Calcium signaling in cardiomyocyte mitochondria

Decoding cytosolic calcium signaling during heart contraction

Summary of mitochondria calcium signaling during ECC.

Beat-to-beat spikes of cytosolic calcium regulate cardiac contraction in a process known as excitation-contraction coupling (ECC). Subsequent relaxation takes place when calcium is removed from the cytosol via different routes. One option involves calcium sequestration by mitochondria. Their apparatus of dedicated channels and transporters enables them to read the cytosolic calcium signals and subsequently transport and store calcium in their matrix compartment. The precise mechanisms by which mitochondria shape the cytosolic calcium concentration is still controversial. Experimental data currently support two models: in the first model (i) the fast cytosolic calcium transients are slowly integrated by transporters of the inner mitochondrial membrane (IMM) resulting in a gradual increase of mitochondrial calcium concentration ([Ca]_m), whereas in the second model (ii) the fast beat-to-beat oscillations of cytosolic calcium are rapidly transmitted into the mitochondrial matrix and therefore reflects into fast changes of [Ca]_m. The mode of mitochondrial calcium uptake determines their capacity to buffer cytosolic calcium spikes. Therefore this has implications on how prominent is the role of mitochondria in regulating cardiac contraction on a beat-to-beat basis. Recent technological advances in the field lead to new discoveries that suggest the co-existence of the two models.

Mitochondria in cardiomyocytes

The metabolic demand of the heart is specific and extreme. At every beat the heart needs to precisley match workload and energy production. In cadriac myocytes, mitochondria occupy approximately one third of the cell volume reflecting the high energy demands of these cells. It has been estimated that every heartbeat uses up approximately 2% of cellular ATP and the mitochondrial oxidative metabolism has to be finely synchonized in time and intensity to the contractile activity of the cardiomyocytes. In cardiac disease, such as arrhythmia and heart failure, mitochondria undergo remodelling, which result in detrimental effects on their structure and function.

Mitochondrial calcium transport mechanisms

Mitochondrial Ca influx accross the IMM is important for the regulation of ATP synthetsis, mitochondrial fission-fusion, dynamics. opening of the mitochondrial permeability transition pore and generation of reactive oxygen species.

The Mitochondrial Calcium Uniporter (MCU) is the most characterised pathway for mitochondrial calcium uptake, driven by the mitochondrial membrane potential for Ca accross the IMM.

Other methods include:

Mitochondrial Ryanodine receptor type 1 (mRyR1)

• Located in the IMM, mRyR1 is proposed to be one of the mitochondrial Ca uptake pathways. It has been shown to demonstrate biochemical, pharmacological and functional properties similar to the skeletal muscle RyR.

Leucine-zipper-EF-hand-containing transmembrane protein 1 (LETM1)

• First identified as a mitochondrial potassium/proton antipoerter, LETM1 is highly debated to function as a mitochondrial Ca influx mechanism.

Rapid mode of uptake (RaM)

• Demonstrated in isolated isolated cardiac mitochondria

Other proposed methods of uptake include rapid uptake by uncoupling proteins UCP2 and 3, Ca-selective conductance's mCa1 and mCa2, and Co-enzyme Q10.

Mitochondrial Ca efflux can be mediated by:

Na-dependent calcium extrusion pathway, mNCX1

• The sodium/calcium exchanger is the primary calcium extrusion pathway in cardiac mitochondria. It plays an important role in modulating the steady state balance between extra- and intramitochondrial Ca.

Mitochondrial permeability transistion pore (mPTP)

• Although controversial, the mPTP has been postulated to function in Ca efflux. Ca and reactive oxygen species are known inducers of mPTP and the channel is cyclosporin A (CsA) sensitive. Molecular identity is undefined, however, VDAC and cyclophillin D are proposed components.

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