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Specific cell shapes are fundamental to the organization and function of multicellular organisms. Fibroblast Growth Factor (FGF) signaling induces the elongation of lens fiber cells during vertebrate lens development. Nonetheless, exactly how this extracellular FGF signal is transmitted to the cytoskeletal network has previously not been determined. Here, we show that the Crk family of adaptor proteins, Crk and Crkl, are required for mouse lens morphogenesis but not differentiation. Genetic ablation and epistasis experiments demonstrated that Crk and Crkl play overlapping roles downstream of FGF signaling in order to regulate lens fiber cell elongation. Upon FGF stimulation, Crk proteins were found to interact with Frs2, Shp2 and Grb2. The loss of Crk proteins was partially compensated for by the activation of Ras and Rac signaling. These results reveal that Crk proteins are important partners of the Frs2/Shp2/Grb2 complex in mediating FGF signaling, specifically promoting cell shape changes., Competing Interests: TC, YM, HL, MB, AH, GF, FW, LQ, LC, TP, TC, XZ No competing interests declared, (© 2018, Collins et al.)

CT10 regulator of kinase (Crk) and Crk-like (CrkL) are the cellular counterparts of the viral oncogene v-Crk Elevated levels of Crk and CrkL have been observed in many human cancers; inhibition of Crk and CrkL expression reduced the tumor-forming potential of cancer cell lines. Despite a close relationship between the Crk family proteins and tumorigenesis, how Crk and CrkL contribute to cell growth is unclear. We ablated endogenous Crk and CrkL from cultured fibroblasts carrying floxed alleles of Crk and CrkL by transfection with synthetic Cre mRNA (synCre). Loss of Crk and CrkL induced by synCre transfection blocked cell proliferation and caused shrinkage of the cytoplasm and the nucleus, formation of adherens junctions, and reduced cell motility. Ablation of Crk or CrkL alone conferred a much more modest reduction in cell proliferation. Reintroduction of CrkI, CrkII, or CrkL individually rescued cell proliferation in the absence of the endogenous Crk and CrkL, suggesting that Crk and CrkL play overlapping functions in regulating fibroblast growth. Serum and basic FGF induced phosphorylation of Akt, MAP kinases, and S6 kinase and Fos expression in the absence of Crk and CrkL, suggesting that cells lacking Crk and CrkL are capable of initiating major signal transduction pathways in response to extracellular stimuli. Furthermore, cell cycle and cell death analyses demonstrated that fibroblasts lacking Crk and CrkL become arrested at the G 1 -S transition and undergo a modest apoptosis. Taken together, our results suggest that Crk and CrkL play essential overlapping roles in fibroblast growth., (© 2016 by The American Society for Biochemistry and Molecular Biology, Inc.)

Cytosolic proteins containing SH2 and SH3 domains, such as Crk and Crk-like (CrkL), are broadly expressed adapters that interact with a variety of proteins to fulfill key roles in signal transduction pathways triggered by activation of receptor and non-receptor tyrosine kinases. Crk and CrkL are similar to each other in structure and biochemical function, although they provide both distinct, as well as overlapping, biological roles during development. We developed a systematic approach to investigate Crk family functions at the cellular level by generating a conditional knock-out system for ablation of Crk and CrkL in cultured fibroblasts. The loss of both Crk and CrkL from fibroblasts resulted in reduced cell surface area and adoption of a rounded, refractile cellular phenotype. These morphological alterations were accompanied by a decrease in focal adhesion sites, reduced actin stress fibers and a collapse of microtubule structures. In addition, cells exhibited decreases in spontaneous motility and wound-healing behavior. Reduced p130Cas phosphorylation and actin levels closely followed the loss of Crk and CrkL, and stabilization of polymerized actin by jasplakinolide suppressed the morphological conversion. Ablation of Crk or CrkL alone conferred a much more modest phenotype suggesting that Crk and CrkL have overlapping functions that are critical for maintaining cell structure. The morphological alterations could be partially rescued by reintroduction of CrkII, and, to a lesser extent, CrkL. Taken together, our results suggest that Crk and CrkL have critical roles in cell structure and motility by maintaining cytoskeletal integrity.