Cloning, expression and chromosomal localization of a new putative receptor-like protein tyrosine phosphatase

Regulation of Endothelial Adherens Junctions by Tyrosine Phosphorylation

Endothelial cells form a semipermeable, regulated barrier that limits the passage of fluid, small molecules, and leukocytes between the bloodstream and the surrounding tissues. The adherens junction, a major mechanism of intercellular adhesion, is comprised of transmembrane cadherins forming homotypic interactions between adjacent cells and associated cytoplasmic catenins linking the cadherins to the cytoskeleton. Inflammatory conditions promote the disassembly of the adherens junction and a loss of intercellular adhesion, creating openings or gaps in the endothelium through which small molecules diffuse and leukocytes transmigrate. Tyrosine kinase signaling has emerged as a central regulator of the inflammatory response, partly through direct phosphorylation and dephosphorylation of the adherens junction components. This review discusses the findings that support and those that argue against a direct effect of cadherin and catenin phosphorylation in the disassembly of the adherens junction. Recent findings indicate a complex interaction between kinases, phosphatases, and the adherens junction components that allow a fine regulation of the endothelial permeability to small molecules, leukocyte migration, and barrier resealing.

A protease storm cleaves a cell-cell adhesion molecule in cancer: multiple proteases converge to regulate PTPmu in glioma cells

Cleavage of the cell-cell adhesion molecule, PTPµ, occurs in human glioblastoma multiforme brain tumor tissue and glioma cell lines. PTPµ cleavage is linked to increased cell motility and growth factor independent survival of glioma cells in vitro. Previously, PTPµ was shown to be cleaved by furin in the endoplasmic reticulum to generate membrane associated E- (extracellular) and P- (phosphatase) subunits, and by ADAMs and the gamma secretase complex at the plasma membrane. We also identified the presence of additional extracellular and intracellular PTPµ fragments in brain tumors. We set out to biochemically analyze PTPµ cleavage in cancer cells. We determined that, in addition to the furin-processed form of PTPµ, a pool of 200 kDa full-length PTPµ exists at the plasma membrane that is cleaved directly by ADAM to generate a larger shed form of the PTPµ extracellular segment. Notably, in glioma cells, full-length PTPµ is also subject to calpain cleavage, which generates novel PTPµ fragments not found in other immortalized cells. We also observed glycosylation and phosphorylation differences in the cancer cells. Our data suggest that an additional serine protease also contributes to PTPµ shedding in glioma cells. We hypothesize that a "protease storm" occurs in cancer cells whereby multiple proteases converge to reduce the presence of cell-cell adhesion molecules at the plasma membrane and to generate protein fragments with unique biological functions. As a consequence, the "protease storm" could promote the migration and invasion of tumor cells.

Regulation of development and cancer by the R2B subfamily of RPTPs and the implications of proteolysis

The initial cloning of receptor protein tyrosine phosphatases (RPTPs) was met with excitement because of their hypothesized function in counterbalancing receptor tyrosine kinase signaling. In recent years, members of a subfamily of RPTPs with homophilic cell-cell adhesion capabilities, known as the R2B subfamily, have been shown to have functions beyond that of counteracting tyrosine kinase activity, by independently influencing cell signaling in their own right and by regulating cell adhesion. The R2B subfamily is composed of four members: PTPmu (PTPRM), PTPrho (PTPRT), PTPkappa (PTPRK), and PCP-2 (PTPRU). The effects of this small subfamily of RPTPs is far reaching, influencing several developmental processes and cancer. In fact, R2B RPTPs are predicted to be tumor suppressors and are among the most frequently mutated protein tyrosine phosphatases (PTPs) in cancer. Confounding these conclusions are more recent studies suggesting that proteolysis of the full-length R2B RPTPs result in oncogenic extracellular and intracellular protein fragments. This review discusses the current knowledge of the role of R2B RPTPs in development and cancer, with special detail given to the mechanisms and implications that proteolysis has on R2B RPTP function. We also touch upon the concept of exploiting R2B proteolysis to develop cancer imaging tools, and consider the effects of R2B proteolysis on axon guidance, perineural invasion and collective cell migration.

Inactivation of the catalytic phosphatase domain of PTPRT/RPTPρ increases social interaction in mice

Receptor protein tyrosine phosphatase rho (RPTPρ, gene symbol PTPRT) is a transmembrane protein expressed at high levels in the developing hippocampus, olfactory bulb, cortex, and cerebellum. It has an extracellular domain that interacts with other cell adhesion molecules, and it has two intracellular phosphatase domains, one of which is catalytically active. In a recent genome-wide association study, PTPRT was identified as a potential candidate gene for autism spectrum disorder (ASD) susceptibility. Mutation of a critical aspartate to alanine (D1046A) in the PTPRT catalytic domain inactivates phosphatase function but retains substrate binding. We have generated a knockin mouse line carrying the PTPRT D1046A mutation. The D1046A mutation in homozygous knockin mice did not significantly change locomotor activities or anxiety-related behaviors. In contrast, male homozygous mice had significantly higher social approach scores than wild-type animals. Our results suggest that PTPRT phosphatase function is important in modulating neural pathways involved in mouse social behaviors relevant to the symptoms in human ASD patients.

Single cell molecular recognition of migrating and invading tumor cells using a targeted fluorescent probe to receptor PTPmu

Detection of an extracellular cleaved fragment of a cell-cell adhesion molecule represents a new paradigm in molecular recognition and imaging of tumors. We previously demonstrated that probes that recognize the cleaved extracellular domain of receptor protein tyrosine phosphatase mu (PTPmu) label human glioblastoma brain tumor sections and the main tumor mass of intracranial xenograft gliomas. In this article, we examine whether one of these probes, SBK2, can label dispersed glioma cells that are no longer connected to the main tumor mass. Live mice with highly dispersive glioma tumors were injected intravenously with the fluorescent PTPmu probe to test the ability of the probe to label the dispersive glioma cells in vivo. Analysis was performed using a unique three-dimensional (3D) cryo-imaging technique to reveal highly migratory and invasive glioma cell dispersal within the brain and the extent of colabeling by the PTPmu probe. The PTPmu probe labeled the main tumor site and dispersed cells up to 3.5 mm away. The cryo-images of tumors labeled with the PTPmu probe provide a novel, high-resolution view of molecular tumor recognition, with excellent 3D detail regarding the pathways of tumor cell migration. Our data demonstrate that the PTPmu probe recognizes distant tumor cells even in parts of the brain where the blood-brain barrier is likely intact. The PTPmu probe has potential translational significance for recognizing tumor cells to facilitate molecular imaging, a more complete tumor resection and to serve as a molecular targeting agent to deliver chemotherapeutics to the main tumor mass and distant dispersive tumor cells.

Receptor type protein tyrosine phosphatases (RPTPs) - roles in signal transduction and human disease

Protein tyrosine phosphorylation is a fundamental regulatory mechanism controlling cell proliferation, differentiation, communication, and adhesion. Disruption of this key regulatory mechanism contributes to a variety of human diseases including cancer, diabetes, and auto-immune diseases. Net protein tyrosine phosphorylation is determined by the dynamic balance of the activity of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Mammals express many distinct PTKs and PTPs. Both of these families can be sub-divided into non-receptor and receptor subtypes. Receptor protein tyrosine kinases (RPTKs) comprise a large family of cell surface proteins that initiate intracellular tyrosine phosphorylation-dependent signal transduction in response to binding of extracellular ligands, such as growth factors and cytokines. Receptor-type protein tyrosine phosphatases (RPTPs) are enzymatic and functional counterparts of RPTKs. RPTPs are a family of integral cell surface proteins that possess intracellular PTP activity, and extracellular domains that have sequence homology to cell adhesion molecules. In comparison to extensively studied RPTKs, much less is known about RPTPs, especially regarding their substrate specificities, regulatory mechanisms, biological functions, and their roles in human diseases. Based on the structure of their extracellular domains, the RPTP family can be grouped into eight sub-families. This article will review one representative member from each RPTP sub-family.

Receptor protein tyrosine phosphatases and cancer: new insights from structural biology

There is general agreement that many cancers are associated with aberrant phosphotyrosine signaling, which can be caused by the inappropriate activities of tyrosine kinases or tyrosine phosphatases. Furthermore, incorrect activation of signaling pathways has been often linked to changes in adhesion events mediated by cell surface receptors. Among these receptors, receptor protein tyrosine phosphatases (RPTPs) both antagonize tyrosine kinases as well as engage extracellular ligands. A recent wealth of data on this intriguing family indicates that its members can fulfill either tumor suppressing or oncogenic roles. The interpretation of these results at a molecular level has been greatly facilitated by the recent availability of structural information on the extra- and intracellular regions of RPTPs. These structures provide a molecular framework to understand how alterations in extracellular interactions can inactivate RPTPs in cancers or why the overexpression of certain RPTPs may also participate in tumor progression.

Diverse injurious stimuli reduce protein tyrosine phosphatase-μ expression and enhance epidermal growth factor receptor signaling in human airway epithelia

In response to injury, airway epithelia utilize an epidermal growth factor (EGF) receptor (EGFR) signaling program to institute repair and restitution. Protein tyrosine phosphatases (PTPs) counterregulate EGFR autophosphorylation and downstream signaling. PTPμ is highly expressed in lung epithelia and can be localized to intercellular junctions where its ectodomain homophilically interacts with PTPμ ectodomain expressed on neighboring cells. We asked whether PTPμ expression might be altered in response to epithelial injury and whether altered PTPμ expression might influence EGFR signaling. In A549 cells, diverse injurious stimuli dramatically reduced PTPμ protein expression. Under basal conditions, small interfering RNA (siRNA)-induced silencing of PTPμ increased EGFR Y992 and Y1068 phosphorylation. In the presence of EGF, PTPμ knockdown increased EGFR Y845, Y992, Y1045, Y1068, Y1086, and Y1173 but not Y1148 phosphorylation. Reduced PTPμ expression increased EGF-stimulated phosphorylation of Y992, a docking site for phospholipase C (PLC)γ(1), activation of PLCγ(1) itself, and increased cell migration in both wounding and chemotaxis assays. In contrast, overexpression of PTPμ decreased EGF-stimulated EGFR Y992 and Y1068 phosphorylation. Therefore, airway epithelial injury profoundly reduces PTPμ expression, and PTPμ depletion selectively increases phosphorylation of specific EGFR tyrosine residues, PLCγ(1) activation, and cell migration, providing a novel mechanism through which epithelial integrity may be restored.

Characterization of the adhesive properties of the type IIb subfamily receptor protein tyrosine phosphatases

Receptor protein tyrosine phosphatases (RPTPs) have cell adhesion molecule-like extracellular domains coupled to cytoplasmic tyrosine phosphatase domains. PTPmu is the prototypical member of the type IIb subfamily of RPTPs, which includes PTPrho, PTPkappa, and PCP-2. The authors performed the first comprehensive analysis of the subfamily in one system, examining adhesion and antibody recognition. The authors evaluated if antibodies that they developed to detect PTPmu also recognized other subfamily members. Notably, each antibody recognizes distinct subsets of type IIb RPTPs. PTPmu, PTPrho, and PTPkappa have all been shown to mediate cell-cell aggregation, and prior work with PCP-2 indicated that it can mediate bead aggregation in vitro. This study reveals that PCP-2 is unique among the type IIb RPTPs in that it does not mediate cell-cell aggregation via homophilic binding. The authors conclude from these experiments that PCP-2 is likely to have a distinct biological function other than cell-cell aggregation.

Proteolytic cleavage of protein tyrosine phosphatase mu regulates glioblastoma cell migration

Glioblastoma multiforme (GBM), the most common malignant primary brain tumor, represents a significant disease burden. GBM tumor cells disperse extensively throughout the brain parenchyma, and the need for tumor-specific drug targets and pharmacologic agents to inhibit cell migration and dispersal is great. The receptor protein tyrosine phosphatase mu (PTPmu) is a homophilic cell adhesion molecule. The full-length form of PTPmu is down-regulated in human glioblastoma. In this article, overexpression of full-length PTPmu is shown to suppress migration and survival of glioblastoma cells. Additionally, proteolytic cleavage is shown to be the mechanism of PTPmu down-regulation in glioblastoma cells. Proteolysis of PTPmu generates a series of proteolytic fragments, including a soluble catalytic intracellular domain fragment that translocates to the nucleus. Only proteolyzed PTPmu fragments are detected in human glioblastomas. Short hairpin RNA-mediated down-regulation of PTPmu fragments decreases glioblastoma cell migration and survival. A peptide inhibitor of PTPmu function blocks fragment-induced glioblastoma cell migration, which may prove to be of therapeutic value in GBM treatment. These data suggest that loss of cell surface PTPmu by proteolysis generates catalytically active PTPmu fragments that contribute to migration and survival of glioblastoma cells.

Receptor protein tyrosine phosphatase micro: measuring where to stick

We review here recent results on the structure and function of a receptor protein tyrosine phosphatase, RPTPmicro. In addition to their intercellular catalytic domains which bear the phosphatase activity, the RPTPs are cell-surface-receptor-type molecules and in many cases have large extracellular regions. What role can these extracellular regions play in function? For RPTPmicro, the extracellular region is known to mediate homophilic adhesion. Sequence analysis indicates that it comprises six domains: an N-terminal MAM (meprin/A5/micro), one immunoglobulin-like domain and four fibronectin type III (FN) repeats. We have determined the crystal structure of the entire extracellular region for RPTPmicro in the form of a functional adhesion dimer. The physical characteristics and dimensions of the adhesion dimer suggest a mechanism by which the location of this phosphatase can be influenced by cell-cell spacings.

Intracellular substrates of brain-enriched receptor protein tyrosine phosphatase rho (RPTPrho/PTPRT)

Receptor protein tyrosine phosphatase rho (RPTPrho/PTPRT) is a transmembrane protein that is highly expressed in the developing and adult central nervous system. It is a member of the RPTP R2B subfamily, which includes PTPkappa, PTPmu and PCP-2. Glutathione-S-transferase (GST) pulldown assays were used to show that RPTPrho interacts with several adherens junctional proteins in brain, including E-cadherin, N-cadherin, VE-cadherin (cadherin-5), desmoglein, alpha, beta and gamma catenin, p120(ctn) and alpha-actinin. With the exception of E-cadherin and alpha-actinin, binding was considerably reduced at high sodium concentrations. Furthermore, immunoprecipitation phosphatase assays indicated that E-cadherin, and to a far lesser extent p120(ctn), were tyrosine dephosphorylated by a recombinant RPTPrho intracellular fragment, and thus, were likely to be primary substrates for RPTPrho. The interaction of RPTPrho with adherens junctional components suggests that this phosphatase may transduce extracellular signals to the actin cytoskeleton and thereby play a role in regulating cadherin-mediated cell adhesion in the central nervous system.

Identification of Brain Cell Death Associated Proteins in Human Post-mortem Cerebrospinal Fluid

Following any form of brain insult, proteins are released from damaged tissues into the cerebrospinal fluid (CSF). This body fluid is therefore an ideal sample to use in the search for biomarkers of neurodegenerative disorders and brain damage. In this study, we used human post-mortem CSF as a model of massive brain injury and cell death for the identification of such protein markers. Pooled post-mortem CSF samples were analyzed using a protocol that combined immunoaffinity depletion of abundant CSF proteins, off-gel electrophoresis, SDS-PAGE and protein identification by LC-MS/MS. A total of 299 proteins were identified, of which 172 proteins were not previously described to be present in CSF. Of these 172 proteins, more than 75% have been described as intracellular proteins suggesting that they were released from damaged cells. Immunoblots of a number of proteins were performed on individual post-mortem CSF samples and confirmed elevated concentrations in post-mortem CSF compared to ante-mortem CSF. Interestingly, among the proteins specifically identified in the post-mortem CSF, several have been previously described as biochemical markers of brain damage.

Molecular analysis of receptor protein tyrosine phosphatase mu-mediated cell adhesion

Type IIB receptor protein tyrosine phosphatases (RPTPs) are bi-functional cell surface molecules. Their ectodomains mediate stable, homophilic, cell-adhesive interactions, whereas the intracellular catalytic regions can modulate the phosphorylation state of cadherin/catenin complexes. We describe a systematic investigation of the cell-adhesive properties of the extracellular region of RPTPmu, a prototypical type IIB RPTP. The crystal structure of a construct comprising its N-terminal MAM (meprin/A5/mu) and Ig domains was determined at 2.7 A resolution; this assigns the MAM fold to the jelly-roll family and reveals extensive interactions between the two domains, which form a rigid structural unit. Structure-based site-directed mutagenesis, serial domain deletions and cell-adhesion assays allowed us to identify the four N-terminal domains (MAM, Ig, fibronectin type III (FNIII)-1 and FNIII-2) as a minimal functional unit. Biophysical characterization revealed at least two independent types of homophilic interaction which, taken together, suggest that there is the potential for formation of a complex and possibly ordered array of receptor molecules at cell contact sites.

The Receptor Protein-tyrosine Phosphatase PTPμ Interacts with IQGAP1

The receptor protein-tyrosine phosphatase PTPmu is a member of the Ig superfamily of cell adhesion molecules. The extracellular domain of PTPmu contains motifs commonly found in cell adhesion molecules. The intracellular domain of PTPmu contains two conserved catalytic domains, only the membrane-proximal domain has catalytic activity. The unique features of PTPmu make it an attractive molecule to transduce signals upon cell-cell contact. PTPmu has been shown to regulate cadherin-mediated cell adhesion, neurite outgrowth, and axon guidance. Protein kinase C is a component of the PTPmu signaling pathway utilized to regulate these events. To aid in the further characterization of PTPmu signaling pathways, we used a series of GST-PTPmu fusion proteins, including catalytically inactive and substrate trapping mutants, to identify PTPmu-interacting proteins. We identified IQGAP1, a known regulator of the Rho GTPases, Cdc42 and Rac1, as a novel PTPmu-interacting protein. We show that this interaction is due to direct binding. In addition, we demonstrate that amino acid residues 765-958 of PTPmu, which include the juxtamembrane domain and 35 residues of the first phosphatase domain, mediate the binding to IQGAP1. Furthermore, we demonstrate that constitutively active Cdc42, and to a lesser extent Rac1, enhances the interaction of PTPmu and IQGAP1. These data indicate PTPmu may regulate Rho-GTPase-dependent functions of IQGAP1 and suggest that IQGAP1 is a component of the PTPmu signaling pathway. In support of this, we show that a peptide that competes IQGAP1 binding to Rho GTPases blocks PTPmu-mediated neurite outgrowth.

Receptor protein tyrosine phosphatase micro regulates the paracellular pathway in human lung microvascular endothelia

The pulmonary vascular endothelial paracellular pathway and zonula adherens (ZA) integrity are regulated, in part, through protein tyrosine phosphorylation. ZA-associated protein tyrosine phosphatase (PTP)s are thought to counterregulate tyrosine phosphorylation events within the ZA multiprotein complex. One such receptor PTP, PTPmu, is highly expressed in lung tissue and is almost exclusively restricted to the endothelium. We therefore studied whether PTPmu, in pulmonary vascular endothelia, associates with and/or regulates both the tyrosine phosphorylation state of vascular endothelial (VE)-cadherin and the paracellular pathway. PTPmu was expressed in postconfluent human pulmonary artery and lung microvascular endothelial cells (ECs) where it was almost exclusively restricted to EC-EC boundaries. In human lung microvascular ECs, knockdown of PTPmu through RNA interference dramatically impaired barrier function. In immortalized human microvascular ECs, overexpression of wild-type PTPmu enhanced barrier function. PTPmu-VE-cadherin interactions were demonstrated through reciprocal co-immunoprecipitation assays and co-localization with double-label fluorescence microscopy. When glutathione S-transferase-PTPmu was incubated with purified recombinant VE-cadherin, and when glutathione S-transferase-VE-cadherin was incubated with purified recombinant PTPmu, PTPmu directly bound to VE-cadherin. Overexpression of wild-type PTPmu decreased tyrosine phosphorylation of VE-cadherin. Therefore, PTPmu is expressed in human pulmonary vascular endothelia where it directly binds to VE-cadherin and regulates both the tyrosine phosphorylation state of VE-cadherin and barrier integrity.

Impaired flow-induced dilation in mesenteric resistance arteries from receptor protein tyrosine phosphatase-mu-deficient mice

The transmembrane receptor-like protein tyrosine phosphatase-mu (RPTPmu) is thought to play an important role in cell-cell adhesion-mediated processes. We recently showed that RPTPmu is predominantly expressed in the endothelium of arteries and not in veins. Its involvement in the regulation of endothelial adherens junctions and its specific arterial expression suggest that RPTPmu plays a role in controlling arterial endothelial cell function and vascular tone. To test this hypothesis, we analyzed myogenic responsiveness, flow-induced dilation, and functional integrity of mesenteric resistance arteries from RPTPmu-deficient (RPTPmu(-/-)) mice and from wild-type littermates. Here, we show that cannulated mesenteric arteries from RPTPmu(-/-) mice display significantly decreased flow-induced dilation. In contrast, mechanical properties, myogenic responsiveness, responsiveness to the vasoconstrictors phenylephrine or U-46619, and responsiveness to the endothelium-dependent vasodilators methacholine or bradykinin were similar in both groups. Our results imply that RPTPmu is involved in the mechanotransduction or accessory signaling pathways that control shear stress responses in mesenteric resistance arteries.

The Conserved Immunoglobulin Domain Controls the Subcellular Localization of the Homophilic Adhesion Receptor Protein-tyrosine Phosphatase μ

The receptor protein-tyrosine phosphatase mu (PTPmu) is a homophilic adhesion protein thought to regulate cell-cell adhesion in the vascular endothelium through dephosphorylation of cell junction proteins. In subconfluent cell cultures, PTPmu resides in an intracellular membrane pool; however, as culture density increases and cell contacts form, the phosphatase localizes to sites of cell-cell contact, and its expression level increases. These characteristics of PTPmu, which are consistent with a role in cell-cell adhesion, suggest that control of subcellular localization is an important mechanism to regulate the function of this phosphatase. To gain a better understanding of how PTPmu is regulated, we examined the importance of the conserved immunoglobulin domain, containing the homophilic binding site, in control of the localization of the enzyme. Deletion of the immunoglobulin domain impaired localization of PTPmu to the cell-cell contacts in endothelial and epithelial cells. In addition, deletion of the immunoglobulin domain affected the distribution of PTPmu in subconfluent endothelial cells when homophilic binding to another PTPmu molecule on an apposing cell was not possible, resulting in an accumulation of the mutant phosphatase at the cell surface with a concentration at the cell periphery in the region occupied by focal adhesions. This aberrant localization correlated with reduced survival and alterations in normal focal adhesion and cytoskeleton morphology. This study therefore illustrates the critical role of the immunoglobulin domain in regulation of the localization of PTPmu and the importance of such control for the maintenance of normal cell physiology.

Casein Kinase II Alpha Subunit and C1-Inhibitor Are Independent Predictors of Outcome in Patients with Squamous Cell Carcinoma of the Lung

PURPOSE

Gene expression profiling has been shown to be a valuable tool for prognostication and identification of cancer-associated genes in human malignancies. We aimed to identify potential prognostic marker(s) in non-small cell lung cancers using global gene expression profiles.

EXPERIMENTAL DESIGN

Twenty-one previously untreated patients with non-small cell lung cancer were analyzed using the Affymetrix GeneChip high-density oligonucleotide array and comparative genomic hybridization. Identified candidate genes were validated in an independent cohort of 45 patients using quantitative real-time reverse transcription-PCR and Western blot analyses. Follow-up data for these patients was collected and used to assess outcome correlations.

RESULTS

Hierarchical clustering analysis yielded three distinct subgroups based on gene expression profiling. Cluster I consisted of 4 patients with adenocarcinoma and 1 with squamous cell carcinoma (squamous cell carcinoma); clusters II and III consisted of 6 and 10 patients with squamous cell carcinoma, respectively. Outcome analysis was performed on the cluster groups containing solely squamous cell carcinoma, revealing significant differences in disease-specific survival rates. Moreover, patients having a combination of advanced Tumor-Node-Metastasis stage and assigned to the poor prognosis cluster group (cluster II) had significantly poorer outcomes. Comparative genomic hybridization analysis showed recurrent chromosomal losses at 1p, 3p, 17, 19, and 22 and gains/amplifications at 3q, 5p, and 8q, which did not vary significantly between the cluster groups. We internally and externally validated a subset of 11 cluster II (poor prognosis)-specific genes having corresponding chromosomal aberrations identified by comparative genomic hybridization as prognostic markers in an independent cohort of patients with lung squamous cell carcinoma identifying CSNK2A1 and C1-Inh as independent predictors of outcome.

CONCLUSION

CSNK2A1 and C1-Inh are independent predictors of survival in lung squamous cell carcinoma patients and may be useful as prognostic markers.

The MAM (meprin/A5-protein/PTPmu) domain is a homophilic binding site promoting the lateral dimerization of receptor-like protein-tyrosine phosphatase mu

The MAM (meprin/A5-protein/PTPmu) domain is present in numerous proteins with diverse functions. PTPmu belongs to the MAM-containing subclass of protein-tyrosine phosphatases (PTP) able to promote cell-to-cell adhesion. Here we provide experimental evidence that the MAM domain is a homophilic binding site of PTPmu. We demonstrate that the MAM domain forms oligomers in solution and binds to the PTPmu ectodomain at the cell surface. The presence of two disulfide bridges in the MAM molecule was evidenced and their integrity was found to be essential for MAM homophilic interaction. Our data also indicate that PTPmu ectodomain forms oligomers and mediates the cellular adhesion, even in the absence of MAM domain homophilic binding. Reciprocally, MAM is able to interact homophilically in the absence of ectodomain trans binding. The MAM domain therefore contains independent cis and trans interaction sites and we predict that its main role is to promote lateral dimerization of PTPmu at the cell surface. This finding contributes to the understanding of the signal transduction mechanism in MAM-containing PTPs.

Genomic structure and alternative splicing of murine R2B receptor protein tyrosine phosphatases (PTPkappa, mu, rho and PCP-2)

Background

Four genes designated as PTPRK (PTPκ), PTPRL/U (PCP-2), PTPRM (PTPμ) and PTPRT (PTPρ) code for a subfamily (type R2B) of receptor protein tyrosine phosphatases (RPTPs) uniquely characterized by the presence of an N-terminal MAM domain. These transmembrane molecules have been implicated in homophilic cell adhesion. In the human, the PTPRK gene is located on chromosome 6, PTPRL/U on 1, PTPRM on 18 and PTPRT on 20. In the mouse, the four genes ptprk, ptprl, ptprm and ptprt are located in syntenic regions of chromosomes 10, 4, 17 and 2, respectively.

Results

The genomic organization of murine R2B RPTP genes is described. The four genes varied greatly in size ranging from ~64 kb to ~1 Mb, primarily due to proportional differences in intron lengths. Although there were also minor variations in exon length, the number of exons and the phases of exon/intron junctions were highly conserved. In situ hybridization with digoxigenin-labeled cRNA probes was used to localize each of the four R2B transcripts to specific cell types within the murine central nervous system. Phylogenetic analysis of complete sequences indicated that PTPρ and PTPμ were most closely related, followed by PTPκ. The most distant family member was PCP-2. Alignment of RPTP polypeptide sequences predicted putative alternatively spliced exons. PCR experiments revealed that five of these exons were alternatively spliced, and that each of the four phosphatases incorporated them differently. The greatest variability in genomic organization and the majority of alternatively spliced exons were observed in the juxtamembrane domain, a region critical for the regulation of signal transduction.

Conclusions

Comparison of the four R2B RPTP genes revealed virtually identical principles of genomic organization, despite great disparities in gene size due to variations in intron length. Although subtle differences in exon length were also observed, it is likely that functional differences among these genes arise from the specific combinations of exons generated by alternative splicing.

PTP mu-dependent growth cone rearrangement is regulated by Cdc42

PTP mu is expressed in the developing nervous system and promotes growth and guidance of chick retinal ganglion cells. Using a newly developed growth cone rearrangement assay, we examined whether the small G-proteins were involved in PTP mu-dependent signaling. The stimulation of retinal cultures with purified PTP mu resulted in a striking morphological change in the growth cone, which becomes dominated by filopodia within 5 min of addition. This rearrangement in response to PTP mu stimulation was mediated by homophilic binding. We perturbed GTPase signaling using Toxin B, which inhibits Cdc42, Rac, and Rho, as well as the toxin Exoenzyme C3 that inhibits Rho. The PTP mu-induced growth cone rearrangement was blocked by Toxin B, but not by Exoenzyme C3. This result suggests that either Cdc42 or Rac are required but not Rho. To determine which GTPase was involved in PTP mu signaling, we utilized dominant-negative mutants of Cdc42 and Rac. Dominant-negative Cdc42 blocked PTP mu-induced rearrangement, while wild-type Cdc42 and dominant-negative Rac did not. Together, these results suggest a molecular signaling cascade beginning with PTP mu homophilic binding at the plasma membrane and the activation of Cdc42, which acts on the actin cytoskeleton to result in rearrangement of the growth cone.

Gene expression profile of human lymphoid CEM cells sensitive and resistant to glucocorticoid-evoked apoptosis

Three closely related clones of leukemic lymphoid CEM cells were compared for their gene expression responses to the glucocorticoid dexamethasone (Dex). All three contained receptors for Dex, but only two responded by undergoing apoptosis. After a time of exposure to Dex that ended late in the interval preceding onset of apoptosis, gene microarray analyses were carried out. The results indicate that the expression of a limited, distinctive set of genes was altered in the two apoptosis-prone clones, not in the resistant clone. That clone showed altered expression of different sets of genes, suggesting that a molecular switch converted patterns of gene expression between the two phenotypes: apoptosis-prone and apoptosis-resistant. The results are consistent with the hypothesis that altered expression of a distinctive network of genes after glucocorticoid administration ultimately triggers apoptosis of leukemic lymphoid cells. The altered genes identified provide new foci for study of their role in cell death.

Structure, expression, and developmental function of early divergent forms of metalloproteinases in hydra

Metalloproteinases have a critical role in a broad spectrum of cellular processes ranging from the breakdown of extracellular matrix to the processing of signal transduction-related proteins. These hydrolytic functions underlie a variety of mechanisms related to developmental processes as well as disease states. Structural analysis of metalloproteinases from both invertebrate and vertebrate species indicates that these enzymes are highly conserved and arose early during metazoan evolution. In this regard, studies from various laboratories have reported that a number of classes of metalloproteinases are found in hydra, a member of Cnidaria, the second oldest of existing animal phyla. These studies demonstrate that the hydra genome contains at least three classes of metalloproteinases to include members of the 1) astacin class, 2) matrix metalloproteinase class, and 3) neprilysin class. Functional studies indicate that these metalloproteinases play diverse and important roles in hydra morphogenesis and cell differentiation as well as specialized functions in adult polyps. This article will review the structure, expression, and function of these metalloproteinases in hydra.

Dynamic Changes in the Expression of Protein Tyrosine Phosphatases During Preimplantation Mouse Development: Semi-Quantification by Real-Time PCR

Protein tyrosine phosphatase (PTP) expression was examined in preimplantation mouse embryos. We previously reported that SHP-2, LAR, PTPT9, SHP-1, and mRPTPB were expressed in preimplantation mouse embryos. Here, we examined changes in the expression levels of these PTPs during preimplantation development. cDNA was obtained by reverse transcription of embryo mRNA, amplified with 10 PCR cycles, and then subjected to real-time fluorescence-monitored PCR. Experiments with an mRNA dilution series revealed that the data obtained matched the quantities of mRNA used. The measurements obtained with real-time fluorescence-monitored PCR showed that the expression of each PTP mRNA changed dynamically, and that each had a different expression pattern. This suggests that PTPs are involved in the regulation of growth and differentiation during preimplantation development.

Protein tyrosine phosphatase-mu differentially regulates neurite outgrowth of nasal and temporal neurons in the retina

Cell adhesion molecules play an important role in the development of the visual system. The receptor-type protein tyrosine phosphatase, PTPmu is a cell adhesion molecule that mediates cell aggregation and may signal in response to adhesion. PTPmu is expressed in the chick retina during development and promotes neurite outgrowth from retinal ganglion cell (RGC) axons in vitro (Burden-Gulley and Brady-Kalnay, 1999). The axons of RGC neurons form the optic nerve, which is the sole output from the retina to the optic tectum in the chick. In this study, we observed that PTPmu expression in RGC axons occurs as a step gradient, with temporal axons expressing the highest level of PTPmu. PTPmu expression in the optic tectum occurred as a smooth descending gradient from anterior to posterior regions during development. Because temporal RGC axons innervate anterior tectal regions, PTPmu may regulate the formation of topographic projections to the tectum. In agreement with this hypothesis, a differential response of RGC neurites to a PTPmu substrate was also observed: RGCs of temporal retina were unable to extend neurites on PTPmu compared with neurites of nasal retina. When given a choice between PTPmu and a second substrate, the growth cones of temporal neurites clustered at the PTPmu border and stalled, thus avoiding additional growth on the PTPmu substrate. In contrast, PTPmu was permissive for growth of nasal neurites. Finally, application of soluble PTPmu to retinal cultures resulted in the collapse of temporal but not nasal growth cones. Therefore, PTPmu may specifically signal to temporal RGC axons to cease their forward growth after reaching the anterior tectum, thus allowing for subsequent innervation of deeper tectal layers.

Expression of the receptor protein-tyrosine phosphatase, PTPmu, restores E-cadherin-dependent adhesion in human prostate carcinoma cells

Normal prostate expresses the receptor protein-tyrosine phosphatase, PTPmu, whereas LNCaP prostate carcinoma cells do not. PTPmu has been shown previously to interact with the E-cadherin complex. LNCaP cells express normal levels of E-cadherin and catenins but do not mediate either PTPmu- or E-cadherin-dependent adhesion. Re-expression of PTPmu restored cell adhesion to PTPmu and to E-cadherin. A mutant form of PTPmu that is catalytically inactive was re-expressed, and it also restored adhesion to PTPmu and to E-cadherin. Expression of PTPmu-extra (which lacks most of the cytoplasmic domain) induced adhesion to PTPmu but not to E-cadherin, demonstrating a requirement for the presence of the intracellular domains of PTPmu to restore E-cadherin-mediated adhesion. We previously observed a direct interaction between the intracellular domain of PTPmu and RACK1, a receptor for activated protein kinase C (PKC). We demonstrate that RACK1 binds to both the catalytically active and inactive mutant form of PTPmu. In addition, we determined that RACK1 binds to the PKCdelta isoform in LNCaP cells. We tested whether PKC could be playing a role in the ability of PTPmu to restore E-cadherin-dependent adhesion. Activation of PKC reversed the adhesion of PTPmuWT-expressing cells to E-cadherin, whereas treatment of parental LNCaP cells with a PKCdelta-specific inhibitor induced adhesion to E-cadherin. Together, these studies suggest that PTPmu regulates the PKC pathway to restore E-cadherin-dependent adhesion via its interaction with RACK1.

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.

Genomic organization and alternative splicing of the human and mouse RPTPrho genes

BACKGROUND

Receptor protein tyrosine phosphatase rho (RPTPrho, gene symbol PTPRT) is a member of the type IIB RPTP family. These transmembrane molecules have been linked to signal transduction, cell adhesion and neurite extension. The extracellular segment contains MAM, Ig-like and fibronectin type III domains, and the intracellular segment contains two phosphatase domains. The human RPTPrho gene is located on chromosome 20q12-13.1, and the mouse gene is located on a syntenic region of chromosome 2. RPTPrho expression is restricted to the central nervous system.

RESULTS

The cloning of the mouse cDNA, identification of alternatively spliced exons, detection of an 8 kb 3'-UTR, and the genomic organization of human and mouse RPTPrho genes are described. The two genes are comprised of at least 33 exons. Both RPTPrho genes span over 1 Mbp and are the largest RPTP genes characterized. Exons encoding the extracellular segment through the intracellular juxtamembrane 'wedge' region are widely spaced, with introns ranging from 9.7 to 303.7 kb. In contrast, exons encoding the two phosphatase domains are more tightly clustered, with 15 exons spanning approximately 60 kb, and introns ranging in size from 0.6 kb to 13.1 kb. Phase 0 introns predominate in the intracellular, and phase 1 in the extracellular segment.

CONCLUSIONS

We report the first genomic characterization of a RPTP type IIB gene. Alternatively spliced variants may result in different RPTPrho isoforms. Our findings suggest that RPTPrho extracellular and intracellular segments originated as separate modular proteins that fused into a single transmembrane molecule during a later evolutionary period.

Cellular signaling by neural cell adhesion molecules of the immunoglobulin superfamily

Neural cell adhesion molecules (CAMs) of the immunoglobulin superfamily nucleate and maintain groups of cells at key sites during early development and in the adult. In addition to their adhesive properties, binding of CAMs can affect intracellular signaling. Their ability to influence developmental events, including cell migration, proliferation, and differentiation can therefore result both from their adhesive as well as their signaling properties. This review focuses on the two CAMs for which the most information is known, the neural CAM, N-CAM, and L1. N-CAM was the first CAM to be characterized and, therefore, has been studied extensively. The binding of N-CAM to cells leads to a number of signaling events, some of which result in changes in gene expression. Interest in L1 derives from the fact that mutations in its gene lead to human genetic diseases including mental retardation. Much is known about modifications of the L1 cytoplasmic domain and its interaction with cytoskeletal molecules. The study of CAM signaling mechanisms has been assay-dependent rather than molecule-dependent, with particular emphasis on assays of neurite outgrowth and gene expression, an emphasis that is maintained throughout the review. The signals generated following CAM binding that lead to alterations in cell morphology and gene expression have been linked directly in only a few cases. We also review information on other CAMs, giving special consideration to those that are anchored in the membrane by a phospholipid anchor. These proteins, including a form of N-CAM, are presumed to be localized in lipid rafts, membrane substructures that include distinctive subsets of cytoplasmic signaling molecules such as members of the src-family of nonreceptor protein tyrosine kinases. In the end, these studies may reveal that what CAMs do after they bind cells together may have as profound consequences for the cells as the adhesive interactions themselves. This area will therefore remain a rich ground for future studies.

Intramolecular interactions between the juxtamembrane domain and phosphatase domains of receptor protein-tyrosine phosphatase RPTPmu. Regulation of catalytic activity

RPTPmu is a receptor-like protein-tyrosine phosphatase (RPTP) whose ectodomain mediates homotypic cell-cell interactions. The intracellular part of RPTPmu contains a relatively long juxtamembrane domain (158 amino acids; aa) and two conserved phosphatase domains (C1 and C2). The membrane-proximal C1 domain is responsible for the catalytic activity of RPTPmu, whereas the membrane-distal C2 domain serves an unknown function. The regulation of RPTP activity remains poorly understood, although dimerization has been proposed as a general mechanism of inactivation. Using the yeast two-hybrid system, we find that the C1 domain binds to an N-terminal noncatalytic region in RPTPmu, termed JM (aa 803-955), consisting of a large part of the juxtamembrane domain (120 aa) and a small part of the C1 domain (33 aa). When co-expressed in COS cells, the JM polypeptide binds to both the C1 and the C2 domain. Strikingly, the isolated JM polypeptide fails to interact with either full-length RPTPmu or with truncated versions of RPTPmu that contain the JM region, consistent with the JM-C1 and JM-C2 interactions being intramolecular rather than intermolecular. Furthermore, we find that large part of the juxtamembrane domain (aa 814-922) is essential for C1 to be catalytically active. Our findings suggest a model in which RPTPmu activity is regulated by the juxtamembrane domain undergoing intramolecular interactions with both the C1 and C2 domain.

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.

Receptor protein-tyrosine phosphatase RPTPmu binds to and dephosphorylates the catenin p120(ctn)

RPTPmu is a prototypic receptor-like protein-tyrosine phosphatase (RPTP) that mediates homotypic cell-cell interactions. Intracellularly, RPTPmu consists of a relatively large juxtamembrane region and two phosphatase domains, but little is still known about its substrate(s). Here we show that RPTPmu associates with the catenin p120(ctn), a tyrosine kinase substrate and an interacting partner of cadherins. No interaction is detectable between RPTPmu and beta-catenin. Furthermore, we show that tyrosine-phosphorylated p120(ctn) is dephosphorylated by RPTPmu both in vitro and in intact cells. Complex formation between RPTPmu and p120(ctn) does not require tyrosine phosphorylation of p120(ctn). Mutational analysis reveals that both the juxtamembrane region and the second phosphatase domain of RPTPmu are involved in p120(ctn) binding. The RPTPmu-interacting domain of p120(ctn) maps to its unique N terminus, a region distinct from the cadherin-interacting domain. A mutant form of p120(ctn) that fails to bind cadherins can still associate with RPTPmu. Our findings indicate that RPTPmu interacts with p120(ctn) independently of cadherins, and they suggest that this interaction may serve to control the tyrosine phosphorylation state of p120(ctn) at sites of cell-cell contact.

RPTPmu and protein tyrosine phosphorylation regulate K(+) channel mRNA expression in adult cardiac myocytes

Previously, we reported that cell-cell contact regulates K(+) channel mRNA expression in cultured adult rat cardiac myocytes. Here we show that exposing cardiac myocytes to tyrosine kinase inhibitors (genistein, tyrphostin A25), but not inactive analogs, prevents downregulation of Kv1.5 mRNA and upregulation of Kv4.2 mRNA normally observed when they are cultured under low-density conditions. Furthermore, cardiac myocytes cocultured with cells that endogenously (Mv 1 Lu) or heterologously (Chinese hamster ovary cells) express the receptor-type protein tyrosine phosphatase mu (RPTPmu) display Kv1.5 mRNA levels paralleling that which was observed in myocytes cultured under high-density conditions and in intact tissue. In contrast, myocytes cocultured with control cells failed to produce this response. Finally, it is shown that Kv4.2 mRNA expression is unaffected by RPTPmu. These findings reveal that multiple tyrosine phosphorylation-dependent mechanisms control cardiac myocyte K(+) channel genes. Furthermore, we conclude that RPTPmu specifically regulates cardiac myocyte Kv1.5 mRNA expression. Thus this receptor protein tyrosine phosphatase may be important in responses to pathological conditions associated with the loss of cell-cell interactions in the heart.

Identification and characterization of hydra metalloproteinase 2 (HMP2): a meprin-like astacin metalloproteinase that functions in foot morphogenesis

Several members of the newly emerging astacin metalloproteinase family have been shown to function in a variety of biological events, including cell differentiation and morphogenesis during both embryonic development and adult tissue differentiation. We have characterized a new astacin proteinase, hydra metalloproteinase 2 (HMP2) from the Cnidarian, Hydra vulgaris. HMP2 is translated from a single mRNA of 1.7 kb that contains a 1488 bp open reading frame encoding a putative protein product of 496 amino acids. The overall structure of HMP2 most closely resembles that of meprins, a subgroup of astacin metalloproteinases. The presence of a transient signal peptide and a putative prosequence indicates that HMP2 is a secreted protein that requires post-translational processing. The mature HMP2 starts with an astacin proteinase domain that contains a zinc binding motif characteristic of the astacin family. Its COOH terminus is composed of two potential protein-protein interaction domains: an “MAM” domain (named after meprins, A-5 protein and receptor protein tyrosine phosphatase mu) that is only present in meprin-like astacin proteinases; and a unique C-terminal domain (TH domain) that is also present in another hydra metalloproteinase, HMP1, in Podocoryne metalloproteinase 1 (PMP1) of jellyfish and in toxins of sea anemone. The spatial expression pattern of HMP2 was determined by both mRNA whole-mount in situ hybridization and immunofluorescence studies. Both morphological techniques indicated that HMP2 is expressed only by the cells in the endodermal layer of the body column of hydra. While the highest level of HMP2 mRNA expression was observed at the junction between the body column and the foot process, immunofluorescence studies indicated that HMP2 protein was present as far apically as the base of the tentacles. In situ analysis also indicated expression of HMP2 during regeneration of the foot process. To test whether the higher levels of HMP2 mRNA expression at the basal pole related to processes underlying foot morphogenesis, antisense studies were conducted. Using a specialized technique named localized electroporation (LEP), antisense constructs to HMP2 were locally introduced into the endodermal layer of cells at the basal pole of polyps and foot regeneration was initiated and monitored. Treatment with antisense to HMP2 inhibited foot regeneration as compared to mismatch and sense controls. These functional studies in combination with the fact that HMP2 protein was expressed not only at the junction between the body column and the foot process, but also as far apically as the base of the tentacles, suggest that this meprin-class metalloproteinase may be multifunctional in hydra.

The receptor tyrosine phosphatase CRYPalpha promotes intraretinal axon growth

Retinal ganglion cell axons grow towards the optic fissure in close contact with the basal membrane, an excellent growth substratum. One of the ligands of receptor tyrosine phosphatase CRYPalpha is located on the retinal and tectal basal membranes. To analyze the role of this RPTP and its ligand in intraretinal growth and guidance of ganglion cell axons, we disrupted ligand- receptor interactions on the retinal basal membrane in culture. Antibodies against CRYPalpha strongly reduced retinal axon growth on the basal membrane, and induced a dramatic change in morphology of retinal growth cones, reducing the size of growth cone lamellipodia. A similar effect was observed by blocking the ligand with a CRYPalpha ectodomain fusion protein. These effects did not occur, or were much reduced, when axons were grown either on laminin-1, on matrigel or on basal membranes with glial endfeet removed. This indicates that a ligand for CRYPalpha is located on glial endfeet. These results show for the first time in vertebrates that the interaction of a receptor tyrosine phosphatase with its ligand is crucial not only for promotion of retinal axon growth but also for maintenance of retinal growth cone lamellipodia on basal membranes.

The Receptor Protein Tyrosine Phosphatase, PTP-RO, Is Upregulated During Megakaryocyte Differentiation and Is Associated With the c-Kit Receptor

We have recently isolated a cDNA encoding a novel human receptor-type tyrosine phosphatase, termed PTP-RO (for a protein tyrosine phosphatase receptor omicron), from 5-fluorouracil–treated murine bone marrow cells. PTP-RO is a human homologue of murine PTPλ and is related to the homotypically adhering κ and μ receptor-type tyrosine phosphatases. PTP-RO is expressed in human megakaryocytic cell lines, primary bone marrow megakaryocytes, and stem cells. PTP-RO mRNA and protein expression are upregulated upon phorbol 12-myristate 13-acetate (PMA) treatment of the megakaryocytic cell lines CMS, CMK, and Dami. To elucidate the function of PTP-RO in megakaryocytic cells and its potential involvement in the stem cell factor (SCF)/c-Kit receptor pathway, COS-7 and 293 cells were cotransfected with the cDNAs of both the c-Kit tyrosine kinase receptor and PTP-RO. PTP-RO was found to be associated with the c-Kit receptor in these transfected cells and the SCF/Kit ligand induced a rapid tyrosine phosphorylation of PTP-RO. Interestingly, these transfected cells demonstrated a decrease in their proliferative response to the SCF/Kit ligand. In addition, we assessed the association of PTP-RO with c-Kit in vivo. The results demonstrated that PTP-RO associates with c-Kit but not with the tyrosine kinase receptor FGF-R and that PTP-RO is tyrosine-phosphorylated after SCF stimulation of Mo7e and CMK cells. Antisense oligonucleotides directed against PTP-RO mRNA sequences significantly inhibited megakaryocyte progenitor proliferation. Therefore, these data show that the novel tyrosine kinase phosphatase PTP-RO is involved in megakaryocytopoiesis and that its function is mediated by the SCF/c-Kit pathway.

The Receptor Protein Tyrosine Phosphatase, PTP-RO, Is Upregulated During Megakaryocyte Differentiation and Is Associated With the c-Kit Receptor

We have recently isolated a cDNA encoding a novel human receptor-type tyrosine phosphatase, termed PTP-RO (for a protein tyrosine phosphatase receptor omicron), from 5-fluorouracil–treated murine bone marrow cells. PTP-RO is a human homologue of murine PTPλ and is related to the homotypically adhering κ and μ receptor-type tyrosine phosphatases. PTP-RO is expressed in human megakaryocytic cell lines, primary bone marrow megakaryocytes, and stem cells. PTP-RO mRNA and protein expression are upregulated upon phorbol 12-myristate 13-acetate (PMA) treatment of the megakaryocytic cell lines CMS, CMK, and Dami. To elucidate the function of PTP-RO in megakaryocytic cells and its potential involvement in the stem cell factor (SCF)/c-Kit receptor pathway, COS-7 and 293 cells were cotransfected with the cDNAs of both the c-Kit tyrosine kinase receptor and PTP-RO. PTP-RO was found to be associated with the c-Kit receptor in these transfected cells and the SCF/Kit ligand induced a rapid tyrosine phosphorylation of PTP-RO. Interestingly, these transfected cells demonstrated a decrease in their proliferative response to the SCF/Kit ligand. In addition, we assessed the association of PTP-RO with c-Kit in vivo. The results demonstrated that PTP-RO associates with c-Kit but not with the tyrosine kinase receptor FGF-R and that PTP-RO is tyrosine-phosphorylated after SCF stimulation of Mo7e and CMK cells. Antisense oligonucleotides directed against PTP-RO mRNA sequences significantly inhibited megakaryocyte progenitor proliferation. Therefore, these data show that the novel tyrosine kinase phosphatase PTP-RO is involved in megakaryocytopoiesis and that its function is mediated by the SCF/c-Kit pathway.

Transcriptional regulation of a receptor protein tyrosine phosphatase gene hPTP-J by PKC-mediated signaling pathways in Jurkat and Molt-4 T lymphoma cells

The recently cloned type II receptor protein tyrosine phosphatase (RPTP) gene hPTP-J is a new member of the MAM (meprin, A5, PTPmicro) domain subfamily. We previously reported that hPTP-J mRNA was detected significantly in Jurkat T lymphoma cells and its expression was completely down-regulated by phorbol myristate acetate (PMA). In this study, we investigated what signaling pathways/molecules are involved in the transcriptional regulation of hPTP-J expression in Jurkat and Molt-4 T cell lines. The hPTP-J transcription was transiently up-regulated 20 min after the addition of PMA (20 ng/ml) to the Jurkat culture, followed by the complete down-regulation in 8 h after PMA addition. The transient up-regulation and the complete down-regulation induced by PMA was blocked by a PKC-specific inhibitor, GF109203X, suggesting that the regulatory effect of PMA on the hPTP-J transcription depends on protein kinase C activation. hPTP-J transcription was down-regulated not only by PMA but also by several signaling modulators including 1-oleoyl-2-acetylglycerol, forskolin, orthovanadate, manumycin and okadaic acid. Therefore, several signaling molecules such as protein tyrosine phosphatases, PP2A/CaMKIV and Ras are required for hPTP-J transcription in Jurkat and Molt-4 cells.

Protein tyrosine dephosphorylation and the maintenance of cell adhesions in the pancreas

Cell-cell contacts are important regulatory elements in tissue development, organ morphogenesis and malignant tumor invasion. In recent in vivo studies we have identified the members of the cadherin/catenin family of cell adhesion proteins that are differentially expressed in the pancreas and have determined their cell biological dynamics during dissociation and repair of adherens junctions. To further characterize these events, epithelial cell culture systems were used and a number of type II protein tyrosine phosphatases (PTPs) were found to colocalize and interact with the cadherin/catenin complex. These observations suggest that tyrosine dephosphorylation in general and PTPs in particular are involved in cell contact formation. Our most recent experiments indicate 1) that inhibition of PTPs alone dissociates pancreatic adherens junctions, 2) that cytosolic and transmembrane PTPs are differentially expressed in acinar cells, and 3) that a subset of them can associate with proteins of the cadherin/catenin complex at pancreatic cell-cell adhesions.

Receptor-type protein-tyrosine phosphatase mu is expressed in specific vascular endothelial beds in vivo

We investigated the localization of receptor-type protein-tyrosine phosphatase mu (RPTPmu) in tissues by immunofluorescence. RPTPmu immunoreactivity was found almost exclusively within vascular endothelial cells. RPTPmu was more abundant in the arterial tree than in the venous circulation. This pattern of expression was opposite to that of the von Willebrand factor and demonstrated a lack of difference in expression of VE-cadherin. RPTPmu was undetectable in the endocardium. In agreement with previous work on nonendothelial cell lines, RPTPmu was exclusively at the lateral aspects of endothelial cells in vivo and at cell-cell contacts as well as ex vivo in two- or three-dimensional endothelial cell cultures, and expression levels were upregulated by cell density. RPTPmu was detected in few other cells: bronchial and biliary epithelia and cardiocytes (intercalated discs). Our results identify RPTPmu as a new marker of endothelial cell heterogeneity and suggest a possible role in endothelial-specific functions, involving cell-cell contact.

Expression of receptor tyrosine phosphatases during development of the retinotectal projection of the chick

Receptor tyrosine kinases and receptor protein tyrosine phosphatases (RPTPs) appear to coordinate many aspects of neural development, including axon growth and guidance. Here, we focus on the possible roles of RPTPs in the developing avian retinotectal system. Using both in situ hybridization analysis and immunohistochemistry, we show for the first time that five RPTP genes--CRYPalpha, CRYP-2, PTPmu, PTPgamma, and PTPalpha--have different but overlapping expression patterns throughout the retina and the tectum. PTPalpha is restricted to Muller glia cells and radial glia of the tectum, indicating a possible function in controlling neuronal migration. PTPgamma expression is restricted to amacrine neurons. CRYPalpha and CRYP-2 mRNAs in contrast are expressed throughout the retinal ganglion cell layer from where axons grow out to their tectal targets. PTPmu is expressed in a subset of these ganglion cells. CRYPalpha, CRYP-2, and PTPmu proteins are also localized in growth cones of retinal ganglion cell axons and are present in defined laminae of the tectum. Thus, the spatial and temporal expression of three distinct RPTP subtypes--CRYPalpha, CRYP-2, and PTPmu--are consistent with the possibility of their involvement in axon growth and guidance of the retinotectal projection.

Identification of different gene expression patterns in low and high grade non-Hodgkin's lymphomas by differential display

We have compared the patterns of gene expression in non-Hodgkin's lymphoma (NHL) biopsy samples from patients with either low grade or high grade disease, by the polymerase chain reaction (PCR) based technique of differential display. By using a combination of 30 primer pairs we estimate that we were able to survey over 3,000 genes expressed in these tissues. In this study we compared a group of three low grade follicular centre lymphomas with a group of two high grade diffuse large cell lymphomas and scored only those PCR products that were represented in all samples of each group. In doing so we were able to avoid many of the problems associated with the occurence of false PCR-positives. 139 differences were then scored as representing genes which may be differentially expressed during the transformation from low to high grade disease. However, as many of these might simply reflect changing populations of cells, we focused on only those genes which appeared to be expressed exclusively in either low grade or high grade disease. We have identified 14 such genes, of which 10 were low grade specific and 4 were high grade specific. This approach therefore appears to offer a systematic method for the identification and characterisation of differentially expressed genes, which are characteristic of different NHL sub-types.

PTPmu regulates N-cadherin-dependent neurite outgrowth

Cell adhesion is critical to the establishment of proper connections in the nervous system. Some receptor-type protein tyrosine phosphatases (RPTPs) have adhesion molecule-like extracellular segments with intracellular tyrosine phosphatase domains that may transduce signals in response to adhesion. PTPmu is a RPTP that mediates cell aggregation and is expressed at high levels in the nervous system. In this study, we demonstrate that PTPmu promotes neurite outgrowth of retinal ganglion cells when used as a culture substrate. In addition, PTPmu was found in a complex with N-cadherin in retinal cells. To determine the physiological significance of the association between PTPmu and N-cadherin, the expression level and enzymatic activity of PTPmu were perturbed in retinal explant cultures. Downregulation of PTPmu expression through antisense techniques resulted in a significant decrease in neurite outgrowth on an N-cadherin substrate, whereas there was no effect on laminin or L1-dependent neurite outgrowth. The overexpression of a catalytically inactive form of PTPmu significantly decreased neurite outgrowth on N-cadherin. These data indicate that PTPmu specifically regulates signals required for neurites to extend on an N-cadherin substrate, implicating reversible tyrosine phosphorylation in the control of N-cadherin function. Together, these results suggest that PTPmu plays a dual role in the regulation of neurite outgrowth.

Identification and characterization of RPTP rho, a novel RPTP mu/kappa-like receptor protein tyrosine phosphatase whose expression is restricted to the central nervous system

We describe the cloning, chromosomal localization and characterization of RPTPrho, a new member of the RPTPmu/kappa phosphatase subfamily. Receptor tyrosine phosphatases in this subfamily are comprised of a MAM domain near the N-terminal, an immunoglobulin-like domain, four fibronectin type III repeats, a single transmembrane domain, and a large juxtamembrane segment followed by two intracellular phosphatase domains. An alternatively spliced mini-exon was identified in the extracellular segment of RPTPrho, between the fourth fibronectin type III repeat and the transmembrane domain. The RPTPrho gene was mapped to human chromosome 20 and mouse chromosome 2. Northern blot analysis demonstrated that RPTPrho expression was restricted to the central nervous system, and in situ hybridization studies showed that the RPTPrho transcript was distributed throughout the murine brain and spinal cord. Exceptionally high levels of the transcript were present in the cortex and olfactory bulbs during perinatal development, but were down-regulated during postnatal week two. The motifs found in the extracellular segment of type II receptor protein tyrosine phosphatases are commonly found in neural cell adhesion molecules, suggesting that RPTPrho may be involved in both signal transduction and cellular adhesion in the central nervous system.

The second catalytic domain of protein tyrosine phosphatase delta (PTP delta) binds to and inhibits the first catalytic domain of PTP sigma

The LAR family protein tyrosine phosphatases (PTPs), including LAR, PTP delta, and PTP sigma, are transmembrane proteins composed of a cell adhesion molecule-like ectodomain and two cytoplasmic catalytic domains: active D1 and inactive D2. We performed a yeast two-hybrid screen with the first catalytic domain of PTP sigma (PTP sigma-D1) as bait to identify interacting regulatory proteins. Using this screen, we identified the second catalytic domain of PTP delta (PTP delta-D2) as an interactor of PTP sigma-D1. Both yeast two-hybrid binding assays and coprecipitation from mammalian cells revealed strong binding between PTP sigma-D1 and PTP delta-D2, an association which required the presence of the wedge sequence in PTP sigma-D1, a sequence recently shown to mediate D1-D1 homodimerization in the phosphatase RPTP alpha. This interaction was not reciprocal, as PTP delta-D1 did not bind PTP sigma-D2. Addition of a glutathione S-transferase (GST)-PTP delta-D2 fusion protein (but not GST alone) to GST-PTP sigma-D1 led to approximately 50% inhibition of the catalytic activity of PTP sigma-D1, as determined by an in vitro phosphatase assay against p-nitrophenylphosphate. A similar inhibition of PTP sigma-D1 activity was obtained with coimmunoprecipitated PTP delta-D2. Interestingly, the second catalytic domains of LAR (LAR-D2) and PTP sigma (PTP sigma-D2), very similar in sequence to PTP delta-D2, bound poorly to PTP sigma-D1. PTP delta-D1 and LAR-D1 were also able to bind PTP delta-D2, but more weakly than PTP sigma-D1, with a binding hierarchy of PTP sigma-D1 >> PTP delta-D1 > LAR-D1. These results suggest that association between PTP sigma-D1 and PTP delta-D2, possibly via receptor heterodimerization, provides a negative regulatory function and that the second catalytic domains of this and likely other receptor PTPs, which are often inactive, may function instead to regulate the activity of the first catalytic domains.

Dynamic interaction of PTPmu with multiple cadherins in vivo

There is a growing body of evidence to implicate reversible tyrosine phosphorylation as an important mechanism in the control of the adhesive function of cadherins. We previously demonstrated that the receptor protein tyrosine phosphatase PTPmu associates with the cadherin-catenin complex in various tissues and cells and, therefore, may be a component of such a regulatory mechanism (Brady-Kalnay, S. M., D.L. Rimm, and N.K. Tonks. 1995. J. Cell Biol. 130:977- 986). In this study, we present further characterization of this interaction using a variety of systems. We observed that PTPmu interacted with N-cadherin, E-cadherin, and cadherin-4 (also called R-cadherin) in extracts of rat lung. We observed a direct interaction between PTPmu and E-cadherin after coexpression in Sf9 cells. In WC5 cells, which express a temperature-sensitive mutant form of v-Src, the complex between PTPmu and E-cadherin was dynamic, and conditions that resulted in tyrosine phosphorylation of E-cadherin were associated with dissociation of PTPmu from the complex. Furthermore, we have demonstrated that the COOH-terminal 38 residues of the cytoplasmic segment of E-cadherin was required for association with PTPmu in WC5 cells. Zondag et al. (Zondag, G., W. Moolenaar, and M. Gebbink. 1996. J. Cell Biol. 134: 1513-1517) have asserted that the association we observed between PTPmu and the cadherin-catenin complex in immunoprecipitates of the phosphatase arises from nonspecific cross-reactivity between BK2, our antibody to PTPmu, and cadherins. In this study we have confirmed our initial observation and demonstrated the presence of cadherin in immunoprecipitates of PTPmu obtained with three antibodies that recognize distinct epitopes in the phosphatase. In addition, we have demonstrated directly that the anti-PTPmu antibody BK2 that we used initially did not cross-react with cadherin. Our data reinforce the observation of an interaction between PTPmu and E-cadherin in vitro and in vivo, further emphasizing the potential importance of reversible tyrosine phosphorylation in regulating cadherin function.

Novel receptor protein tyrosine phosphatase (RPTPrho) and acidic fibroblast growth factor (FGF-1) transcripts delineate a rostrocaudal boundary in the granule cell layer of the murine cerebellar cortex

We have identified a novel receptor-like protein tyrosine phosphatase (RPTPrho) transcript whose expression in the cerebellar cortex is restricted to the granule cell layer of lobules 1-6. Acidic fibroblast growth factor (FGF-1) mRNA follows a similar cerebellar expression pattern. Together, the two markers define a sharp boundary in lobule 6, slightly caudal to the primary fissure. Anterior and posterior compartments became discernible only during postnatal weeks two and six, for RPTPrho and FGF-1, respectively. A rostrocaudal boundary in lobule 6 of the murine cerebellar cortex has also been identified morphologically by the effects of the meander tail mutation. The position of the RPTPrho and FGF-1 boundary on the rostrocaudal axis of the cerebellar cortex was close to, but not coincident with, the caudal extent of the disorganized anterior lobe of meander tail and the rostral extent of Otx-2 expression. The restricted pattern of FGF-1 and RPTPrho implies that these molecules may have specific signaling roles in the tyrosine phosphorylation/dephosphorylation pathway in the anterior compartment of the adult cerebellar cortex.