Mitochondrial Ca2+ Uptake Requires Sustained Ca2+ Release from the Endoplasmic Reticulum

Abstract

We analyzed the role of inositol 1,4,5-trisphosphateinduced Ca 2؉ release from the endoplasmic reticulum (ER) (i) in powering mitochondrial Ca 2؉ uptake and (ii) in maintaining a sustained elevation of cytosolic Ca 2؉ concentration ([Ca 2؉ ] c ). For this purpose, we expressed in HeLa cells aequorin-based Ca 2؉ -sensitive probes targeted to different intracellular compartments and studied the effect of two agonists: histamine, acting on endogenous H 1 receptors, and glutamate, acting on co-transfected metabotropic glutamate receptor (mGluR1a), which rapidly inactivates through protein kinase C-dependent phosphorylation and thus causes transient inositol 1,4,5-trisphosphate production. from intracellular stores (the ER and Golgi apparatus), the latter is sustained by Ca 2ϩ entry from the extracellular space, which may be directly receptor activated or controlled by the filling state of the intracellular stores (store-operated Ca 2ϩ influx (SOC); for a review, see Refs. 2 and 3). Indeed, removal of Ca 2ϩ from the extracellular medium abolishes the sustained plateau phase but not the initial Ca 2ϩ release. However, the necessity of a continuous influx from the extracellular medium for maintaining a prolonged Ca 2ϩ rise does not imply that Ca 2ϩ directly diffuses through the cytosol to the intracellular targets. Rather, Ca 2ϩ entry could serve the purpose of filling ER cisternae in proximity of the plasma membrane; Ca 2ϩ diffusion through the ER would then provide the driving force for continuous Ca 2ϩ release in different cytosolic domains, including ER/mitochondria contact sites. Two recent observations support this scenario. First, examples of intracellular Ca 2ϩ handling in different cell types show that influx-dependent ER refilling from the subplasma membrane space, followed by rapid diffusion of Ca 2ϩ in the ER lumen, and IP 3 -dependent Ca 2ϩ release at distant sites may represent a general paradigm that allows the maintenance of sustained [Ca 2ϩ ] rises in the cell body (4, 5) (for reviews, see Refs. 6 and 7). Second, some cytosolic effectors appear to "sense" very efficiently the release of stored Ca 2ϩ , of which an excellent example is provided by mitochondria. Work from numerous laboratories has demonstrated that when a Ca 2ϩ signal is elicited in the cytosol by the stimulation with IP 3 -generating agonists, the cytosolic rise is always paralleled by Ca 2ϩ uptake into the mitochondrial matrix (for reviews, see Refs. 8 and 9). A major increase in the mitochondrial matrix Ca 2ϩ concentration ([Ca 2ϩ ] m ) is thus observed (ranging from 5 M in neuronal cells to 500 M in chromaffin cells), which appears in contrast with the low affinity of the mitochondrial uptake mechanisms (the electrogenic uniporter of the inner membrane). The steep dependence of the mitochondrial Ca 2ϩ uptake machinery on the extramitochondrial [Ca 2ϩ ] has been well studied in isolated mitochondria (for reviews, see Refs. 10 and 11) and intact or digitonin-permeabilized cells (12)(13)(14). According to these studies, in order to ob...

Sustained Ca2+ Transfer across Mitochondria Is Essential for Mitochondrial Ca2+ Buffering, Store-operated Ca2+ Entry, and Ca2+ Store Refilling

“…Because the initial mitochondrial Ca 2ϩ transient remained unchanged even in the absence of extracellular Ca 2ϩ , it is tempting to speculate that this Ca 2ϩ spike is predominantly due to uptake of Ca 2ϩ released from the ER. Similar data have been described recently in HeLa cells (43). Our findings that histamine-initiated elevation in [Ca 2ϩ ] mito turned out to be long lasting when NCX mito was inhibited with CGP 37157 (44) is in line with reports demonstrating that NCX mito is the main mechanism of mitochondrial Ca 2ϩ extrusion in endothelial cells (37).…”

The Role of Mitochondria for Ca2+ Refilling of the Endoplasmic Reticulum

“…Furthermore, it has been shown that Ca 2ϩ , which has been released from the ER by a continuous IP 3 generation, is sequestered by mitochondria and, thus, yields elevation in [Ca 2ϩ ] mito (40) (Fig. 7A).…”

Gangliosides as apoptotic signals in ER stress response

“…Ca 2 þ influx from the cytosol to the ER lumen is mediated by the sarco/endoplasmic reticulum Ca 2 þ -ATPase (SERCA). 75,76 Fluctuations in Ca 2 þ concentration help regulate normal cell functions, 8 but ER Ca 2 þ imbalance and acute Ca 2 þ release from the ER lead to cell death. The aforementioned pump and channels at the ER membrane may, at least in part, control both of these conditions.…”