Pancreatic β‐cell Na⁺channels control global Ca²⁺signaling and oxidative metabolism by inducing Na⁺and Ca²⁺responses that are propagated into mitochondria

Abstract

Communication between the plasma membrane and mitochondria is essential for initiating the Ca(2+) and metabolic signals required for secretion in β cells. Although voltage-dependent Na(+) channels are abundantly expressed in β cells and activated by glucose, their role in communicating with mitochondria is unresolved. Here, we combined fluorescent Na(+), Ca(2+), and ATP imaging, electrophysiological analysis with tetrodotoxin (TTX)-dependent block of the Na(+) channel, and molecular manipulation of mitochondrial Ca(2+) transporters to study the communication between Na(+) channels and mitochondria. We show that TTX inhibits glucose-dependent depolarization and blocks cytosolic Na(+) and Ca(2+) responses and their propagation into mitochondria. TTX-sensitive mitochondrial Ca(2+) influx was largely blocked by knockdown of the mitochondrial Ca(2+) uniporter (MCU) expression. Knockdown of the mitochondrial Na(+)/Ca(2+) exchanger (NCLX) and Na(+) dose response analysis demonstrated that NCLX mediates the mitochondrial Na(+) influx and is tuned to sense the TTX-sensitive cytosolic Na(+) responses. Finally, TTX blocked glucose-dependent mitochondrial Ca(2+) rise, mitochondrial metabolic activity, and ATP production. Our results show that communication of the Na(+) channels with mitochondria shape both global Ca(2+) and metabolism signals linked to insulin secretion in β cells.- Nita, I. I., Hershfinkel, M., Kantor, C., Rutter, G. A., Lewis, E. C., Sekler, I. Pancreatic β-cell Na(+) channels control global Ca(2+) signaling and oxidative metabolism by inducing Na(+) and Ca(2+) responses that are propagated into mitochondria.

Modulation of Ionic Channels and Insulin Secretion by Drugs and Hormones in Pancreatic Beta Cells

“…This idea is supported by whole cell recordings of beta cells in slices from rat pancreas, where concentrations of ATP (2-8 mM) in the recording pipette shift the inactivation curve to more positive potentials and accelerate the recovery from inactivation of VGSC (Zou et al, 2013). In the other direction, the TTXsensitive depolarizing Na 1 influx contributes to the cytosolic Na 1 transients (Nita et al, 2014), which, together with Ca 21 transients, propagate to the mitochondria and modulate ATP synthesis. Thus, a glucose-induced uptake of Na 1 through the mitochondrial Na 1 /Ca 21 exchanger promotes Ca 21 extrusion and reverses the divalent cation uptake by the mitochondrial Ca 21 uniporter (Nita et al, 2014).…”

“…In the other direction, the TTXsensitive depolarizing Na 1 influx contributes to the cytosolic Na 1 transients (Nita et al, 2014), which, together with Ca 21 transients, propagate to the mitochondria and modulate ATP synthesis. Thus, a glucose-induced uptake of Na 1 through the mitochondrial Na 1 /Ca 21 exchanger promotes Ca 21 extrusion and reverses the divalent cation uptake by the mitochondrial Ca 21 uniporter (Nita et al, 2014). After four decades of study on VGSC in pancreatic beta cells, we have advanced in the understanding of how Na 1 currents contribute to cell depolarization, voltage and Ca 21 oscillations in beta cells, and, finally, in the promotion of glucose-induced insulin secretion (Fig.…”

The Pancreatic β-Cell: A Bioenergetic Perspective

“…The INS-1 832/13 cell is notable for the ion channels that are apparently not required. While the cells possess tetrodotoxin-sensitive Na ϩ channels, the toxin is virtually without effect on the oscillation parameters; this contrasts with MIN6 cells (389). Likewise, oscillations continue in the combined presence of 4-aminopyridine and tetraethylammonium (175) that between them inhibit Ͼ90% of the K ATP -independent conductance of INS-1 cells (498).…”

Standing of giants shoulders the story of the mitochondrial Na+Ca2+ exchanger

“…By controlling the activity of NCLX, it paces the duration of the mitochondrial Ca 2þ transients. It further controls the duration and magnitude of cytosolic and mitochondrial Na þ transients [23]. This latter aspect lags behind the current focus on Ca 2þ signaling, primarily because of the inferior quality of Na þ reporters but is of major importance and warrants many more studies.…”