Aspects of the energetics and renal physiology of some African arid-adapted rodents

Bibliography: leaves 358-363.The following aspects of physiological adaptation and resource usage were examined in several species of arid-adapted African rodents: The effect of temperature on thermoregulation, metabolism, evaporative water loss and thermal conductance was studied in the North East African pigmy gerbil, gerbillus ppusillus; This gerbil did not strictly maintain homoeothermy and like most arid-adapted rodents had a reduced basal metabolic rate. When G. pusillus was maintained on an ad lib. food and water diet, its daily energy expenditure was similar to that predicted by mass. This increased to 116% of the expected allometric value when deprived of water. On a diet of air-dried millet seeds, increased kidney concentrating ability, reduced pulmocutaneous evaporation and a reduction in faecal water loss, were sufficient to impart virtual independence of exogenous water and the maintenance of a positive water balance. If food was restricted, G. pusillus utilized torpor to precisely balance energy expenditure with that available and so maintained a constant body mass. During torpor, assimilation efficiency was significantly reduced. The cost of maintaining coenothermia for the full duration of food processing would be prohibitive and it was estimated that it would have utilised approximately 50% of the daily energy available. The cost of employing torpor (0.5kJ.day-1) on the other hand was insignificant in comparison with the savings accrued by the use of torpor (32.25kJ. day -1). Parsimony of water loss with food restriction enabled the maintenance of a positive water balance during this period. The importance of micro-habitat in thermoregulation was investigated in Gerbillurus paeba, inhabiting the thermally stable confines of a plugged burrow, and in Aethomys namaquensis, a crevice dwelling rock rat. The rock rat, living in an environment characterized by large diet ranges, precisely controlled body temperature. It did this by compensating for its reduced basal metabolic rate, with a low rate of thermal conductance. Gerbillurus paeba tolerated a labile body temperature and used a high rate of thermal conductance to remove metabolic heat, for the high humidities it encountered in its milieu would impede the use of evaporative cooling. Daily energy expenditure of both Namib rodents. Was monitored when they were maintained on a low fibre food source (golden millet) and then when this was replaced with a high fibre food source (high-bulk bran). Aethomys namaquensis showed a DEE similar to that predicted by mass and this remained constant, irrespective of the food source or the presence or absence of water. The DEE of G. paeba was more variable. When water was freely available DEE was higher than that predicted by mass and was similar to that predicted for an insectivore of similar body mass. This might reflect its propensity towards an insectivorous diet. When water-stressed on a high fibre diet, DEE of G. paeba declined by half, suggesting that G. paeba employed torpor when conditions were stressful and when the quality of resources was reduced. On a low fibre diet, both species were able to maintain a positive water balance when deprived of water, showing similar reductions in water loss to that of G. pusillus. Low assimilation efficiency and the concomitant increase in faecal water loss precluded the maintenance of a positive water balance when they were fed a high fibre food source. Despite increased urine concentrations and reduced evaporative water loss both species succumbed to chronic water-deprivation. Daily water turnovers and urine concentrating ability was monitored in 17 species of African rodents from arid areas. In general, rodents belonging to the Cricetidae exhibited lower water turnovers and voided smaller volumes of urine at significantly greater concentrations than the murids. All these cricetid rodents were found to produce a crystalline precipitate of allantoin in their urine when water stressed. The quantities of crystalline allantoin excreted were far greater than that expected from purine catabolism and accounted for 30% of the total nitrogenous wastes of the cricetids when deprived of water. It was thought that the substantial quantities of allantoin excreted were responsible for the lower water turnover rates in the cricetids. It is suggested, therefore, when water is limiting, there is a shift in the nitrogen pathway from urea to allantoin, so conferring an eco-physiological advantage to these long term residents of the arid regions of Africa

RNA Sequencing Reveals Differential Expression of Mitochondrial and Oxidation Reduction Genes in the Long-Lived Naked Mole-Rat When Compared to Mice

The naked mole-rat (Heterocephalus glaber) is a long-lived, cancer resistant rodent and there is a great interest in identifying the adaptations responsible for these and other of its unique traits. We employed RNA sequencing to compare liver gene expression profiles between naked mole-rats and wild-derived mice. Our results indicate that genes associated with oxidoreduction and mitochondria were expressed at higher relative levels in naked mole-rats. The largest effect is nearly 300-fold higher expression of epithelial cell adhesion molecule (Epcam), a tumour-associated protein. Also of interest are the protease inhibitor, alpha2-macroglobulin (A2m), and the mitochondrial complex II subunit Sdhc, both ageing-related genes found strongly over-expressed in the naked mole-rat. These results hint at possible candidates for specifying species differences in ageing and cancer, and in particular suggest complex alterations in mitochondrial and oxidation reduction pathways in the naked mole-rat. Our differential gene expression analysis obviated the need for a reference naked mole-rat genome by employing a combination of Illumina/Solexa and 454 platforms for transcriptome sequencing and assembling transcriptome contigs of the non-sequenced species. Overall, our work provides new research foci and methods for studying the naked mole-rat's fascinating characteristics

Enhanced Protein Homeostasis Mechanisms in Naked Mole Rats Cells

Phylogenic studies suggest that cells from long-lived species are more resistant to a variety of stressors than short-lived species. However, there is little information on the cellular mechanisms that give rise to increased resistance to stress. Our previous studies have shown that liver proteins from a long-lived species have lower levels of protein ubiquitination which is associated with increased proteasome activity, suggesting that mechanisms of protein quality control could play a critical role in assuring longevity of long-lived species. In this study, we evaluated whether autophagy and heat shock chaperones proteins (HSPs) are associated with longevity in rodents using skin fibroblasts isolated from mice and naked mole rats (NMR); two species that are similar in body size but differ almost 10 fold in longevity. Our results indicate that macroautophagy induced by serum-starvation is significantly enhanced in NMR compared to mouse which correlates with an inhibition of the mTOR pathway and increased LC3 conversion. We also found that several HSPs (e.g., Hsp90, Hsp70, Hsp40, Hsp 27) were significantly higher at both basal and after heat shock conditions. These observations suggest that NMR, a long-lived species, has increased mechanisms to ensure protein quality (autophagy and HSPs) and support the idea that protein homeostasis could play an important role in promoting longevity

The First International Mini-Symposium on Methionine Restriction and Lifespan

It has been 20 years since the Orentreich Foundation for the Advancement of Science, under the leadership Dr. Norman Orentreich, first reported that low methionine (Met) ingestion by rats extends lifespan (Orentreich et al., 1993). Since then, several studies have replicated the effects of dietary methionine restricted (MR) in delaying age-related diseases (Richie et al., 1994; Miller et al., 2005; Ables et al., 2012; Sanchez-Roman and Barja, 2013). We report the abstracts from the First International Mini-Symposium on Methionine Restriction and Lifespan held in Tarrytown, NY, September 2013. The goals were (1) to gather researchers with an interest in MR and lifespan, (2) to exchange knowledge, (3) to generate ideas for future investigations, and (4) to strengthen relationships within this community. The presentations highlighted the importance of research on cysteine, growth hormone (GH), and ATF4 in the paradigm of aging. In addition, the effects of dietary restriction or MR in the kidneys, liver, bones, and the adipose tissue were discussed. The symposium also emphasized the value of other species, e.g., the naked mole rat, Brandt's bat, and Drosophila, in aging research. Overall, the symposium consolidated scientists with similar research interests and provided opportunities to conduct future collaborative studies (Figure 3)

Blunted Neuronal Calcium Response to Hypoxia in Naked Mole-Rat Hippocampus

Naked mole-rats are highly social and strictly subterranean rodents that live in large communal colonies in sealed and chronically oxygen-depleted burrows. Brain slices from naked mole-rats show extreme tolerance to hypoxia compared to slices from other mammals, as indicated by maintenance of synaptic transmission under more hypoxic conditions and three fold longer latency to anoxic depolarization. A key factor in determining whether or not the cellular response to hypoxia is reversible or leads to cell death may be the elevation of intracellular calcium concentration. In the present study, we used fluorescent imaging techniques to measure relative intracellular calcium changes in CA1 pyramidal cells of hippocampal slices during hypoxia. We found that calcium accumulation during hypoxia was significantly and substantially attenuated in slices from naked mole-rats compared to slices from laboratory mice. This was the case for both neonatal (postnatal day 6) and older (postnatal day 20) age groups. Furthermore, while both species demonstrated more calcium accumulation at older ages, the older naked mole-rats showed a smaller calcium accumulation response than even the younger mice. A blunted intracellular calcium response to hypoxia may contribute to the extreme hypoxia tolerance of naked mole-rat neurons. The results are discussed in terms of a general hypothesis that a very prolonged or arrested developmental process may allow adult naked mole-rat brain to retain the hypoxia tolerance normally only seen in neonatal mammals

Oxidative stress and life histories: unresolved issues and current needs.

Life-history theory concerns the trade-offs that mold the patterns of investment by animals between reproduction, growth, and survival. It is widely recognized that physiology plays a role in the mediation of life-history trade-offs, but the details remain obscure. As life-history theory concerns aspects of investment in the soma that influence survival, understanding the physiological basis of life histories is related, but not identical, to understanding the process of aging. One idea from the field of aging that has gained considerable traction in the area of life histories is that life-history trade-offs may be mediated by free radical production and oxidative stress. We outline here developments in this field and summarize a number of important unresolved issues that may guide future research efforts. The issues are as follows. First, different tissues and macromolecular targets of oxidative stress respond differently during reproduction. The functional significance of these changes, however, remains uncertain. Consequently there is a need for studies that link oxidative stress measurements to functional outcomes, such as survival. Second, measurements of oxidative stress are often highly invasive or terminal. Terminal studies of oxidative stress in wild animals, where detailed life-history information is available, cannot generally be performed without compromising the aims of the studies that generated the life-history data. There is a need therefore for novel non-invasive measurements of multi-tissue oxidative stress. Third, laboratory studies provide unrivaled opportunities for experimental manipulation but may fail to expose the physiology underpinning life-history effects, because of the benign laboratory environment. Fourth, the idea that oxidative stress might underlie life-history trade-offs does not make specific enough predictions that are amenable to testing. Moreover, there is a paucity of good alternative theoretical models on which contrasting predictions might be based. Fifth, there is an enormous diversity of life-history variation to test the idea that oxidative stress may be a key mediator. So far we have only scratched the surface. Broadening the scope may reveal new strategies linked to the processes of oxidative damage and repair. Finally, understanding the trade-offs in life histories and understanding the process of aging are related but not identical questions. Scientists inhabiting these two spheres of activity seldom collide, yet they have much to learn from each other

Altered Composition of Liver Proteasome Assemblies Contributes to Enhanced Proteasome Activity in the Exceptionally Long-Lived Naked Mole-Rat

The longest-lived rodent, the naked mole-rat (Bathyergidae; Heterocephalus glaber), maintains robust health for at least 75% of its 32 year lifespan, suggesting that the decline in genomic integrity or protein homeostasis routinely observed during aging, is either attenuated or delayed in this extraordinarily long-lived species. The ubiquitin proteasome system (UPS) plays an integral role in protein homeostasis by degrading oxidatively-damaged and misfolded proteins. In this study, we examined proteasome activity in naked mole-rats and mice in whole liver lysates as well as three subcellular fractions to probe the mechanisms behind the apparently enhanced effectiveness of UPS. We found that when compared with mouse samples, naked mole-rats had significantly higher chymotrypsin-like (ChT-L) activity and a two-fold increase in trypsin-like (T-L) in both whole lysates as well as cytosolic fractions. Native gel electrophoresis of the whole tissue lysates showed that the 20S proteasome was more active in the longer-lived species and that 26S proteasome was both more active and more populous. Western blot analyses revealed that both 19S subunits and immunoproteasome catalytic subunits are present in greater amounts in the naked mole-rat suggesting that the observed higher specific activity may be due to the greater proportion of immunoproteasomes in livers of healthy young adults. It thus appears that proteasomes in this species are primed for the efficient removal of stress-damaged proteins. Further characterization of the naked mole-rat proteasome and its regulation could lead to important insights on how the cells in these animals handle increased stress and protein damage to maintain a longer health in their tissues and ultimately a longer life

The naked truth:a comprehensive clarification and classification of current 'myths' in naked mole-rat biology

The naked mole-rat (Heterocephalus glaber) has fascinated zoologists for at least half a century. It has also generated considerable biomedical interest not only because of its extraordinary longevity, but also because of unusual protective features (e.g. its tolerance of variable oxygen availability), which may be pertinent to several human disease states, including ischemia/reperfusion injury and neurodegeneration. A recent article entitled 'Surprisingly long survival of premature conclusions about naked mole-rat biology' described 28 'myths' which, those authors claimed, are a 'perpetuation of beautiful, but falsified, hypotheses' and impede our understanding of this enigmatic mammal. Here, we re-examine each of these 'myths' based on evidence published in the scientific literature. Following Braude et al., we argue that these 'myths' fall into four main categories: (i) 'myths' that would be better described as oversimplifications, some of which persist solely in the popular press; (ii) 'myths' that are based on incomplete understanding, where more evidence is clearly needed; (iii) 'myths' where the accumulation of evidence over the years has led to a revision in interpretation, but where there is no significant disagreement among scientists currently working in the field; (iv) 'myths' where there is a genuine difference in opinion among active researchers, based on alternative interpretations of the available evidence. The term 'myth' is particularly inappropriate when applied to competing, evidence-based hypotheses, which form part of the normal evolution of scientific knowledge. Here, we provide a comprehensive critical review of naked mole-rat biology and attempt to clarify some of these misconceptions

Skin morphology and its role in thermoregulation in mole-rats, Heterocephalus glaber and Cryptomys hottentotus

The skin structure of 2 Bathyergid rodents, the naked mole-rat (Heterocephalus glaber) and the common mole-rat (Cryptomys hottentotus) is compared, to investigate whether thermoregulatory differences may be attributed to different skin features. Histological and ultrastructural studies of the dorsal skin of these closely related species show morphological and structural similarities but differences in the degree of skin folding, thickness of the integument and dermal infrastructure were evident. The skin of the common mole-rat conforms with expected morphological/histological arrangements that are commonly found in mammalian skin. Many features of the skin of the naked mole-rat, such as the lack of an insulating layer and the loosely folded morphological arrangement contribute to poikilothermic responses to changing temperatures of this mammal. Further evidence for poikilothermy in the naked mole-rat is indicated by the presence of pigment containing cells in the dermis, rather than the epidermis, as commonly occurs in homeotherms. Lack of fur is compensated by a thicker epidermal layer and a marked reduction in sweat glands. Differences in skin morphology thus contribute substantially to the different thermoregulatory abilities of the 2 Bathyergids. The skin morphology is related to the poor thermoinsulatory ability of the animals while simultaneously facilitating heat transfer from the environment to the animal by thigmothermy and/or other behavioural means

Naked mole-rat mortality rates defy Gompertzian laws by not increasing with age

The longest-lived rodent, the naked mole-rat (Heterocephalus glaber), has a reported maximum lifespan of >30 years and exhibits delayed and/or attenuated age-associated physiological declines. We questioned whether these mouse-sized, eusocial rodents conform to Gompertzian mortality laws by experiencing an exponentially increasing risk of death as they get older. We compiled and analyzed a large compendium of historical naked mole-rat lifespan data with >3000 data points. Kaplan-Meier analyses revealed a substantial portion of the population to have survived at 30 years of age. Moreover, unlike all other mammals studied to date, and regardless of sex or breeding-status, the age-specific hazard of mortality did not increase with age, even at ages 25-fold past their time to reproductive maturity. This absence of hazard increase with age, in defiance of Gompertz’s law, uniquely identifies the naked mole-rat as a non-aging mammal, confirming its status as an exceptional model for biogerontology