Loss of cell-cell and cell-substrate contacts in single pancreatic β-cells divert insulin release to intracellular vesicular compartments.

Cell-cell or cell-substrate interactions are lost when cells are dissociated in culture, or during pathophysiological breakdowns, therefore impairing their structure and polarity, and affecting their function. We show that single rat β-cells, cultured under non-adhesive conditions, form intracytoplasmic vacuoles increasing in number and size over time. We characterized these structures and their implication in β-cell function. Ultrastructurally, the vacuoles resemble vesicular apical compartments and are delimited by a membrane, containing microvilli and expressing markers of the plasma membrane, including glucose transporter 2 and actin. When insulin secretion is stimulated, insulin accumulates in the lumen of the vacuoles. By contrast, when the cells are incubated under low calcium levels, the hormone is undetectable in vesicular compartments. Insulin release studies from single cells revealed that vacuole-containing cells release less insulin as compared to control cells. When added to the medium, a non-permeant fluid phase marker becomes trapped within vacuoles. Inhibition of vesicular trafficking and exocytosis as well as dynamin-dependent endocytosis changed the percentage of vacuole-containing cells, suggesting that both endocytic and exocytic track contribute to their formation. These results suggest that loss of cell-cell and cell-substrate contacts in isolated β-cells affect normal vesicular trafficking and redirects insulin secretion to intracellular vesicular compartments. Our study reveals for the first time that single β-cells develop vacuolar compartments when cultured in suspension and redirect their insulin secretion to these vacuoles. This may underlie a compensatory process for cultured cells who lost their interactions with adhesive substrates or neighbouring cells.