Mechanisms of ion selectivity and throughput in the mitochondrial calcium uniporter

Fluoride induces spermatocyte apoptosis by IP3R1/MCU-mediated mitochondrial calcium overload through MAMs

Excessive fluoride exposure has been shown to induce diminished sperm quality and mitochondrial dysfunction. The interaction between mitochondria and the endoplasmic reticulum (ER) is critical for regulating mitochondrial function in spermatogenic cells. Therefore, this study was designed to investigate the molecular events involved in mitochondria-associated ER membranes (MAMs) in mice exposed to 25, 50, and 100 mg/L NaF for 60 days, and in GC-2spd treated with 1.5, 2.0, and 2.5 mM NaF for 24 hours. Mitochondrial stress tests revealed a significant reduction in basal respiration, maximal respiration, and ATP production, suggesting mitochondrial dysfunction following fluoride exposure. Results further indicated that fluoride exposure significantly enhanced ER-mitochondria contacts, mitochondrial Ca2+ levels, and the expressions of IP3R1, GRP75, VDAC1, and MCU, while reduced the levels of MFN1, MFN2, VAPB, and PTPIP51, along with an increase in Cytochrome C and Caspase-3. Treatment with the Ru360 and IP3R1 siRNA restored mitochondrial membrane potential, while reduced mitochondrial Ca2+ levels and apoptosis rates, indicating that both MCU and IP3R1 play a role in regulating fluoride-induced the formation of MAMs. Collectively, these findings proved that fluoride promoted Ca2+ transfer through MAMs in spermatocytes via the IP3R1-GRP75-VDAC1-MCU axis, and inhibiting IP3R1/MCU might be a potential therapeutic target in fluorosis.

Mitochondrial calcium uniporter complex: An emerging therapeutic target for cardiovascular diseases (Review)

Cardiovascular disease (CVD) is currently a major factor affecting human physical and mental health. In recent years, the relationship between intracellular Ca2+ and CVD has been extensively studied. Ca2+ movement across the mitochondrial inner membrane plays a vital role as an intracellular messenger, regulating energy metabolism and calcium homeostasis. It is also involved in pathological processes such as cardiomyocyte apoptosis, hypertrophy and fibrosis in CVD. The selective mitochondrial calcium uniporter complex (MCU complex) located in the inner membrane is essential for mitochondrial Ca2+ uptake. Therefore, the MCU complex is a potential therapeutic target for CVD. In this review, recent research progress on the pathophysiological mechanisms and therapeutic potential of the MCU complex in various CVDs was summarized, including myocardial ischemia‑reperfusion injury, pulmonary arterial hypertension, other peripheral vascular diseases, myocardial remodeling and arrhythmias. This review contributes to a deeper understanding of these mechanisms at the molecular level and highlights potential intervention targets for CVD treatment in clinical practice.

The pentameric chloride channel BEST1 is activated by extracellular GABA

Bestrophin 1 (BEST1) is chloride channel expressed in the eye, central nervous system (CNS), and other tissues in the body. A link between BEST1 and the principal inhibitory neurotransmitter γ-aminobutyric acid (GABA) has been proposed. The most appreciated receptors for extracellular GABA are the GABAB G-protein coupled receptors and the pentameric GABAA chloride channels, both of which have fundamental roles in the CNS. Here, we demonstrate that BEST1 is directly activated by GABA. Through functional studies and atomic-resolution structures of human and chicken BEST1, we identify a GABA binding site on the channel's extracellular side and determine the mechanism by which GABA binding induces opening of the channel's central gate. This same gate is activated by intracellular [Ca2+], indicating that BEST1 is controlled by ligands from both sides of the membrane. The studies demonstrate that BEST1, which shares no structural homology with GABAA, is a GABA-activated chloride channel. The physiological implications of this finding remain to be studied.

Unraveling the role and mechanism of mitochondria in postoperative cognitive dysfunction: a narrative review

Postoperative cognitive dysfunction (POCD) is a frequent neurological complication encountered during the perioperative period with unclear mechanisms and no effective treatments. Recent research into the pathogenesis of POCD has primarily focused on neuroinflammation, oxidative stress, changes in neural synaptic plasticity and neurotransmitter imbalances. Given the high-energy metabolism of neurons and their critical dependency on mitochondria, mitochondrial dysfunction directly affects neuronal function. Additionally, as the primary organelles generating reactive oxygen species, mitochondria are closely linked to the pathological processes of neuroinflammation. Surgery and anesthesia can induce mitochondrial dysfunction, increase mitochondrial oxidative stress, and disrupt mitochondrial quality-control mechanisms via various pathways, hence serving as key initiators of the POCD pathological process. We conducted a review on the role and potential mechanisms of mitochondria in postoperative cognitive dysfunction by consulting relevant literature from the PubMed and EMBASE databases spanning the past 25 years. Our findings indicate that surgery and anesthesia can inhibit mitochondrial respiration, thereby reducing ATP production, decreasing mitochondrial membrane potential, promoting mitochondrial fission, inducing mitochondrial calcium buffering abnormalities and iron accumulation, inhibiting mitophagy, and increasing mitochondrial oxidative stress. Mitochondrial dysfunction and damage can ultimately lead to impaired neuronal function, abnormal synaptic transmission, impaired synthesis and release of neurotransmitters, and even neuronal death, resulting in cognitive dysfunction. Targeted mitochondrial therapies have shown positive outcomes, holding promise as a novel treatment for POCD.

Electrostatic Modulation for Enhanced Ion Selectivity in Gate-All-Around Multilayer Stacked Graphene Nanopore

Biological ion channels exhibit exceptional gating capabilities for regulated transport and filtration across cell membranes. This study explores similar gating functions in artificial nanopores using graphene membranes. By applying direct voltage, we can dynamically control ion distribution around nanopores, allowing for real-time triggering, dynamic flow control, and adaptability to varying pore sizes. We investigate electrostatic modulation of ion transport in a stacked nanoporous graphene configuration, which mitigates defects from growth and transfer processes. Nanopores are created using oxygen plasma, enabling fine-tuning of ion transport. External voltage enhances ion conductivity at positive voltages and reduces it at negative voltages, demonstrating significant modulation by the surface potential-induced electric double layer (EDL). Voltage-dependent ion enrichment and depletion within the nanopores affect the effective surface charge density, facilitating controllable ion sieving. Results show that nanopores, with sizes comparable to hydrated ion diameters, achieve high and tunable voltage-gating functionality, enabling efficient on-demand ion transport. Voltage-gating effectively tunes ion selectivity in multilayer stacked graphene membranes, with negative voltages impeding divalent cations and positive voltages mimicking biological K+ nanochannels. This research lays the foundation for developing nanopores with tunable ion selectivity for applications in energy conversion, ion separation, and nanofluidics.

The MCU and MCUb amino-terminal domains tightly interact: mechanisms for low conductance assembly of the mitochondrial calcium uniporter complex

The mitochondrial calcium (Ca2+) uniporter (MCU) complex is regulated via integration of the MCU dominant negative beta subunit (MCUb), a low conductance paralog of the main MCU pore forming protein. The MCU amino (N)-terminal domain (NTD) also modulates channel function through cation binding to the MCU regulating acidic patch (MRAP). MCU and MCUb have high sequence similarities, yet the structural and functional roles of MCUb-NTD remain unknown. Here, we report that MCUb-NTD exhibits α-helix/β-sheet structure with a high thermal stability, dependent on protein concentration. Remarkably, MCU- and MCUb-NTDs heteromerically interact with ∼nM affinity, increasing secondary structure and stability and structurally perturbing MRAP. Further, we demonstrate MCU and MCUb co-localization is suppressed upon NTD deletion concomitant with increased mitochondrial Ca2+ uptake. Collectively, our data show that MCU:MCUb NTD tight interactions are promoted by enhanced regular structure and stability, augmenting MCU:MCUb co-localization, lowering mitochondrial Ca2+ uptake and implicating an MRAP-sensing mechanism.

Enhanced Rectification Performance in Bipolar Janus Graphene Oxide Channels by Lateral Electric Fields

Improving the ionic rectification in nanochannels enables versatile applications such as biosensors, energy harvesting, and fluidic diodes. While previous work mostly focused on the effect of channel geometry and surface charge, in this work via a series of molecular dynamics simulations, we find a striking phenomenon that the ionic current rectification (ICR) ratio in Janus graphene oxide (GO) channels can be tremendously promoted by lateral electric fields. First, under a given axial electric field, an additional lateral electric field can improve the ICR ratio by several times to an order, depending on the channel symmetry. The symmetric channel has an obviously greater ICR ratio because it maintains a more pronounced ion transport disparity at opposite axial fields. The underlying mechanism for the function of the lateral electric field is that it promotes the lateral migration of ions and thus amplifies the ion-residue electrostatic interaction at opposite axial fields, enlarging the ion dynamical difference. Furthermore, for different axial electric fields, the ICR ratio can always be improved by lateral electric fields (up to two orders), suggesting that the ICR improvement is universal. Our results demonstrate that applying a lateral electric field could be a new method to improve the rectification performance of nanochannels, providing valuable guidance for the design of efficient ionic diode devices.

A mechanism of lysosomal calcium entry

Lysosomal calcium (Ca2+) release is critical to cell signaling and is mediated by well-known lysosomal Ca2+ channels. Yet, how lysosomes refill their Ca2+ remains hitherto undescribed. Here, from an RNA interference screen in Caenorhabditis elegans, we identify an evolutionarily conserved gene, lci-1, that facilitates lysosomal Ca2+ entry in C. elegans and mammalian cells. We found that its human homolog TMEM165, previously designated as a Ca2+/H+ exchanger, imports Ca2+ pH dependently into lysosomes. Using two-ion mapping and electrophysiology, we show that TMEM165, hereafter referred to as human LCI, acts as a proton-activated, lysosomal Ca2+ importer. Defects in lysosomal Ca2+ channels cause several neurodegenerative diseases, and knowledge of lysosomal Ca2+ importers may provide previously unidentified avenues to explore the physiology of Ca2+ channels.

Ru360 Alleviates Postoperative Cognitive Dysfunction in Aged Mice by Inhibiting MCU-Mediated Mitochondrial Dysfunction

Purpose

Ru360, a selective inhibitor of mitochondrial calcium uptake, maintains mitochondrial calcium homeostasis. To evaluate whether mitochondrial calcium uniporter (MCU)-mediated mitochondrial function is associated with the pathological process of Postoperative cognitive dysfunction (POCD), elucidate its relationship with neuroinflammation, and observe whether the relevant pathological process can be improved with Ru360.

Methods

Aged mice underwent experimental open abdominal surgery after anesthesia. Open field tests, Novel object recognition tests and Y Maze Tests were used to conduct behavioral experiments. The reactive oxygen species (ROS) content, the levels of inflammatory cytokines interleukin-1β (IL-1β), interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α), intra-mitochondrial calcium, mitochondrial membrane potential (MMP) and the activity of antioxidant superoxide dismutase (SOD) in the hippocampus of mice were detected using kits. The expression of proteins was detected using Western blot.

Results

After treatment with Ru360, MCU-mediated mitochondrial dysfunction was inhibited, neuroinflammation was reduced, and the learning ability of the mice was improved after surgery.

Conclusion

Our study demonstrated that mitochondrial function plays a crucial role in the pathology of POCD, and using Ru360 to improve mitochondrial function may be a new and necessary direction for the treatment of POCD.

The mitochondrial calcium uniporter transports Ca 2+ via a ligand-relay mechanism

The mitochondrial calcium uniporter (mtCU) is a multicomponent Ca 2+ -specific channel that imparts mitochondria with the capacity to sense the cytosolic calcium signals. The metazoan mtCU comprises the pore-forming subunit MCU and the essential regulator EMRE, arranged in a tetrameric channel complex, and the Ca 2+ sensing peripheral proteins MICU1-3. The mechanism of mitochondrial Ca 2+ uptake by mtCU and its regulation is poorly understood. Our analysis of MCU structure and sequence conservation, combined with molecular dynamics simulations, mutagenesis, and functional studies, led us to conclude that the Ca 2+ conductance of MCU is driven by a ligand-relay mechanism, which depends on stochastic structural fluctuations in the conserved DxxE sequence. In the tetrameric structure of MCU, the four glutamate side chains of DxxE (the E-ring) chelate Ca 2+ directly in a high-affinity complex (site 1), which blocks the channel. The four glutamates can also switch to a hydrogen bond-mediated interaction with an incoming hydrated Ca 2+ transiently sequestered within the D-ring of DxxE (site 2), thus releasing the Ca 2+ bound at site 1. This process depends critically on the structural flexibility of DxxE imparted by the adjacent invariant Pro residue. Our results suggest that the activity of the uniporter can be regulated through the modulation of local structural dynamics. A preliminary account of this work was presented at the 67 th Annual Meeting of the Biophysical Society in San Diego, CA, February 18-22, 2023.

MICU1 occludes the mitochondrial calcium uniporter in divalent-free conditions

Mitochondrial Ca2+ uptake is mediated by the mitochondrial uniporter complex (mtCU) that includes a tetramer of the pore-forming subunit, MCU, a scaffold protein, EMRE, and the EF-hand regulatory subunit, MICU1 either homodimerized or heterodimerized with MICU2/3. MICU1 has been proposed to regulate Ca2+ uptake via the mtCU by physically occluding the pore and preventing Ca2+ flux at resting cytoplasmic [Ca2+] (free calcium concentration) and to increase Ca2+ flux at high [Ca2+] due to cooperative activation of MICUs EF-hands. However, mtCU and MICU1 functioning when its EF-hands are unoccupied by Ca2+ is poorly studied due to technical limitations. To overcome this barrier, we have studied the mtCU in divalent-free conditions by assessing the Ru265-sensitive Na+ influx using fluorescence-based measurement of mitochondrial matrix [Na+] (free sodium concentration) rise and the ensuing depolarization and swelling. We show an increase in all these measures of Na+ uptake in MICU1KO cells as compared to wild-type (WT) and rescued MICU1KO HEK cells. However, mitochondria in WT cells and MICU1 stable-rescued cells still allowed some Ru265-sensitive Na+ influx that was prevented by MICU1 in excess upon acute overexpression. Thus, MICU1 restricts the cation flux across the mtCU in the absence of Ca2+, but even in cells with high endogenous MICU1 expression such as HEK, some mtCU seem to lack MICU1-dependent gating. We also show rearrangement of the mtCU and altered number of functional channels in MICU1KO and different rescues, and loss of MICU1 during mitoplast preparation, that together might have obscured the pore-blocking function of MICU1 in divalent-free conditions in previous studies.