Ageing-Dependent Thyroid Hormone Receptor α Reduction Activates IP3R1-Meditated Ca²⁺ Transfer in MAM and Exacerbates...

Sarcopenia profoundly impacts the quality of life and longevity in elderly populations. Notably, alterations in thyroid hormone (TH) levels during ageing are intricately linked to the development of sarcopenia. In skeletal muscle, the primary action of TH is mediated through the thyroid hormone receptor alpha (TRα). Emerging evidence suggests that decreased TRα expression may precipitate mitochondrial dysfunction in ageing skeletal muscle tissues. Yet, the precise mechanisms and the potential causative role of TRα deficiency in sarcopenia are not fully understood. This study suggests that TRα may regulate mitochondrial calcium (Ca²⁺) transport across membranes by targeting the inositol 1,4,5-trisphosphate receptor 1 (IP3R1), as evidenced by ChIP-seq and RNA-seq analyses. Experiments using naturally aged mice, skeletal muscle-specific TRα knockout (SKT) mice, and C2C12 myoblasts were conducted to investigate this process further. Findings include increased IP3R1, mitochondria-associated endoplasmic reticulum membranes (MAM), and mitochondrial Ca²⁺ in aged skeletal muscle. Additionally, SKT mice exhibited smaller muscle fibres, increased IP3R1 and MAM, and mitochondrial dysfunction. ChIP-qPCR and TRα manipulation in C2C12 cells showed that TRα negatively regulates IP3R1 transcription. Moreover, TRα knockdown cells exhibited increased Ca²⁺ transfer in MAM and mitochondrial dysfunction, which was ameliorated by the IP3R1 inhibitor 2-aminoethoxydiphenyl borate. Reintroduction of TRα improved IP3R1-mediated mitochondrial Ca²⁺ overload in aged cells. Our findings uncover a novel mechanism by which TRα deficiency induces mitochondrial Ca²⁺ overload through IP3R1-mediated Ca²⁺ transfer in MAM, exacerbating skeletal muscle atrophy during ageing. The TRα/IP3R1 pathway in MAM Ca²⁺ transfer presents a potential therapeutic target for sarcopenia.