Reproduction Abstracts

Importin protein family members are best known for serving critical roles in mediating regulated nucleocytoplasmic transport, and many are essential to developmental events including gametogenesis. We proposed that coordinated expression of an importin and a cell-specific transcription factor would drive a developmental switch by enabling transcription and other nuclear factors to enter the nucleus appropriately. Analyses of importin alpha (IMPα) and beta (IMPβ) mRNAs and proteins revealed their synthesis and subcellular localization are tightly regulated throughout spermatogenesis. Although normally cytoplasmic, certain IMPαs are nuclear-localized in meiotic and haploid male germ cells. IMPα-specific binding partners differ between these germ cells; intriguingly, many are not nuclear proteins, and several are essential for male fertility. IMP proteins localize to distinct regions in mature sperm, highlighting their potential to serve as adaptors for protein trafficking to sites other than the nucleus. Our HeLa cell work showed that IMPα proteins become nuclear-localised sequestered following transient exposure to cellular stressors and that importins can function in the nucleus to control gene transcription. This led to identification of STK35 as a unique transcriptional target for nuclear-localized IMPα proteins that is also highly expressed in testes, and Stk35 transcripts are relatively high in germ cell subtypes that contain nuclear-localized IMPα proteins. In addition, our data implicate importins in germ cell stress responses. Interrogation of our unique mouse models with higher and lower IMPα4 levels in spermatids has provided in vivo evidence that nuclear IMPα proteins affect cell fate in response to oxidative stress. Thus interrogation of IMPα functions during spermatogenesis showed that importins function at several levels to modify cellular fate. Our growing understanding is that this can occur through regulating transcription factor access to the nucleus, by acting within the nucleus to directly influence transcription, and through modulating subcellular localization of proteins required for fertility.