Tnfr1-associated Signaling Proteins in Mature Human Spermatozoa

Tumor necrosis factor-alpha (TNF-α) is a pleiotropic cytokine that activates the extrinsic apoptotic pathway through TNFR1, leading to recruitment of adaptor proteins and caspase activation in most somatic cell types. Although TNF-α has been implicated in inflammation-associated sperm dysfunction, the expression and functional competence of TNFR1 in mature human spermatozoa remain poorly defined. Here, we report that human spermatozoa express TNFR1 and that the receptor localizes along the flagellum, where it colocalizes with caveolin-1 within membrane microdomains. However, exposure to TNF-α across a wide concentration range did not affect sperm motility or mitochondrial membrane potential and failed to induce activation of caspase-8 or caspase-3. Molecular analyses revealed markedly reduced expression of the adaptor proteins TRADD and FADD and an absence of detectable pro-caspase-8 in spermatozoa compared with Jurkat cells. These findings indicate that, despite preserved receptor expression and membrane compartmentalization, the downstream extrinsic apoptotic machinery is profoundly limited in mature sperm cells. Our data support a model in which death receptor expression and downstream signaling competence become developmentally uncoupled during terminal differentiation, reflecting selective remodeling of apoptotic signaling pathways.