We devised a strategy to disrupt the PGC-1β gene in a skeletal muscle-selective manner in a generalized PGC-1α-deficient mouse background. As described herein, we found that PGC-1α and PGC-1β serve overlapping roles in skeletal muscle and together are necessary for exercise performance (even minimal exertion) and maintenance of mitochondrial structure and function, but are dispensable for fundamental fiber type determination and normal muscle insulin sensitivity and glucose disposal.
Our findings demonstrated that the cooperative and overlapping actions of PGC-1α and PGC-1β are required for muscle mitochondrial function and structure. Transcriptional profiling studies revealed that the expression of a broad array of genes involved in multiple mitochondrial energy transduction and OXPHOS pathways are dependent on having at least one functional PGC-1 gene. PGC-1α/β deficiency resulted in dramatic derangements in mitochondrial structure and respiratory function, indicating that the PGC-1 coactivators are required for maintaining a healthy population of normal mitochondria. This latter function has been ascribed to the ongoing dynamics of fusion and fission, which serve to maintain mitochondrial quality control (Chen et al., 2005
; Waterham et al., 2007
; Zhang and Chan, 2007
). Consistent with this conclusion, we found that the expression of genes involved in fusion (Mfn1, Mfn2) and fission (Drp1) was downregulated in the muscle of PGC-1α−/−
mice compared to the other genotypes. Others have recently shown that PGC-1α and PGC-1β are capable of activating transcription of the Mfn2 gene by coactivating the nuclear receptor ERRα (Soriano et al., 2006
; Liesa et al., 2008
). Taken together, these results suggest that the PGC-1 coactivators are necessary for maintaining high level coordinated expression of genes involved in mitochondrial energy transduction and ATP production pathways, as well as maintenance of a healthy population of muscle mitochondria.
The impact of combined PGC-1α and PGC-1β deficiency on exercise performance was dramatic. Our results strongly suggest that the exercise phenotype is due to severe mitochondrial dysfunction forcing reliance on anaerobic glycolysis for ATP production, leading to rapid depletion of glycogen and premature fatigue. The marked elevation in circulating lactate levels in the PGC-1α/β-deficient mice post-exercise is consistent with this conclusion. Interestingly, lactate levels were also increased post-exercise in the PGC-1α−/−mice, albeit to a lesser extent than the PGC-1α/β-deficient animals. This latter observation could reflect the fact that hepatic PGC-1α deficiency results in an altered Cori cycle response. It is also possible that PGC-1 coactivators play a primary role in glycogen metabolism. However, the protein levels of the enzymes involved in glycogen synthesis and degradation were not significantly altered in the PGC-1α−/−βf/f/MLC-Cre mice (data not shown).
Surprisingly, combined PGC-1α/β deficiency results in a remarkable uncoupling of muscle fiber type and oxidative programs. Specifically, phenotypic analysis of the PGC-1α−/−
mice did not reveal a shift towards a “detrained” muscle fiber type profile, as would be predicted by PGC-1 overexpression studies. Rather, we found a modest increase in type I fibers in the muscles of both PGC-1α−/−
lines. The observed increase in type I fibers in the mutant mice suggests that an independent, currently unidentified pathway involved in fundamental fiber type is compensatorily activated. PGC-1 overexpression in mice has been shown to inhibit the protein degradation known to occur in disuse atrophy (Wenz et al., 2009
; Brault et al., 2010
). However, we did not find an overt abnormality in muscle mass in the PGC-1α−/−
mice. Thus, whereas PGC-1 coactivators are capable of driving a shift towards oxidative fibers and muscle growth, they are not required for fundamental muscle development or fiber type specification. One caveat to our conclusions is that a small amount of a mutant form of a naturally-occurring alternatively spliced form of PGC-1α referred to as NT- PGC-1α (Zhang et al., 2009
), could be theoretically expressed in the PGC-1α-deficient line used in this study. However, RT-PCR analysis demonstrated that levels of this transcript are barely detectable in muscle as we have shown in the generalized PGC-1α-deficient line (Leone et al., 2005
). Moreover, given the severe mitochondrial and exercise phenotype shown here, it would seem unlikely that this could account for a completely normal fiber phenotype.
Significant evidence suggests a link between skeletal muscle mitochondrial dysfunction and the development of insulin resistance (Lowell and Shulman, 2005
; Morino et al., 2005
; Petersen et al., 2005
). In addition, several studies in humans have shown reduced expression of PGC-1α (Mootha et al., 2003
; Patti et al., 2003
) in skeletal muscle of insulin resistant and diabetic subjects. These studies implicate muscle mitochondrial dysfunction and altered PGC-1 signaling in the pathogenesis of insulin resistance. The studies shown herein demonstrate that total skeletal muscle PGC-1 deficiency in mice does not lead to insulin resistance or glucose intolerance on standard chow or high fat diet. Moreover, given the mitochondrial derangements present in PGC-1α−/−
muscle, our results suggest that mitochondrial dysfunction is unlikely to contribute to insulin resistance. It is certainly possible, however, that insulin resistance drives muscle mitochondrial dysfunction.