Major Ca²⁺ transporters and channels at the mitochondrion and ER. Ca²⁺ released upon the activation of IP3 receptors or ryanodine receptors at the ER is taken up into mitochondria via VDAC and Ca²⁺ uniporters at the OMM and IMM, respectively. Ca²⁺ extrusion from mitochondria is mediated via Na²⁺-dependent and Na²⁺-independent mechanisms. The Ca²⁺/Na⁺ exchanger responsible for the Na⁺-dependent mechanism has been extensively studied, whereas the characteristics of the molecule involved in Na²⁺-independent mechanism is elusive (indicated by?). The H⁺ gradient established by the Na⁺/H⁺ exchanger is thought to be important for the Na²⁺-dependent mechanism. PTP, which is often activated by pathological conditions, increases the permeability of IMM to small ions and molecules, leading to a collapse of the mitochondrial membrane potential as well as the membrane architecture. The ryanodine receptor is similar to the IP3 receptor in function except that the former is controlled under different mechanisms. Abbreviations: Ca²⁺-ATPase, ATP-dependent Ca²⁺ pump; IMM, inner mitochondrial membrane; IR3R, inositol 1,4,5-trisphosphate receptor; OMM, outer mitochondrial membrane; PTP, permeability transition pore; RYR, ryanodine receptor; VDAC, voltage-dependent anion channel.

Convergence of signal transduction and metabolism at the MAM. Ca²⁺-signaling molecules and lipid metabolizing enzymes are highly compartmentalized at the physical interface between the mitochondria-associated ER membrane (MAM) and the outer mitochondrial membrane (OMM). IP3 receptors (IP3R) are often seen at the MAM. In certain cells, such as CHO cells, type-3 IP3Rs are more enriched at MAM. IP3Rs at the MAM couple sigma-1 receptor chaperones (Sig-1Rs) at the ER lumen and grp75 in the cytosol. Sig-1Rs stabilize ligand-activated type-3 IP3Rs, whereas grp75 links IP3Rs to the voltage-dependent anion channel (VDAC) at the OMM. Cytochrome c (Cyt-c) released from mitochondria associates with IP3Rs, leading to the overloading of mitochondrial Ca²⁺. The MAM contains high levels of the Ca²⁺-binding chaperones calnexin (CNX), calreticulin (CRT) and BiP, thus, perhaps serving as a high-capacity Ca²⁺ reservoir at the ER-mitochondrion interface. Ca²⁺ released from IP3Rs at the MAM creates microdomains of high Ca²⁺ concentrations that, in turn, activate the Ca²⁺ uniporter for Ca²⁺ uptake into the mitochondrial matrix. IP3 receptors are also known to be regulated by another ER chaperone, ERp44. Ca²⁺ in the mitochondrial matrix activates certain enzymes in the TCA cycle, leading to an enhanced ATP production. ATP in mitochondria is released to the cytosol via the adenine nucleotide transporter (ANT) and VDAC, causing the activationof the Ca²⁺-ATPase at the ER. The activity of Ca²⁺-ATPase is also regulated by several Ca²⁺-binding proteins including calreticulin and ERp57. Mitochondrial Ca²⁺ also activates Mn²⁺-dependent superoxide dismutase (MnSOD) and phospholipase A2 (PLA2). Fatty acids (FAs) produced via the PLA2 activation facilitate the pore formation on the OMM. The ER-mitochondrion interface also serves to facilitate the intermembrane transport of phospholipids. The EF-hand (helix-loop-helix)-type Ca²⁺-binding protein S100B and ubiquitin (Ub) ligase at the interface might regulate phospholipid transport. Cholesterol (Chol) and ceramides (Cer) might also use the interface for their transport and metabolism. The vesicular sorting protein PACS and the ubiquitin ligase AMF-R are suggested to regulate the association of the MAM and the OMM. Abbreviations: Preg, pregnenolone; Prog, progesterone.

The chaperone machinery regulating mitochondrial Ca²⁺ signaling and bioenergetics. Ca²⁺ signaling and its regulation at the MAM are crucial for the proper functioning of mitochondria specifically regarding their roles in the bioenergetics and apoptosis. Many chaperones are known to participate in the regulation of Ca²⁺ transporters and channels at the MAM particularly on the Ca²⁺-ATPase, which uptakes Ca²⁺ from the cytosol into the ER and the IP3 receptor, which transmits high concentration Ca²⁺ ‘puffs’ into mitochondria. ERp57 collaborates with another chaperone calreticulin (CRT) to attenuate the activity of Ca²⁺-ATPase. ERp57 does so by modulating the redox state of the Ca²⁺-ATPase and thus provides dynamic control of ER Ca²⁺ homeostasis. Another chaperone, ERp44, can sense the environment in the ER lumen and inhibits type-1 IP3 receptors, which reside mainly outside the MAM, for instance, in CHO cells. The chaperone grp75 serves to link the VDAC and IP3 receptor, thus, shortening the distance between the MAM and the mitochondria. The cytosolic sorting protein PACS-2 can cause the translocation of calnexin chaperone from the ER to the plasma membrane, thus, indirectly affecting the Ca²⁺ homeostasis in the ER lumen. Another chaperone player at the MAM is the newly identified receptor chaperone called the sigma-1 receptor. Under normal, resting conditions, the sigma-1 receptor (Sig-1R) chaperone, residing specifically at the MAM, forms a complex with BiP when the ER Ca²⁺ concentration is 0.5-1.0 mM. Ca²⁺ seems to facilitate the association between the Sig-1R and BiP. The Sig-1R in the complex is essentially in a dormant state with regard to chaperone activity. When IP3 receptors are activated, however, the subsequent drop of the ER Ca²⁺ concentration causes the dissociation of Sig-1Rs from BiP, unleashing the chaperone activity of the receptor. In the presence of high concentrations of cytosolic IP3, activated IP3 receptors are unstable and are readily ubiquitylated and degraded by proteasomes. The free form of Sig-1Rs associates with type-3 IP3 receptors (IP3R3) at the MAM, thus, preventing IP3R3 from being degraded by proteasomes. Sig-1Rs apparently do not chaperone type-1 IP3 receptors (IP3R1) at the bulk ER membrane. The stabilization of IP3R3 by Sig-1Rs therefore ensures the proper Ca²⁺ influx into mitochondria, presumably leading to the enhancement of ATP production in the TCA cycle or the electron transport chain. The refilling of the ER Ca²⁺ pool inactivates Sig-1R chaperones by promoting the re-association of the Sig-1R with BiP. Chaperone machinery on both sides of the ER and mitochondria thus works in concert, partly by sensing the ER Ca²⁺ concentration, to strengthen the interaction between the ER and mitochondrion, facilitating interorganelle signal transduction, metabolic regulation and the bioenergetics of the cell.