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Understanding the organisation and role of the extracellular matrix (ECM) in islets of Langerhans is critical for maintaining pancreatic β-cells, and to recognise and revert the physiopathology of diabetes. Indeed, integrin-mediated adhesion signalling in response to the pancreatic ECM plays crucial roles in β-cell survival and insulin secretion, two major functions, which are affected in diabetes. Here, we would like to present an update on the major components of the pancreatic ECM, their role during integrin-mediated cell-matrix adhesions and how they are affected during diabetes. To treat diabetes, a promising approach consists in replacing β-cells by transplantation. However, efficiency is low, because β-cells suffer of anoikis, due to enzymatic digestion of the pancreatic ECM, which affects the survival of insulin-secreting β-cells. The strategy of adding ECM components during transplantation, to reproduce the pancreatic microenvironment, is a challenging task, as many of the regulatory mechanisms that control ECM deposition and turnover are not sufficiently understood. A better comprehension of the impact of the ECM on the adhesion and integrin-dependent signalling in β-cells is primordial to improve the healthy state of islets to prevent the onset of diabetes as well as for enhancing the efficiency of the islet transplantation therapy. [ABSTRACT FROM AUTHOR]

Kit ligand (KitL) and its tyrosine kinase receptor c-kit are critical for germ cells, melanocytes, mastocytes, and hematopoietic stem cells. Alternative splicing of KitL generates membrane-bound KitL (mb-KitL) or soluble KitL, providing survival or cell migration, respectively. Here we analyzed whether c-kit can function both as an adhesion and signaling receptor to mb-KitL presented by the environmental niche. At contacts between fibroblasts and MC/9 mast cells, mb-KitL, and c-kit formed ligand/receptor clusters that formed stable complexes, which resisted dissociation by c-kit blocking mAbs and provided cell anchorage under physiological shear stresses. Ousters recruited tyrosine-phosphorylated proteins and induced spatially restricted F-actin polymerization. Mutational analysis of c-kit demonstrated kinase-independent mb-KitL/c-kit clustering, anchorage, F-actin polymerization, and Tyr567-dependent cluster phosphorylation. Kinase inhibition of c-kit by imatinib reduced cluster coalescence, but allowed cluster phosphorylation and F-actin polymerization, which required PI3K recruitment and a newly identified juxtamembrane residue. Synergies between integrin and c-kit-mediated spreading and adhesion of MC/9 cells were studied in vitro on immobilized-KitL/ fibronectin surfaces. While c-kit blocking antibodies prevented spreading, imatinib blocked spreading induced by soluble- but not immobilized KitL. Thus, "mechanical" activation of c-kit provides signaling, niche-anchorage, and synergies with integrin-mediated adhesion, which is independent of kinase function and resistant to c-kit kinase inhibitors. [ABSTRACT FROM AUTHOR]

CD56bright NK cells express receptors for IL-2, IL-7, IL-15, and SCF. We found that human peripheral blood CD56bright NK cells responded to IL-2, IL-7, IL-15 by phosphorylating STAT-5, ERK, and Akt but did not respond to SCF. However, CD56bright NK cells in culture upregulated c-kit transcription three to fourfold, which led to a steady increase in c-kit and a concomitant acquisition of responsiveness to SCF. After 44 h, CD56bright NK cells had upregulated c-kit approximately 20-fold and phosphorylated ERK and Akt in response to SCF concentrations well below levels present in plasma. CD56bright NK cells cultured in IL-15 maintained c-kit transcription/expression at ex vivo levels and did not become responsive to SCF. Furthermore, SCF-responsive, CD56brightc-kithigh NK cells swiftly downregulated c-kit and stopped responding to SCF after IL-15 stimulation. However, commitment of CD56bright NK cells to a c-kit-negative, SCF-unresponsive state did not occur, as after 5 days of culture, withdrawal of IL-15 restored c-kit to maximal levels and reestablished SCF-responsiveness. CD56bright NK cells that had upregulated c-kit firmly adhered to COS cells transfected with the membrane form of SCF. Furthermore, SCF signaling significantly increased the capacity of CD56bright NK cells to degranulate. Collectively, our data suggest that c-kit on human CD56bright NK cells is a functional receptor that is downregulated in peripheral blood, possibly to render CD56bright NK cells unresponsive to the SCF therein. [ABSTRACT FROM AUTHOR]

Morphogenesis, as illustrated by melanocyte migration and homing to the skin, requires cadherin adhesion, integrin-dependent migration and Kit-ligand growth factor signaling. However, it is not known how Kit ligand regulates integrin or cadherin-dependent intraepidermal melanocyte behavior. To answer this question, we developed specific 2-dimensional (2D) and 3D culture systems analyzing how soluble or immobilized Kit-ligandregulated melanocyte migration on vitronectin and laminin, or within a monolayer of kidney epithelial cells. In a 2D system, soluble Kit ligand stimulated integrindependent melanoblast migration and chemotaxis and accelerated integrin turnover. In contrast, immobilized, but not soluble, Kit ligand, enhanced integrin-dependent melanocyte spreading on suboptimal laminin concentrations. In 3D, membrane-bound Kit ligand induced intraepithelial melanocyte proliferation, survival, and tight adhesion to epithelial cells, while cleavable Kit ligand was less effective. In contrast, melanocyte motility was independent of membrane-bound Kit ligand, but increased in the presence of the cleavable Kit-ligand isoform. Transmembrane- dimerization or basolateral-targeting mutants of Kit ligand altered intraepithelial melanocyte localization, survival, and adhesion to epithelial cells. These data and the identification of c-kit/Kit-ligand clusters at cell contacts suggest that membrane-bound Kit ligand captures cell surface-expressed c-kit, providing mechanical anchoring and survival signaling within intraepithelial niches, and thereby defining a new mechanism for melanocyte homeostasis and requirement for environmental niches. [ABSTRACT FROM AUTHOR]

From the onset of melanocyte specification from the neural crest, throughout their migration during embryogenesis and until they reside in their niche in the basal keratinocyte layer, melanocytes interact in dynamic ways with the extracellular environment of the growing embryo. To recognize and to adhere to their environment, melanocytes depend on heterodimeric cell surface receptors of the family of integrins. In addition to the control of adhesive interactions between melanocytes and the extracellular matrix scaffold secreted by fibroblasts and keratinocytes, the integrin receptors allow cells also to sense the mechanical condition of the extracellular environment, responding by intracellular signaling, triggering cell survival, proliferation or migration events. In this review, we summarize the recently emerged concepts that explain integrin-dependent adhesion and how this adhesion system interfaces with integrin-dependent signaling events. The gained information will help to understand melanocyte behavior in pathological situations such as melanoma growth and metastasis formation. [ABSTRACT FROM AUTHOR]

Kit-ligand (Kitl), also known as stem cell factor, is a membrane-anchored, noncovalently bound dimer signaling via the c-kit receptor tyrosine kinase, required for migration, survival, and proliferation of hematopoietic stem and germ cells, melanocytes, and mastocytes. Despite its fundamental role in morphogenesis and stem cell biology, the mechanisms that regulate Kitl dimerization are not well understood. By employing cell-permeable cross-linker and quantitative bimolecular fluorescence complementation of wild-type and truncated forms of Kitl, we determined that Kitl dimerization is initiated in the endoplasmic reticulum and mediated to similar levels by the transmembrane and the extracellular growth factor domain. Further biochemical and mutational analysis revealed a conserved Ser-Gly-Gly-Tyr-containing motif that is required for transmembrane domain dimerization and efficient cell-surface expression of Kitl. A novel intracellular capture assay with the Kitl transmembrane domain as bait revealed specific interactions with Kitl, but not with unrelated transmembrane proteins. During evolution, the transmembrane dimerization motif appeared in Kitl at the transition from teleosts to tetrapods, which correlates with the emergence of Kitl as a supporter of stem cell populations. Thus, transmembrane-mediated association of membrane-anchored growth factors consists of a novel mechanism to improve paracrine signaling and morphogenesis. [ABSTRACT FROM AUTHOR]

Synaptotagmins (SYT) are calcium-binding proteins that participate in regulated exocytosis. Although SYTI to IX isoforms are expressed in insulin-producing cell lines, hitherto only SYTIX has been associated with native β-cell insulin granules and implicated in exocytosis. SYTVII was also proposed to regulate insulin exocytosis, but its subcellular location and number of alternative splice variants produced remain controversial. Only transcripts of SYTVII α, β, and a novel splice variant 8 are expressed in β-cells and INS-1E cells. Western blotting revealed that INS-1E cells predominantly produced SYTVII α and low levels of SYTVII β, whereas SYTVII δ was undectable. The protein colocalized with insulin granules but not with synaptic-like microvesicles. Overexpression of SYTVII α resulted in decreased insulin granule content with a concomitant translocation of the variant to the plasma membrane, while SYTVII β retained largely a granular pattern. Overexpressed SYTVII δ exhibited a distribution different to that of insulin granules and inhibited exocytosis when assessed by whole cell patch clamp capacitance recording. Silencing of SYTVII α by targeted RNA interference suppressed secretion, while repression of β slightly increased release. Our results demonstrate that SYTVII is expressed on insulin granides and that only SYTVII α is implicated in exocytosis under physiological conditions. [ABSTRACT FROM AUTHOR]

The study of free-living amoebae has proven valuable to explain the molecular mechanisms controlling phagocytosis, cell adhesion and motility. In this study, we identified a new adhesion molecule in Dictyostelium amoebae. The SibA (Similar to Integrin Beta) protein is a type I transmembrane protein, and its cytosolic, transmembrane and extracellular domains contain features also found in integrin β chains. In addition, the conserved cytosolic domain of SibA interacts with talin, a well-characterized partner of mammalian integrins. Finally, genetic inactivation of SIBA affects adhesion to phagocytic particles, as well as cell adhesion and spreading on its substrate. It does not visibly alter the organization of the actin cytoskeleton, cellular migration or multicellular development. Our results indicate that the SibA protein is a Dictyostelium cell adhesion molecule presenting structural and functional similarities to metazoan integrin β chains. This study sheds light on the molecular mechanisms controlling cell adhesion and their establishment during evolution. [ABSTRACT FROM AUTHOR]

Kit-ligand (Kitl) also known as steel factor, stem cell factor and mast cell growth factor plays a crucial role in the development and maintenance of the melanocyte lineage in adult skin. Kitl exerts permanent survival, proliferation and migration functions in Kit receptor-expressing melanocytes. A comprehensive overview over the differential roles of Kitl in melanocyte development and homeostasis is provided. I discuss species-specific differences of the Kitl/Kit signalling system, regulation at the transcriptional level and also covering the regulation of cell surface Kitl presentation by cytoplasmic targeting sequences. In addition, recent studies evoked the importance of Kitl misexpression in some hyperpigmented lesions that may open the avenue for Kitl-dependent treatment of pathological skin conditions. [ABSTRACT FROM AUTHOR]

Reviews the regulation of neural crest cell migration patterns in avian and mammalian embryos based on the expression and activity of receptor tyrosine kinases during embryogenesis. Differentiation of neural crest derivatives; Mechanisms of specific pathway choices; Localization of growth factor ligand activity in the embryo.