1772-P: Insulin Granule Biogenesis at the Golgi Requires a Metabolically-Regulated, Non-centrosomal Subpopulation of Microtubules

Insulin undergoes biogenesis through the ER and Golgi, ultimately leading to packaging into vesicles that are secreted in response to glucose. The movement between these membranes and the process of budding from the Golgi are vital steps. Previous work has found a role for microtubules (MTs) in these processes in both pancreatic β-cells and other cell types. Here, we show that the last stages of this process are regulated by a specific subpopulation of MTs that are non-centrosomal. Their formation, or nucleation, at the Golgi membrane is regulated by a glucose signal-transduction pathway through cAMP and its effector EPAC2. This effect is especially pronounced during the first, rapid phase of insulin secretion, during which EPAC2 levels increase at the sites of their nucleation. Preventing new nucleation of this specific population dramatically affects the pipeline of insulin production, storage and release. There is an overall reduction of β-cell insulin content, and the remaining insulin becomes retained within the Golgi, likely because of stalling of insulin-granule budding. This diminished granule availability substantially effects the level of both basal and stimulated insulin secretion, though not the ratio between these levels during the first wave. Constant dynamic maintenance of this network is therefore critical for normal β-cell physiology. This is the first demonstration that the biogenesis of post-Golgi carriers, particularly large secretory granules, requires ongoing nucleation/replenishment of this network. We are further looking at the role of MTs in β-cell secretory activity in intact islets. We have observed that MTs promote β-cell heterogeneity by restricting secretion from a specific β-cell sub-population, where insulin release can be activated upon disruption of the entire MT network by the drug nocodazole.