Topological and Developmental Expression Gradients of Kir2.1, an Inward Rectifier K+ Channel, in Spiral Ganglion and Cochlear Hair Cells of Mouse Inner Ear

Abstract

Objectives: Inwardly rectifying potassium current plays critical roles in setting resting membrane potential and thus modulating the excitability of many excitable cells including the hair cells in inner ears, which are excitable during early development. Up to 7 subfamilies have been identified as channels for this current. The present study investigated the developmental and spatial expression of one member, Kir2.1, in the sensorineural epithelia and spiral ganglion neurons of mouse cochleae starting from neonatal stage to the maturation of hearing function, in an attempt to verify its role in the postnatal cochlear development. Methods: The distribution of Kir2.1 protein and mRNA was observed using immunofluorescence staining and in situ hybridization on samples of surface preparation and cross sections of cochleae. The protein and mRNA were further measured with semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) and Western blotting to show the developmental change of this channel in the cochlea of mice. Results: The expression of Kir2.1 appears to have a clear temporal pattern: increasing from the first postnatal day (PD0) to PD12 and then quickly decreasing after that. The expression cannot be detected on PD20. Throughout this developmental pattern, the expression was stronger at the basal turn on PD0 and shifted upwards (longitudinal gradient), so that stronger signals were generally seen in the apical parts of the cochlea after PD0. A radial gradient of the Kir2.1 protein was also evident with a stronger and consistent signal usually occurring on row 3 outer hair cells. The spatial and developmental changes of Kir2.1 mRNA in in situ hybridization exhibited similar patterns as seen for the Kir2.1 protein on PD8 and PD12. Western blot data showed a slightly higher concentration of the protein in the apical half of the cochlea and a slight increase from PD4 to PD12. This result is consistent with the quantification of mRNA in RT-PCR. No significant expression of Kir2.1 was found in spiral ganglion neurons. Conclusions: Kir2.1 exhibits a clear and temporal expression in the hair cells of mouse cochleae which may be related to the functional maturation of the hair cells and the neurons.