Receptor tyrosine phosphatases in axon growth and guidance

Decoding triancestral origins, archaic introgression, and natural selection in the Japanese population by whole-genome sequencing

We generated Japanese Encyclopedia of Whole-Genome/Exome Sequencing Library (JEWEL), a high-depth whole-genome sequencing dataset comprising 3256 individuals from across Japan. Analysis of JEWEL revealed genetic characteristics of the Japanese population that were not discernible using microarray data. First, rare variant-based analysis revealed an unprecedented fine-scale genetic structure. Together with population genetics analysis, the present-day Japanese can be decomposed into three ancestral components. Second, we identified unreported loss-of-function (LoF) variants and observed that for specific genes, LoF variants appeared to be restricted to a more limited set of transcripts than would be expected by chance, with PTPRD as a notable example. Third, we identified 44 archaic segments linked to complex traits, including a Denisovan-derived segment at NKX6-1 associated with type 2 diabetes. Most of these segments are specific to East Asians. Fourth, we identified candidate genetic loci under recent natural selection. Overall, our work provided insights into genetic characteristics of the Japanese population.

ZINC40099027 activates human focal adhesion kinase by accelerating the enzymatic activity of the FAK kinase domain

Focal adhesion kinase (FAK) regulates gastrointestinal epithelial restitution and healing. ZINC40099027 (Zn27) activates cellular FAK and promotes intestinal epithelial wound closure in vitro and in mice. However, whether Zn27 activates FAK directly or indirectly remains unknown. We evaluated Zn27 potential modulation of the key phosphatases, PTP-PEST, PTP1B, and SHP2, that inactivate FAK, and performed in vitro kinase assays with purified FAK to assess direct Zn27-FAK interaction. In human Caco-2 cells, Zn27-stimulated FAK-Tyr-397 phosphorylation despite PTP-PEST inhibition and did not affect PTP1B-FAK interaction or SHP2 activity. Conversely, in vitro kinase assays demonstrated that Zn27 directly activates both full-length 125 kDa FAK and its 35 kDa kinase domain. The ATP-competitive FAK inhibitor PF573228 reduced basal and ZN27-stimulated FAK phosphorylation in Caco-2 cells, but Zn27 increased FAK phosphorylation even in cells treated with PF573228. Increasing PF573228 concentrations completely prevented activation of 35 kDa FAK in vitro by a normally effective Zn27 concentration. Conversely, increasing Zn27 concentrations dose-dependently activated kinase activity and overcame PF573228 inhibition of FAK, suggesting the direct interactions of Zn27 with FAK may be competitive. Zn27 increased the maximal activity (Vmax ) of FAK. The apparent Km of the substrate also increased under laboratory conditions less relevant to intracellular ATP concentrations. These results suggest that Zn27 is highly potent and enhances FAK activity via allosteric interaction with the FAK kinase domain to increase the Vmax of FAK for ATP. Understanding Zn27 enhancement of FAK activity will be important to redesign and develop a clinical drug that can promote mucosal wound healing.

Role of Chondroitin Sulfation Following Spinal Cord Injury

Traumatic spinal cord injury produces long-term neurological damage, and presents a significant public health problem with nearly 18,000 new cases per year in the U.S. The injury results in both acute and chronic changes in the spinal cord, ultimately resulting in the production of a glial scar, consisting of multiple cells including fibroblasts, macrophages, microglia, and reactive astrocytes. Within the scar, there is an accumulation of extracellular matrix (ECM) molecules—primarily tenascins and chondroitin sulfate proteoglycans (CSPGs)—which are considered to be inhibitory to axonal regeneration. In this review article, we discuss the role of CSPGs in the injury response, especially how sulfated glycosaminoglycan (GAG) chains act to inhibit plasticity and regeneration. This includes how sulfation of GAG chains influences their biological activity and interactions with potential receptors. Comprehending the role of CSPGs in the inhibitory properties of the glial scar provides critical knowledge in the much-needed production of new therapies.

Elucidating the genetic architecture of reproductive ageing in the Japanese population

Population studies elucidating the genetic architecture of reproductive ageing have been largely limited to European ancestries, restricting the generalizability of the findings and overlooking possible key genes poorly captured by common European genetic variation. Here, we report 26 loci (all P < 5 × 10^–8) for reproductive ageing, i.e. puberty timing or age at menopause, in a non-European population (up to 67,029 women of Japanese ancestry). Highlighted genes for menopause include GNRH1, which supports a primary, rather than passive, role for hypothalamic-pituitary GnRH signalling in the timing of menopause. For puberty timing, we demonstrate an aetiological role for receptor-like protein tyrosine phosphatases by combining evidence across population genetics and pre- and peri-pubertal changes in hypothalamic gene expression in rodent and primate models. Furthermore, our findings demonstrate widespread differences in allele frequencies and effect estimates between Japanese and European associated variants, highlighting the benefits and challenges of large-scale trans-ethnic approaches.

The timing of female reproductive capacity is influenced by genetic and environmental factors. Here, in genome-wide association studies, the authors identify genetic loci for age at menarche and onset of menopause in Japanese women, and highlight differences with European populations.

Expression of receptor protein tyrosine phosphatase δ, PTPδ, in mouse central nervous system

Protein tyrosine phosphate δ (PTPδ), one of the receptor type IIa protein tyrosine phosphates, is known for its roles in axon guidance, synapse formation, cell adhesion, and tumor suppression. Alternative splicing of this gene generates at least four (A-D) isoforms; however, the major isoform in vivo is yet to be determined. The protein localization has neither been revealed. We have generated anti-mouse PTPδ-specific monoclonal antibody and analyzed the protein expression in wild-type and Ptpδ knockout mice. Immunoblot analysis of various organs revealed that neuronal tissues express both C-and D-isoforms of PTPδ, whereas non-neuronal tissues express only C-isoform. Immunohistochemistry of wild-type or Ptpδ heterozygous sections showed that olfactory bulb, cerebral cortex, hippocampus, cerebellum, and several nuclei in brain stem exhibit moderate to strong positive signals. These signals were absent in Ptpδ knockout specimens. Higher magnification revealed differences between expression patterns of PTPδ mRNA and its protein product. In hippocampus, weak mRNA expression in CA1 stratum pyramidale but strong immunostaining in the stratum lacunosum moleculare was observed, suggesting the axonal expression of PTPδ in the entorhinal cortical afferents. Olfactory mitral cells exhibited mRNA expression in cell bodies and protein localization in their dendritic fields, glomerular and external plexiform layers. Nissl staining showed that the external plexiform layer was reduced in Ptpδ knockout mice. Golgi-impregnation confirmed the poor dendritic growth of homozygous mitral cells. These results suggest that PTPδ may localize in axons as well as in dendrites to regulate their elaboration in the central nervous system.

A Synthetic Lethal Screen Identifies a Role for Lin-44/Wnt in C. elegans Embryogenesis

BACKGROUND

The C. elegans proteins PTP-3/LAR-RPTP and SDN-1/Syndecan are conserved cell adhesion molecules. Loss-of-function (LOF) mutations in either ptp-3 or sdn-1 result in low penetrance embryonic developmental defects. Work from other systems has shown that syndecans can function as ligands for LAR receptors in vivo. We used double mutant analysis to test whether ptp-3 and sdn-1 function in a linear genetic pathway during C. elegans embryogenesis.

RESULTS

We found animals with LOF in both sdn-1 and ptp-3 exhibited a highly penetrant synthetic lethality (SynLet), with only a small percentage of animals surviving to adulthood. Analysis of the survivors demonstrated that these animals had a synergistic increase in the penetrance of embryonic developmental defects. Together, these data strongly suggested PTP-3 and SDN-1 function in parallel during embryogenesis. We subsequently used RNAi to knockdown ~3,600 genes predicted to encode secreted and/or transmembrane molecules to identify genes that interacted with ptp-3 or sdn-1. We found that the Wnt ligand, lin-44, was SynLet with sdn-1, but not ptp-3. We used 4-dimensional time-lapse analysis to characterize the interaction between lin-44 and sdn-1. We found evidence that loss of lin-44 caused defects in the polarization and migration of endodermal precursors during gastrulation, a previously undescribed role for lin-44 that is strongly enhanced by the loss of sdn-1.

CONCLUSIONS

PTP-3 and SDN-1 function in compensatory pathways during C. elegans embryonic and larval development, as simultaneous loss of both genes has dire consequences for organismal survival. The Wnt ligand lin-44 contributes to the early stages of gastrulation in parallel to sdn-1, but in a genetic pathway with ptp-3. Overall, the SynLet phenotype provides a robust platform to identify ptp-3 and sdn-1 interacting genes, as well as other genes that function in development, yet might be missed in traditional forward genetic screens.

Molecular mechanisms of scar-sourced axon growth inhibitors

Astrogliosis is a defense response of the CNS to minimize primary damage and to repair injured tissues, but it ultimately generates harmful effects by upregulating inhibitory molecules to suppress neuronal elongation and forming potent barriers to axon regeneration. Chondroitin sulfate proteoglycans (CSPGs) are highly expressed by reactive scars and are potent contributors to the non-permissive environment in mature CNS. Surmounting strong inhibition by CSPG-rich scar is an important therapeutic goal for achieving functional recovery after CNS injuries. Currently, enzymatic digestion of CSPGs with locally applied chondroitinase ABC is the main in vivo approach to overcome scar inhibition, but several disadvantages may prevent using this bacterial enzyme as a therapeutic option for patients. A better understanding of molecular mechanisms underlying CSPG function may facilitate development of new effective therapies to overcome scar-mediated inhibition. Previous studies support that CSPGs act by non-specifically hindering the binding of matrix molecules to their cell surface receptors through steric interactions, but two members of the leukocyte common antigen related (LAR) phosphatase subfamily, protein tyrosine phosphatase σ and LAR, are functional receptors that bind CSPGs with high affinity and mediate CSPG inhibition. CSPGs may also act by binding two receptors for myelin-associated growth inhibitors, Nogo receptors 1 and 3. Thus, CSPGs inhibit axon growth through multiple mechanisms, making them especially potent and difficult therapeutic targets. Identification of CSPG receptors is not only important for understanding the scar-mediated growth suppression, but also for developing novel and selective therapies to promote axon sprouting and/or regeneration after CNS injuries. This article is part of a Special Issue entitled SI: Spinal cord injury.

Oxidative stress-associated protein tyrosine kinases and phosphatases in Fanconi anemia

SIGNIFICANCE

Fanconi anemia (FA) is a genetic disorder featuring chromosomal instability, developmental defects, progressive bone marrow failure, and predisposition to cancer. Besides the predominant role in DNA damage response and/or repair, many studies have linked FA proteins to oxidative stress. Oxidative stress, defined as imbalance in pro-oxidant and antioxidant homeostasis, has been considered to contribute to disease development, including FA.

RECENT ADVANCES

A variety of signaling pathways may be influenced by oxidative stress, particularly the equilibrium between protein kinases and phosphatases, consequently leading to an aberrant phosphorylation state of cellular proteins. Dysfunction of kinases/phosphatases has been implicated in the pathophysiology of human diseases. In FA, evidence is emerging that links abnormal phosphorylation/de-phosphorylation of signaling molecules to clinical complications and malformations.

CRITICAL ISSUES

In this study, we review the recent findings on the oxidative stress-related kinases and phosphatases, particularly tyrosine phosphatases in FA.

FUTURE DIRECTIONS

Understanding the role of oxidative stress-related kinases and phosphatases in FA may provide unique and generic possibilities for the future development of therapeutic strategies by targeting the dysregulated protein kinases and phosphatases in a clinical setting.

Axonal growth towards Xenopus skin in vitro is mediated by matrix metalloproteinase activity

We have previously demonstrated that the growth of peripheral nervous system axons is strongly attracted towards limb buds and skin explants in vitro. Here, we show that directed axonal growth towards skin explants of Xenopus laevis in matrigel is associated with expression of matrix metalloproteinase (MMP)-18 and also other MMPs, and that this long-range neurotropic activity is inhibited by the broad-spectrum MMP inhibitors BB-94 and GM6001. We also show that forced expression of MMP-18 in COS-7 cell aggregates enhances axonal growth from Xenopus dorsal root ganglia explants. Nidogen is the target of MMPs released by cultured skin in matrigel, whereas other components remain intact. Our results suggest a novel link between MMP activity and extracellular matrix breakdown in the control of axonal growth.

Scar-mediated inhibition and CSPG receptors in the CNS

Severed axons in adult mammals do not regenerate appreciably after central nervous system (CNS) injury due to developmentally determined reductions in neuron-intrinsic growth capacity and extracellular environment for axon elongation. Chondroitin sulfate proteoglycans (CSPGs), which are generated by reactive scar tissues, are particularly potent contributors to the growth-limiting environment in mature CNS. Thus, surmounting the strong inhibition by CSPG-rich scar is an important therapeutic goal for achieving functional recovery after CNS injuries. As of now, the main in vivo approach to overcoming inhibition by CSPGs is enzymatic digestion with locally applied chondroitinase ABC (ChABC), but several disadvantages may prevent using this bacterial enzyme as a therapeutic option for patients. A better understanding of the molecular mechanisms underlying CSPG action is needed in order to develop more effective therapies to overcome CSPG-mediated inhibition of axon regeneration and/or sprouting. Because of their large size and dense negative charges, CSPGs were thought to act by non-specifically hindering the binding of matrix molecules to their cell surface receptors through steric interactions. Although this may be true, recent studies indicate that two members of the leukocyte common antigen related (LAR) phosphatase subfamily, protein tyrosine phosphatase σ (PTPσ) and LAR, are functional receptors that bind CSPGs with high affinity and mediate CSPG inhibitory effects. CSPGs also may act by binding to two receptors for myelin-associated growth inhibitors, Nogo receptors 1 and 3 (NgR1 and NgR3). If confirmed, it would suggest that CSPGs have multiple mechanisms by which they inhibit axon growth, making them especially potent and difficult therapeutic targets. Identification of CSPG receptors is not only important for understanding the scar-mediated growth suppression, but also for developing novel and selective therapies to promote axon sprouting and/or regeneration after CNS injuries, including spinal cord injury (SCI).

PTPBR7 binding proteins in myelinating neurons of the mouse brain

Mouse protein tyrosine phosphatase PTPBR7 is a receptor-like, transmembrane protein that is localized on the surface of neuronal cells. Its protein phosphatase activity is reduced upon multimerization, and PTPBR7-deficient mice display motor coordination defects. Extracellular molecules that may influence PTPBR7 activity, however, remain to be determined. We here show that the PTPBR7 extracellular domain binds to highly myelinated regions in mouse brain, in particular the white matter tracks in cerebellum. PTPBR7 deficiency does not alter this binding pattern, as witnessed by RAP in situ staining of Ptprr^-/- mouse brain sections. Additional in situ and in vitro experiments also suggest that sugar moieties of heparan sulphate and chondroitin sulphate glycosaminoglycans are not critical for PTPBR7 binding. Candidate binding proteins were affinity-purified exploiting the PTPBR7 extracellular domain and identified by mass spectrometric means. Results support the suggested link between PTPRR isoforms and cerebellar calcium ion homeostasis, and suggest an additional role in the process of cell-cell adhesion.

Identification of novel, less toxic PTP-LAR inhibitors using in silico strategies: pharmacophore modeling, SADMET-based virtual screening and docking

Human leukocyte antigen-related (PTP-LAR) is a receptor-like transmembrane phosphatase and a potential target for diabetes, obesity and cancer. In the present study, a sequence of in silico strategies (pharmacophore mapping, a 3D database searching, SADMET screening, and docking and toxicity studies) was performed to identify eight novel nontoxic PTP-LAR inhibitors. Twenty different pharmacophore hypotheses were generated using two methods; the best (hypothesis 2) consisted of three hydrogen-bond acceptor (A), one ring aromatic (R), and one hydrophobic aliphatic (Z) features. This hypothesis was used to screen molecules from several databases, such as Specs, IBS, MiniMaybridge, NCI, and an in-house PTP inhibitor database. In order to overcome the general bioavailability problem associated with phosphatases, the hits obtained were filtered by Lipinski's rule of five and SADMET properties and validated by molecular docking studies using the available crystal structure 1LAR. These docking studies suggested the ligand binding pattern and interactions required for LAR inhibition. The docking analysis also revealed that sulfonylurea derivatives with an isoquinoline or naphthalene scaffold represent potential LAR drugs. The screening protocol was further validated using ligand pharmacophore mapping studies, which showed that the abovementioned interactions are indeed crucial and that the screened molecules can be presumed to possess potent inhibitory activities.

Profiling muscle-specific microRNA expression after peripheral denervation and reinnervation in a rat model

MicroRNAs (miRNAs) are a class of highly conserved, non-coding RNAs involved in post-transcriptional gene regulation. The muscle-specific miRNAs, miR-1, miR-133a, and miR-206, are expressed in skeletal muscles and have been shown to contribute to muscle development. To profile their expression after sciatic nerve denervation and reinnervation, the soleus muscles of the rats were analyzed with quantitative real-time PCR at 1 week, 1 month, 2 months, and 4 months after the experiments. In addition, a combined approach using computational prediction by the miRanda website and the Agilent Whole Rat Genome 4 x 44 k oligo microarray experiment was performed to investigate the potential target genes of these three miRNAs in the denervated and reinnervated muscles. The results revealed that with the first downregulation of miR-1 and miR-133a within 1 month in the denervated muscle, the expression of miR-1 and miR-133 increased by approximately 2-fold at 4 months after denervation and reinnervation; on the other hand, the expression of miR-206 was significantly increased to approximately 3-fold 1 month later only following reinnervation but not following denervation, and lasted at least for 4 months. The expression pattern of miR-206 was different from that of miR-1 and miR-133a. Notably, two genes (Hnrpu and Npy) and one gene (Ptprd) were potentially regulated both in the denervated and reinnervated muscle by miR-1 and miR-133a, respectively. There were six potential target genes (Hnrpu, Lsamp, MGC108776, Mef2, Npy, and Ppfibp2) of the upregulated miR-206 in the reinnervated muscle. Among these, three (Hnrpu, Npy, and MGC108776) were potentially regulated by both miR-1 and miR-206. Because the Mef2 transcription factor was reported to promote the transformation of type II fast glycolytic fibers into type I slow oxidative fibers, the upregulation of miR-206 with decreased expression of the Mef2 transcript in the 4 month reinnervated muscle, which presented type II fiber predominance 4 months after nerve microanastomosis, might indicate the role of miR-206 in determining the fiber type after peripheral nerve regeneration.

Dimerization of tyrosine phosphatase PTPRO decreases its activity and ability to inactivate TrkC

Receptor-protein tyrosine phosphatases (RPTPs), like receptor tyrosine kinases, regulate neuronal differentiation. While receptor tyrosine kinases are dimerized and activated by extracellular ligands, the extent to which RPTPs dimerize, and the effects of dimerization on phosphatase activity, are poorly understood. We have examined a neuronal type III RPTP, PTPRO; we find that PTPRO can form dimers in living cells, and that disulfide linkages in PTPROs intracellular domain likely regulate dimerization. Dimerization of PTPROs transmembrane and intracellular domains, achieved by ligand binding to a chimeric fusion protein, decreases activity toward artificial peptides and toward a putative substrate, tropomyosin-related kinase C (TrkC). Dephosphorylation of TrkC by PTPRO may be physiologically relevant, as it is efficient, and TrkC and PTPRO can be co-precipitated from transfected cells. Inhibition of PTPROs phosphatase activity by dimerization is interesting, as dimerization of a related RPTP, CD148/PTPRJ, increases activity. Thus, our results suggest a complex relationship between dimerization and activity in type III RPTPs.

Protein tyrosine phosphatases expression during development of mouse superior colliculus

Protein tyrosine phosphatases (PTPs) are key regulators of different processes during development of the central nervous system. However, expression patterns and potential roles of PTPs in the developing superior colliculus remain poorly investigated. In this study, a degenerate primer-based reverse transcription-polymerase chain reaction (RT-PCR) approach was used to isolate seven different intracellular PTPs and nine different receptor-type PTPs (RPTPs) from embryonic E15 mouse superior colliculus. Subsequently, the expression patterns of 11 PTPs (TC-PTP, PTP1C, PTP1D, PTP-MEG2, PTP-PEST, RPTPJ, RPTPε, RPTPRR, RPTPσ, RPTPκ and RPTPγ) were further analyzed in detail in superior colliculus from embryonic E13 to postnatal P20 stages by quantitative real-time RT-PCR, Western blotting and immunohistochemistry. Each of the 11 PTPs exhibits distinct spatiotemporal regulation of mRNAs and proteins in the developing superior colliculus suggesting their versatile roles in genesis of neuronal and glial cells and retinocollicular topographic mapping. At E13, additional double-immunohistochemical analysis revealed the expression of PTPs in collicular nestin-positive neural progenitor cells and RC-2-immunoreactive radial glia cells, indicating the potential functional importance of PTPs in neurogenesis and gliogenesis.

Redox control of neural function: background, mechanisms, and significance

The redox environment within neural cells is dependent on a series of redox couples. The glutathione disulfide/ glutathione (GSSG/GSH) redox pair forms the major redox couple in cells and as such plays a critical role in regulating redox-dependent cellular functions. Not only does GSH act as an antioxidant but it also can modulate the activity of a variety of different proteins via S-glutathionylation of cysteine sulfhydryl groups. The thioredoxin system also makes a significant contribution to the redox environment by reducing inter- and intrachain protein disulfide bonds as well as maintaining the activity of important antioxidant enzymes such as peroxiredoxins and methionine sulfoxide reductases. The redox environment affects the activity and function of a number of different protein phosphatases, protein kinases, and transcription factors. The sum of these effects will determine how changes in the redox environment alter overall cellular function, thereby playing a fundamental role in regulating neural cell fate and physiology.

N-cadherin is an in vivo substrate for protein tyrosine phosphatase sigma (PTPsigma) and participates in PTPsigma-mediated inhibition of axon growth

Protein tyrosine phosphatase sigma (PTPsigma) belongs to the LAR family of receptor tyrosine phosphatases and was previously shown to negatively regulate axon growth. The substrate for PTPsigma and the effector(s) mediating this inhibitory effect were unknown. Here we report the identification of N-cadherin as an in vivo substrate for PTPsigma. Using brain lysates from PTPsigma knockout mice, in combination with substrate trapping, we identified a hyper-tyrosine-phosphorylated protein of approximately 120 kDa in the knockout animals (relative to sibling controls), which was identified by mass spectrometry and immunoblotting as N-cadherin. beta-Catenin also precipitated in the complex and was also a substrate for PTPsigma. Dorsal root ganglion (DRG) neurons, which highly express endogenous N-cadherin and PTPsigma, exhibited a faster growth rate in the knockout mice than in the sibling controls when grown on laminin or N-cadherin substrata. However, when N-cadherin function was disrupted by an inhibitory peptide or lowering calcium concentrations, the differential growth rate between the knockout and sibling control mice was greatly diminished. These results suggest that the elevated tyrosine phosphorylation of N-cadherin in the PTPsigma(-/-) mice likely disrupted N-cadherin function, resulting in accelerated DRG nerve growth. We conclude that N-cadherin is a physiological substrate for PTPsigma and that N-cadherin (and likely beta-catenin) participates in PTPsigma-mediated inhibition of axon growth.

Cell surface nucleolin on developing muscle is a potential ligand for the axonal receptor protein tyrosine phosphatase-sigma

Reversible tyrosine phosphorylation, catalyzed by receptor tyrosine kinases and receptor tyrosine phosphatases, plays an essential part in cell signaling during axonal development. Receptor protein tyrosine phosphatase-sigma has been implicated in the growth, guidance and repair of retinal axons. This phosphatase has also been implicated in motor axon growth and innervation. Insect orthologs of receptor protein tyrosine phosphatase-sigma are also implicated in the recognition of muscle target cells. A potential extracellular ligand for vertebrate receptor protein tyrosine phosphatase-sigma has been previously localized in developing skeletal muscle. The identity of this muscle ligand is currently unknown, but it appears to be unrelated to the heparan sulfate ligands of receptor protein tyrosine phosphatase-sigma. In this study, we have used affinity chromatography and tandem MS to identify nucleolin as a binding partner for receptor protein tyrosine phosphatase-sigma in skeletal muscle tissue. Nucleolin, both from tissue lysates and in purified form, binds to receptor protein tyrosine phosphatase-sigma ectodomains. Its expression pattern also overlaps with that of the receptor protein tyrosine phosphatase-sigma-binding partner previously localized in muscle, and nucleolin can also be found in retinal basement membranes. We demonstrate that a significant amount of muscle-associated nucleolin is present on the cell surface of developing myotubes, and that two nucleolin-binding components, lactoferrin and the HB-19 peptide, can block the interaction of receptor protein tyrosine phosphatase-sigma ectodomains with muscle and retinal basement membranes in tissue sections. These data suggest that muscle cell surface-associated nucleolin represents at least part of the muscle binding site for axonal receptor protein tyrosine phosphatase-sigma and that nucleolin may also be a necessary component of basement membrane binding sites of receptor protein tyrosine phosphatase-sigma.

Loss of phosphatase activity in Ptp69D alleles supporting axon guidance defects

PTP69D is a receptor protein tyrosine phosphatase that was identified as a key regulator of neuromuscular axon guidance in Drosophila, and has subsequently been shown to play a similar role in the central nervous system and retina. Three Ptp69D alleles with mutations involving catalytically important residues exhibit a high degree of phenotypic variation with viability of mutant adult flies ranging from 0 to 96%, and ISNb motor nerve defects ranging from 11 to 57% [Desai and Purdy, 2003]. To determine whether mutations in Ptp69D affecting axon guidance and viability demonstrate losses of phosphatase activity and whether differences in catalytic potential underlie phenotypic variability, we expressed full-length wild-type and mutant PTP69D protein in Schneider 2 cells, and assessed phosphatase activity using the fluorogenic substrate 6,8-difluoro-4-methylumbelliferone phosphate (DiFMUP). Detailed biochemical characterization of wild-type PTP69D, including an examination of sensitivity to various inhibitors, in vitro catalytic efficiency, and the pH-k(cat) profile of the enzyme, suggests a common tyrosine phosphatase reaction mechanism despite lack of sequence conservation in the WPD loop. Analysis of mutant proteins revealed that every mutant had less than 1% activity relative to the wild-type enzyme, and these rates did not differ significantly from one another. These results indicate that mutations in Ptp69D resulting in axon guidance defects and lethality significantly compromise catalytic activity, yet the range of biological activity exhibited by Ptp69D mutants cannot be explained by differences in catalytic activity, as gauged by their ability to hydrolyze the substrate DiFMUP.

Neural stem cells from protein tyrosine phosphatase sigma knockout mice generate an altered neuronal phenotype in culture

Background

The LAR family Protein Tyrosine Phosphatase sigma (PTPσ) has been implicated in neuroendocrine and neuronal development, and shows strong expression in specific regions within the CNS, including the subventricular zone (SVZ). We established neural stem cell cultures, grown as neurospheres, from the SVZ of PTPσ knockout mice and sibling controls to determine if PTPσ influences the generation and the phenotype of the neuronal, astrocyte and oligodendrocyte cell lineages.

Results

The neurospheres from the knockout mice acquired heterogeneous developmental characteristics and they showed similar morphological characteristics to the age matched siblings. Although Ptprs expression decreases as a function of developmental age in vivo, it remains high with the continual renewal and passage of the neurospheres. Stem cells, progenitors and differentiated neurons, astrocytes and oligodendrocytes all express the gene. While no apparent differences were observed in developing neurospheres or in the astrocytes and oligodendrocytes from the PTPσ knockout mice, the neuronal migration patterns and neurites were altered when studied in culture. In particular, neurons migrated farther from the neurosphere centers and the neurite outgrowth exceeded the length of the neuronal processes from age matched sibling controls.

Conclusion

Our results imply a specific role for PTPσ in the neuronal lineage, particularly in the form of inhibitory influences on neurite outgrowth, and demonstrate a role for tyrosine phosphatases in neuronal stem cell differentiation.

Protein-tyrosine phosphatase (PTP) wedge domain peptides: a novel approach for inhibition of PTP function and augmentation of protein-tyrosine kinase function

Inhibition of protein-tyrosine phosphatases (PTPs) counterbalancing protein-tyrosine kinases (PTKs) offers a strategy for augmenting PTK actions. Conservation of PTP catalytic sites limits development of specific PTP inhibitors. A number of receptor PTPs, including the leukocyte common antigen-related (LAR) receptor and PTPmu, contain a wedge-shaped helix-loop-helix located near the first catalytic domain. Helix-loop-helix domains in other proteins demonstrate homophilic binding and inhibit function; therefore, we tested the hypothesis that LAR wedge domain peptides would exhibit homophilic binding, bind to LAR, and inhibit LAR function. Fluorescent beads coated with LAR or PTPmu wedge peptides demonstrated PTP-specific homophilic binding, and LAR wedge peptide-coated beads precipitated LAR protein. Administration of LAR wedge Tat peptide to PC12 cells resulted in increased proliferation, decreased cell death, increased neurite outgrowth, and augmented Trk PTK-mediated responses to nerve growth factor (NGF), a phenotype matching that found in PC12 cells with reduced LAR levels. PTPmu wedge Tat peptide had no effect on PC12 cells but blocked the PTPmu-dependent phenotype of neurite outgrowth of retinal ganglion neurons on a PTPmu substrate, whereas LAR wedge peptide had no effect. The survival- and neurite-promoting effect of the LAR wedge peptide was blocked by the Trk inhibitor K252a, and reciprocal co-immunoprecipitation demonstrated LAR/TrkA association. The addition of LAR wedge peptide inhibited LAR co-immunoprecipitation with TrkA, augmented NGF-induced activation of TrkA, ERK, and AKT, and in the absence of exogenous NGF, induced activation of TrkA, ERK, and AKT. PTP wedge domain peptides provide a unique PTP inhibition strategy and offer a novel approach for augmenting PTK function.

Identification of an ectodomain within the LAR protein tyrosine phosphatase receptor that binds homophilically and activates signalling pathways promoting neurite outgrowth

Elucidation of mechanisms by which receptor protein tyrosine phosphatases (PTPs) regulate neurite outgrowth will require characterization of ligand-receptor interactions and identification of ligand-induced signalling components mediating neurite outgrowth. The first identified ligand of the leucocyte common antigen-related (LAR) receptor PTP consists of a 99-residue ectodomain isoform, termed LARFN5C, which undergoes homophilic binding to LAR and promotes neurite outgrowth. We employed peptide mapping of LARFN5C to identify an active neurite-promoting domain of LAR. A peptide mimetic consisting of 37 residues (L59) and corresponding to the fifth LAR fibronectin type III (FNIII) domain prevented LARFN5C homophilic binding, demonstrated homophilic binding to itself and promoted neurite outgrowth of mouse E16-17 hippocampal neurons and of dorsal root ganglia explants. Response to L59 was partially lost when using neurons derived from LAR-deficient (-/-) mice or neurons treated with LAR siRNA, consistent with homophilic interaction of L59 with LAR. L59 neurite-promoting activity was decreased in the presence of inhibitors of Src, Trk, PLCgamma, PKC, PI3K and MAPK. L59 activated Src (a known substrate of LAR), FAK and TrkB and also activated downstream signalling intermediates including PKC, ERK, AKT and CREB. BDNF augmented the maximal neurite-promoting activity of L59, a finding consistent with the presence of shared and distinct signalling pathways activated by L59 with BDNF and L59 with TrkB. These studies are the first to identify an ectodomain of LAR (located within the fifth FNIII domain) capable of promoting neurite outgrowth and point to novel approaches for promotion of neurite outgrowth.

Receptor protein tyrosine phosphatase sigma inhibits axon regrowth in the adult injured CNS

Recently, receptor protein tyrosine phosphatase-sigma (RPTPsigma) has been shown to inhibit axon regeneration in injured peripheral nerves. Unlike the peripheral nervous system (PNS), central nervous system (CNS) neurons fail to regenerate their axons after injury or in disease. In order to assess the role of RPTPsigma in CNS regeneration, we used the retinocollicular system of adult mice lacking RPTPsigma to evaluate retinal ganglion cell (RGC) axon regrowth after optic nerve lesion. Quantitative analysis demonstrated a significant increase in the number of RGC axons that crossed the glial scar and extended distally in optic nerves from RPTPsigma (-/-) mice compared to wild-type littermate controls. Although we found that RPTPsigma is expressed by adult RGCs in wild-type mice, the retinas and optic nerves of adult RPTPsigma (-/-) mice showed no histological defects. Furthermore, the time-course of RGC death after nerve lesion was not different between knockout and wild-type animals. Thus, enhanced axon regrowth in the absence of RPTPsigma could not be attributed to developmental defects or increased neuronal survival. Finally, we show constitutively elevated activity of mitogen-activated protein kinase (MAPK) and Akt kinase in adult RPTPsigma (-/-) mice retinas, suggesting that these signaling pathways may contribute to promoting RGC axon regrowth following traumatic nerve injury. Our results support a model in which RPTPsigma inhibits axon regeneration in the adult injured CNS.

Effects of LAR and PTP-BL phosphatase deficiency on adult mouse retinal cells activated by lens injury

Using intact and lens-lesioned wildtype, leucocyte common antigen-related phosphatase deficient (LARDeltaP) and protein tyrosine phosphatase (PTP)-BAS-like phosphatase deficient (PTP-BLDeltaP) mice, we have evaluated the role of LAR and PTP-BL in retinal ganglion cell survival and neuritogenesis, and survival of activated retinal glia in vitro. There were no differences in in vitro retinal ganglion cell neuritogenesis and survival, as well as in activated retinal glia survival between intact wildtype and intact LARDeltaP or PTP-BLDeltaP mutant mice. In wildtype, LARDeltaP, and PTP-BLDeltaP retinal cultures, pre-conditioning by lens injury significantly increased retinal ganglion cell neuritogenesis and activated retinal glia numbers. However, in retinal cultures from lens-lesioned LARDeltaP and PTP-BLDeltaP mice, significantly smaller percentages of retinal ganglion cells grew neurites compared to lens-lesioned wildtype cultures. Significantly increased numbers of retinal ganglion cells survived in retinal cultures from lens-lesioned LARDeltaP mice compared to lens-lesioned wildtypes. PTP-BL phosphatase deficiency did not affect retinal ganglion cell survival in retinal cultures from lens-lesioned mice, though activated retinal glia numbers were significantly reduced in cultures from lens-lesioned PTP-BLDeltaP mice compared to lens-lesioned wildtypes. In summary, a functional phenotype was found in LARDeltaP and PTP-BLDeltaP mice, that was only obvious in lens lesion-stimulated retinal cultures. These observations suggest that LAR enhances retinal ganglion cell neurite initiation whilst suppressing retinal ganglion cell survival, and that PTP-BL facilitates both retinal ganglion cell neurite initiation and survival of activated retinal glia.

Expression of a catalytically inactive transmembrane protein tyrosine phosphatase epsilon (tm-PTP epsilon) delays optic nerve myelination

Reversible tyrosine phosphorylation is integral to the process of oligodendrocyte differentiation. To interfere with the subset of the phosphorylation cycle overseen by protein tyrosine phosphatase epsilon (PTP epsilon) in oligodendrocytes, we applied a substrate-trapping approach in the development of transgenic mice overexpressing a catalytically inactive, transmembrane PTP epsilon-hemaglutinin (tm-PTP epsilon-HA) from the dual promoter element of the gene encoding the myelin protein 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP). Transgene expression peaked during the active myelinating period, at 2-3 weeks postnatal. Two tyrosine phosphoproteins, alpha-enolase and beta-actin, were phosphorylated to a greater degree in transgenic mice. Despite a high degree of tm-PTP epsilon-HA expression, myelin was grossly normal in nearly all axonal tracts. Phenotypic abnormalities were limited to optic nerve, where a decrease in the degree of myelination was reflected by reduced levels of myelin proteins on postnatal day 21 (PND21), as well as a decrease in the density of differentiated oligodendrocytes. The optic chiasm was reduced in thickness in transgenic mice; optic nerves similarly exhibited a reduction in transverse width. Further analyses of the optic pathway demonstrated that transgenic protein was unexpectedly present in retinal ganglion cells, whose axons are the targets of myelination by optic nerve oligodendrocytes. On PND28, transgenic protein declined dramatically in both oligodendrocytes and retinal ganglion cells contributing to the recovery of optic nerve myelination. Thus, delayed myelination arises only when tm-PTP epsilon-HA is simultaneously expressed in myelin-forming glia and their neuronal targets. While tm-PTP epsilon related signaling pathways may figure in axon-glial interactions, interfering with tm-PTP epsilon activity does not perceptibly affect the development or myelinating capacity of most oligodendrocytes.

Receptor protein tyrosine phosphatases regulate neural development and axon guidance

The regulation of tyrosine phosphorylation is recognized as an important developmental mechanism. Both addition and removal of phosphate moieties on tyrosine residues are tightly regulated during development. Originally, most attention focused on the role of tyrosine kinases during development, but more recently, the developmental importance of tyrosine phosphatases has been gaining interest. Receptor protein tyrosine phosphatases (RPTPs) are of particular interest to developmental biologists because the extracellular domains of RPTPs are similar to those of cell adhesion molecules (CAMs). This suggests that RPTPs may have functions in development similar to CAMs. This review focuses on the role of RPTPs in development of the nervous system in processes such as axon guidance, synapse formation, and neural tissue morphogenesis.

Functional significance of the LAR receptor protein tyrosine phosphatase family in development and diseases

The protein tyrosine phosphatases (PTPs) have emerged as critical players in diverse cellular functions. The focus of this review is the leukocyte common antigen-related (LAR) subfamily of receptor PTPs (RPTPs). This subfamily is composed of three vertebrate homologs, LAR, RPTP-sigma, and RPTP-delta, as well as few invertebrates orthologs such as Dlar. LAR-RPTPs have a predominant function in nervous system development that is conserved throughout evolution. Proteolytic cleavage of LAR-RPTP proproteins results in the noncovalent association of an extracellular domain resembling cell adhesion molecules and intracellular tandem PTPs domains, which is likely regulated via dimerization. Their receptor-like structures allow them to sense the extracellular environment and transduce signals intracellularly via their cytosolic PTP domains. Although many interacting partners of the LAR-RPTPs have been identified and suggest a role for the LAR-RPTPs in actin remodeling, very little is known about the mechanisms of action of RPTPs. LAR-RPTPs recently raised a lot of interest when they were shown to regulate neurite growth and nerve regeneration in transgenic animal models. In addition, LAR-RPTPs have also been implicated in metabolic regulation and cancer. This RPTP subfamily is likely to become important as drug targets in these various human pathologies, but further understanding of their complex signal transduction cascades will be required.Key words: protein tyrosine phosphatase, LAR, signal transduction, nervous system development.

Normal patterns of spontaneous activity are required for correct motor axon guidance and the expression of specific guidance molecules

Rhythmic spontaneous electrical activity occurs in many parts of the developing nervous system, where it plays essential roles in the refinement of neural connections. By blocking or slowing this bursting activity, via in ovo drug applications at precise developmental periods, we show that such activity is also required at much earlier stages for spinal motoneurons to accurately execute their first major dorsal-ventral pathfinding decision. Blockade or slowing of rhythmic bursting activity also prevents the normal expression patterns of EphA4 and polysialic acid on NCAM, which may contribute to the pathfinding errors observed. More prolonged (E2-5) blockade resulted in a downregulation of LIM homeodomain transcription factors, but since this occurred only after the pathfinding errors and alterations in guidance molecules, it cannot have contributed to them.

Receptor protein tyrosine phosphatase alpha is essential for hippocampal neuronal migration and long-term potentiation

Despite clear indications of their importance in lower organisms, the contributions of protein tyrosine phosphatases (PTPs) to development or function of the mammalian nervous system have been poorly explored. In vitro studies have indicated that receptor protein tyrosine phosphatase alpha (RPTPalpha) regulates SRC family kinases, potassium channels and NMDA receptors. Here, we report that absence of RPTPalpha compromises correct positioning of pyramidal neurons during development of mouse hippocampus. Thus, RPTPalpha is a novel member of the functional class of genes that control radial neuronal migration. The migratory abnormality likely results from a radial glial dysfunction rather than from a neuron-autonomous defect. In spite of this aberrant development, basic synaptic transmission from the Schaffer collateral pathway to CA1 pyramidal neurons remains intact in Ptpra(-/-) mice. However, these synapses are unable to undergo long-term potentiation. Mice lacking RPTPalpha also underperform in the radial-arm water-maze test. These studies identify RPTPalpha as a key mediator of neuronal migration and synaptic plasticity.

Leukocyte antigen-related protein tyrosine phosphatase receptor: a small ectodomain isoform functions as a homophilic ligand and promotes neurite outgrowth

The identities of ligands interacting with protein tyrosine phosphatase (PTP) receptors to regulate neurite outgrowth remain mainly unknown. Analysis of cDNA and genomic clones encoding the rat leukocyte common antigen-related (LAR) PTP receptor predicted a small, approximately 11 kDa ectodomain isoform, designated LARFN5C, containing a novel N terminal followed by a C-terminal segment of the LAR fifth fibronectin type III domain. RT-PCR and Northern blot analysis confirmed the presence of LARFN5C transcripts in brain. Transfection of COS cells with LARFN5C-Fc cDNA resulted in expression of the predicted protein, and Western blot analysis verified expression of approximately 11 kDa LARFN5C protein in vivo and its developmental regulation. Beads coated with rLARFN5C demonstrated aggregation consistent with homophilic binding, and pull-down and immunoprecipitation assays demonstrated that rLARFN5C associates with the LAR receptor. rLARFN5C binding to COS cells was dependent on LAR expression, and rLARFN5C binding to LAR +/+ hippocampal neurons was fivefold greater than that found by using LAR-deficient (-/-) neurons. Substratum-bound rLARFN5C had potent neurite-promoting effects on LAR +/+ neurons, with a fivefold loss in potency with the use of LAR -/- neurons. rLARFN5C in solution at low nanomolar concentrations inhibited neurite outgrowth induced by substratum-bound rLARFN5C, consistent with receptor-based function. These studies suggest that a small ectodomain isoform of a PTP receptor can function as a ligand for the same receptor to promote neurite outgrowth.

Delayed peripheral nerve regeneration and central nervous system collateral sprouting in leucocyte common antigen-related protein tyrosine phosphatase-deficient mice

Cell adhesion molecule-like receptor-type protein tyrosine phosphatases have been shown to be important for neurite outgrowth and neural development in several animal models. We have previously reported that in leucocyte common antigen-related (LAR) phosphatase deficient (LAR-deltaP) mice the number and size of basal forebrain cholinergic neurons, and their innervation of the hippocampal area, is reduced. In this study we compared the sprouting response of LAR-deficient and wildtype neurons in a peripheral and a central nervous system lesion model. Following sciatic nerve crush lesion, LAR-deltaP mice showed a delayed recovery of sensory, but not of motor, nerve function. In line with this, neurofilament-200 immunostaining revealed a significant reduction in the number of newly outgrowing nerve sprouts in LAR-deltaP animals. Morphometric analysis indicated decreased axonal areas in regenerating LAR-deltaP nerves when compared to wildtypes. Nonlesioned nerves in wildtype and LAR-deltaP mice did not differ regarding myelin and axon areas. Entorhinal cortex lesion resulted in collateral sprouting of septohippocampal cholinergic fibres into the dentate gyrus outer molecular layer in both genotype groups. However, LAR-deltaP mice demonstrated less increase in acetylcholinesterase density and fibre number at several time points following the lesion, indicating a delayed collateral sprouting response. Interestingly, a lesion-induced reduction in number of (septo-entorhinal) basal forebrain choline acetyltransferase-positive neurons occurred in both groups, whereas in LAR-deltaP mice the average cell body size was reduced as well. Thus, regenerative and collateral sprouting is significantly delayed in LAR-deficient mice, reflecting an important facilitative role for LAR in peripheral and central nervous system axonal outgrowth.

Receptor protein tyrosine phosphatases in nervous system development

Receptor protein tyrosine phosphatases (RPTPs) are key regulators of neuronal morphogenesis in a variety of different vertebrate and invertebrate systems, yet the mechanisms by which these proteins regulate central nervous system development are poorly understood. In the past few years, studies have begun to outline possible models for RPTP function by demonstrating in vivo roles for RPTPs in axon outgrowth, guidance, and synaptogenesis. In addition, the crystal structures of several RPTPs have been solved, numerous downstream effectors of RPTP signaling have been identified, and a small number of RPTP ligands have been described. In this review, we focus on how RPTPs transduce signals from the extracellular environment to the cytoplasm, using a detailed comparative analysis of the different RPTP subfamilies. Focusing on the roles RPTPs play in the development of the central nervous system, we discuss how the elucidation of RPTP crystal structures, the biochemical analysis of phosphatase enzyme catalysis, and the characterization of complex signal transduction cascades downstream of RPTPs have generated testable models of RPTP structure and function.

Isoform-specific binding of the tyrosine phosphatase PTPsigma to a ligand in developing muscle

PTPsigma is a receptor tyrosine phosphatase that is expressed widely in the developing nervous system and that controls the growth and retinotopic mapping of retinal axons. PTPsigma is also expressed in motor neurons where its function is unclear. Given that invertebrate relatives of PTPsigma can control motor axon guidance, target contact, and synaptogenesis, we have asked if extracellular ligands exist for cPTPsigma, the avian PTPsigma orthologue, in the neuromuscular system. Of the two major isoforms cPTPsigma1 and cPTPsigma2, only the shorter cPTPsigma1 isoform is expressed in developing spinal motor neurons and their axons. We show that ectodomains of cPTPsigma1, but not of cPTPsigma2, bind specifically to developing skeletal myotubes. The putative myotube ligand is not related to the previously described binding of cPTPsigma to heparan sulfates within the proteoglycans agrin and collagen XVIII, since heparinase treatment of myotubes does not alter cPTPsigma1 binding and since most mutations that abolish binding of cPTPsigma1 to heparin do not affect myotube binding. The expression of cPTPsigma1 in motor axons and its direct binding to target myotubes suggest an isoform-specific role for axonally expressed cPTPsigma1 during establishment or maintenance of neuromuscular contacts.

Modulators of axonal growth and guidance at the brain midline with special reference to glial heparan sulfate proteoglycans

Bilaterally symmetric organisms need to exchange information between the left and right sides of their bodies to integrate sensory input and to coordinate motor control. Thus, an important choice point for developing axons is the Central Nervous System (CNS) midline. Crossing of this choice point is influenced by highly conserved, soluble or membrane-bound molecules such as the L1 subfamily, laminin, netrins, slits, semaphorins, Eph-receptors and ephrins, etc. Furthermore, there is much circumstantial evidence for a role of proteoglycans (PGs) or their glycosaminoglycan (GAG) moieties on axonal growth and guidance, most of which was derived from simplified models. A model of intermediate complexity is that of cocultures of young neurons and astroglial carpets (confluent cultures) obtained from medial and lateral sectors of the embryonic rodent midbrain soon after formation of its commissures. Neurite production in these cocultures reveals that, irrespective of the previous location of neurons in the midbrain, medial astrocytes exerted an inhibitory or non-permissive effect on neuritic growth that was correlated to a higher content of both heparan and chondroitin sulfates (HS and CS). Treatment with GAG lyases shows minor effects of CS and discloses a major inhibitory or non-permissive role for HS. The results are discussed in terms of available knowledge on the binding of HSPGs to interative proteins and underscore the importance of understanding glial polysaccharide arrays in addition to its protein complement for a better understanding of neuron-glial interactions.

Association of LAR-like receptor protein tyrosine phosphatases with an enabled homolog in Hirudo medicinalis

Receptor protein tyrosine phosphatases (RPTPs) are thought to play important functions in pathfinding and target recognition by growing neuronal processes. The leech RPTPs HmLAR1 and HmLAR2 are expressed selectively by central neurons, Comb cells, and peripheral muscle tissues in the Hirudo medicinalis embryo. To explore the functions of HmLARs, we have sought to determine their physiological substrates. We report here the cloning and embryonic expression of Lena, the leech homolog of Enabled, a cytosolic protein implicated in actin-based cell motility. Lena is expressed in embryonic central neurons and in the Comb cell. We present experimental evidences indicating that Lena associates selectively with the intracellular domain of HmLAR1 and HmLAR2. Additionally, RNA interference (RNAi) of HmLAR1 in intact leech embryos leads to the hyperphosphorylation of Lena. We propose, therefore, that Lena is an in vivo substrate of HmLAR1 in neurons and perhaps of HmLAR2 in the Comb cells.

Control of photoreceptor axon target choice by transcriptional repression of Runt

Drosophila photoreceptor neurons (R cells) project their axons to one of two layers in the optic lobe, the lamina or the medulla. The transcription factor Runt (Run) is normally expressed in the two inner R cells (R7 and R8) that project their axons to the medulla. Here we examine the relationship between Run and the ubiquitously expressed nuclear protein Brakeless (Bks), which has previously been shown to be important for axon termination in the lamina. We report that Bks represses Run in two of the outer R cells: R2 and R5. Expression of Run in R2 and R5 causes axonal mistargeting of all six outer R cells (R1-R6) to the inappropriate layer, without altering expression of cell-specific developmental markers.

Enhanced rate of nerve regeneration and directional errors after sciatic nerve injury in receptor protein tyrosine phosphatase sigma knock-out mice

The receptor protein tyrosine phosphatase sigma (PTPsigma) is a member of the mammalian leukocyte common antigen-related (LAR) family. Its expression is developmentally regulated in neuronal tissues. The Drosophila homolog of the mammalian LAR family of phosphatases (DLAR) controls axon guidance during Drosophila embryogenesis. We have demonstrated previously that mice deficient in PTPsigma have CNS and peripheral nervous system abnormalities. The sciatic nerve in the PTPsigma(-/-) mice demonstrates an increased number of small diameter fibers and slower nerve conduction velocities compared with PTPsigma(+/+) or PTPsigma(+/-) controls. To study whether peripheral nerve regeneration is affected by PTPsigma activity, we assessed nerve regeneration in the PTPsigma(-/-) mouse after three standard models of sciatic nerve injury. We report that after sciatic nerve crush injury, nerve regeneration was significantly faster in the PTPsigma(-/-) animals, as determined by histologic, electrophysiologic, and neuromuscular testing. After sciatic nerve transection with immediate microsurgical repair or allografting, PTPsigma(-/-) nerve fibers demonstrated errors in directional growth compared with controls. We propose that PTPsigma regulates the axonal regeneration rate and guidance of regenerating fibers.

Chick PTPsigma regulates the targeting of retinal axons within the optic tectum

Chick PTPsigma (cPTPsigma), also known as CRYPalpha, is a receptor-like protein tyrosine phosphatase found on axons and growth cones. Putative ligands for cPTPsigma are distributed within basement membranes and on glial end feet of the retina, optic nerve, and optic tectum, suggesting that cPTPsigma signaling is occurring along the whole retinotectal pathway. We have shown previously that cPTPsigma plays a role in supporting the retinal phase of axon outgrowth. Here we have now addressed the role of cPTPsigma within retinal axons as they undergo growth and topographic targeting in the optic tectum. With the use of retroviruses, a secretable cPTPsigma ectodomain was ectopically expressed in ovo in the developing chick optic tectum, with the aim of directly disrupting the function of endogenous cPTPsigma. In ovo, the secreted ectodomains accumulated at tectal sites in which cPTPsigma ligands are also specifically found, suggesting that they are binding to these endogenous ligands. Anterograde labeling of retinal axons entering these optic tecta revealed abnormal axonal phenotypes. These included the premature stalling and arborization of fibers, excessive pretectal arbor formation, and diffuse termination zones. Most of the defects were rostral of the predicted termination zone, indicating that cPTPsigma function is necessary for sustaining the growth of retinal axons over the optic tectum and for directing axons to their correct sites of termination. This demonstrates that regulation of cPTPsigma signaling in retinal axons is required for their topographic mapping, the first evidence of this function for a receptor-like protein tyrosine phosphatase in the retinotectal projection.

The paired-like homeodomain protein, Arix, mediates protein kinase A-stimulated dopamine beta-hydroxylase gene transcription through its phosphorylation status

The homeodomain transcription factor Arix/Phox2a plays a critical role in the specification of noradrenergic neurons by inducing the expression of dopamine beta-hydroxylase (DBH), the terminal enzyme for noradrenaline biosynthesis. In reporter assays, Arix together with activation of cAMP-dependent protein kinase (PKA) potentiates DBH gene transcription. We have evaluated whether post-translational modification of Arix regulates PKA-mediated DBH gene transcription. We found that Arix is constitutively phosphorylated in vivo at the basal level and that the phosphorylation level is substantially decreased upon stimulation of the PKA pathway. The change in the Arix phosphorylation state coincides with DNA binding activity of Arix. Treatment of cells with forskolin results in a robust enhancement of the DNA binding of Arix, which is reversed by treatment with serine/threonine and tyrosine phosphatase inhibitors. Consistent with the DNA binding activity of Arix, treatment of cultured cells with phosphatase inhibitors diminishes transcriptional activation with Arix plus forskolin. Amino acid analysis demonstrates the presence of phosphoserine within Arix. The results collectively suggest that dephosphorylation of Arix is a necessary event to fully activate PKA-mediated DBH transcription. Thus, the present study demonstrates that Arix can integrate extrinsic signals through post-translational modification, regulating DBH gene transcription in response to activation of the PKA pathway.

Axon guidance: the cytoplasmic tail

Recent advances in the study of axon guidance have begun to clarify the intricate signalling mechanisms utilised by receptors that mediate path-finding. Many of these axon guidance receptors, including Plexin B, EphA, ephrin B and Robo, regulate the Rho family of GTPases, to effect changes in motility. Recent studies demonstrate a critical role for the cytoplasmic tails of guidance receptors in signalling and also reveal the potential for a great deal of crosstalk between the various receptor-signalling pathways.

Drosophila liprin-alpha and the receptor phosphatase Dlar control synapse morphogenesis

Here, we examine the synaptic function of the receptor protein tyrosine phosphatase (RPTP), Dlar, and an associated intracellular protein, Dliprin-alpha, at the Drosophila larval neuromuscular junction. We show that Dliprin-alpha and Dlar are required for normal synaptic morphology. We also find that synapse complexity is proportional to the amount of Dlar gene product, suggesting that Dlar activity determines synapse size. Ultrastructural analysis reveals that Dliprin-alpha and Dlar are required to define the size and shape of the presynaptic active zone. Accordingly, there is a concomitant decrease in synaptic transmission in both mutants. Finally, epistasis analysis indicates that Dliprin-alpha is required for Dlar's action at the synapse. These data suggest a model where Dliprin-alpha and Dlar cooperate to regulate the formation and/or maintenance of a network of presynaptic proteins.

Tyrosine phosphatase STEP is a tonic brake on induction of long-term potentiation

The functional roles of protein tyrosine phosphatases (PTPs) in the developed CNS have been enigmatic. Here we show that striatal enriched tyrosine phosphatase (STEP) is a component of the N-methyl-D-aspartate receptor (NMDAR) complex. Functionally, exogenous STEP depressed NMDAR single-channel activity in excised membrane patches. STEP also depressed NMDAR-mediated synaptic currents whereas inhibiting endogenous STEP enhanced these currents. In hippocampal slices, administering STEP into CA1 neurons did not affect basal glutamatergic transmission evoked by Schaffer collateral stimulation but prevented tetanus-induced long-term potentiation (LTP). Conversely, inhibiting STEP in CA1 neurons enhanced transmission and occluded LTP induction through an NMDAR-, Src-, and Ca(2+)-dependent mechanism. Thus, STEP acts as a tonic brake on synaptic transmission by opposing Src-dependent upregulation of NMDARs.

Eye defects in receptor protein-tyrosine phosphatase alpha knock-down zebrafish

Receptor protein-tyrosine phosphatase alpha (RPTP alpha) is highly expressed in the developing retina of different species, but little is known about its function there. Here, we report that injection of antisense morpholinos in zebrafish embryos reduced RPTP alpha expression to almost nondetectable levels up to 3 days postfertilization (dpf). RPTP alpha was detectable again from 4 dpf onward. RPTP alpha knock-down resulted in smaller eyes. Examination of sections of the retina at different developmental stages demonstrated that already at 28 hours postfertilization (hpf) fewer cells were present in the retina of RPTP alpha-morpholino-injected embryos. At 3 dpf, the layered organization of the retina was absent. In addition, the morphology and labeling with an axon specific antibody, acetylated tubulin, demonstrated that most cells appeared to be undifferentiated. Strikingly, at 5 dpf the lamination of the retina was partially restored, concomitant with re-expression of RPTP alpha protein. Although cells in the retina were now differentiated, the layering of the retina remained disrupted and significant gaps were observed in the amacrine cell layer. Therefore, knock-down of RPTP alpha protein provides evidence that RPTP alpha is essential for normal retinal development.

Heparan sulfate proteoglycans are ligands for receptor protein tyrosine phosphatase sigma

RPTPsigma is a cell adhesion molecule-like receptor protein tyrosine phosphatase involved in nervous system development. Its avian orthologue, known as cPTPsigma or CRYPalpha, promotes intraretinal axon growth and controls the morphology of growth cones. The molecular mechanisms underlying the functions of cPTPsigma are still to be determined, since neither its physiological ligand(s) nor its substrates have been described. Nevertheless, a major class of ligand(s) is present in the retinal basal lamina and glial endfeet, the potent native growth substrate for retinal axons. We demonstrate here that cPTPsigma is a heparin-binding protein and that its basal lamina ligands include the heparan sulfate proteoglycans (HSPGs) agrin and collagen XVIII. These molecules interact with high affinity with cPTPsigma in vitro, and this binding is totally dependent upon their heparan sulfate chains. Using molecular modelling and site-directed mutagenesis, a binding site for heparin and heparan sulfate was identified in the first immunoglobulin-like domain of cPTPsigma. HSPGs are therefore a novel class of heterotypic ligand for cPTPsigma, suggesting that cPTPsigma signaling in axons and growth cones is directly responsive to matrix-associated cues.

Protein kinase C delta (PKCdelta) is required for protein tyrosine phosphatase mu (PTPmu)-dependent neurite outgrowth

Protein tyrosine phosphatase mu (PTPmu) is an adhesion molecule in the immunoglobulin superfamily and is expressed in the developing nervous system. We have shown that PTPmu can promote neurite outgrowth of retinal ganglion cells and it regulates neurite outgrowth mediated by N-cadherin (S. M. Burden-Gulley and S. M. Brady-Kalnay, 1999, J. Cell Biol. 144, 1323-1336). We previously demonstrated that PTPmu binds to the scaffolding protein RACK1 in yeast and mammalian cells (T. Mourton et al., 2001, J. Biol. Chem. 276, 14896-14901). RACK1 is a receptor for activated protein kinase C (PKC). In this article, we demonstrate that PKC is involved in PTPmu-dependent signaling. PTPmu, RACK1, and PKCdelta exist in a complex in cultured retinal cells and retinal tissue. Using pharmacologic inhibition of PKC, we demonstrate that PKCdelta is required for neurite outgrowth of retinal ganglion cells on a PTPmu substrate. These results suggest that PTPmu signaling via RACK1 requires PKCdelta activity to promote neurite outgrowth.

Receptor protein tyrosine phosphatases regulate retinal ganglion cell axon outgrowth in the developing Xenopus visual system

Receptor protein tyrosine phosphatases (RPTPs) are regulators of axon outgrowth and guidance in a variety of different vertebrate and invertebrate systems. Three RPTPs, CRYP-alpha, PTP-delta, and LAR, are expressed in overlapping but distinct patterns in the developing Xenopus retina, including expression in retinal ganglion cells (RGCs) as they send axons to the tectum (Johnson KG, Holt CE. 2000. Expression of CRYP-alpha, LAR, PTP-delta, and PTP-rho in the developing Xenopus visual system. Mech Dev 92:291-294). In order to examine the role of these RPTPs in visual system development, putative dominant negative RPTP mutants (CS-CRYP-alpha, CS-PTP-delta, and CS-LAR) were expressed either singly or in combination in retinal cells. No effect was found on either retinal cell fate determination or on gross RGC axon guidance to the tectum. However, expression of these CS-RPTP constructs differentially affected the rate of RGC axon outgrowth. In vivo, expression of all three CS-RPTPs or CS-PTP-delta alone inhibited RGC axon outgrowth, while CS-LAR and CS-CRYP-alpha had no significant effect. In vitro, expression of CS-CRYP-alpha enhanced neurite outgrowth, while CS-PTP-delta inhibited neurite outgrowth in a substrate-dependent manner. This study provides the first in vivo evidence that RPTPs regulate retinal axon outgrowth.