Time course of auditory impairment in mice lacking the electroneutral sodium bicarbonate cotransporter NBC3 (slc4a7)

Mice with a targeted disruption of the gene encoding the stilbene-insensitive electroneutral sodium bicarbonate cotransporter (NBC3; slc4a7) exhibit cochlear and retinal degeneration. To establish the progressive nature of sensory cells loss in slc4a7^−/− deficient mice, we studied the morphology of cochleas of slc4a7^−/− and slc4a7^+/+ mice from postnatal day two (P2) to ninety (P90). Cell death was evaluated in slc4a7^−/− cochleas using the TUNEL technique and caspase-3 immunoreactivity. The time course of NBC3 expression in the cochlea was assessed by immunohistochemistry using an antibody against NBC3. Between P2 and P8, slc4a7^−/− mice cochlea exhibit normal morphology. There was a normal complement of inner and outer hair cells from the hook to the apical region. At P15, slc4a7^−/− mice cochlea inner and outer hair cells were still present at the hook region, and vacuoles were seen underneath Hensen's cells. At P21, inner and outer hair cells were degenerated in this region. Between P30 and P90, there was a pronounced loss of hair cells and spiral ganglia neurons. Morphological analysis of the spiral ligament showed a progressive loss of type II and IV fibrocytes beginning at day 21. Transmission electron microscopy observations at P30 and P90 revealed that type II and IV fibrocytes showed shrinkage and vacuolization. In addition, hair cells were deteriorated with evidence of shrinkage and picnotic nuclei. TUNEL staining showed apoptotic cells at P8 in the organ of Corti at the basal region of the cochlea. At P15, caspase-3 immunoreactivity was present in supporting cells of the organ of Corti. NBC3 mild immunoreactivity was detected in the organ of Corti at P11. There was an increase in the expression of NBC3 in the spiral ligament between P17 and P19. From P21 to P90, NBC3 expression was confined to the spiral ligament and inner and outer sulcus cells. The vestibular sensory epithelia from slc4a7^−/− mice were normal from P2 to P90. Damage of the sensory epithelia at the high frequency zone of the cochlea suggests that NBC3 may play an important physiological role in this region.