SHP-2 mediates target-regulated axonal termination and NGF-dependent neurite growth in sympathetic neurons

Strategy for Leukemia Treatment Targeting SHP-1,2 and SHIP

Protein tyrosine phosphatases (PTPs) are modulators of cellular functions such as differentiation, metabolism, migration, and survival. PTPs antagonize tyrosine kinases by removing phosphate moieties from molecular signaling residues, thus inhibiting signal transduction. Two PTPs, SHP-1 and SHP-2 (SH2 domain-containing phosphatases 1 and 2, respectively) and another inhibitory phosphatase, SH2 domain-containing inositol phosphatase (SHIP), are essential for cell function, which is reflected in the defective phenotype of mutant mice. Interestingly, SHP-1, SHP-2, and SHIP mutations are identified in many cases of human leukemia. However, the impact of these phosphatases and their mutations regarding the onset and progression of leukemia is controversial. The ambiguity of the role of these phosphatases imposes challenges on the development of targeting therapies for leukemia. This fundamental problem, confronted by the expanding investigational field of leukemia, will be addressed in this review, which will include a discussion of the molecular mechanisms of SHP-1, SHP-2, and SHIP in normal hematopoiesis and their role in leukemia. Clinical development of leukemic therapies achieved by targeting these phosphatases will be addressed as well.

Proteomic investigation of Cbl and Cbl-b in neuroblastoma cell differentiation highlights roles for SHP-2 and CDK16

Neuroblastoma is a highly heterogeneous embryonal solid tumor of the sympathetic nervous system. As some tumors can be treated to undergo differentiation, investigating this process can guide differentiation-based therapies of neuroblastoma. Here, we studied the role of E3 ubiquitin ligases Cbl and Cbl-b in regulation of long-term signaling responses associated with extracellular signal-regulated kinase phosphorylation and neurite outgrowth, a morphological marker of neuroblastoma cell differentiation. Using quantitative mass spectrometry (MS)-based proteomics, we analyzed how the neuroblastoma cell line proteome, phosphoproteome, and ubiquitylome were affected by Cbl and Cbl-b depletion. To quantitatively assess neurite outgrowth, we developed a high-throughput microscopy assay that was applied in combination with inhibitor studies to pinpoint signaling underlying neurite outgrowth and to functionally validate proteins identified in the MS data sets. Using this combined approach, we identified a role for SHP-2 and CDK16 in Cbl/Cbl-b-dependent regulation of extracellular signal-regulated kinase phosphorylation and neurite outgrowth, highlighting their involvement in neuroblastoma cell differentiation.

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Multi-layered proteomics captures cellular changes induced by Cbl/Cbl-b depletion

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SHP-2 and CDK16 protein and phosphorylation levels increase upon Cbl/Cbl-b depletion

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SHP-2 and CDK16 regulate phospho-ERK and neurite outgrowth in neuroblastoma cells

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Inhibition of SHP-2 or CDK16 reverts Cbl/Cbl-b knockdown effects on differentiation

SHP-2 deletion in postmigratory neural crest cells results in impaired cardiac sympathetic innervation

Autonomic innervation is an essential component of cardiovascular regulation that is first established from the neural crest (NC) lineage in utero and continues developing postnatally. Although in vitro studies have indicated that SH2-containing protein tyrosine phosphatase 2 (SHP-2) is a signaling factor critical for regulating sympathetic neuron differentiation, this has yet to be shown in the complex in vivo environment of cardiac autonomic innervation. Targeting SHP-2 within postmigratory NC lineages resulted in a fully penetrant mouse model of diminished sympathetic cardiac innervation and concomitant bradycardia. Immunohistochemistry of the sympathetic nerve marker tyrosine hydroxylase revealed a progressive loss of adrenergic ganglionic neurons and reduction of cardiac sympathetic axon density in Shp2 cKOs. Molecularly, Shp2 cKOs exhibit lineage-specific suppression of activated phospo-ERK1/2 signaling but not of other downstream targets of SHP-2 such as pAKT. Genetic restoration of the phosphorylated-extracellular signal-regulated kinase (pERK) deficiency via lineage-specific expression of constitutively active MEK1 was sufficient to rescue the sympathetic innervation deficit and its physiological consequences. These data indicate that SHP-2 signaling specifically through pERK in postmigratory NC lineages is essential for development and maintenance of sympathetic cardiac innervation postnatally.

A sympathetic neuron autonomous role for Egr3-mediated gene regulation in dendrite morphogenesis and target tissue innervation

Egr3 is a nerve growth factor (NGF)-induced transcriptional regulator that is essential for normal sympathetic nervous system development. Mice lacking Egr3 in the germline have sympathetic target tissue innervation abnormalities and physiologic sympathetic dysfunction similar to humans with dysautonomia. However, since Egr3 is widely expressed and has pleiotropic function, it has not been clear whether it has a role within sympathetic neurons and if so, what target genes it regulates to facilitate target tissue innervation. Here, we show that Egr3 expression within sympathetic neurons is required for their normal innervation since isolated sympathetic neurons lacking Egr3 have neurite outgrowth abnormalities when treated with NGF and mice with sympathetic neuron-restricted Egr3 ablation have target tissue innervation abnormalities similar to mice lacking Egr3 in all tissues. Microarray analysis performed on sympathetic neurons identified many target genes deregulated in the absence of Egr3, with some of the most significantly deregulated genes having roles in axonogenesis, dendritogenesis, and axon guidance. Using a novel genetic technique to visualize axons and dendrites in a subpopulation of randomly labeled sympathetic neurons, we found that Egr3 has an essential role in regulating sympathetic neuron dendrite morphology and terminal axon branching, but not in regulating sympathetic axon guidance to their targets. Together, these results indicate that Egr3 has a sympathetic neuron autonomous role in sympathetic nervous system development that involves modulating downstream target genes affecting the outgrowth and branching of sympathetic neuron dendrites and axons.

Central regulation of metabolism by protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs) are important regulators of intracellular signaling pathways via the dephosphorylation of phosphotyrosyl residues on various receptor and non-receptor substrates. The phosphorylation state of central nervous system (CNS) signaling components underlies the molecular mechanisms of a variety of physiological functions including the control of energy balance and glucose homeostasis. In this review, we summarize the current evidence implicating PTPs as central regulators of metabolism, specifically highlighting their interactions with the neuronal leptin and insulin signaling pathways. We discuss the role of a number of PTPs (PTP1B, SHP2, TCPTP, RPTPe, and PTEN), reviewing the findings from genetic mouse models and in vitro studies which highlight these phosphatases as key central regulators of energy homeostasis.

Kinase/phosphatase overexpression reveals pathways regulating hippocampal neuron morphology

Kinases and phosphatases that regulate neurite number versus branching versus extension are weakly correlated.

The kinase family that most strongly enhances neurite growth is a family of non-protein kinases; sugar kinases related to NADK.

Pathway analysis revealed that genes in several cancer pathways were highly active in enhancing neurite growth.

In neural development, neuronal precursors differentiate, migrate, extend long axons and dendrites, and finally establish connections with their targets. Clinical conditions such as spinal cord injury, traumatic brain injury, stroke, multiple sclerosis, Parkinson's disease, Huntington's disease, and Alzheimer's disease are often associated with a loss of axon and/or dendrite connectivity and treatment strategies would be enhanced by new therapies targeting cell intrinsic mechanisms of axon elongation and regeneration.

Phosphorylation controls most cellular processes, including the cell cycle, proliferation, metabolism, and apoptosis. Neuronal differentiation, including axon formation and elongation, is also regulated by a wide range of kinases and phosphatases. For example, the non-receptor tyrosine kinase Src is required for cell adhesion molecule-dependent neurite outgrowth. In addition to individual kinases and phosphatases, signaling pathways like the MAPK, growth factor signaling, PIP3, cytoskeletal, and calcium-dependent pathways have been shown to impinge on or control neuronal process development. Recent results have implicated GSK3 and PTEN as therapeutically relevant targets in axonal regeneration after injury. However, these and other experiments have studied only a small fraction of the total kinases and phosphatases in the genome. Because of recent advances in genomic knowledge, large-scale cDNA production, and high-throughput phenotypic analysis, it is now possible to take a more comprehensive approach to understanding the functions of kinases and phosphatases in neurons.

We performed a large, unbiased set of experiments to answer the question ‘what effect does the overexpression of genes encoding kinases, phosphatases, and related proteins have on neuronal morphology?' We used ‘high-content analysis' to obtain detailed results about the specific phenotypes of neurons. We studied embryonic rat hippocampal neurons because of their stereotypical development in vitro (Dotti et al, 1988) and their widespread use in studies of neuronal differentiation and signaling. We transfected over 700 clones encoding kinases and phosphatases into hippocampal neurons and analyzed the resulting changes in neuronal morphology.

Many known genes, including PP1a, ERK1, ErbB2, atypical PKC, Calcineurin, CaMK2, IGF1R, FGFR, GSK3, and PIK3 were observed to have significant effects on neurite outgrowth in our system, consistent with earlier findings in the literature.

We obtained quantitative data for many cellular and neuronal morphological parameters from each neuron imaged. These included nuclear morphology (nuclear area and Hoechst dye intensity), soma morphology (tubulin intensity, area, and shape), and numerous parameters of neurite morphology (e.g. tubulin intensity along the neurites, number of primary neurites, neurite length, number of branches, distance from the cell body to the branches, number of crossing points, width and area of the neurites, and longest neurite; Supplementary Figure 1). Other parameters were reported on a ‘per well' basis, including the percentage of transfected neurons in a condition, as well as the percentage of neurons initiating neurite growth. Data for each treatment were normalized to a control (pSport CAT) within the same experiment, then aggregated across replicate experiments.

Correlations among the 19 normalized parameters were analyzed for neurons transfected with all kinase and phosphatase clones (Figure 2). On the basis of this analysis, the primary variables that define the neurite morphology are primary neurite count, neurite average length, and average branches. Interestingly, primary neurite count was not well correlated with neurite length or branching. The Pearson correlation coefficient (r²) between the number of primary neurites and the average length of the neurites was 0.3, and between the number of primary neurites and average branching was 0.2. In contrast, the correlation coefficient of average branching with neurite average length was 0.7. The most likely explanation is that signaling mechanisms underlying the neurite number determination are different than those controlling length/branching of the neurites.

Related proteins are often involved in similar neuronal functions. For example, families of receptor protein tyrosine phosphatases are involved in motor axon extension and guidance in both Drosophila and in vertebrates, and a large family of Eph receptor tyrosine kinases regulates guidance of retinotectal projections, motor axons, and axons in the corpus callosum. We therefore asked whether families of related genes produced similar phenotypes when overexpressed in hippocampal neurons. Our set of genes covered 40% of the known protein kinases, and many of the non-protein kinases and phosphatases.

Gene families commonly exhibit redundant function. Redundant gene function has often been identified when two or more knockouts are required to produce a phenotype. Our technique allowed us to measure whether different members of gene families had similar (potentially redundant) or distinct effects on neuronal phenotype.

To determine whether groups of related genes affect neuronal morphology in similar ways, we used sequence alignment information to construct gene clusters (Figure 6). Genes were clustered at nine different thresholds of similarity (called ‘tiers'). The functional effect for a particular parameter was then averaged within each cluster of a given tier, and statistics were performed to determine the significance of the effect. We analyzed the results for three key neurite parameters (average neurite length, primary neurite count, and average branching). Genes that perturbed each of these phenotypes are grouped in Figure 6. Eight families, most with only a few genes, produced significant changes for one or two parameters. A diverse family of non-protein kinases had a positive effect on neurite outgrowth in three of the four parameters analyzed. This family of kinases consisted of a variety of enzymes, mostly sugar and lipid kinases. A similar analysis was performed using pathway cluster analysis with pathways from the KEGG database, rather than sequence homology. Interestingly, pathways involved in cancer cell proliferation potentiated neurite extension and branching.

Our studies have identified a large number of kinases and phosphatases, as well as structurally and functionally defined families of these proteins, that affect neuronal process formation in specific ways. We have provided an analytical methodology and new tools to analyze functional data, and have implicated genes with novel functions in neuronal development. Our studies are an important step towards the goal of a molecular description of the intrinsic control of axodendritic growth.

Development and regeneration of the nervous system requires the precise formation of axons and dendrites. Kinases and phosphatases are pervasive regulators of cellular function and have been implicated in controlling axodendritic development and regeneration. We undertook a gain-of-function analysis to determine the functions of kinases and phosphatases in the regulation of neuron morphology. Over 300 kinases and 124 esterases and phosphatases were studied by high-content analysis of rat hippocampal neurons. Proteins previously implicated in neurite growth, such as ERK1, GSK3, EphA8, FGFR, PI3K, PKC, p38, and PP1a, were confirmed to have effects in our functional assays. We also identified novel positive and negative neurite growth regulators. These include neuronal-developmentally regulated kinases such as the activin receptor, interferon regulatory factor 6 (IRF6) and neural leucine-rich repeat 1 (LRRN1). The protein kinase N2 (PKN2) and choline kinase α (CHKA) kinases, and the phosphatases PPEF2 and SMPD1, have little or no established functions in neuronal function, but were sufficient to promote neurite growth. In addition, pathway analysis revealed that members of signaling pathways involved in cancer progression and axis formation enhanced neurite outgrowth, whereas cytokine-related pathways significantly inhibited neurite formation.

PTP1B and SHP2 in POMC neurons reciprocally regulate energy balance in mice

Protein tyrosine phosphatase 1B (PTP1B) and SH2 domain-containing protein tyrosine phosphatase-2 (SHP2) have been shown in mice to regulate metabolism via the central nervous system, but the specific neurons mediating these effects are unknown. Here, we have shown that proopiomelanocortin (POMC) neuron-specific deficiency in PTP1B or SHP2 in mice results in reciprocal effects on weight gain, adiposity, and energy balance induced by high-fat diet. Mice with POMC neuron-specific deletion of the gene encoding PTP1B (referred to herein as POMC-Ptp1b-/- mice) had reduced adiposity, improved leptin sensitivity, and increased energy expenditure compared with wild-type mice, whereas mice with POMC neuron-specific deletion of the gene encoding SHP2 (referred to herein as POMC-Shp2-/- mice) had elevated adiposity, decreased leptin sensitivity, and reduced energy expenditure. POMC-Ptp1b-/- mice showed substantially improved glucose homeostasis on a high-fat diet, and hyperinsulinemic-euglycemic clamp studies revealed that insulin sensitivity in these mice was improved on a standard chow diet in the absence of any weight difference. In contrast, POMC-Shp2-/- mice displayed impaired glucose tolerance only secondary to their increased weight gain. Interestingly, hypothalamic Pomc mRNA and alpha-melanocyte-stimulating hormone (alphaMSH) peptide levels were markedly reduced in POMC-Shp2-/- mice. These studies implicate PTP1B and SHP2 as important components of POMC neuron regulation of energy balance and point to what we believe to be a novel role for SHP2 in the normal function of the melanocortin system.

Dimerization of tyrosine phosphatase PTPRO decreases its activity and ability to inactivate TrkC

Receptor-protein tyrosine phosphatases (RPTPs), like receptor tyrosine kinases, regulate neuronal differentiation. While receptor tyrosine kinases are dimerized and activated by extracellular ligands, the extent to which RPTPs dimerize, and the effects of dimerization on phosphatase activity, are poorly understood. We have examined a neuronal type III RPTP, PTPRO; we find that PTPRO can form dimers in living cells, and that disulfide linkages in PTPROs intracellular domain likely regulate dimerization. Dimerization of PTPROs transmembrane and intracellular domains, achieved by ligand binding to a chimeric fusion protein, decreases activity toward artificial peptides and toward a putative substrate, tropomyosin-related kinase C (TrkC). Dephosphorylation of TrkC by PTPRO may be physiologically relevant, as it is efficient, and TrkC and PTPRO can be co-precipitated from transfected cells. Inhibition of PTPROs phosphatase activity by dimerization is interesting, as dimerization of a related RPTP, CD148/PTPRJ, increases activity. Thus, our results suggest a complex relationship between dimerization and activity in type III RPTPs.

PirB is a functional receptor for myelin inhibitors of axonal regeneration

A major barrier to regenerating axons after injury in the mammalian central nervous system is an unfavorable milieu. Three proteins found in myelin--Nogo, MAG, and OMgp--inhibit axon regeneration in vitro and bind to the glycosylphosphatidylinositol-anchored Nogo receptor (NgR). However, genetic deletion of NgR has only a modest disinhibitory effect, suggesting that other binding receptors for these molecules probably exist. With the use of expression cloning, we have found that paired immunoglobulin-like receptor B (PirB), which has been implicated in nervous system plasticity, is a high-affinity receptor for Nogo, MAG, and OMgp. Interfering with PirB activity, either with antibodies or genetically, partially rescues neurite inhibition by Nogo66, MAG, OMgp, and myelin in cultured neurons. Blocking both PirB and NgR activities leads to near-complete release from myelin inhibition. Our results implicate PirB in mediating regeneration block, identify PirB as a potential target for axon regeneration therapies, and provide an explanation for the similar enhancements of visual system plasticity in PirB and NgR knockout mice.

The Src homology 2 domain tyrosine phosphatases SHP-1 and SHP-2: diversified control of cell growth, inflammation, and injury

Interest in the diverse biology of protein tyrosine phosphatases that are encoded by more than 100 genes in the human genome continues to grow at an accelerated pace. In particular, two cytoplasmic protein tyrosine phosphatases composed of two Src homology 2 (SH2) NH2-terminal domains and a C-terminal protein-tyrosine phosphatase domain referred to as SHP-1 and SHP-2 are known to govern a host of cellular functions. SHP-1 and SHP-2 modulate progenitor cell development, cellular growth, tissue inflammation, and cellular chemotaxis, but more recently the role of SHP-1 and SHP-2 to directly control cell survival involving oxidative stress pathways has come to light. SHP-1 and SHP-2 are fundamental for the function of several growth factor and metabolic pathways yielding far reaching implications for disease pathways and disorders such as diabetes, neurodegeneration, and cancer. Although SHP-1 and SHP-2 can employ similar or parallel cellular pathways, these proteins also clearly exert opposing effects upon downstream cellular cascades that affect early and late apoptotic programs. SHP-1 and SHP-2 modulate cellular signals that involve phosphatidylinositol 3-kinase, Akt, Janus kinase 2, signal transducer and activator of transcription proteins, mitogen-activating protein kinases, extracellular signal-related kinases, c-Jun-amino terminal kinases, and nuclear factor-kappaB. Our progressive understanding of the impact of SHP-1 and SHP-2 upon multiple cellular environments and organ systems should continue to facilitate the targeted development of treatments for a variety of disease entities.

Tyrosine phosphatases such as SHP-2 act in a balance with Src-family kinases in stabilization of postsynaptic clusters of acetylcholine receptors

BACKGROUND

Development of neural networks requires that synapses are formed, eliminated and stabilized. At the neuromuscular junction (NMJ), agrin/MuSK signaling, by triggering downstream pathways, causes clustering and phosphorylation of postsynaptic acetylcholine receptors (AChRs). Postnatally, AChR aggregates are stabilized by molecular pathways that are poorly characterized. Gain or loss of function of Src-family kinases (SFKs) disassembles AChR clusters at adult NMJs in vivo, whereas AChR aggregates disperse rapidly upon withdrawal of agrin from cultured src-/-;fyn-/- myotubes. This suggests that a balance between protein tyrosine phosphatases (PTPs) and protein tyrosine kinases (PTKs) such as those of the Src-family may be essential in stabilizing clusters of AChRs.

RESULTS

We have analyzed the role of PTPs in maintenance of AChR aggregates, by adding and then withdrawing agrin from cultured myotubes in the presence of PTP or PTK inhibitors and quantitating remaining AChR clusters. In wild-type myotubes, blocking PTPs with pervanadate caused enhanced disassembly of AChR clusters after agrin withdrawal. When added at the time of agrin withdrawal, SFK inhibitors destabilized AChR aggregates but concomitant addition of pervanadate rescued cluster stability. Likewise in src-/-;fyn-/- myotubes, in which agrin-induced AChR clusters form normally but rapidly disintegrate after agrin withdrawal, pervanadate addition stabilized AChR clusters. The PTP SHP-2, known to be enriched at the NMJ, associated and colocalized with MuSK, and agrin increased this interaction. Specific SHP-2 knockdown by RNA interference reduced the stability of AChR clusters in wild-type myotubes. Similarly, knockdown of SHP-2 in adult mouse soleus muscle by electroporation of RNA interference constructs caused disassembly of pretzel-shaped AChR-rich areas in vivo. Finally, we found that src-/-;fyn-/- myotubes contained elevated levels of SHP-2 protein.

CONCLUSION

Our data are the first to show that the fine balance between PTPs and SFKs is a key aspect in stabilization of postsynaptic AChR clusters. One phosphatase that acts in this equilibrium is SHP-2. Thus, PTPs such as SHP-2 stabilize AChR clusters under normal circumstances, but when these PTPs are not balanced by SFKs, they render clusters unstable.

You can't go home again: transcriptionally driven alteration of cell signaling by NGF

Here we review findings indicating that neurotrophins such as NGF promote changes in gene transcription that in turn influence the ways that cells subsequently respond to trophic factors. As a result, initial responses of "naïve" cells to NGF and other trophic agents differ from those of cells with prior NGF exposure. We discuss specific examples based on reports in the literature as well as on data derived from a serial analysis of gene expression (SAGE) study of NGF-promoted transcriptional changes in PC12 pheochromocytoma cells.

ERK1/2 is a negative regulator of homeodomain protein Arix/Phox2a

The homeodomain protein Arix/Phox2a plays a role in the development and maintenance of the noradrenergic cell type by regulating the transcription of genes involved in the biosynthesis and metabolism of noradrenaline. Previous work has shown that Arix/Phox2a is a phosphoprotein, and the phosphorylated form of Arix/Phox2a exhibits poorer DNA-binding activity than does the dephosphorylated form. Here, we demonstrate that Arix/Phox2a is phosphorylated by extracellular signal-related kinase (ERK)1/2 at two sites within the N-terminal transactivation domain. The phosphorylation level of Arix in cultured SH-SY5Y neuroblastoma cells is reduced when cells are treated with the mitogen activated protein kinase kinase 1 (MEK1) inhibitor UO126. Treatment of sympathetic neurons with the MEK1 inhibitor, PD98059, results in an elevation of mRNAs encoding noradrenergic proteins, dopamine beta-hydroxylase (DBH) and norepinephrine transporter (NET), but not tyrosine hydroyxlase (TH). Treatment of neuroblastoma cultures with PD98059 increases the interaction of Arix with DBH and NET genes, but not the TH gene. Together, these results suggest that phosphorylation of Arix by ERK1/2 inhibits its ability to interact with target genes, and that both specificity of expression and modulation by external stimuli are monitored through the same transcription factor.

SIRPα negatively regulates differentiation of PC12 cell

Signal regulatory protein alpha (SIRPalpha) is an Ig superfamily protein whose cytoplasmic region contains immunoreceptor tyrosine-based inhibitory motif (ITIM), which when tyrosine phosphorylated binds the SH2-domain containing phosphatase 2 (SHP-2). Both SIRPalpha and SHP-2 are highly expressed in brain. Murine cerebellar cells cultured on SIRPalpha-coated surface exhibit enhanced neurite outgrowth and SIRPalpha is localized at sites of synaptogenesis in postnatal mouse brain. In this study, we show that nerve growth factor (NGF) stimulation resulted in elevated SIRPalpha expression during PC12 differentiation. We also show that NGF-induced morphological differentiation, but not growth arrest response, was inhibited by ectopic SIRPalpha expression. PC12 cells stably expressing SIRPalpha proliferated more rapidly than mock-transfected cells. The activity of c-jun N-terminal kinase (JNK) decreased in SIRPalpha-transfected PC12 cells, whereas nuclear factor-kappaB (NF-kappaB) activity increased. Collectively, our results suggest that SIRPalpha may stabilize synaptic connections by inhibiting improper neurite outgrowth and might realize its neuronal function, at least in part, by modulating JNK and NF-kappaB activity.