The movement of Ca²⁺ is vital to the function of any cell type but has a special role to play in pancreatic β-cells due to its importance in the process of insulin release. Insulin production and subsequent release from these cells is controlled by multiple players, including glucose, neurotransmitters, peptide hormones, and other compounds [13,14]. Briefly, the elevation in blood glucose levels that follows food intake is sensed by these cells, which subsequently take glucose up from the blood and metabolize it to more fuel for the mitochondria to shunt towards ATP production, increased levels of which result in the inhibition of the cell´s K_ATP channels. This ultimately leads to depolarization at the plasma membrane (PM), an electrical change that functions to activate L-type Ca²⁺ channels, which allows an influx of Ca²⁺ into the cell. Finally, this wave of Ca²⁺ triggers the release of secretory granules containing insulin, to be released from the cell by exocytosis (Figure 1).

Figure 1. Schematic representation of glucose stimulated insulin secretion (GSIS) and subcellular Ca²⁺ dynamics in pancreatic β-cells. Ca²⁺ transporters within a pancreatic β-cell responsible for balancing Ca²⁺ homeostasis are the following: in the plasma membrane: PMCA: plasma membrane Ca²⁺-ATPase, NCX: Na⁺/Ca²⁺ exchanger; within the endoplasmic reticulum: SERCA: sarco/endoplasmic reticulum Ca²⁺-ATPase, RyR: ryanodine receptor, IP₃R: inositol 1,4,5-trisphsphate receptor, VDAC: voltage dependent anion-selective channel; within mitochondria: NCLX: mitochondrial Na⁺/Ca²⁺ exchanger, PTP: permeability transition pore, MCUC: mitochondrial Ca²⁺ uniporter complex. Red arrows depict the process of glucose-stimulated insulin secretion. After uptake of glucose via GLUT2/1 mitochondrial ATP production is boosted leading to a closing of plasma membrane located ATP-sensitive K⁺ channels (K_ATP). The resulting shift in membrane potential activates PM voltage-dependent Ca²⁺ channels (VDCC) stimulating Ca²⁺-induced Ca²⁺ release which ultimately leads to exocytosis of insulin-containing granules.

Previous work has also put forth that glucose may directly affect the activity of voltage-dependent Ca²⁺ and K⁺ channels [15]. Recent investigation has also shown that cellular insulin secretion stimulated by glucose is tied to mitochondrial Ca²⁺ dynamics. Namely, the proper functioning of mitochondrial Ca²⁺ uptake 1 (MICU1) and mitochondrial calcium uniporter (MCU) proteins is necessary for the physiological function of β-cells [16].

The well-established insulin release-triggering pathway outlined above is not the only mechanism wherein glucose stimulates insulin release. Another, as yet not completely understood pathway is also evident, though it depends on the K_ATP-dependent mechanism due to the need for high cytosolic Ca²⁺ levels. Therefore, it is poignant to consider the fact that any of β-cells´ channels that may affect membrane potential could also have an effect on Ca²⁺ movement into the cytosol and thus insulin secretion.

Importantly, β-cells have a wide array of channels involved in Ca²⁺ influx across their plasma membranes. The placement of these excitable cells´ voltage-dependent channels has been observed to be erratically distributed across the PM, allowing for micro-domains containing high levels of Ca²⁺ to form [17]. In certain species, the electrical activity of the β-cells contained within an islet as a whole is synchronized due to coupling through the presence of gap junctions [18]. This leads to synchronized Ca²⁺ oscillations between the cells as well, which subsequently induce insulin secretion waves observable at the level of a single islet. This does not apply fully to human islets, because while there is some coupling apparent between adjacent β-cells, there is a more heterogeneous spread of the many cell types present in each islet, thus affecting the ability of β-cells separated by other cell types to behave synchronously [18].

The extrusion of Ca²⁺ from β-cells does not differ significantly from the majority of other cell types. Ca²⁺ in the cytosol is removed through a combination of Na⁺/Ca²⁺ exchanger(s) (NCX) and plasma membrane Ca²⁺–ATPase(s) (PMCA) activity [19–22]. As each of these transporters have complementary characteristics (PMCA has a high affinity for Ca²⁺ and a low extrusion capacity, while NCX displays exactly the opposite traits [23]), each exhibits different levels of activity under cellular conditions. Namely, NCX acts as the main Ca²⁺ extrusion pathway at high cytosolic Ca²⁺ concentrations, while PMCA takes over most extrusion activity when low levels are observed [24].

ER Ca²⁺ homeostasis is vital to proper function in most cell types [25]. In the case of β-cells, Ca²⁺ is taken up by the ER through SERCA activity, specifically isoforms SERCA2b and SERCA3, which are expressed ubiquitously and only in β-cells found within pancreatic islets, respectively [26]. Insofar as Ca²⁺ release from the ER goes, this organelle is known to exhibit considerable Ca²⁺ leakage into the cytosol, which must ultimately be corrected for by SERCA pump activity [27]. A likely mechanism for the ER Ca²⁺ leak in β-cells was recently outlined to involve presenilin-1 (PS 1) [28].

As in various other cell types, ER Ca²⁺ release in β-cells is mediated by inositol 1,4,5-triphosphate (IP₃R) and ryanodine receptors (RyR) [29]. The latter can be activated by (local) Ca²⁺ elevations representing the so-called “calcium-induced calcium release” (CICR) processes [30,31]. In regard to the Ca²⁺ release from the ER in pancreatic β-cells, it is unclear whether they exhibit CICR as triggered by RyRs on the ER membrane. These cells show expression of all three of IP₃R isoforms, but their expression and the overall function of RyR is reported as being underwhelming [32,33]. It has been discussed that RyR expression in β-cells is much lower than in most other tissues. Nevertheless, it is possible that lower expression of RyR may be sufficient to allow β-cells´ to accomplish CICR, because of their large-conductance capacity [34].

Another very important aspect of pancreatic β-cell Ca²⁺ signaling is the great role of stromal interaction molecules 1 (STIM1/2) and its pairing with Orai1/2/3. Generally speaking, Orais are ion channels that are selective for Ca²⁺, and are activated following this ion’s depletion from intracellular Ca²⁺ stores [35,36]. The depletion of ER Ca²⁺ is first sensed by the ER STIM1 protein, which is then induced to oligomerize at ER/plasma membrane junctions. STIM1 subsequently activates the Orai1 Ca²⁺-release-activated channel (CRAC) through direct protein interaction, thus inducing store-operated Ca²⁺ entry (SOCE) [37]. STIM has been observed to have two highly homologous isoforms, STIM1 and STIM2. Despite their degree of similarity, the two isoforms have differing functions. Namely, STIM1 serves as the main activator of SOCE channels, while STIM2 serves as a feedback regulator maintaining ER and cytosolic Ca²⁺ concentrations within a narrow range [38,39]. The Orai protein also exists in various forms: Orai1, Orai2, and Orai3. Orai2 and 3 are not as thoroughly characterized as Orai1, but they have been shown to have similar roles in modifying SOCE across different cell types [40–42].

Since the discovery of its mechanism in the early 2000s, the STIM/Orai Ca²⁺-signaling pair has been found to play a distinct role in pancreatic β-cell insulin secretion. It has been demonstrated in rat β-cells that a complex is formed by Orai1, STIM1, and TRPC1 proteins in response to ER Ca²⁺ depletion [43]. Blocking the activity of Orai1 or TRPC1 was shown to impair GSIS in these cells [44], indicating that Orai1 and TRCP1 are vital to the formation of store-operated Ca²⁺ channels (SOCs), which, combined with their activation by STIM1, are necessary for the physiological response of cells to acetylcholine (Ach) in insulin secretion. Furthermore, the group postulated that the effective activation of these SOCs may be reduced in T2DM, thus confirming the importance of Orai1 to the pancreatic cell’s optimal functionality [44]. Notably, the importance of basal mitochondrial Ca²⁺ entry, possibly via a TRPC-mediated mechanism has been postulated to be fundamental for the responsiveness to increased glucose [28].

Beyond the basic mechanisms involved in β-cell Ca²⁺ signaling, the bigger picture must of course also be considered. What kind of signals are these channels and pumps propagating? What effects can they exert on the cell? Ca²⁺ sparks, the creation of Ca²⁺ microdomains, the potential effect of Ca²⁺ on processes ranging from channel activity to gene expression are but a few of the many ways that this ion exerts its effect on β-cell function. Oscillations are also observable in β-cells, as well as almost universally across cell types, including cells performing substantially varied functions, such as fibroblasts and astrocytes [45–47]. Importantly, glucose-stimulated insulin release has been associated with transient changes to these cells´ cytosolic Ca²⁺ concentration. This increase in Ca²⁺ levels, and the subsequently observed oscillations are induced by cellular glucose metabolism [48]. It has been determined that in primary mouse islets, for example, the addition of glucose to cells can induce the generation of IP₃. Every time that cytoplasmic Ca²⁺ increased, so did IP₃, indicating clearly that these processes are related [45].

Cellular Ca²⁺ oscillations have also been linked to effects on gene expression. For example, Dolmetsch and colleagues [49], demonstrated that cytoplasmic Ca²⁺ oscillations effectively reduced the Ca²⁺ threshold required for the activation of a specific set of transcription factors. Whether this holds true in β-cells is as yet unclear, though other breakthroughs in the role of Ca²⁺ in β-cell gene expression are evident in the literature. It was discussed that the signals resulting from glucose flux into the cells are transmitted through the insulin enhancer, activation of which could result from direct protein modification along the lines of phosphorylation or similar processes, or through changes in local co-factor concentrations through Ca²⁺ signals [50]. Others have shown that extended exposure of β-cells to glibenclamide, an ATP-sensitive K⁺-channel inhibitor, which depolarizes the β-cells and thereby stimulates the cells to secrete insulin (often enlisted as a treatment tool for diabetes) [51], causes a prolonged increase in the cells´ basal insulin production, and showed that this was a Ca²⁺-dependent effect [52]. Ultimately, it was determined that extended exposure of the cells to this insulin secretagogue activated protein translation through Ca²⁺-regulated signaling pathways mTOR, MEK, and PKA [52].

The important role of Ca²⁺ in β-cells´ general function, indicates that dysregulation of this cation likely contributes strongly to the development of pathophysiological conditions.

The publication can be found here: https://www.mdpi.com/1422-0067/20/24/6110/htm