Calcium (Ca 2+) is a versatile secondary messenger involved in the regulation of a plethora of different signaling pathways for cell maintenance. Specifically, intracellular Ca 2+ homeostasis is mainly regulated by the endoplasmic reticulum and the mitochondria, whose Ca 2+ exchange is mediated by appositions, termed endoplasmic reticulum–mitochondria-associated membranes (MAMs), formed by proteins resident in both compartments. These tethers are essential to manage the mitochondrial Ca 2+ influx that regulates the mitochondrial function of bioenergetics, mitochondrial dynamics, cell death, and oxidative stress. However, alterations of these pathways lead to the development of multiple human diseases, including neurological disorders, such as amyotrophic lateral sclerosis, Friedreich’s ataxia, and Charcot–Marie–Tooth. A common hallmark in these disorders is mitochondrial dysfunction, associated with abnormal mitochondrial Ca 2+ handling that contributes to neurodegeneration. In this work, we highlight the importance of Ca 2+ signaling in mitochondria and how the mechanism of communication in MAMs is pivotal for mitochondrial maintenance and cell homeostasis. Lately, we outstand potential targets located in MAMs by addressing different therapeutic strategies focused on restoring mitochondrial Ca 2+ uptake as an emergent approach for neurological diseases.
Calcium (Ca 2+) is the most ubiquitous secondary messenger in intracellular signaling of most living cells, acting as a key connection between extracellular signals and intracellular responses. The most remarkable property of Ca 2+ is that such a simple bivalent ion is involved in a plethora of different signaling pathways. Its versatility is achieved by its rich dynamics in concentration changes, which can be caused either by Ca 2+ entry from the extracellular space or Ca 2+ release from intracellular storage compartments or by other side pumping Ca 2+ out of the cell or to intracellular organelles. The main Ca 2+ storage in mammal cells is, depending on the cell type, the sarcoplasmic–endoplasmic reticulum (SR/ER). Intracellular concentration of Ca 2+ is in the range of nM, whereas extracellular Ca 2+ is in the range of mM. Changes in intracellular Ca 2+ levels are required for different structures, cell compartments, receptors, channels, Ca 2+-binding proteins, pumps, transporters, enzymes, and transcription factors. In addition, when intracellular levels rise above physiological concentration, a number of deleterious cellular processes can be triggered.
In non-excitable cells, the pathways regulated by these Ca 2+ signals encompass a wide variety of processes, including from gene expression to fertilization, secretion, protein folding, energy metabolism, and cell cycle regulation. In excitable cells, the signal depends on Ca 2+ entry through voltage or ligand-operated channels, which regulates muscle contraction, postsynaptic potentials, memory formation in neurons (long term potentiation), and insulin secretion from beta cells.
Due to the huge amount of Ca 2+-dependent events occurring in cells, alteration of its signaling pathways contributes to the development of multiple human disorders. Therefore, the study of Ca 2+ signaling is essential for understanding the pathophysiology of many diseases, including diabetes, carcinogenesis, cardio- and cerebrovascular diseases including endothelial dysfunction, as well as neurodegenerative disorders.
In this review, we describe the importance of Ca 2+ signaling in mitochondria and how the mechanism of communication between the ER and the mitochondria is pivotal to the mitochondria. Lately, we address different therapeutic strategies targeting mitochondrial Ca 2+ uptake as an emergent therapeutic approach for neurological disorders.
Mitochondria and the ER are structures that experience continuous remodeling to coordinate complex mechanisms of signal transduction and gene expression, generating physical interactions that facilitate a fast and efficient regulation of these processes. Termed endoplasmic reticulum–mitochondria-associated membranes (MAMs), the contact sites between the two compartments are dynamic structures that are highly sensitive to the physiological changes of the cell.
The association between the ER and the mitochondria was described in the 1950s, when Copeland and Dalton observed a precise orientation of the ER with respect to the mitochondria. The distance between membranes in this region is 10–30 nm depending on the cell type and cell conditions. Besides, it is estimated that, in physiological conditions, 5–20% of the mitochondrial surface is transiently connected to the ER and these contacts are signaling-dependent.
MAMs encompass an extensive variety of different proteins. The first independent proteomic studies identified 911 and 1212 proteins localized in the tethers, but only 44% of them were common. During the last decade, different authors have contributed to increment the list by different molecular approaches, such as microscopy or subcellular fractionation. The development of new techniques has facilitated the proteomic analysis of subcellular domains in-depth. The group of Alice Y Ting has recently identified more than 100 new proteins located in MAMs by means of TurboID technique. This approach was developed to study the interactome of a protein of interest in a specific cell compartment. This emphasizes the complexity of these structures, specialized in each cell type and organism. Indeed, the set of proteins involved in MAMs provides important information about the functions regulated in this domain. As proteins involved in essential cellular processes belong to both the ER and the mitochondrial membranes, the contacts between the organelles enable a coordinated regulation of events, such as lipid biosynthesis, mitochondrial biogenesis, mitochondrial dynamics, and Ca 2+ transfer.
Ca 2+ exchange between the ER and the mitochondria requires the formation of a protein bridge composed by proteins of both compartments. In particular, the formation of microdomains localized in the ER–mitochondria contact sites promotes a rapid and efficient exchange of Ca 2+, fundamental for mitochondrial function, dynamics, and the regulation of apoptosis. In 1993, Rosario Rizzuto and colleagues reported the increase in mitochondrial Ca 2+ upon the cation mobilization through the ER channel IP 3 R (inositol 1,4,5-trisphosphate receptor). Recently, the spatial relation between the ER and the mitochondria was described by the same group. They observed numerous close appositions between these two organelles that contributed to Ca 2+ entry into the mitochondria in Hela cells.
The lumen of the ER is one of the main storages of free Ca 2+ in the cell (about 100–500 µM) compared to the cytosol (~100 nM). Ca 2+ is released to the cytosolic space upon the input signals from the ER through the IP 3 R and through the RyR (ryanodine receptor) in the case of the SR. Furthermore, Sig-1R (Sigma non-opioid intracellular receptor 1 or shortly Sigma 1R), located in the ER, is also involved in Ca 2+ signaling regulation. Sig-1R is enriched in MAMs and stabilizes activated IP 3 R, promoting Ca 2+ influx into the mitochondria.
For the mitochondria, Ca 2+ must cross both mitochondrial membranes. The outer mitochondrial membrane (OMM) is Ca 2+ permeable due to VDAC (voltage-dependent anion channel), which enables different metabolites (succinate, malate, pyruvate, NADH, ATP, and phosphate) to cross from the cytosol to the mitochondria. In connection with the inner mitochondrial membrane (IMM), Ca 2+ enters the mitochondrial matrix through the mitochondrial calcium uniporter (MCU) since this layer is ion-impermeable.
In addition, another key protein that stabilizes the connections of both compartments is glucose-regulated protein 75 (GRP75), which chaperones IP 3 R and VDAC, maintaining the junction and ensuring an efficient transfer of Ca 2+ to the mitochondria. Altogether, all these attributes highlight MAMs as a coordinated domain that requires an optimal communication between the ER and the mitochondria.
IP 3 R-GRP75-VDAC-MCU is one of the complexes in MAMs that is not only essential for the regulation of Ca 2+ homeostasis, but also for the control of mitochondrial function in the regulation of bioenergetics, mitochondrial dynamics, and cell death. Mitochondria are considered the powerhouse of th
