Mitochondrial signals in glucose-stimulated insulin secretion in the beta cell

Abstract

Glucose-induced insulin secretion is determined by signals generated in the mitochondria. The elevation of ATP is necessary for the membrane-dependent increase in cytosolic Ca2+, the main trigger of insulin exocytosis. Beta cells depleted of mitochondrial DNA fail to respond to glucose while still secreting insulin in response to membrane depolarisation. This cell model resembles the situation of defective insulin secretion in patients with mitochondrial diabetes. On the other hand, infants with activating mutations in the mitochondrial enzyme glutamate dehydrogenase are characterised by hyperinsulinism and hypoglycaemia. We have recently proposed that glutamate, generated by this enzyme, participates in insulin secretion as a glucose-derived metabolic messenger. In this model, glutamate acts downstream of the mitochondria by sensitising the exocytotic process to Ca2+. The evidence in favour of such a role for glutamate is discussed in the present review.

Figure 1. Plasma membrane potential and mitochondrial membrane potential (ΔΨ_m) in INS-1 and INS-1 ρ° cells

Suspensions of cells were loaded either with the fluorescent probe bisoxonol to monitor plasma membrane potential in INS-1 (A and E) or INS-1 ρ° cells (B and F) or with Rh123 to monitor ΔΨ_m in INS-1 (C and G) or INS-1 ρ° cells (D and H). met-Suc, methyl-succinate. Reproduced from Kennedy et al. (1998), with permission.

Figure 2. Effects of the TCA cycle intermediate succinate on [Ca²⁺]_m and insulin secretion in permeabilised INS-1 cells

Cells expressing the Ca²⁺-sensitive photoprotein aequorin targeted to the mitochondria were permeabilised with α-toxin and perifused with an intracellular buffer containing 500 nM free Ca²⁺ and high ATP (10 mm). The effects of succinate (Suc) on [Ca²⁺]_m (A) and insulin secretion (B) were measured simultaneously. Influx of Ca²⁺ into the mitochondria via the uniporter was blocked using Ruthenium Red (RR) before the addition of Suc during the measurement of [Ca²⁺]_m (C) and insulin secretion (D). Modified with permission from Maechler et al. (1997).

Figure 3. Effects of oligomycin on insulin secretion in permeabilised INS-1 cells

Cells were permeabilised with α-toxin and perifused with an intracellular buffer at 500 nM [Ca²⁺]_c and 1 mm ATP. Oligomycin (1 μg ml⁻¹) suppresses the effect of succinate (1 mm, A) but not of glutamate (1 mm, B) on insulin secretion. Conditions as in Maechler & Wollheim (1999); the traces are representative of three independent experiments.

Figure 4. Proposed model for coupling of glucose metabolism to insulin secretion in the beta cell

Glycolysis converts glucose to pyruvate (Pyr), which enters the mitochondrion and feeds the TCA cycle, resulting in the transfer of reducing equivalents (red.equ.) to the electron (e⁻) transport chain, hyperpolarisation of ΔΨ_m and generation of ATP. Subsequently, closure of K_ATP channels depolarises the plasma membrane which opens voltage-sensitive Ca²⁺ channels, raising [Ca²⁺]_c and triggering insulin exocytosis. Concomitantly, under conditions of elevated [Ca²⁺]_c, hyperpolarised ΔΨ_m promotes [Ca²⁺]_m rise, further activating the TCA cycle. Glutamate is then formed from α-ketoglutarate (αKG) by glutamate dehydrogenase (GDH). Glutamate uptake by granules would participate in the second phase of insulin secretion. Reproduced from Maechler & Wollheim (1999), with permission.