Increased Degradation Rates in the Components of the Mitochondrial Oxidative Phosphorylation Chain in the Cerebellum of Old Mice

Abstract

Brain structures differ in the magnitude of age-related neuron loss with the cerebellum being more affected. An underlying cause could be an age-related decline in mitochondrial bioenergetics. Successful aging of mitochondria reflects a balanced turnover of proteins involved in mitochondrial biogenesis and mitophagy. Thus, an imbalance in mitochondrial turnover can contribute to the diminution of cellular function seen during aging. Mitochondrial biogenesis and mitophagy are mediated by a set of proteins including MFN1, MFN2, OPA1, DRP1, FIS1 as well as DMN1l and DNM1, all of which are required for mitochondrial fission. Using N15 labeling, we report that the turnover rates for DMN1l and FIS1 go in opposite directions in the cerebellum of 22-month-old C57BL6j mice as compared to 3-month-old mice. Previous studies have reported decreased turnover rates for the mitochondrial respiratory complexes of aged rodents. In contrast, we found increased turnover rates for mitochondrial proteins of the oxidative phosphorylation chain in the aged mice as compared to young mice. Furthermore, the turnover rate of the components that are most affected by aging -complex III components (ubiquinol cytochrome C oxidoreductase) and complex IV components (cytochrome C oxidase)- was significantly increased in the senescent cerebellum. However, the turnover rates of proteins involved in mitophagy (i.e., the proteasomal and lysosomal degradation of damaged mitochondria) were not significantly altered with age. Overall, our results suggest that an age-related imbalance in the turnover rates of proteins involved in mitochondrial biogenesis and mitophagy (DMN1l, FIS1) in conjunction with an age-related imbalance in the turnover rates of proteins of the complexes III and IV of the electron transfer chain, might impair cerebellar mitochondrial bioenergetics in old mice.

Keywords: aging; cerebellum; mice; mitochondria; proteins; turnover.

FIGURE 1

Gene ontology (GO) analysis of proteins with enhanced degradation in old mice. Log2 ratios of degradation rates between old and young mice cerebellum samples were measured (see Materials and Methods). A hierarchical GO enrichment analysis was conducted for proteins with enhanced degradation in the old mice. The statistical significance (P-value) of gene enrichment of differentially expressed genes within each GO accession is indicated by the color shading. The hierarchical relationship between accessions is indicated by arrows. The analysis was conducted using the algorithm GOrilla (2). The table provides additional information about each accession, including the false discovery rate and enrichment level. This figure is limited to the cellular component GO category. The complete results of the GO analysis are tabulated in Supplementary Table S2. A list of proteins belonging to the GO term “mitochondrial protein complex” that are enriched among the subset of proteins with enhanced degradation rates in old mice is provided.

FIGURE 2

Distribution of log2 ratios of degradation rates between old and young mice for a select group of functional categories related to protein homeostasis. Log2 ratios of degradation rates between old and young mice cerebellum samples were measured (see Materials and Methods). Box plots indicate the distribution of log₂ ratios degradation rates for each category. The box indicates the interquartile range (IQR) and the line indicates the median. Far outliers (>1.5^∗IQR) were excluded.

FIGURE 3

Distribution of degradation rates for old and young mice for components of the mitochondrial transport chain. Box plots indicate the distribution of degradation rates for all subunits quantified for each complex. The box indicates the interquartile range (IQR) and the line indicates the median. Far outliers (>1.5^∗IQR) were excluded. P-values for comparisons between old and young mice were conducted by two-sided Mann–Whitney U test, considering values from different subunits as independent measurements of k_degradation (d^-1) for a given complex. For complex 2, a single subunit was quantifiable, and the data is shown as that single measurement rather than a distribution.