Effects of PUFA on hibernation and ageing

Hibernators save energy by substantially decreasing metabolic rate and body temperature (Tb), but spend ~80% of their energy expenditure to repeatedly rewarm during winter. However, the function of these arousals remains a mystery. Dietary uptake of polyunsaturated fatty acids (PUFA), especially of Linoleic acid (LA, C18:2 n-6), strongly affects torpor duration and hence the frequency of arousals, suggesting a limiting effect of PUFA on the time individuals can remain torpid. We hypothesize that high n-6 PUFA content in membrane phospholipids (PL) mitigates the effects of low temperature on the activity of the cardiac sarcoplasmic reticulum (SR) calcium ATPase (SERCA2a), the enzyme responsible for removing calcium into the SR and for proper cardiac function. Whereas protein degradation is not entirely blocked at low Tb, protein synthesis, including SERCA2a, is drastically down-regulated during torpor entrance and only resumed during arousals. A high LA content could partially compensate for a reduction in SERCA2a activity resulting from proteolysis over time, prolonging the time hibernators can remain torpid until the need to synthetize SERCA2a forces them to rewarm to euthermic Tb. The intake of LA may involve a trade-off, however, since extremely high LA levels above an apparent optimum trigger a decrease in hibernation. This could be due to increased PUFA-related oxidative damage during the torpor-arousal cycle, impacting negatively on SERCA2a activity and on somatic maintenance in general. In view of these potential trade-offs, we propose to experimentally test the following hypothetical concept, using dietary manipulation of LA intake: Optimal levels of LA will enable hibernators to maintain high levels of SERCA2a activity. This in turn will lead to increased torpor duration, compared with animals on very low or extremely high LA diets. Individuals will re-synthesize SERCA2a during euthermic phases, leading to higher SERCA2a activity in early subsequent torpor, compared with later stages in a bout. An increase in membrane LA beyond optimal levels will lead to a rise in oxidative stress, affecting negatively SERCA2a activity, forcing individuals to rewarm more often and to lengthen their time spent euthermic. Frequent arousals will lead to increased levels of markers of oxidative stress, cellular damage, and accelerated ageing.

In seasonal environments, animal species have developed strong physiological and behavioural adaptations to face recurrent periods of reduced energy availability and ambient temperatures. For instance, hibernators enter a state of torpor, by substantially reducing their metabolic rate and body temperature, during several months in winter. Together with ambient temperature, polyunsaturated fatty acids (PUFAs) strongly affect the behaviour of torpor and hibernation. In particular, linoleic acid (LA, C18:2), the major fatty acid of the n-6 family, has strong impact on torpor bout duration. We suggested that n-6 PUFAs (including LA) exert their effect on sustaining cardiac function during deep torpor. The project aimed at determining regulatory mechanisms by which PUFAs and temperatures affect cardiac function, oxidative stress and somatic maintenance, e.g. telomeres, during hibernation. In garden dormice, we found positive effect of LA on the activity of cardiac sarcoplasmic reticulum (SR) calcium ATPase (SERCA), a pump responsible for continued cardiac function, and on the minimal body temperature reached by hibernators during torpor. However, we found no implication of LA in the generation of oxidative stress and no effect of dietary fatty acids on telomeres during hibernation. Alternatively, high levels of n-3 PUFAs, namely docosahexaenoic acid (DHA, C22:6), prevented garden dormice from entering into torpor and forced them to delay hibernation onset. This supports the existence of a seasonal remodelling of tissue fatty acid composition in hibernators prior and/or during hibernation. Therefore, we investigated the seasonal shift in lipid composition in a large hibernator - the brown bear - which hibernation contrasts with the one of small hibernators. We found that bears show marked seasonal changes of tissue lipid composition during hibernation, similar to small-bodied hibernators. Also, we observed in the brown bear significant seasonal changes in specific n-3 and n-6 precursors, namely linolenic acid (LNA, C18:3) and LA (C18:2), during hibernation. These changes occurred along with marked seasonal alterations of eicosanoids levels, pro- and inflammatory molecules derived from n-6 and n-3 PUFAs, in tissues relevant for hibernation. We further investigated the effects of ambient temperature and food availability on telomere dynamics in hibernating garden dormice. We found that hibernation at low temperatures comes with a trade-off between energy savings and somatic maintenance, which can be compensated by food availability. Finally, we studied the effect of seasonal timing of reproduction on physiological processes linked to hibernation behaviour in juvenile garden dormice. We found marked effects of the timing of birth on the development, fattening, lipid metabolism and hibernation of the individuals. Taken together, these findings support the view that the seasonal strategy of hibernation is a well-regulated adaptive mechanism enabling animals to survive recurrent events of limited resource availabilities and changes in environmental temperatures.