Testis-specific cytochrome c-null mice produce functional sperm but undergo early testicular atrophy

Differentiating male germ cells express a testis-specific form of cytochrome c (Cyt c_T) that is distinct from the cytochrome c expressed in somatic cells (Cyt C_S). To examine the role of Cyt c_T in germ cells, we generated mice null for Cyt c_T. Homozygous Cyt c_T^-/- pups were statistically underrepresented (21%) but developed normally and were fertile. However, spermatozoa isolated from the cauda epididymis of Cyt c_T-null animals were less effective in fertilizing oocytes in vitro and contain reduced levels of ATP compared to wild-type sperm. Sperm from Cyt c_T-null mice contained a greater number of immotile spermatozoa than did samples from control mice, i.e., 53.1% ± 13.7% versus 33.2% ± 10.3% (P < 0.0001) for vas deferens sperm and 40.1% ± 9.6% versus 33.2% ± 7.5% (P = 0.0104) for epididymal sperm. Cyt c_T-null mice often exhibit early atrophy of the testes after 4 months of age, losing germ cells as a result of increased apoptosis. However, no difference in the activation of caspase-3, -8, or -9 was detected between the Cyt c_T^-/- testes and controls. Our data indicate that the Cyt c_T-null testes undergo early atrophy equivalent to that which occurs during aging as a consequence of a reduction in oxidative phosphorylation.