Novel Role for Calcium Channel β 3 Subunit in Insulin Secretion

Abstract

The accessory β subunits of voltage-gated calcium channels regulate various aspects of channel activity, from delivery to the plasma membrane to the biophysical properties of the channel. Berggren et al. show that the β 3 subunit of the L-type calcium channel serves to negatively regulate insulin secretion and that β 3 —/— mice had increased insulin secretion and more efficient glucose uptake than wild-type mice. Surprisingly, the biophysical properties and activity of the L-type Ca 2+ channel in the knockout and wild-type pancreatic β cells were identical. Measurement of the changes in intracellular calcium concentration ([Ca 2+ ] i ) in response to high glucose indicated that a higher proportion of the knockout cells exhibited calcium oscillations and that the oscillations were a higher frequency than those observed in wild-type cells. Indeed repetitive depolarizations produced calcium oscillations and an increase in exocytotic activity (based on an increase in membrane capacitance), whereas a single-step depolarization produced a nonoscillating increase in [Ca 2+ ] i and a smaller increase in exocytotic activity. The calcium oscillations seemed to be due to the release of calcium from internal stores, because pharmacological inhibition of the endoplasmic reticulum (ER) calcium pump or the inositol trisphosphate receptor (IP 3 R) altered the calcium signal. Islets from the knockout mice showed an increase in [Ca 2+ ] i in response to stimulation of muscarinic acetylcholine receptors with carbachol, which couples to IP 3 through the activation of phospholipase C. When the β 3 subunit was transfected into cells that do not express the L-type calcium channel, IP 3 -induced increase in [Ca 2+ ] i was decreased. Confocal microscopy of pancreatic β cells indicated that the β 3 subunit was present on the ER adjacent to the IP 3 R, but the two proteins could not be coimmunoprecipitated; thus, it is not clear whether the regulation of IP 3 -stimulated calcium release by the β subunit is mediated by a direct interaction. Islets from the knockout mice showed increased IP 3 production in response to carbachol. Thus, the β subunit appears to be a negative regulator of calcium signaling in pancreatic β cells, and the mechanism may not involve the L-type calcium channel but rather may be mediated through regulation of release of calcium from internal stores. Although the exact mechanism remains to be determined, the β subunit may be an attractive target for increasing insulin release in diabetes. P. O. Berggren, S. N. Yang, M. Murakami, A. M. Efanov, S. Uhles, M. Kohler, T. Moede, A. Fernstrom, I. B. Appelskog, C. A. Aspinwall, S. V. Zaitsev, O. Larsson, L. M. de Vargas, C. Fecher-Trost, P. Weissgerber, A. Ludwig, B. Leibiger, L. Juntti-Berggren, C. J. Barker, J. Gromada, M. Freichel, I. B. Leibiger, V. Flockerzi, Removal of Ca 2+ channel β 3 subunit enhances Ca 2+ oscillation frequency and insulin exocytosis. Cell 119 , 273-284 (2004). [Online Journal]