MCU, originally known as CCDC109A, is widely recognized as the gene responsible for encoding a pore-forming subunit of a Ca²⁺-selective channel, mitochondrial Ca²⁺ uniporter complex (mtCUC). While MCU expression is typically highly mitochondrial-specific, we report here a protein variant derived from the MCU gene, termed MCU-S, which lacks the mitochondria-targeting sequence (MTS) and forms a Ca²⁺-permeable channel outside of mitochondria. The mRNA of MCU-S was ubiquitously expressed in all cell types/tissues tested, with particularly high expression in human platelets. MCU-S protein formed Ca²⁺ channels at the plasma membrane, which exhibited similar channel properties to those observed in mtCUC. MCU-S channels at the plasma membrane served as an additional Ca²⁺ influx pathway for platelet activation. Our findings show that the MCU-S functions are completely distinct from the originally reported functions of the MCU gene and provide additional insights into the molecular importance of MCU variant-dependent cellular Ca²⁺ handling.

Keywords: apoptosis; cell death; cell signaling; mitochondria.

Endoplasmic reticulum (ER)-mitochondrial (ER-Mito) interface, termed mitochondrial-ER contacts (MERCs), plays significant roles in the maintenance of bioenergetics and basal cell functions via the exchange of lipids, Ca²⁺, and reactive oxygen species (ROS) in various cell types/tissues. Genetic deletion of mitofusin 2 (Mfn2), one of the key components of ER-Mito tethering, in cardiomyocytes (CMs) in vivo revealed the importance of the microdomains between mitochondria and sarcoplasmic reticulum (SR), a differentiated form of ER in muscle cells, for maintaining normal mitochondrial Ca²⁺ (mtCa²⁺) handling and bioenergetics in the adult heart. However, key questions remain to be answered: 1) What tethering proteins sustain SR-Mito contact site structure in SR-Mito contact sites in the adult ventricular CMs (AVCMs), the predominant cell type in the adult heart? 2) Which MERC proteins operate in AVCMs to mediate specific microdomain functions under physiological conditions? and 3) How are the MERC protein expression profile and function altered in cardiac pathophysiology? In this review, we summarize current knowledge regarding the structure, function, and regulation of SR-Mito microdomains in the heart, with particular focus on AVCMs, which display unique membrane organization and Ca²⁺ handling compared with other cell types. We further explore molecular mechanisms underpinning microdomain dysfunction in cardiac diseases and highlight the emerging roles of MERC proteins in the development and progression of cardiac pathology.

Cardiac fibroblasts (CFs) are an attractive target for reducing pathological cardiac remodeling, and understanding the underlying mechanisms of these processes is the key to develop successful therapies for treating the pressure-overloaded heart. CF-specific knockout (KO) mouse lines with a Cre recombinase under the control of human TCF21 (hTCF21) promoter and/or an adeno-associated virus serotype 9 (AAV9)-hTCF21 system provide a powerful tool for understanding CF biology in vivo. Although a variety of rat disease models are vital for the research of cardiac fibrosis similar to mouse models, there are few rat models that employ cardiac cell-specific conditional gene modification, which has hindered the development and translational relevance of cardiac disease models. In addition, to date, there are no reports of gene manipulation specifically in rat CFs in vivo. Here, we report a simplified CF-specific rat transgenic model using an AAV9-hTCF21 system that achieved a CF-specific expression of transgene in adult rat hearts. Moreover, we successfully applied this approach to specifically manipulate mitochondrial morphology in quiescent CFs. In summary, this model will allow us to develop fast and simple rat CF-specific transgenic models for studying cardiovascular diseases in vivo.

Intermittent fasting (IF) extends life span via pleotropic mechanisms, but one important molecular mediator is adenosine monophosphate-activated protein kinase (AMPK). AMPK enhances lipid metabolism and modulates microtubule dynamics. Dysregulation of these molecular pathways causes right ventricular (RV) failure in patients with pulmonary arterial hypertension. In rodent pulmonary arterial hypertension, IF activates RV AMPK, which restores mitochondrial and peroxisomal morphology and restructures mitochondrial and peroxisomal lipid metabolism protein regulation. In addition, IF increases electron transport chain protein abundance and activity in the right ventricle. Echocardiographic and hemodynamic measures of RV function are positively associated with fatty acid oxidation and electron transport chain protein levels. IF also combats heightened microtubule density, which normalizes transverse tubule structure.

MCU is widely recognized as a responsible gene for encoding a pore-forming subunit of highly mitochondrial-specific and Ca 2+ -selective channel, mitochondrial Ca 2+ uniporter complex (mtCUC). Here, we report a novel short variant derived from the MCU gene (termed MCU-S) which lacks mitochondria-targeted sequence and forms a Ca 2+ - permeable channel outside of mitochondria. MCU-S was ubiquitously expressed in all cell-types/tissues, with particularly high expression in human platelets. MCU-S formed Ca 2+ channels at the plasma membrane, which exhibited similar channel properties to those observed in mtCUC. MCU-S channels at the plasma membrane served as an additional Ca 2+ influx pathway for platelet activation. Our finding is completely distinct from the originally reported MCU gene function and provides novel insights into the molecular basis of MCU variant-dependent cellular Ca 2+ handling.