NRSA for Mohammed Hoque: The Roles of Cby1L and ciBAR1 in Spermatogenesis

Project Summary The sperm flagellum is a modified motile cilium that is essential for locomotion and fertilization of the egg. As such, defective flagellogenesis is linked to infertility. Although not completely equivalent, similar mechanisms are thought to operate during flagellogenesis and ciliogenesis. Defects in ciliogenesis are associated with human disorders, known as ciliopathies, such as primary ciliary dyskinesia, and many of the patients also present with fertility issues. Despite tremendous research efforts to understand the role of ciliary proteins in these diseases, their physiological roles in male fertility remain poorly defined. We reported that mice lacking the critical ciliary protein Chibby 1 (Cby1) display signs of classical ciliopathies. Molecularly, Cby1 interacts with the membrane binding Bin/Amphiphysin/Rvs (BAR) domain proteins, Cby1-interacting BAR domain-containing 1 and 2 (ciBAR1 and 2, formerly known as FAM92A and B), to facilitate ciliogenesis. More recently, we found that the Chibby family member, Cby1-Like (Cby1L, also known as Cby3), is expressed abundantly and almost exclusively in the testis. I found that Cby1L and ciBAR1 localize to the annulus, a septin-based ring structure on the flagellum, in the testis. Both Cby1L-/- and ciBAR1-/- male mice are infertile with the majority of sperm showing kinked tails. In addition, I found that Cby1L physically interacts with ciBAR1 and the annulus septin SEPT4. Interestingly, in the absence of Cby1L or ciBAR1, the annulus is present but fails to stop at the midpiece/principal piece (MP/PP) junction and progresses posteriorly into the PP. These data suggest that Cby1L and ciBAR1 form a complex that assists in proper positioning of the annulus at the MP/PP junction. Although the annulus is typically bound to membranes during migration, in Cby1L-/- testis, the annulus is partially detached from the flagellar membrane, suggesting that Cby1L is involved in anchoring the annulus to the membrane. Collectively, my preliminary data suggest a working model in which a complex of Cby1L and ciBAR1 plays an indispensable role in the accurate positioning of the annulus at the MP/PP junction. The overall goal of this proposal is to understand the molecular roles of Cby1L and ciBAR1 in spermatogenesis and annulus positioning. To achieve this goal, I propose the following Specific Aims: 1, Define the role of Cby1L in flagellogenesis and annulus migration. I will determine the subcellular localization of Cby1L during spermatogenesis and investigate the molecular and cellular basis of the male infertility of Cby1L-/- mice. 2, Investigate Cby1L/ciBAR1/septin interactions and ciBAR1’s role in sperm tail segmentation. I predict that Cby1L forms a complex with ciBAR1 and annulus septins to mediate the attachment of the annulus to the flagellar membrane, which contributes to the proper compartmentalization of the sperm tail. In parallel, I will determine the cause of male infertility in ciBAR1-/- male mice. Overall, I expect that my proposed research will unravel a novel mechanism for the proper positioning of the annulus, by ciliary proteins, which contributes to proper sperm tail segmentation.