Calcium is a prominent regulator for diverse processes such as gene transcription, proliferation, cell motility, cell signaling, neuronal regulation, autophagy and apoptosis [1]. To perform such a broad spectrum of functions, the cells have evolved multiple unique mechanisms that are modulated by different proteins that regulate cellular Ca²⁺ levels. In addition, the spatial and temporal regulation of Ca²⁺ levels that is maintained by various Ca²⁺ channels, transporters, pumps, and their binding to key proteins, play an essential role in maintaining a tight control on intracellular Ca²⁺ levels. The transient receptor potential (TRP) channels have been studied extensively and play a prominent role in regulating various cellular functions [2,3,4]. The TRPC subfamily consists of seven members (TRPC1-7) with diverse modes of regulation and physiological function. Intracellular Ca²⁺ plays a crucial role in both basal and induced autophagy [5,6]. Plenty of evidence has suggested a complex role of Ca²⁺ in the regulation of autophagy as well as in the regulation of apoptosis [7]. However, the mechanism by which Ca²⁺ controls autophagy and apoptosis remains controversial [8]. Previous studies have shown a negative role of Ca²⁺ in regulating autophagy that induces apoptosis [5,9], while many studies showed a positive role of Ca²⁺ in activating autophagy [1,8,10,11]. Nonetheless, Ca²⁺-permeable channels have emerged as important factors in modulating both basal and induced autophagy that can also prevent apoptosis. In mammalian cells, starvation causes autophagosomes to form omegasomes at the endoplasmic reticulum (ER). Autophagy is induced either by mammalian target of rapamycin (mTOR) inactivation, or adenosine monophosphate (AMP) activated protein kinase (AMPK) activation, which causes distinct Unc-51-like autophagy activating kinase 1 (ULK1) activation [12,13,14] (Figure 1A). Altered Ca²⁺ transmission has been implicated in a variety of processes fundamental for both non-excitable and excitable cells. Here, we review what is known about Ca²⁺-channel-induced Ca²⁺ signaling and how this fundamental second messenger regulates life and death decisions, with particular attention directed to cell autophagy in both excitable and non-excitable cells.

Intracellular Ca²⁺ plays a crucial role in both basal and induced autophagy along with modulating apoptosis [5,6] (Figure 1A,B). Autophagy, or “self-eating”, is a highly conserved and dynamic catabolic process termed as a fundamental cell survival system, intracellular clearance pathway and major intracellular degradation mechanism. Cellular stress, including damaged mitochondria, protein aggregation, bacterial and viral infections, tumors, hypoxia, aging and metabolic stress, activate the cellular autophagy pathway in the cells, resulting in a modified, surplus, or harmful intracellular components being sequestered in autophagosomes and surrendered to the lysosome for the degradation and recycling of intracellular components to regenerate the macromolecule and produce energy during the dynamic program of stem cell proliferation, self-renewal and differentiation [8,15].

Apoptosis is a genetically controlled and evolutionarily conserved form of cell death that is important for normal embryonic development and for the maintenance of tissue homeostasis in the adult organism. Programmed cell death or apoptosis is a genetically determined cell routine in which cells undergo an unexpected decline in homeostasis and functionality, triggering several intracellular pathways and ultimately cell death. The role of Ca²⁺ as a death trigger has been long proposed, and it has been shown that the entry of excess Ca²⁺ into cells might be the mechanism underlying the tissue pathology [1,6]. Ca²⁺ release from the ER, as well as Ca²⁺ entry that is taken up by the mitochondria, is pivotal in triggering apoptotic signals and is one of the mechanisms through which the overexpression of antiapoptotic proteins (or ablation of proapoptotic ones) counteracts cell death. The amount of releasable Ca²⁺—rather than the Ca²⁺ concentration of the ER—seems to be the relevant parameter for the transduction of the death signal, as it controls the ‘amplitude’ of the signal reaching the mitochondria [1,5,8,16,17,18].

Apoptosis is a well-characterized mechanism of programmed cell death and is functionally distinct from autophagy. Notably, in many cell types and disease conditions, the activation of autophagy inhibits the apoptotic mediated cell death, whereas autophagy inhibition activates the apoptotic process [19,20]. However, in some physiological conditions, the proteins originally involved in regulating autophagy might also induce apoptosis [19]. Interestingly, many stimuli that ultimately cause apoptosis also trigger autophagy in the same cell, where autophagy occurs first, and if not resolved, it may proceed to apoptosis [21,22,23]. Apoptosis can be triggered by various cellular signals, especially the intracellular Ca²⁺. Several studies have provided strong experimental evidence that supports the involvement of intracellular Ca²⁺ homeostasis in the induction of apoptosis [24,25,26]. Fleckenstein et al. provided the first evidence that showed the intracellular Ca²⁺ overload caused by excessive Ca²⁺ influx induces cell death in myocytes [27]. Similarly, the lethal effect of intracellular Ca²⁺ overload in cell survival was scrutinized using Ca²⁺ ionophore/chelator in thymocytes. Notably, Ca²⁺ ionophore A23187 mimics the apoptotic effect of glucocorticoid, and the removal of extracellular Ca²⁺ by EGTA diminishes the cytolytic action of glucocorticoid and A23187, suggesting that Ca²⁺ influx and intracellular Ca²⁺ concentration play a crucial role in cell survival/apoptosis [28]. Several other studies have shown that exposing the cells to different apoptotic inducers causes a sustained increase in intracellular Ca²⁺ concentration, which in turn activates the endonuclease and initiates the programed cell death cascade [29,30,31]. The increase in intracellular Ca²⁺ concentration in cells is mainly achieved either by increased Ca²⁺ influx through the plasma membrane Ca²⁺ channels or Ca²⁺ release from the internal storage such as the endoplasmic reticulum (ER) and mitochondria. In addition to intracellular Ca²⁺ overload, other studies have shown that apoptosis can be triggered by both Ca²⁺ depletion from the ER and Ca²⁺ entry through the plasma membrane [32].

The ER is a major Ca²⁺ storage organelle in mammalian cells, which maintain a concentration of around 1mM compared with 100 nM in the cytosol and mitochondria. Ca²⁺ homeostasis in the ER is essential for the activity of molecular chaperones and the folding of enzymes. Dysregulation in ER Ca²⁺ content affects protein folding, leading to the accumulation of misfolded proteins in the ER and inducing ER stress. Cells activate defense mechanisms such as the unfolded protein response (UPR) to remove the unfolded and/or incorrectly folded accumulated proteins in the ER [33,34]. Therefore, mild or transient ER stress activates several pathways to combat the ER stress, including the activation of chaperones, folding enzymes, reactive oxygen species (ROS) scavengers, and degradative pathways, such as autophagy. In contrast, these pathways can also induce apoptosis when the cells experience severe or prolonged ER stress [35,36].

ER Ca²⁺ homeostasis is tightly regulated by active Ca²⁺ uptake via Ca²⁺ pumps from the cytosol and Ca²⁺ release from the ER by Ca²⁺ leak channels. The Ca²⁺ release from the ER occurs primarily via inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) or ryanodine receptors (RyRs); however, other Ca²⁺ leak channels also exist in the ER, and these channels are restricted to excitable cell types [37,38]. IP₃R is predominantly localized on the ER, is one of the major ER Ca²⁺ leak channels, and IP₃R-mediated Ca²⁺ release induces the release of several pro-apoptotic factors from the mitochondria and activates apoptosis. IP₃R1-deficient T cells showed resistance to apoptosis induced by dexamethasone, T cell receptor (TCR) stimulation, ionizing radiation, and Fas stimulation [39]. Interestingly, increasing the intracellular Ca²⁺ concentration by thapsigargin makes the IP₃R1-deficient T cells susceptible to TCR-mediated apoptosis. Interestingly, the removal of external Ca²⁺ has no effect on the TCR-mediated apoptosis in thapsigargin-treated cells, suggesting that ER store depletion is sufficient and Ca²⁺ entry is not required to trigger TCR-mediated apoptosis [39]. In another study, the expression of IP₃R was upregulated in lymphocytes exposed with apoptotic inducers, and the augmented receptor population was localized to the plasma membrane. Notably, the antisense-mediated silencing of type 3 IP₃R (IP₃R3), but not type 1 IP₃R, inhibited dexamethasone-induced apoptosis in the mouse S49 T-lymphoma cell line, suggesting that Ca²⁺ release via IP₃R3 is required to trigger autophagy that inhibited apoptosis [29].

Ca²⁺ release from the ER transmits Ca²⁺ to the surrounding mitochondria via the mitochondria-associated ER membranes (MAMs). The stimulation of IP₃R releases Ca²⁺ from the ER, which generates high-Ca²⁺ microdomains at the ER–mitochondrial junction, leading to the activation of mitochondrial Ca²⁺ uniporter and subsequent mitochondrial Ca²⁺ uptake. Ca²⁺ homeostasis in the mitochondria is required in regulating the activity of Krebs cycle enzymes and transporters, impacting its bioenergetic and biosynthetic function [40,41,42]. Various apoptotic stimuli cause the release of excessive Ca²⁺ from the ER and subsequent Ca²⁺ overload into the mitochondria, resulting in the constant opening of the mitochondrial permeability transition pore (PTP), that leads to mitochondrial swelling with perturbation or rupture of the outer mitochondrial membrane (OMM) and alters the mitochondrial membrane potential [43,44,45]. The rupture of OMM activates the release of several pro-apoptotic proteins such as cytochrome c, apoptosis-inducing factor (AIF), procaspase 9, Smac/DIABLO, and endonuclease G into the cytosol, where it partners with other proteins and initiates the signaling cascade, leading to apoptosis [35,46,47,48,49]. The critical role of Ca²⁺ and apoptosis was further reinforced while studying the mechanism of action of the anti-apoptotic proteins, such as B cell lymphoma 2 (Bcl-2), B cell lymphoma extra-large (Bcl-XL) and myeloid cell leukemia 1 (Mcl-1). Bcl-2 is a critical regulator in controlling apoptotic cell death by neutralizing proapoptotic Bcl-2 family members at the mitochondria. Bcl2 also regulates Ca²⁺ homeostasis in the ER and mitochondria and controls the cell death pathway [50]. Interestingly, Bcl-2 overexpression has been shown to inhibit cell death by decreasing the steady-state Ca²⁺ levels in the ER, thereby reducing Ca²⁺ transfer from the ER to the mitochondria [51]. On the other hand, proapoptotic BAX promotes cell death by increasing the Ca²⁺ mobilization from the ER to the mitochondria. Moreover, the loss of BAX reduced resting ER Ca²⁺ levels that resulted in diminished mitochondrial Ca²⁺ uptake and were highly resistant to apoptosis [52].