß lobe overextension phenotype of ptpmeg¹, and any differences between ΔFERM and wild-type ptpmeg transgene rescue of the ß lobe defect were not of statistical significance. By contrast, the ΔFERM variant did not significantly rescue the α lobe reduction of ptpmeg¹, and there was a highly significant difference between ΔFERM and wild-type transgenes for α lobe rescue. These data suggest that the FERM domain is important for Ptpmeg's role in α lobe maintenance, but not essential for ß lobe maintenance and EB pathfinding (Whited, 2007).

The function of the Ptpmeg PDZ domain was examined by mutating residues in the GLGF motif that forms part of the substrate-binding pocket of other PDZ domains. As a GF to AA mutation in the GLGF motif of the PDZ domain protein Enigma disrupts its ability to bind ligand, these amino acids were mutated in Ptpmeg, creating Ptpmeg[G494A,F495A]. Ptpmeg[G494A,F495A] rescued the EB defects of ptpmeg¹ mutants as effectively as a wild-type Ptpmeg cDNA. Ptpmeg[G494A,F495A] also rescued both the MB α and ß lobe defects. However, the ability of Ptpmeg[G494A,F495A] to rescue the MB defects was reduced compared to wild-type Ptpmeg for both the α lobe and ß lobe, suggesting that the PDZ domain contributes to the effectiveness of Ptpmeg in maintenance of the MBs (Whited, 2007).

The importance of catalytic activity for ptpmeg function was examined by creating three forms of Ptpmeg in which residues crucial for phosphatase function were mutated. Both Ptpmeg[C877S] and Ptpmeg[Y650F,D787A] contained mutations that disrupt catalysis, whereas Ptpmeg[R883M] contained a mutation predicted to disrupt substrate binding. Whereas expression of a wild-type Ptpmeg cDNA in neurons completely rescued the EB axon defects of ptpmeg mutants, none of the three phosphatase domain mutants significantly rescued EB defects. Similarly, none of the phosphatase mutants rescued either the α lobe or ß lobe defects in the MB. In no case did expression of a mutant form of Ptpmeg cause a dominant EB axon or MB axon phenotype in an otherwise normal animal. Control experiments demonstrated that each mutant protein was expressed at a level comparable to wild-type transgenic protein as detected by western blot. These results demonstrate that the phosphatase activity of Ptpmeg is crucial for all of the ptpmeg functions observed in this study, including EB axon pathfinding and the stabilization of MB axon branching, where Ptpmeg inhibits retraction of dorsal lobe branches and prevents overextension of medial lobe branches (Whited, 2007).

EVOLUTIONARY HOMOLOGS

A gene encoding a protein tyrosine phosphatase (PTP) contains sequence homology to protein 4.1, designated PTPMEG, has been cloned. Recombinant protein and amino- and carboxyl-terminal peptides were used to obtain polyclonal antibodies against PTPMEG to identify endogenous PTPMEG in A172 cells and to show that the enzyme is primarily localized to the membrane and cytoskeletal fractions of these cells. Recombinant protein was prepared in Sf9 and COS-7 cells to further characterize it. The protein was phosphorylated in both cell types on serine and threonine residues. The multiple sites of phosphorylation were all within the intermediate domain of the protein between amino acids 386 and 503. This region also contains two PEST sequences and two proline-rich motifs that may confer binding to Src homology 3 domains. The recombinant protein was cleaved by trypsin and calpain in this region and thereby activated 4-8-fold as assayed using Raytide as substrate. The protein was immunoprecipitated from human platelets with both amino- and carboxyl-terminal antipeptide antibodies to assess the state of the enzyme in these cells. The full-length molecule was found in extracts from unstimulated platelets, whereas extracts from both calcium ionophore- and thrombin-treated platelets contained proteolyzed and activated forms of the enzyme, indicating that proteolysis by calpain is evoked in response to thrombin. Prior incubation of platelets with calpeptin, an inhibitor of calpain, blocked the agonist-induced proteolysis (Gu, 1996a).

Stable COS-7 cell lines overexpressing recombinant PTPMEG and an inactive mutant form were established in which the active site cysteine is mutated to serine (PTPMEGCS). Both endogenous and recombinant enzymes were primarily located in the membrane and cytoskeletal fractions of COS-7 cells. Endogenous PTPMEG accounts for only 1/3000th of the total tyrosine phosphatase activity in COS-7 cells and transfected cells expressed 2- to 7-fold higher levels of the enzyme. These levels of overexpression did not result in detectable changes in either total tyrosine phosphatase activity or the state of protein tyrosine phosphorylation as determined by immunoblotting of cell homogenates with anti-phosphotyrosine antibodies. Despite the low levels of activity for PTPMEG, it was found that overexpressing cells grew slower and reached confluence at a lower density than vector transfected cells. Surprisingly, PTPMEGCS-transfected cells also reach confluence at a lower density than vector-transfected cells, although they grow to higher density than PTPMEG-transfected cells. Both constructs inhibited the ability of COS-7 cells to form colonies in soft agar, with the native PTPMEG having a greater effect (30-fold) than PTPMEGCS (10-fold). These results indicate that in COS-7 cells both PTPMEG and PTPMEGCS inhibit cell proliferation, reduce the saturation density, and block the ability of these cells to grow without adhering to a solid matrix (Gu, 1996b).

Glutamate receptor (GluR) delta2 is selectively expressed in cerebellar Purkinje cells and plays a crucial role in cerebellum-dependent motor learning. Although GluRdelta2 belongs to an ionotropic GluR family, little is known about its pharmacological features and downstream signaling cascade. To study molecular mechanisms underlying GluRdelta2-dependent motor learning, yeast two-hybrid screening was employed to isolate GluRdelta2-interacting molecules and identified protein-tyrosine phosphatase PTPMEG. PTPMEG is a family member of band 4.1 domain-containing protein-tyrosine phosphatases and is expressed prominently in brain. In situ hybridization analysis showed that the PTPMEG mRNA is enriched in mouse thalamus and Purkinje cells. PTPMEG interacts with GluRdelta2 as well as with N-methyl-D-aspartate receptor GluRepsilon1 in cultured cells and in brain. PTPMEG bound to the putative C-terminal PDZ target sequence of GluRdelta2 and GluRepsilon1 via its PDZ domain. Examination of the effect of PTPMEG on tyrosine phosphorylation of GluRepsilon1 unexpectedly revealed that PTPMEG enhanced Fyn-mediated tyrosine phosphorylation of GluRepsilon1 in its PTPase activity-dependent manner. Thus, it is concluded that PTPMEG associates directly with GluRdelta2 and GluRepsilon1. Moreover, the data suggest that PTPMEG plays a role in signaling downstream of the GluRs and/or in regulation of their activities through tyrosine dephosphorylation (Hironaka, 2000).

Protein tyrosine phosphorylation is regulated by protein tyrosine kinase and protein tyrosine phosphatase activities. These two counteracting proteins are implicated in cell growth and transformation. Using polymerase chain reaction with degenerate primers, a novel mouse protein tyrosine phosphatase (PTP) was identified. This cDNA contains a single open reading frame of the predicted 926 amino acids. Those predicted amino acids showed significant identity with human megakaryocyte protein-tyrosine phosphatase by 91% in nucleotide sequences and 94% in amino acid sequences. Expression of this PTP is highly enriched in the testis in mouse and human and has been termed 'testis-enriched phosphatase' (TEP). Northern analysis detected two mRNA species of 3.7 and 3.2kb for this PTP in mouse testis and the expression of TEP is regulated during development. The recombinant phosphatase domain possesses protein tyrosine phosphatase activity when expressed in Escherichia coli. Immunohistochemical analysis of the cellular localization of TEP on mouse testis sections showed that this PTP is specifically expressed in spermatocytes and spermatids within seminiferous tubules, suggesting an important role in spermatogenesis (Park, 2000).

PTP-FERM is a protein tyrosine phosphatase (PTP) of Caenorhabditis elegans containing a FERM domain and a PDZ domain. This study reports the characterization of PTP-FERM and the essential role of its FERM domain in the localization of PTP-FERM in the worm. There are at least three alternatively spliced PTP-FERM isoforms, all of which contain a band 4.1/FERM domain, a PDZ domain, and a catalytic domain. PTP-FERM possesses phosphatase activity. PTP-FERM is expressed predominantly in neurons in the nerve ring and the ventral nerve cord. PTP-FERM is found in the nerve processes and is enriched in the peri-membrane region. Studies using various deletion mutants revealed that the FERM domain is essential and sufficient for the subcellular localization. These results suggest the essential role of the FERM domain in the function of PTP-FERM in the neurons of C. elegans (Uchida, 2002).

Oncoproteins from DNA tumor viruses associate with critical cellular proteins to regulate cell proliferation, survival, and differentiation. Human papillomavirus (HPV) E6 oncoproteins have been shown to associate with a cellular HECT domain ubiquitin ligase termed E6AP (UBE3A; see Drosophila Ube3a). This study shows that the E6-E6AP complex associates with and targets the degradation of the protein tyrosine phosphatase PTPN3 (PTPH1) in vitro and in living cells. PTPN3 is a membrane-associated tyrosine phosphatase with FERM, PDZ, and PTP domains implicated in regulating tyrosine phosphorylation of growth factor receptors and p97 VCP (valosin-containing protein, termed Cdc48 in Saccharomyces cerevisiae) and is mutated in a subset of colon cancers. Degradation of PTPN3 by E6 requires E6AP, the proteasome, and an interaction between the carboxy terminus of E6 and the PDZ domain of PTPN3. In transduced keratinocytes, E6 confers reduced growth factor requirements, a function that requires the PDZ ligand of E6 and that can in part be replicated by inhibiting the expression of PTPN3. This report demonstrates the potential of E6 to regulate phosphotyrosine metabolism through the targeted degradation of a tyrosine phosphatase (Jing, 2007).

PTPN3 (PTPH1) is a cytoskeletal protein tyrosine phosphatase that has been implicated as a negative regulator of early TCR signal transduction and T cell activation. To determine whether PTPN3 functions as a physiological negative regulator of TCR signaling in primary T cells, gene-trapped and gene-targeted mouse strains were generated that lack expression of catalytically active PTPN3. PTPN3 phosphatase-negative mice were born in expected Mendelian ratios and exhibited normal growth and development. Furthermore, numbers and ratios of T cells in primary and secondary lymphoid organs were unaffected by the PTPN3 mutations and there were no signs of spontaneous T cell activation in the mutant mice with increasing age. TCR-induced signal transduction, cytokine production, and proliferation was normal in PTPN3 phosphatase-negative mice. This was observed using both quiescent T cells and recently stimulated T cells where expression of PTPN3 is substantially up-regulated. It is concluded, therefore, that the phosphatase activity of PTPN3 is dispensable for negative regulation of TCR signal transduction and T cell activation (Bauler, 2007).

Protein-tyrosine phosphatase PTPN3 is a membrane-associated non-receptor protein-tyrosine phosphatase. PTPN3 contains a N-terminal FERM domain, a middle PDZ domain, and a C-terminal phosphatase domain. Upon co-expression of PTPN3, the level of human hepatitis B viral (HBV) RNAs, 3.5 kb, 2.4/2.1 kb, and 0.7 kb transcribed from a replicating HBV expression plasmid is significantly reduced in human hepatoma HuH-7 cells. When the expression of endogenous PTPN3 protein is diminished by specific small interfering RNA, the expression of HBV genes is enhanced, indicating that the endogenous PTPN3 indeed plays a suppressive role on HBV gene expression. PTPN3 can interact with HBV core protein. The interaction is mediated via the PDZ domain of PTPN3 and the carboxyl-terminal last four amino acids of core. Either deletion of PDZ domain of PTPN3 or substitution of PDZ ligand in core has no effect on PTPN3-mediated suppression. These results clearly show that the interaction of PTPN3 with core is not required for PTPN3 suppressive effect. Mutation of (359)serine and (835)serine of 14-3-3beta binding sites to alanine, which slightly reduces the interaction with 14-3-3beta, does not influence the PTPN3 effect. In contrast, mutation of the invariant (842)cysteine residue in phosphatase domain to serine, which makes the phosphatase activity inactive, does not change its subcellular localization and interaction with core or 14-3-3beta, but completely abolishes PTPN3-mediated suppression. Furthermore, deletion of FERM domain does not affect the phosphatase activity or interaction with 14-3-3beta, but changes the subcellular localization from cytoskeleton-membrane interface to cytoplasm and nucleus, abolishes binding to core, and diminishes the PTPN3 effect on HBV gene expression. Taken together, these results demonstrate that the phosphatase activity and FERM domain of PTPN3 are essential for its suppression of HBV gene expression (Hsu, 2007).

REFERENCES

Search PubMed for articles about Drosophila Ptpmeg

Bauler, T. J., et al. (2007). Normal TCR signal transduction in mice that lack catalytically active PTPN3 protein tyrosine phosphatase. J. Immunol. 178(6): 3680-7. Medline abstract: 17339465

Gu, M. and Majerus, P. W. (1996a). The properties of the protein tyrosine phosphatase PTPMEG. J. Biol. Chem. 271(44): 27751-9. Medline abstract: 8910369

Gu, M., Meng, K. and Majerus, P. W. (1996b). The effect of overexpression of the protein tyrosine phosphatase PTPMEG on cell growth and on colony formation in soft agar in COS-7 cells. Proc. Natl. Acad. Sci. 93(23): 12980-5. Medline abstract: 8917530

Hironaka, K., Umemori, H., Tezuka, T., Mishina, M. and Yamamoto, T. (2000). The protein-tyrosine phosphatase PTPMEG interacts with glutamate receptor delta 2 and epsilon subunits. J. Biol. Chem. 275: 16167-16173. Medline abstract: 10748123

Hsu, E. C., et al. (2007). Suppression of hepatitis B viral gene expression by protein-tyrosine phosphatase PTPN3. J. Biomed. Sci. [Epub ahead of print]. Medline abstract: 17588219

Jing, M., et al. (2007). Degradation of tyrosine phosphatase PTPN3 (PTPH1) by association with oncogenic human papillomavirus E6 proteins. J. Virol. 81(5): 2231-9. Medline abstract: 17166906

Li, M. Y., Lai, P. L., Chou, Y. T., Chi, A. P., Mi, Y. Z., Khoo, K. H., Chang, G. D., Wu, C. W., Meng, T. C. and Chen, G. C. (2014). Protein tyrosine phosphatase PTPN3 inhibits lung cancer cell proliferation and migration by promoting EGFR endocytic degradation. Oncogene [Epub ahead of print]. PubMed ID: 25263444

Park, K. W., et al. (2000). Molecular cloning and characterization of a protein tyrosine phosphatase enriched in testis, a putative murine homologue of human PTPMEG. Gene 257(1): 45-55. Medline abstract: 11054567

Sahin, M., Slaugenhaupt, S. A., Gusella, J. F. and Hockfield, S. (1995). Expression of PTPH1, a rat protein tyrosine phosphatase, is restricted to the derivatives of a specific diencephalic segment. Proc. Natl. Acad. Sci. USA 92: 7859-7863. Medline abstract: 7644504

Takeuchi, K., Kawashima, A., Nagafuchi, A. and Tsukita, S. (1994). Structural diversity of band 4.1 superfamily members. J. Cell Sci. 107: 1921-1928. Medline abstract: 7983158

Takeuchi, T., Miyazaki, T., Watanabe, M., Mori, H., Sakimura, K. and Mishina, M. (2005). Control of synaptic connection by glutamate receptor delta2 in the adult cerebellum. J. Neurosci. 25: 2146-2156. Medline abstract: 15728855

Uchida, Y., Ogata, M., Mori, Y., Oh-hora, M., Hatano, N. and Hamaoka, T. (2002). Localization of PTP-FERM in nerve processes through its FERM domain. Biochem. Biophys. Res. Commun. 292: 13-19. Medline abstract: 11890665

Wang, Z., et al. (2004). Mutational analysis of the tyrosine phosphatome in colorectal cancers. Science 304: 1164-1166. Medline abstract: 15155950

Whited, J. L., Robichaux, M. B., Yang, J. C. and Garrity, P. A. (2007). Ptpmeg is required for the proper establishment and maintenance of axon projections in the central brain of Drosophila. Development 134(1): 43-53. Medline abstract: 17138662

Biological Overview

date revised: 25 November 2014