An mTOR-Stat3-Stathmin pathway controls centrosome and microtubule dynamics for oocyte polarization

In animals and plants, egg production is essential for fertility, reproduction, and embryonic development. However, our understanding of the regulatory mechanisms underlying the cellular and developmental programs of oogenesis is lacking. Here, we aimed to identify overlooked regulators of early oogenesis in zebrafish. First, we established a long-term ovary culture system that enables physiological oocyte development from oogonia to primordial follicles, providing an effective ex vivo platform for rapid investigation. Next, we utilized this system for functional screening of candidates from stage-specific oocyte transcriptomic data. We identified mammalian target of rapamycin (mTOR), signal transducer and activator of transcription 3 (Stat3), and the microtubule-destabilizing protein Stathmin as novel regulators of oocyte polarity. In zebrafish and most species, oocyte polarity is essential for oogenesis and embryonic development. In polarization, microtubules control the localization of the polarity regulator Buc to the centrosome, as well as its condensation into the Balbiani body, an oocyte membraneless compartment. Through a combination of genetics and pharmacological manipulations in vivo and ex vivo, we show that loss of mTOR or Stat3 functions, as well as overactivation of Stathmin, disrupted centrosome regulation and destabilized microtubules, leading to dispersed Buc condensates and loss of polarity. We demonstrate that mTOR acts upstream of Stat3 in oocytes, and inhibition of Stathmin in either stat3^-/- or mTOR-deficient ovaries rescued both cytoskeletal and polarity defects. Thus, we established an unpredicted but essential mTOR-Stat3-Stathmin pathway that controls centrosome and microtubule dynamics to drive oocyte polarity. We propose that through cytoskeletal regulation, this pathway provides oocytes with polarization competence, a likely common step in cell polarity.