Mitochondrial calcium cycling in neuronal function and neurodegeneration

Abstract

Mitochondria are essential for proper cellular function through their critical roles in ATP synthesis, reactive oxygen species production, calcium (Ca²⁺) buffering, and apoptotic signaling. In neurons, Ca²⁺ buffering is particularly important as it helps to shape Ca²⁺ signals and to regulate numerous Ca²⁺-dependent functions including neuronal excitability, synaptic transmission, gene expression, and neuronal toxicity. Over the past decade, identification of the mitochondrial Ca²⁺ uniporter (MCU) and other molecular components of mitochondrial Ca²⁺ transport has provided insight into the roles that mitochondrial Ca²⁺ regulation plays in neuronal function in health and disease. In this review, we discuss the many roles of mitochondrial Ca²⁺ uptake and release mechanisms in normal neuronal function and highlight new insights into the Ca²⁺-dependent mechanisms that drive mitochondrial dysfunction in neurologic diseases including epilepsy, Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. We also consider how targeting Ca²⁺ uptake and release mechanisms could facilitate the development of novel therapeutic strategies for neurological diseases.

Keywords: MCU; calcium; mitochondria; neurodegeneration; neuronal calcium homeostasis.

FIGURE 1

Summary of the Ca²⁺ signaling mechanisms in neurons that regulate cytosolic and mitochondrial Ca²⁺ concentrations. Ca²⁺ enters the cell through voltage gated Ca²⁺ channels (VGCC) following depolarization or through glutamate NMDA receptors. As the cytosolic Ca²⁺ concentration increases, the MCU complex opens and Ca²⁺ enters the mitochondria. Ca²⁺ is rapidly removed from the cytosol via Na⁺/Ca²⁺ exchangers (NCX) and plasma membrane Ca²⁺ ATPases (PMCA). As the cytosolic Ca²⁺ concentration decreases Ca²⁺ is slowly released from the mitochondria through Na⁺/Ca²⁺ (mtNCX) exchangers and H⁺/Ca²⁺ exchangers (mtHCX). Proper Ca²⁺ buffering is critical for normal neuronal functions and neuronal plasticity. Impaired buffering and Ca²⁺ overload leads to the opening of the mPTP and activation of apoptotic pathways and is implicated in many neurological diseases. For simplicity, the ER and other Ca²⁺ organelles are omitted from the model.

FIGURE 2

The structure of the greater mitochondrial Ca²⁺ uniporter (MCU) complex and its subunits that can regulate its permeability. (A) The assembled complex containing a tetramer of pore forming MCU and four subunits of the essential MCU regulator (EMRE), which are critical for the proper assembly of the transmembrane MCU to the mitochondrial Ca²⁺ uptake isoforms 1, 2, and 3 (MICU 1–3). The MCU complex also contains the MCU regulator 1 (MCUR1), which is thought to bind to EMRE. (B) The MCU paralog MCUb has been shown to replace subunits of pore forming MCU, which leads to reduced Ca²⁺ through the channel and as more MCUb subunits replace MCU the Ca²⁺ conductivity is reduced. (C) MICU1 and MICU two are critical for the Ca²⁺ sensing function of the channel and MICU2 has been shown to regulate Ca²⁺ sensitivity. When Ca²⁺ is low, MICU1 blocks the pore, but in the presence of high Ca²⁺ the conformational changes in MICU1 and MICU2 result in opening of the channel and allow Ca²⁺ to enter the mitochondria. (D) In neurons, MICU3 is more abundant and has been shown to replace MICU2 in dimers with MICU1. This replacement leads to increeased sensitivity to cytosolic Ca²⁺ of the MCU complex and results in MCU channel opening to lower concentrations of Ca²⁺ than compared to MICU2.

FIGURE 3

Mitochondrial Ca²⁺ overload leading to activation of the mPTP is a common pathway in many neurological diseases. (A) In epilepsy, seizures lead to buildup of cytosolic and mitochondrial Ca²⁺ through NMDA receptors, Ca²⁺ permeable AMPA receptors and Ca_v2 and Ca_v3 voltage-gated Ca²⁺ channels (VGCCs) in neurons that are in a state of hyperexcitability. Long seizures or repeated seizures can lead to Ca²⁺ overload in the mitochondria and trigger activation of the mPTP, ultimately leading to neuronal toxicity. Commonly prescribed antiepileptic drugs target these channels including: ethosuximide, which is a Ca_v3 VGCC inhibitor, as well as gabapentin and pregabalin, which act by binding to the with α2δ-1 subunit of voltage-gated Ca²⁺ channels, which could disrupt plasma membrane expression and function of Ca_v2 VGCCs, NMDA receptors, and Ca²⁺ permeable AMPA receptors. (B) In Alzheimer’s disease, Ca²⁺ overload in the mitochondria occurs through multiple mechanisms. Ca²⁺ entry into the cytosol can be enhanced by binding of Aβ to NMDA receptors. Internal release of Ca²⁺ from the ER through activation of metabotropic glutamate receptors (mGluR) also increases the accumulation of Ca²⁺ into the mitochondria. Enhanced Ca²⁺ extrusion from the ER has also been shown to occur through interactions between presenilin 1/2 (PS1/2) and inositol trisphosphate receptors (IP₃R) and ryanodine receptors (RyR). This interaction seems to occur at mitochondria associated membranes (MAMs), which due to the proximity to the mitochondria, drives Ca²⁺ uptake into the mitochondria. Aβ oligomers also can form a pore in the mitochondria that can lead to increases in Ca²⁺ in the mitochondria. Ca²⁺ extrusion through the mitochondrial Na⁺/Ca²⁺ exchanger (mtNCX) has been shown to be impaired by tau accumulation leading to excessive Ca²⁺ accumulation in the mitochondria. Memantine, one of the FDA approved drugs for AD, inhibits NMDA receptors, which could block excessive Ca²⁺ uptake and to reduce glutamate induced excitotoxicity. (C) In Parkinson’s disease (PD), substantia nigra dopaminergic neurons are at risk of Ca²⁺ overload due to the presence of voltage-gated Ca_v1.3 channels, which are critical for the pacemaking activity of these neurons. One common form of autosomal recessive familial PD occurs through mutations in PARK2, the gene for Parkin. Parkin is responsible for regulating mitochondrial Ca²⁺ uptake isoform 1 (MICU1) degradation and may lead to disrupted MCU assembly altering mitochondrial Ca²⁺ uptake. Another form of autosomal recessive PD is caused by mutations in PINK1, which can impair mtNCX function and increase Ca²⁺ accumulation into the mitochondria. In astrocytes, ER Ca²⁺ release at MAMs may be enhanced in PD due to mutations in leucine-rich repeat kinase 2 (LRRK2), which enhance Ca²⁺ uptake into the ER through activation of sarcoplasmic reticulum Ca²⁺ -ATPase (SERCA). The NMDA receptor antagonist amantadine is used to prevent dyskinesia in PD patients. (D) In amyotrophic lateral sclerosis (ALS), motor neurons of the spinal cord and brain begin to degenerate. These neurons have been shown to have increased Ca²⁺ uptake through glutamate NMDA and Ca²⁺ permeable AMPA receptors, which enhances their susceptibility to mitochondrial Ca²⁺ overload. Excitotoxic glutamate, which signals through NMDA and GluA2-lacking Ca²⁺ permeable AMPA receptors, is the target of Riluzole, one of the few FDA approved drugs for ALS.