Translocation of CrkL to focal adhesions mediates integrin-induced migration downstream of Src family kinases

The p130Cas-Crk/CrkL Axis: A Therapeutic Target for Invasive Cancers Unveiled by Collaboration Among p130Cas, Crk, and CrkL

Numerous studies have documented the involvement of p130Cas (Crk-associated substrate) in a wide range of cellular processes across different types of cells. These processes encompass cell transformation, the connection between the extracellular matrix and the actin cytoskeleton, cell migration and invasion, and cardiovascular development. Moreover, p130Cas has been associated with the regulation of various physiological processes, including mammary, bone, brain, muscle, and liver homeostasis. The diverse functions of p130Cas can be attributed to its possession of multiple protein-protein interaction domains, which sets it apart as a unique class of adaptor protein. It is well established that p130Cas interacts critically with the CT10 regulator of kinase (Crk) adaptor protein family members, including CrkII, CrkI, and Crk-like (CrkL), which is the basis for the naming of the Cas family. The Crk family proteins play a crucial role in integrating signals from various sources, such as growth factors, extracellular matrix molecules, bacterial pathogens, and apoptotic cells. An increasing body of evidence suggests that the dysregulation of Crk family proteins is linked to various human diseases, including cancer and increased susceptibility to pathogen infections. This review focuses primarily on the structural and functional aspects of the interaction between p130Cas and the Crk family proteins, providing insights into how these proteins regulate specific signaling events. Furthermore, we delve into the functions of p130Cas and the Crk family proteins in both normal and tumor cells to gain a comprehensive understanding of their collaborative roles in cellular physiology and pathology. This review demonstrates that tumor cell migration and invasion are the two cellular functions that have been studied the most for the p130Cas-Crk/CrkL axis. Understanding the tumor cell migration and invasion that require both p130Cas and Crk/CrkL is necessary to further evaluate the role of the p130Cas-Crk/CrkL axis in cancer. Establishing the contribution of the p130Cas-Crk/CrkL axis to cancer will facilitate the development of cancer drugs targeting the axis to inhibit cancer cell dissemination and improve patient outcomes.

Digital spatial profiling of segmental outflow regions in trabecular meshwork reveals a role for ADAM15

In this study we used a spatial transcriptomics approach to identify genes specifically associated with either high or low outflow regions in the trabecular meshwork (TM) that could potentially affect aqueous humor outflow in vivo. High and low outflow regions were identified and isolated from organ cultured human anterior segments perfused with fluorescently-labeled 200 nm FluoSpheres. The NanoString GeoMx Digital Spatial Profiler (DSP) platform was then used to identified genes in the paraffin embedded tissue sections from within those regions. These transcriptome analyses revealed that 16 genes were statistically upregulated in high outflow regions and 57 genes were statistically downregulated in high outflow regions when compared to low outflow regions. Gene ontology enrichment analysis indicated that the top three biological categories of these differentially expressed genes were ECM/cell adhesion, signal transduction, and transcription. The ECM/cell adhesion genes that showed the largest differential expression (Log2FC ±1.5) were ADAM15, BGN, LDB3, and CRKL. ADAM15, which is a metalloproteinase that can bind integrins, was upregulated in high outflow regions, while the proteoglycan BGN and two genes associated with integrin signaling (LDB3, and CRKL) were downregulated. Immunolabeling studies supported the differential expression of ADAM15 and showed that it was specifically upregulated in high outflow regions along the inner wall of Schlemm's canal and in the juxtacanalicular (JCT) region of the TM. In addition to these genes, the studies showed that genes for decorin, a small leucine-rich proteoglycan, and the α8 integrin subunit were enriched in high outflow regions. These studies identify several novel genes that could be involved in segmental outflow, thus demonstrating that digital spatial profiling could be a useful approach for understanding segmental flow through the TM. Furthermore, this study suggests that changes in the expression of genes involved in regulating the activity and/or organization of the ECM and integrins in the TM are likely to be key players in segmental outflow.

Adhesome Receptor Clustering is Accompanied by the Colocalization of the Associated Genes in the Cell Nucleus

Proteins on the cell membrane cluster to respond to extracellular signals; for example, adhesion proteins cluster to enhance extracellular matrix sensing; or T-cell receptors cluster to enhance antigen sensing. Importantly, the maturation of such receptor clusters requires transcriptional control to adapt and reinforce the extracellular signal sensing. However, it has been unclear how such efficient clustering mechanisms are encoded at the level of the genes that code for these receptor proteins. Using the adhesome as an example, we show that genes that code for adhesome receptor proteins are spatially co-localized and co-regulated within the cell nucleus. Towards this, we use Hi-C maps combined with RNA-seq data of adherent cells to map the correspondence between adhesome receptor proteins and their associated genes. Interestingly, we find that the transcription factors that regulate these genes are also co-localized with the adhesome gene loci, thereby potentially facilitating a transcriptional reinforcement of the extracellular matrix sensing machinery. Collectively, our results highlight an important layer of transcriptional control of cellular signal sensing.

Crk and Crkl have shared functions in neural crest cells for cardiac outflow tract septation and vascular smooth muscle differentiation

CRK and CRKL encode cytoplasmic adaptors that contribute to the etiology of congenital heart disease. Neural crest cells (NCCs) are required for cardiac outflow tract (OFT) septation and aortic arch formation. The roles of Crk/Crkl in NCCs during mouse cardiovascular development remains unknown. To test this, we inactivated Crk and/or Crkl in NCCs. We found that the loss of Crk, rather than Crkl, in NCCs resulted in double outlet right ventricle, while loss of both Crk/Crkl in NCCs resulted in severe defects with earlier lethality due to failed OFT septation and severe dilation of the pharyngeal arch arteries (PAAs). We found that these defects are due to altered cell morphology resulting in reduced localization of NCCs to the OFT and failed integrity of the PAAs, along with reduced expression of Integrin signaling genes. Further, molecular studies identified reduced differentiation of vascular smooth muscle cells that may in part be due to altered Notch signaling. Additionally, there is increased cellular stress that leads to modest increase in apoptosis. Overall, this explains the mechanism for the Crk/Crkl phenotype.

Crk and CrkL as Therapeutic Targets for Cancer Treatment

Crk and CrkL are cellular counterparts of the viral oncoprotein v-Crk. Crk and CrkL are overexpressed in many types of human cancer, correlating with poor prognosis. Furthermore, gene knockdown and knockout of Crk and CrkL in tumor cell lines suppress tumor cell functions, including cell proliferation, transformation, migration, invasion, epithelial-mesenchymal transition, resistance to chemotherapy drugs, and in vivo tumor growth and metastasis. Conversely, overexpression of tumor cells with Crk or CrkL enhances tumor cell functions. Therefore, Crk and CrkL have been proposed as therapeutic targets for cancer treatment. However, it is unclear whether Crk and CrkL make distinct or overlapping contributions to tumor cell functions in various cancer types because Crk or CrkL have been examined independently in most studies. Two recent studies using colorectal cancer and glioblastoma cells clearly demonstrated that Crk and CrkL need to be ablated individually and combined to understand distinct and overlapping roles of the two proteins in cancer. A comprehensive understanding of individual and overlapping roles of Crk and CrkL in tumor cell functions is necessary to develop effective therapeutic strategies. This review systematically discusses crucial functions of Crk and CrkL in tumor cell functions and provides new perspectives on targeting Crk and CrkL in cancer therapy.

CrkL is required for donor T cell migration to GvHD target organs

The success of cancer therapies based on allogeneic hematopoietic stem cell transplant relies on the ability to separate graft-versus-host disease (GvHD) from graft-versus-tumor (GVT) responses. Controlling donor T cell migration into peripheral tissues is a viable option to limit unwanted tissue damage, but a lack of specific targets limits progress on this front. Here, we show that the adaptor protein CrkL, but not the closely related family members CrkI or CrkII, is a crucial regulator of T cell migration. In vitro, CrkL-deficient T cells fail to polymerize actin in response to the integrin ligand ICAM-1, resulting in defective migration. Using a mouse model of GvHD/GVT, we found that while CrkL-deficient T cells can efficiently eliminate hematopoietic tumors they are unable to migrate into inflamed organs, such as the liver and small intestine, and thus do not cause GvHD. These results suggest a specific role for CrkL in trafficking to peripheral organs but not the lymphatic system. In line with this, we found that although CrkL-deficient T cells could clear hematopoietic tumors, they failed to clear the same tumor growing subcutaneously, highlighting the role of CrkL in controlling T cell migration into peripheral tissues. Our results define a unique role for CrkL in controlling T cell migration, and suggest that CrkL function could be therapeutically targeted to enhance the efficacy of immunotherapies involving allogeneic donor cells.

Novel role for CRK adaptor proteins as essential components of SRC/FAK signaling for epithelial-mesenchymal transition and colorectal cancer aggressiveness

Epithelial-mesenchymal transition (EMT) is a cell plasticity process required for metastasis and chemoresistance of carcinoma cells. We report a crucial role of the signal adaptor proteins CRK and CRKL in promoting EMT and tumor aggressiveness, as well as resistance against chemotherapy in colorectal and pancreatic carcinoma. Genetic loss of either CRKL or CRK partially counteracted EMT in three independent cancer cell lines. Strikingly, complete loss of the CRK family shifted cells strongly toward the epithelial phenotype. Cells exhibited greatly increased E-cadherin and grew as large, densely packed clusters, completely lacked invasiveness and the ability to undergo EMT induced by cytokines or genetic activation of SRC. Furthermore, CRK family-deficiency significantly reduced cell survival, proliferation and chemoresistance, as well as ERK1/2 phosphorylation and c-MYC protein levels. In accordance, MYC-target gene expression was identified as novel hallmark process positively regulated by CRK family proteins. Mechanistically, CRK proteins were identified as pivotal amplifiers of SRC/FAK signaling at focal adhesions, mediated through a novel positive feedback loop depending on RAP1. Expression of the CRK family and the EMT regulator ZEB1 was significantly correlated in samples from colorectal cancer patients, especially in invasive regions. Further, high expression of CRK family genes was significantly associated with reduced survival in locally advanced colorectal cancer, as well as in pan-cancer datasets from the TCGA project. Thus, CRK family adaptor proteins are promising therapeutic targets to counteract EMT, chemoresistance, metastasis formation and minimal residual disease. As proof of concept, CRK family-mediated oncogenic signaling was successfully inhibited by a peptide-based inhibitor.

The Crk adapter protein is essential for Drosophila embryogenesis, where it regulates multiple actin-dependent morphogenic events

Small Src homology domain 2 (SH2) and 3 (SH3) adapter proteins regulate cell fate and behavior by mediating interactions between cell surface receptors and downstream signaling effectors in many signal transduction pathways. The CT10 regulator of kinase (Crk) family has tissue-specific roles in phagocytosis, cell migration, and neuronal development and mediates oncogenic signaling in pathways like that of Abelson kinase. However, redundancy among the two mammalian family members and the position of the Drosophila gene on the fourth chromosome precluded assessment of Crk's full role in embryogenesis. We circumvented these limitations with short hairpin RNA and CRISPR technology to assess Crk's function in Drosophila morphogenesis. We found that Crk is essential beginning in the first few hours of development, where it ensures accurate mitosis by regulating orchestrated dynamics of the actin cytoskeleton to keep mitotic spindles in syncytial embryos from colliding. In this role, it positively regulates cortical localization of the actin-related protein 2/3 complex (Arp2/3), its regulator suppressor of cAMP receptor (SCAR), and filamentous actin to actin caps and pseudocleavage furrows. Crk loss leads to the loss of nuclei and formation of multinucleate cells. We also found roles for Crk in embryonic wound healing and in axon patterning in the nervous system, where it localizes to the axons and midline glia. Thus, Crk regulates diverse events in embryogenesis that require orchestrated cytoskeletal dynamics.

Phosphoproteomic Profiling Identifies Aberrant Activation of Integrin Signaling in Aggressive Non-Type Bladder Carcinoma

Bladder carcinoma is highly heterogeneous and its complex molecular landscape; thus, poses a significant challenge for resolving an effective treatment in metastatic tumors. We computed the epithelial-mesenchymal transition (EMT) scores of three bladder carcinoma subtypes-luminal, basal, and non-type. The EMT score of the non-type indicated a "mesenchymal-like" phenotype, which correlates with a relatively more aggressive form of carcinoma, typified by an increased migration and invasion. To identify the altered signaling pathways potentially regulating this EMT phenotype in bladder cancer cell lines, we utilized liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based phosphoproteomic approach. Bioinformatics analyses were carried out to determine the activated pathways, networks, and functions in bladder carcinoma cell lines. A total of 3125 proteins were identified, with 289 signature proteins noted to be differentially phosphorylated (p ≤ 0.05) in the non-type cell lines. The integrin pathway was significantly enriched and five major proteins (TLN1, CTTN, CRKL, ZYX and BCAR3) regulating cell motility and invasion were hyperphosphorylated. Our study reveals GSK3A/B and CDK1 as promising druggable targets for the non-type molecular subtype, which could improve the treatment outcomes for aggressive bladder carcinoma.

Crk adaptor proteins mediate actin-dependent T cell migration and mechanosensing induced by the integrin LFA-1

T cell entry into inflamed tissue involves firm adhesion, spreading, and migration of the T cells across endothelial barriers. These events depend on "outside-in" signals through which engaged integrins direct cytoskeletal reorganization. We investigated the molecular events that mediate this process and found that T cells from mice lacking expression of the adaptor protein Crk exhibited defects in phenotypes induced by the integrin lymphocyte function-associated antigen 1 (LFA-1), namely, actin polymerization, leading edge formation, and two-dimensional cell migration. Crk protein was an essential mediator of LFA-1 signaling-induced phosphorylation of the E3 ubiquitin ligase c-Cbl and its subsequent interaction with the phosphatidylinositol 3-kinase (PI3K) subunit p85, thus promoting PI3K activity and cytoskeletal remodeling. In addition, we found that Crk proteins were required for T cells to respond to changes in substrate stiffness, as measured by alterations in cell spreading and differential phosphorylation of the force-sensitive protein CasL. These findings identify Crk proteins as key intermediates coupling LFA-1 signals to actin remodeling and provide mechanistic insights into how T cells sense and respond to substrate stiffness.

The Tumor Suppressor SASH1 Interacts With the Signal Adaptor CRKL to Inhibit Epithelial-Mesenchymal Transition and Metastasis in Colorectal Cancer

BACKGROUND & AIMS

The tumor-suppressor sterile α motif- and Src-homology 3-domain containing 1 (SASH1) has clinical relevance in colorectal carcinoma and is associated specifically with metachronous metastasis. We sought to identify the molecular mechanisms linking decreased SASH1 expression with distant metastasis formation.

METHODS

SASH1-deficient, SASH1-depleted, or SASH1-overexpressing HCT116 colon cancer cells were generated by the Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated 9-method, RNA interference, and transient plasmid transfection, respectively. Epithelial-mesenchymal transition (EMT) was analyzed by quantitative reverse-transcription polymerase chain reaction, immunoblotting, immunofluorescence microscopy, migration/invasion assays, and 3-dimensional cell culture. Yeast 2-hybrid assays and co-immunoprecipitation/mass-spectrometry showed V-Crk avian sarcoma virus CT10 oncogene homolog-like (CRKL) as a novel interaction partner of SASH1, further confirmed by domain mapping, site-directed mutagenesis, co-immunoprecipitation, and dynamic mass redistribution assays. CRKL-deficient cells were generated in parental or SASH1-deficient cells. Metastatic capacity was analyzed with an orthotopic mouse model. Expression and significance of SASH1 and CRKL for survival and response to chemotherapy was assessed in patient samples from our department and The Cancer Genome Atlas data set.

RESULTS

SASH1 expression is down-regulated during cytokine-induced EMT in cell lines from colorectal, pancreatic, or hepatocellular cancer, mediated by the putative SASH1 promoter. Deficiency or knock-down of SASH1 induces EMT, leading to an aggressive, invasive phenotype with increased chemoresistance. SASH1 counteracts EMT through interaction with the oncoprotein CRKL, inhibiting CRKL-mediated activation of SRC kinase, which is crucially required for EMT. SASH1-deficient cells form significantly more metastases in vivo, depending entirely on CRKL. Patient tumor samples show significantly decreased SASH1 and increased CRKL expression, associated with significantly decreased overall survival. Patients with increased CRKL expression show significantly worse response to adjuvant chemotherapy.

CONCLUSIONS

We propose SASH1 as an inhibitor of CRKL-mediated SRC signaling, introducing a potentially druggable mechanism counteracting chemoresistance and metastasis formation.

Exploring the mechanistic insights of Cas scaffolding protein family member 4 with protein tyrosine kinase 2 in Alzheimer's disease by evaluating protein interactions through molecular docking and dynamic simulations

Cas scaffolding protein family member 4 and protein tyrosine kinase 2 are signaling proteins, which are involved in neuritic plaques burden, neurofibrillary tangles, and disruption of synaptic connections in Alzheimer's disease. In the current study, a computational approach was employed to explore the active binding sites of Cas scaffolding protein family member 4 and protein tyrosine kinase 2 proteins and their significant role in the activation of downstream signaling pathways. Sequential and structural analyses were performed on Cas scaffolding protein family member 4 and protein tyrosine kinase 2 to identify their core active binding sites. Molecular docking servers were used to predict the common interacting residues in both Cas scaffolding protein family member 4 and protein tyrosine kinase 2 and their involvement in Alzheimer's disease-mediated pathways. Furthermore, the results from molecular dynamic simulation experiment show the stability of targeted proteins. In addition, the generated root mean square deviations and fluctuations, solvent-accessible surface area, and gyration graphs also depict their backbone stability and compactness, respectively. A better understanding of CAS and their interconnected protein signaling cascade may help provide a treatment for Alzheimer's disease. Further, Cas scaffolding protein family member 4 could be used as a novel target for the treatment of Alzheimer's disease by inhibiting the protein tyrosine kinase 2 pathway.

CRKL Mediates p110β-Dependent PI3K Signaling in PTEN-Deficient Cancer Cells

The p110β isoform of PI3K is preferentially activated in many tumors deficient in the phosphatase and tensin homolog (PTEN). However, the mechanism(s) linking PTEN loss to p110β activation remain(s) mysterious. Here, we identify CRKL as a member of the class of PI3Kβ-interacting proteins. Silencing CRKL expression in PTEN-null human cancer cells leads to a decrease in p110β-dependent PI3K signaling and cell proliferation. In contrast, CRKL depletion does not impair p110α-mediated signaling. Further study showed that CRKL binds to tyrosine-phosphorylated p130Cas in PTEN-null cancer cells. Since Src family kinases are known both to be regulated by PTEN and to phosphorylate and activate p130Cas, we tested and found that Src inhibition cooperated with p110β inhibition to suppress the growth of PTEN-null cells. These data suggest both a potential mechanism linking PTEN loss to p110β activation and the possible benefit of dual inhibition of Src and PI3K for PTEN-null tumors.

Rho GTPase signaling complexes in cell migration and invasion

Cell migration is dependent on the dynamic formation and disassembly of actin filament-based structures, including lamellipodia, filopodia, invadopodia, and membrane blebs, as well as on cell-cell and cell-extracellular matrix adhesions. These processes all involve Rho family small guanosine triphosphatases (GTPases), which are regulated by the opposing actions of guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Rho GTPase activity needs to be precisely tuned at distinct cellular locations to enable cells to move in response to different environments and stimuli. In this review, we focus on the ability of RhoGEFs and RhoGAPs to form complexes with diverse binding partners, and describe how this influences their ability to control localized GTPase activity in the context of migration and invasion.

miR-193b-Regulated Signaling Networks Serve as Tumor Suppressors in Liposarcoma and Promote Adipogenesis in Adipose-Derived Stem Cells

Well-differentiated and dedifferentiated liposarcomas (WDLS/DDLS) account for approximately 13% of all soft tissue sarcoma in adults and cause substantial morbidity or mortality in the majority of patients. In this study, we evaluated the functions of miRNA (miR-193b) in liposarcoma in vitro and in vivo Deep RNA sequencing on 93 WDLS, 145 DDLS, and 12 normal fat samples demonstrated that miR-193b was significantly underexpressed in DDLS compared with normal fat. Reintroduction of miR-193b induced apoptosis in liposarcoma cells and promoted adipogenesis in human adipose-derived stem cells (ASC). Integrative transcriptomic and proteomic analysis of miR-193b-target networks identified novel direct targets, including CRK-like proto-oncogene (CRKL) and focal adhesion kinase (FAK). miR-193b was found to regulate FAK-SRC-CRKL signaling through CRKL and FAK. miR-193b also stimulated reactive oxygen species signaling by targeting the antioxidant methionine sulfoxide reductase A to modulate liposarcoma cell survival and ASC differentiation state. Expression of miR-193b in liposarcoma cells was downregulated by promoter methylation, resulting at least in part from increased expression of the DNA methyltransferase DNMT1 in WDLS/DDLS. In vivo, miR-193b mimetics and FAK inhibitor (PF-562271) each inhibited liposarcoma xenograft growth. In summary, miR-193b not only functions as a tumor suppressor in liposarcoma but also promotes adipogenesis in ASC. Furthermore, this study reveals key tyrosine kinase and DNA methylation pathways in liposarcoma, some with immediate implications for therapeutic exploration. Cancer Res; 77(21); 5728-40. ©2017 AACR.

Proteomic responses to elevated ocean temperature in ovaries of the ascidian Ciona intestinalis

Ciona intestinalis, a common sea squirt, exhibits lower reproductive success at the upper extreme of the water temperatures it experiences in coastal New England. In order to understand the changes in protein expression associated with elevated temperatures, and possible response to global temperature change, we reared C. intestinalis from embryos to adults at 18°C (a temperature at which they reproduce normally at our collection site in Rhode Island) and 22°C (the upper end of the local temperature range). We then dissected ovaries from animals at each temperature, extracted protein, and measured proteomic levels using shotgun mass spectrometry (LC-MS/MS). 1532 proteins were detected at a 1% false discovery rate present in both temperature groups by our LC-MS/MS method. 62 of those proteins are considered up- or down-regulated according to our statistical criteria. Principal component analysis shows a clear distinction in protein expression pattern between the control (18°C) group and high temperature (22°C) group. Similar to previous studies, cytoskeletal and chaperone proteins are upregulated in the high temperature group. Unexpectedly, we find evidence that proteolysis is downregulated at the higher temperature. We propose a working model for the high temperature response in C. intestinalis ovaries whereby increased temperature induces upregulation of signal transduction pathways involving PTPN11 and CrkL, and activating coordinated changes in the proteome especially in large lipid transport proteins, cellular stress responses, cytoskeleton, and downregulation of energy metabolism.

A pre-metazoan origin of the CRK gene family and co-opted signaling network

CRK and CRKL adapter proteins play essential roles in development and cancer through their SRC homology 2 and 3 (SH2 and SH3) domains. To gain insight into the origin of their shared functions, we have investigated their evolutionary history. We propose a term, crk/crkl ancestral (crka), for orthologs in invertebrates before the divergence of CRK and CRKL in the vertebrate ancestor. We have isolated two orthologs expressed in the choanoflagellate Monosiga brevicollis, a unicellular relative to the metazoans. Consistent with its highly-conserved three-dimensional structure, the SH2 domain of M. brevicollis crka1 can bind to the mammalian CRK/CRKL SH2 binding consensus phospho-YxxP, and to the SRC substrate/focal adhesion protein BCAR1 (p130^CAS) in the presence of activated SRC. These results demonstrate an ancient origin of the CRK/CRKL SH2-target recognition specificity. Although BCAR1 orthologs exist only in metazoans as identified by an N-terminal SH3 domain, YxxP motifs, and a C-terminal FAT-like domain, some pre-metazoan transmembrane proteins include several YxxP repeats in their cytosolic region, suggesting that they are remotely related to the BCAR1 substrate domain. Since the tyrosine kinase SRC also has a pre-metazoan origin, co-option of BCAR1-related sequences may have rewired the crka-dependent network to mediate adhesion signals in the metazoan ancestor.

The ubiquitin-proteasome system regulates focal adhesions at the leading edge of migrating cells

Cell migration requires the cyclical assembly and disassembly of focal adhesions. Adhesion induces phosphorylation of focal adhesion proteins, including Cas (Crk-associated substrate/p130Cas/BCAR1). However, Cas phosphorylation stimulates adhesion turnover. This raises the question of how adhesion assembly occurs against opposition from phospho-Cas. Here we show that suppressor of cytokine signaling 6 (SOCS6) and Cullin 5, two components of the CRL5^SOCS6 ubiquitin ligase, inhibit Cas-dependent focal adhesion turnover at the front but not rear of migrating epithelial cells. The front focal adhesions contain phospho-Cas which recruits SOCS6. If SOCS6 cannot access focal adhesions, or if cullins or the proteasome are inhibited, adhesion disassembly is stimulated. This suggests that the localized targeting of phospho-Cas within adhesions by CRL5^SOCS6 and concurrent cullin and proteasome activity provide a negative feedback loop, ensuring that adhesion assembly predominates over disassembly at the leading edge. By this mechanism, ubiquitination provides a new level of spatio-temporal control over cell migration.

DOI:http://dx.doi.org/10.7554/eLife.17440.001

Animal cells can move in the body, for example to heal a wound, by protruding a leading edge forwards, attaching it to the surroundings and then pulling against these new attachments while disassembling the older ones. Mechanical forces regulate the assembly and disassembly of these attachments, known as focal adhesions, and so do signals from outside the cell that are transmitted to the adhesions via specialized proteins. However, it was not clear how the assembly and disassembly of adhesions is coordinated.

CRL5 is a ubiquitin ligase, an enzyme that can mark other proteins for destruction. Cells migrate more quickly if CRL5 is inhibited, and so Teckchandani and Cooper set out to uncover whether CRL5 affects the assembly and disassembly of focal adhesions. The experiments showed that human cells lacking a crucial component of the CRL5 complex, SOCS6, disassemble adhesions faster than normal cells, but only at their leading edge and not at the rear.

Teckchandani and Cooper also found that SOCS6 localizes to the leading edge by binding to a focal adhesion protein called Cas. Shortly after the attachments assemble, the Cas protein becomes tagged with a phosphate group and then acts to promote the adhesion to disassemble. Further experiments indicated that Cas was marked by the CRL5 complex and possibly destroyed while in or very close to the leading edge adhesions, slowing their disassembly.

Together, these findings suggest that by binding Cas, SOCS6 regulates the turnover of adhesions, specifically by inhibiting disassembly and allowing adhesions to grow at the leading edge. Since SOCS6 is not present in adhesions outside of the leading edge, this may help explain how the older adhesions are disassembled. Future studies could next focus on the exact sequence of events that occur in focal adhesions after the CRL5 complex binds to Cas as the cell migrates.

DOI:http://dx.doi.org/10.7554/eLife.17440.002

Vasodilator-Stimulated Phosphoprotein (VASP)-dependent and -independent pathways regulate thrombin-induced activation of Rap1b in platelets

Background

Vasodilator-Stimulated Phosphoprotein (VASP) is involved in the inhibition of agonist-induced platelet aggregation by cyclic nucleotides and the adhesion of platelets to the vascular wall. α_IIbβ₃ is the main integrin responsible for platelet activation and Rap1b plays a key role in integrin signalling. We investigated whether VASP is involved in the regulation of Rap1b in platelets since VASP-null platelets exhibit augmented adhesion to endothelial cells in vivo.

Methods

Washed platelets from wild type and VASP-deficient mice were stimulated with thrombin, the purinergic receptors agonist ADP, or the thromboxane A2 receptor agonist U46619 and Rap1b activation was measured using the GST-RalGDS-RBD binding assay. Interaction of VASP and Crkl was investigated by co-immunoprecipitation, confocal microscopy, and pull-down assays using Crkl domains expressed as GST-fusion proteins.

Results

Surprisingly, we found that activation of Rap1b in response to thrombin, ADP, or U46619 was significantly reduced in platelets from VASP-null mice compared to platelets from wild type mice. However, inhibition of thrombin-induced activation of Rap1b by nitric oxide (NO) was similar in platelets from wild type and VASP-null mice indicating that the NO/cGMP/PKG pathway controls inhibition of Rap1b independently from VASP. To understand how VASP regulated Rap1b, we investigated association between VASP and the Crk-like protein (Crkl), an adapter protein which activates the Rap1b guanine nucleotide exchange factor C3G. We demonstrated the formation of a Crkl/VASP complex by showing that: 1) Crkl co-immunoprecipitated VASP from platelet lysates; 2) Crkl and VASP dynamically co-localized at actin-rich protrusions reminiscent of focal adhesions, filopodia, and lamellipodia upon platelet spreading on fibronectin; 3) recombinant VASP bound directly to the N-terminal SH3 domain of Crkl; 4) Protein Kinase A (PKA) -mediated VASP phosphorylation on Ser157 abrogated the binding of Crkl.

Conclusions

We identified Crkl as a novel protein interacting with VASP in platelets. We propose that the C3G/Crkl/VASP complex plays a role in the regulation of Rap1b and this explains, at least in part, the reduced agonist-induced activation of Rap1b in VASP-null platelets. In addition, the fact that PKA-dependent VASP phosphorylation abrogated its interaction with Crkl may provide, at least in part, a rationale for the PKA-dependent inhibition of Rap1b and platelet aggregation.

The effects of Micro-429 on inhibition of cervical cancer cells through targeting ZEB1 and CRKL

MicroRNA-429 (miR-429) has been suggested to inhibit epithelial-mesenchymal transition (EMT), mainly due to targeting of ZEB1 and ZEB2, which are repressors of the cell to cell contact protein, E-cadherin. In this study, we indicated that regulation of miR-429 in cervical cancer cells modulates cell migration, elongation, as well as transforming growth factor β (TGF-β)-induced stress fiber formation through regulating the cytoskeleton reorganization which is likely independent of the zinc finger E-box binding homeobox (ZEB)/E-cadherin axis. ZEB1 and Crk-like adapter protein (CRKL), as novel targets of miR-429 and direct regulators of the actin cytoskeleton were identified. Remarkably, expression levels of ZEB1 and CRKL were inversely associated with the level of miR-429 in cervical cancer cell lines. In addition, individual knockdown and over-expression of these targeting genes phenocopied the roles of miR-429 over-expression and inhibition on cell elongation, migration, stress fiber formation, and invasion. Targeting of ZEB1 by miR-429 led to a decreased expression and transcriptional activity of CRB3, regulated by interference with the translocation of the CRB3. This finally led to decreasing of the expression of Crumbs 3 (CRB3), which is needed for the formation of stress fiber and contractility. Therefore, miR-429 affects cervical cancer by modulating some EMT-related processes. And in this study, evidences were provided to support a role for miR-429 as a novel target suppressing invasion and migration of human cervical cancer cells through modulation of its targeting genes ZEB1 and CRKL. Taken together, our data indicate that miR-429 plays a pivotal role in cervical cancer progression, which is a potential therapeutic target for patients.

The microRNA-200/Zeb1 axis regulates ECM-dependent β1-integrin/FAK signaling, cancer cell invasion and metastasis through CRKL

Tumor cell metastasis is a complex process that has been mechanistically linked to the epithelial-mesenchymal transition (EMT). The double-negative feedback loop between the microRNA-200 family and the Zeb1 transcriptional repressor is a master EMT regulator, but there is incomplete understanding of how miR-200 suppresses invasion. Our recent efforts have focused on the tumor cell-matrix interactions essential to tumor cell activation. Herein we utilized both our Kras/p53 mutant mouse model and human lung cancer cell lines to demonstrate that upon miR-200 loss integrin β1-collagen I interactions drive 3D in vitro migration/invasion and in vivo metastases. Zeb1-dependent EMT enhances tumor cell responsiveness to the ECM composition and activates FAK/Src pathway signaling by de-repression of the direct miR-200 target, CRKL. We demonstrate that CRKL serves as an adaptor molecule to facilitate focal adhesion formation, mediates outside-in signaling through Itgβ1 to drive cell invasion, and inside-out signaling that maintains tumor cell-matrix contacts required for cell invasion. Importantly, CRKL levels in pan-cancer TCGA analyses were predictive of survival and CRKL knockdown suppressed experimental metastases in vivo without affecting primary tumor growth. Our findings highlight the critical ECM-tumor cell interactions regulated by miR-200/Zeb1-dependent EMT that activate intracellular signaling pathways responsible for tumor cell invasion and metastasis.

LASP1 is a novel BCR-ABL substrate and a phosphorylation-dependent binding partner of CRKL in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is characterized by a genomic translocation generating a permanently active BCR-ABL oncogene with a complex pattern of atypically tyrosine-phosphorylated proteins that drive the malignant phenotype of CML. Recently, the LIM and SH3 domain protein 1 (LASP1) was identified as a component of a six gene signature that is strongly predictive for disease progression and relapse in CML patients. However, the underlying mechanisms why LASP1 expression correlates with dismal outcome remained unresolved. Here, we identified LASP1 as a novel and overexpressed direct substrate of BCR-ABL in CML. We demonstrate that LASP1 is specifically phosphorylated by BCR-ABL at tyrosine-171 in CML patients, which is abolished by tyrosine kinase inhibitor therapy. Further studies revealed that LASP1 phosphorylation results in an association with CRKL - another specific BCR-ABL substrate and bona fide biomarker for BCR-ABL activity. pLASP1-Y171 binds to non-phosphorylated CRKL at its SH2 domain. Accordingly, the BCR-ABL-mediated pathophysiological hyper-phosphorylation of LASP1 in CML disrupts normal regulation of CRKL and LASP1, which likely has implications on downstream BCR-ABL signaling. Collectively, our results suggest that LASP1 phosphorylation might serve as an additional candidate biomarker for assessment of BCR-ABL activity and provide a first step toward a molecular understanding of LASP1 function in CML.

CrkL meditates CCL20/CCR6-induced EMT in gastric cancer

BACKGROUND

In recent years, Crk-like adapter protein (CrkL) has been identified as a key regulator in the epithelial-to-mesenchymal transition (EMT). However, the molecular mechanisms underlying the CC chemokine receptor 6 (CCR6) and chemokine (C-C motif) ligand 20 (CCL20)-induced EMT in gastric cancer are still unclear.

METHODS

We conducted the immunohistochemistry and immunoblotting to detect the expression of CCR6 and CrkL in 90 cases of gastric cancer tissues and five kinds of cell lines. And then, gastric cancer cells were subjected to small interfering RNA (siRNA) treatment and in vitro assay.

RESULTS

Both CCR6 and CrkL were aberrantly expressed in gastric cancer specimens and closely correlated with differentiation of cell lines. The expression of CCR6 and CrkL was also significantly associated with metastasis, stage, and poor prognosis of gastric cancer. In addition, we validated CCL20 activated the expression of p-CrkL, p-Erk1/2, p-Akt, vimentin, N-cadherin and MMP2 in MGC803 cells in a dose-dependent manner. However, si-CrkL abrogated the CCL20-induced p-Erk1/2, vimentin, N-cadherin and MMP2 expression. Most importantly, the knockdown of CrkL decreased migration and invasion of MGC803 cells.

CONCLUSIONS

CrkL mediates CCL20/CCR6-induced EMT via Akt pathway, instead of Erk1/2 pathway in development of gastric cancer, which indicated CCL20/CCR6-CrkL-Erk1/2-EMT pathway may be targeted to antagonize the progression of gastric cancer.

MUC1-mediated motility in breast cancer: a review highlighting the role of the MUC1/ICAM-1/Src signaling triad

Breast cancer is the most common cancer in women with the leading cause of death being metastasis, the spread of cancer to distant organs. For those patients with high-risk estrogen receptor positive (ER+) breast cancer, an increased expression of the glycoprotein MUC1 is associated with resistance to anti-hormonal therapy, metastasis and death. Tumor cells may use MUC1 to metastasize by exploiting the vascular adhesion pathways used by leukocytes during the inflammatory response. MUC1 is a type 1 transmembrane protein whose cytoplasmic tail acts as a scaffold for several signaling pathways including the non-receptor kinase Src, a signaling molecule involved in cell differentiation, proliferation, adhesion and motility. This review will highlight our current knowledge of how MUC1/ICAM-1 binding can lead to the recruitment and activation of Src and propose a novel role for lipid raft microdomains in this promigratory signaling. Improved understanding of the mechanism of metastases and the underlying signaling cascade is a prerequisite to the discovery of therapeutic targets to prevent metastasis and death in ER+ breast cancer patients.

CRK proteins selectively regulate T cell migration into inflamed tissues

Effector T cell migration into inflamed sites greatly exacerbates tissue destruction and disease severity in inflammatory diseases, including graft-versus-host disease (GVHD). T cell migration into such sites depends heavily on regulated adhesion and migration, but the signaling pathways that coordinate these functions downstream of chemokine receptors are largely unknown. Using conditional knockout mice, we found that T cells lacking the adaptor proteins CRK and CRK-like (CRKL) exhibit reduced integrin-dependent adhesion, chemotaxis, and diapedesis. Moreover, these two closely related proteins exhibited substantial functional redundancy, as ectopic expression of either protein rescued defects in T cells lacking both CRK and CRKL. We determined that CRK proteins coordinate with the RAP guanine nucleotide exchange factor C3G and the adhesion docking molecule CASL to activate the integrin regulatory GTPase RAP1. CRK proteins were required for effector T cell trafficking into sites of inflammation, but not for migration to lymphoid organs. In a murine bone marrow transplantation model, the differential migration of CRK/CRKL-deficient T cells resulted in efficient graft-versus-leukemia responses with minimal GVHD. Together, the results from our studies show that CRK family proteins selectively regulate T cell adhesion and migration at effector sites and suggest that these proteins have potential as therapeutic targets for preventing GVHD.

Contribution of Crk adaptor proteins to host cell and bacteria interactions

The Crk adaptor family of proteins comprises the alternatively spliced CrkI and CrkII isoforms, as well as the paralog Crk-like (CrkL) protein, which is encoded by a different gene. Initially thought to be involved in signaling during apoptosis and cell adhesion, this ubiquitously expressed family of proteins is now known to play essential roles in integrating signals from a wide range of stimuli. In this review, we describe the structure and function of the different Crk proteins. We then focus on the emerging roles of Crk adaptors during Enterobacteriaceae pathogenesis, with special emphasis on the important human pathogens Salmonella, Shigella, Yersinia, and enteropathogenic Escherichia coli. Throughout, we remark on opportunities for future research into this intriguing family of proteins.

miR-429 inhibits migration and invasion of breast cancer cells in vitro

Accumulating evidence indicates that microRNAs (miRNAs) are involved in regulating cancer invasion and metastasis, and an increasing number of research demonstrates that miRNAs can promote or inhibit cell motility depending on genetic background of different cancers and the microenvironment. In the present study, we established an in vivo bone metastasis model of breast cancer by injecting MDA-MB-231 cells into the left ventricle of nude mice, and then screened the differentially expressed miRNAs between parental and bone-metastatic MDA-MB-231 cells using miRNA array. The results revealed that decreased expression of miR-429 was probably involved in negatively regulating bone metastasis of breast cancer cells. On the other hand, overexpression of miR-429 in MDA-MB-231 cells remarkably suppressed invasion in vitro. We identified ZEB1 and CRKL as potential targets of miR-429 by analyzing combined results from in silico search and global expression array of the same RNA samples. Immunoblot assay confirmed that miR-429 reduced their expression at protein level. Taken together, our results offer an opportunity for further understanding of the recondite mechanisms underlying the bone metastasis of breast cancer.

Src-inducible association of CrkL with procaspase-8 promotes cell migration

Procaspase-8, the zymogen form of the apoptosis-initiator caspase-8, undergoes phosphorylation following integrin-mediated cell attachment to an extracellular matrix substrate. Concordant with cell attachment to fibronectin, a population of procaspase-8 becomes associated with a peripheral insoluble compartment that includes focal complexes and lamellar microfilaments. Phosphorylation of procaspase-8 both impairs its maturation to the proapoptotic form and can promote cell migration. Here we show that the cytoskeletal adaptor protein CrkL promotes caspase-8 recruitment to the peripheral spreading edge of cells, and that the catalytic domain of caspase-8 directly interacts with the SH2 domain of CrkL. We show that the interaction is abolished by shRNA-mediated silencing of Src, in Src-deficient MEFs, and by pharmacologic inhibitors of the kinase. The results provide insight into how tyrosine kinases may act to coordinate the suppression caspase-8 mediated apoptosis, while promoting cell invasion.

Targeting Src family kinases inhibits bevacizumab-induced glioma cell invasion

Anti-VEGF antibody therapy with bevacizumab provides significant clinical benefit in patients with recurrent glioblastoma multiforme (GBM). Unfortunately, progression on bevacizumab therapy is often associated with a diffuse disease recurrence pattern, which limits subsequent therapeutic options. Therefore, there is an urgent need to understand bevacizumab's influence on glioma biology and block it's actions towards cell invasion.

To explore the mechanism(s) of GBM cell invasion we have examined a panel of serially transplanted human GBM lines grown either in short-term culture, as xenografts in mouse flank, or injected orthotopically in mouse brain. Using an orthotopic xenograft model that exhibits increased invasiveness upon bevacizumab treatment, we also tested the effect of dasatinib, a broad spectrum SFK inhibitor, on bevacizumab-induced invasion.

We show that 1) activation of Src family kinases (SFKs) is common in GBM, 2) the relative invasiveness of 17 serially transplanted GBM xenografts correlates strongly with p120 catenin phosphorylation at Y228, a Src kinase site, and 3) SFK activation assessed immunohistochemically in orthotopic xenografts, as well as the phosphorylation of downstream substrates occurs specifically at the invasive tumor edge. Further, we show that SFK signaling is markedly elevated at the invasive tumor front upon bevacizumab administration, and that dasatinib treatment effectively blocked the increased invasion induced by bevacizumab.

Our data are consistent with the hypothesis that the increased invasiveness associated with anti-VEGF therapy is due to increased SFK signaling, and support testing the combination of dasatinib with bevacizumab in the clinic.

Focal adhesion kinase is required for neural crest cell morphogenesis during mouse cardiovascular development

Neural crest cells (NCCs) participate in the remodeling of the cardiac outflow tract and pharyngeal arch arteries during cardiovascular development. Focal adhesion kinase (FAK) mediates signal transduction by integrin and growth factor receptors, each of which is important for normal cardiovascular development. To investigate the role of FAK in NCC morphogenesis, we deleted it in murine NCCs using Wnt1cre, yielding craniofacial and cardiovascular malformations resembling those observed in individuals with DiGeorge syndrome. In these mice, we observed normal cardiac NCC migration but reduced differentiation into smooth muscle within the aortic arch arteries and impaired cardiac outflow tract rotation, which resulted in a dextroposed aortic root. Moreover, within the conotruncal cushions, Fak-deficient NCCs formed a less organized mesenchyme, with reduced expression of perlecan and semaphorin 3C, and exhibited disorganized F-actin stress fibers within the aorticopulmonary septum. Additionally, absence of Fak resulted in reduced in vivo phosphorylation of Crkl and Erk1/2, components of a signaling pathway essential for NCC development. Consistent with this, both TGF-beta and FGF induced FAK and Crkl phosphorylation in control but not Fak-deficient NCCs in vitro. Our results indicate that FAK plays an essential role in cardiac outflow tract development by promoting the activation of molecules such as Crkl and Erk1/2.

Structural and functional basis of a role for CRKL in a fibroblast growth factor 8-induced feed-forward loop

The adapter protein CRKL is required for the normal development of multiple tissues that rely on fibroblast growth factor 8 (FGF8). The precise role of CRKL in receptor signaling has been unclear, however. To address this issue, we first modeled the three-dimensional structure of CRKL by molecular dynamics. By taking advantage of structural simulations, we performed in silico analysis of the interactions of the autophosphorylation sites of FGR receptor 1 (FGFR1) with the SH2 domain of CRKL or a highly related protein, CRK. As predicted by simulations, we confirm the specific physical interaction of phosphorylated Y463 (pY463) in FGFR1 with the CRKL SH2 domain at an affinity approximately 30-fold stronger than that of CRK. We also provide evidence that interactions outside of the core YXXP motif have a significant impact on physical association, which is consistent with predictions from molecular-dynamics simulations. Furthermore, we identify CRKL as an essential component of an FGF8-induced feed-forward loop permissive for efficient activation of the mitogen-activated protein kinase Erk1/2, as well as FGF8-induced anchorage-independent cell growth, using Crkl-deficient cells or a pY463 synthetic peptide. Although many cells generally require cell-matrix adhesion, our results demonstrate that CRKL permits cells to bypass the strict need for adhesion in response to FGF8 through direct interaction with receptor.

Fibulin-1 is required for morphogenesis of neural crest-derived structures

Here we report that mouse embryos homozygous for a gene trap insertion in the fibulin-1 (Fbln1) gene are deficient in Fbln1 and exhibit cardiac ventricular wall thinning and ventricular septal defects with double outlet right ventricle or overriding aorta. Fbln1 nulls also display anomalies of aortic arch arteries, hypoplasia of the thymus and thyroid, underdeveloped skull bones, malformations of cranial nerves and hemorrhagic blood vessels in the head and neck. The spectrum of malformations is consistent with Fbln1 influencing neural crest cell (NCC)-dependent development of these tissues. This is supported by evidence that Fbln1 expression is associated with streams of cranial NCCs migrating adjacent to rhombomeres 2-7 and that Fbln1-deficient embryos display patterning anomalies of NCCs forming cranial nerves IX and X, which derive from rhombomeres 6 and 7. Additionally, Fbln1-deficient embryos show increased apoptosis in areas populated by NCCs derived from rhombomeres 4, 6 and 7. Based on these findings, it is concluded that Fbln1 is required for the directed migration and survival of cranial NCCs contributing to the development of pharyngeal glands, craniofacial skeleton, cranial nerves, aortic arch arteries, cardiac outflow tract and cephalic blood vessels.

MUC1 initiates Src-CrkL-Rac1/Cdc42-mediated actin cytoskeletal protrusive motility after ligating intercellular adhesion molecule-1

MUC1, a transmembrane glycoprotein of the mucin family, when aberrantly expressed on breast cancer cells is correlated with increased lymph node metastases. We have previously shown that MUC1 binds intercellular adhesion molecule-1 (ICAM-1) on surrounding accessory cells and facilitates transendothelial migration of MUC1-bearing cells. Nevertheless, the underlying molecular mechanism is still obscure. In the present study, we used a novel assay of actin cytoskeletal reorganization to show that by ligating ICAM-1, MUC1 triggers Rac1- and Cdc42-dependent actin cytoskeletal protrusive activity preferentially at the heterotypic cell-cell contact sites. Further, we show that these MUC1/ICAM-1 interaction-initiated lamellipodial and filopodial protrusions require Src family kinase and CT10 regulator of kinase like (CrkL) accompanied by the rapid formation of a Src-CrkL signaling complex at the MUC1 cytoplasmic domain. Through inhibition of Src kinase activity, we further revealed that Src is required for recruiting CrkL to the MUC1 cytoplasmic domain as well as mediating the observed actin cytoskeleton dynamics. These findings suggest a novel MUC1-Src-CrkL-Rac1/Cdc42 signaling cascade following ICAM-1 ligation, through which MUC1 regulates cytoskeletal reorganization and directed cell motility during cell migration.

Tyrosine phosphatase PTPalpha regulates focal adhesion remodeling through Rac1 activation

We characterized the role of protein tyrosine phosphatase (PTP)-alpha in focal adhesion (FA) formation and remodeling using wild-type and PTPalpha-deficient (PTPalpha(-/-)) cells. Compared with wild-type cells, spreading PTPalpha(-/-) fibroblasts displayed fewer leading edges and formed elongated alpha-actinin-enriched FA at the cell periphery. These features suggest the presence of slowly remodeling cell adhesions and were phenocopied in human fibroblasts in which PTPalpha was knocked down using short interfering RNA (siRNA) or in NIH-3T3 fibroblasts expressing catalytically inactive (C433S/C723S) PTPalpha. Fluorescence recovery after photobleaching showed slower green fluorescence protein-alpha-actinin recovery in the FA of PTPalpha(-/-) than wild-type cells. These alterations correlated with reduced cell spreading, adhesion, and polarization and retarded contraction of extracellular matrices in PTPalpha(-/-) fibroblasts. Activation of Rac1 and its recruitment to FA during spreading were diminished in cells expressing C433S/C723S PTPalpha. Rac1(-/-) cells also displayed abnormally elongated and peripherally distributed FA that failed to remodel. Conversely, expression of constitutively active Rac1 restored normal FA remodeling in PTPalpha(-/-) cells. We conclude that PTPalpha is required for remodeling of FA during cell spreading via a pathway involving Rac1.

Function of the N-terminus of zizimin1: autoinhibition and membrane targeting

Rho family small GTPases are critical regulators of multiple cellular functions. Dbl-homology-domain-containing proteins are the classical GEFs (guanine nucleotide exchange factors) responsible for activation of Rho proteins. Zizimin1 is a Cdc42-specific GEF that belongs to a second family of mammalian Rho-GEFs, CZH [CDM (Ced-5/DOCK180/Myoblast city)-zizimin homology] proteins, which possess a novel type of GEF domain. CZH proteins can be divided into a subfamily related to DOCK 180 and a subfamily related to zizimin1. The two groups share two conserved regions named the CZH1 (or DHR1) domain and the CZH2 (DHR2 or DOCKER) domains, the latter exhibiting GEF activity. We now show that limited proteolysis of zizimin1 suggests the existence of structural domains that do not correspond to those identified on the basis of homologies. We demonstrate that the N-terminal half binds to the GEF domain through three distinct areas, including the CZH1, to inhibit the interaction with Cdc42. The N-terminal PH (pleckstrin homology) domain binds phosphoinositides and mediates zizimin1 membrane targeting. These results define two novel functions for the N-terminal region of zizimin1.

Conditional deletion of focal adhesion kinase leads to defects in ventricular septation and outflow tract alignment

To examine a role for focal adhesion kinase (FAK) in cardiac morphogenesis, we generated a line of mice with a conditional deletion of FAK in nkx2-5-expressing cells (herein termed FAKnk mice). FAKnk mice died shortly after birth, likely resulting from a profound subaortic ventricular septal defect and associated malalignment of the outflow tract. Additional less penetrant phenotypes included persistent truncus arteriosus and thickened valve leaflets. Thus, conditional inactivation of FAK in nkx2-5-expressing cells leads to the most common congenital heart defect that is also a subset of abnormalities associated with tetralogy of Fallot and the DiGeorge syndrome. No significant differences in proliferation or apoptosis between control and FAKnk hearts were observed. However, decreased myocardialization was observed for the conal ridges of the proximal outflow tract in FAKnk hearts. Interestingly, chemotaxis was significantly attenuated in isolated FAK-null cardiomyocytes in comparison to genetic controls, and these effects were concomitant with reduced tyrosine phosphorylation of Crk-associated substrate (CAS). Thus, it is possible that ventricular septation and appropriate outflow tract alignment is dependent, at least in part, upon FAK-dependent CAS activation and subsequent induction of polarized myocyte movement into the conal ridges. Future studies will be necessary to determine the precise contributions of the additional nkx2-5-derived lineages to the phenotypes observed.

SET and MYND domain-containing protein 3 decreases sensitivity to dexamethasone and stimulates cell adhesion and migration in NIH3T3 cells

SET and MYND domain-containing protein 3 (SMYD3) is a novel histone methyltransferase that plays an important role in transcriptional regulation in human carcinogenesis. It activates the transcription of a set of downstream genes. Of these downstream genes, there are several oncogenes and genes associated with cell adhesion (including those of N-Myc, CrkL, Wnt10b, L-selectin, CD31 and galectin-4), which have been shown to have effects on cell viability, adhesion, migration and metastasis by many studies. To determine whether SMYD3 has such functions, in this study, we examined these types of biological activities in mouse fibroblast NIH3T3 cells by stably transfecting the human SMYD3 gene. The SMYD3-gene-transfected cells showed an increased proliferation rate and became more resistant to cell death induced by dexamethasone. Furthermore, the SMYD3-transfected cells also exhibited increased rates of cell adhesion to both type IV collagen and endothelial cells, and enhanced cell migration ability in both two-dimensional and three-dimensional assays. This study is the first to show that the overexpression of the SMYD3 gene affects cell viability, adhesion and migration, indicating that SMYD3 may be a promising new target of therapeutic intervention for the treatment of cancers or other pathological processes associated with cell adhesion and migration.

The role of connexins in controlling cell growth and gene expression

The purpose of this paper is to provide a brief overview of current thinking on the role of connexins, in particular Cx43, in growth regulation, and a more detailed discussion as to potential mechanisms involved with an emphasis on gene expression. While the precise molecular mechanism by which connexins can affect the growth of normal or tumor cells remains elusive, a number of exciting reports have expanded our understanding and are presented in some detail. Thus, we will discuss (Section 2): the role of protein-protein interactions in integrating connexins into multiple signal transduction pathways; phosphorylation at specific sites and reversal of growth inhibition; the role of the carboxy-terminal regulatory domain as a signaling molecule. Some of our latest work on the potential functions of endogenously produced carboxy-terminal fragments of Cx43 are also presented (Section 3). Finally, Section 4 will pay tribute to the rapidly emerging realization that connexins such as Cx43 and Cx32 exert important and extensive effects on gene expression, particularly those genes linked to growth regulation.

CrkL plays a role in SDF-1-induced activation of the Raf-1/MEK/Erk pathway through Ras and Rac to mediate chemotactic signaling in hematopoietic cells

Intracellular signaling mechanisms regulating SDF-1-induced chemotaxis of hematopoietic cells have remained elusive. Here we demonstrate that overexpression of the adaptor molecule CrkL enhances SDF-1-induced chemotaxis of hematopoietic BaF3 and 32Dcl3 cells. Overexpression of CrkL also enhanced SDF-1-induced activation of the Raf-1/MEK/Erk signaling pathway as well as that of the small GTPases Ras, Rap1, and Rac, while a dominant negative mutant of Ras or Rac suppressed CrkL-enhanced Erk activation. SDF-1 stimulation induced tyrosine phosphorylation of CrkL, which was inhibited by the Src family kinase inhibitor PP1 or by dominant negative mutants of Lyn, thus indicating that Lyn mediated SDF-1-induced phosphorylation of CrkL. However, inhibition of the Lyn kinase activity failed to affect SDF-1-induced activation of the small GTPases and Erk. On the other hand, SDF-1-induced activation of the Erk signaling pathway as well as chemotaxis was inhibited by overexpression of a CrkL mutant lacking the N-terminal SH3 domain, which mediates interaction with various signaling molecules including guanine nucleotide exchange factors for the Ras and Rho family GTPases. SDF-1-induced chemotaxis was also inhibited by the dominant negative Ras or Rac mutant as well as by the MEK inhibitor PD98059. These results indicate that CrkL mediates SDF-1-induced activation of the Raf-1/MEK/Erk signaling pathway through Ras as well as Rac in hematopoietic cells and, thereby, plays important roles in the induction of chemotactic response.

Mouse models for investigating the developmental basis of human birth defects

Clinicians and basic scientists share an interest in discovering how genetic or environmental factors interact to perturb normal development and cause birth defects and human disease. Given the complexity of such interactions, it is not surprising that 4% of human infants are born with a congenital malformation, and cardiovascular defects occur in nearly 1%. Our research is based on the fundamental hypothesis that an understanding of normal and abnormal development will permit us to generate effective strategies for both prevention and treatment of human birth defects. Animal models are invaluable in these efforts because they allow one to interrogate the genetic, molecular and cellular events that distinguish normal from abnormal development. Several features of the mouse make it a particularly powerful experimental model: it is a mammalian system with similar embryology, anatomy and physiology to humans; genes, proteins and regulatory programs are largely conserved between human and mouse; and finally, gene targeting in murine embryonic stem cells has made the mouse genome amenable to sophisticated genetic manipulation currently unavailable in any other model organism.

Role of Tyrosine Kinase Csk in G Protein-coupled Receptor- and Receptor Tyrosine Kinase-induced Fibroblast Cell Migration

Tyrosine kinase Csk is essential for mouse embryonic development. Csk knock-out mice died at early stages of embryogenesis (around embryonic day 10). The molecular mechanism for this defect is not completely understood. Here we report that Csk deficiency in mouse embryonic fibroblast cells blocked cell migration induced by lysophosphatidic acid through G protein-coupled receptors, by platelet-derived growth factor and epidermal growth factor through receptor tyrosine kinases, and by serum. Re-expression of Csk in these Csk-deficient cells rescued the migratory phenotype. Furthermore, deletion of Csk did not interfere with Rac activation and lamellipodia formation, but impaired the focal adhesions. Our data demonstrate a critical role for Csk in cell migration.

Chemical rescue of a mutant enzyme in living cells

The restoration of catalytic activity to mutant enzymes by small molecules is well established for in vitro systems. Here, we show that the protein tyrosine kinase Src arginine-388-->alanine (R388A) mutant can be rescued in live cells with the use of the small molecule imidazole. Cellular rescue of a viral Src homolog was rapid and reversible and conferred predicted oncogenic properties. Using chemical rescue in combination with mass spectrometry, we confirmed six known Src kinase substrates and identified several new protein targets. Chemical rescue data suggest that cellular Src is active under basal conditions. Rescue of R388A cellular Src provided insights into the mitogen-activated protein kinase pathway. This chemical rescue approach will likely have many applications in cell signaling.

Cell surface transglutaminase promotes RhoA activation via integrin clustering and suppression of the Src-p190RhoGAP signaling pathway

Tissue transglutaminase (tTG) is a multifunctional protein that serves as cross-linking enzyme and integrin-binding adhesion coreceptor for fibronectin on the cell surface. Previous work showed activation of small GTPase RhoA via enzymatic transamidation by cytoplasmic tTG. Here, we report an alternative nonenzymatic mechanism of RhoA activation by cell surface tTG. Direct engagement of surface tTG with specific antibody or the fibronectin fragment containing modules I(6)II(1,2)I(7-9) increases RhoA-GTP levels. Integrin-dependent signaling to RhoA and its downstream target Rho-associated coiled-coil containing serine/threonine protein kinase (ROCK) is amplified by surface tTG. tTG expression on the cell surface elevates RhoA-GTP levels in nonadherent and adherent cells, delays maximal RhoA activation upon cell adhesion to fibronectin and accelerates a rise in RhoA activity after binding soluble integrin ligands. These data indicate that surface tTG induces integrin clustering regardless of integrin-ligand interactions. This notion is supported by visualization of integrin clusters, increased susceptibility of integrins to chemical cross-linking, and biochemical detection of large integrin complexes in cells expressing tTG. In turn, integrin aggregation by surface tTG inhibits Src kinase activity and decreases activation of the Src substrate p190RhoGAP. Moreover, pharmacological inhibition of Src kinase reveals inactivation of Src signaling as the primary cause of elevated RhoA activity in cells expressing tTG. Together, these findings show that surface tTG amplifies integrin-mediated signaling to RhoA/ROCK via integrin clustering and down-regulation of the Src-p190RhoGAP regulatory pathway.

Crkl deficiency disrupts Fgf8 signaling in a mouse model of 22q11 deletion syndromes

Deletions on chromosome 22q11.21 disrupt pharyngeal and cardiac development and cause DiGeorge and related human syndromes. CRKL (CRK-Like) lies within 22q11.21, and Crkl-/- mice have phenotypic features of 22q11 deletion (del22q11) syndromes. While human FGF8 does not localize to 22q11, deficiency of Fgf8 also generates many features of del22q11 syndrome in mice. Since Fgf8 signals via receptor-type tyrosine kinases, and Crk family adaptor proteins transduce intracellular signals downstream of tyrosine kinases, we investigated whether Crkl mediates Fgf8 signaling. In addition to discovering genetic interactions between Crkl and Fgf8 during morphogenesis of structures affected in del22q11 syndrome, we found that Fgf8 induces tyrosine phosphorylation of FgfRs 1 and 2 and their binding to Crkl. Crkl is required for normal cellular responses to Fgf8, including survival and migration, Erk activation, and target gene expression. These findings provide mechanistic insight into disrupted intercellular interactions in the pathogenesis of malformations seen in del22q11 syndrome.

CZH proteins: a new family of Rho-GEFs

The Rho family of small GTPases are important regulators of multiple cellular activities and, most notably, reorganization of the actin cytoskeleton. Dbl-homology (DH)-domain-containing proteins are the classical guanine nucleotide exchange factors (GEFs) responsible for activation of Rho GTPases. However, members of a newly discovered family can also act as Rho-GEFs. These CZH proteins include: CDM (Ced-5, Dock180 and Myoblast city) proteins, which activate Rac; and zizimin proteins, which activate Cdc42. The family contains 11 mammalian proteins and has members in many other eukaryotes. The GEF activity is carried out by a novel, DH-unrelated domain named the DOCKER, CZH2 or DHR2 domain. CZH proteins have been implicated in cell migration, phagocytosis of apoptotic cells, T-cell activation and neurite outgrowth, and probably arose relatively early in eukaryotic evolution.

Crk-associated substrate tyrosine phosphorylation sites are critical for invasion and metastasis of SRC-transformed cells

Crk-associated substrate (CAS, p130Cas) is a major tyrosine phosphorylated protein in cells transformed by v-crk and v-src oncogenes. We recently reported that reexpression of CAS in CAS-deficient mouse embryo fibroblasts transformed by oncogenic Src promoted an invasive phenotype associated with enhanced cell migration through Matrigel, organization of actin into large podosome ring and belt structures, activation of matrix metalloproteinase-2, and elevated tyrosine phosphorylation of the focal adhesion proteins FAK and paxillin. We have now extended these studies to examine the mechanism by which CAS achieves these changes and to evaluate the potential role for CAS in promoting in vivo tumor growth and metastasis. Whereas the presence or absence of CAS did not alter the primary growth of subcutaneous-injected Src-transformed mouse embryo fibroblasts, CAS expression was required to promote lung metastasis following removal of the primary tumor. The substrate domain YxxP tyrosines, the major sites of CAS phosphorylation by Src that mediate interactions with Crk, were found to be critical for promoting both invasive and metastatic properties of the cells. The ability of CAS to promote Matrigel invasion, formation of large podosome structures, and tyrosine phosphorylation of Src substrates, including FAK, paxillin, and cortactin, was also strictly dependent on the YxxP tyrosines. In contrast, matrix metalloproteinase-2 activation was most dependent on the CAS SH3 domain, whereas the substrate domain YxxP sites also contributed to this property. Thus multiple CAS-mediated signaling events are implicated in promoting invasive and metastatic properties of Src-transformed cells.

Differences in gene expression profiles in dermal fibroblasts from control and patients with age-related macular degeneration elicited by oxidative injury

The pathogenesis of age-related macular degeneration (AMD) is still unknown but there is growing evidence that a combination of both oxidative injury and genetic factors may play a role. One particle hypothesis proposes that dysregulation of multiple genes in response to an oxidative injury could contribute to the development of AMD. While direct examination of ocular cells from AMD patients is difficult, AMD also appears to have a systemic component. Therefore, as is the case with other central nervous diseases, peripheral sites may also manifest any underlying genetic abnormalities. For the present study, biopsy-derived fibroblasts from 4 patients with the early form and 4 patients with the late form of AMD and 3 age-matched control patients were grown in culture and treated with a nonlethal dose of the oxidative stimulus menadione. Gene expression patterns were quantitatively and qualitatively examined using Human Genome U95A GeneChips (Affymetrix) and verified by real-time PCR analysis. In response to the oxidative injury 755 genes were found to be upregulated at least twofold in one of the patients groups. Cluster analysis of expression profiles detected six patterns of dysregulation initiated by oxidative injury specific for the disease groups (98 genes total). Clusters of genes dysregulated by the sublethal oxidative injury in either early and/or late AMD groups were further categorized by overrepresentation of GO "biological process" categories using Expression Analysis Systematic Explorer (EASE) software. This approach demonstrated that four major functional gene groups including inflammatory/innate immune response, transcriptional regulation, cell cycle, and proliferation were significantly overrepresented (Fisher test ranging from 0.0393 to 0.00018) in both AMD patients groups in response to the oxidative injury. Despite the small number of patients in the study, specific biological and statistical differences in gene expression profiles between control and AMD patients were identified but only in the presence of an environmental stimulus.

Inactivation of TGFbeta signaling in neural crest stem cells leads to multiple defects reminiscent of DiGeorge syndrome

Specific inactivation of TGFbeta signaling in neural crest stem cells (NCSCs) results in cardiovascular defects and thymic, parathyroid, and craniofacial anomalies. All these malformations characterize DiGeorge syndrome, the most common microdeletion syndrome in humans. Consistent with a role of TGFbeta in promoting non-neural lineages in NCSCs, mutant neural crest cells migrate into the pharyngeal apparatus but are unable to acquire non-neural cell fates. Moreover, in neural crest cells, TGFbeta signaling is both sufficient and required for phosphorylation of CrkL, a signal adaptor protein implicated in the development of DiGeorge syndrome. Thus, TGFbeta signal modulation in neural crest differentiation might play a crucial role in the etiology of DiGeorge syndrome.