A Novel mTOR-Stat3-Stathmin Pathway Establishes Oocyte Polarization Competence by Orchestrating Centrosome Regulation and Microtubule Organization

In animals and plants, egg production is essential for fertility, reproduction, and embryonic development. The production of functional eggs in sufficient numbers for a female lifespan requires dynamic and precisely coordinated cellular and developmental programs in early oogenesis. However, our understanding of their underlying regulatory mechanisms is critically lacking. Here, we aimed to identify overlooked essential regulators of early oogenesis in zebrafish. First, we present the establishment of a long-term ovary culture system which enables physiological oocyte development from oogonia to primordial follicles in cultured ovaries, providing an invaluable ex-vivo platform for rapid investigation. Next, we utilized this system for robust functional screening of candidates from stage-specific oocyte transcriptomic data. We identified mTOR, Stat3, and Stathmin as novel regulators of oocyte polarity. By using a combination of genetics, pharmacological manipulations ex-vivo, and rescue experiments, we established an essential mTOR-Stat3-Stathmin pathway that orchestrates centrosome regulation and microtubule organization to facilitate oocyte polarity. Microtubules control the localization and condensation of the essential polarity regulator Bucky ball (Buc). Loss of mTOR or Stat3 functions, as well as overactivation of Stathmin (a microtubule destabilizing protein) resulted in aberrant centrosome regulation and destabilization of microtubules, leading to dispersed mis-localized Buc condensates and loss of polarity. We show that mTOR acts upstream of Stat3 in oocytes, and that inhibition of Stathmin in stat3-/-or mTOR deficient ovaries rescued both cytoskeletal and polarity defects. We propose a novel mTOR-Stat3-Stathmin pathway which through cytoskeletal regulation, provides oocytes with polarization competence, a step likely widely conserved in biology. mTOR, Stat3, and Stathmin are known for their roles in cellular growth and cancer. Our work reveals their novel unpredicted functions in cell polarity during animal post-embryonic development.