Mitochondria-endoplasmic reticulum contact sites in hepatocytic senescence

Inter-organelle communication via membrane contact sites (MCSs) is essential for the efficient functioning of eukaryotic cells, facilitating coordination among approximately 20 distinct organelles, each with unique metabolic profiles. Among these interactions, mitochondria-endoplasmic reticulum (ER) contacts (MERCs) are particularly significant, encompassing about 5% of the mitochondrial surface. Key proteins involved in MERCs include inositol 1,4,5-trisphosphate receptor (IP3R), voltage-dependent anion channel (VDAC), glucose-regulated protein 75 (GRP75), Sigma1 receptor (Sig-1R), vesicle-associated membrane protein (VAMP)-associated protein B (VAPB), protein deglycase DJ-1, and protein tyrosine phosphatase interacting protein 51 (PTPIP51), with new proteins continually being identified for their roles in these structures. At these contact sites, metabolic exchanges involve calcium (Ca²⁺), lipids, reactive oxygen species (ROS), and proteins. MERCs enable efficient molecular exchanges through temporary bridges mainly formed by the ER, the organelle with the largest surface area. These contacts are crucial for maintaining mitochondrial dynamics, which is essential for cellular homeostasis, and they are notably impacted in pathological states such as metabolic dysfunction-associated steatotic liver disease (MASLD), alcohol-related liver diseases (ALD), and viral hepatitis. Dysfunctional MERCs can lead to mitochondrial fragmentation, increased ROS production, impaired autophagy, and disrupted protein trafficking, thereby exacerbating senescence and cellular aging. Senescence is a cell fate initiated by stress, characterized by stable cell-cycle arrest and a hypersecretory state, and is an underlying cause of aging and many chronic conditions, including liver diseases. The hallmarks of senescence-such as macromolecular damage, cell cycle withdrawal, deregulated metabolism, and a secretory phenotype-are well established. However, recent studies have demonstrated that senescence is a heterogeneous process, with molecular markers varying according to the stressors that induce it. This review focuses on the functional aspects of MERCs in hepatic senescence and their impact on liver diseases, and explores the potential of targeting MERCs to address hepatocytic senescence.