Domain-specific function of ShcC docking protein in neuroblastoma cells

Phosphorylation of RasGRP1 by Shc3 prevents RasGRP1 degradation and contributes to Ras/c-Jun activation in hepatocellular carcinoma

Ras guanine nucleotide-releasing protein 1 (RasGRP1), a Ras activator, is upregulated in hepatocellular carcinoma (HCC) and other kinds of cancer and is associated with the poor prognosis of patients. However, little is known about the underlying regulatory mechanisms of RasGRP1 in the context of cancer. Here, we report that RasGRP1 physically interacted with the adaptor protein Src homolog and collagen homolog 3 (Shc3). Moreover, RasGRP1 C-terminus domain (aa 607-797) bound to the central collagen-homology 1 (CH1) domain of Shc3. Subsequently, Shc3 enhanced the RasGRP1 tyrosine phosphorylation rate and stability by inhibiting its ubiquitination. Notably, the phosphorylation-mimicking mutants of RasGRP1, RasGRP1 Y704A, and Y748A, rescued the phosphorylation and ubiquitination levels of RasGRP1 in HCC cells. Further investigation showed that the RasGRP1 and Shc3 interaction induced activation of Ras and c-Jun, resulting in cell proliferation in vitro. Moreover, the regulation of Shc3/RasGRP1/Ras/c-Jun signal transduction was confirmed in vivo using the subcutaneous xenograft mouse model. Thus, we propose that continuous Shc3 overexpression may be a possible mechanism for maintaining RasGRP1 stability and that persistent activation of Ras/c-Jun signaling through the interaction of RasGRP1 and Shc3 is a key event increasing cell proliferation. Our findings suggest that the interaction of RasGRP1 and Shc3 plays an important role in HCC tumorigenesis and suggests the potential clinical usage of novel biomarkers and therapeutic targets in HCC.

The Long Non-Coding RNA AC006329.1 Facilitates Hepatocellular Carcinoma Progression and Metastasis by Regulating miR-127-5p/SHC3/ERK Axis

Purpose

Hepatocellular carcinoma(HCC) is the most common type of liver cancer and the sixth largest common cancer worldwide. Although surgical resection, hepatic arterial chemoembolization, targeted drugs and immunotherapy are currently available, the mortality of advanced patients remains high. Therefore, new therapeutic targets are urgently needed. In recent years, many studies have found that The long non-coding RNA(lncRNA) has multiple functions in human tumors, including participating in epigenetic, transcriptional, post-transcriptional and translational regulation, and is closely related to the progression of HCC. The purpose of this study was to investigate the role of AC006329.1 in HCC progression and provide theoretical guidance for finding new targets.

Patients and Methods

AC006329.1 was screened out by transcriptome sequencing and quantitative real-time polymerase chain reaction (qRT-PCR). Then a series of functional tests in vivo and in vitro were conducted to investigate the effects of AC006329.1 on HCC progression and metastasis. Epithelial-mesenchymal transformation (EMT) of HCC was detected by Western blot and immunofluorescence staining. The targeted miRNA and downstream gene of AC006329.1 were predicted by databases and the pathway regulation axis eventually validated by dual luciferase reporter assays, qRT-PCR and WB.

Results

AC006329.1 was found high expressed in HCC tissues and cell lines by qRT-PCR. The prognosis of HCC patients with high expressed AC006329.1 was poor. In vitro and in vivo, overexpression of AC006329.1 can promote the progression, metastasis and EMT of HCC by acting as a sponge of miR-127-5p to increase the expression of SHC3. In addition, up-regulation of miR-127-5p or knockdown of SHC3 can both reverse the promoting effects of AC006329.1 on progression, metastasis and EMT of HCC. Finally, WB and qRT-PCR analysis was discovered that AC006329.1 can facilitate HCC progression, EMT and metastasis by competitively inhibiting miR-127-5p to activate SHC3/ERK signaling pathway.

Conclusion

These above experimental results confirmed that AC006329.1 can facilitate HCC progression, EMT and metastasis by acting as a competing endogenous RNA (ceRNA) to inhibit miR-127-5p and activate SHC3/ERK signaling pathway.

Shc3 promotes hepatocellular carcinoma stemness and drug resistance by interacting with β-catenin to inhibit its ubiquitin degradation pathway

Hepatocellular carcinoma (HCC) is one of the most common cancers with an insidious onset, strong invasiveness, insensitivity to chemotherapy, and poor prognosis, thus makes clinical treatment challenging. The mechanisms require further elucidation for developing novel therapies and targeting drug resistance. Here, we observed high Shc3 expression in patients with chemoresistant and recurrent HCCs. Shc3 overexpression induced a significant increase in MDR1/P-glycoprotein expression, whereas Shc3 knockdown impaired this expression. Further, Shc3 inhibition significantly restored HCC cell sensitivity to doxorubicin and sorafenib. Mechanistically, Shc3 interacted with β-catenin, inhibited destruction complex stability, promoted β-catenin release, and dampened β-catenin ubiquitination. Shc3 bound β-catenin and facilitated its nuclear translocation, prompting the β-catenin/TCF pathway to elevate MDR1 transcription. β-catenin blockage abolished the discrepancy in drug resistance between Shc3-depleted HCC cells and control cells, which further validating that β-catenin is required for Shc3-mediated liver chemotherapy. We also determined the effect of Shc3 on the sensitivity of HCC to chemotherapy in vivo. Collectively, this study provides a potential strategy to target these pathways concurrently with systemic chemotherapy that can improve the clinical treatment of HCC.

The Shc protein Rai enhances T-cell survival under hypoxia

Hypoxia occurs in physiological and pathological conditions. T cells experience hypoxia in pathological and physiological conditions as well as in lymphoid organs. Indeed, hypoxia-inducible factor 1α (HIF-1α) affects T cell survival and functions. Rai, an Shc family protein member, exerts pro-survival effects in hypoxic neuroblastoma cells. Since Rai is also expressed in T cells, we here investigated its role in hypoxic T cells. In this work, hypoxia differently affected cell survival, proapoptotic, and metabolic programs in T cells, depending upon Rai expression. By using Jurkat cells stably expressing Rai and splenocytes from Rai^-/- mice, we demonstrated that Rai promotes T cell survival and affects cell metabolism under hypoxia. Upon exposure to hypoxia, Jurkat T cells expressing Rai show (a) higher HIF-1α protein levels; (b) a decreased cell death and increased Akt/extracellular-signal-regulated kinase phosphorylation; (c) a decreased expression of proapoptotic markers, including caspase activities and poly(ADP-ribose) polymerase cleavage; (d) an increased glucose and lactate metabolism; (e) an increased activation of nuclear factor-kB pathway. The opposite effects were observed in hypoxic splenocytes from Rai^-/- mice. Thus, Rai plays an important role in hypoxic signaling and may be relevant in the protection of T cells against hypoxia.

Novel Bioluminescent Activatable Reporter for Src Tyrosine Kinase Activity in Living Mice

Aberrant activation of the Src kinase is implicated in the development of a variety of human malignancies. However, it is almost impossible to monitor Src activity in an in vivo setting with current biochemical techniques. To facilitate the noninvasive investigation of the activity of Src kinase both in vitro and in vivo, we developed a genetically engineered, activatable bioluminescent reporter using split-luciferase complementation. The bioluminescence of this reporter can be used as a surrogate for Src activity in real time. This hybrid luciferase reporter was constructed by sandwiching a Src-dependent conformationally responsive unit (SH2 domain-Srcpep) between the split luciferase fragments. The complementation bioluminescence of this reporter was dependent on the Src activity status. In our study, Src kinase activity in cultured cells and tumor xenografts was monitored quantitatively and dynamically in response to clinical small-molecular kinase inhibitors, dasatinib and saracatinib. This system was also applied for high-throughput screening of Src inhibitors against a kinase inhibitor library in living cells. These results provide unique insights into drug development and pharmacokinetics/phoarmocodynamics of therapeutic drugs targeting Src signaling pathway enabling the optimization of drug administration schedules for maximum benefit. Using both Firefly and Renilla luciferase imaging, we have successfully monitored Src tyrosine kinase activity and Akt serine/threonine kinase activity concurrently in one tumor xenograft. This dual luciferase reporter imaging system will be helpful in exploring the complex signaling networks in vivo. The strategies reported here can also be extended to study and image other important kinases and the cross-talks among them.

Cathepsin Inhibition Prevents Autophagic Protein Turnover and Downregulates Insulin Growth Factor-1 Receptor-Mediated Signaling in Neuroblastoma

Inhibition of the major lysosomal proteases, cathepsins B, D, and L, impairs growth of several cell types but leads to apoptosis in neuroblastoma. The goal of this study was to examine the mechanisms by which enzyme inhibition could cause cell death. Cathepsin inhibition caused cellular accumulation of fragments of the insulin growth factor 1 (IGF-1) receptor. The fragments were located in dense organelles that were characterized as autophagosomes. This novel discovery provides the first clear link between lysosomal function, autophagy, and IGF-1- mediated cell proliferation. A more in-depth analysis of the IGF1 signaling pathway revealed that the mitogen-activated protein kinase (MAPK) cell-proliferation pathway was impaired in inhibitor treated cells, whereas the Akt cell survival pathway remained functional. Shc, an adapter protein that transmits IGF-1 signaling through the MAPK pathway, was sequestered in autophagosomes; whereas IRS-2, an adapter protein that transmits IGF-1 signaling through the Akt pathway, was unaffected by cathepsin inhibition. Furthermore, Shc was sequestered in autophagosomes as its active form, indicating that autophagy is a key mechanism for downregulating IGF-1-induced cell proliferation. Cathepsin inhibition had a greater effect on autophagic sequestration of the neuronal specific adapter protein, Shc-C, than ubiquitously expressed Shc-A, providing mechanistic support for the enhanced sensitivity of neuronally derived tumor cells. We also observed impaired activation of MAPK by epidermal growth factor treatment in inhibitor-treated cells. The Shc adapter proteins are central to transducing proliferation signaling by a range of receptor tyrosine kinases; consequently, cathepsin inhibition may become an important therapeutic approach for treating neuroblastoma and other tumors of neuronal origin.

ShcC proteins: brain aging and beyond

To date, most studies of Shc family of signaling adaptor proteins have been focused on the near-ubiquitously expressed ShcA, indicating its relevance to age-related diseases and longevity. Although the role of the neuronal ShcC protein is much less investigated, accumulated evidence suggests its importance for neuroprotection against such aging-associated conditions as brain ischemia and oxidative stress. Here, we summarize more than decade of studies on the ShcC expression and function in normal brain, age-related brain pathologies and immune disorders with a focus on the interactions of ShcC with signaling proteins/pathways, and the possible implications of these interactions for changes associated with aging.

Mechanistic insight into ALK receptor tyrosine kinase in human cancer biology

The burgeoning field of anaplastic lymphoma kinase (ALK) in cancer encompasses many cancer types, from very rare cancers to the more prevalent non-small-cell lung cancer (NSCLC). The common activation of ALK has led to the use of the ALK tyrosine kinase inhibitor (TKI) crizotinib in a range of patient populations and to the rapid development of second-generation drugs targeting ALK. In this Review, we discuss our current understanding of ALK function in human cancer and the implications for tumour treatment.

miR-99b-targeted mTOR induction contributes to irradiation resistance in pancreatic cancer

Background

Radiation exerts direct antitumor effects and is widely used in clinics, but the efficacy is severely compromised by tumor resistance. Therefore uncovering the mechanism of radioresistance might promote the development of new strategies to overcome radioresistance by manipulating activity of the key molecules.

Methods

Immunohistochemistry were used to find whether mTOR were over-activated in radioresistant patients’ biopsies. Then Western blot, real-time PCR and transfection were used to find whether radiotherapy regulates the expression and activity of mTOR by modulating its targeting microRNA in human pancreatic cancer cell lines PANC-1, Capan-2 and BxPC-3. Finally efficacy of radiation combined with mTOR dual inhibitor AZD8055 was assessed in vitro and in vivo.

Results

Ionizing radiation promoted mTOR expression and activation in pancreatic cancer cells through reducing miR-99b expression, which negatively regulated mTOR. Novel mTOR inhibitor, AZD8055 (10 nM, 100 nM, 500 nM) synergistically promoted radiation (0–10 Gy) induced cell growth inhibition and apoptosis. In human pancreatic cancer xenografts, fractionated radiation combined with AZD8055 treatment further increased the anti-tumor effect, the tumor volume was shrinked to 278 mm³ after combination treatment for 3 weeks compared with single radiation (678 mm³) or AZD8055 (708 mm³) treatment (P < 0.01).

Conclusions

Our data provide a rationale for overcoming radio-resistance by combined with mTOR inhibitor AZD8055 in pancreatic cancer therapy.

Novel adaptor protein Shf interacts with ALK receptor and negatively regulates its downstream signals in neuroblastoma

Our neuroblastoma cDNA project previously identified Src homology 2 domain containing F (Shf) as one of the genes expressed at high levels in favorable neuroblastoma. Shf is an adaptor protein containing four putative tyrosine phosphorylation sites and an SH2 domain. In this study, we found that Shf interacted with anaplastic lymphoma kinase (ALK), an oncogenic receptor tyrosine kinase in neuroblastoma. Real-time PCR analysis showed that Shf mRNA is highly expressed in non-metastatic neuroblastomas compared to metastatic tumor samples (P < 0.030, n = 106). Interestingly, patients showing high ALK and low Shf mRNA expressions showed poor prognosis, whereas low ALK and high Shf expressions were related to better prognosis (P < 0.023, n = 38). Overexpression of ALK and siRNA-mediated knockdown of Shf yielded similar results, such as an increase in cellular growth and phosphorylation of ALK, in addition to Erk1/2 and signal transducer and activator of transcription 3 (STAT3) that are downstream signals of the ALK-initiated phospho-transduction pathway. Knockdown of Shf also increased the cellular mobility and invasive capability of neuroblastoma cells. These results suggest that Shf interacts with ALK and negatively regulates the ALK-initiated signal transduction pathway in neuroblastoma. We thus propose that Shf inhibits phospho-transduction signals mediated by ALK, which is one of the major key players on neuroblastoma development, resulting in better prognosis of the tumor.

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.

Anaplastic lymphoma kinase in human cancer

The receptor tyrosine kinases (RTKs) play a critical role, controlling cell proliferation, survival, and differentiation of normal cells. Their pivotal function has been firmly established in the pathogenesis of many cancers as well. The anaplastic lymphoma kinase (ALK), a transmembrane RTK, originally identified in the nucleophosmin (NPM)-ALK chimera of anaplastic large cell lymphoma, has emerged as a novel tumorigenic player in several human cancers. In this review, we describe the expression of the ALK-RTK, its related fusion proteins, and their molecular mechanisms of activation. Novel tailored strategies are briefly illustrated for the treatment of ALK-positive neoplasms.

EDG3 and SHC3 on chromosome 9q22 are co-amplified in human ependymomas

By qPCR we found that EDG3 and SHC3 were amplified in 60% of ependymomas but none in choroid plexus papillomas. In ependymomas EDG3 and SHC3 amplification increased Shc3 protein levels while EDG3 was less affected. Both proteins were co-immunoprecipitated from ependymoma and Shc3 was tyrosine phosphorylated thus presumably active. We showed by digestion with N-glycosidase-F that EDG3 was glycosylated indicating that EDG3 protein was not retained in the endoplasmic reticulum. The co-immunoprecipitation of Shc3 and EDG3 proteins from ependymomas with amplification of SHC3 and EDG3 genes suggests that the two proteins co-operate and are important for ependymomas in vivo.

RET protein promotes non-adherent growth of NB-39-nu neuroblastoma cell line

The receptor tyrosine kinase RET is expressed in a number of neuroblastoma tissues and cell lines, but its role in neuroblastoma remains to be determined. In this study, we examined the roles of RET protein in neuroblastoma by the RNA interference technique using the NB-39-nu neuroblastoma cell line. NB-39-nu neuroblastoma cells show high expression and elevated tyrosine phosphorylation of RET, although short interfering RNA against RET (RET siRNA) did not significantly inhibit cell proliferation or suppression of basal levels of phosphorylation of extracellular regulated kinase (ERK)1/2 or protein kinase B (AKT). By the addition of glial cell line-derived neurotrophic factor (GDNF), both the expression and phosphorylation of RET and the phosphorylation of ERK1/2 and AKT were further increased, whereas cell proliferation was not stimulated under normal culture conditions. However, proliferation of cells cultured under non-adherent conditions was significantly increased by GDNF. The increased proliferation was suppressed by RET siRNA, which also caused inhibition of the phosphorylation of ERK1/2 and AKT. These results suggest that RET signaling plays an important role in GDNF-induced enhancement of non-adherent proliferation of NB-39-nu cells, which might contribute to the metastasis of neuroblastoma.

Genome-wide analysis of genetic alterations in Barrett's adenocarcinoma using single nucleotide polymorphism arrays

We performed genome-wide analysis of copy-number changes and loss of heterozygosity (LOH) in Barrett's esophageal adenocarcinoma by single nucleotide polymorphism (SNP) microarrays to identify associated genomic alterations. DNA from 27 esophageal adenocarcinomas and 14 matching normal tissues was subjected to SNP microarrays. The data were analyzed using dChipSNP software. Copy-number changes occurring in at least 25% of the cases and LOH occurring in at least 19% were regarded as relevant changes. As a validation, fluorescence in situ hybridization (FISH) of 8q24.21 (CMYC) and 8p23.1 (SOX7) was performed. Previously described genomic alterations in esophageal adenocarcinomas could be confirmed by SNP microarrays, such as amplification on 8q (CMYC, confirmed by FISH) and 20q13 or deletion/LOH on 3p (FHIT) and 9p (CDKN2A). Moreover, frequent gains were detected on 2p23.3, 7q11.22, 13q31.1, 14q32.31, 17q23.2 and 20q13.2 harboring several novel candidate genes. The highest copy numbers were seen on 8p23.1, the location of SOX7, which could be demonstrated to be involved in amplification by FISH. A nuclear overexpression of the transcription factor SOX7 could be detected by immunohistochemistry in two amplified tumors. Copy-number losses were seen on 18q21.32 and 20p11.21, harboring interesting candidate genes, such as CDH20 and CST4. Finally, a novel LOH region could be identified on 6p in at least 19% of the cases. In conclusion, SNP microarrays are a valuable tool to detect DNA copy-number changes and LOH at a high resolution. Using this technique, we identified several novel genes and DNA regions associated with esophageal adenocarcinoma.

Anaplastic lymphoma kinase: role in cancer pathogenesis and small-molecule inhibitor development for therapy

Anaplastic lymphoma kinase (ALK), a receptor tyrosine kinase in the insulin receptor superfamily, was initially identified in constitutively activated oncogenic fusion forms - the most common being nucleophosmin-ALK - in anaplastic large-cell lymphomas, and subsequent studies have identified ALK fusions in diffuse large B-cell lymphomas, systemic histiocytosis, inflammatory myofibroblastic tumors, esophageal squamous cell carcinomas and non-small-cell lung carcinomas. More recently, genomic DNA amplification and protein overexpression, as well as activating point mutations, of ALK have been described in neuroblastomas. In addition to those cancers for which a causative role for aberrant ALK activity is well validated, more circumstantial links implicate the full-length, normal ALK receptor in the genesis of other malignancies - including glioblastoma and breast cancer - via a mechanism of receptor activation involving autocrine and/or paracrine growth loops with the reported ALK ligands, pleiotrophin and midkine. This review summarizes normal ALK biology, the confirmed and putative roles of ALK in the development of human cancers and efforts to target ALK using small-molecule kinase inhibitors.

Distinct role of ShcC docking protein in the differentiation of neuroblastoma

The biological and clinical heterogeneity of neuroblastoma is closely associated with signaling pathways that control cellular characteristics such as proliferation, survival and differentiation. The Shc family of docking proteins is important in these pathways by mediating cellular signaling. In this study, we analysed the expression levels of ShcA and ShcC proteins in 46 neuroblastoma samples and showed that a significantly higher level of ShcC protein is observed in neuroblastomas with poor prognostic factors such as advanced stage and MYCN amplification (P<0.005), whereas the expression level of ShcA showed no significant association with these factors. Using TNB1 cells that express a high level of ShcC protein, it was demonstrated that knockdown of ShcC by RNAi caused elevation in the phosphorylation of ShcA, which resulted in sustained extracellular signal-regulated kinase activation and neurite outgrowth. The neurites induced by ShcC knockdown expressed several markers of neuronal differentiation suggesting that the expression of ShcC potentially has a function in inhibiting the differentiation of neuroblastoma cells. In addition, marked suppression of in vivo tumorigenicity of TNB1 cells in nude mice was observed by stable knockdown of ShcC protein. These findings indicate that ShcC is a therapeutic target that might induce differentiation in the aggressive type of neuroblastomas.

Hyperphosphorylated cortactin in cancer cells plays an inhibitory role in cell motility

Cortactin is frequently overexpressed in cancer cells, and changes of the levels of its tyrosine phosphorylation have been observed in several cancer cells. However, how the expression level and phosphorylation state of cortactin would influence the ultimate cellular function of cancer cells is unknown. In this study, we analyzed the role of cortactin in gastric and breast cancer cell lines using RNA interference technique and found that knockdown of cortactin inhibited cell migration in a subset of gastric cancer cells with a lower level of its tyrosine phosphorylation, whereas it greatly enhanced cell migration and increased tyrosine phosphorylation of p130Cas in other subsets of cells with hyperphosphorylated cortactin. Consistent results were obtained when hyperphosphorylation of cortactin was induced in MCF7 breast cancer cells by expressing Fyn tyrosine kinase. Additionally, immunostaining analysis showed that knockdown of hyperphosphorylated cortactin resulted in the recruitment of p130Cas to focal adhesions. These results suggest that cortactin hyperphosphorylation suppresses cell migration possibly through the inhibition of membrane localization and tyrosine phosphorylation of p130Cas.

AKT pathway in neuroblastoma and its therapeutic implication

Neuroblastoma is a frequent pediatric tumor with a poor outcome in spite of aggressive treatment, even with autologous hematopoietic stem cell transplantation. The overall cure rate of 40% is unsatisfactory and new therapeutic strategies are urgently needed. AKT is a major mediator of survival signals that protect cells from apoptosis and regulate cell proliferation. The AKT signaling network is considered a key determinant of the biological aggressiveness of these tumors. In this article, the authors discuss the relation between activators of AKT in neuroblastoma, in particular, growth factors such as IGF-1, TRK, GDNF, VEGF and EGF, and their effects on tumoral proliferation, differentiation and apoptosis. Numerous other proteins interact with AKT in neuroblastoma. Several are relatively well characterized, such as PTEN and retinoic acid; others are new and potentially interesting, such as PKC and anaplastic lymphoma kinase. Specific inhibition of AKT has been studied, such as with LY249002, with significant effects on cell progression and apoptosis in tumoral cells. Moreover, a series of new drugs, such as geldanamycin and rapamycin, directly modify the expression of AKT in tumoral cells. Few specific inhibitors of AKT are available; less specific inhibitors are probably unsuitable therapeutic options in neuroblastoma. Drugs with a direct or indirect inhibitory effect on the AKT pathway, used alone or in combination with other drugs, seem to hold great promise as a new therapeutic modality in neuroblastoma.

Force sensing by mechanical extension of the Src family kinase substrate p130Cas

How physical force is sensed by cells and transduced into cellular signaling pathways is poorly understood. Previously, we showed that tyrosine phosphorylation of p130Cas (Cas) in a cytoskeletal complex is involved in force-dependent activation of the small GTPase Rap1. Here, we mechanically extended bacterially expressed Cas substrate domain protein (CasSD) in vitro and found a remarkable enhancement of phosphorylation by Src family kinases with no apparent change in kinase activity. Using an antibody that recognized extended CasSD in vitro, we observed Cas extension in intact cells in the peripheral regions of spreading cells, where higher traction forces are expected and where phosphorylated Cas was detected, suggesting that the in vitro extension and phosphorylation of CasSD are relevant to physiological force transduction. Thus, we propose that Cas acts as a primary force sensor, transducing force into mechanical extension and thereby priming phosphorylation and activation of downstream signaling.

The docking protein Cas links tyrosine phosphorylation signaling to elongation of cerebellar granule cell axons

Crk-associated substrate (Cas) is a tyrosine-phosphorylated docking protein that is indispensable for the regulation of the actin cytoskeletal organization and cell migration in fibroblasts. The function of Cas in neurons, however, is poorly understood. Here we report that Cas is dominantly enriched in the brain, especially the cerebellum, of postnatal mice. During cerebellar development, Cas is highly tyrosine phosphorylated and is concentrated in the neurites and growth cones of granule cells. Cas coimmunoprecipitates with Src family protein tyrosine kinases, Crk, and cell adhesion molecules and colocalizes with these proteins in granule cells. The axon extension of granule cells is inhibited by either RNA interference knockdown of Cas or overexpression of the Cas mutant lacking the YDxP motifs, which are tyrosine phosphorylated and thereby interact with Crk. These findings demonstrate that Cas acts as a key scaffold that links the proteins associated with tyrosine phosphorylation signaling pathways to the granule cell axon elongation.