Abstract:
Mitochondria have a critical role in supplying energy and mediating adaptive signalling
following exercise. Recently, short open-reading frames within the mitochondrial genome
have been found to encode several functional polypeptides termed ‘mitochondrial-derived
peptides (MDPs)’ named humanin, MOTS-c and small humanin-like peptides (SHLP) 1-6).
Exogenous treatment with several of these peptides has been shown to promote system-wide
enhancements in cytoprotection and metabolism and had been shown to activate similar
molecular signalling pathways to aerobic exercise in vitro and in rodent models of disease.
Although MDPs are shown to be relatively ubiquitously expressed throughout the body and
in circulation, little is known regarding their endogenous regulation. Given that mitochondria
are sensitive to metabolic perturbations induced by exercise stressors and activate similar
pathways to exercise known to promote health, we hypothesized that MDPs are likely
endogenously regulated by exercise.
The first aim of this thesis was to determine the effect of exercise and short-term training on
endogenous intramuscular and plasma expression of several MDPs within humans. Within a
cohort of healthy young men, it was shown that MOTS-c and humanin were transiently
increased within skeletal muscle and plasma immediately following acute high intensity
interval exercise but not with training. As these responses of MOTS-c and humanin were
characteristic of myokines, the next aim was to determine if skeletal muscle was capable of
producing MOTS-c and/or humanin following isolated contraction, ex vivo. It was observed
that MOTS-c and humanin expression increased following isolated contraction relative to a
rested control in EDL but not soleus, suggesting that skeletal muscle contains an intrinsic
mechanism capable of producing MDPs, although this appears to be differentially modulated
depending on fibre type composition of the muscle.
As the possibility of intramuscular uptake from the circulation within the in vivo model of
exercise in humans could not be eliminated, the next aim was to determine the effect of
exercise on both skeletal muscle and non-muscle tissues as well as plasma by utilising two
acute exercise bouts of differing intensity and duration. It was observed that both skeletal
muscle and non-muscle tissues were differentially modulated by exercise and this was
dependent on tissue type, exercise intensity and duration. As liver MOTS-c expression was
observed to be significantly higher after exercise, the following aim was to determine whether
circulating MOTS-c uptake was enhanced specifically amongst several tissues using a
radiolabelled MOTS-c peptide approach following exercise. Following optimisation,
significantly higher uptake of radiolabelled MOTS-c into the liver, pancreas, and
subcutaneous fat was observed following exercise, suggesting that these tissues are targeted
following acute exercise, which may be a part of the endogenous function of MOTS-c.
Finally, given MOTS-c and humanin were reported to activate similar signalling pathways to
exercise that promote enhancements in mitochondrial biogenesis and function, the final aim
of the thesis was to determine whether these MDPs could promote ergogenic effects on
exercise performance in mice during sedentary or exercise trained conditions and whether
this may be related to mitochondrial adaptations. Only 20mg/kg/day of exogenous MOTS-c,
but not humanin, treatment promoted enhancements in treadmill exercise performance,
increased basal core temperature, lowered subcutaneous fat, lowered blood lactate at
exhaustion from exercise and increased complex I and IV mitochondrial respiratory complex
expression in gastrocnemius but only in sedentary treated mice. When investigating the
mechanism behind these ergogenic effects, there was no difference in mitochondrial
respiration, hydrogen peroxide production, respiratory exchange ratio or signalling protein
activation/expression of canonical mitochondrial biogenesis markers, in vitro. Taken
together, these finding suggest MOTS-c is capable of evoking ergogenic effects on exercise
performance although this occurs independently of functional mitochondrial adaptation.
These findings provide evidence that the mitochondrial-derived peptides, namely MOTS-c
and humanin, are acutely exercise-sensitive in skeletal muscles, non-muscle tissues and
plasma. Additionally, we provide evidence that circulatory MOTS-c uptake into the liver,
pancreas and subcutaneous fat is significantly increased following acute exercise, suggesting
that these tissues may be targeted in response to exercise-induced stress as part of the
endogenous function of MOTS-c. Lastly, we demonstrated the potential ergogenic effects of
MOTS-c on exercise performance in sedentary mice and provide evidence that these effects
appear independent of markers of mitochondrial adaptation.