Activated mutants of SHP-2 preferentially induce elongation of Xenopus animal caps

Allosteric inhibition rescues hydrocephalus caused by catalytically inactive Shp2

SHP2, a protein tyrosine phosphatase (PTP) crucial in Ras-MAPK signaling, is associated with various human congenital diseases and cancers. Here, we show that the catalytically inactive Shp2 C459S mutation results in communicating hydrocephalus, similar to the catalytically activating Shp2 E76K and Mek1 DD mutants. Unlike previous mutants, however, Shp2 C459S/+ mutation uniquely affects ciliary development rather than neurogenesis, leading to reduced cilia density and impaired ciliary motility. Differential scanning fluorimetry revealed that SHP2 C459S , SHP2 E76K and SHP2 C459S/E76K mutations all induce an open SHP2 conformation, but only SHP2 C459S leads to aberrant GAB1 phosphorylation in cells expressing wild-type SHP2. This distinctive signaling pattern correlates with our observations in brain ventricular tissues of Shp2 C459S/+ mice, where Erk and Stat3 activities remain normal but Gab1 phosphorylation is elevated. Critically, we show that the hydrocephalus phenotype in Shp2 C459S mice can be mitigated by allosteric inhibition of Shp2. These findings suggest that Shp2-associated hydrocephalus is driven by conformational changes rather than altered catalytic activity. Our results underscore the therapeutic potential of conformation-specific allosteric inhibitors in targeting both catalytically active and inactive SHP2 mutants.

Ionizable networks mediate pH-dependent allostery in SH2 signaling proteins

Transient intracellular pH dynamics1regulate mammalian proliferation2,3, migration4, and differentiation5. However, for many pH-dependent cell processes, the molecular mediators are unknown6. Prior work identified histidine residues as molecular switches in pH-sensitive proteins, but how other ionizable residues contribute to pH-dependent protein allostery is understudied. Here, we develop anin silicocomputational pipeline to identify putative pH-sensitive proteins and their molecular mechanisms. We first apply this pipeline to SHP2, a known pH-sensitive signaling protein with an uncharacterized molecular mechanism. We show wild-type SHP2 phosphatase activity is pH-sensitivein vitroand in cells, and mutation of identified H116 and E252 to non-titratable alanine residues abolishes pH-sensitive function. We also show that c-Src is a previously unrecognized pH-dependent kinase, and mutation of the identified ionizable network again abolishes pH-sensitive activity. Constant pH molecular dynamics simulations support a conserved allosteric mechanism of pH-dependent binding of inhibitory SH2 domains to the functional catalytic domains of SHP2 and c-Src. We apply our computational pipeline across SH2 domain-containing signaling proteins and identify evolutionarily conserved putative pH-sensing networks. Our results reveal that pH is an allosteric regulator of SH2 domain-containing signaling proteins providing insight into normal pH-dependent cell biology and diseases where pHi is dysregulated, such as cancer.

The Toxoplasma secreted effector TgWIP modulates dendritic cell motility by activating host tyrosine phosphatases Shp1 and Shp2

The obligate intracellular parasite Toxoplasma gondii causes life-threatening toxoplasmosis to immunocompromised individuals. The pathogenesis of Toxoplasma relies on its swift dissemination to the central nervous system through a 'Trojan Horse' mechanism using infected leukocytes as carriers. Previous work found TgWIP, a protein secreted from Toxoplasma, played a role in altering the actin cytoskeleton and promoting cell migration in infected dendritic cells (DCs). However, the mechanism behind these changes was unknown. Here, we report that TgWIP harbors two SH2-binding motifs that interact with tyrosine phosphatases Shp1 and Shp2, leading to phosphatase activation. DCs infected with Toxoplasma exhibited hypermigration, accompanying enhanced F-actin stress fibers and increased membrane protrusions such as filopodia and pseudopodia. By contrast, these phenotypes were abrogated in DCs infected with Toxoplasma expressing a mutant TgWIP lacking the SH2-binding motifs. We further demonstrated that the Rho-associated kinase (Rock) is involved in the induction of these phenotypes, in a TgWIP-Shp1/2 dependent manner. Collectively, the data uncover a molecular mechanism by which TgWIP modulates the migration dynamics of infected DCs in vitro.

The Toxoplasma secreted effector TgWIP modulates dendritic cell motility by activating host tyrosine phosphatases Shp1 and Shp2

The obligate intracellular parasite Toxoplasma gondii causes life-threatening toxoplasmosis to immunocompromised individuals. The pathogenesis of Toxoplasma relies on its swift dissemination to the central nervous system through a 'Trojan Horse' mechanism using infected leukocytes as carriers. Previous work found TgWIP, a protein secreted from Toxoplasma, played a role in altering the actin cytoskeleton and promoting cell migration in infected dendritic cells (DCs). However, the mechanism behind these changes was unknown. Here, we report that TgWIP harbors two SH2-binding motifs that interact with tyrosine phosphatases Shp1 and Shp2, leading to phosphatase activation. DCs infected with Toxoplasma exhibited hypermigration, accompanying enhanced F-actin stress fibers and increased membrane protrusions such as filopodia and pseudopodia. By contrast, these phenotypes were abrogated in DCs infected with Toxoplasma expressing a mutant TgWIP lacking the SH2-binding motifs. We further demonstrated that the Rho-associated kinase (Rock) is involved in the induction of these phenotypes, in a TgWIP-Shp1/2 dependent manner. Collectively, the data uncover a molecular mechanism by which TgWIP modulates the migration dynamics of infected DCs in vitro.

The pathogenic T42A mutation in SHP2 rewires the interaction specificity of its N-terminal regulatory domain

Mutations in the tyrosine phosphatase SHP2 are associated with a variety of human diseases. Most mutations in SHP2 increase its basal catalytic activity by disrupting auto-inhibitory interactions between its phosphatase domain and N-terminal SH2 (phosphotyrosine recognition) domain. By contrast, some disease-associated mutations located in the ligand-binding pockets of the N- or C-terminal SH2 domains do not increase basal activity and likely exert their pathogenicity through alternative mechanisms. We lack a molecular understanding of how these SH2 mutations impact SHP2 structure, activity, and signaling. Here, we characterize five SHP2 SH2 domain ligand-binding pocket mutants through a combination of high-throughput biochemical screens, biophysical and biochemical measurements, and molecular dynamics simulations. We show that, while some of these mutations alter binding affinity to phosphorylation sites, the T42A mutation in the N-SH2 domain is unique in that it also substantially alters ligand-binding specificity, despite being 8-10 Å from the specificity-determining region of the SH2 domain. This mutation exerts its effect on sequence specificity by remodeling the phosphotyrosine binding pocket, altering the mode of engagement of both the phosphotyrosine and surrounding residues on the ligand. The functional consequence of this altered specificity is that the T42A mutant has biased sensitivity toward a subset of activating ligands and enhances downstream signaling. Our study highlights an example of a nuanced mechanism of action for a disease-associated mutation, characterized by a change in protein-protein interaction specificity that alters enzyme activation.

THEMIS is a substrate and allosteric activator of SHP1, playing dual roles during T cell development

THEMIS plays an indispensable role in T cells, but its mechanism of action has remained highly controversial. Using the systematic proximity labeling methodology PEPSI, we identify THEMIS as an uncharacterized substrate for the phosphatase SHP1. Saturated mutagenesis assays and mass spectrometry analysis reveal that phosphorylation of THEMIS at the evolutionally conserved Tyr34 residue is oppositely regulated by SHP1 and the kinase LCK. Similar to THEMIS-/- mice, THEMISY34F/Y34F knock-in mice show a significant decrease in CD4 thymocytes and mature CD4 T cells, but display normal thymic development and peripheral homeostasis of CD8 T cells. Mechanistically, the Tyr34 motif in THEMIS, when phosphorylated upon T cell antigen receptor activation, appears to act as an allosteric regulator, binding and stabilizing SHP1 in its active conformation, thus ensuring appropriate negative regulation of T cell antigen receptor signaling. However, cytokine signaling in CD8 T cells fails to elicit THEMIS Tyr34 phosphorylation, indicating both Tyr34 phosphorylation-dependent and phosphorylation-independent roles of THEMIS in controlling T cell maturation and expansion.

Gastrulation morphogenesis in synthetic systems

Recent advances in pluripotent stem cell culture allow researchers to generate not only most embryonic cell types, but also morphologies of many embryonic structures, entirely in vitro. This recreation of embryonic form from naïve cells, known as synthetic morphogenesis, has important implications for both developmental biology and regenerative medicine. However, the capacity of stem cell-based models to recapitulate the morphogenetic cell behaviors that shape natural embryos remains unclear. In this review, we explore several examples of synthetic morphogenesis, with a focus on models of gastrulation and surrounding stages. By varying cell types, source species, and culture conditions, researchers have recreated aspects of primitive streak formation, emergence and elongation of the primary embryonic axis, neural tube closure, and more. Here, we describe cell behaviors within in vitro/ex vivo systems that mimic in vivo morphogenesis and highlight opportunities for more complete models of early development.

Intramolecular Interaction with the E6 Region Stabilizes the Closed Conformation of the N-SH2 Domain and Concurs with the Self-Inhibitory Docking in Downregulating the Activity of the SHP2 Tyrosine Phosphatase: A Molecular Dynamics Study

The localization and activity of the SHP2 tyrosine phosphatase across different cellular compartments to the target substrates are steered by the binding of phosphotyrosine (pY) peptides to the tandem SH2 domains. The most N-terminal domain (N-SH2) can also keep the enzyme inactive by intramolecular occlusion of the catalytic site. Enzyme activity can be recovered by an allosteric disruption of this self-inhibitory docking upon the binding of pY peptides to the N-SH2 domain. Prior to this, the N-SH2 domain must abandon the closed conformation because it impedes the access of pY peptides to the binding cleft. Although it cooperates with the self-inhibitory docking in the negative regulation of the phosphatase activity, the structural determinants of the stability of the closed conformation in the self-inhibited phosphatase are still elusive. To address this issue, a molecular dynamics simulation study is carried out. It is shown that the closed conformation is stabilized by the interaction of the N-SH2 domain with a conserved peptide portion in the region encoded by PTPN11 exon 6 (E6).

In vivo Functional Genomics for Undiagnosed Patients: The Impact of Small GTPases Signaling Dysregulation at Pan-Embryo Developmental Scale

While individually rare, disorders affecting development collectively represent a substantial clinical, psychological, and socioeconomic burden to patients, families, and society. Insights into the molecular mechanisms underlying these disorders are required to speed up diagnosis, improve counseling, and optimize management toward targeted therapies. Genome sequencing is now unveiling previously unexplored genetic variations in undiagnosed patients, which require functional validation and mechanistic understanding, particularly when dealing with novel nosologic entities. Functional perturbations of key regulators acting on signals' intersections of evolutionarily conserved pathways in these pathological conditions hinder the fine balance between various developmental inputs governing morphogenesis and homeostasis. However, the distinct mechanisms by which these hubs orchestrate pathways to ensure the developmental coordinates are poorly understood. Integrative functional genomics implementing quantitative in vivo models of embryogenesis with subcellular precision in whole organisms contribute to answering these questions. Here, we review the current knowledge on genes and mechanisms critically involved in developmental syndromes and pediatric cancers, revealed by genomic sequencing and in vivo models such as insects, worms and fish. We focus on the monomeric GTPases of the RAS superfamily and their influence on crucial developmental signals and processes. We next discuss the effectiveness of exponentially growing functional assays employing tractable models to identify regulatory crossroads. Unprecedented sophistications are now possible in zebrafish, i.e., genome editing with single-nucleotide precision, nanoimaging, highly resolved recording of multiple small molecules activity, and simultaneous monitoring of brain circuits and complex behavioral response. These assets permit accurate real-time reporting of dynamic small GTPases-controlled processes in entire organisms, owning the potential to tackle rare disease mechanisms.

Grb2 binding induces phosphorylation-independent activation of Shp2

The regulation of phosphatase activity is fundamental to the control of intracellular signalling and in particular the tyrosine kinase-mediated mitogen-activated protein kinase (MAPK) pathway. Shp2 is a ubiquitously expressed protein tyrosine phosphatase and its kinase-induced hyperactivity is associated with many cancer types. In non-stimulated cells we find that binding of the adaptor protein Grb2, in its monomeric state, initiates Shp2 activity independent of phosphatase phosphorylation. Grb2 forms a bidentate interaction with both the N-terminal SH2 and the catalytic domains of Shp2, releasing the phosphatase from its auto-inhibited conformation. Grb2 typically exists as a dimer in the cytoplasm. However, its monomeric state prevails under basal conditions when it is expressed at low concentration, or when it is constitutively phosphorylated on a specific tyrosine residue (Y160). Thus, Grb2 can activate Shp2 and downstream signal transduction, in the absence of extracellular growth factor stimulation or kinase-activating mutations, in response to defined cellular conditions. Therefore, direct binding of Grb2 activates Shp2 phosphatase in the absence of receptor tyrosine kinase up-regulation.

Swimming toward solutions: Using fish and frogs as models for understanding RASopathies

The RAS signaling pathway regulates cell growth, survival, and differentiation, and its inappropriate activation is associated with disease in humans. The RASopathies, a set of developmental syndromes, arise when the pathway is overactive during development. Patients share a core set of symptoms, including congenital heart disease, craniofacial anomalies, and neurocognitive delay. Due to the conserved nature of the pathway, animal models are highly informative for understanding disease etiology, and zebrafish and Xenopus are emerging as advantageous model systems. Here we discuss these aquatic models of RASopathies, which recapitulate many of the core symptoms observed in patients. Craniofacial structures become dysmorphic upon expression of disease-associated mutations, resulting in wider heads. Heart defects manifest as delays in cardiac development and changes in heart size, and behavioral deficits are beginning to be explored. Furthermore, early convergence and extension defects cause elongation of developing embryos: this phenotype can be quantitatively assayed as a readout of mutation strength, raising interesting questions regarding the relationship between pathway activation and disease. Additionally, the observation that RAS signaling may be simultaneously hyperactive and attenuated suggests that downregulation of signaling may also contribute to etiology. We propose that models should be characterized using a standardized approach to allow easier comparison between models, and a better understanding of the interplay between mutation and disease presentation.

Canine histiocytic sarcoma cell lines with SHP2 p.Glu76Gln or p.Glu76Ala mutations are sensitive to allosteric SHP2 inhibitor SHP099

Some canine cases of histiocytic sarcoma (HS) carry an activating mutation in the src homology two domain-containing phosphatase 2 (SHP2) encoded by PTPN11. SHP099 is an allosteric inhibitor of SHP2 that stabilizes SHP2 in a folded, auto-inhibited conformation. Here, we examined the expression and mutation status of SHP2 in five canine HS cell lines and evaluated the growth inhibitory properties of SHP099 against these cell lines. All five of the canine HS cell lines expressed SHP2, with three of the lines each harbouring a distinct mutation in PTPN11/SHP2 (p.Glu76Gln, p.Glu76Ala and p.Gly503Val). In silico analysis suggested that p.Glu76Gln and p.Glu76Ala, but not p.Gly503Val, promote shifting of the SHP2 conformation from folded to open-active state. SHP099 potently suppressed the growth of two of the mutant cell lines (harbouring SHP2 p.Glu76Gln or p.Glu76Ala) but not that of the other three cell lines. In addition, SHP099 suppressed ERK activation in the cell line harbouring the SHP2 p.Glu76Ala mutation. The SHP2 p.Glu76Gln and p.Glu76Ala mutations are considered to be activating mutations, and the signal from SHP2 p.Glu76Ala is inferred to be transduced primarily via the ERK pathway. Moreover, SHP099-sensitive HS cells, including those with SHP2 p.Glu76Gln or p.Glu76Ala mutations, may depend on these mutations for growth. Therefore, targeting cells harbouring SHP2 p.Glu76Gln and p.Glu76Ala with SHP099 may be an approach for the treatment of canine HS.

Excitatory neuron-specific SHP2-ERK signaling network regulates synaptic plasticity and memory

Mutations in RAS signaling pathway components cause diverse neurodevelopmental disorders, collectively called RASopathies. Previous studies have suggested that dysregulation in RAS-extracellular signal-regulated kinase (ERK) activation is restricted to distinct cell types in different RASopathies. Some cases of Noonan syndrome (NS) are associated with gain-of-function mutations in the phosphatase SHP2 (encoded by PTPN11); however, SHP2 is abundant in multiple cell types, so it is unclear which cell type(s) contribute to NS phenotypes. Here, we found that expressing the NS-associated mutant SHP2^D61G in excitatory, but not inhibitory, hippocampal neurons increased ERK signaling and impaired both long-term potentiation (LTP) and spatial memory in mice, although endogenous SHP2 was expressed in both neuronal types. Transcriptomic analyses revealed that the genes encoding SHP2-interacting proteins that are critical for ERK activation, such as GAB1 and GRB2, were enriched in excitatory neurons. Accordingly, expressing a dominant-negative mutant of GAB1, which reduced its interaction with SHP2^D61G, selectively in excitatory neurons, reversed SHP2^D61G-mediated deficits. Moreover, ectopic expression of GAB1 and GRB2 together with SHP2^D61G in inhibitory neurons resulted in ERK activation. These results demonstrate that RAS-ERK signaling networks are notably different between excitatory and inhibitory neurons, accounting for the cell type-specific pathophysiology of NS and perhaps other RASopathies.

Exploring the effect of N308D mutation on protein tyrosine phosphatase-2 cause gain-of-function activity by a molecular dynamics study

One of the most common protein tyrosine phosphatase-2 (SHP2) mutations in Noonan syndrome is the N308D mutation, and it increases the activity of the protein. However, the molecular basis of the activation of N308D mutation on SHP2 conformations is poorly understood. Here, molecular dynamic simulations were performed on SHP2 and SHP2-N308D to explore the effect of N308D mutation on SHP2 cause gain of function activity, respectively. The principal component analysis, dynamic cross-correlation map, secondary structure analysis, residue interaction networks, and solvent accessible surface area analysis suggested that the N308D mutation distorted the residues interactions network between the allosteric site (residue Gly244-Gly246) and C-SH2 domain, including the hydrogen bond formation and the binding energy. Meanwhile, the activity of catalytic site (residue Gly503-Val505) located in the Q-loop in mutant increased due to this region's high fluctuations. Therefore, the substrate had more chances to access to the catalytic activity site of the precision time protocol domain of SHP2-N308D, which was easy to be exposed. In addition, we had speculated that the Lys244 located in the allosteric site was the key residue which lead to the protein conformation changes. Consequently, overall calculations presented in this study ultimately provide a useful understanding of the increased activity of SHP2 caused by the N308D mutation.

Noonan syndrome: lessons learned from genetically modified mouse models

Noonan syndrome is a RASopathy that results from activating mutations in different members of the RAS/MAPK signaling pathway. At least eleven members of this pathway have been found mutated, PTPN11 being the most frequently mutated gene affecting about 50% of the patients, followed by SOS1 (10%), RAF1 (10%) and KRAS (5%). Recently, even more infrequent mutations have been newly identified by next generation sequencing. This spectrum of mutations leads to a broad variety of clinical symptoms such as cardiopathies, short stature, facial dysmorphia and neurocognitive impairment. The genetic variability of this syndrome makes it difficult to establish a genotype-phenotype correlation, which will greatly help in the clinical management of the patients. Areas covered: Studies performed with different genetically engineered mouse models (GEMMs) developed up to date. Expert commentary: GEMMs have helped us understand the role of some genes and the effect of the different mutations in the development of the syndrome. However, few models have been developed and more characterization of the existing ones should be performed to learn about the impact of the different modifiers in the phenotypes, the potential cancer risk in patients, as well as preventative and therapeutic strategies.

A Global Analysis of the Receptor Tyrosine Kinase-Protein Phosphatase Interactome

Receptor tyrosine kinases (RTKs) and protein phosphatases comprise protein families that play crucial roles in cell signaling. We used two protein-protein interaction (PPI) approaches, the membrane yeast two-hybrid (MYTH) and the mammalian membrane two-hybrid (MaMTH), to map the PPIs between human RTKs and phosphatases. The resulting RTK-phosphatase interactome reveals a considerable number of previously unidentified interactions and suggests specific roles for different phosphatase families. Additionally, the differential PPIs of some protein tyrosine phosphatases (PTPs) and their mutants suggest diverse mechanisms of these PTPs in the regulation of RTK signaling. We further found that PTPRH and PTPRB directly dephosphorylate EGFR and repress its downstream signaling. By contrast, PTPRA plays a dual role in EGFR signaling: besides facilitating EGFR dephosphorylation, it enhances downstream ERK signaling by activating SRC. This comprehensive RTK-phosphatase interactome study provides a broad and deep view of RTK signaling.

Inhibition of SHP2 ameliorates the pathogenesis of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a devastating multisystemic autoimmune disorder. However, the molecular mechanisms underlying its pathogenesis remain elusive. Some patients with Noonan syndrome, a congenital disorder predominantly caused by gain-of-function mutations in the protein tyrosine phosphatase SH2 domain-containing PTP (SHP2), have been shown to develop SLE, suggesting a functional correlation between phosphatase activity and systemic autoimmunity. To test this directly, we measured SHP2 activity in spleen lysates isolated from lupus-prone MRL/lpr mice and found it was markedly increased compared with that in control mice. Similar increases in SHP2 activity were seen in peripheral blood mononuclear cells isolated from lupus patients relative to healthy patients. To determine whether SHP2 alters autoimmunity and related immunopathology, we treated MRL/lpr mice with an SHP2 inhibitor and found increased life span, suppressed crescentic glomerulonephritis, reduced spleen size, and diminished skin lesions. SHP2 inhibition also reduced numbers of double-negative T cells, normalized ERK/MAPK signaling, and decreased production of IFN-γ and IL-17A/F, 2 cytokines involved in SLE-associated organ damage. Moreover, in cultured human lupus T cells, SHP2 inhibition reduced proliferation and decreased production of IFN-γ and IL-17A/F, further implicating SHP2 in lupus-associated immunopathology. Taken together, these data identify SHP2 as a critical regulator of SLE pathogenesis and suggest targeting of its activity as a potent treatment for lupus patients.

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.

Molecular mechanisms of SH2- and PTB-domain-containing proteins in receptor tyrosine kinase signaling

Intracellular signaling is mediated by reversible posttranslational modifications (PTMs) that include phosphorylation, ubiquitination, and acetylation, among others. In response to extracellular stimuli such as growth factors, receptor tyrosine kinases (RTKs) typically dimerize and initiate signaling through phosphorylation of their cytoplasmic tails and downstream scaffolds. Signaling effectors are recruited to these phosphotyrosine (pTyr) sites primarily through Src homology 2 (SH2) domains and pTyr-binding (PTB) domains. This review describes how these conserved domains specifically recognize pTyr residues and play a major role in mediating precise downstream signaling events.

Protein tyrosine phosphatases--from housekeeping enzymes to master regulators of signal transduction

There are many misconceptions surrounding the roles of protein phosphatases in the regulation of signal transduction, perhaps the most damaging of which is the erroneous view that these enzymes exert their effects merely as constitutively active housekeeping enzymes. On the contrary, the phosphatases are critical, specific regulators of signalling in their own right and serve an essential function, in a coordinated manner with the kinases, to determine the response to a physiological stimulus. This review is a personal perspective on the development of our understanding of the protein tyrosine phosphatase family of enzymes. I have discussed various aspects of the structure, regulation and function of the protein tyrosine phosphatase family, which I hope will illustrate the fundamental importance of these enzymes in the control of signal transduction.

Reciprocal regulation of axonal Filopodia and outgrowth during neuromuscular junction development

Background

The assembly of the vertebrate neuromuscular junction (NMJ) is initiated when nerve and muscle first contact each other by filopodial processes which are thought to enable close interactions between the synaptic partners and facilitate synaptogenesis. We recently reported that embryonic Xenopus spinal neurons preferentially extended filopodia towards cocultured muscle cells and that basic fibroblast growth factor (bFGF) produced by muscle activated neuronal FGF receptor 1 (FGFR1) to induce filopodia and favor synaptogenesis. Intriguingly, in an earlier study we found that neurotrophins (NTs), a different set of target-derived factors that act through Trk receptor tyrosine kinases, promoted neuronal growth but hindered presynaptic differentiation and NMJ formation. Thus, here we investigated how bFGF- and NT-signals in neurons jointly elicit presynaptic changes during the earliest stages of NMJ development.

Methodology/Principal Findings

Whereas forced expression of wild-type TrkB in neurons reduced filopodial extension and triggered axonal outgrowth, expression of a mutant TrkB lacking the intracellular kinase domain enhanced filopodial growth and slowed axonal advance. Neurons overexpressing wild-type FGFR1 also displayed more filopodia than control neurons, in accord with our previous findings, and, notably, this elevation in filopodial density was suppressed when neurons were chronically treated from the beginning of the culture period with BDNF, the NT that specifically activates TrkB. Conversely, inhibition by BDNF of NMJ formation in nerve-muscle cocultures was partly reversed by the overexpression of bFGF in muscle.

Conclusions

Our results suggest that the balance between neuronal FGFR1- and TrkB-dependent filopodial assembly and axonal outgrowth regulates the establishment of incipient NMJs.

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.

Counteracting effects operating on Src homology 2 domain-containing protein-tyrosine phosphatase 2 (SHP2) function drive selection of the recurrent Y62D and Y63C substitutions in Noonan syndrome

Activating mutations in PTPN11 cause Noonan syndrome, the most common nonchromosomal disorder affecting development and growth. PTPN11 encodes SHP2, an Src homology 2 (SH2) domain-containing protein-tyrosine phosphatase that positively modulates RAS function. Here, we characterized functionally all possible amino acid substitutions arising from single-base changes affecting codons 62 and 63 to explore the molecular mechanisms lying behind the largely invariant occurrence of the Y62D and Y63C substitutions recurring in Noonan syndrome. We provide structural and biochemical data indicating that the autoinhibitory interaction between the N-SH2 and protein-tyrosine phosphatase (PTP) domains is perturbed in both mutants as a result of an extensive structural rearrangement of the N-SH2 domain. Most mutations affecting Tyr⁶³ exerted an unpredicted disrupting effect on the structure of the N-SH2 phosphopeptide-binding cleft mediating the interaction of SHP2 with signaling partners. Among all the amino acid changes affecting that codon, the disease-causing mutation was the only substitution that perturbed the stability of the inactive conformation of SHP2 without severely impairing proper phosphopeptide binding of N-SH2. On the other hand, the disruptive effect of the Y62D change on the autoinhibited conformation of the protein was balanced, in part, by less efficient binding properties of the mutant. Overall, our data demonstrate that the selection-by-function mechanism acting as driving force for PTPN11 mutations affecting codons 62 and 63 implies balancing of counteracting effects operating on the allosteric control of the function of SHP2.

Shp2 controls female body weight and energy balance by integrating leptin and estrogen signals

In mammals, leptin regulates food intake and energy balance mainly through the activation of LepRb in the hypothalamus, and estrogen has a leptin-like effect in the hypothalamic control of metabolism. However, it remains to be elucidated how estrogen signaling is intertwined with the leptin pathway. We show here that Shp2, a nonreceptor tyrosine phosphatase, acts to integrate leptin and estrogen signals. The expression of a dominant-active mutant (Shp2(D61A)) in forebrain neurons conferred female, but not male, transgenic mice resistance to high-fat diet (HFD)-induced obesity and liver steatosis, accompanied by improved insulin sensitivity and glucose homeostasis. Fed with either HFD or regular chow food, Shp2(D61A) female mice showed dramatically enhanced leptin sensitivity. Microinjection of Shp2(D61A)-expressing adeno-associated virus into mediobasal hypothalamus elicited a similar antiobese effect in female mice. Biochemical analyses showed a physical association of Shp2 with estrogen receptor alpha, which is necessary for the synergistic and persistent activation of Erk by leptin and estrogen. Together, these results elucidate a mechanism for the direct cross talk of leptin and estrogen signaling and offer one explanation for the propensity of postmenopausal women to develop obesity.

Reactive oxygen species and epidermal growth factor are antagonistic cues controlling SHP-2 dimerization

The SHP-2 tyrosine phosphatase plays key regulatory roles in the modulation of the cell response to growth factors and cytokines. Over the past decade, the integration of genetic, biochemical, and structural data has helped in interpreting the pathological consequences of altered SHP-2 function. Using complementary approaches, we provide evidence here that endogenous SHP-2 can dimerize through the formation of disulfide bonds that may also involve the catalytic cysteine. We show that the fraction of dimeric SHP-2 is modulated by growth factor stimulation and by the cell redox state. Comparison of the phosphatase activities of the monomeric self-inhibited and dimeric forms indicated that the latter is 3-fold less active, thus pointing to the dimerization process as an additional mechanism for controlling SHP-2 activity. Remarkably, dimers formed by different SHP-2 mutants displaying diverse biochemical properties were found to respond differently to epidermal growth factor (EGF) stimulation. Although this differential behavior cannot be rationalized mechanistically yet, these findings suggest a possible regulatory role of dimerization in SHP-2 function.

Molecular mechanism of interleukin-2-induced mucosal homeostasis

Sustained damage to the mucosal lining in patients with inflammatory bowel disease (IBD) facilitates translocation of intestinal microbes to submucosal immune cells leading to chronic inflammation. Previously, we demonstrated the role of Jak3 in IL-2-induced intestinal epithelial cell (IEC) migration, one of the early events during intestinal wound repair. In this study, we demonstrate that IL-2 also plays a role in IEC homeostasis through concentration-dependent regulation of IEC proliferation and cell death. At lower concentrations (≤50 U/ml), IL-2 promoted proliferation, while at higher concentrations (100 U/ml), it promoted apoptosis. Activation by IL-2 led to tyrosine phosphorylation-dependent interactions between Jak3 and p52ShcA only at lower concentrations. Phosphatase SHP1 dephosphorylated IL-2-induced phosphorylated p52ShcA. Higher concentrations of IL-2 decreased the phosphorylation of Jak3 and p52ShcA, disrupted their interactions, redistributed Jak3 to the nucleus, and induced apoptosis in IEC. IL-2 also induced dose-dependent upregulation of p52shcA and downregulation of jak3-mRNA. Constitutive overexpression and mir-shRNA-mediated knockdown studies showed that expression of both Jak3 and p52ShcA were necessary for IL-2-induced proliferation of IEC. Doxycycline-regulated sh-RNA expression demonstrated that IL-2-induced downregulation of jak3-mRNA was responsible for higher IL-2-induced apoptosis in IEC. Collectively, these data demonstrate a novel mechanism of IL-2-induced mucosal homeostasis through posttranslational and transcriptional regulation of Jak3 and p52ShcA.

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.

Protein tyrosine phosphatase SHP-2: a proto-oncogene product that promotes Ras activation

SHP-2 is a cytoplasmic protein tyrosine phosphatase (PTP) that contains two Src homology 2 (SH2) domains. Although PTPs are generally considered to be negative regulators on the basis of their ability to oppose the effects of protein tyrosine kinases, SHP-2 is unusual in that it promotes the activation of the Ras-MAPK signaling pathway by receptors for various growth factors and cytokines. The molecular basis for the activation of SHP-2 is also unique: In the basal state, the NH(2)-terminal SH2 domain of SHP-2 interacts with the PTP domain, resulting in autoinhibition of PTP activity; the binding of SHP-2 via its SH2 domains to tyrosine-phosphorylated growth factor receptors or docking proteins, however, results in disruption of this intramolecular interaction, leading to exposure of the PTP domain and catalytic activation. Indeed, SHP-2 proteins with artificial mutations in the NH(2)-terminal SH2 domain have been shown to act as dominant active mutants in vitro. Such activating mutations of PTPN11 (human SHP-2 gene) were subsequently identified in individuals with Noonan syndrome, a human developmental disorder that is sometimes associated with juvenile myelomonocytic leukemia. Furthermore, somatic mutations of PTPN11 were found to be associated with pediatric leukemia. SHP-2 is also thought to participate in the development of other malignant disorders, but in a manner independent of such activating mutations. Biochemical and functional studies of SHP-2 and genetic analysis of PTPN11 in human disorders have thus converged to provide new insight into the pathogenesis of cancer as well as potential new targets for cancer treatment.

Noonan, Costello and cardio–facio–cutaneous syndromes: dysregulation of the Ras–MAPK pathway

A class of developmental disorders caused by dysregulation of the Ras-induced mitogen-activated protein kinase (MAPK) cascade (the Ras–MAPK pathway) has emerged. Three of these disorders – Noonan, Costello and cardio–facio–cutaneous syndromes – have overlapping phenotypic features characterised by distinctive facial dysmorphia, cardiac defects, musculoskeletal and cutaneous abnormalities, and neurocognitive delay. The germline mutations associated with these disorders are in genes that encode proteins of the Ras–MAPK pathway. In vitro studies have determined that the overwhelming majority of these mutations result in increased signal transduction down the pathway, but usually to a lesser degree than somatic mutations in the same genes that are associated with cancer. The Ras–MAPK pathway is essential in the regulation of the cell cycle, differentiation, growth and senescence, so it is not surprising that germline mutations that affect its function have profound effects on development. Here we review the clinical consequences of the known molecular lesions associated with Noonan syndrome, Costello syndrome and cardio–facio–cutaneous syndrome, and explore possible therapeutic modalities for treatment.

Role of ERK1/2 signaling in congenital valve malformations in Noonan syndrome

Noonan syndrome (NS) is the most common nonchromosomal genetic disorder associated with cardiovascular malformations. The most prominent cardiac defects in NS are pulmonary valve stenosis and hypertrophic cardiomyopathy. Gain-of-function mutations in the protein tyrosine phosphatase Shp2 have been identified in 50% of NS families. We created a NS mouse model with selective overexpression of mutant Shp2 (Q79R-Shp2) in the developing endocardial cushions. In our model, Cre recombinase driven by the Tie2 promoter irreversibly activates transgenic Q79R-Shp2 expression in the endothelial-derived cell lineage. Q79R-Shp2 expression resulted in embryonic lethality by embryonic day 14.5. Importantly, mutant embryos showed significantly enlarged endocardial cushions in the atrioventricular canal and in the outflow tract. In contrast, overexpression of wild-type Shp2 protein at comparable levels did not enhance endocardial cushion growth or alter the morphology of the mature adult valves. Expression of Q79R-Shp2 was accompanied by increased ERK1/2 activation in a subset of cells within the cushion mesenchyme, suggesting that hyperactivation of this signaling pathway may play a pathogenic role. To test this hypothesis in vivo, Q79R-Shp2-expressing mice were crossed with mice carrying either a homozygous ERK1 or a heterozygous ERK2 deletion. Deletion of ERK1 completely rescued the endocardial cushion phenotype, whereas ERK2 protein reduction did not affect endocardial cushion size. Constitutive hyperactivation of ERK1/2 signaling alone with a transgenic approach resulted in a phenocopy of the valvular phenotype. The data demonstrate both necessity and sufficiency of increased ERK activation downstream of Shp2 in mediating abnormal valve development in a NS mouse model.

Noonan syndrome/leukemia-associated gain-of-function mutations in SHP-2 phosphatase (PTPN11) enhance cell migration and angiogenesis

Mutations in SHP-2 phosphatase (PTPN11) that cause hyperactivation of its catalytic activity have been identified in Noonan syndrome and various childhood leukemias. Recent studies suggest that the gain-of-function (GOF) mutations of SHP-2 play a causal role in the pathogenesis of these diseases. However, the molecular mechanisms by which GOF mutations of SHP-2 induce these phenotypes are not fully understood. Here, we show that GOF mutations in SHP-2, such as E76K and D61G, drastically increase spreading and migration of various cell types, including hematopoietic cells, endothelial cells, and fibroblasts. More importantly, in vivo angiogenesis in SHP-2 D61G knock-in mice is also enhanced. Mechanistic studies suggest that the increased cell migration is attributed to the enhanced beta1 integrin outside-in signaling. In response to beta1 integrin cross-linking or fibronectin stimulation, activation of ERK and Akt kinases is greatly increased by SHP-2 GOF mutations. Also, integrin-induced activation of RhoA and Rac1 GTPases is elevated. Interestingly, mutant cells with the SHP-2 GOF mutation (D61G) are more sensitive than wild-type cells to the suppression of cell motility by inhibition of these pathways. Collectively, these studies reaffirm the positive role of SHP-2 phosphatase in cell motility and suggest a new mechanism by which SHP-2 GOF mutations contribute to diseases.

The function of Shp2 tyrosine phosphatase in the dispersal of acetylcholine receptor clusters

Background

A crucial event in the development of the vertebrate neuromuscular junction (NMJ) is the postsynaptic enrichment of muscle acetylcholine (ACh) receptors (AChRs). This process involves two distinct steps: the local clustering of AChRs at synapses, which depends on the activation of the muscle-specific receptor tyrosine kinase MuSK by neural agrin, and the global dispersal of aneural or "pre-patterned" AChR aggregates, which is triggered by ACh or by synaptogenic stimuli. We and others have previously shown that tyrosine phosphatases, such as the SH2 domain-containing phosphatase Shp2, regulate AChR cluster formation in muscle cells, and that tyrosine phosphatases also mediate the dispersal of pre-patterned AChR clusters by synaptogenic stimuli, although the specific phosphatases involved in this latter step remain unknown.

Results

Using an assay system that allows AChR cluster assembly and disassembly to be studied separately and quantitatively, we describe a previously unrecognized role of the tyrosine phosphatase Shp2 in AChR cluster disassembly. Shp2 was robustly expressed in embryonic Xenopus muscle in vivo and in cultured myotomal muscle cells, and treatment of the muscle cultures with an inhibitor of Shp2 (NSC-87877) blocked the dispersal of pre-patterned AChR clusters by synaptogenic stimuli. In contrast, over-expression in muscle cells of either wild-type or constitutively active Shp2 accelerated cluster dispersal. Significantly, forced expression in muscle of the Shp2-activator SIRPα1 (signal regulatory protein α1) also enhanced the disassembly of AChR clusters, whereas the expression of a truncated SIRPα1 mutant that suppresses Shp2 signaling inhibited cluster disassembly.

Conclusion

Our results suggest that Shp2 activation by synaptogenic stimuli, through signaling intermediates such as SIRPα1, promotes the dispersal of pre-patterned AChR clusters to facilitate the selective accumulation of AChRs at developing NMJs.

The tyrosine phosphatase Shp2 (PTPN11) in cancer

Diverse cellular processes are regulated by tyrosyl phosphorylation, which is controlled by protein-tyrosine kinases (PTKs) and protein-tyrosine phosphatases (PTPs). De-regulated tyrosyl phosphorylation, evoked by gain-of-function mutations and/or over-expression of PTKs, contributes to the pathogenesis of many cancers and other human diseases. PTPs, because they oppose the action of PTKs, had been considered to be prime suspects for potential tumor suppressor genes. Surprisingly, few, if any, tumor suppressor PTPs have been identified. However, the Src homology-2 domain-containing phosphatase Shp2 (encoded by PTPN11) is a bona fide proto-oncogene. Germline mutations in PTPN11 cause Noonan and LEOPARD syndromes, whereas somatic PTPN11 mutations occur in several types of hematologic malignancies, most notably juvenile myelomonocytic leukemia and, more rarely, in solid tumors. Shp2 also is an essential component in several other oncogene signaling pathways. Elucidation of the events underlying Shp2-evoked transformation may provide new insights into oncogenic mechanisms and novel targets for anti-cancer therapy.

Diverse driving forces underlie the invariant occurrence of the T42A, E139D, I282V and T468M SHP2 amino acid substitutions causing Noonan and LEOPARD syndromes

Missense PTPN11 mutations cause Noonan and LEOPARD syndromes (NS and LS), two developmental disorders with pleiomorphic phenotypes. PTPN11 encodes SHP2, an SH2 domain-containing protein tyrosine phosphatase functioning as a signal transducer. Generally, different substitutions of a particular amino acid residue are observed in these diseases, indicating that the crucial factor is the residue being replaced. For a few codons, only one substitution is observed, suggesting the possibility of specific roles for the residue introduced. We analyzed the biochemical behavior and ligand-binding properties of all possible substitutions arising from single-base changes affecting codons 42, 139, 279, 282 and 468 to investigate the mechanisms underlying the invariant occurrence of the T42A, E139D and I282V substitutions in NS and the Y279C and T468M changes in LS. Our data demonstrate that the isoleucine-to-valine change at codon 282 is the only substitution at that position perturbing the stability of SHP2's closed conformation without impairing catalysis, while the threonine-to-alanine change at codon 42, but not other substitutions of that residue, promotes increased phosphopeptide-binding affinity. The recognition specificity of the C-SH2 domain bearing the E139D substitution differed substantially from its wild-type counterpart acquiring binding properties similar to those observed for the N-SH2 domain, revealing a novel mechanism of SHP2's functional dysregulation. Finally, while functional selection does not seem to occur for the substitutions at codons 279 and 468, we point to deamination of the methylated cytosine at nucleotide 1403 as the driving factor leading to the high prevalence of the T468M change in LS.

Shp2 knockdown and Noonan/LEOPARD mutant Shp2-induced gastrulation defects

Shp2 is a cytoplasmic protein-tyrosine phosphatase that is essential for normal development. Activating and inactivating mutations have been identified in humans to cause the related Noonan and LEOPARD syndromes, respectively. The cell biological cause of these syndromes remains to be determined. We have used the zebrafish to assess the role of Shp2 in early development. Here, we report that morpholino-mediated knockdown of Shp2 in zebrafish resulted in defects during gastrulation. Cell tracing experiments demonstrated that Shp2 knockdown induced defects in convergence and extension cell movements. In situ hybridization using a panel of markers indicated that cell fate was not affected by Shp2 knock down. The Shp2 knockdown-induced defects were rescued by active Fyn and Yes and by active RhoA. We generated mutants of Shp2 with mutations that were identified in human patients with Noonan or LEOPARD Syndrome and established that Noonan Shp2 was activated and LEOPARD Shp2 lacked catalytic protein-tyrosine phosphatase activity. Expression of Noonan or LEOPARD mutant Shp2 in zebrafish embryos induced convergence and extension cell movement defects without affecting cell fate. Moreover, these embryos displayed craniofacial and cardiac defects, reminiscent of human symptoms. Noonan and LEOPARD mutant Shp2s were not additive nor synergistic, consistent with the mutant Shp2s having activating and inactivating roles in the same signaling pathway. Our results demonstrate that Shp2 is required for normal convergence and extension cell movements during gastrulation and that Src family kinases and RhoA were downstream of Shp2. Expression of Noonan or LEOPARD Shp2 phenocopied the craniofacial and cardiac defects of human patients. The finding that defective Shp2 signaling induced cell movement defects as early as gastrulation may have implications for the monitoring and diagnosis of Noonan and LEOPARD syndrome.

Noonan syndrome-associated SHP-2/Ptpn11 mutants enhance SIRPalpha and PZR tyrosyl phosphorylation and promote adhesion-mediated ERK activation

Noonan syndrome (NS) is an autosomal dominant disorder that is associated with multiple developmental abnormalities. Activated mutations of the protein-tyrosine phosphatase, SHP-2/PTPN11, have been reported in approximately 50% of NS cases. Despite being activated, NS-associated SHP-2 mutants require plasma membrane proximity to evoke disease-associated signaling. Here we show that NS-associated SHP-2 mutants induce hypertyrosyl phosphorylation of the transmembrane glycoproteins, SIRPalpha (signal-regulatory protein alpha) and PZR (protein zero-related), resulting in their increased association with NS-associated SHP-2 mutants. NS-associated SHP-2 mutants enhanced SIRPalpha and PZR tyrosyl phosphorylation either by impairing SIRPalpha dephosphorylation or by promoting PZR tyrosyl phosphorylation. Importantly, during embryogenesis in a mouse model of NS, SIRPalpha and PZR were hypertyrosyl-phosphorylated and bound increased levels of the NS-associated SHP-2 mutant. SIRPalpha and PZR have been implicated in extracellular matrix-dependent signaling. Mouse embryonic fibroblasts derived from a mouse model of NS displayed enhanced ERK activation in response to fibronectin plating. Knockdown of SIRPalpha and PZR in these cells attenuated the enhanced activation of ERK following fibronectin plating. Thus, SIRPalpha and PZR serve as scaffolds that facilitate plasma membrane recruitment and signaling of NS-associated SHP-2 mutants.

Regulation of ACh receptor clustering by the tyrosine phosphatase Shp2

At the vertebrate neuromuscular junction (NMJ), postsynaptic aggregation of muscle acetylcholine receptors (AChRs) depends on the activation of MuSK, a muscle-specific tyrosine kinase that is stimulated by neural agrin and regulated by muscle-intrinsic tyrosine kinases and phosphatases. We recently reported that Shp2, a tyrosine phosphatase containing src homology two domains, suppressed MuSK-dependent AChR clustering in cultured myotubes, but how this effect of Shp2 is controlled has remained unclear. In this study, biochemical assays showed that agrin-treatment of C2 mouse myotubes enhanced the tyrosine phosphorylation of signal regulatory protein alpha1 (SIRPalpha1), a known activator of Shp2, and promoted SIRPalpha1's interaction with Shp2. Moreover, in situ experiments revealed that treatment of myotubes with the Shp2-selective inhibitor NSC-87877 increased spontaneous and agrin-induced AChR clustering, and that AChR clustering was also enhanced in myotubes ectopically expressing inactive (dominant-negative) Shp2; in contrast, AChR clustering was reduced in myotubes expressing constitutively active Shp2. Significantly, expression of truncated (nonShp2-binding) and full-length (Shp2-binding) forms of SIRPalpha1 in myotubes also increased and decreased AChR clustering, respectively, and coexpression of truncated SIRPalpha1 with active Shp2 and full-length SIRPalpha1 with inactive Shp2 reversed the actions of the exogenous Shp2 proteins on AChR clustering. These results suggest that SIRPalpha1 is a novel downstream target of MuSK that activates Shp2, which, in turn, suppresses AChR clustering. We propose that an inhibitory loop involving both tyrosine kinases and phosphatases sets the level of agrin/MuSK signaling and constrains it spatially to help generate high-density AChR clusters selectively at NMJs.

Mediating ERK 1/2 signaling rescues congenital heart defects in a mouse model of Noonan syndrome

Noonan syndrome (NS) is an autosomal dominant disorder characterized by a wide spectrum of defects, which most frequently include proportionate short stature, craniofacial anomalies, and congenital heart disease (CHD). NS is the most common nonchromosomal cause of CHD, and 80%-90% of NS patients have cardiac involvement. Mutations within the protein tyrosine phosphatase Src homology region 2, phosphatase 2 (SHP2) are responsible for approximately 50% of the cases of NS with cardiac involvement. To understand the developmental stage- and cell type-specific consequences of the NS SHP2 gain-of-function mutation, Q79R, we generated transgenic mice in which the mutated protein was expressed during gestation or following birth in cardiomyocytes. Q79R SHP2 embryonic hearts showed altered cardiomyocyte cell cycling, ventricular noncompaction, and ventricular septal defects, while, in the postnatal cardiomyocyte, Q79R SHP2 expression was completely benign. Fetal expression of Q79R led to the specific activation of the ERK1/2 pathway, and breeding of the Q79R transgenics into ERK1/2-null backgrounds confirmed the pathway's necessity and sufficiency in mediating mutant SHP2's effects. Our data establish the developmental stage-specific effects of Q79R cardiac expression in NS; show that ablation of subsequent ERK1/2 activation prevents the development of cardiac abnormalities; and suggest that ERK1/2 modulation could have important implications for developing therapeutic strategies in CHD.

Vertebrate Ctr1 coordinates morphogenesis and progenitor cell fate and regulates embryonic stem cell differentiation

Embryogenesis involves two distinct processes. On the one hand, cells must specialize, acquiring fates appropriate to their positions (differentiation); on the other hand, they must physically construct the embryo through coordinated mechanical activity (morphogenesis). In early vertebrate development, fibroblast growth factor (FGF) regulates multiple embryonic events, including germ layer differentiation and morphogenesis; the cellular components that direct FGF signaling to evoke these different responses remain largely unknown. We show here that the copper transporter 1 (Ctr1) protein is a critical router of FGF signals during early embryogenesis. Ctr1 both promotes the differentiation and inhibits the morphogenesis of mesoderm and neurectoderm in embryos of the frog Xenopus laevis, thereby coordinating normal development. Signal sorting by Ctr1 involves the activation of the Ras-MAP kinase cascade and appears to be independent of its role in copper transport. Mouse embryonic stem (ES) cells deficient for Ctr1 (Ctr1(-/-)) retain characteristics of pluripotency under conditions that favor differentiation in wild-type ES cells, indicating a conserved role for Ctr1 during amphibian and mammalian cell fate determination. Our studies support a model in which vertebrate Ctr1 functions as a key regulator of the differentiation capacity of both stem and progenitor cell populations.

The tyrosine phosphatase SHP-2 regulates differentiation and apoptosis of individual primary T lymphocytes

Although phosphatases are key players of intracellular processes, not much is known about the phosphatase SHP-2 during T cell differentiation. Here we show that ectopic over-expression of SHP-2 in primary T helper cells directly reduced the frequency of individual lymphocytes expressing pro-inflammatory cytokines after antigen-specific stimulation by a mechanism impairing activation of protein kinase C. In addition we demonstrate that SHP-2 mediates enhanced migration upon CXCR4 signaling in a G-protein-dependent manner. Most strikingly, SHP-2 mediated a dramatic increase in apoptosis by highly enhanced activation of caspases. Co-immunoprecipitations of SHP-2 and c-Cbl from primary T helper cells demonstrated that SHP-2 strongly interacts with the ubiquitin ligase c-Cbl, indicating that c-Cbl could mediate the negative signals of SHP-2. Our results show that SHP-2 signal transduction regulates central checkpoints of T cell differentiation by the activation of distinct signaling cascades.

Structure-based kinetic models of modular signaling protein function: focus on Shp2

We present here a computational, rule-based model to study the function of the SH2 domain-containing protein tyrosine phosphatase, Shp2, in intracellular signal transduction. The two SH2 domains of Shp2 differentially regulate the enzymatic activity by a well-characterized mechanism, but they also affect the targeting of Shp2 to signaling receptors in cells. Our kinetic model integrates these potentially competing effects by considering the intra- and intermolecular interactions of the Shp2 SH2 domains and catalytic site as well as the effect of Shp2 phosphorylation. Even for the isolated Shp2/receptor system, which may seem simple by certain standards, we find that the network of possible binding and phosphorylation states is composed of over 1000 members. To our knowledge, this is the first kinetic model to fully consider the modular, multifunctional structure of a signaling protein, and the computational approach should be generally applicable to other complex intermolecular interactions.

Deregulated Ras signaling in developmental disorders: new tricks for an old dog

Ras proteins regulate cell proliferation, survival and differentiation and are constitutively activated by somatic point mutations in many cancers. Previous studies of neurofibromatosis type 1 and Noonan syndrome also implicated hyperactive Ras in developmental disorders. Recently, germline mutations in H-RAS and K-RAS and in genes encoding other molecules in the Ras-Raf-MEK-ERK cascade were shown to underlie cases of Noonan, cardio-facio-cutaneous, and Costello syndromes. These disorders share phenotypic traits that include abnormal facial features, heart defects, and impaired growth and development. Many of these germline, disease-associated mutations encode novel Ras, Raf and MEK proteins. These studies underscore a crucial role of Ras signaling in human development.

The role of Shp2 (PTPN11) in cancer

Tyrosyl phosphorylation, which is controlled by protein-tyrosine kinases (PTKs) and protein-tyrosine phosphatases (PTPs), regulates numerous cellular processes. Altered expression and/or mutations in PTKs are linked to many forms of cancer, yet until recently little was known about the roles of PTPs in normal cells or in cancer. Earlier work established that a member of the PTP superfamily, PTEN, is an important tumor suppressor gene. We now know that at least one other PTP, the SH2 domain-containing phosphatase Shp2, is a bona fide oncogene that is mutated in several types of leukemia and hyperactivated by other mechanisms in some solid tumors. Understanding how Shp2 and other PTPs contribute to oncogenesis should provide new insights into pathogenesis and might suggest new targets for anti-neoplastic drugs.

Activation of SHP2 Protein-tyrosine Phosphatase Increases HoxA10-induced Repression of the Genes Encoding gp91PHOX and p67PHOX

The CYBB and NCF2 genes encode the phagocyte oxidase proteins gp91(PHOX) and p67(PHOX), respectively. These genes are transcribed after the promyelocyte stage of differentiation, and transcription continues until cell death. In undifferentiated myeloid cells, homologous cis-elements in the CYBB and NCF2 genes are repressed by the homeodomain transcription factor HoxA10. During cytokine-induced myelopoiesis, tyrosine phosphorylation of HoxA10 decreases binding affinity for the CYBB and NCF2 cis-elements. This abrogates HoxA10-induced transcriptional repression as differentiation proceeds. Therefore, mechanisms involved in differentiation stage-specific HoxA10 tyrosine phosphorylation are of interest because HoxA10 phosphorylation modulates myeloid-specific gene transcription. In this study, we found that HoxA10 is a substrate for SHP2 protein-tyrosine phosphatase in undifferentiated myeloid cells. In contrast, HoxA10 is a substrate for a constitutively active mutant form of SHP2 in both undifferentiated and differentiating myeloid cells. Expression of such SHP2 mutants results in persistent HoxA10 repression of CYBB and NCF2 transcription during myelopoiesis. Both HoxA10 overexpression and activating SHP2 mutations have been described in human myeloid malignancies. Therefore, our results suggest that these mutations could cooperate, leading to decreased myeloid-specific gene transcription and functional differentiation block in myeloid cells with both defects.

Structural and functional effects of disease-causing amino acid substitutions affecting residues Ala72 and Glu76 of the protein tyrosine phosphatase SHP-2

Mutations of the protein tyrosine phosphatase SHP-2 are implicated in human diseases, causing Noonan syndrome (NS) and related developmental disorders or contributing to leukemogenesis depending on the specific amino acid substitution involved. SHP-2 is composed by a catalytic (PTP) and two regulatory (N-SH2 and C-SH2) domains that bind to signaling partners and control the enzymatic activity by limiting the accessibility of the catalytic site. Wild type SHP-2 and four disease-associated mutants recurring in hematologic malignancies (Glu76Lys and Ala72Val) or causing NS (Glu76Asp and Ala72Ser), with affected residues located in the PTP-interacting region of the N-SH2 domain, were analyzed by molecular dynamics simulations and in vitro biochemical assays. Simulations demonstrate that mutations do not affect significantly the conformation of the N-SH2 domain. Rather they destabilize the interaction of this domain with the catalytic site, with more evident effects in the two leukemia associated mutants. Consistent with this structural evidence, mutants exhibit an increased level of basal phosphatase activity in the order Glu76Lys > Ala72Val > Glu76Asp > Ala72Ser > WT. The experimental data also show that the mutants with higher basal activity are more responsive to an activating phosphopeptide. A thermodynamic analysis demonstrates that an increase in the overall phosphopeptide affinity of mutants can be explained by a shift in the equilibrium between the inactive and active SHP-2 structure. These data support the view that an increase in the affinity of SHP-2 for its binding partners, caused by destabilization of the closed, inactive conformation, rather than protein basal activation per se, would represent the molecular mechanism, leading to pathogenesis in these mutants.

PTPN11 is the first identified proto-oncogene that encodes a tyrosine phosphatase

Elucidation of the molecular mechanisms underlying carcinogenesis has benefited tremendously from the identification and characterization of oncogenes and tumor suppressor genes. One new advance in this field is the identification of PTPN11 as the first proto-oncogene that encodes a cytoplasmic tyrosine phosphatase with 2 Src-homology 2 (SH2) domains (Shp2). This tyrosine phosphatase was previously shown to play an essential role in normal hematopoiesis. More recently, somatic missense PTPN11 gain-of-function mutations have been detected in leukemias and rarely in solid tumors, and have been found to induce aberrant hyperactivation of the Ras-Erk pathway. This progress represents another milestone in the leukemia/cancer research field and provides a fresh view on the molecular mechanisms underlying cell transformation.

Leukemia-associated, constitutively active mutants of SHP2 protein tyrosine phosphatase inhibit NF1 transcriptional activation by the interferon consensus sequence binding protein

Deficiency in either the interferon consensus sequence binding protein (ICSBP) or neurofibromin 1 (Nf1) increases the proliferative response of myeloid progenitor cell to hematopoietic cytokines. Consistent with this, we previously demonstrated that ICSBP activates transcription of the gene encoding Nf1 (the NF1 gene). In the studies presented here, we determine that ICSBP tyrosine phosphorylation is necessary for the activation of NF1 transcription. Since ICSBP is tyrosine phosphorylated in response to hematopoietic cytokines, these studies identify a novel pathway by which cytokine-induced posttranslational modification of ICSBP results in NF1 transcription. Nf1 subsequently inactivates cytokine-activated Ras, thereby creating a negative feedback mechanism for cytokine-induced proliferation. In these studies, we also determine that ICSBP is a substrate for SHP2 protein tyrosine phosphatase (SHP2-PTP). We find that wild-type SHP2-PTP dephosphorylates ICSBP only in undifferentiated myeloid cells. In contrast, a leukemia-associated, constitutively activated mutant form of SHP2-PTP dephosphorylates ICSBP in both myeloid progenitors and differentiating myeloid cells. Activated SHP2-PTP mutants thereby inhibit ICSBP-dependent NF1 transcription, impairing this negative feedback mechanism on cytokine-activated Ras. Therefore, these studies suggest that leukemia-associated ICSBP deficiency cooperates with leukemia-associated activating mutants of SHP2-PTP to contribute to the proliferative phenotype in myeloid malignancies.

Mutations of the PTPN11 and RAS genes in rhabdomyosarcoma and pediatric hematological malignancies

PTPN11 has been identified as a causative gene in Noonan syndrome (NS), responsible for about 50% of cases of NS. Given the association between NS and an increased risk of some malignancies, notably leukemia and probably some solid tumors including neuroblastoma (NB) and rhabdomyosarcoma (RMS), recent studies have reported that gain-of-function somatic mutations in PTPN11 occur in some hematological malignancies, especially de novo juvenile myelomonocytic leukemia (JMML) and in some solid tumors such as NB, although at a low frequency. In a screen for mutations of PTPN11 in 7 cell lines and 30 fresh tumors of RMS and in 25 cell lines and 40 fresh tumors of NB, we identified a missense mutation (A72T) in an embryonal RMS patient. In the RMS samples, we also detected mutations of NRAS in 1 cell line and 1 patient; both mutations were in embryonal RMSs and had no PTPN11 mutations. No mutations of PTPN11 were detected in NB. In 95 leukemia cell lines and 261 fresh leukemia samples including 22 JMMLs, 9 kinds of missense mutations were detected in 17 leukemia samples, which included 11 (50.0%) mutations in JMML samples and lower frequencies in other hematological malignancies. Furthermore, we identified 4 (18.2%) NRAS mutations and 1 (4.5%) KRAS mutation in 5 JMML samples, 1 of which had a concomitant PTPN11 mutation. Our data suggest that mutations of PTPN11 as well as RAS play a role in the pathogenesis of not only myeloid hematological malignancies but also a subset of RMS malignancies.

Modulation of alpha-catenin Tyr phosphorylation by SHP2 positively effects cell transformation induced by the constitutively active FGFR3

The Src homology 2 phosphotyrosyl phosphatase (SHP2) is a nonreceptor-type phosphatase that acts as a positive transducer of receptor Tyr kinase (RTK) signaling, particularly the Ras-REK and PI3K-Akt pathways. Recently, we have demonstrated that SHP2 is required for cell transformation induced by the constitutively active fibroblast growth factor receptor 3 (K/E-FR3) (Oncogene, 22, 6909-6918). In that study, we had detected a phosphotyrosyl protein of approximately 100 KDa (p100) in cells expressing dominant-negative SHP2 (R/E-SHP2), but its identity and relevance in SHP2-meditaed transformation was not known. Here, we report the identification of p100 as alpha-catenin, a vinculin-related protein involved in adherens junction-mediated intercellular adhesion. We show that alpha-catenin becomes Tyr phosphorylated in intercellular adhesion-dependent manner and this event is counteracted by SHP2. Substrate trapping in intact cells and immunocomplex phosphatse assays confirmed that alpha-catenin is in deed an SHP2 substrate. Tyr phosphorylation of alpha-catenin enhances its translocation to the plasma membrane and its interaction with beta-catenin, leading to enhanced actin polymerization and stabilization of adherens junction-mediated intercellular adhesion, a phenomenon commensurate with loss of the transformation phenotype. Site-directed mutagenesis studies also suggested that Tyr phosphorylation of alpha-catenin enhances its inhibitory role on cell transformation. Based on our previous work and the current report, we demonstrate that mediation of cell transformation by SHP2 is a complex process that involves modulation of the Ras-ERK and PI3K-Akt signaling pathways, intercellular adhesion, focal adhesion and actin cytoskeletal reorganization. To our knowledge, this is the first report showing regulation of alpha-catenin function by Tyr phosphorylation and its inhibitory effect on cell transformation.