Insulin Receptors and Intracellular Ca²⁺ Form a Double-Negative Regulatory Feedback Loop Controlling Insulin Sensitivity

Introduction

Since the discovery of insulin¹ and insulin receptors (IR)² in the brain in 1978, numerous studies have revealed a fundamental role of IR in the central nervous system (CNS). IR-mediated signaling is implicated in the regulation of diverse functions in the CNS, including synaptic plasticity, long-term potentiation and depression, neuroprotection, learning and memory, and energy balance³. Central insulin resistance has been found in neurodegenerative diseases such as Alzheimer’s disease (AD)^4,5 and Parkinson’s disease (PD)⁶, stroke, and traumatic brain injury (TBI)⁷. Impaired insulin signaling in AD is evident in the activation states of IR and downstream signaling molecules⁵. Compared with control cases, insulin in AD brains induced 24–58% less activation at the level of IR and 90% less activation of insulin receptor substrate 1 (IRS-1)⁵. It has been presumed⁵ that the inhibition of IR activation is mediated by Aβ oligomer-triggered Ca²⁺ influx, in part by activating N-methyl-D-aspartate receptors (NMDARs)⁸, followed by a rise in Akt1 pS^{473 9}, which can inhibit insulin-induced IR activation through Thr phosphorylation of the IR β subunit¹⁰. Aβ oligomers may activate the NMDAR-gated Ca²⁺ influx directly¹¹ or indirectly through the intermediate release of glutamate, a ligand of NMDAR^11–15. This suggests that the rise in intracellular free Ca²⁺ concentration ([Ca²⁺]_i), evoked by either Aβ oligomers or glutamate, leads to dysfunctional activation of IR in AD. In the present opinion article, we highlight evidence that IR and [Ca²⁺]_i form a double-negative regulatory feedback loop controlling insulin sensitivity, and mitochondria have a key role in this feedback loop, being involved in adenosine triphosphate (ATP) synthesis and IR activation.

Glutamate-evoked rise in [Ca²⁺]_i causes inhibition of IR signaling

Glutamate serves as the major excitatory neurotransmitter in the CNS. Its excessive accumulation in a synaptic cleft can trigger excitotoxicity, a pathologic process leading to neuronal cell death. Glutamate-induced activation of the NMDAR-gated Ca²⁺ influx is generally considered central to the development of excitotoxicity¹⁶. Prolonged glutamate exposure causes a rapid initial increase in the [Ca²⁺]_i, followed by a larger secondary [Ca²⁺]_i increase concomitant with a decrease in the mitochondrial inner membrane potential (ΔΨ_m)^17–19. We recently found that on Ca²⁺-induced mitochondrial depolarization, insulin induced 48% less activation of IR (assessed by pY^1150/1151) compared with control²⁰. Earlier, we showed that a decrease in ΔΨ_m can abrogate IR activation¹⁸, since the ΔΨ_m-dependent mitochondrial signal at complex II is involved in the activation of IR in neurons^21–23. Thus, the glutamate-evoked increase in [Ca²⁺]_i, followed by the drop in ΔΨ_m, leads to the inhibition of insulin-induced activation of IR.

Insulin prevents glutamate-evoked rise in [Ca²⁺]_i

Normally, the NMDAR-gated Ca²⁺ influx is counterbalanced with Ca²⁺ efflux, which is governed by plasma membrane Ca²⁺ ATPase and the Na⁺/Ca²⁺ exchanger (NCX)^24,25. NCX-mediated Ca²⁺ efflux is also ATP-dependent, since NCX exchanges one Ca²⁺ for three Na⁺, and the three Na⁺ are then pumped out by the Na⁺/K⁺ ATPase at the expense of one ATP. In excitotoxicity, prolonged stimulation with glutamate leads to ATP depletion and an abnormal rise in [Ca²⁺]_i, since the massive Ca²⁺ influx is no longer counterbalanced by Ca²⁺ efflux²⁶. Therefore, maintenance of ATP production is crucial for preventing the rise in [Ca²⁺]_i in excitotoxicity. We found recently that pre-treatment with insulin prevents neurons from glutamate-evoked ATP depletion due to its protective effect on spare respiratory capacity (SRC), a measure that relates to the amount of extra ATP that can be produced via oxidative phosphorylation in case of increased energy demand¹⁹. The effect of insulin on SRC relates to its action on mitochondrial metabolism. It has long been known that the tricarboxylic acid cycle is the intracellular site of insulin action and that insulin acutely stimulates succinate oxidation at mitochondrial complex II^26,27. Succinate oxidation at mitochondrial complex II has been identified recently as the main source of SRC²⁸. In line with this, insulin prevented the glutamate-evoked rise in [Ca²⁺]_i in our experiments with glutamate excitotoxicity¹⁹.

IR and [Ca²⁺]_i form a double-negative feedback loop controlling insulin sensitivity

Collectively, this evidence suggests that a double-negative regulatory feedback loop exists between IR and [Ca²⁺]_i. The glutamate-evoked rise in [Ca²⁺]_i inhibits activation of IR and, vice versa, insulin-induced activation of IR inhibits the glutamate-evoked rise in [Ca²⁺]_i (Figure 1a).

Figure 1. A double-negative feedback loop between insulin receptors (IR) and intracellular free Ca²⁺ concentration [Ca²⁺]_i generates bistability.

(A) glutamate triggers NMDA receptor–gated Ca²⁺ influx, inhibiting IR activation, and insulin triggers activation of IR, inhibiting [Ca²⁺]_i rise; (B) glutamate-triggered stable steady state with high [Ca²⁺]_i and nonactive IR (pY-IR↓); (C) insulin-triggered stable steady state with active IR (pY-IR↑) and low [Ca²⁺]_i.

In theory, a double-negative feedback loop generates bistability²⁹. Thus, a stable steady state could exist with high [Ca²⁺]_i and nonactive IR (Figure 1b), or with active IR and low [Ca²⁺]_i (Figure 1c), but no stable steady state is possible with both high [Ca²⁺]_i and active IR. Such a circuit could toggle between a high [Ca²⁺]_i state and an active IR state in response to glutamate and insulin, respectively.

This double-negative feedback loop model predicts that any condition leading to an increase in [Ca²⁺]_i may trigger insulin resistance. It appears to explain why central insulin resistance is implicated in the pathogenesis of disorders such as AD^4,5, PD⁶, stroke, and TBI⁷, with which glutamate excitotoxicity is a comorbid condition³⁰. The model also predicts that any intervention aiming to prevent Ca²⁺ influx of or enhance efflux of Ca²⁺ from neurons, thereby maintaining low [Ca²⁺]_i, may be useful for treating central insulin resistance. Given that Ca²⁺ efflux is ATP-dependent, any intervention directed to enhance ATP production in neurons may be especially useful to improve insulin sensitivity in the brain.

Data availability

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