Vazquez-Martinez Rafael, Cruz-Garcia David, Duran-Prado Mario, Peinado Juan R, Castaño Justo P, Malagon Maria M
Department of Cell Biology, Physiology and Immunology, University of Cordoba, 14014 Cordoba, Spain.
Traffic. 2007 Jul;8(7):867-82. doi: 10.1111/j.1600-0854.2007.00570.x. Epub 2007 May 4.
Rab proteins comprise a complex family of small GTPases involved in the regulation of intracellular membrane trafficking and reorganization. In this study, we identified Rab18 as a new inhibitory player of the secretory pathway in neuroendocrine cells. In adrenal chromaffin PC12 cells and pituitary AtT20 cells, Rab18 is located at the cytosol but associates with a subpopulation of secretory granules after stimulation of the regulated secretory pathway, strongly suggesting that induction of secretion provokes Rab18 activation and recruitment to these organelles. In support of this, a dominant-inactive Rab18 mutant was found to distribute diffusely in the cytosol, whereas a dominant-active Rab18 mutant was predominantly associated to secretory granules. Furthermore, interaction of Rab18 with secretory granules was associated to an inhibition in the secretory activity of PC12 and AtT20 cells in response to stimulatory challenges. Association of Rab18 with secretory granules was also observed by immunoelectron microscopy in normal, non-tumoral endocrine cells (pituitary melanotropes), wherein Rab18 protein content is inversely correlated to the level of secretory activity of cells. Taken together, these findings suggest that, in neuroendocrine cells, Rab18 acts as a negative regulator of secretory activity, likely by impairing secretory granule transport.
Rab蛋白构成了一个复杂的小GTP酶家族，参与细胞内膜运输和重组的调控。在本研究中，我们鉴定出Rab18是神经内分泌细胞分泌途径中的一个新的抑制因子。在肾上腺嗜铬PC12细胞和垂体AtT20细胞中，Rab18位于胞质溶胶中，但在调节性分泌途径受到刺激后，它与一部分分泌颗粒相关联，这强烈表明分泌的诱导会引发Rab18的激活并使其募集到这些细胞器中。支持这一观点的是，发现一个显性失活的Rab18突变体在胞质溶胶中呈弥散分布，而一个显性激活的Rab18突变体主要与分泌颗粒相关联。此外，Rab18与分泌颗粒的相互作用与PC12和AtT20细胞在受到刺激挑战时的分泌活性受到抑制有关。在正常的非肿瘤内分泌细胞（垂体促黑素细胞）中，通过免疫电子显微镜也观察到Rab18与分泌颗粒的关联，其中Rab18蛋白含量与细胞的分泌活性水平呈负相关。综上所述，这些发现表明，在神经内分泌细胞中，Rab18可能通过损害分泌颗粒运输而作为分泌活性的负调节因子。
