Close similarity between receptor-linked tyrosine phosphatase and rat brain proteoglycan

The Protein Tyrosine Phosphatase Rptpζ Suppresses Osteosarcoma Development in Trp53-Heterozygous Mice

Osteosarcoma (OS), a highly aggressive primary bone tumor, belongs to the most common solid tumors in growing children. Since specific molecular targets for OS treatment remain to be identified, surgical resection combined with multimodal (neo-)adjuvant chemotherapy is still the only way to help respective individuals. We have previously identified the protein tyrosine phosphatase Rptpζ as a marker of terminally differentiated osteoblasts, which negatively regulates their proliferation in vitro. Here we have addressed the question if Rptpζ can function as a tumor suppressor protein inhibiting OS development in vivo. We therefore analyzed the skeletal phenotype of mice lacking Ptprz1, the gene encoding Rptpζ on a tumor-prone genetic background, i.e. Trp53-heterozygosity. By screening a large number of 52 week old Trp53-heterozygous mice by contact radiography we found that Ptprz1-deficiency significantly enhanced OS development with 19% of the mice being affected. The tumors in Ptprz1-deficient Trp53-heterozygous mice were present in different locations (spine, long bones, ribs), and their OS nature was confirmed by undecalcified histology. Likewise, cell lines derived from the tumors were able to undergo osteogenic differentiation ex vivo. A comparison between Ptprz1-heterozygous and Ptprz1-deficient cultures further revealed that the latter ones displayed increased proliferation, a higher abundance of tyrosine-phosphorylated proteins and resistance towards the influence of the growth factor Midkine. Our findings underscore the relevance of Rptpζ as an attenuator of proliferation in differentiated osteoblasts and raise the possibility that activating Rptpζ-dependent signaling could specifically target osteoblastic tumor cells.

Sugar glues for broken neurons

Proteoglycans (PGs) regulate diverse functions in the central nervous system (CNS) by interacting with a number of growth factors, matrix proteins, and cell surface molecules. Heparan sulfate (HS) and chondroitin sulfate (CS) are two major glycosaminoglycans present in the PGs of the CNS. The functionality of these PGs is to a large extent dictated by the fine sulfation patterns present on their glycosaminoglycan (GAG) chains. In the past 15 years, there has been a significant expansion in our knowledge on the role of HS and CS chains in various neurological processes, such as neuronal growth, regeneration, plasticity, and pathfinding. However, defining the relation between distinct sulfation patterns of the GAGs and their functionality has thus far been difficult. With the emergence of novel tools for the synthesis of defined GAG structures, and techniques for their characterization, we are now in a better position to explore the structure-function relation of GAGs in the context of their sulfation patterns. In this review, we discuss the importance of GAGs on CNS development, injury, and disorders with an emphasis on their sulfation patterns. Finally, we outline several GAG-based therapeutic strategies to exploit GAG chains for ameliorating various CNS disorders.

A complex between contactin-1 and the protein tyrosine phosphatase PTPRZ controls the development of oligodendrocyte precursor cells

The six members of the contactin (CNTN) family of neural cell adhesion molecules are involved in the formation and maintenance of the central nervous system (CNS) and have been linked to mental retardation and neuropsychiatric disorders such as autism. Five of the six CNTNs bind to the homologous receptor protein tyrosine phosphatases gamma (PTPRG) and zeta (PTPRZ), but the biological roles of these interactions remain unclear. We report here the cocrystal structure of the carbonic anhydrase-like domain of PTPRZ bound to tandem Ig repeats of CNTN1 and combine these structural data with binding assays to show that PTPRZ binds specifically to CNTN1 expressed at the surface of oligodendrocyte precursor cells. Furthermore, analyses of glial cell populations in wild-type and PTPRZ-deficient mice show that the binding of PTPRZ to CNTN1 expressed at the surface of oligodendrocyte precursor cells inhibits their proliferation and promotes their development into mature oligodendrocytes. Overall, these results implicate the PTPRZ/CNTN1 complex as a previously unknown modulator of oligodendrogenesis.

Molecular dissection of NRG1-ERBB4 signaling implicates PTPRZ1 as a potential schizophrenia susceptibility gene

Neuregulin and the neuregulin receptor ERBB4 have been genetically and functionally implicated in schizophrenia. In this study, we used the yeast two-hybrid system to identify proteins that interact with ERBB4, to identify genes and pathways that might contribute to schizophrenia susceptibility. We identified the MAGI scaffolding proteins as ERBB4-binding proteins. After validating the interaction of MAGI proteins with ERBB4 in mammalian cells, we demonstrated that ERBB4 expression, alone or in combination with ERBB2 or ERBB3, led to the tyrosine phosphorylation of MAGI proteins, and that this could be further enhanced with receptor activation by neuregulin. As MAGI proteins were previously shown to interact with receptor phosphotyrosine phosphatase beta/zeta (RPTPbeta), we postulated that simultaneous binding of MAGI proteins to RPTPbeta and ERBB4 forms a phosphotyrosine kinase/phosphotyrosine phosphatase complex. Studies in cultured cells confirmed both a spatial and functional association between ERBB4, MAGI and RPTPbeta. Given the evidence for this functional association, we examined the genes coding for MAGI and RPTPbeta for genetic association with schizophrenia in a Caucasian United Kingdom case-control cohort (n= approximately 1400). PTPRZ1, which codes for RPTPbeta, showed significant, gene-wide and hypothesis-wide association with schizophrenia in our study (best individual single-nucleotide polymorphism allelic P=0.0003; gene-wide P=0.0064; hypothesis-wide P=0.026). The data provide evidence for a role of PTPRZ1, and for RPTPbeta signaling abnormalities, in the etiology of schizophrenia. Furthermore, the data indicate a role for RPTPbeta in the modulation of ERBB4 signaling that may in turn provide further support for an important role of neuregulin/ERBB4 signaling in the molecular basis of schizophrenia.

Expression of hypoxia-inducible carbonic anhydrases in brain tumors

Malignant brain tumors exhibit distinct metabolic characteristics. Despite high levels of lactate, the intracellular pH of brain tumors is more alkaline than normal brain. Additionally, with increasing malignancy, brain tumors display intratumoral hypoxia. Carbonic anhydrase (CA) IX and XII are transmembrane isoenzymes that are induced by tissue hypoxia. They participate in regulation of pH homeostasis by catalyzing the reversible hydration of carbon dioxide. The aim of our study was to investigate whether brain tumors of different histology and grade of malignancy express elevated levels of CA IX and XII as compared to normal brain. We analyzed 120 tissue specimens from brain tumors (primary and metastatic) and normal brain for CA IX and XII expression by immunohistochemistry, Western blot, and in situ hybridization. Whereas normal brain tissue showed minimal levels of CA IX and XII expression, expression in tumors was found to be upregulated with increased level of malignancy. Hemangioblastomas, from patients with von Hippel-Lindau disease, also displayed high levels of CA IX and XII expression. Comparison of CA IX and XII staining with HIF-1alpha staining revealed a similar microanatomical distribution, indicating hypoxia as a major, but not the only, induction factor. The extent of CA IX and XII staining correlated with cell proliferation, as indicated by Ki67 labeling. The results demonstrate that CA IX and XII are upregulated in intrinsic and metastatic brain tumors as compared to normal brain tissue. This may contribute to the management of tumor-specific acid load and provide a therapeutic target.

Contactin supports synaptic plasticity associated with hippocampal long-term depression but not potentiation

BACKGROUND

Changes in synaptic efficacy are believed to mediate the processes of learning and memory formation. Accumulating evidence implicates cell adhesion molecules in activity-dependent synaptic modifications associated with long-term potentiation (LTP); however, there is no precedence for the selective role of this molecule class in long-term depression (LTD). The mechanisms that modulate these processes still remain unclear.

RESULTS

We report a novel role for glycosylphosphatidyl inositol (GPI)-anchored contactin in hippocampal CA1 synaptic plasticity. Contactin selectively supports paired-pulse facilitation (PPF) and NMDA (N-methyl-D-aspartate) receptor-dependent LTD but is not required for synaptic morphology, basal transmission, or LTP. Molecular analyses indicate that contactin is essential for the membrane and synaptic targeting of the contactin-associated protein (Caspr/paranodin) and for the proper distribution of a presumptive ligand, receptor protein tyrosine phosphatase beta (RPTPbeta)/phosphacan.

CONCLUSIONS

These results indicate that contactin plays a selective role in synaptic plasticity and identify PPF and LTD, but not LTP, as contactin-dependent processes. Engagement of the contactin-Caspr complex with RPTPbeta may thus regulate cell-cell interactions contributing to specific synaptic plasticity forms.

Proteoglycans in the developing brain: new conceptual insights for old proteins

Proteoglycans are a heterogeneous class of proteins bearing sulfated glycosaminoglycans. Some of the proteoglycans have distinct core protein structures, and others display similarities and thus may be grouped into families such as the syndecans, the glypicans, or the hyalectans (or lecticans). Proteoglycans can be found in almost all tissues being present in the extracellular matrix, on cellular surfaces, or in intracellular granules. In recent years, brain proteoglycans have attracted growing interest due to their highly regulated spatiotemporal expression during nervous system development and maturation. There is increasing evidence that different proteoglycans act as regulators of cell migration, axonal pathfinding, synaptogenesis, and structural plasticity. This review summarizes the most recent data on structures and functions of brain proteoglycans and focuses on new physiological concepts for their potential roles in the developing central nervous system.

Molecular cloning, expression, and distribution of glomerular epithelial protein 1 in developing mouse kidney

BACKGROUND

Glomerular epithelial protein 1 (GLEPP1) is a receptor-like membrane protein tyrosine phosphatase (RPTP) with a large ectodomain consisting of multiple fibronectin type III repeats, a single transmembrane segment, and a single cytoplasmic phosphatase active site sequence. In adult human and rabbit kidneys, GLEPP1 is found exclusively on apical membranes of podocytes and especially on surfaces of foot processes. Although neither ligand nor function for this protein is known, other RPTPs with similar topologies have been implicated in mediating adherence behavior of cells.

METHODS

To evaluate potential roles of GLEPP1 further, we cloned the full-length mouse GLEPP1 cDNA and examined its expression patterns in developing kidney by Northern blot analysis, in situ hybridization, and immunofluorescence microscopy.

RESULTS

Nucleotide sequencing showed that mouse GLEPP1 was approximately 80% identical to rabbit and human GLEPP1 and approximately 91% identical at the amino acid level. The membrane-spanning and phosphatase domains of mouse GLEPP1 shared> 99% homology with PTPphi, a murine macrophage cytoplasmic phosphatase. Northern analysis identified a single GLEPP1 transcript of approximately 5.5 kb in fetal kidney that became approximately threefold more abundant in adults. In situ hybridization of newborn mouse kidney revealed GLEPP1 mRNA in visceral epithelial cells (developing podocytes) of comma- and S-shaped nephric figures, and expression increased in capillary loop and maturing stage glomeruli. Beginning on embryonic day 14, GLEPP1 protein was first observed on cuboidal podocytes of capillary loop stage glomeruli, but nascent podocytes of earlier comma- and S-shaped nephric figures were negative. At later stages of glomerular maturation, where foot process elongation and interdigitation occurs, GLEPP1 immunolabeling intensified on podocytes and then persisted at high levels in fully developed glomeruli.

CONCLUSION

Our findings are consistent with a role for GLEPP1 in mediating and maintaining podocyte differentiation specifically.

Regulated expression during development and following sciatic nerve injury of mRNAs encoding the receptor tyrosine phosphatase HPTPzeta/RPTPbeta

Three major isoforms of the receptor protein tyrosine phosphatase HPTPzeta/RPTPbeta (RPTPzeta/beta) have been previously identified, two with identical transmembrane and intracellular catalytic domains that differ by virtue of a long cysteine-free extracellular region, and a soluble proteoglycan called phosphacan that lacks the transmembrane and carboxy-terminal catalytic domains. To determine whether these RPTPzeta/beta variants are produced by alternative mRNA splicing of a common primary transcript, we performed genomic Southern analysis and characterized several rat cDNA and genomic RPTPzeta/beta clones. These studies indicated that the three major transcripts which encode phosphacan and the two RPTPzeta/beta phosphatase variants are encoded by a single gene, and further that additional alternative mRNA splicing is likely to result in the deletion of a 7 amino acid insert from the intracellular juxtamembrane region of both long and short phosphatase isoforms. Simultaneous quantitation of the three major isoforms by RNase protection analysis indicated that the mRNA encoding phosphacan had the highest relative abundance in the CNS while that encoding the short phosphatase isoform was most abundant relative to the other RPTPzeta/beta variants in the PNS. Following peripheral nerve crush, all RPTPzeta/beta mRNAs, including phosphacan and the phosphatase variants with and without the 21 base insert, were significantly induced in the distal segments of the sciatic nerve with a time course that correlated well with the response of Schwann cells to this injury.

Immunohistochemistry of carbonic anhydrase isozyme IX (MN/CA IX) in human gut reveals polarized expression in the epithelial cells with the highest proliferative capacity

MN/CA IX is a recently discovered member of the carbonic anhydrase (CA) gene family that has been identified in the plasma membranes of certain tumor and epithelial cells and found to promote cell proliferation when transfected into NIH3T3 cells. This study presents localization of MN/CA IX in human gut and compares its distribution to those of CA I, II, and IV, which are known to be expressed in the intestinal epithelium. The specificity of the monoclonal antibody for MN/CA IX was confirmed by Western blots and immunostaining of COS-7 cells transfected with MN/CA IX cDNA. Immunohistochemical stainings of human gut revealed prominent polarized staining for MN/CA IX in the basolateral surfaces of the enterocytes of duodenum and jejunum, the reaction being most intense in the crypts. A moderate reaction was also seen in the crypts of ileal mucosa, whereas the staining became generally weaker in the large intestine. The results indicate isozyme-specific regulation of MN/CA IX expression along the cranial-caudal axis of the human gut and place the protein at the sites of rapid cell proliferation. The unique localization of MN/CA IX on the basolateral surfaces of proliferating crypt enterocytes suggests that it might serve as a ligand or a receptor for another protein that regulates intercellular communication or cell proliferation. Furthermore, MN/CA IX has a completely conserved active site domain of CAs suggesting that it could also participate in carbon dioxide/bicarbonate homeostasis.

Cell-contact-dependent signalling in axon growth and guidance: Eph receptor tyrosine kinases and receptor protein tyrosine phosphatase beta

The growth and guidance of axons involves the recognition of complex environmental cues by receptor proteins on the surface of the growth cone and their interpretation by cellular machinery, leading to changes in cellular behaviour. Recent advances have demonstrated that the ligands for Eph receptor tyrosine kinases, the ephrins, act as repulsive axon guidance cues, and that Eph receptors are required for correct axonal navigation in vivo. Members of the receptor protein tyrosine phosphatase (RPTP) family also play important roles in axon guidance and growth. RPTP beta and Eph receptors interact with cell-surface-bound ligands, and there is increasing evidence that both transmembrane ephrins and contactin, a ligand for RPTP beta, may possess an intrinsic signalling function. Thus, the cell-contact-dependent interactions between these receptors and ligands may lead to initiation of bidirectional signals that regulate axonal growth and migration.

Expression of a receptor protein tyrosine phosphatase in human glial tumors

We have analyzed expression of a receptor protein tyrosine phosphatase (RPTPzeta/beta) in tissue samples from 23 human gliomas. Using the reverse transcription-polymerase chain reaction (RT-PCR) technique, we assayed for the presence or absence of mRNA transcripts encoding the intact receptor and 2 alternatively spliced forms of RPTPzeta/beta. Transcripts encoding the intact and truncated receptors were expressed in all of the lower grade gliomas (WHO grade 1-3) analyzed, but not in 55% of the grade 4 glioblastomas multiforme (GBM). However, this subset of GBMs did express an alternatively spliced secreted form comprised of only the RPTPzeta/beta extracellular domain. Our data suggests there may be a correlation between the loss of transcripts encoding the receptor forms of RPTPzeta/beta and progression from low to high grade gliomas. This work provides additional evidence for the importance of phosphatase isoform expression in human tumors.

Receptor protein tyrosine phosphatases: involvement in cell-cell interaction and signaling

Receptor protein tyrosine phosphatases (RPTPs) represent a relatively new family of cell-surface receptors consisting of a variable, putative ligand-binding ectodomain followed by a single transmembrane segment and one or two intracellular catalytic domains. The RPTPs are thought to transduce extracellular signals by dephosphorylating tyrosine-phosphorylated intracellular substrates. As such, they are the enzymatic counterparts of the well studied receptor tyrosine kinases. However, little is known about the signaling mechanisms and biological functions of the RPTPs. Recent studies show that the extracellular domain of certain RPTPs can mediate either homophilic or heterophilic interactions and suggest a role in cadherin-mediated cell-cell adhesion, possibly via an action on catenins. This review will focus on the role of RPTPs in cell-cell interaction and the possible biological implications.

Identification of a novel contactin-associated transmembrane receptor with multiple domains implicated in protein-protein interactions

Receptor protein tyrosine phosphatase beta (RPTPbeta) expressed on the surface of glial cells binds to the glycosylphosphatidylinositol (GPI)-anchored recognition molecule contactin on neuronal cells leading to neurite outgrowth. We describe the cloning of a novel contactin-associated transmembrane receptor (p190/Caspr) containing a mosaic of domains implicated in protein-protein interactions. The extracellular domain of Caspr contains a neurophilin/coagulation factor homology domain, a region related to fibrinogen beta/gamma, epidermal growth factor-like repeats, neurexin motifs as well as unique PGY repeats found in a molluscan adhesive protein. The cytoplasmic domain of Caspr contains a proline-rich sequence capable of binding to a subclass of SH3 domains of signaling molecules. Caspr and contactin exist as a complex in rat brain and are bound to each other by means of lateral (cis) interactions in the plasma membrane. We propose that Caspr may function as a signaling component of contactin, enabling recruitment and activation of intracellular signaling pathways in neurons. The binding of RPTPbeta to the contactin-Caspr complex could provide a mechanism for cell-cell communication between glial cells and neurons during development.

Induction of neurite outgrowth through contactin and Nr-CAM by extracellular regions of glial receptor tyrosine phosphatase beta

Receptor protein tyrosine phosphatase beta (RPTPbeta) is expressed as soluble and receptor forms with common extracellular regions consisting of a carbonic anhydrase domain (C), a fibronectin type III repeat (F), and a unique region called S. We showed previously that a recombinant Fc fusion protein with the C domain (beta C) binds to contactin and supports neuronal adhesion and neurite growth. As a substrate, betaCFS was less effective in supporting cell adhesion, but it was a more effective promoter of neurite outgrowth than betaCF. betaS had no effect by itself, but it potentiated neurite growth when mixed with betaCF. Neurite outgrowth induced by betaCFS was inhibited by antibodies against Nr-CAM and contactin, and these cell adhesion molecules formed a complex that bound betaCFS. NIH-3T3 cells transfected to express betaCFS on their surfaces induced neuronal differentiation in culture. These results suggest that binding of glial RPTPbeta to the contactin/Nr-CAM complex is important for neurite growth and neuronal differentiation.

Comparison of RPTP zeta/beta, phosphacan, and trkB mRNA expression in the developing and adult rat nervous system and induction of RPTP zeta/beta and phosphacan mRNA following brain injury

The receptor protein tyrosine phosphatase (RPTP) zeta/beta and a major isoform, phosphacan, a chondroitin sulfate proteoglycan that contains the RPTP zeta/beta extracellular domain but not the transmembrane and intracellular phosphatase domains, are expressed abundantly in the nervous system, primarily by astroglia. Because of similarities in the expression patterns of RPTP zeta/beta and the receptor tyrosine kinase TrkB, we investigated whether RNAs encoding these proteins were co-localized during development, which would suggest that these molecules might functionally interact in vivo. By in-situ hybridization, we noted extensive areas of overlap in the expression of trkB and RPTP zeta/beta mRNAs in the developing peripheral and central nervous systems. Analysis with a probe specific for the catalytic TrkB isoform suggested that RPTP zeta/beta and non-catalytic trkB mRNAs were co-expressed in particular regions of the nervous system while the catalytic trkB and RPTP zeta/beta transcripts were also, but to a lesser extent. RPTP zeta/beta and phosphacan expression were extremely similar, differing particularly in the level of expression in the ventricular and subventricular zones, hippocampus, and ependyma. Furthermore, both RPTP zeta/beta and phosphacan mRNAs were found in several subsets of neurons as well as astrocytes. Following CNS injury, we observed robust induction of RPTP zeta/beta mRNA in areas of axonal sprouting, and of both RPTP zeta/beta and phosphacan mRNAs in areas of glial scarring, implying that the encoded proteins and the cell adhesion molecules and extracellular matrix proteins to which they bind may contribute to recovery from injury and perhaps regulation of axonal regrowth in the nervous system.

Three forms of RPTP-beta are differentially expressed during gliogenesis in the developing rat brain and during glial cell differentiation in culture

In situ hybridization and Northern analysis demonstrate that the three splicing variants of RPTP-beta have different spatial and temporal patterns of expression in the developing brain. The 9.5-kb and 6.4-kb transcripts, which encode transmembrane protein tyrosine phosphatases with different extracellular domains, are predominantly expressed in glial progenitors located in the subventricular zone (SVZ). The 8.4-kb transcript, which encodes a secreted chondroitin sulfate proteoglycan (phosphacan), is expressed at high levels by more mature glia that have migrated out of the SVZ. The three transcripts are also differentially expressed in glial cell cultures; O2A progenitors express high levels of the 9.5- and 8.4-kb transcript, whereas type 1 astrocyte progenitors predominantly express the 6.4-kb transcript. C6 gliomas also express high levels of the 6.4-kb transcript. Treating C6 cells with the differentiating agent dibutyryl cyclic-AMP (DBcAMP), induces a decrease in the 6.4-kb transcript and a corresponding increase in the 8.4-kb transcript. O2A cells grown in the presence of basic fibroblast growth factor (bFGF) and platelet-derived growth factor (PDGF) remain highly proliferative and undifferentiated, and continue to express high levels of RPTP-beta. However, when O2A cells are grown in conditions that induce oligodendrocyte differentiation, there is a marked decrease in the expression of the transmembrane forms of RPTP-beta, as determined by immunofluorescence. These results demonstrate that RPTP-beta expression is regulated during glial cell differentiation and suggest that the different forms of RPTP-beta perform distinct functions during brain development.

Expression of polypeptide variants of receptor-type protein tyrosine phosphatase beta: the secreted form, phosphacan, increases dramatically during embryonic development and modulates glial cell behavior in vitro

Glial cells express three splicing variants of a receptor-type protein tyrosine phosphatase called RPTP beta. Two are receptor forms that differ in a large extracellular domain. The third is a secreted proteoglycan called phosphacan that lacks the cytoplasmic phosphatase domains. We have now identified, by immunoblotting, proteins corresponding to these three forms of RPTP beta in rat C6 glioma cells and brain. The short receptor form is much more prevalent than the full-length receptor in C6 glioma cells. Phosphacan is much more abundant than either of the receptor forms in rat brain, and its expression increases progressively during embryonic development, while the receptor forms show only moderate changes. In contrast to the long form and phosphacan that were detected as proteoglycans, the short receptor form, lacking the large alternatively spliced domain, was not detected as a chondroitin sulfate proteoglycan. We recently showed that phosphacan binds to the neuron-glia cell adhesion molecule, Ng-CAM, and we now report that glia expressing RPTP beta adhere and extend processes on substrates coated with Ng-CAM. After one day in culture, however, the glia retract their processes and often lift off the substrate. Conditioned medium from glial cells, which contains large amounts of phosphacan, inhibits glial adhesion to Ng-CAM, and depletion of phosphacan from the conditioned medium by immunoadsorption reduces the inhibitory activity. The results show that phosphacan increases dramatically during development, and indicate that secreted forms of RPTP beta can modulate glial cell adhesion and behavior.

The carbonic anhydrase domain of receptor tyrosine phosphatase beta is a functional ligand for the axonal cell recognition molecule contactin

Receptor-type protein tyrosine phosphatase beta (RPTP beta) is expressed in the developing nervous system and contains a carbonic anhydrase (CAH) domain as well as a fibronectin type III repeat in its extracellular domain. Fusion proteins containing these domains were used to search for ligands of RPTP beta. The CAH domain bound specifically to a 140 kDa protein expressed on the surface of neuronal cells. Expression cloning in COS7 cells revealed that this protein is contactin, a GPI membrane-anchored neuronal cell recognition molecule. The CAH domain of RPTP beta induced cell adhesion and neurite growth of primary tectal neurons, and differentiation of neuroblastoma cells. These responses were blocked by antibodies against contactin, demonstrating that contactin is a neuronal receptor for RPTP beta. These experiments show that an individual domain of RPTP beta acts as a functional ligand for the neuronal receptor contactin. The interaction between contactin and RPTP beta may generate unidirectional or bidirectional signals during neural development.

Seven protein tyrosine phosphatases are differentially expressed in the developing rat brain

Regulation of protein function through tyrosine phosphorylation is critical in the control of many developmental processes, such as cellular proliferation and differentiation. Growing evidence suggests that tyrosine phosphorylation also regulates key events in neural development. Although a large body of data has demonstrated that protein tyrosine kinases play an important role in neural development, much less is known about their counterparts, protein tyrosine phosphatases (PTPases). Using polymerase chain reaction (PCR) with degenerate primers and a neonatal rat cortex cDNA library, we have identified seven PTPases expressed in the developing rat brain. Four of these are transmembrane PTPases: LAR, LRP, RPTP gamma, and CPTP1. Three are nonreceptor PTPases: PTP-1, P19-PTP, and SHP. Northern hybridization analysis demonstrates that only CPTP1 is preferentially expressed in neural tissues, whereas the others are found abundantly in nonneural tissues as well as in the brain. Within the embryonic and early postnatal brain, the seven PTPases have overlapping, yet unique, distributions. For example, LAR mRNA is highly expressed by both proliferating and postmitotic cells in the cerebral cortex at embryonic day 17 and in all layers of the cortex at postnatal day 4. In contrast, RPTP gamma mRNA is expressed by postmitotic neurons in the embryo and predominantly by neurons in the superficial layers of the postnatal cortex. Several of the PTPases examined here are expressed at very high levels in the embryonic cortical plate and postnatal neocortex, including the subplate and subventricular zone. The spatial and temporal regulation of PTPase gene expression suggests that these PTPases have important roles in signal transduction during early neuronal differentiation and neural development.

Molecular cloning and tissue-specific RNA processing of a murine receptor-type protein tyrosine phosphatase

The molecular cloning of a murine receptor-type protein tyrosine phosphatase, termed PTP NU-3, with an extracellular cell-adhesion-molecule-like domain is reported. NU-3 was isolated from 11.5-day total mouse embryonic RNA by reverse-transcriptase PCR using degenerate oligonucleotides flanking the conserved protein tyrosine phosphatase catalytic domain. This produced a 280-bp DNA probe which was subsequently employed to screen a mouse embryonic kidney library. Several overlapping cDNA clones were isolated, collectively forming a cDNA of 6.0 kb that encodes a putative 211-kDa protein. Northern-blot analysis of total RNA from adult and embryonic mouse tissues indicates the existence of two major PTP NU-3 transcripts of approximately 6 kb and 7 kb. Both messages are expressed predominantly in brain tissues and neuronal-derived cell lines, although detectable levels of the 7-kb message were found in other non-neuronal tissues. We have identified a unique 132-bp exon segment that is present in the 7-kb message but is completely absent in the 6-kb transcript, suggesting tissue-specific levels of expression and RNA processing. Analysis of the amino acid sequence encoded by the 132-bp segment reveals that it completes a partial fibronectin type-III element resulting in a protein with a total of nine such elements. Bacterial expression of the two catalytic domains demonstrated that only the first domain possesses enzymic activity towards a tyrosine phosphorylated substrate.