A New Player in the Spermiogenesis Pathway of Caenorhabditis elegans

Abstract

Precise timing of sperm activation ensures the greatest likelihood of fertilization. Precision in Caenorhabditis elegans sperm activation is ensured by external signaling, which induces the spherical spermatid to reorganize and extend a pseudopod for motility. Spermatid activation, also called spermiogenesis, is prevented from occurring prematurely by the activity of SPE-6 and perhaps other proteins, termed "the brake model." Here, we identify the spe-47 gene from the hc198 mutation that causes premature spermiogenesis. The mutation was isolated in a suppressor screen of spe-27(it132ts), which normally renders worms sterile, due to defective transduction of the activation signal. In a spe-27(+) background, spe-47(hc198) causes a temperature-sensitive reduction of fertility, and in addition to premature spermiogenesis, many mutant sperm fail to activate altogether. The hc198 mutation is semidominant, inducing a more severe loss of fertility than do null alleles generated by CRISPR-associated protein 9 (Cas9) technology. The hc198 mutation affects an major sperm protein (MSP) domain, altering a conserved amino acid residue in a b-strand that mediates MSP-MSP dimerization. Both N-and C-terminal SPE-47 reporters associate with the forming fibrous body (FB)-membranous organelle, a specialized sperm organelle that packages MSP and other components during spermatogenesis. Once the FB is fully formed, the SPE-47 reporters dissociate and disappear. SPE-47 reporter localization is not altered by either the hc198 mutation or a C-terminal truncation deleting the MSP domain. The disappearance of SPE-47 reporters prior to the formation of spermatids requires a reevaluation of the brake model for prevention of premature spermatid activation.

Functionally non-redundant paralogs spe-47 and spe-50 encode FB-MO associated proteins and interact with him-8

“…When we first discovered that spe-47 harbored the hc198 mutation that suppressed spe-27(it132ts) sterility by inducing premature spermatid activation [ 18 ], we became aware that there was a closely-related paralog present in the genome: Y48B6A.5 ( Fig 1A and 1B ). The SPE-47 and Y48B6A.5 proteins exhibit a high degree of sequence conservation, with both having an N-terminus of unknown function and a C-terminal MSP domain that lacks the final β strand ( Fig 1B ).…”

Functionally non-redundant paralogsspe-47andspe-50encode FB-MO associated proteins and interact withhim-8

“…When we first discovered that spe-47 harbored the hc198 mutation that suppressed spe-27(it132ts) sterility by inducing premature spermatid activation [18], we became aware that there was a closely-related paralog present in the genome: Y48B6A.5 (Fig 1 A&B). The SPE-47 and SPE-50 proteins exhibit a high degree of sequence conservation, with both having an N-terminus of unknown function and a C-terminal MSP domain that lacks the final ß strand (Fig 1 B).…”

“…The spe-27 mutation causes hermaphrodite sterility because the self-spermatids are unresponsive to the signal to activate [32]. The spe-47(hc198) mutation recovered from the screen causes some spermatids to activate precociously without the need for an activation signal, overcoming the sterility of spe-27(it132ts) (Fig 4) [18]. If SPE-50 is functionally redundant to SPE-47, then a mutation similar to hc198 in spe-50 should also result in precocious spermatid activation.…”

“…Even though the spe-50(ttTi4488) transposon insertion disrupts spe - 50 , it had essentially no effect on fertility (Fig 5). In our previous study of spe-47 , we created a knockout allele, spe-47(zq19) (Fig 1A), which caused only a slight reduction in fertility [18]. In the interim, a second isoform of the spe-47 transcript, which is unaffected by the spe-47(zq19) mutation, was identified.…”

Genome Editing in C. elegans and Other Nematode Species

“…These pathways functioned redundantly in ancestral males [ 29 ]. Characterizations of an identified spe-47 ( hc198 ) allele using the CRISPR system showed that the hc198 mutation is able to bypass the SPE-8 signaling pathway to cause premature activation of a small fraction of sperm [ 63 ]. In addition, the authors have found that the nucleosome remodeling factor (NURF) complex has acquired a unique role in the sperm/oocyte decision of C. briggsae [ 62 ].…”