Cell-cell signaling: The ins and outs of receptor tyrosine phosphatases

Mechanistic Insights for Drug Repurposing and the Design of Hybrid Drugs for Alzheimer's Disease

The heterogeneity and complex nature of Alzheimer's disease (AD) is attributed to several genetic risk factors and molecular culprits. The slow pace and increasing failure rate of conventional drug discovery has led to the exploration of complementary strategies based on repurposing approved drugs to treat AD. Drug repurposing (DR) is a cost-effective, low-risk, and efficient approach for identifying novel therapeutic candidates for AD treatment. Similarly, hybrid drug design through the integration of distinct pharmacophores from known or failed drugs and natural products is an interesting strategy to target the multifactorial nature of AD. In this Perspective, we discuss the potential of DR and highlight promising drug candidates that can be advanced for clinical trials, backed by a detailed discussion on their plausible mechanisms of action. Our article fosters research on the hidden potential of DR and hybrid drug design with the goal of unravelling new drugs and targets to tackle AD.

Deficiency of Src homology 2-containing phosphatase 1 results in abnormalities in murine neutrophil function: studies in motheaten mice

Neutrophils, an essential component of the innate immune system, are regulated in part by signaling pathways involving protein tyrosine phosphorylation. While protein tyrosine kinase functions in regulating neutrophil behavior have been extensively investigated, little is known about the role for specific protein tyrosine phosphatases (PTP) in modulating neutrophil signaling cascades. A key role for Src homology 2 domain-containing phosphatase 1 (SHP-1), a PTP, in neutrophil physiology is, however, implied by the overexpansion and inappropriate activation of granulocyte populations in SHP-1-deficient motheaten (me/me) and motheaten viable (me(v)/me(v)) mice. To directly investigate the importance of SHP-1 to phagocytic cell function, bone marrow neutrophils were isolated from both me/me and me(v)/me(v) mice and examined with respect to their responses to various stimuli. The results of these studies revealed that both quiescent and activated neutrophils from motheaten mice manifested enhanced tyrosine phosphorylation of cellular proteins in the 60- to 80-kDa range relative to that detected in wild-type congenic control neutrophils. MOTHEATEN: neutrophils also demonstrated increased oxidant production, surface expression of CD18, and adhesion to protein-coated plastic. Chemotaxis, however, was severely diminished in the SHP-deficient neutrophils relative to control neutrophils, which was possibly attributable to a combination of defective deadhesion and altered actin assembly. Taken together, these results indicate a significant role for SHP-1 in modulating the tyrosine phosphorylation-dependent signaling pathways that regulate neutrophil microbicidal functions.

Regulation of Integrin-Mediated T Cell Adhesion by the Transmembrane Protein Tyrosine Phosphatase CD45

The transmembrane protein tyrosine phosphatase CD45 is required for Ag receptor signal transduction in lymphocytes. Recently, a role for CD45 in the regulation of macrophage adhesion has been demonstrated as well. To investigate further the role of CD45 in the regulation of adhesion, we examined integrin-mediated adhesion to fibronectin of two T cell lines and their CD45-deficient variants. The absence of CD45 correlated with enhanced adhesion to fibronectin via integrin α5β1 (VLA-5), but not α4β1 (VLA-4) in both cell lines. Adhesion returned to normal levels upon transfection of wild-type CD45 into the CD45-deficient lines. Transfection of chimeric or mutant molecules expressing some, but not all, CD45 domains and activities demonstrated that both the transmembrane domain and the tyrosine phosphatase activity of CD45 were required for regulation of integrin-dependent adhesion, but the highly glycosylated extracellular domain was dispensable. In contrast, only a catalytically active CD45 cytoplasmic domain was required for TCR signaling. Transfectants that restored normal levels of adhesion to fibronectin coimmunoprecipitated with the transmembrane protein known as CD45-associated protein. These studies demonstrate a novel role for CD45 in adhesion regulation and suggest a possible function for its association with CD45-associated protein.

Negative Regulation of Myeloid Cell Proliferation and Function by the SH2 Domain-Containing Tyrosine Phosphatase-1

The SH2 domain containing tyrosine phosphatase SHP-1 has been implicated in the regulation of a multiplicity of signaling pathways involved in hemopoietic cell growth, differentiation, and activation. A pivotal contribution of SHP-1 in the modulation of myeloid cell signaling cascades has been revealed by the demonstration that SHP-1 gene mutation is responsible for the overexpansion and inappropriate activation of myelomonocytic populations in motheaten mice. To investigate the role of SHP-1 in regulation of myeloid leukocytes, an HA epitope-tagged dominant negative (interfering) SHP-1 (SHP-1C453S) was expressed in the myelo-monocytic cell line U937 using the pcDNA3 vector. Overexpression of this protein in SHP-1C453S transfectants was demonstrated by Western blot analysis and by detection of decreased specific activity. Growth, proliferation, and IL-3-induced proliferative responses were substantially increased in the SHP-1C453S-overexpressing cells relative to those in control cells. The results of cell cycle analysis also revealed that the proportion of cells overexpressing SHP-1C453S in S phase was greater than that of control cells. The SHP-1C453S-expressing cells also displayed diminished rates of apoptosis as detected by flow cytometric analysis of propidium iodide-stained cells and terminal deoxynucleotidyltransferase-mediated fluorescein-dUTP nick end-labeling assay. While motility and phagocytosis were not affected by SHP-1C453S overexpression, adhesion and the oxidative burst in response to PMA were enhanced in the SHP-1C453S compared with those in the vector alone transfectants. Taken together, these results suggest that SHP-1 exerts an important negative regulatory influence on cell proliferation and activation while promoting spontaneous cell death in myeloid cells.

Airway nerves and protein phosphatases

Much attention has focused on the important role played by phosphatases in the control of gene transcription, cell differentiation and memory regulation. It is also clear that phosphatases may regulate a number of biochemical pathways which can modulate cellular function. Of particular interest is the role of phosphatases in the control of neuronal function. Alterations in neuronal function may contributed to the heightened airways responsiveness observed in asthma to a number of physiological stimuli including distilled water, sulfur dioxide, metabisulfite, hypertonic saline, exercise, allergens, viruses and cold air. An understanding of the mechanisms which regulate the function of sensory nerves could have important clinical implications. In this review we will highlight a number of studies that have investigated the role of phosphatases in the regulation of airway nerve function.

Protein-tyrosine phosphatase alpha regulates Src family kinases and alters cell-substratum adhesion

The roles of protein-tyrosine phosphatases (PTPs) in processes such as cell growth and adhesion are poorly understood. To explore the ability of specific PTPs to regulate cell signaling pathways initiated by stimulation of growth factor receptors, we expressed the receptor-like PTP, PTPalpha, in A431 epidermoid carcinoma cells. These cells express high levels of the epidermal growth factor (EGF) receptor and proliferate in response to the autocrine production of transforming growth factor-alpha. Conversely, EGF stimulation of A431 cells in vitro leads to growth inhibition and triggers the rapid detachment of these cells from the substratum. Although PTPalpha expression did not alter the growth characteristics of either unstimulated or EGF-stimulated cells, this phosphatase was associated with increased cell-substratum adhesion. Furthermore, PTPalpha-expressing A431 cells were strikingly resistant to EGF-induced cell rounding. Overexpression of PTPalpha in A431 cells was associated with the dephosphorylation/activation of specific Src family kinases, suggesting a potential mechanism for the observed alteration in A431 cell-substratum adhesion. Src kinase activation was dependent on the D1 catalytic subunit of PTPalpha, and there was evidence of association between PTPalpha and Src kinase(s). PTPalpha expression also led to increased association of Src kinase with the integrin-associated focal adhesion kinase, pp125(FAK). In addition, paxillin, a Src and/or pp125(FAK) substrate, displayed increased levels of tyrosine phosphorylation in PTPalpha-expressing cells and was associated with elevated amounts of Csk. In view of these alterations in focal adhesion-associated molecules in PTPalpha-expressing A431 cells, as well as the changes in adhesion demonstrated by these cells, we propose that PTPalpha may have a role in regulating cell-substratum adhesion.

LAR tyrosine phosphatase receptor: proximal membrane alternative splicing is coordinated with regional expression and intraneuronal localization

Examination of null-mutant Drosophila and Leukocyte Common Antigen-Related (LAR)-deficient transgenic mice has demonstrated that the LAR protein tyrosine phosphatase (PTP) receptor promotes neurite outgrowth. In the absence of known ligands, the mechanisms by which LAR-type PTP receptors are regulated are unknown. We hypothesized that an alternatively spliced eleven amino acid proximal membrane segment of LAR (LAR alternatively spliced element-a; LASE-a) contributes to regulation of LAR function. Human, rat and mouse LAR cDNA sequences demonstrated that the predicted eleven amino acid inserts in rat and mouse are identical and share nine of eleven residues with the human insert. LASE-a splicing led to the introduction of a Ser residue into LAR at a position analogous to Ser residues undergoing regulated phosphorylation in other PTPs. In-situ studies revealed increasingly region-specific expression of LASE-a containing LAR transcripts during postnatal development. RT-PCR analysis of cortical and hippocampal tissue confirmed that the proportion of LAR transcripts containing LASE-a decreases during development. Immunostaining of cultured PC12 cells, cerebellar granule neurons, dorsal root ganglia and sciatic nerve sections with antibody directed against the LASE-a insert demonstrated signal in cell bodies but little if any along neurites. In contrast, staining with antibody directed to a separate domain of LAR showed accumulation of LAR along neurites. The findings that LASE-a splicing is conserved across human, rat and mouse, that the LASE-a insert introduces a Ser at a site likely to be targeted for regulated phosphorylation and that developmentally regulated splicing is coordinated with specific regional and intraneuronal localization point to important novel potential mechanisms regulating LAR-type tyrosine phosphatase receptor function in the nervous system.

LAR Tyrosine Phosphatase Receptor: A Developmental Isoform Is Present in Neurites and Growth Cones and Its Expression Is Regional- and Cell-Specific

Transgenic mice and Drosophila mutant studies demonstrate that the leukocyte common antigen-related (LAR) protein tyrosine phosphatase (PTPase) receptor is required for formation of neural networks. We assessed the hypothesis that alternative splicing of the LAR extracellular region contributes to this function by establishing temporospatial expression patterns of LAR isoforms containing an alternatively spliced extracellular nine amino acid segment (LAR alternatively spliced element-c; LASE-c). LASE-c was present in multiple alternatively spliced and truncated LAR transcripts. In contrast to LAR isoforms without LASE-c, levels of LAR transcripts and protein isoforms containing LASE-c were primarily present during development, suggesting a mechanism for developmental regulation of LAR function. In situ analysis demonstrated increasingly region- and cell-specific expression of LASE-c during maturation. Immunostaining revealed LASE-c-containing LAR protein along neurites and in growth cones. The discovery of highly regulated, temporospatial extracellular domain alternative splicing of LAR-type PTPase receptors points to a novel mechanism by which these receptors might influence network formation. Copyright 1998 Academic Press.

Neurotactin functions in concert with other identified CAMs in growth cone guidance in Drosophila

We have isolated and characterized mutations in Drosophila neurotactin, a gene that encodes a cell adhesion protein widely expressed during neural development. Analysis of both loss and gain of gene function conditions during embryonic and postembryonic development revealed specific requirements for neurotactin during axon outgrowth, fasciculation, and guidance. Furthermore, embryos of some double mutant combinations of neurotactin and other genes encoding adhesion/signaling molecules, including neuroglian, derailed, and kekkon1, displayed phenotypic synergy. This result provides evidence for functional cooperativity in vivo between the adhesion and signaling pathways controlled by neurotactin and the other three genes.

Transmembrane glycoprotein gp150 is a substrate for receptor tyrosine phosphatase DPTP10D in Drosophila cells

We have begun to explore the downstream signaling pathways of receptor protein tyrosine phosphatases (RPTPs) that control axon guidance decisions in the Drosophila central nervous system. We have focused our studies on the adhesion molecule-like gp150 protein, which binds directly to and is an in vitro substrate for the RPTP DPTP10D. Here we show that gp150 and DPTP10D form stable complexes in Drosophila Schneider 2 (S2) cells and in wild-type larval tissue. We also demonstrate that the DPTP10D cytoplasmic domain is sufficient to confer binding to gp150. gp150 has a short cytoplasmic domain containing four tyrosines, all found within sequences similar to immunoreceptor family tyrosine-based activation motifs (ITAMs). We demonstrate that gp150 is tyrosine phosphorylated in wild-type larvae. In S2 cells, gp150 becomes tyrosine phosphorylated following incubation with PTP inhibitors or upon coexpression of the Dsrc tyrosine kinase. Phosphorylated Dsrc and an unknown 40-kDa phosphoprotein form stable complexes with gp150, thereby implicating them in a putative gp150 signaling pathway. When coexpressed with gp150, either full-length DPTP10D or its cytoplasmic domain mediates gp150 dephosphorylation whereas a catalytically inactive DPTP10D cytoplasmic domain does not. The neural RPTP DPTP99A can also induce gp150 dephosphorylation but does not coimmunoprecipitate with gp150. Taken together, the results suggest that gp150 transduces signals via phosphorylation of its ITAM-like elements. Phosphotyrosines on gp150 might function as binding sites for downstream signaling molecules, thereby initiating a signaling cascade that could be modulated in vivo by RPTPs such as DPTP10D.

CD45 regulates Src family member kinase activity associated with macrophage integrin-mediated adhesion

BACKGROUND

Adhesion of leukocytes to the extracellular matrix and to other cells is mediated by members of the integrin family of adhesion molecules. Src family kinases are activated upon integrin-mediated adhesion. In lymphocytes, CD45 is a leukocyte-specific transmembrane protein tyrosine phosphatase that activates Src family kinases associated with B-cell and T-cell antigen receptor signaling by constitutive dephosphorylation of the inhibitory carboxy-terminal tyrosine phosphorylation site. Here, we show that CD45 is also important in downregulating the kinase activity of Src family members during integrin-mediated adhesion in macrophages.

RESULTS

We found that CD45 colocalized with beta2 integrin and the Src family kinase p53/56(lyn) to adhesion sites in bone marrow-derived macrophages. Macrophages from CD45(-/-) mice were unable to maintain integrin-mediated adhesion. In adherent macrophages, absence of CD45 led to the hyperphosphorylation and hyperactivation of p56/59(hck) and p53/56(lyn), but not of p58(c-fgr). CD45 directly inactivated p59(hck) but not p56(lck) in transient transfection assays. Furthermore, coexpression of CD45 with p59(hck) or p56(lyn) containing a tyrosine to phenylalanine mutation at the carboxy-terminal negative regulatory site resulted in decreased tyrosine phosphorylation of the Src family member kinases due to dephosphorylation of the potentiating tyrosine phosphorylation site within the kinase domain.

CONCLUSIONS

Using primary bone marrow macrophages, these studies demonstrate that CD45 regulates Src family kinases and is required to maintain macrophage adhesion. CD45 decreases Src family kinase activity by dephosphorylating the tyrosine residue located within the kinase domain.

Competition and cooperation among receptor tyrosine phosphatases control motoneuron growth cone guidance in Drosophila

The neural receptor tyrosine phosphatases DPTP69D, DPTP99A and DLAR are involved in motor axon guidance in the Drosophila embryo. Here we analyze the requirements for these three phosphatases in growth cone guidance decisions along the ISN and SNb motor pathways. Any one of the three suffices for the progression of ISN pioneer growth cones beyond their first intermediate target in the dorsal muscle field. DLAR or DPTP69D can facilitate outgrowth beyond a second intermediate target, and DLAR is uniquely required for formation of a normal terminal arbor. A different pattern of partial redundancy among the three phosphatases is observed for the SNb pathway. Any one of the three suffices to allow SNb axons to leave the common ISN pathway at the exit junction. When DLAR is not expressed, however, SNb axons sometimes bypass their ventrolateral muscle targets after leaving the common pathway, instead growing out as a separate bundle adjacent to the ISN. This abnormal guidance decision can be completely suppressed by also removing DPTP99A, suggesting that DLAR turns off or counteracts a DPTP99A signal that favors the bypass axon trajectory. Our results show that the relationships among the tyrosine phosphatases are complex and dependent on cellular context. At growth cone choice points along one nerve, two phosphatases cooperate, while along another nerve these same phosphatases can act in opposition to one another.

Peroxovanadate induces tyrosine phosphorylation of multiple signaling proteins in mouse liver and kidney

The intraperitoneal injection of a vanadate/H2O2 mixture (peroxovanadate) into mice resulted within minutes in the appearance of numerous tyrosine-phosphorylated proteins in the liver and kidney. These effects are presumably due to the inhibition of phosphotyrosine phosphatase activity. Three of the tyrosine-phosphorylated proteins have been identified as the receptors for epidermal growth factor, insulin, and hepatocyte growth factor. The injection of peroxovanadate also enhanced the tyrosine phosphorylation of many of the proteins known to function downstream of these receptors, including SHC, signal transducer and activator of transcription (Stat) 1alpha,beta, Stat 3, Stat 5, phospholipase C-gamma, insulin receptor substrate 1, GTPase-activating protein, beta-catenin, gamma-catenin, p120cas, SHP-1, and SHP-2. The administration of peroxovanadate also induced nuclear translocation of a number of tyrosine-phosphorylated Stat proteins. In addition, the global effects on tyrosine phosphorylation permitted the detection of a number of novel intracellular protein interactions, including an association of Tyk2 with beta-catenin. The in situ administration of peroxovanadate may prove useful in the search for novel tyrosine-phosphorylated proteins and the identification of new interactions between previously identified tyrosine-phosphorylated substrates.

Axon guidance: motor-way madness

Recent results have revealed for the first time that receptor-like protein tyrosine phosphatases help to control the navigation of motor axons in the Drosophila nervous system.