ReviewSuspended in time: Molecular responses to hibernation also promote longevity
Introduction
Aging is an inevitable process that occurs in all living organisms and is characterized by a global and time-dependent decline in physiological and biological functions that lead to an increased risk of mortality (Demetrius, 2006). At the physiological level, aging is associated with progressive decline in cardiovascular, pulmonary, renal and gastrointestinal functions and has been linked to a reduction in hepatobiliary function, alterations in the endocrine and immune system, and increased risk of neurodegeneration (Aalami et al., 2003) amongst others. At the cellular level, aging is associated with a decrease in reparative and regenerative abilities in tissues and organs. For example, when the body is under stress, it uses its physiological reserves to restore homeostasis (e.g. repair damaged DNA and proteins, restore redox homeostasis, repair damaged tissue, etc.); with greater stress requiring the use of more physiological reserves. Over time, physiological reserves deplete and when the aging body is faced with stress, it is unable to effectively restore homeostasis and instead undergoes homeostenosis (Karp et al., 2008). Homeostenosis is the attenuation of the body's ability to repair itself both at the physiological and molecular level and leads to the accumulation of cellular damage that overtime, results in serious impairment in cellular functions manifesting in accelerated aging and age-dependent disorders (Khan et al., 2017). Although the advancement in technology and medicine is responsible for extending the average life span of humans by proximately 30 years compared to 1900; despite this advancement, age-associated medical complications are burdening older adults around the world (Crimmins, 2015). In fact, chronic conditions such as heart diseases, cancers, respiratory diseases, strokes, diabetes and neurodegenerative disorders are the leading cause of mortality amongst older adults and account for about 70% of death (Johnson, 2015; Kalyani et al., 2017; Tinetti et al., 2012). Therefore, it is necessary to understand aging at the molecular level and find potential therapies that could decelerate the aging process to allow individuals to remain healthy for longer periods of time.
Numerous studies point to metabolic rates as one of the main contributors of aging and age-associated conditions (Azzu and Valencak, 2017; Jumpertz et al., 2011; Schrack et al., 2014). Indeed, these studies have independently confirmed that higher metabolic rates and higher energy turnover contribute to accelerated aging and age-associated diseases (Azzu and Valencak, 2017; Jumpertz et al., 2011; Schrack et al., 2014); however, the molecular mechanism behind metabolic rate regulation is not clearly understood. Scientists are currently using numerous animal models (fruit flies, nematodes, mice, rats, etc.) to study the effect of specific manipulations on metabolic rates and longevity, and while important information on the role of specific regulatory factors (e.g. proteins, RNA, reversible modifications, etc.) has been collected, there is no consensus on how a global reduction in metabolic rates promote longevity and healthy aging. This is because a global reduction in metabolic rates is a multifactorial process that requires coordination between all biological stages, a process that cannot be established with individual genomic manipulations. Therefore, studying novel animal models capable of naturally reducing their metabolic rates may help to decipher the type of regulations involved in successfully slowing down aging or preventing age-associated conditions. For this review, we will be focusing our discussion on the molecular regulation of metabolic rate depression during hibernation in mammalian models, but we invite the reader to read our previous work on the relevance of other factors (e.g. anoxia tolerance, caloric restriction, etc.) with respect to aging in other animals (Krivoruchko and Storey, 2010a; Storey and Storey, 2004a; Wu and Storey, 2016).
Metabolic rate depression (MRD) is a phenomenon used by many species when faced with unfavourable environmental conditions to increase their chances of survival (Storey and Storey, 2004b, Storey and Storey, 2007). For example, some species of turtles can suppress their metabolic rate by over 90% under oxygen deprived environments and survive for weeks and up to months at a time (Herbert and Jackson, 1985). In response to prolonged seasonal droughts, some species of lungfish, frogs, snails and toads undergo estivation; whereas other organisms use diapause to reduce their metabolism (Denlinger, 2000; Storey, 2002; Storey and Storey, 2012). In response to high daily energy expenditure and unfavourable environmental conditions, some birds and several species of mammals also undergo a hypometabolic state that ranges from daily torpor to prolonged seasonal hibernation that could last for months (Geiser, 1988; Ruf and Geiser, 2015). While there are many forms of metabolic rate depression, the underlying theory remains the same. In general, metabolic rate depression (MRD) refers to a strategic process whereby energy expensive non-critical and or harmful cellular and metabolic processes (e.g. apoptosis, cell cycle, and DNA damage mechanisms) are greatly reduced and the finite amount of energy reserves available are allocated to promoting pro-survival processes (e.g. cytoprotective mechanisms, antioxidant responses, and anti-apoptotic responses) (Logan and Storey, 2016; Wu et al., 2015, Wu et al., 2018; Zhang et al., 2013). MRD is regulated at several stages of gene expression: during transcription by epigenetic modifiers and regulation of trans-regulatory elements, post-transcriptionally by microRNAs, at the translational level by ribosome availability and function, and at the post-translational level by reversible protein modification (Fig. 1) (Biggar and Storey, 2018; Storey, 2015a; Storey and Storey, 2004b; Tessier et al., 2017a). Successful implementation of MRD relies on the synchrony between all regulatory stages and dysregulations in any of these stages could result in catastrophic events for the organisms. For the purposes of this review, we will be discussing the regulation of MRD in response to mammalian hibernation and daily torpor in different animals and outline its importance in promoting longevity and healthy aging.
To date, eight mammalian groups have been reported to undergo hibernation or daily torpor. These species include: monotremes (Tachyglossus aculeatus), marsupials (Didelphimorphia, Dasyuridae, Petauridae, and many more), rodents (Ictidomys tridecemlineatus, and many more), bats (Myotis lucifugus and many more), insectivores (Erinaceinae), carnivores (Ursus americanus and Ursus arctos), elephant shrews (Macroscelididae) and primates (Microcebus murinus and a few others) (Geiser, 1994; Grigg et al., 1989; Ruf and Geiser, 2015; Storey, 2015b; Storey and Storey, 2010). Hibernation is generally characterized by several physiological, biochemical and molecular changes that occur simultaneously, manifesting in a great reduction in core body temperature (Tb), decrease in vital signs such as breathing, and heartbeat, albeit at different levels generally correlated with body size. For larger animals such as brown and black bears, these changes occur in a more subtle manner. For example, hibernating Ursidae decrease their Tb only by 3–5 °C, reduce their heat rates from ~ 55 bpm (beats per minute) to 9 bpm, and slow their breathing to 1–2 breaths per minute during hibernation (Evans et al., 2016; Toien et al., 2011). Small hibernators such as bats, marmots or squirrels greatly reduce their Tb from 35 to 38 °C to <5 °C, slow their heart beat from 350 to 400 bpm to only 5–10 bpm, and drop their breathing rate from >40 breaths per minute to <1 (Geiser, 2004; Milsom and Jackson, 2011). Larger hibernators reduce their metabolic rates (usually measured by oxygen consumption levels) by ~75% and smaller hibernators reduce their rates by 95–99% compared to their euthermic counterparts (Geiser, 2004; Toien et al., 2011). Overall, the reduction in metabolic rates during hibernation is accompanied by physiological and molecular changes that account for ~90% saving in energy expenditure in small mammals (Wang and Wolowyk, 1988). While the duration and the mode of hibernation differs with respect to the animal, a typical hibernation cycle consists of multiple rounds of torpor bouts that range from days to several weeks at a time, where metabolic rates are greatly reduced (Carey et al., 2003). In several animals, torpor is interrupted by brief periods of arousal, where metabolic rates increase and Tb rises to ~36 °C for 12–24 h before retreating into torpor (Carey et al., 2003).
Animals that hibernate for prolonged periods of time must build up intrinsic fuel reserves to use during hibernation. Indeed, these hibernators substantially increase their body fat by undergoing hyperphagia in late summer and store the acquired fat as triglycerides in their white adipose tissue (Dark, 2005). Furthermore, oxidation of fatty acids via β-oxidation becomes the primary source of energy supply during hibernation and all tissues, including the brain (using ketone bodies), become dependent on lipid metabolism to meet their energy demands (Dark, 2005; Lang-Ouellette et al., 2014; Serkova et al., 2007).
Hibernation possess several cellular stresses that would otherwise be lethal for non-hibernating animals. For example, prolonged periods of hypothermia could negatively influence protein stability, membrane fluidity and enzymatic function and promote cardiac hypertrophy to accommodate for pumping cold and viscous blood throughout the body ((Zhang et al., 2016) for review (Carey et al., 2003; Krivoruchko and Storey, 2010b)). In addition, prolonged periods of inactivity promote conditions of muscle atrophy, which could have long-lasting effects if not dealt with appropriately. At the molecular level, hibernation causes a very low tissue perfusion rates (down to <10% of euthermia in small hibernators) resulting in metabolic stress, reactive oxygen species (ROS) production and ischemic damage. Similarly, during periodic arousal back to euthermia, the rapid rise in Tb and the sudden increase in perfusion rates cause an outburst of ROS and increase the risk of ischemia-reperfusion injuries in organs. Miraculously, these animals do not show any measurable damage over the hibernation-arousal cycle, and thus studying the molecular mechanisms that hibernators use to navigate through these stresses would provide invaluable information on their metabolic plasticity and may hold the key to solving medical complications (e.g. prolonging the storage of transplantable organs, minimizing damage due to ischemia/reperfusion, preventing muscle atrophy, and reversing cardiac hypertrophy) associated with these conditions.
Mammalian hibernators have a longer lifespan than their non-hibernating counterparts. For example, it was shown that 18 of the 19 mammalian species that outlive humans compared to their body size are different species of bats capable of hibernation (Austad and Fischer, 1991). Myotis brandtii can live for up to 41 years, approximately 9.8 times longer than its expected life expectancy compared to its weight with no measurable signs of senescence (Fleischer et al., 2017; Podlutsky et al., 2005). In addition, other small mammalian hibernators (e.g. gray mouse lemur, 13-lined ground squirrels, etc.) also live longer than their non-hibernating mammalian counterparts of similar body size (e.g. house mouse) (Wu and Storey, 2016). It is important to note that hibernation also decreases the risk of predation, which could also partly account for the increase in longevity and lifespan of these mammals (Turbill et al., 2011). Nonetheless, the molecular mechanisms that protect against stresses associated with hibernation (e.g. oxidative damage, telomere shortening, genomic instability, etc.) have coevolved with mechanisms that promote longevity and decrease cellular senescence in these hibernators (Blanco and Zehr, 2015; Ruf et al., 2012). In support of this, several studies have found strong positive correlations between daily torpor or prolonged hibernation and longevity (Koizumi et al., 1992; Lyman et al., 1981; Walford and Spindler, 1997). For example, it has been previously established that telomere length decreases with age in humans, and recent studies on adult edible dormice (Glis glis), and Djungarian hamsters (Phodopus sungorus) show that these animals increase the length of their telomeres in active season following hibernation (Hoelzl et al., 2016; Turbill et al., 2012, Turbill et al., 2013). The increase in telomere length could partly contribute to the increase in longevity in these animals. As such, understanding the molecular mechanisms that regulate hibernation could in part help us understand aging, and devise remedies that could decelerate the aging process and promote longevity.
To successfully survive the hibernation/arousal cycle, hibernators must undergo physiological and biochemical changes to not only survive hibernation and to also recover fully upon arousal. Most of these changes are conserved amongst hibernators, but differences do exist with respect to species and the duration of hibernation. In this review, we will discuss the molecular regulation of hibernation at the epigenetic level and its influence in suppressing global gene transcription, the role of microRNAs and the translational machinery in regulating protein synthesis, and the role of post-translational modifications (PTMs) in regulating protein and enzymatic activity over the torpor arousal cycle. We will also briefly discuss current medical research on aging on non-hibernating models and compare the regulation of the mechanisms between the two groups.
Section snippets
Epigenetic and transcriptional control of hibernation
Gene transcription is an energy expensive process, and depending on the tissue type, it costs the cell 1–10% of its total energy (Rolfe and Brown, 1997). As such, animals in hypometabolic states globally suppress gene transcription in efforts to conserve energy, and only promote the expression of genes necessary for survival. Global suppression of transcription relies on modulating the activity of the transcriptional machinery and their accessibility of the DNA. For example, it was shown that
Post-transcriptional and translational regulation of hibernation
Molecular responses to hibernation are also regulated at the post-transcriptional and post-translational level. Even under extreme hypometabolic states where global level of gene expression is reduced, a select group of cytoprotective and pro-survival genes are expressed in a tissue specific manner. For example, the transcript levels of peroxiredoxin 2, were shown to increase in brown adipose tissue and heart by 1.7 and 3.7-folds respectively during torpor in 13-LGS (Morin and Storey, 2007).
Post-translational regulation of hibernation
Despite the rigorous control of mRNA translation during torpor or prolonged hibernation, several transcripts are indeed translated, and these transcripts are generally associated with pro-survival and cytoprotective proteins necessary to overcome cellular damage during hibernation and early arousal. Transcripts translated during torpor are generally grouped into several categories based on their function: those that are involved in fatty acid metabolism, antioxidant responses and chaperons,
Prospective and future direction
As demonstrated in this review, hibernating mammals and other animals capable of natural stress-tolerance are able to strategically launch an adaptive response that demonstrates a perfect synchrony between all regulatory stages (transcription, translation and post-translation), that allows for inducing a hypometabolic response that favours survival during hibernation. In the same manner, these molecular responses protect the animal during early stages of recovery where an increase in oxidative
Acknowledgements
We thank Jan M. Storey for editorial review of this manuscript. This work is supported by a discovery grant from the Natural Science and Engineering Research Council of Canada (#6793) awarded to KBS. KBS holds the Canada Research Chair in Molecular Physiology. RA holds an Ontario Graduate Scholarship.
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