From patterning to secretion: Kv2.1 subunits as regulators of zebrafish hatching gland morphogenesis and function
Zebrafish hatching, a critical developmental milestone, occurs around 48-72 hours post-fertilization (hpf). It is regulated by the specialized secretory organ called the hatching gland. Voltage-gated potassium channels (Kv) are known for their roles in maintaining plasma membrane potential and regulating intracellular protein traffic and secretion. Previous studies on zebrafish mutants of Kv2.1 channel subunits - the electrically active subunit Kcnb1 and the modulatory subunit Kcng4b - revealed antagonistic functions in the development of brain ventricles, ear, and Reissner fiber. In this study, we investigated their functional role in the hatching gland. The loss of either subunit resulted in a significant delay in normal hatching. Using in situ hybridization and immunohistochemistry, we show that both mutants exhibited severe defects in the hatching gland patterning, including a reduced number of hatching gland cells. The mutants displayed changes in the transcript levels of several hatching gland markers and reduced cell proliferation in this organ. These developmental defects were intensified by a late-stage functional failure characterized by decreased cathepsin synthesis, reduced proteolytic activity, and delay in the period of secretion in both mutants. Together, our findings establish that Kv2.1 subunits, Kcnb1, and Kcng4b are essential during the development of the zebrafish hatching gland and its secretion.