Three-dimensional FIB-SEM reconstruction of microtubule-organelle interaction in whole primary mouse beta cells

Microtubules play a major role in intracellular trafficking of cargo vesicles in endocrine cells and detailed knowledge of the microtubule network organization and its relation to other cell constituents is crucial for understanding primary cell function. However, their role in insulin transport and secretion is currently under debate. Here, we use FO_SCPLOWIBC_SCPLOW-SO_SCPLOWEMC_SCPLOW to image insulin secreting pancreatic islet beta cells in their entirety at an unprecedented resolution. We reconstruct all mitochondria, Golgi apparati, centrioles, insulin secretory granules and microtubules of seven beta cells, and generate a comprehensive spatial map of microtubule-organelle interactions. We find that microtubules form non-radial networks that are predominantly not connected to either centrioles or endomembranes. Moreover, microtubule number and length, but not microtubule density, vary with glucose stimulation. Furthermore, we show that insulin secretory granules are enriched near the plasma membrane where they associate with microtubules. In summary, we provide the first three-dimensional reconstructions of complete microtubule networks in primary mammalian cells together with evidence regarding their importance for insulin secretory granule positioning and thus supportive role in insulin secretion.