Rab3a ablation related changes in morphology of secretory vesicles in major endocrine pancreatic cells, pituitary melanotroph cells and adrenal gland chromaffin cells in mice

In this work we have compared the ultrastructural characteristics of major pancreatic endocrine cells, pituitary melanotrophs and adrenal chromaffin cells in the normal mouse strain (wild type, WT) and mice with a known secretory deficit, the Rab3a knockout strain (Rab3a KO). For this purpose, pancreata, pituitary glands and adrenal glands from the Rab3a KO and from the WT mice were analysed, using conventional transmission electron microscopy (TEM). In order to assess the significance of the presence of Rab3a proteins in the relevant cells, we focused primarily on their secretory vesicle morphology and distribution. Our results showed a comparable general morphology in Rab3a KO and WT in all assessed endocrine cell types. In all studied cell types, the distribution of secretory granules along the plasma membrane (number of docked and almost-docked vesicles) was comparable between Rab3a KO and WT mice. Specific differences were found in the diameters of their secretory vesicles, diameters of their electron-dense cores and the presence of autophagic structures in the cells of Rab3A KO mice only. Occasionally, individual electron-dense round vesicles were present inside autophagosome-like structures; these were possibly secretory vesicles or their remnants. The differences found in the diameters of the secretory vesicles confirm the key role of Rab3a proteins in controlling the balance between secretory vesicle biogenesis and degradation, and suggest that the ablation of this protein probably changes the nature of the reservoir of secretory vesicles available for regulated exocytosis.

► General morphology in Rab3a KO and WT endocrine cell types was comparable.
► Significant differences were found in the diameters of their secretory vesicles and electron-dense cores.
► The electron-dense cores of the secretory granules from Rab3a KO mice are significantly bigger than those from WT mice.
► In all cell types, the number of docked granules in Rab3a KO mice and WT mice was comparable.
► Autophagosome-like structures could contribute to maintaining cell homeostasis.