hnRNP I is required to generate the Ca²⁺ signal that causes egg activation in zebrafish

Egg activation is an important cellular event required to prevent polyspermy and initiate development of the zygote. Egg activation in all animals examined is elicited by a rise in free Ca²⁺ in the egg cytosol at fertilization. This Ca²⁺ rise is crucial for all subsequent egg activation steps, such as cortical granule exocytosis, which modifies the vitelline membrane to prevent polyspermy. The cytosolic Ca²⁺ rise is primarily initiated by inositol 1,4,5-trisphosphate (IP₃)-mediated Ca²⁺ release from the endoplasmic reticulum. The genes involved in regulating the IP₃-mediated Ca²⁺ release during egg activation remain largely unknown. Here we report on a zebrafish maternal-effect mutant, brom bones, which is defective in the cytosolic Ca²⁺ rise and subsequent egg activation events, including cortical granule exocytosis and cytoplasmic segregation. We show that the egg activation defects in brom bones can be rescued by providing Ca²⁺ or the Ca²⁺-release messenger IP₃, suggesting that brom bones is a regulator of IP₃-mediated Ca²⁺ release at fertilization. Interestingly, brom bones mutant embryos also display defects in dorsoventral axis formation accompanied by a disorganized cortical microtubule network, which is known to be crucial for dorsal axis formation. We provide evidence that the impaired microtubule organization is associated with non-exocytosed cortical granules from the earlier egg activation defect. Positional cloning of the brom bones gene reveals that a premature stop codon in the gene encoding hnRNP I (referred to here as hnrnp I) underlies the abnormalities. Our studies therefore reveal an important new role of hnrnp I in regulating the fundamental process of IP₃-mediated Ca²⁺ release at egg activation.