Characterization of chicken protein tyrosine phosphatase alpha and its expression in the central nervous system

Purification and Characterization of a Novel Thermostable Extracellular Protein Tyrosine Phosphatase fromMetarhizium anisopliaeStrain CQMa102

An extracellular phosphatase was purified to homogeneity from the entomopathogenic fungus Metarhizium anisopliae with a 41.0% yield. The molecular mass and isoelectric point of the purified enzyme were about 82.5 kDa and 9.5 respectively. The optimum pH and temperature were about 5.5 and 75 degrees C when using O-phospho-L-tyrosine as substrate. The protein displayed high stability in a pH range 3.0-9.5 at 30 degrees C and was remarkably thermostable at 70 degrees C. The purified enzyme showed high activity on O-phospho-L-tyrosine and protein tyrosine phosphatase substrate monophosphate (a specific substrate of protein tyrosine phosphatase). Although one peptide of the phosphatase shared identity with one alkaline phosphatase of Neurospora crassa, its substrate specificity and inhibitor sensitivity indicate that the enzyme is a protein tyrosine phosphatase.

Receptor protein tyrosine phosphatases are expressed by cycling retinal progenitor cells and involved in neuronal development of mouse retina

Receptor protein tyrosine phosphatases (RPTPs) appear to coordinate many aspects of neural development, including cell proliferation, migration and differentiation. Here we investigated potential roles of RPTPs in the developing mouse retina. Using a degenerate oligonucleotide-based reverse transcription polymerase chain reaction approach, we identified 11 different RPTPs in the retina at embryonic stage 13 (E13). Subsequently, the expression patterns of RPTPkappa, RPTPJ, RPTPRR, RPTPsigma, RPTPepsilon and RPTPgamma in the retina from embryonic stages to adult were analyzed in detail using quantitative real-time-PCR, in situ hybridization, immunohistochemistry and Western blotting. At E13, all six RPTPs are expressed in actively cycling retinal progenitor cells and postmitotic newborn retinal neurons. With ongoing maturation, RPTPkappa, RPTPJ, RPTPRR, RPTPsigma, RPTPepsilon and RPTPgamma display a different spatiotemporal regulation of mRNAs and proteins in the pre- and postnatal retina. Finally, in adulthood these six RPTPs localize to distinct cellular compartments of multiple retinal neurons. Additional studies in RPTPgamma(-/-) and RPTPbeta/zeta(-/-) (also known as PTPRZ1, RPTPbeta or RPTPzeta) mice at postnatal stage P1 reveal no apparent differences in retinal laminar organization or in the expression pattern of specific retinal cell-type markers when compared with wild type. However, in RPTPbeta/zeta(-/-) retinas, immunoreactivity of vimentin, a marker of Müller glial cells, is selectively reduced and the morphology of vimentin-immunoreactive radial processes of Müller cells is considerably disturbed. Our results suggest distinct roles of RPTPs in cell proliferation and establishing phenotypes of different retinal cells during retinogenesis as well as later in the maintenance of mature retina.

Extracellular domain splice variants of a transforming protein tyrosine phosphatase alpha mutant differentially activate Src-kinase dependent focus formation

The extracellular domains of receptor-type protein-tyrosine phosphatases (PTPs) contain a diverse range of protein modules like fibronectin- or immunoglobulin-like structures. These are frequently expressed in a tissue- and development specific manner as splice variants. The extracellular domain of PTPalpha is rather short and heavily glycosylated. Two splice variants are known, which it differs by an exon encoding nine amino acids within the extracellular domain. We have analyzed the expression pattern of both variants and found that the smaller form is ubiquitously expressed while the larger form was found at an increased level only in brain, some skeletal muscle and differentiating cells like granule neurons, adipocytes and myotubes. The phosphatase activity of both forms was similar when tested in vitro using para-nitrophenylphosphate as a substrate and in a transient expression system with the substrates c-Fyn or c-Src. In a quantitative focus formation assay the capability of the larger form to activate Src-dependent focus formation in intact cells was increased more than twofold whereas the capability to dephosphorylate the insulin receptor in a BHK cell system was similar. We conclude that the two splice variants of PTPalpha are expressed differentially and regulate c-Src activity in different ways.

Protein tyrosine phosphatase alpha (PTP alpha) knockout mice show deficits in Morris water maze learning, decreased locomotor activity, and decreases in anxiety

Receptor PTPalpha is a widely expressed transmembrane enzyme enriched in brain. PTPalpha knockout (PTPalpha(-/-)) mice are viable and display no gross abnormalities. Brain and embryo derived fibroblast src and fyn activity is reduced to <50% in PTPalpha(-/-) mice. These protein kinases are implicated in multiple aspects of neuronal development and function. However, the effect of the loss of function of the PTPalpha gene on behavior has yet to be investigated. PTPalpha(-/-) and WT mice were tested for anxiety, swimming ability, spatial learning, cued learning, locomotor activity, and novel object recognition (NOR). PTPalpha(-/-) mice were indistinguishable from WT in swimming ability, cued learning and novel object recognition. Knockout mice showed decreased anxiety without an increase in head dips and stretch-attend movements. During Morris water maze (MWM) learning, PTPalpha(-/-) mice had increased latencies to reach the goal compared to WT on acquisition, but no memory deficit on probe trials. On reversal learning, knockout mice showed no significant effects. PTPalpha(-/-) mice showed decreased exploratory locomotor activity, but responded normally to a challenge dose of D-methamphetamine. The data suggest that PTPalpha serves a regulatory function in learning and other forms of neuroplasticity.

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

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

Multipathways for transdifferentiation of human prostate cancer cells into neuroendocrine-like phenotype

The neuroendocrine (NE) cell is a minor cell population in normal human prostate glands. The number of NE cells is increased in advanced hormone-refractory prostate carcinomas (PCA). The mechanism of increased NE cell population in these advanced tumors is poorly understood. We examined molecular mechanisms which may be involved in the regulation of the transdifferentiation process of human PCA cells leading to a NE phenotype. We compared PCA cell lines LNCaP and PC-3 in the following medium conditions: steroid-reduced (SR), interleukin-6 (IL-6)-supplemented, or dibutyrate cAMP (db-cAMP)-supplemented. We found that androgen-responsive C-33 LNCaP cells responded to all treatments, having a neuronal-like morphology. In contrast, C-81 LNCaP cells, having a decreased androgen responsiveness, had a less pronounced effect although followed a similar trend. Androgen-unresponsive PC-3 cells showed little change in their morphology. Grown in the SR condition, the level of neuron-specific enolase (NSE), a marker of neuronal cells, was upregulated in C-33 LNCaP cells, while to a lesser degree in the presence of IL-6. In the presence of db-cAMP, the NSE level in C-33 cells was decreased, lower than that in control cells. An opposite effect was observed for C-81 LNCaP cells. Nevertheless, the NSE level was only elevated in db-cAMP-treated PC-3 cells, but no change was found in PC-3 cells grown in the SR- or IL-6-supplemented medium. Thus, a similar gross phenotypic change may correlate with differential molecular expressions. We also analyzed the expression of protein tyrosine phosphatase alpha (RPTPalpha) since it plays a critical role in normal neuronal differentiation and signaling. Our results showed that the expression of RPTPalpha correlates with the NE phenotypic change of LNCaP cells in the SR condition. In summary, our data clearly show that the molecular process by which cultured human prostate cancer cells undergo a transdifferentiation process to a NE cell-like phenotype is accompanied by differential expressions of different markers, and a gross NE cell-like phenotype can occur by exposing PCA cells to different pharmacological agents.

Expression of receptor protein tyrosine phosphatases in embryonic chick spinal cord

Receptor-like protein tyrosine phosphatases potentially play a crucial role in axon growth and targeting. We focus here on their role within the embryonic avian spinal cord, in particular the development and outgrowth of motorneurons. We have used in situ mRNA hybridization to examine the spatiotemporal expression of eight receptor-like protein tyrosine phosphatases and find that it is both dynamic and highly varied, including novel, isoform-specific expression patterns. CRYP alpha 1 is expressed in all of the ventral motorneuron pools, whereas CRYP2, RPTP gamma, and RPTP alpha are only expressed in specific subsets of these neurons. CRYP alpha 2, RPTP psi, and RPTP delta are neuronally expressed elsewhere in the cord, but not in ventral motorneurons, whereas RPTP mu is unique in being restricted to capillaries. The developmentally regulated expression of these genes strongly suggests that the encoded phosphatases play numerous roles during neurogenesis and axonogenesis in the vertebrate spinal cord.

Tissue specific expression of Yrk kinase: implications for differentiation and inflammation

The Src family of proto-oncogenes is a highly conserved group of non-receptor tyrosine kinases with very similar, but not identical, tissue distributions and functions. Yrk is a recently discovered new member of this family. Here we report the patterns of expression of this kinase in a variety of chicken tissues during development and after hatching, and experiments that correlate some of the observed patterns of expression with potential functions. The results show that the Yrk protein is primarily found in neuronal and epithelial cells and in monocyte/macrophages. In neuronal tissues of hatched chicks, Yrk is expressed in Purkinje cells, in the gigantocellularis of the brain-stem, and in retinal ganglion cells. In addition, staining for this kinase is also seen as thread-like and punctate patterns suggesting staining in neurites and growth cones. Epithelial cells express Yrk in the stomach during late developmental stages and after hatching but, in other epithelia such as in the peridermis, intestine and kidney, expression is high during development but low (skin) or undetectable (intestine and kidney) after hatching. These results suggest that Yrk may have several functional roles, specifically in cell migration and or differentiation during neuronal and epithelial cell development and in maintenance of the differentiated phenotype. In this study we also show that significant levels of Yrk are detected in monocytes of the blood and in tissue macrophages. Analysis of chicken hematopoietic cell lines confirmed the expression of Yrk in cells of monocyte/macrophage lineage and show for the first time in experimentally-induced inflammation that Yrk kinase activity is high during the period of monocyte infiltration, raising the possibility that this kinase plays a role in inflammation and/or response to injury.

Protein tyrosine phosphatase alpha (PTPalpha) and contactin form a novel neuronal receptor complex linked to the intracellular tyrosine kinase fyn

Glycosyl phosphatidylinositol (GPI)-linked receptors and receptor protein tyrosine phosphatases (RPTPs), both play key roles in nervous system development, although the molecular mechanisms are largely unknown. Despite lacking a transmembrane domain, GPI receptors can recruit intracellular src family tyrosine kinases to receptor complexes. Few ligands for the extracellular regions of RPTPs are known, relegating most to the status of orphan receptors. We demonstrate that PTPalpha, an RPTP that dephosphorylates and activates src family kinases, forms a novel membrane-spanning complex with the neuronal GPI-anchored receptor contactin. PTPalpha and contactin associate in a lateral (cis) complex mediated through the extracellular region of PTPalpha. This complex is stable to isolation from brain lysates or transfected cells through immunoprecipitation and to antibody-induced coclustering of PTPalpha and contactin within cells. This is the first demonstration of a receptor PTP in a cis configuration with another cell surface receptor, suggesting an additional mode for regulation of a PTP. The transmembrane and catalytic nature of PTPalpha indicate that it likely forms the transducing element of the complex, and we postulate that the role of contactin is to assemble a phosphorylation-competent system at the cell surface, conferring a dynamic signal transduction capability to the recognition element.

Retinotectal ligands for the receptor tyrosine phosphatase CRYPalpha

The cell adhesion molecule-like tyrosine phosphatase CRYPalpha is localized on retinal axons and their growth cones. We present evidence that two isoforms of this type IIa phosphatase, CRYPalpha1 and CRYPalpha2, have extracellular ligands along the developing retinotectal pathway. Using alkaline phosphatase fusion proteins containing the CRYPalpha1 ectodomain, we detect a prominent ligand on basement membranes of the early retina, optic stalk, and chiasm. A second ligand is observed in the endfeet region of radial processes in the developing stratum opticum, the site of initial retinal axon invasion. This latter ligand binds CRYPalpha2 preferentially. Further ligand interactions are detected for both CRYPalpha protein isoforms in retinorecipient tectal laminae and on retinal fibers themselves. CRYPalpha thus has cell- and matrix-associated ligands along the entire retinotectal projection. Moreover, these ligands appear to be heterotypic and interact with CRYPalpha through both its immunoglobulin and fibronectin type III regions. The anteroposterior levels of the ligands are relatively uniform within the retina and tectum, suggesting that the CRYPalpha protein within retinal axons does not directly recognise topographically graded guidance cues. We propose that CRYPalpha may have a permissive role in promoting retinal axon growth across the eye and tectum and that its functions are modulated temporally and spatially by isoform-specific interactions with cell- and matrix-associated ligands.

Protein-tyrosine phosphatases in development

One of the most important mechanisms of eukaryotic signalling is protein phosphorylation on tyrosine residues, which plays a pivotal role in development by regulating cell proliferation, differentiation and migration. Cellular phosphotyrosine (P.Tyr) levels are regulated by the antagonistic activities of the protein-tyrosine kinases (PTKs) and protein-tyrosine phosphatases (PTPs). We have good insight into the function of PTKs at the molecular level and into the role of PTK-mediated signalling in development. Intuitively, PTPs and PTKs are equally important in development. Over the past decade, much emphasis has been placed on elucidation of the function of PTPs, which has led to good insights into the mechanism of PTP-mediated dephosphorylation. Although still relatively little is known about the role of PTPs in cell signalling and development, evidence is now emerging that several PTPs are crucial for proper development. Here I will introduce PTP-mediated signalling and discuss recent findings regarding the function of PTPs in development.

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.

Overexpression of protein tyrosine phosphatase-alpha (PTP-alpha) but not PTP-kappa inhibits translocation of GLUT4 in rat adipose cells

Protein tyrosine phosphatases (PTPases) are likely to play important roles in insulin action. We recently demonstrated that the nontransmembrane PTPase PTP1B can act as a negative modulator of insulin-stimulated translocation of GLUT4. We now examine the role of PTP-alpha and PTP-kappa (two transmembrane PTPases) in this metabolic action of insulin. Rat adipose cells were transfected with either PTP-alpha or PTP-kappa and effects of these PTPases on the translocation of a cotransfected epitope-tagged GLUT4 were studied. Cells overexpressing wild-type PTP-alpha had significantly lower levels of cell surface GLUT4 in response to insulin and a threefold decrease in insulin sensitivity when compared with control cells expressing only tagged GLUT4. Co-overexpression of PTP-alpha and PTP1B did not have additive effects, suggesting that these PTPases share common substrates. Cells overexpressing either wild-type PTP-kappa or catalytically inactive mutants of PTP-alpha had dose-response curves similar to those of control cells. Since overexpression of PTP-alpha, but not PTP-kappa, had effects on translocation of GLUT4, our data suggest that PTPalpha may be a specific negative modulator of insulin-stimulated glucose transport.

Protein tyrosine phosphatases and neural development

During neural development, cells interact dynamically with each other and with the extracellular matrix, using cell signaling to control differentiation, axonogenesis, and survival. Enzymes that regulate protein tyrosine phosphorylation often lie at the core of such cell signaling. Protein tyrosine phosphatases (PTPases) are recognized as being of central importance here, and a growing family of PTPases are now known to be expressed in embryonic neurons and glia. Both receptor-like and cytoplasmic enzymes have been identified. The receptor family includes immunoglobulin superfamily members that influence cell-cell adhesion, proteoglycans that control neurite growth, and enzymes in Drosophila that regulate axon guidance and target cell recognition. Cytoplasmic PTPases are implicated in nerve cell commitment and potentially in the regulation of cell survival. This review outlines what we currently know about PTPases in the nervous system and presents concepts concerning their possible modes of action.

Physical and functional interactions between receptor-like protein-tyrosine phosphatase alpha and p59fyn

We have examined the in vivo activity of receptor-like protein-tyrosine phosphatase alpha (PTPalpha) toward p59(fyn), a widely expressed Src family kinase. In a coexpression system, PTPalpha effected a dose-dependent tyrosine dephosphorylation and activation of p59(fyn), where maximal dephosphorylation correlated with a 5-fold increase in kinase activity. PTPalpha expression resulted in increased accessibility of the p59(fyn) SH2 domain, consistent with a PTPalpha-mediated dephosphorylation of the regulatory C-terminal tyrosine residue of p59(fyn). No p59(fyn) dephosphorylation was observed with an enzymatically inactive mutant form of PTPalpha or with another receptor-like PTP, CD45. Many enzyme-linked receptors are complexed with their substrates, and we examined whether PTPalpha and p59(fyn) underwent association. Reciprocal immunoprecipitations and assays detected p59(fyn) and an appropriate kinase activity in PTPalpha immunoprecipitates and PTPalpha and PTP activity in p59(fyn) immunoprecipitates. No association between CD45 and p59(fyn) was detected in similar experiments. The PTPalpha-mediated activation of p59(fyn) is not prerequisite for association since wild-type and inactive mutant PTPalpha bound equally well to p59(fyn). Endogenous PTPalpha and p59(fyn) were also found in association in mouse brain. Together, these results demonstrate a physical and functional interaction of PTPalpha and p59(fyn) that may be of importance in PTPalpha-initiated signaling events.

Protein tyrosine phosphatases in the developing nervous system

Protein tyrosine phosphatases (PTPs) constitute a diverse family of intracellular and transmembrane proteins. Expression data and recent genetic analyses indicate that many PTPs play important roles in different aspects of nervous system development. Although PTP mechanisms are still poorly understood, current data suggest considerable complexity in these signaling pathways.