A K(+)-selective CNG channel orchestrates Ca(2+) signalling in zebrafish sperm

Area of Science:

• Reproductive Biology
• Ion Channel Physiology
• Aquatic Animal Reproduction

Background:

• Calcium (Ca2+) signaling is vital for sperm navigation in marine invertebrates and mammals, but poorly understood in fish.
• In sea urchins, cyclic nucleotide-gated potassium channels (CNGK) regulate Ca2+ influx via cGMP-induced hyperpolarization.

Purpose of the Study:

• To identify and characterize novel ion channels involved in sperm function in the freshwater fish Danio rerio (zebrafish).
• To investigate the role of these channels in sperm navigation and fertilization mechanisms specific to fish.

Main Methods:

• Identification of a novel CNGK family member in zebrafish sperm.
• Analysis of channel properties, including localization, activation by intracellular pH, and response to cyclic nucleotides.
• Observation and analysis of sperm swimming behavior induced by Ca2+ influx.

Main Results:

• A novel CNGK family member was identified in the sperm head, not the flagellum, of Danio rerio.
• This channel is activated by intracellular pH (alkalization) and mediates Ca2+ entry, independent of cyclic nucleotides.
• Ca2+ influx triggers a unique 'spinning' swimming mode, distinct from other species, likely aiding micropyle navigation.

Conclusions:

• Zebrafish sperm utilize a unique pH-gated ion channel for Ca2+ regulation, leading to a specialized swimming behavior for fertilization.
• This discovery reveals diverse activation mechanisms and functional adaptations of sperm ion channels across different species.
• Sperm channel evolution reflects species-specific adaptations to fertilization challenges.