Localization of the mitochondrial uncoupling protein family in the rat inner ear

Uncoupling proteins (UCPs) are a proton transporter family located in the mitochondrial inner membrane. The molecular expression and activity of UCPs in brown adipose tissue and skeletal muscle are regulated by factors as diverse as chronic overeating and cold exposure, suggesting roles in energy expenditure and heat production. Although UCP2, UCP4 and brain mitochondrial carrier protein-1 (BMCP-1, i.e. UCP5) mRNAs are expressed in the central nervous system, their central function is unknown. This study presents the first evidence on localization and quantitative expression of UCPs in the rat inner ear by real-time PCR and immunohistochemistry. Real-time PCR studies revealed that UCP2 mRNA was expressed in the vestibular and spiral ganglia more abundantly than any other UCP. Neocortex, by contrast, contained UCP2 and UCP4 equally. Notably, UCP3 and UCP4 mRNAs were expressed in inner ear ganglia, but brain UCP3 mRNA expression level was undetectable by simple PCR. Immunohistochemical studies confirmed that both UCP2- and UCP3-like immunoreactivities were detected in vestibular and spiral ganglion cells and co-localized with a mitochondrial marker, MitoFluorGreen. According to previous reports, UCP2 and UCP3 are thermogenic in yeast and brain UCP2 has been suggested to modulate pre- and post-synaptic events by axonal thermogenesis. It has also been reported recently that UCP2 and UCP3 responses to superoxide application may be an antioxidant protective mechanism. Therefore, it is suggested that mitochondrial UCPs (UCP2, UCP3, UCP4) may play both a protective role against oxidative damage and a thermal signaling role in the eighth nerve.