HCN channels in the mammalian cochlea: Expression pattern, subcellular location, and age-dependent changes

Area of Science:

• Neuroscience
• Auditory Physiology
• Molecular Biology

Background:

• Neuronal diversity in the cochlea is influenced by ion channels, particularly hyperpolarization-activated cyclic nucleotide-gated (HCN) channels.
• The precise functions and localization of HCN channels in auditory systems remain controversial and poorly understood.

Purpose of the Study:

• To create a detailed map of the subcellular location and co-expression patterns of all four HCN channel subunits across various mammalian species.
• To investigate the correlation between HCN subunit expression levels and age-related hearing deterioration in specific mouse strains.
• To elucidate the spatiotemporal expression of HCN channels during auditory neuron development and their role in hair cell innervation.

Main Methods:

• Subcellular localization and co-expression analysis of HCN subunits (HCN1-4) in auditory neurons from mice (CBA/J, C57Bl/6N, Ly5.1), guinea pigs, cats, and humans.
• Correlation of age-related hearing loss with HCN subunit expression in auditory neurons.
• Analysis of spatiotemporal expression during murine postnatal development.
• Immunostaining for HCN channel subunits.

Main Results:

• A comprehensive map of HCN subunit localization and co-expression was established across species, highlighting neuron clusters as key expression sites.
• HCN2 and HCN4 subunits are involved in a critical phase of hair cell innervation during postnatal development.
• While subunit diversity is high, heteromeric pairing was limited along the perisomatic membrane and axon initial segments.
• Age-related changes in HCN expression varied by mouse strain and did not directly correlate with audiometric data, showing either up- or downregulation.
• HCN3 channels were unexpectedly found in outer hair cells, overlapping with prestin, suggesting a role in outer hair cell function.

Conclusions:

• HCN channels play a significant, diverse role in auditory neuron function and development across mammals.
• The specific localization and expression patterns suggest an active role for auditory neurons in modulating their electrical properties via HCN channels.
• Understanding HCN channel involvement in auditory pathways, including outer hair cells, may offer new strategies for optimizing electrical stimulation in cochlear implants.