Loss of Calmodulin- and Radial-Spoke-Associated Complex Protein CFAP251 Leads to Immotile Spermatozoa Lacking Mitochondria and Infertility in Men

Flagella and motile cilia share a 9 + 2 microtubule-doublet axoneme structure, and asthenozoospermia (reduced spermatozoa motility) is found in 76% of men with primary ciliary dyskinesia (PCD). Nevertheless, causal genetic variants in a conserved axonemal component have been found in cases of isolated asthenozoospermia: 30% of men with multiple morphological anomalies of sperm flagella (MMAF) carry bi-allelic mutations in DNAH1, encoding one of the seven inner-arm dynein heavy chains of the 9 + 2 axoneme. To further understand the basis for isolated asthenozoospermia, we used whole-exome and Sanger sequencing to study two brothers and two independent men with MMAF. In three men, we found bi-allelic loss-of-function mutations in WDR66, encoding cilia- and flagella-associated protein 251 (CFAP251): the two brothers were homozygous for the frameshift chr12: g.122359334delA (p.Asp42Metfs^∗4), and the third individual was compound heterozygous for chr12: g.122359542G>T (p.Glu111^∗) and chr12: g.122395032_122395033delCT (p.Leu530Valfs^∗4). We show that CFAP251 is normally located along the flagellum but is absent in men carrying WDR66 mutations and reveal a spermatozoa-specific isoform probably generated during spermatozoon maturation. CFAP251 is a component of the calmodulin- and radial-spoke- associated complex, located adjacent to DNAH1, on the inner surface of the peripheral microtubule doublets of the axoneme. In Tetrahymena, the CFAP251 ortholog is necessary for efficient coordinated ciliary beating. Using immunofluorescent and transmission electron microscopy, we provide evidence that loss of CFAP251 affects the formation of the mitochondrial sheath. We propose that CFAP251 plays a structural role during biogenesis of the spermatozoon flagellum in vertebrates.

LEM-domain proteins are lost during human spermiogenesis but BAF and BAF-L persist

During spermiogenesis the spermatid nucleus is elongated, and dramatically reduced in size with protamines replacing histones to produce a highly compacted chromatin. After fertilisation, this process is reversed in the oocyte to form the male pronucleus. Emerging evidence, including the coordinated loss of the nuclear lamina (NL) and the histones, supports the involvement of the NL in spermatid nuclear remodelling, but how the NL links to the chromatin is not known. In somatic cells, interactions between the NL and the chromatin have been demonstrated: LEM-domain proteins and LBR interact with the NL and respectively, the chromatin proteins BAF and HP1. We therefore sought to characterise the lamina-chromatin interface during spermiogenesis, by investigating the localisation of six LEM-domain proteins, two BAF proteins and LBR, in human spermatids and spermatozoa. Using RT-PCR, IF and western blotting, we show that six of the proteins tested are present in spermatids: LEMD1, LEMD2 (a short isoform), ANKLE2, LAP2β, BAF and BAF-L, and three absent: Emerin, LBR and LEMD3. The full-length LEMD2 isoform, required for nuclear integrity in somatic cells, is absent. In spermatids, no protein localised to the nuclear periphery, but five were nucleoplasmic, receding towards the posterior nuclear pole as spermatids matured. Our study therefore establishes that the lamina-chromatin interface in human spermatids is radically distinct from that defined in somatic cells. In ejaculated spermatozoa, we detected only BAF and BAF-L, suggesting that they might contribute to the shaping of the spermatozoon nucleus and, after fertilisation, its transition to the male pronucleus.