Analysis of a Shc family adaptor protein, ShcD/Shc4, that associates with muscle-specific kinase

Identification of the Factor That Leads Human Mesenchymal Stem Cell Lines into Decellularized Bone

Hematopoiesis is maintained by the interaction of hematopoietic stem cells (HSCs) and bone marrow mesenchymal stem cells (MSCs) in bone marrow microenvironments, called niches. Certain genetic mutations in MSCs, not HSCs, provoke some hematopoietic neoplasms, such as myelodysplastic syndrome. An in vivo bone marrow niche model using human MSC cell lines with specific genetic mutations and bone scaffolds is necessary to elucidate these interactions and the disease onset. We focused on decellularized bone (DCB) as a useful bone scaffold and attempted to induce human MSCs (UE7T-9 cells) into the DCB. Using the CRISPR activation library, we identified SHC4 upregulation as a candidate factor, with the SHC4 overexpression in UE7T-9 cells activating their migratory ability and upregulating genes to promote hematopoietic cell migration. This is the first study to apply the CRISPR library to engraft cells into decellularized biomaterials. SHC4 overexpression is essential for engrafting UE7T-9 cells into DCB, and it might be the first step toward creating an in vivo human–mouse hybrid bone marrow niche model.

SHC4 promotes tumor proliferation and metastasis by activating STAT3 signaling in hepatocellular carcinoma

Background

The Src homology and collagen 4 (SHC4) is an important intracellular adaptor protein that has been shown to play a pro-cancer role in melanoma and glioma. However, the biological function and detailed mechanisms of SHC4 in hepatocellular carcinoma progression are unclear. This study aimed to evaluate the potential prognostic and treatment value of SHC4 in patients with HCC.

Methods

The expression status of SHC4 in HCC tissues were investigated by immunohistochemistry and western blotting. Clinical significance of SHC4 was evaluated in a large cohort of HCC patients. The effects of SHC4 repression or overexpression on migration, invasion, and tumor growth were detected by colony formation assay, wound healing, transwell assays, and xenograft assay. Cell cycle and EMT-related proteins were detected by western blotting and immunofluorescence. In addition, the molecular regulation between SHC4 and STAT3 signaling in HCC were discovered by western blotting, immunofluorescence and xenograft assay.

Results

SHC4 was overexpressed in HCC compared to adjacent normal liver tissues and increased SHC4 expression was associated with high AFP level, incomplete tumor encapsulation, poor tumor differentiation and poor prognosis. SHC4 was shown to enhance cell proliferation, colony formation, cells migration and invasion in vitro, and promotes cell cycle progression and EMT process in HCC cells. Tumor xenograft model assay confirmed the oncogenic role of SHC4 in tumorigenicity in nude mice. Moreover, activation of STAT3 signaling was found in the SHC4 overexpressed HCC cells and HCC tissues. Further intervention of STAT3 confirmed STAT3 as an important signaling pathway for the oncogenic role of SHC4 in HCC.

Conclusions

Together, our results reveal that SHC4 activates STAT3 signaling to promote HCC progression, which may provide new clinical ideas for the treatment of HCC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12935-022-02446-9.

The Role of p66Shc in Diabetes: A Comprehensive Review from Bench to Bedside

It is well-documented that diabetes is an inflammatory and oxidative disease, with an escalating global burden. Still, there is no definite treatment for diabetes or even prevention of its harmful complications. Therefore, understanding the molecular pathways associated with diabetes might help in finding a solution. p66Shc is a member of Shc family proteins, and it is considered as an oxidative stress sensor and regulator in cells. There are inconsistent data about the role of p66Shc in inducing diabetes, but accumulating evidence supports its role in the pathogenesis of diabetes-related complications, including macro and microangiopathies. There is growing hope that by understanding and targeting molecular pathways involved in this network, prevention of diabetes or its complications would be achievable. This review provides an overview about the role of p66Shc in the development of diabetes and its complications.

Downregulation of SHCBP1 Inhibits Proliferation, Migration, and Invasion in Human Nasopharyngeal Carcinoma Cells

Background

SHC SH2 domain-binding protein 1 (SHCBP1), one of the members of Src homolog and collagen homolog (Shc) family, has been reported to be overexpressed in several malignant cancers and involved in tumor progression. However, the expression of SHCBP1 in nasopharyngeal carcinoma (NPC) remains unclear, and its clinical significance remains to be further elucidated.

Methods

The expression of SHCBP1 mRNA in 35 pair samples of NPC and adjacent normal tissues of NPC was detected by RT-qPCR. The expression level of SHCBP1 protein and mRNA in the selected cells was detected by western blot and RT-qPCR, respectively. The effects of SHCBP1 on NPC in vitro were observed by MTT method, colony formation assay, apoptosis assay, cell cycle assay, wound healing assay, transwell migration assay, and transwell invasion assay.

Results

SHCBP1 was highly expressed in clinical tissues and NPC cell lines, and SHCBP1 knockdown significantly inhibited NPC cell proliferation. Overexpression of SHCBP1 promoted NPC cell proliferation, migration, and invasion in NPC cell lines. Silencing SHCBP1 expression can delay cell cycle and inhibit cell apoptosis.

Conclusion

Our results suggest that SHCBP1 may promote proliferation and metastasis of NPC cells, which represents that SHCBP1 may act as a new indicator for predicting the prognosis of NPC and a new target for clinical treatment.

Adaptor Protein ShcD/SHC4 Interacts with Tie2 Receptor to Synergistically Promote Glioma Cell Invasion

Gliomas are characterized by diffuse infiltration of tumor cells into surrounding brain tissue, and this highly invasive nature contributes to disease recurrence and poor patient outcomes. The molecular mechanisms underlying glioma cell invasion remain incompletely understood, limiting development of new targeted therapies. Here, we have identified phosphotyrosine adaptor protein ShcD as upregulated in malignant glioma and shown that it associates with receptor tyrosine kinase Tie2 to facilitate invasion. In human glioma cells, we find that expression of ShcD and Tie2 increases invasion, and this significant synergistic effect is disrupted with a ShcD mutant that cannot bind Tie2 or hyperphosphorylate the receptor. Expression of ShcD and/or Tie2 further increases invadopodia formation and matrix degradation in U87 glioma cells. In a coculture model, we show that U87-derived tumor spheroids expressing both ShcD and Tie2 display enhanced infiltration into cerebral organoids. Mechanistically, we identify changes in focal adhesion kinase phosphorylation in the presence of ShcD and/or Tie2 in U87 cells upon Tie2 activation. Finally, we identify a strong correlation between transcript levels of ShcD and Tie2 signaling components as well as N-cadherin in advanced gliomas and those with classical or mesenchymal subtypes, and we show that elevated expression of ShcD correlates with a significant reduction in patient survival in higher grade gliomas with mesenchymal signature. Altogether, our data highlight a novel Tie2-ShcD signaling axis in glioma cell invasion, which may be of clinical significance. IMPLICATIONS: ShcD cooperates with Tie2 to promote glioma cell invasion and its elevated expression correlates with poor patient outcome in advanced gliomas.

Protective Effects of ShcA Protein Silencing for Photothrombotic Cerebral Infarction

Reactive oxygen species (ROS) exacerbate stroke-induced cell damage. We found that ShcA, a protein that regulates ROS, is highly expressed in a Rose Bengal photothrombosis model. We investigated whether ShcA is essential for mitophagy in ROS-induced cellular damage and determined whether ROS exacerbate mitochondrial dysfunction via ShcA protein expression. Ischemic stroke was generated by Rose Bengal photothrombosis in mice. To silence ShcA protein expression in the mouse brain, ShcA-targeting siRNA-encapsulated nanoparticles were intrathecally injected into the cisterna magna. Upon staining with antibodies against ShcA counterpart caspase-3 or NeuN, we found that the ShcA protein expression was increased in apoptotic neurons. In addition, mitochondrial dysfunction and excessive mitophagy were evident in photothrombotic stroke tissue. Infarct volumes were significantly reduced, and neurological deficits were diminished in the ShcA siRNA nanoparticle-treated group, compared with the negative control siRNA nanoparticle-treated group. We confirmed that the reduction of ShcA expression by nanoparticle treatment rescued the expression of genes, associated with mitochondrial dynamics and mitophagy mediation in a stroke model. This study suggests that the regulation of ShcA protein expression can be a therapeutic target for reducing brain damage with mitochondrial dysfunction caused by thrombotic infarction.

Down-regulation of the insulin signaling pathway by SHC may correlate with congenital heart disease in Chinese populations

BACKGROUND/AIMS

Congenital heart disease (CHD) is one of the most common and severe congenital defects. The incidence of fetal cardiac malformation is increased in the context of maternal gestational diabetes mellitus (GDM). Therefore, we wanted to determine whether abnormalities in the insulin signaling pathway are associated with the occurrence of nonsyndromic CHD (ns-CHD).

METHODS

We used digital gene expression profiling (DGE) of right atrial myocardial tissue samples from eight ns-CHD patients and four controls. The genes potentially associated with CHD were validated by real-time fluorescence quantitative PCR analysis of right atrial myocardial tissues from 37 patients and 10 controls and the H9C2 cell line.

RESULTS

The results showed that the insulin signaling pathway, which is mediated by the SHC gene family, was inhibited in the ns-CHD patients. The expression levels of five genes (PTPRF, SHC4, MAP2K2, MKNK2, and ELK1) in the pathway were significantly down-regulated in the patients' atrial tissues (P<0.05 for all). In vitro, the H9C2 cells cultured in high glucose (33 mmol/l) expressed less SHC4, MAP2K2, and Elk-1 than those cultured in low glucose (25 mmol/l). Furthermore, the high glucose concentration down-regulated the 25 genes associated with blood vessel development based on Gene Ontology (GO) term enrichment analyses of RNA-seq data.

CONCLUSION

We considered that changes in the insulin signaling pathway mediated by SHC might be involved in the heart development process. This mechanism might account for the increase in the incidence of fetal cardiac malformations in the context of GDM.

MuSK antibodies, lessons learned from poly- and monoclonality

Muscle-specific kinase (MuSK) plays a critical role in establishing and maintaining neuromuscular synapses. Antibodies derived from immunizing animals with MuSK were important tools to help detect MuSK and its activity. The role of antibodies in MuSK-related research got an extra dimension when autoantibodies to MuSK were found to cause myasthenia gravis (MG) in 2001. Active immunization with MuSK or passive transfer of polyclonal purified IgG(4) fractions from patients reproduced myasthenic muscle weakness in a range of animal models. Polyclonal patient-purified autoantibodies were furthermore found to block agrin-Lrp4-MuSK signaling, explaining the synaptic disassembly, failure of neuromuscular transmission and ultimately muscle fatigue observed in vivo. MuSK autoantibodies are predominantly of the IgG4 subclass. Low levels of other subclass MuSK antibodies coexist, but their role in the pathogenesis is unclear. Patient-derived monoclonal antibodies revealed that MuSK antibody subclass and valency alters their functional effects and possibly their pathogenicity. Interestingly, recombinant functional bivalent MuSK antibodies might even have therapeutic potential for a variety of neuromuscular disorders, due to their agonistic nature on the MuSK signaling cascade. Thus, MuSK antibodies have proven to be helpful tools to study neuromuscular junction physiology, contributed to our understanding of the pathophysiology of MuSK MG and might be used to treat neuromuscular disorders. The source of MuSK antibodies and consequently their (mixed) polyclonal or monoclonal nature were important confounding factors in these experiments. Here we review the variety of MuSK antibodies described thus far, the insights they have given us and their potential for the future.

Localization of phosphotyrosine adaptor protein ShcD/SHC4 in the adult rat central nervous system

Background

Mammalian Shc (Src homology and collagen) proteins comprise a family of four phosphotyrosine adaptor molecules which exhibit varied spatiotemporal expression and signaling functions. ShcD is the most recently discovered homologue and it is highly expressed in the developing central nervous system (CNS) and adult brain. Presently however, its localization within specific cell types of mature neural structures has yet to be characterized.

Results

In the current study, we examine the expression profile of ShcD in the adult rat CNS using immunohistochemistry, and compare with those of the neuronally enriched ShcB and ShcC proteins. ShcD shows relatively widespread distribution in the adult brain and spinal cord, with prominent levels of staining throughout the olfactory bulb, as well as in sub-structures of the cerebellum and hippocampus, including the subgranular zone. Co-localization studies confirm the expression of ShcD in mature neurons and progenitor cells. ShcD immunoreactivity is primarily localized to axons and somata, consistent with the function of ShcD as a cytoplasmic adaptor. Regional differences in expression are observed among neural Shc proteins, with ShcC predominating in the hippocampus, cerebellum, and some fiber tracts. Interestingly, ShcD is uniquely expressed in the olfactory nerve layer and in glomeruli of the main olfactory bulb.

Conclusions

Together our findings suggest that ShcD may provide a distinct signaling contribution within the olfactory system, and that overlapping expression of ShcD with other Shc proteins may allow compensatory functions in the brain.

Role of TGF‑β in the motility of ShcD‑overexpressing 293 cells

The newly identified Src homology and collagen (Shc) family member ShcD was observed to be upregulated in 50% of vertical growth phase and metastatic melanomas. The aim of the present study was to investigate the mechanism by which ShcD mediates cell motility. 293 cell lines were altered to stably express GFP (GF) or GFP‑ShcD (G5). Treatment of the cells with transforming growth factor (TGF)β2 promoted extracellular signal‑regulated kinase (ERK) phosphorylation and, to a lesser extent, Smad2 phosphorylation in GFP‑ShcD‑expressing cells but not in GFP‑overexpressing cells. GFP‑ShcD‑expressing cells exhibited upregulated expression of certain epithelial‑mesenchymal transition‑related genes, such as snail family transcriptional repressor 1 and SLUG, than GFP‑expressing cells. Higher levels of ERK were found in the nuclear fraction of GFP‑ShcD‑expressing cells than that of GFP‑expressing cells. Overall, GFP‑ShcD‑expressing cells demonstrated enhanced migration compared with GFP‑expressing cells. A slight increase in cell migration was observed in both cell lines (GF and G5) when the cells were allowed to migrate towards conditioned medium derived from TGFβ2‑treated GFP‑ShcD expressing cells. Collectively, ShcD upregulation was proposed to induce cell migration by affecting the expression of certain epithelial‑mesenchymal transition‑related genes. Thus, our findings may improve understanding of the role of ShcD in cell migration.

[Functional roles of phosphotyrosine adaptor Shc in the brain]

Adaptor molecules (adaptor proteins) have indispensable roles in cellular signaling, essential for cellular proliferation, development and metabolism. Shc (Src homology and collagen homology)-family molecule is a group of adaptor molecules, and indicated to be involved in intracellular phosphotyrosine signaling. Shc family has 4 subtypes, ShcA-ShcD, and there are long and short isoforms in ShcA and ShcC whereas ShcB and ShcD have short isoform only. There are three domains conserved in all Shc-family isoforms: phosphotyrosine-binding (PTB) domain, collagen-homology 1 (CH1) domain and Src-homology 2 (SH2) domain, from the N-terminal to C-terminal. PTB and SH2 domains recognize and bind to phosphotyrosine in other molecules, and CH1 domain is recognized and bind to SH2 domain in Grb2, an adaptor molecule, when the tyrosine residues in the domain are phosphorylated. Expression of ShcA is observed in all tissues except for brain in adult animals, although ShcA mRNA is detected in brain during embryonic days. On the other hand, in adult brain, expressions of ShcB, ShcC, and ShcD are observed. Analysis of single knockout mice (ShcA (neuron specific), ShcB, ShcC) and double knockout mice for ShcB and C indicated essential roles of Shc-family molecules in proliferation and survival of cells in various brain regions as well as synaptic plasticity and higher brain functions such as learning and memory. Studies on multiple-knockout mice of Shc-family molecules may further clarify possible involvements of Shc family in physiological and pathophysiological functions in brain.

The biochemistry, signalling and disease relevance of RYK and other WNT-binding receptor tyrosine kinases

The receptor tyrosine kinases (RTKs) are a well-characterized family of growth factor receptors that have central roles in human disease and are frequently therapeutically targeted. The RYK, ROR, PTK7 and MuSK subfamilies make up an understudied subset of WNT-binding RTKs. Numerous developmental, stem cell and pathological roles of WNTs, in particular WNT5A, involve signalling via these WNT receptors. The WNT-binding RTKs have highly context-dependent signalling outputs and stimulate the β-catenin-dependent, planar cell polarity and/or WNT/Ca²⁺ pathways. RYK, ROR and PTK7 members have a pseudokinase domain in their intracellular regions. Alternative signalling mechanisms, including proteolytic cleavage and protein scaffolding functions, have been identified for these receptors. This review explores the structure, signalling, physiological and pathological roles of RYK, with particular attention paid to cancer and the possibility of therapeutically targeting RYK. The other WNT-binding RTKs are compared with RYK throughout to highlight the similarities and differences within this subset of WNT receptors.

The role of the ShcD and RET interaction in neuroblastoma survival and migration

Preliminary screening data showed that the ShcD adaptor protein associates with the proto-oncogene RET receptor tyrosine kinase. In the present study, we aimed to investigate the molecular interaction between ShcD and RET in human neuroblastoma cells and study the functional impact of this interaction. We were able to show that ShcD immunoprecipitated with RET from SK-N-AS neuroblastoma cell lysates upon GDNF treatment. This result was validated by ShcD-RET co-localization, which was visualized using a fluorescence microscope. ShcD-RET coexpression promoted ShcD and RET endosomal localization, resulting in unexpected inhibition of the downstream ERK and AKT pathways. Interestingly, ShcD-RET association reduced the viability and migration of SK-N-AS cells. Although ShcD was previously shown to trigger melanoma cell migration and tumorigenesis, our data showed an opposite role for ShcD in neuroblastoma SK-N-AS cells via its association with RET in GDNF-treated cells. In conclusion, ShcD acts as a switch molecule that promotes contrasting biological responses depending on the stimulus ad cell type.

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The melanoma associated Shc adaptor, ShcD, is found to interact with Ret oncogene receptor in SK-N-AS neuroblastoma cells.

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ShcD and Ret coexpression favoures their endosomal localization.

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ShcD-Ret association has suppressed ERK and AKT signalling.

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The functional consequence of ShcD and Ret interaction was shown to negatively affect cell survival and cellular migration in.

Protein Tyrosine Phosphatase Receptor Type G (PTPRG) Controls Fibroblast Growth Factor Receptor (FGFR) 1 Activity and Influences Sensitivity to FGFR Kinase Inhibitors

Recently, FGFR1 was found to be overexpressed in osteosarcoma and represents an important target for precision medicine. However, because targeted cancer therapy based on FGFR inhibitors has so far been less efficient than expected, a detailed understanding of the target is important. We have here applied proximity-dependent biotin labeling combined with label-free quantitative mass spectrometry to identify determinants of FGFR1 activity in an osteosarcoma cell line. Many known FGFR interactors were identified (e.g. FRS2, PLCG1, RSK2, SRC), but the data also suggested novel determinants. A strong hit in our screen was the tyrosine phosphatase PTPRG. We show that PTPRG and FGFR1 interact and colocalize at the plasma membrane where PTPRG directly dephosphorylates activated FGFR1. We further show that osteosarcoma cell lines depleted for PTPRG display increased FGFR activity and are hypersensitive to stimulation by FGF1. In addition, PTPRG depletion elevated cell growth and negatively affected the efficacy of FGFR kinase inhibitors. Thus, PTPRG may have future clinical relevance by being a predictor of outcome after FGFR inhibitor treatment.

Agrin-LRP4-MuSK signaling as a therapeutic target for myasthenia gravis and other neuromuscular disorders

INTRODUCTION

Signal transduction at the neuromuscular junction (NMJ) is compromised in a diverse array of diseases including myasthenia gravis, Lambert-Eaton myasthenic syndrome, Isaacs' syndrome, congenital myasthenic syndromes, Fukuyama-type congenital muscular dystrophy, amyotrophic lateral sclerosis, and sarcopenia. Except for sarcopenia, all are orphan diseases. In addition, the NMJ signal transduction is impaired by tetanus, botulinum, curare, α-bungarotoxin, conotoxins, organophosphate, sarin, VX, and soman to name a few. Areas covered: This review covers the agrin-LRP4-MuSK signaling pathway, which drives clustering of acetylcholine receptors (AChRs) and ensures efficient signal transduction at the NMJ. We also address diseases caused by autoantibodies against the NMJ molecules and by germline mutations in genes encoding the NMJ molecules. Expert opinion: Representative small compounds to treat the defective NMJ signal transduction are cholinesterase inhibitors, which exert their effects by increasing the amount of acetylcholine at the synaptic space. Another possible therapeutic strategy to enhance the NMJ signal transduction is to increase the number of AChRs, but no currently available drug has this functionality.

The ShcD phosphotyrosine adaptor subverts canonical EGF receptor trafficking

Shc family signalling adaptors connect activated transmembrane receptors to proximal effectors, and most also contain a sequence involved in clathrin-mediated receptor endocytosis. Notably, this AP2 adaptin-binding motif (AD) is absent from the ShcD (also known as Shc4) homolog, which also uniquely promotes ligand-independent phosphorylation of the epidermal growth factor receptor (EGFR). We now report that cultured cells expressing ShcD exhibit reduced EGF uptake, commensurate with a decrease in EGFR surface presentation. Under basal conditions, ShcD colocalises with the EGFR and facilitates its phosphorylation, ubiquitylation and accumulation in juxtanuclear vesicles identified as Rab11-positive endocytic recycling compartments. Accordingly, ShcD also functions as a constitutive binding partner for the E3 ubiquitin ligase Cbl. EGFR phosphorylation and focal accumulation likewise occur upon ShcD co-expression in U87 glioma cells. Loss of ShcD phosphotyrosine-binding function or insertion of the ShcA AD sequence each restore ligand acquisition through distinct mechanisms. The AD region also contains a nuclear export signal, indicating its multifunctionality. Overall, ShcD appears to possess several molecular permutations that actively govern the EGFR, which may have implications in development and disease.

Insights into the Shc Family of Adaptor Proteins

The Shc family of adaptor proteins is a group of proteins that lacks intrinsic enzymatic activity. Instead, Shc proteins possess various domains that allow them to recruit different signalling molecules. Shc proteins help to transduce an extracellular signal into an intracellular signal, which is then translated into a biological response. The Shc family of adaptor proteins share the same structural topography, CH2-PTB-CH1-SH2, which is more than an isoform of Shc family proteins; this structure, which includes multiple domains, allows for the posttranslational modification of Shc proteins and increases the functional diversity of Shc proteins. The deregulation of Shc proteins has been linked to different disease conditions, including cancer and Alzheimer’s, which indicates their key roles in cellular functions. Accordingly, a question might arise as to whether Shc proteins could be targeted therapeutically to correct their disturbance. To answer this question, thorough knowledge must be acquired; herein, we aim to shed light on the Shc family of adaptor proteins to understand their intracellular role in normal and disease states, which later might be applied to connote mechanisms to reverse the disease state.

Signaling adaptor ShcD suppresses extracellular signal-regulated kinase (Erk) phosphorylation distal to the Ret and Trk neurotrophic receptors

Proteins of the Src homology and collagen (Shc) family are typically involved in signal transduction events involving Ras/MAPK and PI3K/Akt pathways. In the nervous system, they function proximal to the neurotrophic factors that regulate cell survival, differentiation, and neuron-specific characteristics. The least characterized homolog, ShcD, is robustly expressed in the developing and mature nervous system, but its contributions to neural cell circuitry are largely uncharted. We now report that ShcD binds to active Ret, TrkA, and TrkB neurotrophic factor receptors predominantly via its phosphotyrosine-binding (PTB) domain. However, in contrast to the conventional Shc adaptors, ShcD suppresses distal phosphorylation of the Erk MAPK. Accordingly, genetic knock-out of mouse ShcD enhances Erk phosphorylation in the brain. In cultured cells, this capacity is tightly aligned to phosphorylation of ShcD CH1 region tyrosine motifs, which serve as docking platforms for signal transducers, such as Grb2. Erk suppression is relieved through independent mutagenesis of the PTB domain and the CH1 tyrosine residues, and successive substitution of these tyrosines breaks the interaction between ShcD and Grb2, thereby promoting TrkB-Grb2 association. Erk phosphorylation can also be restored in the presence of wild type ShcD through Grb2 overexpression. Conversely, mutation of the ShcD SH2 domain results in enhanced repression of Erk. Although the SH2 domain is a less common binding interface in Shc proteins, we demonstrate that it associates with the Ptpn11 (Shp2) phosphatase, which in turn regulates ShcD tyrosine phosphorylation. We therefore propose a model whereby ShcD competes with neurotrophic receptors for Grb2 binding and opposes activation of the MAPK cascade.

MuSK Kinase Activity is Modulated By A Serine Phosphorylation Site in The Kinase Loop

The neuromuscular junction (NMJ) forms when a motor neuron contacts a muscle fibre. A reciprocal exchange of signals initiates a cascade of signalling events that result in pre- and postsynaptic differentiation. At the centre of these signalling events stands muscle specific kinase (MuSK). MuSK activation, kinase activity and subsequent downstream signalling are crucial for NMJ formation as well as maintenance. Therefore MuSK kinase activity is tightly regulated to ensure proper NMJ development. We have identified a novel serine phosphorylation site at position 751 in MuSK that is increasingly phosphorylated upon agrin stimulation. S751 is also phosphorylated in muscle tissue and its phosphorylation depends on MuSK kinase activity. A phosphomimetic mutant of S751 increases MuSK kinase activity in response to non-saturating agrin concentrations . In addition, basal MuSK and AChR phosphorylation as well as AChR cluster size are increased. We believe that the phosphorylation of S751 provides a novel mechanism to relief the autoinhibition of the MuSK activation loop. Such a lower autoinhibition could foster or stabilize MuSK kinase activation, especially during stages when no or low level of agrin are present. Phosphorylation of S751 might therefore represent a novel mechanism to modulate MuSK kinase activity during prepatterning or NMJ maintenance.

Non-canonical dynamic mechanisms of interaction between the p66Shc protein and Met receptor

The present study identifies a novel and unexpected mechanism underscoring the diversification of p66Shc among other Shc (Src homology and collagen homology) proteins, with respect to its mode of interaction with the receptor Met and impacts on key binding effectors of Met-regulated signalling.

Met receptor tyrosine kinase (RTK) is known to bind to the three distinct protein isoforms encoded by the ShcA (Shc) gene. Structure–function studies have unveiled critical roles for p52Shc-dependent signalling pathways in Met-regulated biological functions. The molecular basis of the interaction between the Met and p52Shc proteins is well-defined, but not for the longest protein isoform, p66Shc. In the present study, co-immunoprecipitation assays were performed in human embryonic kidney 293 (HEK293) cells, transiently co-transfected with Met and p66Shc mutants, in order to define the molecular determinants involved in mediating Met–p66Shc interaction. Our results show that p66Shc interacts constitutively with the receptor Met, and the Grb2 (growth factor receptor-bound protein-2) and Gab1 (Grb2-associated binder-1) adaptor proteins. Although its phosphotyrosine-binding domain (PTB) and Src homology 2 (SH2) domains co-ordinate p66Shc binding to non-activated Met receptor, these phosphotyrosine-binding modules, and its collagen homology domain 2 (CH2) region, exert negative constraints. In contrast, p66Shc interaction with the activated Met depends mainly on the integrity of its PTB domain, and to a lesser extent of its SH2 domain. Even though not required for the recruitment of p66Shc, tyrosine phosphorylation of p66Shc by activated Met enhances these interactions by mechanisms not reliant on the integrity of the Met multisubstrate-binding site. In turn, this increases phosphotyrosine-dependent p66Shc–Grb2–Gab1 complex formation away from the receptor, while blocking Grb2 and Gab1 recruitment to activated Met. In conclusion, we identify, for the first time, a novel non-canonical dynamic mode of interaction between Met and the p66 protein isoform of Shc and its effects on rewiring binding effector complexes according to the activation state of the receptor.

Biologically active substances-enriched diet regulates gonadotrope cell activation pathway in liver of adult and old rats

According to the Hippocrates’ theorem “Let food be your medicine and medicine be your food”, dietary interventions may induce changes in the metabolic and inflammatory state by modulating the expression of important genes involved in the chronic disorders. The aim of the present study was to evaluate the influence of long-term (14 months) use of biologically active substances-enriched diet (BASE-diet) on transcriptomic profile of rats’ liver. The experiment was conducted on 36 Sprague–Dawley rats divided into two experimental groups (fed with control or BASE-diet, both n = 18). Control diet was a semi-synthetic diet formulated according to the nutritional requirements for laboratory animals. The BASE-diet was enriched with a mixture of polyphenolic compounds, β-carotene, probiotics, and n-3 and n-6 polyunsaturated fatty acids. In total, n = 3,017 differentially expressed (DE) genes were identified, including n = 218 DE genes between control and BASE groups after 3 months of feeding and n = 1,262 after 14 months. BASE-diet influenced the expression of genes involved particularly in the gonadotrope cell activation pathway and guanylate cyclase pathway, as well as in mast cell activation, gap junction regulation, melanogenesis and apoptosis. Especially genes involved in regulation of GnRH were strongly affected by BASE-diet. This effect was stronger with the age of animals and the length of diet use. It may suggest a link between the diet, reproductive system function and aging.

Cell density modulates SHC3 expression and survival of human glioblastoma cells through Fak activation

Shc3 protein levels are high in human glioblastoma but they decrease in vitro. We found that SHC3 mRNA and protein increased when glioblastoma cells grew as multicellular tumor spheroid (MTS). Shc3 expression was also induced in adherent cultures by increasing cell density. Among the Shc family members, only Shc2 and Shc3 increased with cell density. Shc3 and focal adhesion kinase (Fak) interact as shown by co-immunoprecipitation. Inhibition of Fak activation reduced Shc3 increase and MTS formation and changed Shc3 phosphorylation pattern. Our results suggest that in gliomas cell density modulates Shc3 protein levels and its activity, at least in part, through Fak activation.

The ShcD signaling adaptor facilitates ligand-independent phosphorylation of the EGF receptor

Proto-oncogenic Src homology and collagen (Shc) proteins have been considered archetypal adaptors of epidermal growth factor receptor (EGFR)-mediated signaling. We report that in addition to its role as an EGFR-binding partner and Grb2 platform, ShcD acts noncanonically to promote phosphorylation of select EGFR residues. Unexpectedly, Y1068, Y1148, and Y1173 are subject to ShcD-induced, cell-autonomous hyperphosphorylation in the absence of external stimuli. This response is not elicited by other Shc proteins and requires the intrinsic EGFR kinase, as well as the ShcD phosphotyrosine-binding (PTB) domain. Assessments of Erk, Akt, phospholipase C 1γ, and FAK pathways reveal no apparent distal signaling targets of ShcD. Nevertheless, the capacity of cultured cells to repopulate a wounded monolayer is markedly accelerated by ShcD in an EGFR kinase-dependent manner. Furthermore, detection of overexpressed ShcD coincident with EGFR phosphorylation in human gliomas suggests a clinical application for these findings. We thus demonstrate unique and relevant synergy between ShcD and EGFR that is unprecedented among signaling adaptors.

The adaptor proteins p66Shc and Grb2 regulate the activation of the GTPases ARF1 and ARF6 in invasive breast cancer cells

Signals downstream of growth factor receptors play an important role in mammary carcinogenesis. Recently, we demonstrated that the small GTPases ARF1 and ARF6 were shown to be activated downstream of the epidermal growth factor receptor (EGFR) and act as a key regulator of growth, migration, and invasion of breast cancer cells. However, the mechanism via which the EGFR recruits and activates ARF1 and ARF6 to transmit signals has yet to be fully elucidated. Here, we identify adaptor proteins Grb2 and p66Shc as important regulators mediating ARF activation. We demonstrate that ARF1 can be found in complex with Grb2 and p66Shc upon EGF stimulation of the basal-like breast cancer MDA-MB-231 cell line. However, we report that these two adaptors regulate ARF1 activation differently, with Grb2 promoting ARF1 activation and p66Shc blocking this response. Furthermore, we show that Grb2 is essential for the recruitment of ARF1 to the EGFR, whereas p66Shc hindered ARF1 receptor recruitment. We demonstrate that the negative regulatory role of p66Shc stemmed from its ability to block the recruitment of Grb2/ARF1 to the EGFR. Conversely, p66Shc potentiates ARF6 activation as well as the recruitment of this ARF isoform to the EGFR. Interestingly, we demonstrate that Grb2 is also required for the activation and receptor recruitment of ARF6. Additionally, we show an important role for p66Shc in modulating ARF activation, cell growth, and migration in HER2-positive breast cancer cells. Together, our results highlight a central role for adaptor proteins p66Shc and Grb2 in the regulation of ARF1 and ARF6 activation in invasive breast cancer cells.

Molecular networks in melanoma invasion and metastasis

Metastatic melanoma accounts for approximately 80% of skin cancer-related deaths. Up to now there has been no effective treatment for stage IV melanoma patients due to the complexity and dissemination potential of this disease. Melanomas are heterogeneous tumors in which conventional therapies fail to improve overall survival. Targeted therapies are being developed, but the final outcome can be hampered by the incomplete knowledge of the process of melanoma progression. Even if the intracellular pathways are similar, the interaction of the cells with the surrounding environment should be taken into consideration. This article seeks to highlight some of the advances in the understanding of the molecular mechanisms underlying melanoma dissemination.

A nuclear export signal and oxidative stress regulate ShcD subcellular localisation: a potential role for ShcD in the nucleus

Tumour cells alter their gene expression profile to acquire a more invasive and resistant phenotype. Overexpression of the signalling adaptor protein ShcD in melanoma was found to be a prerequisite for melanoma migration and invasion. In common with other Shc proteins, ShcD has been shown to be involved in coupling receptor tyrosine kinases to the Ras-mitogen activated protein kinase signalling pathway, and to have a predominant cytoplasmic distribution. Here we report that ShcD can exist within the nucleus, and show that its CH2 domain has a critical role in nuclear export of ShcD. Analysis of GFP-tagged ShcD mutants containing deletions or amino acid substitutions within the CH2 domain revealed (83)LCTLIPRM(90) as a functional nuclear export signal. We have further demonstrated that ShcD accumulates in the nucleus upon hydrogen peroxide treatment in FLAG-ShcD expressing HEK293 cells, as well as 518.A2 melanoma cells. Cross linking experiments showed that a proportion of ShcD is associated with DNA. Moreover we have shown that ShcD fused to the GAL4 DNA binding domain can drive transcription of a GAL4 site-driven luciferase reporter, suggesting a role for ShcD in regulating gene transcription. We suggest that ShcD nuclear translocation might provide melanoma cells with a mechanism that enables them to resist DNA damage due to oxidative stress.

Cellular heterogeneity during embryonic stem cell differentiation to epiblast stem cells is revealed by the ShcD/RaLP adaptor protein

The Shc family of adaptor proteins are crucial mediators of a plethora of receptors such as the tyrosine kinase receptors, cytokine receptors, and integrins that drive signaling pathways governing proliferation, differentiation, and migration. Here, we report the role of the newly identified family member, ShcD/RaLP, whose expression in vitro and in vivo suggests a function in embryonic stem cell (ESC) to epiblast stem cells (EpiSCs) transition. The transition from the naïve (ESC) to the primed (EpiSC) pluripotent state is the initial important step for ESCs to commit to differentiation and the mechanisms underlying this process are still largely unknown. Using a novel approach to simultaneously assess pluripotency, apoptosis, and proliferation by multiparameter flow cytometry, we show that ESC to EpiSC transition is a process involving a tight coordination between the modulation of the Oct4 expression, cell cycle progression, and cell death. We also describe, by high-content immunofluorescence analysis and time-lapse microscopy, the emergence of cells expressing caudal-related homeobox 2 (Cdx2) transcription factor during ESC to EpiSC transition. The use of the ShcD knockout ESCs allowed the unmasking of this process as they presented deregulated Oct4 modulation and an enrichment in Oct4-negative Cdx2-positive cells with increased MAPK/extracellular-regulated kinases 1/2 activation, within the differentiating population. Collectively, our data reveal ShcD as an important modulator in the switch of key pathway(s) involved in determining EpiSC identity.

Teaching an old dogma new tricks: twenty years of Shc adaptor signalling

Shc (Src homology and collagen homology) proteins are considered prototypical signalling adaptors in mammalian cells. Consisting of four unique members, ShcA, B, C and D, and multiple splice isoforms, the family is represented in nearly every cell type in the body, where it engages in an array of fundamental processes to transduce environmental stimuli. Two decades of investigation have begun to illuminate the mechanisms of the flagship ShcA protein, whereas much remains to be learned about the newest discovery, ShcD. It is clear, however, that the distinctive modular architecture of Shc proteins, their promiscuous phosphotyrosine-based interactions with a multitude of membrane receptors, involvement in central cascades including MAPK (mitogen-activated protein kinase) and Akt, and unconventional contributions to oxidative stress and apoptosis all require intricate regulation, and underlie diverse physiological function. From early cardiovascular development and neuronal differentiation to lifespan determination and tumorigenesis, Shc adaptors have proven to be more ubiquitous, versatile and dynamic than their structures alone suggest.

In the absence of cellular poly (A) binding protein, the glycolytic enzyme GAPDH translocated to the cell nucleus and activated the GAPDH mediated apoptotic pathway by enhancing acetylation and serine 46 phosphorylation of p53

The cytoplasmic poly (A) binding protein (PABP) interacts with 3' poly (A) tract of eukaryotic mRNA and is important for both translation and stability of mRNA. Previously, we have shown that depletion of PABP by siRNA prevents protein synthesis and consequently leads to cell death through apoptosis. In the present investigation, we studied the mechanism of cell apoptosis. We show that in the absence of PABP, the glycolytic enzyme GAPDH translocated to the cell nucleus and activated the GAPDH mediated apoptotic pathway by enhancing acetylation and serine 46 phosphorylation of p53. As a result, p53 translocated to the mitochondria to initiate Bax mediated apoptosis.

ShcD interacts with TrkB via its PTB and SH2 domains and regulates BDNF-induced MAPK activation

Neurotrophins regulate many aspects of neuronal function through activation of the high affinity Trk receptors. Shc family proteins are implicated in the coupling of RTK to the Ras/mitogen-activated protein kinase signaling cascade. Here we report that the fourth Shc family member, ShcD, associates with TrkB receptor and regulates BDNF-induced MAPK activation. Yeast two-hybrid assay and Co-IP experiments demonstrate ShcD interacts with TrkB in a kinase-activity-dependent manner. Confocal analysis shows ShcD cololizes well with TrkB in transfected 293T cells. Subsequent mapping experiments and mutational analysis indicate that both PTB and SH2 domains are capable of binding to TrkB and PTB domain binds to TrkB NPQY motif. Furthermore, ShcD is involved in BDNF-induced MAPK activation. In summary, we demonstrate that ShcD is a substrate of TrkB and mediates TrkB downstream signaling pathway.

Receptor tyrosine kinase signaling favors a protumorigenic state in breast cancer cells by inhibiting the adaptive immune response

Using transgenic mouse models of breast cancer that ablate Src homology and collagen A (ShcA) expression or oncogene-coupled ShcA signaling, we previously showed that this adaptor is critical for mammary tumor onset and progression. We now provide the first evidence that ShcA regulates mammary tumorigenesis, in part, through its ability to regulate the adaptive immune response. Inactivation of ShcA signaling within tumor cells results in extensive CD4(+) T-cell infiltration and induction of a humoral immune response in mammary tumors. This is associated with a robust CTL response in preneoplastic lesions that are deficient in ShcA signaling. Moreover, mammary tumor progression of ShcA-deficient hyperplasias is accelerated in a T cell-deficient background. We also uncover a clinically relevant correlation between high ShcA expression and low CTL infiltration in human breast cancers. Finally, we define a novel ShcA-regulated immune signature that functions as an independent prognostic marker of survival in human epidermal growth factor receptor 2(+) and basal breast cancers. We reveal a novel role for tumor cell-derived ShcA in the establishment and maintenance of an immunosuppressive state.

The cytoplasmic adaptor protein Dok7 activates the receptor tyrosine kinase MuSK via dimerization

Formation of the vertebrate neuromuscular junction requires, among others proteins, Agrin, a neuronally derived ligand, and the following muscle proteins: LRP4, the receptor for Agrin; MuSK, a receptor tyrosine kinase (RTK); and Dok7 (or Dok-7), a cytoplasmic adaptor protein. Dok7 comprises a pleckstrin-homology (PH) domain, a phosphotyrosine-binding (PTB) domain, and C-terminal sites of tyrosine phosphorylation. Unique among adaptor proteins recruited to RTKs, Dok7 is not only a substrate of MuSK, but also an activator of MuSK's kinase activity. Here, we present the crystal structure of the Dok7 PH-PTB domains in complex with a phosphopeptide representing the Dok7-binding site on MuSK. The structure and biochemical data reveal a dimeric arrangement of Dok7 PH-PTB that facilitates trans-autophosphorylation of the kinase activation loop. The structure provides the molecular basis for MuSK activation by Dok7 and for rationalizing several Dok7 loss-of-function mutations found in patients with congenital myasthenic syndromes.

Positive and negative regulation of antigen receptor signaling by the Shc family of protein adapters

The Shc adapter family includes four members that are expressed as multiple isoforms and participate in signaling by a variety of cell-surface receptors. The biological relevance of Shc proteins as well as their variegated function, which relies on their highly conserved modular structure, is underscored by the distinct and dramatic phenotypic alterations resulting from deletion of individual Shc isoforms both in the mouse and in two model organisms, Drosophila melanogaster and Caenorhabditis elegans. The p52 isoform of ShcA couples antigen and cytokine receptors to Ras activation in both lymphoid and myeloid cells. However, the recognition of the spectrum of activities of p52ShcA in the immune system has been steadily expanding in recent years to other fundamental processes both at the cell and organism levels. Two other Shc family members, p66ShcA and p52ShcC/Rai, have been identified recently in T and B lymphocytes, where they antagonize survival and attenuate antigen receptor signaling. These developments reveal an unexpected and complex interplay of multiple Shc proteins in lymphocytes.

To build a synapse: signaling pathways in neuromuscular junction assembly

Synapses, as fundamental units of the neural circuitry, enable complex behaviors. The neuromuscular junction (NMJ) is a synapse type that forms between motoneurons and skeletal muscle fibers and that exhibits a high degree of subcellular specialization. Aided by genetic techniques and suitable animal models, studies in the past decade have brought significant progress in identifying NMJ components and assembly mechanisms. This review highlights recent advances in the study of NMJ development, focusing on signaling pathways that are activated by diffusible cues, which shed light on synaptogenesis in the brain and contribute to a better understanding of muscular dystrophy.

Linkage and association studies of joint morbidity from rheumatoid arthritis

OBJECTIVE

To investigate the relationship between genetic variations of rheumatoid arthritis (RA) susceptibility in terms of joint morbidity.

METHODS

We used data from Genetic Analysis Workshop 15. The Illumina linkage panel IV included 5858 single-nucleotide polymorphisms (SNP), with 5744 SNP passing quality control filters. The phenotypic variables analyzed were the level of rheumatoid factor (RF) and score on the Joint Alignment and Motion (JAM) scale. We modified the scale, dividing by RF values relevant to disease severity. Linkage analysis for affected sibling pairs was done using the MERLIN program, and family-based association tests were carried out using PLINK and FBAT software.

RESULTS

We found a high peak (LOD = 3.29; NPL Z = 4.07) near the HLA-DRB1 region on chromosome 6. The linkage at 6p24 at rs1410766 [LOD = 2.66; nonparametric linkage (NPL) Z = 3.23] was statistically significant. Two other regions also showed possible linkage peaks: chromosome 7q30 at rs322812 (LOD = 2.47; NPL Z = 3.39) and chromosome 15p34 at rs347117 (LOD = 1.95; NPL Z = 2.80). For the family-based association study, 7 SNP related to clinical RA severity were detected.

CONCLUSION

Genetic variations may lead to an enhanced risk of joint damage and increased levels of RF. Further studies are needed to elucidate the roles of other genes involved in RA and to explore whether the clinical signs of RA are associated with particular genetic variations.

Rai acts as a negative regulator of autoimmunity by inhibiting antigen receptor signaling and lymphocyte activation

Rai (ShcC) belongs to the family of Shc adaptor proteins and is expressed in neuronal cells, where it acts as a survival factor activating the PI3K/Akt survival pathway. In vivo, Rai protects the brain from ischemic damage. In this study, we show that Rai is expressed in T and B lymphocytes. Based on the finding that Rai(-/-) mice consistently develop splenomegaly, the role of Rai in lymphocyte homeostasis and proliferation was addressed. Surprisingly, as opposed to neurons, Rai was found to impair lymphocyte survival. Furthermore, Rai deficiency results in a reduction in the frequency of peripheral T cells with a concomitant increase in the frequency of B cells. Rai(-/-) lymphocytes display enhanced proliferative responses to Ag receptor engagement in vitro, which correlates with enhanced signaling by the TCR and BCR, and more robust responses to allergen sensitization in vivo. A high proportion of Rai(-/-) mice develop a lupus-like autoimmune syndrome characterized by splenomegaly, spontaneous peripheral T and B cell activation, autoantibody production, and deposition of immune complexes in the kidney glomeruli, resulting in autoimmune glomerulonephritis. The data identify Rai as a negative regulator of lymphocyte survival and activation and show that loss of this protein results in breaking of immunological tolerance and development of systemic autoimmunity.

Agrin in the nervous system: synaptogenesis and beyond

The heparan sulfate proteoglycan agrin is best known for its essential role during formation, maintenance and regeneration of the neuromuscular junction. Mutations in agrin-interacting proteins are the genetic basis for a number of neuromuscular disorders. However, agrin is widely expressed in many tissues including neurons and glial cells of the brain, where its precise function is much less understood. Fewer synapses develop in brains that lack agrin, consistent with a function of agrin during CNS synaptogenesis. Recently, a specific transmembrane form of agrin (TM-agrin) was identified that is concentrated at that interneuronal synapses in the brain. Clustering or overexpression of TM-agrin leads to the formation of filopodia-like processes, which might be precursors for CNS synapses. Agrin is subject to defined and activity-dependent proteolytic cleavage by neurotrypsin at synapses and dysregulation of agrin processing might contribute to the development of mental retardation. This review summarizes what is known about the role of agrin during synapse formation at the neuromuscular junction and in the developing CNS and will discuss additional functions of agrin in the adult CNS, in particular during BBB formation, during recovery after traumatic brain injury and in the etiology of diseases, including Alzheimer’s disease and mental retardation.

Dok-7 myasthenia: phenotypic and molecular genetic studies in 16 patients

OBJECTIVE

Detailed analysis of phenotypic and molecular genetic aspects of Dok-7 myasthenia in 16 patients.

METHODS

We assessed our patients by clinical and electromyographic studies, by intercostal muscle biopsies for in vitro microelectrode analysis of neuromuscular transmission and quantitative electron microscopy EM of 409 end plates (EPs), and by mutation analysis, and expression studies of the mutants.

RESULTS

The clinical spectrum varied from mild static limb-girdle weakness to severe generalized progressive disease. The synaptic contacts were single or multiple, and some, but not all, were small. In vitro microelectrode studies indicated variable decreases of the number of released quanta and of the synaptic response to acetylcholine; acetylcholine receptor (AChR) channel kinetics were normal. EM analysis demonstrated widespread and previously unrecognized destruction and remodeling of the EPs. Each patient carries 2 or more heteroallelic mutations: 11 in genomic DNA, 7 of which are novel; and 6 identifiable only in complementary DNA or cloned complementary DNA, 3 of which are novel. The pathogenicity of the mutations was confirmed by expression studies. Although the functions of Dok-7 include AChR beta-subunit phosphorylation and maintaining AChR site density, patient EPs showed normal AChR beta-subunit phosphorylation, and the AChR density on the remaining junctional folds appeared normal.

INTERPRETATION

First, the clinical features of Dok-7 myasthenia are highly variable. Second, some mutations are complex and identifiable only in cloned complementary DNA. Third, Dok-7 is essential for maintaining not only the size but also the structural integrity of the EP. Fourth, the profound structural alterations at the EPs likely contribute importantly to the reduced safety margin of neuromuscular transmission.

Expression of the Rai (Shc C) adaptor protein in the human enteric nervous system

The adaptor protein Rai (ShcC/N-Shc) is almost exclusively present in the nervous system, although little is documented about its expression in the gut and the enteric nervous system (ENS). As Rai is a physiological substrate of Ret, an important factor for the development of ENS, we have evaluated the expression of Rai in the ENS in various segments of the human gastrointestinal tract. The expression of Rai was assessed by immunohistochemistry in disease-free human gut samples (oesophagus, stomach, small bowel and colon) obtained from subjects undergoing surgical procedures. Rai was not expressed in the epithelia or lymphoid tissue, whereas a moderate level of expression was observed in the endothelial cells of blood vessels and on the outer membrane of smooth muscle cells in both the muscularis mucosae and the muscularis propria. In the ENS, strong positivity was observed only in enteric glial cells, overlapping with GFAP and S100. In conclusion, Rai is expressed in the human gut, especially in the enteric glial cells. We conclude that Rai may provide an additional marker for this cell type.

Muscle-specific receptor tyrosine kinase endocytosis in acetylcholine receptor clustering in response to agrin

Agrin, a factor used by motoneurons to direct acetylcholine receptor (AChR) clustering at the neuromuscular junction, initiates signal transduction by activating the muscle-specific receptor tyrosine kinase (MuSK). However, the underlying mechanisms remain poorly defined. Here, we demonstrated that MuSK became rapidly internalized in response to agrin, which appeared to be required for induced AChR clustering. Moreover, we provided evidence for a role of N-ethylmaleimide sensitive factor (NSF) in regulating MuSK endocytosis and subsequent signaling in response to agrin stimulation. NSF interacts directly with MuSK with nanomolar affinity, and treatment of muscle cells with the NSF inhibitor N-ethylmaleimide, mutation of NSF, or suppression of NSF expression all inhibited agrin-induced AChR clustering. Furthermore, suppression of NSF expression and NSF mutation attenuate MuSK downstream signaling. Our study reveals a potentially novel mechanism that regulates agrin/MuSK signaling cascade.