Myelin basic protein induces morphological changes in the endocrine pancreas

To evaluate effects of circulating myelin basic protein (MBP) on the endocrine pancreas, we injected bovine MBP to Djungarian hamsters and studied the morphological changes induced by MBP and its immunocytochemical distribution by electron microscopy. After a treatment time of 5-40 min, some islets appeared severely damaged, especially at their peripheries and near the intraislet capillaries, while others showed minor or no changes. MBP-induced extracellular changes included partial disintegration of the collagen filament network surrounding the islet and the blood vessels. These changes correlated with the association of MBP with the collagen filament bundles and related structures. Intracellularly, the effect of MBP included formation of vacuoles, dilatation of rough ER and Golgi membranes, swelling and aggregation of mitochondria, and disruption of the membranes of part of the insulin and glucagon granules, as well as damage to some plasma membranes. In the damaged B cells, 16-62% of the insulin granules exhibited an enlarged pale core, compared to 1-2% in the control B cells. MBP was shown to associate with mitochondria and with various intracellular membranes in all islet cells. In the B cells, MBP was localized to the membranes of insulin granules, and it also associated with the cores of the granules. In the A cells, the association of MBP to the glucagon granules was mainly with the outsides of the membranes. Interaction of MBP with the secretion granules is suggested to play a role in MBP-induced insulin and glucagon release, and some hormone might be released by leakage. Association of MBP with mitochondria, Golgi structures, and ER may lead to changes in various cellular functions.