Subcellular localization of intracellular protein tyrosine phosphatases in T cells

Discovery of Selective Proteolysis-Targeting Chimera Degraders Targeting PTP1B as Long-Term Hypoglycemic Agents

PTP1B, a promising target for insulin sensitizers in type 2 diabetes treatment, can be effectively degraded using proteolysis-targeting chimera (PROTAC). This approach offers potential for long-acting antidiabetic agents. We report potent bifunctional PROTACs targeting PTP1B through the E3 ubiquitin ligase cereblon. Western blot analysis showed significant PTP1B degradation by PROTACs at concentrations from 5 nM to 5 μM after 48 h. Evaluation of five highly potent PROTACs revealed compound 75 with a longer PEG linker (23 atoms), displaying remarkable degradation activity after 48 and 72 h, with DC50 values of 250 nM and 50 nM, respectively. Compound 75 induced selective degradation of PTP1B, requiring engagement with both the target protein and CRBN E3 ligase, in a ubiquitination and proteasome-dependent manner. It significantly reduced blood glucose AUC0-2h to 29% in an oral glucose tolerance test and activated the IRS-1/PI3K/Akt signaling pathway in HepG2 cells, showing promise for long-term antidiabetic therapy.

PTP4A2 promotes lysophagy by dephosphorylation of VCP/p97 at Tyr805

Overexpression of PTP4A phosphatases are associated with advanced cancers, but their biological functions are far from fully understood due to limited knowledge about their physiological substrates. VCP is implicated in lysophagy via collaboration with specific cofactors in the ELDR complex. However, how the ELDR complex assembly is regulated has not been determined. Moreover, the functional significance of the penultimate and conserved Tyr805 phosphorylation in VCP has not been established. Here, we use an unbiased substrate trapping and mass spectrometry approach and identify VCP/p97 as a bona fide substrate of PTP4A2. Biochemical studies show that PTP4A2 dephosphorylates VCP at Tyr805, enabling the association of VCP with its C-terminal cofactors UBXN6/UBXD1 and PLAA, which are components of the ELDR complex responsible for lysophagy, the autophagic clearance of damaged lysosomes. Functionally, PTP4A2 is required for cellular homeostasis by promoting lysophagy through facilitating ELDR-mediated K48-linked ubiquitin conjugate removal and autophagosome formation on the damaged lysosomes. Deletion of Ptp4a2 in vivo compromises the recovery of glycerol-injection induced acute kidney injury due to impaired lysophagy and sustained lysosomal damage. Taken together, our data establish PTP4A2 as a critical regulator of VCP and uncover an important role for PTP4A2 in maintaining lysosomal homeostasis through dephosphorylation of VCP at Tyr805. Our study suggests that PTP4A2 targeting could be a potential therapeutic approach to treat cancers and other degenerative diseases by modulating lysosomal homeostasis and macroautophagy/autophagy.Abbreviations: AAA+: ATPases associated with diverse cellular activities; AKI: acute kidney injury; CBB: Coomassie Brilliant Blue; CRISPR: clustered regularly interspaced short palindromic repeats; ELDR: endo-lysosomal damage response; GFP: green fluorescent protein; GST: ‎glutathione S-transferase; IHC: immunohistochemistry; IP: immunoprecipitation; LAMP1: lysosomal-associated membrane protein 1; LC-MS: liquid chromatography-mass spectrometry; LGALS3/Gal3: galectin 3; LLOMe: L-leucyl-L-leucine methyl ester; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MEF: mouse embryonic fibroblast; PLAA: phospholipase A2, activating protein; PTP4A2: protein tyrosine phosphatase 4a2; PUB: NGLY1/PNGase/UBA- or UBX-containing protein; PUL: PLAP, Ufd3, and Lub1; TFEB: transcription factor EB; UBXN6/UBXD1: UBX domain protein 6; UPS: ubiquitin-proteasome system; VCP/p97: valosin containing protein; VCPIP1: valosin containing protein interacting protein 1; YOD1: YOD1 deubiquitinase.

Computational Methods in Cooperation with Experimental Approaches to Design Protein Tyrosine Phosphatase 1B Inhibitors in Type 2 Diabetes Drug Design: A Review of the Achievements of This Century

Protein tyrosine phosphatase 1B (PTP1B) dephosphorylates phosphotyrosine residues and is an important regulator of several signaling pathways, such as insulin, leptin, and the ErbB signaling network, among others. Therefore, this enzyme is considered an attractive target to design new drugs against type 2 diabetes, obesity, and cancer. To date, a wide variety of PTP1B inhibitors that have been developed by experimental and computational approaches. In this review, we summarize the achievements with respect to PTP1B inhibitors discovered by applying computer-assisted drug design methodologies (virtual screening, molecular docking, pharmacophore modeling, and quantitative structure–activity relationships (QSAR)) as the principal strategy, in cooperation with experimental approaches, covering articles published from the beginning of the century until the time this review was submitted, with a focus on studies conducted with the aim of discovering new drugs against type 2 diabetes. This review encourages the use of computational techniques and includes helpful information that increases the knowledge generated to date about PTP1B inhibition, with a positive impact on the route toward obtaining a new drug against type 2 diabetes with PTP1B as a molecular target.

The double lives of phosphatases of regenerating liver: A structural view of their catalytic and noncatalytic activities

Phosphatases of regenerating liver (PRLs) are protein phosphatases involved in the control of cell growth and migration. They are known to promote cancer metastasis but, despite over 20 years of study, there is still no consensus about their mechanism of action. Recent work has revealed that PRLs lead double lives, acting both as catalytically active enzymes and as pseudophosphatases. The three known PRLs belong to the large family of cysteine phosphatases that form a phosphocysteine intermediate during catalysis. Uniquely to PRLs, this intermediate is stable, with a lifetime measured in hours. As a consequence, PRLs have very little phosphatase activity. Independently, PRLs also act as pseudophosphatases by binding CNNM membrane proteins to regulate magnesium homeostasis. In this function, an aspartic acid from CNNM inserts into the phosphatase catalytic site of PRLs, mimicking a substrate–enzyme interaction. The delineation of PRL pseudophosphatase and phosphatase activities in vivo was impossible until the recent identification of PRL mutants defective in one activity or the other. These mutants showed that CNNM binding was sufficient for PRL oncogenicity in one model of metastasis, but left unresolved its role in other contexts. As the presence of phosphocysteine prevents CNNM binding and CNNM-binding blocks catalytic activity, these two activities are inherently linked. Additional studies are needed to untangle the intertwined catalytic and noncatalytic functions of PRLs. Here, we review the current understanding of the structure and biophysical properties of PRL phosphatases.

Design, synthesis, kinetic, molecular dynamics, and hypoglycemic effect characterization of new and potential selective benzimidazole derivatives as Protein Tyrosine Phosphatase 1B inhibitors

Protein-tyrosine phosphatase 1B (PTP1B) is a negative regulator of insulin signaling pathway and has been validated as a therapeutic target for type 2 diabetes. A wide variety of scaffolds have been included in the structure of PTP1B inhibitors, one of them is the benzimidazole nucleus. Here, we report the design and synthesis of a new series of di- and tri- substituted benzimidazole derivatives including their kinetic and structural characterization as PTP1B inhibitors and hypoglycemic activity. Results show that compounds 43, 44, 45, and 46 are complete mixed type inhibitors with a Ki of 12.6 μM for the most potent (46). SAR type analysis indicates that a chloro substituent at position 6(5), a β-naphthyloxy at position 5(6), and a p-benzoic acid attached to the linker 2-thioacetamido at position 2 of the benzimidazole nucleus, was the best combination for PTP1B inhibition and hypoglycemic activity. In addition, molecular dynamics studies suggest that these compounds could be potential selective inhibitors from other PTPs such as its closest homologous TCPTP, SHP-1, SHP-2 and CDC25B. Therefore, the compounds reported here are good hits that provide structural, kinetic, and biological information that can be used to develop novel and selective PTP1B inhibitors based on benzimidazole scaffold.

Grass carp (Ctenopharyngodon idellus) SHP2 suppresses IFN I expression via decreasing the phosphorylation of GSK3β in a non-contact manner

As a tyrosine phosphatase, Src homology 2-containing protein tyrosine phosphatase 2 (SHP2) serves as an inhibitor in PI3K-Akt pathway. In mammals, SHP2 can phosphorylate GSK3β at Y216 site to control the expression of IFN. So far, the multiple functions of SHP2 have been reported in mammals. However, little is known about fish SHP2. In this study, we cloned and identified a grass carp (Ctenopharyngodon idellus) SHP2 gene (CiSHP2, MT373151). SHP2 is conserved among different vertebrates by amino acid sequences alignment and the phylogenetic tree analysis. CiSHP2 shared the closest homology with Danio rerio SHP2. Simultaneously, SHP2 was also tested in grass carp tissues and CIK (C. idellus kidney) cells. We found that it responded to poly I:C stimulation. CiSHP2 was located in the cytoplasm just as the same as those of mammals. Interestingly, it inhibited the phosphorylation level of GSK3β in a non-contact manner. Meanwhile CiGSK3β interacted with and directly phosphorylated CiTBK1. In addition, we found that CiSHP2 also reduced the phosphorylation level of CiTBK1 by CiGSK3β, and then it depressed the expression of IFN I via GSK3β-TBK1 axis. These results suggested that CiSHP2 was involved in CiGSK3β and CiTBK1 activity but not regulated their transcriptional level. At the same time, we also found that CiSHP2 also influenced the activity of CiIRF3. Therefore, fish SHP2 inhibited IFN I expression through blocking GSK3β-TBK1 signal axis.

PTPN4 germline variants result in aberrant neurodevelopment and growth

Protein-tyrosine phosphatases (PTPs) are pleomorphic regulators of eukaryotic cellular responses to extracellular signals that function by modulating the phosphotyrosine of specific proteins. A handful of PTPs have been implicated in germline and somatic human disease. Using exome sequencing, we identified missense and truncating variants in PTPN4 in six unrelated individuals with varying degrees of intellectual disability or developmental delay. The variants occurred de novo in all five subjects in whom segregation analysis was possible. Recurring features include postnatal growth deficiency or excess, seizures, and, less commonly, structural CNS, heart, or skeletal anomalies. PTPN4 is a widely expressed protein tyrosine phosphatase that regulates neuronal cell homeostasis by protecting neurons against apoptosis. We suggest that pathogenic variants in PTPN4 confer risk for growth and cognitive abnormalities in humans.

Modulation of TCR Signaling by Tyrosine Phosphatases: From Autoimmunity to Immunotherapy

Early TCR signaling is dependent on rapid phosphorylation and dephosphorylation of multiple signaling and adaptor proteins, leading to T cell activation. This process is tightly regulated by an intricate web of interactions between kinases and phosphatases. A number of tyrosine phosphatases have been shown to modulate T cell responses and thus alter T cell fate by negatively regulating early TCR signaling. Mutations in some of these enzymes are associated with enhanced predisposition to autoimmunity in humans, and mouse models deficient in orthologous genes often show T cell hyper-activation. Therefore, phosphatases are emerging as potential targets in situations where it is desirable to enhance T cell responses, such as immune responses to tumors. In this review, we summarize the current knowledge about tyrosine phosphatases that regulate early TCR signaling and discuss their involvement in autoimmunity and their potential as targets for tumor immunotherapy.

Structural and functional characterization of the PDZ domain of the human phosphatase PTPN3 and its interaction with the human papillomavirus E6 oncoprotein

The human protein tyrosine phosphatase non-receptor type 3 (PTPN3) is a PDZ (PSD-95/Dlg/ZO-1) domain-containing phosphatase with a tumor-suppressive or a tumor-promoting role in many cancers. Interestingly, the high-risk genital human papillomavirus (HPV) types 16 and 18 target the PDZ domain of PTPN3. The presence of a PDZ binding motif (PBM) on E6 confers interaction with a number of different cellular PDZ domain-containing proteins and is a marker of high oncogenic potential. Here, we report the molecular basis of interaction between the PDZ domain of PTPN3 and the PBM of the HPV E6 protein. We combined biophysical, NMR and X-ray experiments to investigate the structural and functional properties of the PDZ domain of PTPN3. We showed that the C-terminal sequences from viral proteins encompassing a PBM interact with PTPN3-PDZ with similar affinities to the endogenous PTPN3 ligand MAP kinase p38γ. PBM binding stabilizes the PDZ domain of PTPN3. We solved the X-ray structure of the PDZ domain of PTPN3 in complex with the PBM of the HPV E6 protein. The crystal structure and the NMR chemical shift mapping of the PTPN3-PDZ/peptide complex allowed us to pinpoint the main structural determinants of recognition of the C-terminal sequence of the E6 protein and the long-range perturbations induced upon PBM binding.

New insights into the Vav1 activation cycle in lymphocytes

Vav1 is a hematopoietic-specific Rho GDP/GTP exchange factor and signaling adaptor. Although these activities are known to be stimulated by direct Vav1 phosphorylation, little information still exists regarding the regulatory layers that influence the overall Vav1 activation cycle. Using a collection of cell models and activation-mimetic Vav1 mutants, we show here that the dephosphorylated state of Vav1 in nonstimulated T cells requires the presence of a noncatalytic, phospholipase Cγ1-Slp76-mediated inhibitory pathway. Upon T cell stimulation, Vav1 becomes rapidly phosphorylated via the engagement of Lck and, to a much lesser extent, other Src family kinases and Zap70. In this process, Lck, Zap70 and the adaptor protein Lat contribute differently to the dynamics and amplitude of the Vav1 phosphorylated pool. Consistent with a multiphosphosite activation mechanism, the optimal stimulation of Vav1 can only be recapitulated by the combination of several activation-mimetic phosphosite mutants. The analysis of these mutants has also unveiled the presence of different Vav1 signaling competent states that are influenced by phosphosites present in the N- and C-terminal domains of the protein.

Regulation of the Human Phosphatase PTPN4 by the inter-domain linker connecting the PDZ and the phosphatase domains

Human protein tyrosine phosphatase non-receptor type 4 (PTPN4) has been shown to prevent cell death. The active form of human PTPN4 consists of two globular domains, a PDZ (PSD-95/Dlg/ZO-1) domain and a phosphatase domain, tethered by a flexible linker. Targeting its PDZ domain abrogates this protection and triggers apoptosis. We previously demonstrated that the PDZ domain inhibits the phosphatase activity of PTPN4 and that the mere binding of a PDZ ligand is sufficient to release the catalytic inhibition. We demonstrate here that the linker connecting the PDZ domain and the phosphatase domain is involved in the regulation of the phosphatase activity in both PDZ-related inhibition and PDZ ligand-related activation events. We combined bioinformatics and kinetic studies to decipher the role of the linker in the PTPN4 activity. By comparing orthologous sequences, we identified a conserved patch of hydrophobic residues in the linker. We showed that mutations in this patch affect the regulation of the PTPN4 bidomain indicating that the PDZ-PDZ ligand regulation of PTPN4 is a linker-mediated mechanism. However, the mutations do not alter the binding of the PDZ ligand. This study strengthens the notion that inter-domain linker can be of functional importance in enzyme regulation of large multi-domain proteins.

The adaptor protein TRAF3 inhibits interleukin-6 receptor signaling in B cells to limit plasma cell development

Tumor necrosis factor receptor-associated factor 3 (TRAF3) is an adaptor protein that inhibits signaling by CD40 and by the receptor for B cell-activating factor (BAFF) and negatively regulates homeostatic B cell survival. Loss-of-function mutations in TRAF3 are associated with human B cell malignancies, in particular multiple myeloma. The cytokine interleukin-6 (IL-6) supports the differentiation and survival of normal and neoplastic plasma cells. We found that mice with a deficiency in TRAF3 specifically in B cells (B-Traf3(-/-) mice) had about twice as many plasma cells as did their littermate controls. TRAF3-deficient B cells had enhanced responsiveness to IL-6, and genetic loss of IL-6 in B-Traf3(-/-) mice restored their plasma cell numbers to normal. TRAF3 inhibited IL-6 receptor (IL-6R)-mediated signaling by facilitating the association of PTPN22 (a nonreceptor protein tyrosine phosphatase) with the kinase Janus-activated kinase 1 (Jak1), which in turn blocked phosphorylation of the transcription factor STAT3 (signal transducer and activator of transcription 3). Consistent with these results, the number of plasma cells in the PTPN22-deficient mice was increased compared to that in the wild-type mice. Our findings identify TRAF3 and PTPN22 as inhibitors of IL-6R signaling in B cells and reveal a previously uncharacterized role for TRAF3 in the regulation of plasma cell differentiation.

A THEMIS:SHP1 complex promotes T-cell survival

THEMIS is critical for conventional T-cell development, but its precise molecular function remains elusive. Here, we show that THEMIS constitutively associates with the phosphatases SHP1 and SHP2. This complex requires the adapter GRB2, which bridges SHP to THEMIS in a Tyr-phosphorylation-independent fashion. Rather, SHP1 and THEMIS engage with the N-SH3 and C-SH3 domains of GRB2, respectively, a configuration that allows GRB2-SH2 to recruit the complex onto LAT. Consistent with THEMIS-mediated recruitment of SHP to the TCR signalosome, THEMIS knock-down increased TCR-induced CD3-ζ phosphorylation, Erk activation and CD69 expression, but not LCK phosphorylation. This generalized TCR signalling increase led to augmented apoptosis, a phenotype mirrored by SHP1 knock-down. Remarkably, a KI mutation of LCK Ser59, previously suggested to be key in ERK-mediated resistance towards SHP1 negative feedback, did not affect TCR signalling nor ligand discrimination in vivo. Thus, the THEMIS:SHP complex dampens early TCR signalling by a previously unknown molecular mechanism that favours T-cell survival. We discuss possible implications of this mechanism in modulating TCR output signals towards conventional T-cell development and differentiation.

Cellular biochemistry methods for investigating protein tyrosine phosphatases

SIGNIFICANCE

The protein tyrosine phosphatases (PTPs) are a family of proteins that play critical roles in cellular signaling and influence many aspects of human health and disease. Although a wealth of information has been collected about PTPs since their discovery, many questions regarding their regulation and function still remain.

CRITICAL ISSUES

Of particular importance are the elucidation of the biological substrates of individual PTPs and understanding of the chemical and biological basis for temporal and spatial resolution of PTP activity within a cell.

RECENT ADVANCES

Drawing from recent advances in both biology and chemistry, innovative approaches have been developed to study the intracellular biochemistry and physiology of PTPs. We provide a summary of PTP-tailored techniques and approaches, emphasizing methodologies to study PTP activity within a cellular context. We first provide a discussion of methods for identifying PTP substrates, including substrate-trapping mutants and synthetic peptide libraries for substrate selectivity profiling. We next provide an overview of approaches for monitoring intracellular PTP activity, including a discussion of mechanistic-based probes, gel-based assays, substrates that can be used intracellularly, and assays tied to cell growth. Finally, we review approaches used for monitoring PTP oxidation, a key regulatory pathway for these enzymes, discussing the biotin switch method and variants of this approach, along with affinity trapping techniques and probes designed to detect PTP oxidation.

FUTURE DIRECTIONS

Further development of approaches to investigate the intracellular PTP activity and functions will provide specific insight into their mechanisms of action and control of diverse signaling pathways.

Tyrosine phosphatase PTPN22: multifunctional regulator of immune signaling, development, and disease

Inheritance of a coding variant of the protein tyrosine phosphatase nonreceptor type 22 (PTPN22) gene is associated with increased susceptibility to autoimmunity and infection. Efforts to elucidate the mechanisms by which the PTPN22-C1858T variant modulates disease risk revealed that PTPN22 performs a signaling function in multiple biochemical pathways and cell types. Capable of both enzymatic activity and adaptor functions, PTPN22 modulates signaling through antigen and innate immune receptors. PTPN22 plays roles in lymphocyte development and activation, establishment of tolerance, and innate immune cell-mediated host defense and immunoregulation. The disease-associated PTPN22-R620W variant protein is likely involved in multiple stages of the pathogenesis of autoimmunity. Establishment of a tolerant B cell repertoire is disrupted by PTPN22-R620W action during immature B cell selection, and PTPN22-R620W alters mature T cell responsiveness. However, after autoimmune attack has initiated tissue injury, PTPN22-R620W may foster inflammation through modulating the balance of myeloid cell-produced cytokines.

The putative role of the C1858T polymorphism of protein tyrosine phosphatase PTPN22 gene in autoimmunity

Autoimmune diseases represent a heterogeneous group of conditions whose incidence is increasing worldwide. This has stimulated studies on their etiopathogenesis, derived from a complex interaction between genetic and environmental factors, in order to improve prevention and treatment of these diseases. An increasing amount of epidemiologic investigations has associated the presence of the C1858T polymorphism in the protein tyrosine phosphatase non-receptor type 22 (PTPN22) gene to the onset of several autoimmune diseases including insulin-dependent diabetes mellitus (Type 1 diabetes). PTPN22 encodes for the lymphoid tyrosine phosphatase Lyp. This belongs to non-receptor-type protein tyrosine phosphatases involved in lymphocyte activation and differentiation. In humans, Lyp may have a role in the negative regulation of T cell receptor signaling. The single nucleotide polymorphism C1858T encodes for a more active phosphatase Lyp R620W. This has the ability to induce a higher negative regulation of T cell receptor signaling. Thus, C1858T could play an important role at the level of thymocyte polarization and escape of autoreactive T lymphocytes, through the positive selection of otherwise negatively selected autoimmune T cells. In this review we discuss the physiological role exerted by the PTPN22 gene and its encoded Lyp product in lymphocyte processes. We highlight the pathogenic significance of the C1858T PTPN22 polymorphism in human autoimmunity with special reference to Type 1 diabetes. Recently the genetic variation in PTPN22 was shown to induce altered function of T and B-lymphocytes. In particular BCR signaling defects and alterations in the B cell compartment were reported in T1D patients. We finally speculate on the possible development of novel therapeutic treatments in human autoimmunity aiming to selectively target the variant Lyp protein in autoreactive T and B lymphocytes.

Regulation of TCR signalling by tyrosine phosphatases: from immune homeostasis to autoimmunity

More than half of the known protein tyrosine phosphatases (PTPs) in the human genome are expressed in T cells, and significant progress has been made in elucidating the biology of these enzymes in T-cell development and function. Here we provide a systematic review of the current understanding of the roles of PTPs in T-cell activation, providing insight into their mechanisms of action and regulation in T-cell receptor signalling, the phenotypes of their genetically modified mice, and their possible involvement in T-cell-mediated autoimmune disease. Our projection is that the interest in PTPs as mediators of T-cell homeostasis will continue to rise with further functional analysis of these proteins, and PTPs will be increasingly considered as targets of immunomodulatory therapies.

Inhibition of hematopoietic protein tyrosine phosphatase augments and prolongs ERK1/2 and p38 activation

The hematopoietic protein tyrosine phosphatase (HePTP) is implicated in the development of blood cancers through its ability to negatively regulate the mitogen-activated protein kinases (MAPKs) ERK1/2 and p38. Small-molecule modulators of HePTP activity may become valuable in treating hematopoietic malignancies such as T cell acute lymphoblastic leukemia (T-ALL) and acute myelogenous leukemia (AML). Moreover, such compounds will further elucidate the regulation of MAPKs in hematopoietic cells. Although transient activation of MAPKs is crucial for growth and proliferation, prolonged activation of these important signaling molecules induces differentiation, cell cycle arrest, cell senescence, and apoptosis. Specific HePTP inhibitors may promote the latter and thereby may halt the growth of cancer cells. Here, we report the development of a small molecule that augments ERK1/2 and p38 activation in human T cells, specifically by inhibiting HePTP. Structure-activity relationship analysis, in silico docking studies, and mutagenesis experiments reveal how the inhibitor achieves selectivity for HePTP over related phosphatases by interacting with unique amino acid residues in the periphery of the highly conserved catalytic pocket. Importantly, we utilize this compound to show that pharmacological inhibition of HePTP not only augments but also prolongs activation of ERK1/2 and, especially, p38. Moreover, we present similar effects in leukocytes from mice intraperitoneally injected with the inhibitor at doses as low as 3 mg/kg. Our results warrant future studies with this probe compound that may establish HePTP as a new drug target for acute leukemic conditions.

Why is PTPN22 a good candidate susceptibility gene for autoimmune disease?

The PTPN22 locus is one of the strongest risk factors outside of the major histocompatability complex that associates with autoimmune diseases. PTPN22 encodes lymphoid protein tyrosine phosphatase (Lyp) which is expressed exclusively in immune cells. A single base change in the coding region of this gene resulting in an arginine to tryptophan amino acid substitution within a polyproline binding motif associates with type 1 diabetes, rheumatoid arthritis, systemic lupus erythematosis, Hashimotos thyroiditis, Graves disease, Addison's disease, Myasthenia Gravis, vitiligo, systemic sclerosis juvenile idiopathic arthritis and psoriatic arthritis. Here, we review the current understanding of the PTPN22 locus from a genetic, geographical, biochemical and functional perspective.

Inhibition of the Hematopoietic Protein Tyrosine Phosphatase by Phenoxyacetic Acids

Protein tyrosine phosphatases (PTPs) have only recently become the focus of attention in the search for novel drug targets despite the fact that they play vital roles in numerous cellular processes and are implicated in many human diseases. The hematopoietic protein tyrosine phosphatase (HePTP) is often found dysregulated in preleukemic myelodysplastic syndrome (MDS), as well as in acute myelogenous leukemia (AML). Physiological substrates of HePTP include the mitogen-activated protein kinases (MAPKs) ERK1/2 and p38. Specific modulators of HePTP catalytic activity will be useful for elucidating mechanisms of MAPK regulation in hematopietic cells, and may also provide treatments for hematopoietic malignancies such as AML. Here we report the discovery of phenoxyacetic acids as inhibitors of HePTP. Structure-activity relationship (SAR) analysis and in silico docking studies reveal the molecular basis of HePTP inhibition by these compounds. We also show that these compounds are able to penetrate cell membranes and inhibit HePTP in human T lymphocytes.

Characterization of protein tyrosine phosphatase H1 knockout mice in animal models of local and systemic inflammation

Background

PTPH1 is a protein tyrosine phosphatase expressed in T cells but its effect on immune response is still controversial. PTPH1 dephosphorylates TCRzeta in vitro, inhibiting the downstream inflammatory signaling pathway, however no immunological phenotype has been detected in primary T cells derived from PTPH1-KO mice. The aim of the present study is to characterize PTPH1 phenotype in two in vivo inflammatory models and to give insights in possible PTPH1 functions in cytokine release.

Methods

We challenged PTPH1-KO mice with two potent immunomodulatory molecules, carrageenan and LPS, in order to determine PTPH1 possible role in inflammatory response in vivo. Cytokine release, inflammatory pain and gene expression were investigated in challenged PTPH1-WT and KO mice.

Results

The present study shows that carrageenan induces a trend of slightly increased spontaneous pain sensitivity in PTPH1-KO mice compared to WT (wild-type) littermates, but no differences in cytokine release, induced pain perception and cellular infiltration have been detected between the two genotypes in this mouse model. On the other hand, LPS-induced TNFα, MCP-1 and IL10 release was significantly reduced in PTPH1-KO plasma compared to WTs 30 and 60 minutes post challenge. No cytokine release modulation was detectable 180 minutes post LPS challenge.

Conclusion

In conclusion, the present study points out a slight potential role for PTPH1 in spontaneous pain sensitivity and it indicates that this phosphatase might play a role in the positive regulation of the LPS-induced cytokines release in vivo, in contrast to previous reports indicating PTPH1 as potential negative regulator of immune response.

Structural basis of substrate recognition by hematopoietic tyrosine phosphatase

Hematopoietic tyrosine phosphatase (HePTP) is one of three members of the kinase interaction motif (KIM) phosphatase family which also includes STEP and PCPTP1. The KIM-PTPs are characterized by a 15 residue sequence, the KIM, which confers specific high-affinity binding to their only known substrates, the MAP kinases Erk and p38, an interaction which is critical for their ability to regulate processes such as T cell differentiation (HePTP) and neuronal signaling (STEP). The KIM-PTPs are also characterized by a unique set of residues in their PTP substrate binding loops, where 4 of the 13 residues are differentially conserved among the KIM-PTPs as compared to more than 30 other class I PTPs. One of these residues, T106 in HePTP, is either an aspartate or asparagine in nearly every other PTP. Using multiple techniques, we investigate the role of these KIM-PTP specific residues in order to elucidate the molecular basis of substrate recognition by HePTP. First, we used NMR spectroscopy to show that Erk2-derived peptides interact specifically with HePTP at the active site. Next, to reveal the molecular details of this interaction, we solved the high-resolution three-dimensional structures of two distinct HePTP-Erk2 peptide complexes. Strikingly, we were only able to obtain crystals of these transient complexes using a KIM-PTP specific substrate-trapping mutant, in which the KIM-PTP specific residue T106 was mutated to an aspartic acid (T106D). The introduced aspartate side chain facilitates the coordination of the bound peptides, thereby stabilizing the active dephosphorylation complex. These structures establish the essential role of HePTP T106 in restricting HePTP specificity to only those substrates which are able to interact with KIM-PTPs via the KIM (e.g., Erk2, p38). Finally, we describe how this interaction of the KIM is sufficient for overcoming the otherwise weak interaction at the active site of KIM-PTPs.

Expression of the putative tumor suppressor gene PTPN13/PTPL1 is an independent prognostic marker for overall survival in breast cancer

Although it is well established that some protein tyrosine kinases have a prognostic value in breast cancer, the involvement of protein tyrosine phosphatases (PTPs) is poorly substantiated for breast tumors. Three of these enzymes (PTP-gamma, LAR, and PTPL1) are already known to be regulated by estrogens or their antagonists in human breast cancer cells. We used a real-time reverse transcriptase polymerase chain reaction method to test the expression levels of PTP-gamma, LAR and its neuronal isoform, and PTPL1 in a training set of RNA from 59 breast tumors. We sought correlations between levels of these molecular markers, current tumor markers, and survival. We then quantified the expression level of the selected phosphatase in 232 additional samples, resulting in a testing set of 291 breast tumor RNAs from patients with a median follow-up of 6.4 years. The Spearman nonparametric test revealed correlations between PTPL1 expression and differentiation markers. Cox univariate analysis of the overall survival studies demonstrated that PTPL1 is a prognostic factor [risk ratio (RR)=0.45], together with the progesterone receptor (PR) (RR=0.52) and node involvement (RR=1.58). In multivariate analyses, PTPL1 and PR retained their prognostic value (RRs of 0.48 and 0.55, respectively). This study demonstrates for the first time that PTPL1 expression level is an independent prognostic indicator of favorable outcome for patients with breast cancer. In conjunction with our mechanistic studies, this finding identifies PTPL1 as an important regulatory element of human breast tumor aggressiveness and sensitivity to treatments such as antiestrogens and antiaromatase.

The FERM and PDZ domain-containing protein tyrosine phosphatases, PTPN4 and PTPN3, are both dispensable for T cell receptor signal transduction

PTPN3 and PTPN4 are two closely-related non-receptor protein tyrosine phosphatases (PTP) that, in addition to a PTP domain, contain FERM (Band 4.1, Ezrin, Radixin, and Moesin) and PDZ (PSD-95, Dlg, ZO-1) domains. Both PTP have been implicated as negative-regulators of early signal transduction through the T cell antigen receptor (TCR), acting to dephosphorylate the TCRzeta chain, a component of the TCR complex. Previously, we reported upon the production and characterization of PTPN3-deficient mice which show normal TCR signal transduction and T cell function. To address if the lack of a T cell phenotype in PTPN3-deficient mice can be explained by functional redundancy of PTPN3 with PTPN4, we generated PTPN4-deficient and PTPN4/PTPN3 double-deficient mice. As in PTPN3 mutants, T cell development and homeostasis and TCR-induced cytokine synthesis and proliferation were found to be normal in PTPN4-deficient and PTPN4/PTPN3 double-deficient mice. PTPN13 is another FERM and PDZ domain-containing non-receptor PTP that is distantly-related to PTPN3 and PTPN4 and which has been shown to function as a negative-regulator of T helper-1 (Th1) and Th2 differentiation. Therefore, to determine if PTPN13 might compensate for the loss of PTPN3 and PTPN4 in T cells, we generated mice that lack functional forms of all three PTP. T cells from triple-mutant mice developed normally and showed normal cytokine secretion and proliferative responses to TCR stimulation. Furthermore, T cell differentiation along the Th1, Th2 and Th17 lineages was largely unaffected in triple-mutants. We conclude that PTPN3 and PTPN4 are dispensable for TCR signal transduction.

Sequence-specific 1H, 13C and 15N backbone resonance assignments of the 34 kDa catalytic domain of human PTPN7

PTPN7 is a protein tyrosine phosphatase responsible for inactivation of MAPK in leukocytes. Here we report the backbone resonance assignments of the 34 kDa phosphatase domain of human PTPN7, which is amplified in myeloid malignancies and deleted in lymphoproliferative disorders.

Knockout mice reveal a role for protein tyrosine phosphatase H1 in cognition

Background

The present study has investigated the protein tyrosine phosphatase H1 (PTPH1) expression pattern in mouse brain and its impact on CNS functions.

Methods

We have previously described a PTPH1-KO mouse, generated by replacing the PTP catalytic and the PDZ domain with a LacZ neomycin cassette. PTPH1 expression pattern was evaluated by LacZ staining in the brain and PTPH1-KO and WT mice (n = 10 per gender per genotype) were also behaviorally tested for CNS functions.

Results

In CNS, PTPH1 is expressed during development and in adulthood and mainly localized in hippocampus, thalamus, cortex and cerebellum neurons. The behavioral tests performed on the PTPH1-KO mice showed an impact on working memory in male mice and an impaired learning performance at rotarod in females.

Conclusion

These results demonstrate for the first time a neuronal expression of PTPH1 and its functionality at the level of cognition.

The protein tyrosine phosphatase PTPN4/PTP-MEG1, an enzyme capable of dephosphorylating the TCR ITAMs and regulating NF-kappaB, is dispensable for T cell development and/or T cell effector functions

T cell receptor signaling processes are controlled by the integrated actions of families of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPases). Several distinct cytosolic protein tyrosine phosphatases have been described that are able to negatively regulate TCR signaling pathways, including SHP-1, SHP-2, PTPH1, and PEP. Using PTPase substrate-trapping mutants and wild type enzymes, we determined that PTPN4/PTP-MEG1, a PTPH1-family member, could complex and dephosphorylate the ITAMs of the TCR zeta subunit. In addition, the substrate-trapping derivative augmented basal and TCR-induced activation of NF-kappaB in T cells. To characterize the contribution of this PTPase in T cells, we developed PTPN4-deficient mice. T cell development and TCR signaling events were comparable between wild type and PTPN4-deficient animals. The magnitude and duration of TCR-regulated ITAM phosphorylation, as well as overall protein phosphorylation, was unaltered in the absence of PTPN4. Finally, Th1- and Th2-derived cytokines and in vivo immune responses to Listeria monocytogenes were equivalent between wild type and PTPN4-deficient mice. These findings suggest that additional PTPases are involved in controlling ITAM phosphorylations.

Analysis of molecular determinants of PRL-3

In order to analyse whether a C-terminal polybasic sequence represents a nuclear localization signal (NLS) we obtained several truncated and mutant forms of protein of regerating liver (PRL)-3 and evaluated their subcellular localization as compared to the wild-type form. Our results invalidate the hypothesis that this is an NLS. We also analysed the influence of the C- and N-terminal residues on the phosphatase activity of PRL-3. Our results provide in vitro evidence that the C-terminal CAAX motif, besides directing the protein farnesylation, plays an additional regulatory role by inhibiting the catalytic efficiency of PRL-3. Taking into account the results we obtained, as well as reported data, we propose a hypothetical molecular mechanism for the nucleocytoplasmic localization and transfer of PRL-3.

Protein tyrosine phosphatases in autoimmunity

Protein tyrosine phosphatases (PTPs) are important regulators of many cellular functions and a growing number of PTPs have been implicated in human disease conditions, such as developmental defects, neoplastic disorders, and immunodeficiency. Here, we review the involvement of PTPs in human autoimmunity. The leading examples include the allelic variant of the lymphoid tyrosine phosphatase (PTPN22), which is associated with multiple autoimmune diseases, and mutations that affect the exon-intron splicing of CD45 (PTPRC). We also find it likely that additional PTPs are involved in susceptibility to autoimmune and inflammatory diseases. Finally, we discuss the possibility that PTPs regulating the immune system may serve as therapeutic targets.

TCR-induced downregulation of protein tyrosine phosphatase PEST augments secondary T cell responses

We report that the protein tyrosine phosphatase PTP-PEST is expressed in resting human and mouse CD4(+) and CD8(+) T cells, but not in Jurkat T leukemia cells, and that PTP-PEST protein, but not mRNA, was dramatically downregulated in CD4(+) and CD8(+) primary human T cells upon T cell activation. This was also true in mouse CD4(+) T cells, but less striking in mouse CD8(+) T cells. PTP-PEST reintroduced into Jurkat at levels similar to those in primary human T cells, was a potent inhibitor of TCR-induced transactivation of reporter genes driven by NFAT/AP-1 and NF-kappaB elements and by the entire IL-2 gene promoter. Introduction of PTP-PEST into previously activated primary human T cells also reduced subsequent IL-2 production by these cells in response to TCR and CD28 stimulation. The inhibitory effect of PTP-PEST was associated with dephosphorylation the Lck kinase at its activation loop site (Y394), reduced early TCR-induced tyrosine phosphorylation, reduced ZAP-70 phosphorylation and inhibition of MAP kinase activation. We propose that PTP-PEST tempers T cell activation by dephosphorylating TCR-proximal signaling molecules, such as Lck, and that down-regulation of PTP-PEST may be a reason for the increased response to TCR triggering of previously activated T cells.

Preliminary evidence for interaction of PTPN12 polymorphism with TSHR genotype and association with Graves' ophthalmopathy

OBJECTIVE

Protein tyrosine phosphatases (PTPs), such as PTPN22, are important regulators of signal transduction from the T cell receptor and have been associated with autoimmunity. PTPN12 interacts with the same T cell activation accessory molecules, Grb2 and Csk kinase, as the Graves' disease (GD) associated PTPN22 encoded lymphoid tyrosine phosphatase (LYP) molecule and also plays a key role in T cell receptor signalling, leading to the hypothesis that it too may be involved in GD susceptibility.

DESIGN

PTPN12 was tested for association in a large well-characterized UK Caucasian case control cohort using seven tagging single nucleotide polymorphisms (SNPs). Patients A total of 1058 GD patients and 864 controls. Measurements Tests for association with the disease.

RESULTS

Despite adequate statistical power to detect an effect if present, none of the seven tag SNPs were associated with GD (P = 0.925-0.089). Three SNPs (rs1468682, rs4729535 and rs17467232), however, demonstrated association with the presence of ophthalmopathy NOSPECS classes 2-4 (P = 0.039-0.004). Four SNPs (rs1468682, rs4729535, rs17155601 and rs17467232) revealed evidence of interaction with the previously associated thyrotropin hormone receptor (TSHR) rs2268458 SNP (P = 0.035-0.002).

CONCLUSIONS

No association was detected between individual PTPN12 tag SNPs and GD but preliminary evidence suggests PTPN12 confers an increased risk of mild/moderate ophthalmopathy (NOSPECS classes 2-4) and that PTPN12 interacts with the TSHR. Replication of these preliminary results is now required in larger independent datasets to validate these findings.

The putative tumor suppressor gene PTPN13/PTPL1 induces apoptosis through insulin receptor substrate-1 dephosphorylation

The protein tyrosine phosphatase (PTP) PTPL1/PTPN13 is a candidate tumor suppressor gene. Indeed, PTPL1 activity has been reported recently to be decreased through somatic mutations, allelic loss, or promoter methylation in some tumors. We showed previously that its expression was necessary for inhibition of Akt activation and induction of apoptosis by antiestrogens in breast cancer cells. Implications of the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway in cancer progression are now well established, and our study was therefore designed to define whether PTPL1 is sufficient to inhibit this pathway and, if so, to identify a direct substrate of this PTP, which may trigger a proapoptotic effect. We first show by complementary approaches that PTPL1 specifically dephosphorylates insulin receptor substrate-1 (IRS-1) in vitro and in cellulo. Next, our experiments using a dominant-negative mutant and RNA interference confirm the crucial role of PTPL1 in IRS-1 dephosphorylation. Finally, we report that PTPL1 expression is sufficient to block the IRS-1/PI3K/Akt signaling pathway, to inhibit the insulin-like growth factor-I effect on cell survival, and to induce apoptosis. Altogether, these data provide the first evidence for a direct positive role of the putative tumor suppressor gene PTPL1/PTPN13 on apoptosis and identify its target in the IRS-1/PI3K/Akt signaling pathway.

Taenia solium: Identification and preliminary characterization of a lipid binding protein with homology to the SEC14 catalytic domain

The objective of this work is to identify proteins of the human and porcine parasite, Taenia solium, which may be exploited for control of the parasite. Through screening a cDNA library of T. solium metacestodes, we have identified a novel Sec-14-like Taenia lipid-binding protein that may play an important role in membrane trafficking. The Sec14-like sequence is a single copy gene, encoding a putative polypeptide of 320 amino acids and 36.1 kDa (sec14Tsol protein). Secondary amino acid structural analysis suggested that the sec14Tsol protein might contain two distinct structural domains, an amino-terminal alpha-helix rich domain and a mixed alpha-helix/beta-stand carboxy-terminal zone, showing homology with the conserved SEC14 domain found in a great number of proteins that bind lipids, as the regulators of membrane trafficking between Golgi membrane bilayers. Significantly, therefore, in a phosphoinositide-binding assay, sec14Tsol purified recombinant protein specifically interacted with important lipid regulators of membrane trafficking, with a preference for PI(3)P(2), PI(3,4)P(2), PI(4,5)P(2) and phosphatidic acid. Moreover, the sec14Tsol protein was localized in the Golgi apparatus of transfected cells and in the spiral canal region of T. solium metacestode tegument. As sec14Tsol protein may play an important role in membrane trafficking, its demonstrated localisation in the intact parasite tegument suggests its involvement in the function of the tegument and thus perhaps interaction with the host.

Characterization of the C-terminal ER membrane anchor of PTP1B

The tyrosine phosphatase PTP1B is an important regulator of cell function. In living cells PTP1B activity is restricted to the vicinity of the endoplasmic reticulum (ER) by post-translational C-terminal attachment of PTP1B to the ER membrane network. In our study we investigated the membrane anchor of PTP1B by use of EGFP fusion proteins. We demonstrate that the membrane anchor of PTP1B cannot be narrowed down to a unique amino acid sequence with a defined start and stop point but rather is moveable within several amino acids. Removal of up to seven amino acids from the C-terminus, as well as exchange of single amino acids in the putative transmembrane sequence did not influence subcellular localization of PTP1B. With the method of bimolecular fluorescence complementation we could demonstrate dimerization of PTP1B in vivo. Homodimerization was, in contrast to other tail-anchored proteins, not dependent on the membrane anchor. Our data demonstrate that the C-terminal membrane anchor of PTP1B is formed by a combination of a single stretch transmembrane domain (TMD) followed by a tail. TMD and tail length are variable and there are no sequence-specific features. Our data for PTP1B are consistent with a concept that explains the ER membrane anchor of tail-anchored proteins as a physicochemical structure.

Suppression of hepatitis B viral gene expression by protein-tyrosine phosphatase PTPN3

Protein-tyrosine phosphatase PTPN3 is a membrane-associated non-receptor protein-tyrosine phosphatase. PTPN3 contains a N-terminal FERM domain, a middle PDZ domain, and a C-terminal phosphatase domain. Upon co-expression of PTPN3, the level of human hepatitis B viral (HBV) RNAs, 3.5 kb, 2.4/2.1 kb, and 0.7 kb transcribed from a replicating HBV expression plasmid is significantly reduced in human hepatoma HuH-7 cells. When the expression of endogenous PTPN3 protein is diminished by specific small interfering RNA, the expression of HBV genes is enhanced, indicating that the endogenous PTPN3 indeed plays a suppressive role on HBV gene expression. PTPN3 can interact with HBV core protein. The interaction is mediated via the PDZ domain of PTPN3 and the carboxyl-terminal last four amino acids of core. Either deletion of PDZ domain of PTPN3 or substitution of PDZ ligand in core has no effect on PTPN3-mediated suppression. These results clearly show that the interaction of PTPN3 with core is not required for PTPN3 suppressive effect. Mutation of (359)serine and (835)serine of 14-3-3beta binding sites to alanine, which slightly reduces the interaction with 14-3-3beta, does not influence the PTPN3 effect. In contrast, mutation of the invariant (842)cysteine residue in phosphatase domain to serine, which makes the phosphatase activity inactive, does not change its subcellular localization and interaction with core or 14-3-3beta, but completely abolishes PTPN3-mediated suppression. Furthermore, deletion of FERM domain does not affect the phosphatase activity or interaction with 14-3-3beta, but changes the subcellular localization from cytoskeleton-membrane interface to cytoplasm and nucleus, abolishes binding to core, and diminishes the PTPN3 effect on HBV gene expression. Taken together, these results demonstrate that the phosphatase activity and FERM domain of PTPN3 are essential for its suppression of HBV gene expression.

The lipid-binding SEC14 domain

Protein-lipid interactions are important for protein targeting, signal transduction, lipid transport, lipid biosynthesis, lipid metabolism, and the maintenance of cellular compartments and membranes. Specific lipid-binding protein domains, such as PH, FYVE, PX, PHD, C2 and SEC14 homology domains, mediate interactions between proteins and specific phospholipids. Here we review the published literature, plus some of our most recent unpublished findings, regarding the biology of the SEC14 domain, also known as CRAL_TRIO domain.

Normal TCR signal transduction in mice that lack catalytically active PTPN3 protein tyrosine phosphatase

PTPN3 (PTPH1) is a cytoskeletal protein tyrosine phosphatase that has been implicated as a negative regulator of early TCR signal transduction and T cell activation. To determine whether PTPN3 functions as a physiological negative regulator of TCR signaling in primary T cells, we generated gene-trapped and gene-targeted mouse strains that lack expression of catalytically active PTPN3. PTPN3 phosphatase-negative mice were born in expected Mendelian ratios and exhibited normal growth and development. Furthermore, numbers and ratios of T cells in primary and secondary lymphoid organs were unaffected by the PTPN3 mutations and there were no signs of spontaneous T cell activation in the mutant mice with increasing age. TCR-induced signal transduction, cytokine production, and proliferation was normal in PTPN3 phosphatase-negative mice. This was observed using both quiescent T cells and recently stimulated T cells where expression of PTPN3 is substantially up-regulated. We conclude, therefore, that the phosphatase activity of PTPN3 is dispensable for negative regulation of TCR signal transduction and T cell activation.

Association of Protein-tyrosine Phosphatase MEG2 via Its Sec14p Homology Domain with Vesicle-trafficking Proteins

The protein-tyrosine phosphatase PTPMEG2 is located on the cytoplasmic face of the enclosing membrane of secretory vesicles, where it regulates vesicle size by promoting homotypic vesicle fusion by dephosphorylating N-ethylmaleimide-sensitive factor, a key regulator of vesicle fusion. Here we address the question of how PTPMEG2 is targeted to this subcellular location. Using a series of deletion mutants, we pinpointed the N-terminal Sec14p homology (SEC14) domain of PTPMEG2, residues 1-261, as the region containing the secretory vesicle targeting signal. This domain, alone or appended to a heterologous protein, was localized to intracellular vesicle membranes. Yeast two-hybrid screening identified a number of secretory vesicle proteins that interacted directly with the SEC14 domain of PTPMEG2, providing a mechanism for PTPMEG2 targeting to secretory vesicles. Two such proteins, mannose 6-phosphate receptor-interacting protein TIP47 and Arfaptin2, were found to alter PTPMEG2 localization when overexpressed, and elimination of TIP47 resulted in loss of PTPMEG2 function. We conclude that the N terminus of PTPMEG2 is necessary for the targeting of this phosphatase to the secretory vesicle compartment by association with other proteins involved in intracellular transport.

Nonreceptor Protein-Tyrosine Phosphatases in Immune Cell Signaling

Tyrosyl phosphorylation plays a critical role in multiple signaling pathways regulating innate and acquired immunity. Although tyrosyl phosphorylation is a reversible process, we know much more about the functions of protein-tyrosine kinases (PTKs) than about protein-tyrosine phosphatases (PTPs). Genome sequencing efforts have revealed a large and diverse superfamily of PTPs, which can be subdivided into receptor-like (RPTPs) and nonreceptor (NRPTPs). The role of the RPTP CD45 in immune cell signaling is well known, but those of most other PTPs remain poorly understood. Here, we review the mechanism of action, regulation, and physiological functions of NRPTPs in immune cell signaling. Such an analysis indicates that PTPs are as important as PTKs in regulating the immune system.

Protein tyrosine phosphatase function: the substrate perspective

It is now well established that the members of the PTP (protein tyrosine phosphatase) superfamily play critical roles in fundamental biological processes. Although there has been much progress in defining the function of PTPs, the task of identifying substrates for these enzymes still presents a challenge. Many PTPs have yet to have their physiological substrates identified. The focus of this review will be on the current state of knowledge of PTP substrates and the approaches used to identify them. We propose experimental criteria that should be satisfied in order to rigorously assign PTP substrates as bona fide. Finally, the progress that has been made in defining the biological roles of PTPs through the identification of their substrates will be discussed.

Cellular localization of PRL-1 and PRL-2 gene expression in normal adult human tissues

Recent evidence suggests that the PRL-1 and -2 phosphatases may be multifunctional enzymes with diverse roles in a variety of tissue and cell types. Northern blotting has previously shown widespread expression of both transcripts; however, little is known about the cell type-specific expression of either gene, especially in human tissues. Therefore, we investigated expression patterns for PRL-1 and -2 genes in multiple normal, adult human tissues using in situ hybridization. Although both transcripts were ubiquitously expressed, they exhibited strikingly different patterns of expression. PRL-2 was expressed heavily in almost every tissue and cell type examined, whereas PRL-1 expression levels varied considerably both between tissue types and between individuals. Widespread expression of PRL-1 and -2 in multiple organ systems suggests an important functional role for these enzymes in normal tissue homeostasis. In addition, the variable patterns of expression for these genes may provide distinct activities in each tissue or cell type.

Cardiac sodium channel Na(v)1.5 interacts with and is regulated by the protein tyrosine phosphatase PTPH1

In order to identify proteins interacting with the cardiac voltage-gated sodium channel Na(v)1.5, we used the last 66 amino acids of the C-terminus of the channel as bait to screen a human cardiac cDNA library. We identified the protein tyrosine phosphatase PTPH1 as an interacting protein. Pull-down experiments confirmed the interaction, and indicated that it depends on the PDZ-domain binding motif of Na(v)1.5. Co-expression experiments in HEK293 cells showed that PTPH1 shifts the Na(v)1.5 availability relationship toward hyperpolarized potentials, whereas an inactive PTPH1 or the tyrosine kinase Fyn does the opposite. The results of this study suggest that tyrosine phosphorylation destabilizes the inactivated state of Na(v)1.5.

Identification of substrates of human protein-tyrosine phosphatase PTPN22

Stimulation of mature T cells activates a downstream signaling cascade involving temporally and spatially regulated phosphorylation and dephosphorylation events mediated by protein-tyrosine kinases and phosphatases, respectively. PTPN22 (Lyp), a non-receptor protein-tyrosine phosphatase, is expressed exclusively in cells of hematopoietic origin, notably in T cells where it represses signaling through the T cell receptor. We used substrate trapping coupled with mass spectrometry-based peptide identification in an unbiased approach to identify physiological substrates of PTPN22. Several potential substrates were identified in lysates from pervanadate-stimulated Jurkat cells using PTPN22-D195A/C227S, an optimized substrate trap mutant of PTPN22. These included three novel PTPN22 substrates (Vav, CD3epsilon, and valosin containing protein) and two known substrates of PEP, the mouse homolog of PTPN22 (Lck and Zap70). T cell antigen receptor (TCR) zeta was also identified as a potential substrate in Jurkat lysates by direct immunoblotting. In vitro experiments with purified recombinant proteins demonstrated that PTPN22-D195A/C227S interacted directly with activated Lck, Zap70, and TCRzeta, confirming the initial substrate trap results. Native PTPN22 dephosphorylated Lck and Zap70 at their activating tyrosine residues Tyr-394 and Tyr-493, respectively, but not at the regulatory tyrosines Tyr-505 (Lck) or Tyr-319 (Zap70). Native PTPN22 also dephosphorylated TCRzeta in vitro and in cells, and its substrate trap variant co-immunoprecipitated with TCRzeta when both were coexpressed in 293T cells, establishing TCRzeta as a direct substrate of PTPN22.

Secretion of the mammalian Sec14p-like phosphoinositide-binding p45 protein

Protein-lipid interactions are important for protein targeting, signal transduction, lipid transport, and the maintenance of cellular compartments and membranes. Specific lipid-binding protein domains, such as PH, FYVE, PX, PHD, C2 and SEC14 homology domains, mediate interactions between proteins and specific phospholipids. We recently cloned a 45-kDa protein from rat olfactory epithelium, which is homologous to the yeast Sec14p phosphatidylinositol (PtdIns) transfer protein and we report here that this protein binds to PtdIns(3,4,5)P3 and far weaker to less phosphorylated derivatives of PtdIns. Expression of the p45 protein in COS-1 cells resulted in accumulation of the protein in secretory vesicles and in the extracellular space. The secreted material contained PtdIns(3,4,5)P3. Our findings are the first report of a Sec14p-like protein involved in transport out of a cell and, to the best of our knowledge, inositol-containing phospholipids have not previously been detected in the extracellular space. Our findings suggest that p45 and phosphoinositides may participate in the formation of the protective mucus on nasal epithelium.