Identification of a vesicle budding mechanism for the release of a meiotic maturation hormone from Caenorhabditis elegans

Abstract

Fertilization is a complex process involving several steps, including sperm activation, oocyte maturation, chemotaxis, gamete recognition, and cell fusion. Many of these essential steps are controlled and regulated by intercellular communication between gametes. This thesis work examines the communication events that occur between sperm and oocytes cells that facilitate fertilization. Oocyte meiotic maturation is one example of a step in fertilization in which intercellular signaling between gametes is required. In many species, sperm prepare the oocyte for fertilization by providing signals for meiotic maturation. Oocyte meiotic maturation is defined by the transition between diakinesis and metaphase I and is accompanied by MAP kinase activation, nuclear envelope breakdown, and meiotic spindle assembly. C. elegans sperm signal oocyte meiotic maturation using the major sperm protein (MSP) as a hormone. Interestingly, the MSP also functions as the central cytoskeletal protein required for the amoeboid motility of nematode sperm. The discovery of MSP’s signaling role raised the question of how sperm export MSP to signal oocytes at a distance. MSP lacks a hydrophobic leader sequence and C. elegans sperm lack many standard secretory components, such as ribosomes, ER, or Golgi. Using light and electron microscopy we analyzed the mechanism of MSP release from sperm. We demonstrate that sperm bud novel MSP vesicles to signal distant oocytes. These 150-300 nm MSP vesicles contain both an inner and an outer membrane, with MSP sandwiched in between. Budding protrusions from the cell body contain MSP, but not the MSD proteins, which counteract MSP filament assembly, suggesting that MSP may generate the protrusive force for its own vesicular export. MSP vesicles are labile structures that generate long-range MSP gradients for signaling at oocyte and sheath cell surfaces. Both spermatozoa and non-motile spermatids bud MSP vesicles, but their stability and signaling properties differ. Spermatozoa generate a long-range, short-acting signal, whereas spermatids generate a long-acting signal. EM results suggest that differential vesicle stability affects the physical and temporal range of signaling. We hypothesize that the MSP vesicle release mechanism is in itself signal dependent and signals derived from the female animal initiate MSP vesicle release.