The nuclear transport factor IPO5 revealed as a critical mediator of male germline development†

The highly conserved nuclear transport protein importin 5 (IPO5) binds cargo implicated in fundamental processes including virus and chromatin assembly, germline development, and cell signaling. It also anchors cell-specific cargo for functional outcomes in development and immune responses. IPO5 displays both spatial and temporal regulation in the male germline, from fetal through to adult ages. Because it transports key early developmental/reproductive factors, including Stella and the BMP signaling SMADs 1/5/9, we hypothesized that targeted IPO5 deletion would impair germline development and viability at specific stages. Here, we demonstrate in vivo functional importance of IPO5 by generating global and conditional IPO5 knockout mice using an Ipo5FL/FL allele flanking exons 9 and 10. Global deletion using CMVCre produced no null embryos at embryonic day (E)12.5, while heterozygous embryo numbers were reduced to 50%, demonstrating it is essential for early embryogenesis. A sex-specific germline requirement for IPO5 was demonstrated following deletion using VasaCre (active from E15.5); adult testes lacked germ cells, while oocytes developed and female fertility was unaffected. Stra8Cre-directed IPO5 deletion (active from postnatal day (PND) 3) caused meiotic failure evident at PND 14; no IPO5-deficient germ cells were present in adults, although niche integrity and function supported emergence of rare IPO5-positive spermatozoa. Novel IPO5 binding proteins identified by immunoprecipitation and mass-spectrometry included SFPQ in fetal testes and XPO2 (exportin 2) in both isolated spermatocytes and spermatids. Remarkably, most IPO5 potential binding proteins are essential for male fertility. These results define IPO5 as crucial for in vivo embryonic development and male fertility.