EMRE Emerges: Concentrations of calcium within mitochondria are tightly regulated and modulate physiological mitochondrial functions, including control of metabolism and cell death. Sancak et al. (p. 1379 , published online 14 November) complete the molecular characterization of the mitochondrial calcium uniporter (MCU), the multicomponent channel that allows concentration of calcium within the organelle. They identified a small protein termed “essential MCU regulator”—or EMRE—which was required for calcium transport activity of the fully assembled uniporter.

Recombinant adenoviruses were used to overexpress green fluorescent protein (GFP)‐fused auxiliary Ca²⁺ channel β subunits (β₁‐β₄) in cultured adult rat heart cells, to explore new dimensions of β subunit functions in vivo. Distinct β‐GFP subunits distributed differentially between the surface sarcolemma, transverse elements, and nucleus in single heart cells. All β‐GFP subunits increased the native cardiac whole‐cell L‐type Ca²⁺ channel current density, but produced distinctive effects on channel inactivation kinetics. The degree of enhancement of whole‐cell current density was non‐uniform between β subunits, with a rank order of potency β_2aαβ₄ > β_1b > β₃. For each β subunit, the increase in L‐type current density was accompanied by a correlative increase in the maximal gating charge (Q_max) moved with depolarization. However, β subunits produced characteristic effects on single L‐type channel gating, resulting in divergent effects on channel open probability (P_o). Quantitative analysis and modelling of single‐channel data provided a kinetic signature for each channel type. Spurred on by ambiguities regarding the molecular identity of the actual endogenous cardiac L‐type channel β subunit, we cloned a new rat β₂ splice variant, β_2b, from heart using 5′ rapid amplification of cDNA ends (RACE) PCR. By contrast with β_2a, expression of β_2b in heart cells yielded channels with a microscopic gating signature virtually identical to that of native unmodified channels. Our results provide novel insights into β subunit functions that are unattainable in traditional heterologous expression studies, and also provide new perspectives on the molecular identity of the β subunit component of cardiac L‐type Ca²⁺ channels. Overall, the work establishes a powerful experimental paradigm to explore novel functions of ion channel subunits in their native environments.

Mitochondrial calcium (Ca²⁺) import is a well-described phenomenon regulating cell survival and ATP production. Of multiple pathways allowing such entry, the mitochondrial Ca²⁺ uniporter is a highly Ca²⁺-selective channel complex encoded by several recently-discovered genes. However, the identity of the pore-forming subunit remains to be established, since knockdown of all the candidate uniporter genes inhibit Ca²⁺ uptake in imaging assays, and reconstitution experiments have been equivocal. To definitively identify the channel, we use whole-mitoplast voltage-clamping, the technique that originally established the uniporter as a Ca²⁺ channel. We show that RNAi-mediated knockdown of the mitochondrial calcium uniporter (MCU) gene reduces mitochondrial Ca²⁺ current (I_MiCa), whereas overexpression increases it. Additionally, a classic feature of I_MiCa, its sensitivity to ruthenium red inhibition, can be abolished by a point mutation in the putative pore domain without altering current magnitude. These analyses establish that MCU encodes the pore-forming subunit of the uniporter channel.

Significance: TRPM7 (transient receptor potential cation channel subfamily M member 7) is required for normal organ development but also mediates anoxic neuronal death. TRPM7 contains a channel that conducts cations into the cytosol and C-terminal kinase that can phosphorylate multiple substrates. The kinase is cleaved to regulate gene expression and apoptosis. The link between TRPM7’s channel and its organismal function remains the least understood aspect of TRPM7. Here, we identify intracellular Zn ²⁺ storage vesicles that contain the majority of TRPM7 protein. TRPM7 senses reactive oxygen species (ROS) to release Zn ²⁺ from these vesicles. Just as the endoplasmic reticulum sequesters and releases Ca ²⁺ , we propose that these vesicles fulfill a similar function for Zn ²⁺ and that TRPM7 coordinates fluctuations in cellular [Zn ²⁺ ] and ROS during development and injury.

Background—: Transient receptor potential ( TRP ) channels are a superfamily of broadly expressed ion channels with diverse physiological roles. TRPC1, TRPC3 , and TRPC6 are believed to contribute to cardiac hypertrophy in mouse models. Human mutations in TRPM4 have been linked to progressive familial heart block. TRPM7 is a divalent-permeant channel and kinase of unknown function, recently implicated in the pathogenesis of atrial fibrillation; however, its function in ventricular myocardium remains unexplored. Methods and Results—: We generated multiple cardiac-targeted knockout mice to test the hypothesis that TRPM7 is required for normal ventricular function. Early cardiac Trpm7 deletion (before embryonic day 9; TnT / Isl1-C re) results in congestive heart failure and death by embryonic day 11.5 as a result of hypoproliferation of the compact myocardium. Remarkably, Trpm7 deletion late in cardiogenesis (about embryonic day 13; αMHC-Cre ) produces viable mice with normal adult ventricular size, function, and myocardial transcriptional profile. Trpm7 deletion at an intermediate time point results in 50% of mice developing cardiomyopathy associated with heart block, impaired repolarization, and ventricular arrhythmias. Microarray analysis reveals elevations in transcripts of hypertrophy/remodeling genes and reductions in genes important for suppressing hypertrophy ( Hdac9) and for ventricular repolarization ( Kcnd2 ) and conduction ( Hcn4 ). These transcriptional changes are accompanied by action potential prolongation and reductions in transient outward current ( I _to ; Kcnd2 ). Similarly, the pacemaker current ( I _f ; Hcn4 ) is suppressed in atrioventricular nodal cells, accounting for the observed heart block. Conclusions—: Trpm7 is dispensable in adult ventricular myocardium under basal conditions but is critical for myocardial proliferation during early cardiogenesis. Loss of Trpm7 at an intermediate developmental time point alters the myocardial transcriptional profile in adulthood, impairing ventricular function, conduction, and repolarization.

Significance: Cells injured by a variety of stressors feature a form of mitochondrial dysfunction termed the permeability transition. During this process, mitochondria swell and become disrupted, ultimately leading to cell death. In excitable cells such as cardiomyocytes or neurons, such injury is often triggered by calcium overload. By screening Drosophila cells, we have found a protein, mitochondrial calcium uniporter regulator 1 (MCUR1), that appears to regulate the amount of calcium required to induce the permeability transition. Modulating the function of this protein acutely may prove beneficial in limiting tissue damage during diseases that feature calcium overload.