Multifactorial processes to slowing the biological clock: Insights from a comparative approach

Comparative analysis of animal lifespan

Comparative studies of aging are a promising approach to identifying general properties of and processes leading to aging. While to date, many comparative studies of aging in animals have focused on relatively narrow species groups, methodological innovations now allow for studies that include evolutionary distant species. However, comparative studies of aging across a wide range of species that have distinct life histories introduce additional challenges in experimental design. Here, we discuss these challenges, highlight the most pressing problems that need to be solved, and provide suggestions based on current approaches to successfully carry out comparative aging studies across the animal kingdom.

The Idiosyncratic Physiological Traits of the Naked Mole-Rat; a Resilient Animal Model of Aging, Longevity, and Healthspan

The subterranean-dwelling naked mole-rat (Heterocephalus glaber) is an extremophilic rodent, able to thrive in the harsh underground conditions of sub-Saharan Northeast Africa. This pelage-free mammal exhibits numerous unusual ecophysiological features including pronounced tolerance of thermolability, hypoxia, hypercapnia and noxious substances. As a mammal, the naked mole-rat provides a proof-of-concept that age-related changes in physiology are avoidable. At ages far beyond their expected lifespans given both their body size and/or the timing of early developmental milestones, naked mole-rats fail to exhibit meaningful changes in physiological health or demographic mortality. Lack of physiological deterioration with age is also evident in lean and fat mass, bone quality, and reproductive capacity. Rather, regardless of age, under basal conditions naked mole-rats appear to "idle on low" with their "shields up" as is manifested by low body temperature, metabolic rate, cardiac output and kidney concentrating ability, enabling better protection of organs and cellular function. When needed, they can nevertheless ramp up these functions, increasing cardiac output and metabolism 2-5 fold. Here we review many unusual aspects of their physiology and examine how these attributes facilitate both tolerance of the diverse suite of hostile conditions encountered in their natural milieu as well as contribute to their extraordinary longevity and resistance to common, age-related chronic diseases.

Telomere Length in Healthy Adults Is Positively Associated With Polyunsaturated Fatty Acids, Including Arachidonic Acid, and Negatively With Saturated Fatty Acids

Lymphocyte telomere length (LTL) is a biomarker of aging that may be modified by dietary factors including fat. Red blood cell fatty acid status is a well-validated indicator of long-term dietary intake of fat from various sources. Recent findings from epidemiological studies of LTL in relation to fatty acids in red blood cells are not conclusive. The present study was carried out to investigate if red blood cell fatty acid status in 174 healthy older South Australians is associated with LTL. Lymphocyte telomere length was measured by real-time qPCR and fatty acid content in red blood cells was measured by gas chromatography. Our results indicate that the majority of saturated fatty acids and monounsaturated fatty acids are negatively associated with LTL, whereas polyunsaturated fatty acids are positively associated with LTL. Multiple regression analysis revealed that arachidonic acid (C20:4n-6) is significantly, independently, positively correlated with LTL (β = 0.262; p = .000). The significant association of fatty acids, particularly C20:4n-6, with telomere length warrants further research.

Probing Pedomorphy and Prolonged Lifespan in Naked Mole-Rats and Dwarf Mice

Pedomorphy, maintenance of juvenile traits throughout life, is most pronounced in extraordinarily long-lived naked mole-rats. Many of these traits (e.g., slow growth rates, low hormone levels, and delayed sexual maturity) are shared with spontaneously mutated, long-lived dwarf mice. Although some youthful traits likely evolved as adaptations to subterranean habitats (e.g., thermolability), the nature of these intrinsic pedomorphic features may also contribute to their prolonged youthfulness, longevity, and healthspan.

Polyunsaturated fatty acids ameliorate aging via redox-telomere-antioncogene axis

Polyunsaturated fatty acids (PUFA), a group of nourishing and health-promoting nutrients, ameliorate age-related chronic diseases. However, how PUFA especially n-3 PUFA exert anti-aging functions remains poorly understood. Here we link fish oil, docosahexaenoic acid (DHA) and arachidonic acid (AA) to the aging etiology via a redox-telomere-antioncogene axis based on D-galactose-induced aging mice. Both fish oil and PUFA enhanced hepatic superoxide dismutase (SOD) and catalase activities and cardiac SOD activities within the range of 18%-46%, 26%-65% and 19%-58%, respectively, whereas reduced cerebral monoamine oxidase activity, plasma F₂-isoprostane level and cerebral lipid peroxidation level by 56%-90%, 20%-79% and 16%-54%, respectively. Thus, PUFA improve the in vivo redox and oxidative stress induced aging process, which however does not exhibit a dose-dependent manner. Notably, both PUFA and fish oil effectively inactivated testicular telomerase and inhibited c-Myc-mediated telomerase reverse transcriptase expression, whereas n-3 PUFA rather than n-6 PUFA protected liver and testes against telomere shortening within the range of 13%-25% and 25%-27%, respectively. Therefore, n-3 PUFA may be better at inhibiting the DNA damage induced aging process. Surprisingly, only DHA significantly suppressed cellular senescence pathway evidenced by testicular antioncogene p16 and p53 expression. This work provides evident support for the crosstalk between PUFA especially n-3 PUFA and the aging process via maintaining the in vivo redox homeostasis, rescuing age-related telomere attrition and down-regulating the antioncogene expression.

Hypofunctional TrkA Accounts for the Absence of Pain Sensitization in the African Naked Mole-Rat

The naked mole-rat is a subterranean rodent lacking several pain behaviors found in humans, rats, and mice. For example, nerve growth factor (NGF), an important mediator of pain sensitization, fails to produce thermal hyperalgesia in naked mole-rats. The sensitization of capsaicin-sensitive TRPV1 ion channels is necessary for NGF-induced hyperalgesia, but naked mole-rats have fully functional TRPV1 channels. We show that exposing isolated naked mole-rat nociceptors to NGF does not sensitize TRPV1. However, the naked mole-rat NGF receptor TrkA displays a reduced ability to engage signal transduction pathways that sensitize TRPV1. Between one- and three-amino-acid substitutions in the kinase domain of the naked mole-rat TrkA are sufficient to render the receptor hypofunctional, and this is associated with the absence of heat hyperalgesia. Our data suggest that evolution has selected for a TrkA variant that abolishes a robust nociceptive behavior in this species but is still compatible with species fitness.

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TRPV1 ion channels in naked mole-rat nociceptors are not sensitized by NGF

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Naked mole-rat TRPV1 channels are sensitized by NGF in mouse nociceptors

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NGF activation of naked mole-rat TrkA receptors does not sensitize TRPV1

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One to three amino acids in the naked mole-rat TrkA receptors may render it hypofunctional

Targeting Protein-Protein Interactions in the Ubiquitin-Proteasome Pathway

The ubiquitin-proteasome pathway (UPP) is a major venue for controlled intracellular protein degradation in Eukaryota. The machinery of several hundred proteins is involved in recognizing, tagging, transporting, and cleaving proteins, all in a highly regulated manner. Short-lived transcription factors, misfolded translation products, stress-damaged polypeptides, or worn-out long-lived proteins, all can be found among the substrates of UPP. Carefully choreographed protein-protein interactions (PPI) are involved in each step of the pathway. For many of the steps small-molecule inhibitors have been identified and often they directly or indirectly target PPI. The inhibitors may destabilize intracellular proteostasis and trigger apoptosis. So far this is the most explored option used as an anticancer strategy. Alternatively, substrate-specific polyubiquitination may be regulated for a precise intervention aimed at a particular metabolic pathway. This very attractive opportunity is moving close to clinical application. The best known drug target in UPP is the proteasome: the end point of the journey of a protein destined for degradation. The proteasome alone is a perfect object to study the mechanisms and roles of PPI on many levels. This giant protease is built from multisubunit modules and additionally utilizes a service from transient protein ligands, for example, delivering substrates. An elaborate set of PPI within the highest-order proteasome assembly is involved in substrate recognition and processing. Below we will outline PPI involved in the UPP and discuss the growing prospects for their utilization in pharmacological interventions.

Unraveling the message: insights into comparative genomics of the naked mole-rat

Animals have evolved to survive, and even thrive, in different environments. Genetic adaptations may have indirectly created phenotypes that also resulted in a longer lifespan. One example of this phenomenon is the preternaturally long-lived naked mole-rat. This strictly subterranean rodent tolerates hypoxia, hypercapnia, and soil-based toxins. Naked mole-rats also exhibit pronounced resistance to cancer and an attenuated decline of many physiological characteristics that often decline as mammals age. Elucidating mechanisms that give rise to their unique phenotypes will lead to better understanding of subterranean ecophysiology and biology of aging. Comparative genomics could be a useful tool in this regard. Since the publication of a naked mole-rat genome assembly in 2011, analyses of genomic and transcriptomic data have enabled a clearer understanding of mole-rat evolutionary history and suggested molecular pathways (e.g., NRF2-signaling activation and DNA damage repair mechanisms) that may explain the extraordinarily longevity and unique health traits of this species. However, careful scrutiny and re-analysis suggest that some identified features result from incorrect or imprecise annotation and assembly of the naked mole-rat genome: in addition, some of these conclusions (e.g., genes involved in cancer resistance and hairlessness) are rejected when the analysis includes additional, more closely related species. We describe how the combination of better study design, improved genomic sequencing techniques, and new bioinformatic and data analytical tools will improve comparative genomics and ultimately bridge the gap between traditional model and nonmodel organisms.