SHP-2 positively regulates myogenesis by coupling to the Rho GTPase signaling pathway

SHP2 as a primordial epigenetic enzyme expunges histone H3 pTyr-54 to amend androgen receptor homeostasis

Mutations that decrease or increase the activity of the tyrosine phosphatase, SHP2 (encoded by PTPN11), promotes developmental disorders and several malignancies by varying phosphatase activity. We uncovered that SHP2 is a distinct class of an epigenetic enzyme; upon phosphorylation by the kinase ACK1/TNK2, pSHP2 was escorted by androgen receptor (AR) to chromatin, erasing hitherto unidentified pY54-H3 (phosphorylation of histones H3 at Tyr54) epigenetic marks to trigger a transcriptional program of AR. Noonan Syndrome with Multiple Lentigines (NSML) patients, SHP2 knock-in mice, and ACK1 knockout mice presented dramatic increase in pY54-H3, leading to loss of AR transcriptome. In contrast, prostate tumors with high pSHP2 and pACK1 activity exhibited progressive downregulation of pY54-H3 levels and higher AR expression that correlated with disease severity. Overall, pSHP2/pY54-H3 signaling acts as a sentinel of AR homeostasis, explaining not only growth retardation, genital abnormalities and infertility among NSML patients, but also significant AR upregulation in prostate cancer patients.

Setting sail: Maneuvering SHP2 activity and its effects in cancer

Since the discovery of tyrosine phosphorylation being a critical modulator of cancer signaling, proteins regulating phosphotyrosine levels in cells have fast become targets of therapeutic intervention. The nonreceptor protein tyrosine phosphatase (PTP) coded by the PTPN11 gene "SHP2" integrates phosphotyrosine signaling from growth factor receptors into the RAS/RAF/ERK pathway and is centrally positioned in processes regulating cell development and oncogenic transformation. Dysregulation of SHP2 expression or activity is linked to tumorigenesis and developmental defects. Even as a compelling anti-cancer target, SHP2 was considered "undruggable" for a long time owing to its conserved catalytic PTP domain that evaded drug development. Recently, SHP2 has risen from the "undruggable curse" with the discovery of small molecules that manipulate its intrinsic allostery for effective inhibition. SHP2's unique domain arrangement and conformation(s) allow for a truly novel paradigm of inhibitor development relying on skillful targeting of noncatalytic sites on proteins. In this review we summarize the biological functions, signaling properties, structural attributes, allostery and inhibitors of SHP2.

A comprehensive review of SHP2 and its role in cancer

Src homology 2-containing protein tyrosine phosphatase 2 (SHP2) is a non-receptor protein tyrosine phosphatase ubiquitously expressed mainly in the cytoplasm of several tissues. SHP2 modulates diverse cell signaling events that control metabolism, cell growth, differentiation, cell migration, transcription and oncogenic transformation. It interacts with diverse molecules in the cell, and regulates key signaling events including RAS/ERK, PI3K/AKT, JAK/STAT and PD-1 pathways downstream of several receptor tyrosine kinases (RTKs) upon stimulation by growth factors and cytokines. SHP2 acts as both a phosphatase and a scaffold, and plays prominently oncogenic functions but can be tumor suppressor in a context-dependent manner. It typically acts as a positive regulator of RTKs signaling with some inhibitory functions reported as well. SHP2 expression and activity is regulated by such factors as allosteric autoinhibition, microRNAs, ubiquitination and SUMOylation. Dysregulation of SHP2 expression or activity causes many developmental diseases, and hematological and solid tumors. Moreover, upregulated SHP2 expression or activity also decreases sensitivity of cancer cells to anticancer drugs. SHP2 is now considered as a compelling anticancer drug target and several classes of SHP2 inhibitors with different mode of action are developed with some already in clinical trial phases. Moreover, novel SHP2 substrates and functions are rapidly growing both in cell and cancer. In view of this, we comprehensively and thoroughly reviewed literatures about SHP2 regulatory mechanisms, substrates and binding partners, biological functions, roles in human cancers, and different classes of small molecule inhibitors target this oncoprotein in cancer.

Double-edged roles of protein tyrosine phosphatase SHP2 in cancer and its inhibitors in clinical trials

Phosphorylation is a reversible post-translational modification regulated by phosphorylase and dephosphorylase to mediate important cellular events. Src homology-2-containing protein tyrosine phosphatase 2 (SHP2) encoded by PTPN11 is the first identified oncogenic protein in protein tyrosine phosphatases family. Serving as a convergent node, SHP2 is involved in multiple cascade signaling pathways including Ras-Raf-MEK-ERK, PI3K-AKT, JAK-STAT and PD-1/PD-L1 pathways. Especially, the double-edged roles of SHP2 based on the substrate specificity in various biological contexts dramatically increase the effect complexity in different SHP2-associated diseases. Evidences suggest that by collaborating with other mutations in associated pathways, dysregulation of SHP2 contributes to the pathogenesis of different cancers, making SHP2 a promising therapeutic target for cancer treatment. SHP2 can either act as oncogenic factor or tumor suppressor in different diseases, and both the conserved catalytic dephosphorylation mechanism and the unique allosteric regulation mechanism of SHP2 provide opportunities for the development of SHP2 inhibitors and activators. To date, several small-molecule SHP2 inhibitors have advanced into clinical trials for mono- or combined therapy of cancers. Moreover, SHP2 activators and proteolysis-targeting chimera (PROTAC)-based degraders also display therapeutic promise. In this review, we comprehensively summarize the overall structures, regulation mechanisms, double-edged roles of SHP2 in both physiological and carcinogenic pathways, and SHP2 inhibitors in clinical trials. SHP2 activators and degraders are also briefly discussed. This review aims to provide in-depth understanding of the biological roles of SHP2 and highlight therapeutic potential of targeting SHP2.

Phosphatase orphan 1 inhibits myoblast proliferation and promotes myogenic differentiation

Myogenesis includes sequential stages of progenitor cell proliferation, myogenic commitment and differentiation, myocyte fusion, and myotube maturation. Different stages of myogenesis are orchestrated and regulated by myogenic regulatory factors and various downstream cellular signaling. Here we identify phosphatase orphan 1 (Phospho1) as a new player in myogenesis. During activation, proliferation, and differentiation of quiescent satellite cells, the expression of Phospho1 gradually increases. Overexpression of Phospho1 inhibits myoblast proliferation but promotes their differentiation and fusion. Conversely, knockdown of Phospho1 accelerates myoblast proliferation but impairs myotube formation. Moreover, knockdown of Phospho1 decreases the OXPHO protein levels and mitochondria density, whereas overexpression of Phospho1 upregulates OXPHO protein levels and promotes mitochondrial oxygen consumption. Finally, we show that Phospho1 expression is controlled by myogenin, which binds to the promoter of Phospho1 to regulate its transcription. These results indicate a key role of Phospho1 in regulating myogenic differentiation and mitochondrial function.

Protein tyrosine phosphatase 11 acts through RhoA/ROCK to regulate eosinophil accumulation in the allergic airway

Src homology domain 2-containing protein tyrosine phosphatase 2 (SHP2) participates in multiple cell functions including cell shape, movement, and differentiation. Therefore, we investigated the potential role of SHP2 in eosinophil recruitment into lungs in allergic airway inflammation and explored the underlying mechanism. Both SHP2 and Ras homolog family member A (RhoA) kinase were robustly activated in the airway eosinophils of children with allergic asthma and of a mouse model with allergic airway inflammation. Moreover, inhibition of SHP2 activity by its specific inhibitors reverses the dephosphorylation of p190-A Rho GTPase-activating protein and in turn attenuates RhoA/Rho-associated protein kinase (ROCK) signaling, resulting in the attenuation of eosinophil migration in response to platelet-activating factor stimulation. Specifically, SHP2 deletion in myeloid cells did not affect the number and classification of circulating leukocytes but significantly attenuated the allergen-induced inflammatory cell, especially eosinophil, infiltration into lungs, and airway hyperreactivity. Notably, genetic interaction between RhoA and SHP2 indicated that RhoA inactivation and SHP2 deletion synergistically attenuated the allergen-induced eosinophil infiltration into lungs and airway hyperreactivity, whereas overexpression of active RhoA robustly restored the SHP2 deletion-resultant attenuation of allergen-induced eosinophil recruitment into lungs and airway hyperreactivity as well. Thus, this study demonstrates that SHP2 via RhoA/ROCK signaling regulates eosinophil recruitment in allergic airway inflammation and possibly in allergic asthma.-Xu, C., Wu, X., Lu, M., Tang, L., Yao, H., Wang, J., Ji, X., Hussain, M., Wu, J., Wu, X. Protein tyrosine phosphatase 11 acts through RhoA/ROCK to regulate eosinophil accumulation in the allergic airway.

SHP-2 phosphatase contributes to KRAS-driven intestinal oncogenesis but prevents colitis-associated cancer development

A major risk factor of developing colorectal cancer (CRC) is the presence of chronic inflammation in the colon. In order to understand how inflammation contributes to CRC development, the present study focused on SHP-2, a tyrosine phosphatase encoded by PTPN11 gene in which polymorphisms have been shown to be markers of colitis susceptibility. Conversely, gain-of-function mutations in PTPN11 gene (E76 residue) have been found in certain sporadic CRC. Results shown herein demonstrate that SHP-2 expression was markedly increased in sporadic human adenomas but not in advanced colorectal tumors. SHP-2 silencing inhibited proliferative, invasive and tumoral properties of both intestinal epithelial cells (IECs) transformed by oncogenic KRAS and of human CRC cells. IEC-specific expression of a SHP-2^E76K activated mutant in mice was not sufficient to induce tumorigenesis but markedly promoted tumor growth under the Apc^Min/+ background. Conversely, mice with a conditional deletion of SHP-2 in IECs developed colitis-associated adenocarcinomas with age, associated with sustained activation of Wnt/β-catenin, NFκB and STAT3 signalings in the colonic mucosae. Moreover, SHP-2 epithelial deficiency considerably increased tumor load in Apc^Min/+ mice, shifting tumor incidence toward the colon. Overall, these results reveal that SHP-2 can exert opposing functions in the large intestine: it can promote or inhibit tumorigenesis depending of the inflammatory context.

SHP2 associates with nuclear localization of STAT3: significance in progression and prognosis of colorectal cancer

Tyrosine phosphatase SHP2, encoded by PTPN11, has been implicated in many physiologic and pathologic processes in neoplastic progression. However, controversies are emerging from many studies, indicating SHP2 has a dual role in different types of tumors. We aimed to explore the role of SHP2 in progression and prognosis of colorectal cancer (CRC). SHP2 inhibited CRC cell proliferation and migration, and the phosphorylation of STAT3 was negatively regulated by SHP2 in CRC. SHP2 and nuclear STAT3 were examined in 270 CRC tissues. SHP2 was significantly correlated with nuclear STAT3 (Spearman’s rho = −0.408, P ≤ 0.001). Based on Cox regression analysis, patients with high levels of SHP2 and low levels of nuclear STAT3 had longer disease-specific survival (DSS) (HR, 0.362; 95% CI, 0.165–0.794) and disease-free survival (DFS) (HR, 0.447; 95% CI, 0.227–0.877). Further, low levels of SHP2 and high levels of nuclear STAT3 were independently associated with adverse outcomes in the whole cohort (DFS; HR, 2.353; 95% CI, 1.199–4.619). These results suggest that combination of SHP2 and nuclear STAT3 is a strong prognostic predictor in CRC.

PTPN11 induces endoplasmic stress and apoptosis in SH-SY5Y cells

PTPN11 is associated with regulation of growth factor signaling pathways in neuronal cells. Using SH-SY5Y neuroblastoma cells, we showed that adeno-associated virus (AAV)-mediated PTPN11 upregulation was associated with TrkB antagonism, reduced neuritogenesis and enhanced endoplasmic reticulum (ER) stress response leading to apoptotic changes. Genetic knock-down of PTPN11 on the other hand leads to increased TrkB phosphorylation in SH-SY5Y cells. ER stress response induced by PTPN11 upregulation was alleviated pharmacologically by a TrkB agonist. Conversely the enhanced ER stress response induced by TrkB receptor antagonism was ameliorated by PTPN11 suppression, providing evidence of cross-talk of PTPN11 effects with TrkB actions. BDNF treatment of neuronal cells with PTPN11 upregulation also resulted in reduced expression of ER stress protein markers. This study provides evidence of molecular interactions between PTPN11 and the TrkB receptor in SH-SY5Y cells. The results reinforce the role played by PTPN11 in regulating neurotrophin protective signaling in neuronal cells and highlight that PTPN11 dysregulation promotes apoptotic activation. Based on these findings we suggest that blocking PTPN11 could have potential beneficial effects to limit the progression of neuronal loss in neurodegenerative disorders.

New and Unexpected Biological Functions for the Src-Homology 2 Domain-Containing Phosphatase SHP-2 in the Gastrointestinal Tract

SHP-2 is a tyrosine phosphatase expressed in most embryonic and adult tissues. SHP-2 regulates many cellular functions including growth, differentiation, migration, and survival. Genetic and biochemical evidence show that SHP-2 is required for rat sarcoma viral oncogene/extracellular signal-regulated kinases mitogen-activated protein kinase pathway activation by most tyrosine kinase receptors, as well as by G-protein-coupled and cytokine receptors. In addition, SHP-2 can regulate the Janus kinase/signal transducers and activators of transcription, nuclear factor-κB, phosphatidyl-inositol 3-kinase/Akt, RhoA, Hippo, and Wnt/β-catenin signaling pathways. Emerging evidence has shown that SHP-2 dysfunction represents a key factor in the pathogenesis of gastrointestinal diseases, in particular in chronic inflammation and cancer. Variations within the gene locus encoding SHP-2 have been associated with increased susceptibility to develop ulcerative colitis and gastric atrophy. Furthermore, mice with conditional deletion of SHP-2 in intestinal epithelial cells rapidly develop severe colitis. Similarly, hepatocyte-specific deletion of SHP-2 induces hepatic inflammation, resulting in regenerative hyperplasia and development of tumors in aged mice. However, the SHP-2 gene initially was suggested to be a proto-oncogene because activating mutations of this gene were found in pediatric leukemias and certain forms of liver and colon cancers. Moreover, SHP-2 expression is up-regulated in gastric and hepatocellular cancers. Notably, SHP-2 functions downstream of cytotoxin-associated antigen A (CagA), the major virulence factor of Helicobacter pylori, and is associated with increased risks of gastric cancer. Further compounding this complexity, most recent findings suggest that SHP-2 also coordinates carbohydrate, lipid, and bile acid synthesis in the liver and pancreas. This review aims to summarize current knowledge and recent data regarding the biological functions of SHP-2 in the gastrointestinal tract.

SHP2 sails from physiology to pathology

Over the two past decades, mutations of the PTPN11 gene, encoding the ubiquitous protein tyrosine phosphatase SHP2 (SH2 domain-containing tyrosine phosphatase 2), have been identified as the causal factor of several developmental diseases (Noonan syndrome (NS), Noonan syndrome with multiple lentigines (NS-ML), and metachondromatosis), and malignancies (juvenile myelomonocytic leukemia). SHP2 plays essential physiological functions in organism development and homeostasis maintenance by regulating fundamental intracellular signaling pathways in response to a wide range of growth factors and hormones, notably the pleiotropic Ras/Mitogen-Activated Protein Kinase (MAPK) and the Phosphoinositide-3 Kinase (PI3K)/AKT cascades. Analysis of the biochemical impacts of PTPN11 mutations first identified both loss-of-function and gain-of-function mutations, as well as more subtle defects, highlighting the major pathophysiological consequences of SHP2 dysregulation. Then, functional genetic studies provided insights into the molecular dysregulations that link SHP2 mutants to the development of specific traits of the diseases, paving the way for the design of specific therapies for affected patients. In this review, we first provide an overview of SHP2's structure and regulation, then describe its molecular roles, notably its functions in modulating the Ras/MAPK and PI3K/AKT signaling pathways, and its physiological roles in organism development and homeostasis. In the second part, we describe the different PTPN11 mutation-associated pathologies and their clinical manifestations, with particular focus on the biochemical and signaling outcomes of NS and NS-ML-associated mutations, and on the recent advances regarding the pathophysiology of these diseases.

Alterations in the phosphoproteomic profile of cells expressing a non-functional form of the SHP2 phosphatase

The phosphatase SHP-2 plays an essential role in growth factor signaling and mutations in its locus is the cause of congenital and acquired pathologies. Mutations of SHP-2 are known to affect the activation of the RAS pathway. Gain-of-function mutations cause the Noonan syndrome, the most common non-chromosomal congenital disorder. In order to obtain a holistic picture of the intricate regulatory mechanisms underlying SHP-2 physiology and pathology, we set out to characterize perturbations of the cell phosphorylation profile caused by an altered localization of SHP-2. To describe the proteins whose activity may be directly or indirectly modulated by SHP-2 activity, we identified tyrosine peptides that are differentially phosphorylated in wild type SHP-2 cells and isogenic cells expressing a non-functional SHP-2 variant that cannot dephosphorylate the physiological substrates due to a defect in cellular localization upon growth factor stimulation. By an iTRAQ based strategy coupled to mass spectrometry, we have identified 63 phosphorylated tyrosine residues in 53 different proteins whose phosphorylation is affected by SHP-2 activity. Some of these confirm already established regulatory mechanisms while many others suggest new possible signaling routes that may contribute to the modulation of the ERK and p38 pathways by SHP-2. Interestingly many new proteins that we found to be regulated by SHP-2 activity are implicated in the formation and regulation of focal adhesions.

Amelioration of cisplatin-induced experimental peripheral neuropathy by a small molecule targeting p75 NTR

Cisplatin is an effective and widely used first-line chemotherapeutic drug for treating cancers. However, many patients sustain cisplatin-induced peripheral neuropathy (CIPN), often leading to a reduction in drug dosages or complete cessation of treatment altogether. Therefore, it is important to understand cisplatin mechanisms in peripheral nerve tissue mediating its toxicity and identify signaling pathways for potential intervention. Rho GTPase activation is increased following trauma in several models of neuronal injury. Thus, we investigated whether components of the Rho signaling pathway represent important neuroprotective targets with the potential to ameliorate CIPN and thereby optimize current chemotherapy treatment regimens. We have developed a novel CIPN model in the mouse. Using this model and primary neuronal culture, we determined whether LM11A-31, a small-molecule, orally bioavailable ligand of the p75 neurotrophin receptor (p75(NTR)), can modulate Rho GTPase signaling and reduce CIPN. Von Frey filament analysis of sural nerve function showed that LM11A-31 treatment prevented decreases in peripheral nerve sensation seen with cisplatin treatment. Morphometric analysis of harvested sural nerves revealed that cisplatin-induced abnormal nerve fiber morphology and the decreases in fiber area were alleviated with concurrent LM11A-31 treatment. Cisplatin treatment increased RhoA activity accompanied by the reduced tyrosine phosphorylation of SHP2, which was reversed by LM11A-31. LM11A-31 also countered the effects of calpeptin, which activated RhoA by inhibiting SHP2 tyrosine phosphatase. Therefore, suppression of RhoA signaling by LM11A-31 that modulates p75(NTR) or activates SHP2 tyrosine phosphatase downstream of the NGF receptor enhances neuroprotection in experimental CIPN in mouse model.

Regulation of bone and skeletal development by the SHP-2 protein tyrosine phosphatase

Src homology-2 protein tyrosine phosphatase (SHP-2) that is encoded by the PTPN11 gene in humans is an intracellular signaling molecule that couples growth factor receptors to activation of the Ras small GTP-binding protein that regulates cell growth, proliferation and differentiation. Germline mutations of PTPN11 are causative of Noonan syndrome and LEOPARD syndrome in humans in which there are recognized skeletal abnormalities that include growth retardation, spinal curvature and chest malformations. In addition, combined somatic and germline PTPN11 mutations have been shown to be responsible for a rare benign bone cartilaginous tumor disease known as metachondromatosis. In parallel, gene targeting studies performed in mice have revealed an essential role for SHP-2 as a regulator of bone and skeletal development. In this review the significance of these findings in mice to the understanding of the pathogenesis of skeletal abnormalities in humans with SHP-2 mutations is discussed.

LEOPARD syndrome-associated SHP2 mutation confers leanness and protection from diet-induced obesity

LEOPARD syndrome (multiple Lentigines, Electrocardiographic conduction abnormalities, Ocular hypertelorism, Pulmonary stenosis, Abnormal genitalia, Retardation of growth, sensorineural Deafness; LS), also called Noonan syndrome with multiple lentigines (NSML), is a rare autosomal dominant disorder associating various developmental defects, notably cardiopathies, dysmorphism, and short stature. It is mainly caused by mutations of the PTPN11 gene that catalytically inactivate the tyrosine phosphatase SHP2 (Src-homology 2 domain-containing phosphatase 2). Besides its pleiotropic roles during development, SHP2 plays key functions in energetic metabolism regulation. However, the metabolic outcomes of LS mutations have never been examined. Therefore, we performed an extensive metabolic exploration of an original LS mouse model, expressing the T468M mutation of SHP2, frequently borne by LS patients. Our results reveal that, besides expected symptoms, LS animals display a strong reduction of adiposity and resistance to diet-induced obesity, associated with overall better metabolic profile. We provide evidence that LS mutant expression impairs adipogenesis, triggers energy expenditure, and enhances insulin signaling, three features that can contribute to the lean phenotype of LS mice. Interestingly, chronic treatment of LS mice with low doses of MEK inhibitor, but not rapamycin, resulted in weight and adiposity gains. Importantly, preliminary data in a French cohort of LS patients suggests that most of them have lower-than-average body mass index, associated, for tested patients, with reduced adiposity. Altogether, these findings unravel previously unidentified characteristics for LS, which could represent a metabolic benefit for patients, but may also participate to the development or worsening of some traits of the disease. Beyond LS, they also highlight a protective role of SHP2 global LS-mimicking modulation toward the development of obesity and associated disorders.

Mining the function of protein tyrosine phosphatases in health and disease

Protein tyrosine phosphatases (PTPs) play a crucial role in the regulation of human health and it is now clear that PTP dysfunction is causal to a variety of human diseases. Research in the PTP field has accelerated dramatically over the last decade fueled by cutting-edge technologies in genomic and proteomic techniques. This system-wide non-biased approach when applied to the discovery of PTP function has led to the elucidation of new and unanticipated roles for the PTPs. These discoveries, driven by genomic and proteomic approaches, have uncovered novel PTP findings that range from those that describe fundamental cell signaling mechanisms to implications for PTPs as novel therapeutic targets for the treatment of human disease. This review will discuss how new PTP functions have been uncovered through studies that have utilized genomic and proteomic technologies and strategies.

The role of the protein tyrosine phosphatase SHP2 in cardiac development and disease

Congenital heart disease is the most common human developmental disorder, affecting ∼1:100 newborns, and is the primary cause of birth-defect related deaths worldwide. As a major regulator of receptor tyrosine kinase (RTK), cytokine and G-protein coupled receptor signaling, the non-receptor protein tyrosine phosphatase SHP2 plays a critical role in normal cardiac development and function. Indeed, SHP2 participates in a wide variety of cellular functions, including proliferation, survival, differentiation, migration, and cell-cell communication. Moreover, human activating and inactivating mutations of SHP2 are responsible for two related developmental disorders called Noonan and LEOPARD Syndromes, respectively, which are both characterized, in part, by congenital heart defects. Structural, enzymologic, biochemical, and SHP2 mouse model studies have together greatly enriched our knowledge of SHP2 and, as such, have also uncovered the diverse roles for SHP2 in cardiac development, including its contribution to progenitor cell specification, cardiac morphogenesis, and maturation of cardiac valves and myocardial chambers. By delineating the precise mechanisms by which SHP2 is involved in regulating these processes, we can begin to better understand the pathogenesis of cardiac disease and find more strategic and effective therapies for treatment of patients with congenital heart disorders.

PZR coordinates Shp2 Noonan and LEOPARD syndrome signaling in zebrafish and mice

Noonan syndrome (NS) is an autosomal dominant disorder caused by activating mutations in the PTPN11 gene encoding Shp2, which manifests in congenital heart disease, short stature, and facial dysmorphia. The complexity of Shp2 signaling is exemplified by the observation that LEOPARD syndrome (LS) patients possess inactivating PTPN11 mutations yet exhibit similar symptoms to NS. Here, we identify "protein zero-related" (PZR), a transmembrane glycoprotein that interfaces with the extracellular matrix to promote cell migration, as a major hyper-tyrosyl-phosphorylated protein in mouse and zebrafish models of NS and LS. PZR hyper-tyrosyl phosphorylation is facilitated in a phosphatase-independent manner by enhanced Src recruitment to NS and LS Shp2. In zebrafish, PZR overexpression recapitulated NS and LS phenotypes. PZR was required for zebrafish gastrulation in a manner dependent upon PZR tyrosyl phosphorylation. Hence, we identify PZR as an NS and LS target. Enhanced PZR-mediated membrane recruitment of Shp2 serves as a common mechanism to direct overlapping pathophysiological characteristics of these PTPN11 mutations.

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.

SHP2 phosphatase promotes mast cell chemotaxis toward stem cell factor via enhancing activation of the Lyn/Vav/Rac signaling axis

SHP2 protein-tyrosine phosphatase (encoded by Ptpn11) positively regulates KIT (CD117) signaling in mast cells and is required for mast cell survival and homeostasis in mice. In this study, we uncover a role of SHP2 in promoting chemotaxis of mast cells toward stem cell factor (SCF), the ligand for KIT receptor. Using an inducible SHP2 knockout (KO) bone marrow-derived mast cell (BMMC) model, we observed defects in SCF-induced cell spreading, polarization, and chemotaxis. To address the mechanisms involved, we tested whether SHP2 promotes activation of Lyn kinase that was previously shown to promote mast cell chemotaxis. In SHP2 KO BMMCs, SCF-induced phosphorylation of the inhibitory C-terminal residue (pY507) was elevated compared with control cells, and phosphorylation of activation loop (pY396) was diminished. Because Lyn also was detected by substrate trapping assays, these results are consistent with SHP2 activating Lyn directly by dephosphorylation of pY507. Further analyses revealed a SHP2- and Lyn-dependent pathway leading to phosphorylation of Vav1, Rac activation, and F-actin polymerization in SCF-treated BMMCs. Treatment of BMMCs with a SHP2 inhibitor also led to impaired chemotaxis, consistent with SHP2 promoting SCF-induced chemotaxis of mast cells via a phosphatase-dependent mechanism. Thus, SHP2 inhibitors may be useful to limit SCF/KIT-induced mast cell recruitment to inflamed tissues or the tumor microenvironment.

Deletion of SHP-2 in mesenchymal stem cells causes growth retardation, limb and chest deformity, and calvarial defects in mice

In mice, induced global disruption of the Ptpn11 gene, which encodes the SHP-2 tyrosine phosphatase, results in severe skeletal abnormalities. To understand the extent to which skeletal abnormalities can be attributed to perturbation of SHP-2 function in bone-forming osteoblasts and chondrocytes, we generated mice in which disruption of Ptpn11 is restricted to mesenchymal stem cells (MSCs) and their progeny, which include both cell types. MSC-lineage-specific SHP-2 knockout (MSC SHP-2 KO) mice exhibited postnatal growth retardation, limb and chest deformity, and calvarial defects. These skeletal abnormalities were associated with an absence of mature osteoblasts and massive chondrodysplasia with a vast increase in the number of terminally differentiated hypertrophic chondrocytes in affected bones. Activation of mitogen activated protein kinases (MAPKs) and protein kinase B (PKB; also known as AKT) was impaired in bone-forming cells of MSC SHP-2 KO mice, which provides an explanation for the skeletal defects that developed. These findings reveal a cell-autonomous role for SHP-2 in bone-forming cells in mice in the regulation of skeletal development. The results add to our understanding of the pathophysiology of skeletal abnormalities observed in humans with germline mutations in the PTPN11 gene (e.g. Noonan syndrome and LEOPARD syndrome).

Heterogeneous nuclear ribonucleoprotein Q is a novel substrate of SH2 domain-containing phosphatase-2

SH2 domain-containing phosphatase-2 (SHP2) is a protein-tyrosine phosphatase implicated in activation of cell signalling such as the Ras/extracellular signal-regulated kinase pathway. The substrates of SHP2 and their roles in cell activation are not fully understood. By using the substrate-trapping method with the phosphatase-dead SHP2 mutant, in which C459 was substituted by serine, and the matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometric analysis, we found that heterogeneous nuclear ribonucleoprotein Q (hnRNP Q), a protein implicated in RNA metabolisms, was a novel substrate of SHP2. Tyrosine-phosphorylated hnRNP Q was detected in HL-60, Jurkat and human peripheral blood mononuclear cells, but not mature neutrophils, treated with pervanadate. Tyrosine-phosphorylated hnRNP Q was directly bound to SHP2 in vivo and in vitro, and dephosphorylated by SHP2 in vitro. These findings suggest that hnRNP Q is a novel substrate of SHP2 and the SHP2 activity may be also involved in RNA metabolisms via dephosphorylation of hnRNP Q.

Interleukin-6 myokine signaling in skeletal muscle: a double-edged sword?

Interleukin (IL)-6 is a cytokine with pleiotropic functions in different tissues and organs. Skeletal muscle produces and releases significant levels of IL-6 after prolonged exercise and is therefore considered as a myokine. Muscle is also an important target of the cytokine. IL-6 signaling has been associated with stimulation of hypertrophic muscle growth and myogenesis through regulation of the proliferative capacity of muscle stem cells. Additional beneficial effects of IL-6 include regulation of energy metabolism, which is related to the capacity of actively contracting muscle to synthesize and release IL-6. Paradoxically, deleterious actions for IL-6 have also been proposed, such as promotion of atrophy and muscle wasting. We review the current evidence for these apparently contradictory effects, the mechanisms involved and discuss their possible biological implications.

Protein tyrosine phosphatase expression profile of rheumatoid arthritis fibroblast-like synoviocytes: a novel role of SH2 domain-containing phosphatase 2 as a modulator of invasion and survival

OBJECTIVE

The fibroblast-like synoviocytes (FLS) in the synovial intimal lining of the joint are key mediators of inflammation and joint destruction in rheumatoid arthritis (RA). In RA, these cells aggressively invade the extracellular matrix, producing cartilage-degrading proteases and inflammatory cytokines. The behavior of FLS is controlled by multiple interconnected signal transduction pathways involving reversible phosphorylation of proteins on tyrosine residues. However, little is known about the role of the protein tyrosine phosphatases (PTPs) in FLS function. This study was undertaken to explore the expression of all of the PTP genes (the PTPome) in FLS.

METHODS

A comparative screening of the expression of the PTPome in FLS from patients with RA and patients with osteoarthritis (OA) was conducted. The functional effect on RA FLS of SH2 domain-containing phosphatase 2 (SHP-2), a PTP that was up-regulated in RA, was then analyzed by knockdown using cell-permeable antisense oligonucleotides.

RESULTS

PTPN11 was overexpressed in RA FLS compared to OA FLS. Knockdown of PTPN11, which encodes SHP-2, reduced the invasion, migration, adhesion, spreading, and survival of RA FLS. Additionally, signaling in response to growth factors and inflammatory cytokines was impaired by SHP-2 knockdown. RA FLS that were deficient in SHP-2 exhibited decreased activation of focal adhesion kinase and mitogen-activated protein kinases.

CONCLUSION

These findings indicate that SHP-2 has a novel role in mediating human FLS function and suggest that it promotes the invasiveness and survival of RA FLS. Further investigation may reveal SHP-2 to be a candidate therapeutic target for RA.

Induction of Siglec-G by RNA viruses inhibits the innate immune response by promoting RIG-I degradation

RIG-I is a critical RNA virus sensor that serves to initiate antiviral innate immunity. However, posttranslational regulation of RIG-I signaling remains to be fully understood. We report here that RNA viruses, but not DNA viruses or bacteria, specifically upregulate lectin family member Siglecg expression in macrophages by RIG-I- or NF-κB-dependent mechanisms. Siglec-G-induced recruitment of SHP2 and the E3 ubiquitin ligase c-Cbl to RIG-I leads to RIG-I degradation via K48-linked ubiquitination at Lys813 by c-Cbl. By increasing type I interferon production, targeted inactivation of Siglecg protects mice against lethal RNA virus infection. Taken together, our data reveal a negative feedback loop of RIG-I signaling and identify a Siglec-G-mediated immune evasion pathway exploited by RNA viruses with implication in antiviral applications. These findings also provide insights into the functions and crosstalk of Siglec-G, a known adaptive response regulator, in innate immunity.

Megakaryocyte-specific deletion of the protein-tyrosine phosphatases Shp1 and Shp2 causes abnormal megakaryocyte development, platelet production, and function

The SH2 domain-containing protein-tyrosine phosphatases Shp1 and Shp2 have been implicated in regulating signaling from a variety of platelet and megakaryocyte receptors. In this study, we investigate the functions of Shp1 and Shp2 in megakaryocytes and platelets. Megakaryocyte/platelet (MP)-specific deletion of Shp1 in mice resulted in platelets being less responsive to collagen-related peptide due to reduced GPVI expression and signaling via the Src family kinase (SFK)-Syk-PLCγ2 pathway, and fibrinogen due to reduced SFK activity. By contrast, deletion of Shp2 in the MP lineage resulted in macrothrombocytopenia and platelets being hyper-responsive to anti-CLEC-2 antibody and fibrinogen. Shp1- and Shp2-deficient megakaryocytes had partial blocks at 2N/4N ploidy; however, only the latter exhibited reduced proplatelet formation, thrombopoietin, and integrin signaling. Mice deficient in both Shp1 and Shp2 were severely macrothrombocytopenic and had reduced platelet surface glycoprotein expression, including GPVI, αIIbβ3, and GPIbα. Megakaryocytes from these mice were blocked at 2N/4N ploidy and did not survive ex vivo. Deletion of the immunoreceptor tyrosine-based inhibition motif-containing receptor G6b-B in the MP lineage phenocopied multiple features of Shp1/2-deficient mice, suggesting G6b-B is a critical regulator of Shp1 and Shp2. This study establishes Shp1 and Shp2 as major regulators of megakaryocyte development, platelet production, and function.

Shp2 plays a crucial role in cell structural orientation and force polarity in response to matrix rigidity

Cells can sense and respond to physical properties of their surrounding extracellular matrix. We have demonstrated here that tyrosine phosphatase Shp2 plays an essential role in the response of mouse embryonic fibroblasts to matrix rigidity. On rigid surfaces, large focal adhesions (FAs) and anisotropically oriented stress fibers are formed, whereas cells plated on compliant substrates form numerous small FAs and radially oriented stress fibers. As a result, traction force is increased and organized to promote cell spreading and elongation on rigid substrates. Shp2-deficient cells do not exhibit the stiffness-dependent increase in FA size and polarized stress fibers nor the intracellular tension and cell shape change. These results indicate the involvement of Shp2 in regulating the FAs and the cytoskeleton for force maintenance and organization. The defect of FA maturation in Shp2-deficient cells was rescued by expressing Y722F Rho-associated protein kinase II (ROCKII), suggesting that ROCKII is the molecular target of Shp2 in FAs for the FA maturation. Thus, Shp2 serves as a key mediator in FAs for the regulation of structural organization and force orientation of mouse embyonic fibroblasts in determining their mechanical polarity in response to matrix rigidity.

PTPN11-associated mutations in the heart: has LEOPARD changed Its RASpots?

In this review, we focus on elucidating the cardiac function of germline mutations in the PTPN11 gene, encoding the Src homology-2 (SH2) domain-containing protein tyrosine phosphatase SHP2. PTPN11 mutations cause LEOPARD syndrome (LS) and Noonan syndrome (NS), two disorders that are part of a newly classified family of autosomal dominant syndromes termed "RASopathies," which are caused by germline mutations in components of the RAS/RAF/MEK/ERK mitogen activating protein kinase pathway. LS and NS mutants have opposing biochemical properties, and yet, in patients, these mutations produce similar cardiac abnormalities. Precisely how LS and NS mutations lead to such similar disease etiology remains largely unknown. Recent complementary in vitro, ex vivo, and in vivo analyses reveal new insights into the functions of SHP2 in normal and pathological cardiac development. These findings also reveal the need for individualized therapeutic approaches in the treatment of patients with LS and NS and, more broadly, patients with the other "RASopathy" gene mutations as well.

The tyrosine phosphatase SHP2 regulates recovery of endothelial adherens junctions through control of β-catenin phosphorylation

Reconstitution of the endothelial barrier involves SHP2-mediated dephosphorylation of VE-cadherin–associated β-catenin, leading to reassembly of adherens junctions and thereby closing the gaps between endothelial cells.

Impaired endothelial barrier function results in a persistent increase in endothelial permeability and vascular leakage. Repair of a dysfunctional endothelial barrier requires controlled restoration of adherens junctions, comprising vascular endothelial (VE)-cadherin and associated β-, γ-, α-, and p120-catenins. Little is known about the mechanisms by which recovery of VE-cadherin–mediated cell–cell junctions is regulated. Using the inflammatory mediator thrombin, we demonstrate an important role for the Src homology 2-domain containing tyrosine phosphatase (SHP2) in mediating recovery of the VE-cadherin–controlled endothelial barrier. Using SHP2 substrate-trapping mutants and an in vitro phosphatase activity assay, we validate β-catenin as a bona fide SHP2 substrate. SHP2 silencing and SHP2 inhibition both result in delayed recovery of endothelial barrier function after thrombin stimulation. Moreover, on thrombin challenge, we find prolonged elevation in tyrosine phosphorylation levels of VE-cadherin–associated β-catenin in SHP2-depleted cells. No disassembly of the VE-cadherin complex is observed throughout the thrombin response. Using fluorescence recovery after photobleaching, we show that loss of SHP2 reduces the mobility of VE-cadherin at recovered cell–cell junctions. In conclusion, our data show that the SHP2 phosphatase plays an important role in the recovery of disrupted endothelial cell–cell junctions by dephosphorylating VE-cadherin–associated β-catenin and promoting the mobility of VE-cadherin at the plasma membrane.

ROCKs cause SHP-wrecks and broken hearts

During embryogenesis, the heart is one of the first organs to develop. Its formation requires a complex combination of migration of cardiac precursors to the ventral midline coupled with the fusion of these cardiogenic fields and subsequent cellular reorganization to form a linear heart tube. A finely controlled choreography of cell proliferation, adhesion, contraction and movement drives the heart tube to loop and subsequently septate to form the four-chambered mammalian heart we are familiar with. Defining how this plethora of cellular processes is controlled both spatially and temporally is a scientific feat that has fascinated researchers for decades. Unfortunately, the complex nature of this organ’s development also makes it a prime target for mutation-induced malformation, as evidenced by the multitude of prevalent congenital heart disorders identified that afflict up to 1% of the population.

P190B RhoGAP Regulates Chromosome Segregation in Cancer Cells

Rho GTPases are overexpressed and hyperactivated in many cancers, including breast cancer. Rho proteins, as well as their regulators and effectors, have been implicated in mitosis, and their altered expression promotes mitotic defects and aneuploidy. Previously, we demonstrated that p190B Rho GTPase activating protein (RhoGAP) deficiency inhibits ErbB2-induced mammary tumor formation in mice. Here we describe a novel role for p190B as a regulator of mitosis. We found that p190B localized to centrosomes during interphase and mitosis, and that it is differentially phosphorylated during mitosis. Knockdown of p190B expression in MCF-7 and Hela cells increased the incidence of aberrant microtubule-kinetochore attachments at metaphase, lagging chromosomes at anaphase, and micronucleation, all of which are indicative of aneuploidy. Cell cycle analysis of p190B deficient MCF-7 cells revealed a significant increase in apoptotic cells with a concomitant decrease in cells in G1 and S phase, suggesting that p190B deficient cells die at the G1 to S transition. Chemical inhibition of the Rac GTPase during mitosis reduced the incidence of lagging chromosomes in p190B knockdown cells to levels detected in control cells, suggesting that aberrant Rac activity in the absence of p190B promotes chromosome segregation defects. Taken together, these data suggest that p190B regulates chromosome segregation and apoptosis in cancer cells. We propose that disruption of mitosis may be one mechanism by which p190B deficiency inhibits tumorigenesis.

Protein tyrosine phosphatase-like A regulates myoblast proliferation and differentiation through MyoG and the cell cycling signaling pathway

Protein tyrosine phosphatase-like A (PTPLa) has been implicated in skeletal myogenesis and cardiogenesis. Mutations in PTPLa correlated with arrhythmogenic right ventricular dysplasia in humans and congenital centronuclear myopathy with severe hypotonia in dogs. The molecular mechanisms of PTPLa in myogenesis are unknown. In this report, we demonstrate that PTPLa is required for myoblast growth and differentiation. The cells lacking PTPLa remained immature and failed to differentiate into mature myotubes. The repressed MyoG expression was responsible for the impaired myoblast differentiation. Meanwhile, impeded cell growth, with an obvious S-phase arrest and compromised G(2)/M transition, was observed in PTPLa-deficient myoblasts. Further study demonstrated that the upregulation of cyclin D1 and cyclin E2 complexes, along with a compromised G(2)/M transition due to the decreased CDK1 (cyclin-dependent kinase 1) activity and upregulated p21, contributed to the mutant cell S-phase arrest and eventually led to the retarded cell growth. Finally, the transcriptional regulation of the PTPLa gene was explored. We identified PTPLa as a new target gene of the serum response factor (SRF). Skeletal- and cardiac-muscle-specific SRF knockouts resulted in significant decreases in PTPLa expression, suggesting a conserved transcriptional regulation of the PTPLa gene in mice.

Smad3 signaling is required for satellite cell function and myogenic differentiation of myoblasts

TGF-β and myostatin are the two most important regulators of muscle growth. Both growth factors have been shown to signal through a Smad3-dependent pathway. However to date, the role of Smad3 in muscle growth and differentiation is not investigated. Here, we demonstrate that Smad3-null mice have decreased muscle mass and pronounced skeletal muscle atrophy. Consistent with this, we also find increased protein ubiquitination and elevated levels of the ubiquitin E3 ligase MuRF1 in muscle tissue isolated from Smad3-null mice. Loss of Smad3 also led to defective satellite cell (SC) functionality. Smad3-null SCs showed reduced propensity for self-renewal, which may lead to a progressive loss of SC number. Indeed, decreased SC number was observed in skeletal muscle from Smad3-null mice showing signs of severe muscle wasting. Further in vitro analysis of primary myoblast cultures identified that Smad3-null myoblasts exhibit impaired proliferation, differentiation and fusion, resulting in the formation of atrophied myotubes. A search for the molecular mechanism revealed that loss of Smad3 results in increased myostatin expression in Smad3-null muscle and myoblasts. Given that myostatin is a negative regulator, we hypothesize that increased myostatin levels are responsible for the atrophic phenotype in Smad3-null mice. Consistent with this theory, inactivation of myostatin in Smad3-null mice rescues the muscle atrophy phenotype.

Substrate specificity of protein tyrosine phosphatases 1B, RPTPα, SHP-1, and SHP-2

We determined the substrate specificities of the protein tyrosine phosphatases (PTPs) PTP1B, RPTPα, SHP-1, and SHP-2 by on-bead screening of combinatorial peptide libraries and solution-phase kinetic analysis of individually synthesized phosphotyrosyl (pY) peptides. These PTPs exhibit different levels of sequence specificity and catalytic efficiency. The catalytic domain of RPTPα has very weak sequence specificity and is approximately 2 orders of magnitude less active than the other three PTPs. The PTP1B catalytic domain has modest preference for acidic residues on both sides of pY, is highly active toward multiply phosphorylated peptides, but disfavors basic residues at any position, a Gly at the pY-1 position, or a Pro at the pY+1 position. By contrast, SHP-1 and SHP-2 share similar but much narrower substrate specificities, with a strong preference for acidic and aromatic hydrophobic amino acids on both sides of the pY residue. An efficient SHP-1/2 substrate generally contains two or more acidic residues on the N-terminal side and one or more acidic residues on the C-terminal side of pY but no basic residues. Subtle differences exist between SHP-1 and SHP-2 in that SHP-1 has a stronger preference for acidic residues at the pY-1 and pY+1 positions and the two SHPs prefer acidic residues at different positions N-terminal to pY. A survey of the known protein substrates of PTP1B, SHP-1, and SHP-2 shows an excellent agreement between the in vivo dephosphorylation pattern and the in vitro specificity profiles derived from library screening. These results suggest that different PTPs have distinct sequence specificity profiles and the intrinsic activity/specificity of the PTP domain is an important determinant of the enzyme's in vivo substrate specificity.

Analysis of protein tyrosine phosphatases and substrates

Protein tyrosine phosphorylation is a reversible post-translational modification that is essential for life in eukaryotic cells. The combinatorial action of both protein tyrosine kinases and protein tyrosine phosphatases (PTPs) determines the net level of cellular tyrosine phosphorylation. This unit discusses methods to determine the level of protein tyrosine phosphatase activity and methods for discovering novel substrates for protein tyrosine phosphatases.

SHP2 mediates the localized activation of Fyn downstream of the α6β4 integrin to promote carcinoma invasion

Src family kinase (SFK) activity is elevated in many cancers, and this activity correlates with aggressive tumor behavior. The α6β4 integrin, which is also associated with a poor prognosis in many tumor types, can stimulate SFK activation; however, the mechanism by which it does so is not known. In the current study, we provide novel mechanistic insight into how the α6β4 integrin selectively activates the Src family member Fyn in response to receptor engagement. Both catalytic and noncatalytic functions of SHP2 are required for Fyn activation by α6β4. Specifically, the tyrosine phosphatase SHP2 is recruited to α6β4 and its catalytic activity is stimulated through a specific interaction of its N-terminal SH2 domain with pY1494 in the β4 subunit. Fyn is recruited to the α6β4/SHP2 complex through an interaction with phospho-Y580 in the C terminus of SHP2. In addition to activating Fyn, this interaction with Y580-SHP2 localizes Fyn to sites of receptor engagement, which is required for α6β4-dependent invasion. Of significance for tumor progression, phosphorylation of Y580-SHP2 and SFK activation are increased in orthotopic human breast tumors that express α6β4 and activation of this pathway is dependent upon Y1494.

The tyrosine phosphatase Shp2 in development and cancer

Deregulation of signaling pathways, through mutation or other molecular changes, can ultimately result in disease. The tyrosine phosphatase Shp2 has emerged as a major regulator of receptor tyrosine kinase (RTK) and cytokine receptor signaling. In the last decade, germline mutations in the human PTPN11 gene, encoding Shp2, were linked to Noonan (NS) and LEOPARD syndromes, two multisymptomatic developmental disorders that are characterized by short stature, craniofacial defects, cardiac defects, and mental retardation. Somatic Shp2 mutations are also associated with several types of human malignancies, such as the most common juvenile leukemia, juvenile myelomonocytic leukemia (JMML). Whereas NS and JMML are caused by gain-of-function (GOF) mutations of Shp2, loss-of-function (LOF) mutations are thought to be associated with LEOPARD syndrome. Animal models that carry conditional LOF and GOF mutations have allowed a better understanding of the mechanism of Shp2 function in disease, and shed light on the role of Shp2 in signaling pathways that control decisive events during embryonic development or during cellular transformation/tumorigenesis.

Role of protein tyrosine phosphatase SHP2 in barrier function of pulmonary endothelium

Pulmonary edema is mediated in part by disruption of interendothelial cell contacts. Protein tyrosine phosphatases (PTP) have been shown to affect both cell-extracellular matrix and cell-cell junctions. The SH2 domain-containing nonreceptor PTP, SHP2, is involved in intercellular signaling through direct interaction with adherens junction proteins. In this study, we examined the role of SHP2 in pulmonary endothelial barrier function. Inhibition of SHP2 promoted edema formation in rat lungs and increased monolayer permeability in cultured lung endothelial cells. In addition, pulmonary endothelial cells demonstrated a decreased level of p190RhoGAP activity following inhibition of SHP2, events that were accompanied by a concomitant increase in RhoA activity. Furthermore, immunofluorescence microscopy confirmed enhanced actin stress fiber formation and diminished interendothelial staining of adherens junction complex-associated proteins upon SHP2 inhibition. Finally, immunoprecipitation and immunoblot analyses demonstrated increased tyrosine phosphorylation of VE-cadherin, beta-catenin, and p190RhoGAP proteins, as well as decreased association between p120-catenin and VE-cadherin proteins. Our findings suggest that SHP2 supports basal pulmonary endothelial barrier function by coordinating the tyrosine phosphorylation profile of VE-cadherin, beta-catenin, and p190RhoGAP and the activity of RhoA, signaling molecules important in adherens junction complex integrity.

High concentrations of HGF inhibit skeletal muscle satellite cell proliferation in vitro by inducing expression of myostatin: a possible mechanism for reestablishing satellite cell quiescence in vivo

Skeletal muscle regeneration and work-induced hypertrophy rely on molecular events responsible for activation and quiescence of resident myogenic stem cells, satellite cells. Recent studies demonstrated that hepatocyte growth factor (HGF) triggers activation and entry into the cell cycle in response to mechanical perturbation, and that subsequent expression of myostatin may signal a return to cell quiescence. However, mechanisms responsible for coordinating expression of myostatin after an appropriate time lag following activation and proliferation are not clear. Here we address the possible role of HGF in quiescence through its concentration-dependent negative-feedback mechanism following satellite cell activation and proliferation. When activated/proliferating satellite cell cultures were treated for 24 h beginning 48-h postplating with 10-500 ng/ml HGF, the percentage of bromodeoxyuridine-incorporating cells decreased down to a baseline level comparable to 24-h control cultures in a HGF dose-dependent manner. The high level HGF treatment did not impair the cell viability and differentiation levels, and cells could be reactivated by lowering HGF concentrations to 2.5 ng/ml, a concentration that has been shown to optimally stimulate activation of satellite cells in culture. Coaddition of antimyostatin neutralizing antibody could prevent deactivation and abolish upregulation of cyclin-dependent kinase (Cdk) inhibitor p21. Myostatin mRNA expression was upregulated with high concentrations of HGF, as demonstrated by RT-PCR, and enhanced myostatin protein expression and secretion were revealed by Western blots of the cell lysates and conditioned media. These results indicate that HGF could induce satellite cell quiescence by stimulating myostatin expression. The HGF concentration required (over 10-50 ng/ml), however, is much higher than that for activation, which is initiated by rapid release of HGF from its extracellular association. Considering that HGF is produced by satellite cells and spleen and liver cells in response to muscle damage, local concentrations of HGF bathing satellite cells may reach a threshold sufficient to induce myostatin expression. This time lag may delay action of the quiescence signaling program in proliferating satellite cells during initial phases of muscle regeneration followed by induction of quiescence in a subset of cells during later phases.

The phosphatase SHP2 regulates the spacing effect for long-term memory induction

A property of long-term memory (LTM) induction is the requirement for repeated training sessions spaced over time. This augmentation of memory formation with spaced resting intervals is called the spacing effect. We now show that in Drosophila, the duration of resting intervals required for inducing LTM is regulated by activity levels of the protein tyrosine phosphatase corkscrew (CSW). Overexpression of wild-type CSW in mushroom body neurons shortens the inter-trial interval required for LTM induction, whereas overexpression of constitutively active CSW proteins prolongs these resting intervals. These gain-of-function csw mutations are associated with a clinical condition of mental retardation. Biochemical analysis reveals that LTM-inducing training regimens generate repetitive waves of CSW-dependent MAPK activation, the length of which appears to define the duration of the resting interval. Constitutively active CSW proteins prolong the resting interval by altering the MAPK inactivation cycle. We thus provide insight into the molecular basis of the spacing effect.

Physiological signaling specificity by protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs) are now recognized to be involved in a multitude of signaling events that control fundamental biological processes such as cell growth, differentiation, apoptosis, and cell movement. PTPs, which were initially thought to be less discriminating in their actions compared with their protein tyrosine kinase counterparts, are now known to regulate these various biological processes in a precise manner. This review will focus on the concept that PTPs exhibit remarkable signaling specificity through intrinsic differences between their PTP domains and through various modes of regulation that endows them with the capacity to promote unique physiological responses.

The tyrosine phosphatase Shp2 (PTPN11) directs Neuregulin-1/ErbB signaling throughout Schwann cell development

The nonreceptor tyrosine phosphatase Shp2 (PTPN11) has been implicated in tyrosine kinase, cytokine, and integrin receptor signaling. We show here that conditional mutation of Shp2 in neural crest cells and in myelinating Schwann cells resulted in deficits in glial development that are remarkably similar to those observed in mice mutant for Neuregulin-1 (Nrg1) or the Nrg1 receptors, ErbB2 and ErbB3. In cultured Shp2 mutant Schwann cells, Nrg1-evoked cellular responses like proliferation and migration were virtually abolished, and Nrg1-dependent intracellular signaling was altered. Pharmacological inhibition of Src family kinases mimicked all cellular and biochemical effects of the Shp2 mutation, implicating Src as a primary Shp2 target during Nrg1 signaling. Together, our genetic and biochemical analyses demonstrate that Shp2 is an essential component in the transduction of Nrg1/ErbB signals.

SHP-2 regulates myogenesis by coupling to FAK signaling pathway

Transient dephosphorylation of FAK at Tyr-397 is required for cell cycle withdrawal early on during myogenesis. Here, we show that upon serum starvation of C2C12 myoblasts, FAK is transiently dephosphorylated in parallel with SHP-2 activation and association with FAK. SHP-2 knockdown by RNA interference suppressed the transient upregulation of SHP-2 and dephosphorylation of FAK during myogenesis. Furthermore, depletion of SHP-2 retarded the cell cycle withdrawal and the differentiation of serum-starved myoblasts into myotubes. These data provide a mechanistic basis for the reduction in FAK activity in differentiating myoblasts, indicating that myogenesis is critically triggered by FAK/SHP-2 complex.

PKCdelta influences p190 phosphorylation and activity: events independent of PKCdelta-mediated regulation of endothelial cell stress fiber and focal adhesion formation and barrier function

BACKGROUND

We have shown that protein kinase Cdelta (PKCdelta) inhibition results in increased endothelial cell (EC) permeability and decreased RhoA activity; which correlated with diminished stress fibers (SF) and focal adhesions (FA). We have also shown co-precipitation of p190RhoGAP (p190) with PKCdelta. Here, we investigated if PKCdelta regulates p190 and whether PKCdelta-mediated changes in SF and FA or permeability were dependent upon p190.

METHODS

Protein-protein interaction and activity analyses were performed using co-precipitation assays. Analysis of p190 phosphorylation was performed using in vitro kinase assays. SF and FA were analyzed by immunofluorescence analyses. EC monolayer permeability was measured using electrical cell impedance sensor (ECIS) technique.

RESULTS

Inhibition of PKCdelta increased p190 activity, while PKCdelta overexpression diminished p190 activity. PKCdelta bound to and phosphorylated both p190FF and p190GTPase domains. p190 protein overexpression diminished SF and FA formation and RhoA activity. Disruption of SF and FA or increased permeability induced upon PKCdelta inhibition, were not attenuated in EC in which the p190 isoforms were suppressed individually or concurrently.

GENERAL SIGNIFICANCE

Our findings suggest that while PKCdelta can regulate p190 activity, possibly at the FF and/or GTPase domains, the effect of PKCdelta inhibition on SF and FA and barrier dysfunction occurs through a pathway independent of p190.

SOCS1, SOCS3, and PIAS1 promote myogenic differentiation by inhibiting the leukemia inhibitory factor-induced JAK1/STAT1/STAT3 pathway

We recently showed that a leukemia inhibitory factor (LIF)-engaged signaling pathway consisting of JAK1, STAT1, and STAT3 plays dual roles in myogenic differentiation: while it participates in myoblast proliferation, it also actively represses differentiation. Downregulation of this pathway is required at the onset of differentiation. However, it remained unclear how this is achieved mechanistically. We now show that SOCS1, SOCS3, and PIAS1 promote myogenic differentiation by specifically inhibiting the LIF-induced JAK1/STAT1/STAT3 pathway via distinct targets; whereas SOCS1 and SOCS3 selectively bind and inhibit JAK1 and gp130, respectively, PIAS1 targets mainly the activated STAT1 and prevents its binding to DNA. We further demonstrated that the SUMO E3-ligase activity of PIAS1 is dispensable for its role in myogenic differentiation. Collectively, our current study revealed a molecular mechanism that explains how the LIF-induced JAK1/STAT1/STAT3 pathway is downregulated upon myogenic differentiation.

Protein tyrosine phosphatase SHP2 is involved in Semaphorin 4D-induced axon repulsion

Semaphorin-4D (Sema4D), a member of class 4 membrane-bound Semaphorins, acts as a chemorepellant to the axons of retinal ganglion cells and hippocampal neurons. Plexin-B1, a neuronal Sema4D receptor, associates with either one of receptor tyrosine kinases, c-Met or ErbB2, to mediate Sema4D-signaling. In contrast to this significance, the involvement of protein tyrosine phosphatases in Semaphorin-signaling remains unknown. We here show that Src homology 2-containing protein-tyrosine phosphatase 2 (SHP2) participates in Sema4D-signaling. SHP2 was localized in the growth cones of chick embryonic retinal ganglion neurons. Phenylarsine oxide, a protein tyrosine phosphatase inhibitor, suppressed Sema4D-induced contractile response in COS-7 cells expressing Plexin-B1. Ectopic expression of a phosphatase-inactive mutant of SHP2 in the retinal ganglion cells attenuated Sema4D-induced growth cone collapse response. A SHP1/2 specific inhibitor, 8-hydroxy-7-(6-sulfonaphthalen-2-yl)diazenyl-quinoline-5-sulfonic acid (NSC-87877), also suppressed this collapse response. These results suggest that SHP2-mediated tyrosine dephosphorylation is an important step in Sema4D-induced axon repulsion.

Hepatocyte growth factor (HGF) signals through SHP2 to regulate primary mouse myoblast proliferation

Niche localized HGF plays an integral role in G(0) exit and the return to mitotic activity of adult skeletal muscle satellite cells. HGF actions are regulated by MET initiated intracellular signaling events that include recruitment of SHP2, a protein tyrosine phosphatase. The importance of SHP2 in HGF-mediated signaling was examined in myoblasts and primary cultures of satellite cells. Myoblasts stably expressing SHP2 (23A2-SHP2) demonstrate increased proliferation rates by comparison to controls or myoblasts expressing a phosphatase-deficient SHP2 (23A2-SHP2DN). By comparison to 23A2 myoblasts, treatment of 23A2-SHP2 cells with HGF does not further increase proliferation rates and 23A2-SHP2DN myoblasts are unresponsive to HGF. Importantly, the effects of SHP2 are independent of downstream ERK1/2 activity as inclusion of PD98059 does not blunt the HGF-induced proliferative response. SHP2 function was further evaluated in primary satellite cell cultures. Ectopic expression of SHP2 in satellite cells tends to decrease proliferation rates and siSHP2 causes an increase the percentage of dividing myogenic cells. Interestingly, treatment of satellite cells with high concentrations of HGF (50 ng/ml) inhibits proliferation, which can be overcome by knockdown of SHP2. From these results, we conclude that HGF signals through SHP2 in myoblasts and satellite cells to directly alter proliferation rates.