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.