Effects of Ageing on the Mitochondrial Genome in Rat Vestibular Organs

Abstract

Background: Deterioration in vestibular function occurs with ageing and is linked to age-related falls. Sensory hair cells located in the inner ear vestibular labyrinth are critical to vestibular function. Vestibular hair cells rely predominantly on oxidative phosphorylation (OXPHOS) for energy production and contain numerous mitochondria. Mitochondrial DNA (mtDNA) mutations and perturbed energy production are associated with the ageing process.

Objective: We investigated the effects of ageing on mtDNA in vestibular hair and support cells, and vestibular organ gene expression, to better understand mechanisms of age-related vestibular deficits.

Methods: Vestibular hair and supporting cell layers were microdissected from young and old rats, and mtDNA was quantified by qPCR. Additionally, vestibular organ gene expression was analysed by microarray and gene set enrichment analyses.

Results: In contrast to most other studies, we found no evidence of age-related mtDNA deletion mutations. However, we found an increase in abundance of major arc genes near the mtDNA control region. There was also a marked age-related reduction in mtDNA copy number in both cell types. Vestibular organ gene expression, gene set enrichment analysis showed the OXPHOS pathway was down regulated in old animals.

Conclusion: Given the importance of mtDNA to mitochondrial OXPHOS and hair cell function, our findings suggest the vestibular organs are potentially on the brink of an energy crisis in old animals.

Keywords: Ageing; balance; hair cell; mitochondria; mtDNA; oxidative phosphorylation; vestibular..

Fig. (1)

Semicircular canal crista showing hair cell (top panels) and supporting cell (lower panels) layers. Left panels: cristae as they appear prior to laser microdissection. Right panels: cristae post microdissection, with hair cell (top) and supporting (lower) cell layers removed. Scale bars = 75µm.

Fig. (2)

Effects of ageing on mtDNA in vestibular cristae hair cell layers. The relative levels of four mtDNA-encoded genes were determined for young (Black bars; n=7) and old (Grey bars; n=7) animals. Bars represent the mean (+SEM) of all possible pair-wise comparisons between the four mtDNA genes for the both the CZ and PZ. The major arc genes, Cytb and Nd4, were significantly more abundant than the two minor arc genes, Nd1 and 12s, in old animals. However, we found no difference in the effects of aging between the CZ and PZ. * p < 0.05, ** p < 0.02.

Fig. (3)

Effects of ageing on mtDNA copy number in vestibular cristae hair cells. The relative abundances of mtDNA (Nd1) and nDNA (psGapdh) were compared between young (Black bars; n=7) and old (Grey bars; n=7) for both CZ and PZ regions. There was a significant reduction in mtDNA copy number in old animals for both regions. * p < 0.05, ** p < 0.01.

Fig. (4)

Effects of ageing on mtDNA in vestibular cristae supporting cells. The relative levels of four mtDNA-encoded genes were determined for young (Black bars; n=14) and old (Grey bars; n=7) animals. Bars represent the mean (+SEM) of all possible pair-wise comparisons between the four mtDNA genes. The major arc gene, Cytb was significantly more abundant than Nd1, Nd4, and 12s. A trend toward an increase in 12s abundance relative to Nd1 was found (p = 0.06 after correction for multiple comparisons). * p < 0.05, ** p < 0.01, *** p < 0.001.

Fig. (5)

Effects of ageing on mtDNA copy number in vestibular cristae supporting cells. The relative abundances of mtDNA (Nd1) and nDNA (psGapdh) were compared between young (Black bars; n=14) and old (Grey bars; n=7). There was a significant reduction in mtDNA copy number in old animals. ** p < 0.01.