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Chromogranins (Cgs) are the major soluble proteins of dense-core secretory vesicles. Chromaffin cells fromChganull mice [chromogranin A knock-out (CgA-KO)] exhibited ∼30% reduction in the content and in the release of catecholamines compared with wild type. This was because of a lower secretion per single exocytotic event, rather than to a lower frequency of exocytotic events. Cell incubation withl-DOPA produced an increase in the vesicular amine content of wild-type, but not CgA-KO vesicles. In contrast, intracellular electrochemistry showed thatl-DOPA produced a significantly larger increase in cytosolic amines in CgA-KO cells than in the wild type. These data indicate that the mechanisms for vesicular accumulation in CgA-KO cells were fully saturated. Patch-amperometry recordings showed a delayed initiation of the amperometric signal after vesicle fusion, whereas no changes were observed in vesicle size or fusion pore kinetics despite the smaller amine content. We conclude that intravesicular proteins are highly efficient systems directly implicated in transmitter accumulation and in the control of neurosecretion.

Receptor Tyrosine Kinases (RTKs) and Focal Adhesion Kinase (FAK) regulate multiple signalling pathways, including mitogen-activated protein (MAP) kinase pathway. FAK interacts with several RTKs but little is known about how FAK regulates their downstream signalling. Here we investigated how FAK regulates signalling resulting from the overexpression of the RTKs RET and EGFR. FAK suppressed RTKs signalling in Drosophila melanogaster epithelia by impairing MAPK pathway. This regulation was also observed in MDA-MB-231 human breast cancer cells, suggesting it is a conserved phenomenon in humans. Mechanistically, FAK reduced receptor recycling into the plasma membrane, which resulted in lower MAPK activation. Conversely, increasing the membrane pool of the receptor increased MAPK pathway signalling. FAK is widely considered as a therapeutic target in cancer biology; however, it also has tumour suppressor properties in some contexts. Therefore, the FAK-mediated negative regulation of RTK/MAPK signalling described here may have potential implications in the designing of therapy strategies for RTK-driven tumours., Author Summary Due to their deregulation in cancer and their potential to be inhibited by small chemical compounds, tyrosine kinases are among the most important targets under consideration for cancer therapeutics. One such oncogenic tyrosine kinase is FAK, which is known to regulate cellular signalling downstream of Integrins and Receptor Tyrosine Kinases (RTK) at the cell surface. In this study, however, we report that FAK can act as a suppressor of oncogenic Receptor Tyrosine Kinases. This mechanism was observed in fruit fly tissues in vivo and human cancer-derived cells in vitro, which additionally suggests it is an evolutionary conserved mechanism in humans. FAK mediated this inhibition by controlling the sub-cellular localisation of receptors, via suppression of receptor recycling to the cell surface. These results suggest that in some particular cancer contexts such as RTK-driven tumours, FAK may act as a tumour suppressor and therefore, may not be a valid drug target.