Henning Wackerhage (1), Philip J Atherton (1,2)

1: School of Life Sciences, University of Dundee
2: Department of Biological Sciences, University of Central Lancashire

https://doi.org/10.36866/pn.56.11

The marathon run is a benchmark endurance test. It was born during the first modern Olympic Games in Athens in 1896, and subsequently stretched in steps to today’s 42.195 km or 26.2 miles. The jogging and running boom of the 70s and 80s, and improvements in endurance training, have changed it from something that was seen as a dangerous, over-exhaustive, males-only activity to a serious test that anyone can do – including the young and old, heart patients and disabled people. Marathon running was also an important stage for female emancipation in sport, starting with unofficial attempts in the 60s up to the present day record of 2:15 h held by Paula Radcliffe.

The high fitness of marathon runners and other endurance athletes, and phenomena such as ‘hitting the wall’, have stimulated a generation of exercise physiologists to study marathon running and endurance training. They focused on the determinants of marathon running performance and on the adaptations that are stimulated by endurance training. An early, major finding was that endurance training promoted ‘healthy’ cardiac growth, resulting in the ‘athlete’s heart’. The endurance athlete’s larger heart can increase its pumping of blood to a maximum of 40 litres per minute (compared to about 20 litres in untrained subjects). The higher capacity for blood transport also means a higher capacity for oxygen and nutrient transport round the body.

Skeletal muscles adapt as well: endurance training promotes a limited fast-to-slow exchange of motor proteins. That is one reason why marathon runners are poor sprinters. Marathon runners, however, have a very high capacity for producing ATP by oxidative phosphorylation because mitochondrial biogenesis is stimulated by endurance exercise. The fuel supply for oxidative phosphorylation also changes, which enables the runner to better sustain the fuel supply during long duration exercise. As a rule of thumb, the concentrations of enzymes that use glycogen decrease, whereas the concentrations of enzymes that synthesize glycogen and utilize fat increase. As a result, marathon runners save their relatively small glycogen store and rely more on the plentiful fat reserves.

Classical exercise physiologists used a ‘black box’ approach to study the adaptation to endurance training. Their model was:

endurance training → black box → adaptation.

Molecular exercise physiologists now seek to open the black box and to identify the mechanisms that regulate the well-described adaptive responses to endurance training. Their aim is to identify the chain of events, starting with an exercise-related signal such as calcium or muscle tension, followed by the activation of a signal transduction pathway and its effect on gene regulation and ending with a known adaptation to exercise.

The major breakthrough in this new field was made by Eva Chin and co-workers in 1998. Chin et al. found that the immunosuppressant drug cyclosporin A increased the percentage of fast muscle fibres in rodents. Cyclosporin A is known to inhibit the calcium-activated calcineurin pathway.

This and other findings suggested that activated calcineurin stimulated the production of proteins such as myoglobin and slow troponin that are known to be upregulated by endurance training. The findings could be summarised as a mechanistic model (Fig. 1):

endurance training → calcium concentration increase → calcineurin activation → increased expression of proteins known to increase in response to endurance training.

Soon after, it became clear that the calcineurin pathway was only part of the story. Murgia et al. (2000) showed that the activated ERK1/2 pathway also increased the percentage of slow muscle fibres (Fig. 1).

Other studies showed that the ERK1/2 pathway is activated by endurance exercise and thus both pathways are likely to co-operatively regulate the adaptive response to endurance exercise.

Much progress has also been made in identifying the mechanisms that regulate the exercise-induced increase in the division of mitochondria, termed mitochondrial biogenesis. Mitochondria are the sites of oxidative phosphorylation. The majority of mitochondrial proteins are encoded in nuclear DNA, but mitochondria have their own 16,600 base pair-long DNA which encodes some of the proteins. This is an evolutionary ‘leftover’ and was the target of the first major human DNA sequencing project. Because of the existence of mitochondrial DNA, the regulation of mitochondrial biogenesis must involve the activation of the expression of genes that are encoded in nuclear and mitochondrial DNA.

Scarpulla (2002) has identified so-called nuclear respiratory factors that were important for the expression of mitochondrial genes encoded in the nuclear DNA. Tiranti et al. (1995) then identified the mitochondrial transcription factor A (mtTFA or TFAM), which is encoded by nuclear DNA but switches on the expression of genes encoded in mitochondrial DNA.

A breakthrough was the discovery of the transcriptional co-factor PGC-1. Puigserver & Spiegelman (2003) found that the overexpression of PGC-1 stimulated mitochondrial biogenesis. Interestingly, mice that overexpress PGC-1 not only have a very high mitochondrial content but also express large amounts of other slow fibre proteins such as myoglobin (their muscles appear red) and slow troponin.

Recently Terada et al. (2002) have shown that endurance exercise and the activation of the AMP kinase increased PGC-1 expression. AMP kinase is activated by increases in the concentration of AMP which is associated with the ‘energy stress’ during exercise and with low glycogen, which results from endurance training.

To summarise, the major regulating events during mitochondrial biogenesis might be formalised as (Fig. 1):

endurance exercise → ‘energy stress’

→ AMP kinase activation → upregulation of PGC-1 → expression of a) nuclear DNA-encoded mitochondrial genes (including mtTFA) via PGC-1 and b) mRNA and mitochondrial DNA-encoded mitochondrial genes via mtTFA → synthesis of all mitochondrial proteins and mitochondrion assembly (more mitochondria) → higher capacity for ATP synthesis by oxidative phosphorylation.

The above findings show that molecular exercise physiology has entered its golden era fuelled by technological advances such as antibodies against phosphoproteins and microarray technology. We are now well on the way to characterising the mechanisms that make athlete’s hearts grow, their muscle capillaries sprout and their bones and their cartilage more resistant to mechanical impact. In addition, genotype chips are available that allow researchers to link an individual’s genotype to performance-related characteristics.

The future will show whether this increase in knowledge can be exploited for guiding marathon runners at all levels towards better performances.

Acknowledgements

We should like to thank the Wellcome Trust, the University of Dundee and the University of Central Lancashire for supporting our research.

References

Chin ER, Olson EN, Richardson JA, Yang Q, Humphries C, Shelton JM, Wu H, Zhu W, Bassel-Duby R & Williams RS (1998). A calcineurin-dependent transcriptional pathway controls skeletal muscle fiber type. Genes Dev 12, 2499-2509.

Murgia M, Serrano AL, Calabria E, Pallafacchina, G, Lomo T & Schiaffino S (2000). Ras is involved in nerve-activity-dependent regulation of muscle genes. Nat Cell Biol 2, 142-147.

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