Genetic factors derived from the MRL/MpJ mouse function to maintain the integrity of spermatogenesis after heat exposure.

MRL/MpJ mice possess highly heat-shock-resistant spermatocytes (HRS) in comparison with C57BL/6 mice. This resistance depends on the MRL/MpJ-type loci at the 81 cM region of Chromosome (Chr) 1 and the 40 cM region of Chr 11. To evaluate the functions of these loci in detail, we examined the histopathological changes resulting from experimental cryptorchidism or transient scrotal heat stress (SHS) in the testes of C57BL/6-based congenic strains (B6.MRLc1, B6.MRLc11, and B6.MRLc1c11) carrying the MRL/MpJ-derived loci responsible for HRS. Among cryptorchid testes from congenic strains, those in B6.MRLc1c11 mice showed the highest heat resistance, indicating that the genetic interactions between MRL/MpJ-derived HRS loci on Chrs 1 and 11 may be important for maintaining spermatogenesis under continuous testicular hyperthermia. In contrast, immediately after SHS induction, germ cell loss via apoptosis was inhibited in B6.MRLc11 and B6.MRLc1c11 mice, similar to that in MRL/MpJ mice. However, this HRS phenotype was not observed in C57BL/6 or B6.MRLc1 mice after SHS induction. Furthermore, testicular calcification owing to long-term damage by SHS induction was inhibited in all congenic strains in comparison with that in C57BL/6 mice, indicating that each MRL/MpJ-derived locus on Chrs 1 and 11 acted independently to facilitate the recovery of heat-induced testicular damage by inhibiting calcification. B6.MRLc11 and B6.MRLc1c11 mice showed greater recovery in spermatogenesis than B6.MRLc1 mice 60 days after SHS induction. Therefore, the MRL/MpJ-derived HRS locus on Chr 11 might play an important role in recovery from heat stress damage. On the basis of these results, we concluded that MRL/MpJ-derived loci on Chrs 1 and 11 cooperatively or independently regulate testicular heat sensitivity depending on the various heat stresses.