Calcium elevation in mouse pancreatic beta cells evoked by extracellular human islet amyloid polypeptide involves activation of the mechanosensitive ion channel TRPV4

To investigate the mechanism by which human islet amyloid polypeptide (hIAPP) fibril formation results in calcium influx across the plasma membrane of pancreatic beta cells, and its association with apoptosis. Cytoplasmic intracellular calcium concentrations ([Ca2+](i)) were monitored for 2 h as the 340/380 nm fluorescence ratio in fura-2 loaded cells of the MIN6 mouse pancreatic beta cell line. Cell morphology was evaluated by transmission electron microscopy, and viability by FACS. hIAPP (10 mu mol/l) increased [Ca2+](i) in 21% of MIN6 cells in standard buffer, and in 8% of cells in Na+-free buffer. Transient receptor potential (TRP) channel inhibitors (gadolinium and ruthenium red) prevented the [Ca2+](i) rise under both conditions, whilst nifedipine was only effective in the presence of Na+. hIAPP increased apoptosis in both insulinoma cells and islets in primary culture, and cell viability was partially rescued by ruthenium red (p<0.001). By RT-PCR, we detected expression of the mechanosensitive TRP cation channel subfamily V member 4 (Trpv4) in MIN6 cells and mouse pancreas. Small interference RNA against Trpv4 prevented hIAPP-induced [Ca2+](i) rises, decreased hIAPP-triggered expression of the endoplasmic reticulum (ER) stress response, and reduced hIAPP-triggered cell death by 50% (p<0.05). Alterations in [Ca2+](i) play a key role in hIAPP-induced beta cell cytotoxicity. By electron microscopy, we detected extracellular hIAPP aggregates adjacent to irregular invaginated regions of the plasma membrane. We propose that TRPV4 channels may sense physical changes in the plasma membrane induced by hIAPP aggregation, enabling Ca2+ entry, membrane depolarisation and activation of L-type Ca2+ channels. Decreasing the activity of TRPV4 prevented hIAPP-induced [Ca2+](i) changes, reduced hIAPP-triggered ER stress and improved cell viability.