Physical Activity by Draper Nick; Stratton Gareth; & Gareth Stratton

Author:Draper, Nick; Stratton, Gareth; & Gareth Stratton
Language: eng
Format: epub
Publisher: Routledge

Physical activity and protein modulation

Seminal work by Holloszy demonstrated that muscles from treadmill-trained rats expressed higher mitochondrial protein content than from untrained rats (Holloszy, 1967). This and much work since supports the general idea that muscle is adaptive to exercise. At a biochemical level, the mechanism of this adaptive response involves the conversion of electrical, mechanical, and chemical consequences of muscle contraction into cellular signals that impact various cellular processes thus changing the underlying characteristics of muscle.

The signals that stimulate downstream cellular pathways involved in muscle adaptation include increased intracellular Ca2+, reactive oxygen species, AMP, ADP, and fatty acids as well as decreased levels of creatine phosphate and glycogen (Hawley et al., 2006). Altered ratios of NAD+:NADH and aberrant redox conditions have also been implicated. While this list is certainly not exhaustive, and more signaling molecules will certainly be identified, it is clear that the molecules involved in muscle adaptive signaling are many and varied.

The story becomes even more complex when we begin to dissect the various signaling pathways that become activated in response to the contraction-induced changes (described above). Exercise is known to activate a variety of signaling cascades including Ca2+/calmodulin dependent protein kinase (CamK), calcineurin, mitogen-activated protein kinases (MAPK), AMP-activated protein kinase (AMPK), and mammalian target of rapamycin (mTOR) (Fujii et al., 2000; Rose & Hargreaves, 2003; Widegren et al., 1998; Winder & Hardie, 1996). Activation of these pathways is mediated by direct modulation of proteins within the cascade. Ultimately, these pathways impact regulation of genes involved in various aspects of muscle function (Hawley, Hargreaves, Joyner, & Zierath, 2014).

While a detailed accounting of the varied cellular consequences of exercise-induced signaling cascades is beyond the scope of this chapter, one highly studied factor in this response is peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α), which has emerged as a key regulator of mitochondrial biogenesis in response to repeated bouts of exercise. PGC1α is regulated by both AMPK and p38 MAPK signaling pathways. Importantly, both pathways result in the direct phosphorylation and activation of PGC1α (Jäger, Handschin, Pierre, & Spiegelman, 2007; Puigserver et al., 2001). Additionally, AMPK phosphorylates the transcriptional repressor HDAC5, which relieves inhibition of MEF2, a known transcriptional activator of PGC1α (McGee & Hargreaves, 2010). p38 MAPK phosphorylates ATF-2, which is a transcriptional activator of PGC1α (Akimoto et al., 2005). Therefore, PGC1α protein activity is directly stimulated via phosphorylation by both pathways, and transcription of PGC1α transcript is also activated by both pathways. PGC1α then induces expression of key mitochondria proteins encoded by both nuclear and mitochondrial encoded genes which stimulates mitochondrial biogenesis and increases the total mitochondrial content in exercise trained muscle (Handschin & Spiegelman, 2006).

Recently, the use of mass-spectrometry based approaches has dramatically expanded our ability to monitor the proteomic consequences of exercise. In a recent study, a mass spectrometry-based analysis of global protein phosphorylation was used to compare phosphorylation both before and immediately following a single bout of intense exercise (Hoffman et al., 2015). Importantly, this single-bout of exercise induced phosphorylation of >550 unique proteins, many of which were phosphorylated at multiple sites.

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