Mitochondrion - Structure - Mitochondria-associated ER Membrane (MAM) - Calcium Signaling

Calcium Signaling

A critical role for the ER in calcium signaling was acknowledged before such a role for the mitochondria was widely accepted, in part because the low affinity of Ca2+ channels localized to the outer mitochondrial membrane seemed to fly in the face of this organelle’s purported responsiveness to changes in intracellular Ca2+ flux. But the presence of the MAM resolves this apparent contradiction: the close physical association between the two organelles results in Ca2+ microdomains at contact points that facilitate efficient Ca2+ transmission from the ER to the mitochondria. Transmission occurs in response to so-called “Ca2+ puffs” generated by spontaneous clustering and activation of IP3R, a canonical ER membrane Ca2+ channel.

The fate of these puffs—in particular, whether they remain restricted to isolated locales or integrated into Ca2+ waves for propagation throughout the cell—is determined in large part by MAM dynamics. Although reuptake of Ca2+ by the ER (concomitant with its release) modulates the intensity of the puffs, thus insulating mitochondria to a certain degree from high Ca2+ exposure, the MAM often serves as a firewall that essentially buffers Ca2+ puffs by acting as a sink into which free ions released into the cytosol can be funneled. This Ca2+ tunneling occurs through the low-affinity Ca2+ receptor VDAC1, which recently has been shown to be physically tethered to the IP3R clusters on the ER membrane and enriched at the MAM. The ability of mitochondria to serve as a Ca2+ sink is a result of the electrochemical gradient generated during oxidative phosphorylation, which makes tunneling of the cation an exergonic process.

But transmission of Ca2+ is not unidirectional; rather, it is a two-way street. The properties of the Ca2+ pump SERCA and the channel IP3R present on the ER membrane facilitate feedback regulation coordinated by MAM function. In particular, clearance of Ca2+ by the MAM allows for spatio-temporal patterning of Ca2+ signaling because Ca2+ alters IP3R activity in a biphasic manner. SERCA is likewise affected by mitochondrial feedback: uptake of Ca2+ by the MAM stimulates ATP production, thus providing energy that enables SERCA to reload the ER with Ca2+ for continued Ca2+ efflux at the MAM. Thus, the MAM is not a passive buffer for Ca2+ puffs; rather it helps modulate further Ca2+ signaling through feedback loops that affect ER dynamics.

Regulating ER release of Ca2+ at the MAM is especially critical because only a certain window of Ca2+ uptake sustains the mitochondria, and consequently the cell, at homeostasis. Sufficient intraorganelle Ca2+ signaling is required to stimulate metabolism by activating dehydrogenase enzymes critical to flux through the citric acid cycle. However, once Ca2+ signaling in the mitochondria passes a certain threshold, it stimulates the intrinsic pathway of apoptosis in part by collapsing the mitochondrial membrane potential required for metabolism. Studies examining the role of pro- and anti-apoptotic factors support this model; for example, the anti-apoptotic factor Bcl-2 has been shown to interact with IP3Rs to reduce Ca2+ filling of the ER, leading to reduced efflux at the MAM and preventing collapse of the mitochondrial membrane potential post-apoptotic stimuli. Given the need for such fine regulation of Ca2+ signaling, it is perhaps unsurprising that dysregulated mitochondrial Ca2+ has been implicated in several neurodegenerative diseases, while the catalogue of tumor suppressors includes a few that are enriched at the MAM.

Read more about this topic:  Mitochondrion, Structure, Mitochondria-associated ER Membrane (MAM)

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