Nucleoporin NUP210L and BAF-paralogue BAF-L redundantly ensure nuclear integrity and manchette microtubule organisation in mouse spermatids

During spermiogenesis, round spermatids differentiate into spermatozoa. This involves nuclear elongation and condensation, acrosome spreading, and flagellum formation, orchestrated by cytoskeletal elements-acroplaxome and manchette-that attach to the nuclear envelope (NE) except at the caudal pole where nuclear pore complexes (NPCs) form a dense array. Evidence that the NPC contributes structurally to nuclear remodelling comes from a unique case of human male infertility, lacking the spermatid-specific nucleoporin NUP210L, where most sperm heads are round and uncondensed. Mice lacking NUP210L are fertile, however, producing essentially normal spermatozoa. Here, we reveal function at the caudal NE in mice lacking NUP210L and BAF-L, a spermatid-specific paralogue, and interactor, of chromatin protein BAF. Like NUP210L, BAF-L localises at the caudal NE in human elongating spermatids, and its inactivation in mouse has little impact on sperm morphology or fertility. We show here, however, that in mice lacking NUP210L, two copies of BAF-L become essential for fertility. In Banf2+/-, Nup210l-/- and Banf2-/-, Nup210l-/- mice, most spermatids arrest during elongation (step 10-11), manchette microtubules frequently invaginate elongating nuclei from the caudal pole, and the NPC array forms at steps 8-10 but shows reduced stability later at steps 12-16. We show that BAF-L interactor, BAF, also concentrates at the caudal pole (step 8-9), independently of NPCs. We propose that BAF-L and NUP210L optimise interactions between NPC, chromatin and the cytoskeleton, to ensure nuclear integrity and fertility. Our study implies that digenic inheritance of mutations in non-homologous genes should be considered when investigating genetic causes of human male infertility.