Mitochondrial function is impaired in the skeletal muscle of pre-frail elderly

Abstract

Aging is accompanied by a gradual decline in both muscle mass and strength over time, which can eventually lead to pathologies, such as frailty and sarcopenia. While these two conditions are well characterized, further investigation of the early biological signs present in pre-frail elderly is still needed to help identify strategies for preventative therapeutic intervention. The goal of the present clinical study was to evaluate the level of mitochondrial (dys)function in a well-defined population of pre-frail elderly (>60 years of age). Pre-frail elderly were compared with an age-matched population of active elderly. Muscle mitochondrial function was assessed in vivo using phosphorus magnetic resonance spectroscopy (³¹P-MRS) and a comprehensive set of biological biomarkers were measured ex vivo in vastus lateralis muscle biopsies. In pre-frail subjects, phosphocreatine recovery was impaired and mitochondrial respiratory complex protein and activity levels were significantly lower when compared with active elderly. Analysis of microarray data showed that mitochondrial genes were also significantly down-regulated in muscle of pre-frail compared to active elderly. These results show that mitochondrial impairment is a hallmark of pre-frailty development and the onset of decline in muscle function in the elderly.

Figure 1

Flow diagram of the subject enrollment and analysis. Subjects were screened for eligibility up to 45 days before study enrollment. Out of the 38 elderly subjects assessed, 22 were included into the study of which 11 were pre-frail and 11 were active elderly. 21 subjects completed the study and were included in the final analysis. One pre-frail subject who was non-compliant with the study protocol dietary restriction between the visits on Day 1 and Day 14 was excluded from all analysis except the genomic expression via microarray (^∗as muscle biopsy had already been collected on Day 1), and was considered a drop-out.

Figure 2

Mitochondrial function is lower in vastus lateralis of pre-frail subjects. (A) Phosphorus Magnetic Resonance Spectroscopy (³¹P-MRS) was used to measure PCr recovery time, a marker of mitochondrial function, in the right posterior calf muscle. (B) Mitochondrial respiratory complexes I, IV and V abundance in vastus lateralis. (C) Mitochondrial respiratory complexes I, II and IV activity in vastus lateralis. (D) Relative abundance of mitochondrial DNA (mtDNA) over nuclear DNA (nuDNA) measured by qPCR. The ratio was calculated by comparing the relative abundance of mitochondrial encoded NADH dehydrogenase 1 (MTND1) to the nuclear encoded gene RNA 18S ribosomal N1 (RNA18SN1). Data represent mean ± SEM. ^*P < 0.05; ^∗∗P < 0.01; ^∗∗∗P < 0.001 after a two-tailed Student t test.

Figure 3

Network representing the genesets downregulated in the skeletal muscle of pre-frail subjects, organized by geneset similarity. Every node is a geneset that is significantly downregulated in pre-frail subjects. The size of the node is dependent on the size of the geneset. The nodes are connected depending on geneset similarity, i.e. on the number of genes that they have in common. The more opaque nodes correspond to the highest normalized enrichment score. Several functions were found more represented than others and are highlighted in color (Mitochondrion; Mitochondrial ribosome and translation; RNA processing and translation; Fatty acid oxidation; Amino acid metabolism; Histone H4 acetyltransferase activity; Proteasome). This figure shows that the Mitochondrion functional group is the most represented and the most downregulated in the prefrail muscle.

Figure 4

Gene expression in muscle biopsies measured by qPCR. Expression of genes, as measured by qPCR, that were in the top 100 of the most differentially expressed genes between pre-frail and active elderly, and in the genesets significantly downregulated in pre-frail subjects. Solute carrier family 25 member 20, SLC25A20; acyl-CoA synthetase long chain family member 1, ACSL1; carnitine palmitoyltransferase 1B, CPT1B; fatty acid binding protein 3, FABP3; coenzyme Q3 methyltransferase, COQ3; lactate dehydrogenase B, LDHB; creatine kinase, mitochondrial 2, CKMT2; and cytochrome c oxidase copper chaperone, COX17. Bargraphs represent mean ± SEM. ^∗P < 0.05 after a two-tailed Student t test.