SMCP maintains ROS homeostasis during sperm mitochondrial sheath assembly.

In brief: In brief Sperm motility and male fertility depend on the proper assembly and function of the mitochondrial sheath during late spermiogenesis. This study shows that sperm mitochondria-associated cysteine-rich protein (SMCP) maintains reactive oxygen species homeostasis during mitochondrial sheath assembly, thereby supporting mitochondrial sheath function, sperm motility, and male fertility. Abstract: Abnormal morphology or function of the sperm mitochondrial sheath is a major cause of asthenozoospermia, yet the mechanisms governing its assembly and functional maturation remain poorly understood. Sperm mitochondria-associated cysteine-rich protein (SMCP) is a mitochondrial sheath-associated protein, and its regulation at the protein level during late spermiogenesis has attracted substantial interest. Here, we show that SMCP is expressed during mitochondrial sheath assembly in late spermiogenesis. SMCP is initially detected in the cytoplasm of elongating spermatids and, following cytoplasm removal, a fraction is retained in the mitochondrial sheath of mature spermatozoa. SMCP deficiency leads to markedly reduced sperm motility and male infertility without overt alterations in mitochondrial sheath morphology. Integrative proteomic and functional analyses further reveal that SMCP loss impairs mitochondrial respiratory chain function and reduces ATP production. Mechanistically, SMCP associates with glutaredoxin-1 (GLRX), and SMCP deficiency disrupts GLRX localization, accompanied by elevated reactive oxygen species levels during mitochondrial sheath assembly and in mature sperm. These redox defects likely contribute to compromised mitochondrial sheath function. Collectively, our findings identify SMCP as a regulator of functional module assembly within the sperm mitochondrial sheath and support the concept that mitochondrial sheath assembly represents not only a morphogenetic process but also a developmental window for functional maturation and redox programming, with the SMCP-GLRX complex serving as a localized antioxidant module during this period.