Camsap3-Mediated Microtubules Maintain Transzonal Projections Essential for Germline-Soma Communication during Ovarian Follicle Maturation in Mice

Oocytes are surrounded and nursed by layers of maternal somatic granulosa cells (GCs) in ovarian follicles. Within a follicle, GCs extend actin-containing transzonal projections (TZPs) to an oocyte across the zona pellucida to establish communication. In contrast, microtubules have rarely been observed in some TZPs, and their significance in TZP organisation and functions for follicular maturation remain largely unknown. Here, using super-resolution microscopy, we visualised the presence of microtubules alongside F-actin in most TZPs. Knockout (KO) mice of the microtubule minus-end binding protein Camsap3 (calmodulin-regulated spectrin-associated protein 3) exhibited infertility with fewer developing late-stage follicles. In earlier stages of Camsap3-KO follicles, the number of TZPs was reduced compared to that in wild-type follicles, and microtubules in TZPs were disorganised, which led to a decrease in physical contact between GCs and oocytes. We also found that TZP morphology in wild-type follicles transforms over time during follicle development, and Camsap3 modulates TZP morphology by assisting the organisation of microtubules as well as F-actin. Our findings thus suggest that Camsap3-mediated microtubules play a crucial role in governing the number and morphology of TZPs to ensure successful follicle development for the production of fertile oocytes.