Dogs are an interesting species when it comes to the study of aging. Firstly they are much closer to human metabolism and cellular biochemistry than mice, and secondly selective breeding has generated lineages with a very wide range of sizes and life spans. Thirdly, they occupy a good compromise position in the range of life spans, study cost, and similarity to humans. Mice live short lives, so studies are rapid and comparatively cheap, but there are sizable, important differences between mouse and human biochemistry. Humans live so long that most studies of aging are simply out of the question. Even in non-human primates that live half or less as long as we do, a study of aging and calorie restriction has lasted for decades, and few organizations can or will commit to that sort of effort.
Interest has picked up in recent years in the dog as a model of aging, to be used in the development of therapies to slow or reverse progression of aging. This is illustrated by the activities of the Dog Aging Project, for example, which seeks to obtain data on mTOR inhibitor therapies via their use in companion animals. Given this increased interest, researchers have started to catalog the holes in present knowledge. Even though dogs are very well studied, there is plenty to room to improve the understanding of how the mechanisms of aging progress and are influenced by genetics in this species.
Several genes have been shown to affect the body size variability of dogs, which is unmatched by any other mammalian species. Importantly, dogs also show marked differences in their expected lifespan in connection with body mass. On average, giant sized breeds (above 50 kg) have an expected lifespan of 6-8 years, while small sized breeds (below 10 kg) can live up to 14-16 years. This wide range of expected lifespans, together with other aspects, has made dogs promising as model organisms for aging research. Despite the huge progress in understanding the genetic basis of morphological variability of dogs, still very little is known about the functional relevance of canine homologs of conserved longevity genes. Currently, this may stand as an obstacle in the way of effectively utilizing dogs as aging models. As dogs can provide unique insights into many aspects of human aging, the current lack of detailed information about the canine genetic pathways of aging should be overcome by future research approaches. In this review, we provide an overview of the evolutionary conserved biological mechanisms that contribute to aging, following the Hallmarks of Aging classification, and we summarize current knowledge about these pathways in dogs.
The DNA repair machinery involves divergent pathways, each aimed to correct certain forms of DNA damage. These protective mechanisms have been in the focus of cancer and aging research for a long time. Polymorphisms in several genes of the DNA damage response machinery have been linked to longevity in humans. Intriguingly, no canine progeria syndrome, resulting from DNA repair deficiency, has been documented in the scientific literature. On the other hand, several studies that investigated various forms of canine cancer revealed alterations in the DNA repair machinery, which corresponded to findings in human cancers. While these findings clearly promote the dog as a natural model of human cancers, it is still unclear how exactly variations in DNA repair capacity contribute to the expected lifespan of dogs.
Telomere shortening is a characteristic only of somatic cells, while in germ line cells, telomere sequences are constantly restored by telomerase enzymes. The limited proliferative potential of somatic cells may seem disadvantageous for an individual, yet it may increase fitness by limiting the growth of malignant cells. Contrary to mice, dogs were reported to have low or no telomerase expression in normal somatic tissues, a pattern similar to that in humans. Tumors in dogs often showed high levels of telomerase expression, similarly to human malignancies. Although very little is known about the molecular mechanisms regulating telomere maintenance and cell cycle arrest in dogs, such findings indicate that dogs may also share basic telomere biology with humans. Importantly, telomere length was shown to be variable across different dog breeds and was in correlation with expected lifespan. Also, telomere length in individual dogs was found to decrease with age, similarly as described in humans.
Although age associated changes in chromatin structure and DNA methylation patterns have been reported in several model animals, there can be major differences between species. For example, epigenetic regulation in C. elegans seems to be limited to chromatin remodeling by histone modifications, limiting its utilization as a model to study epigenetic changes in aging. In dogs, an increasing body of evidence has suggested epigenetic regulation is behind species and breed-specific traits. Importantly, a recent study demonstrated that changes in methylation status in DNA regions, which were homologous to regions with known age-sensitive methylation patterns in humans, were in strong correlation with chronological age in dogs and wolves. This finding supported the applicability of the dog as a model of age-related epigenetic changes, while it also provided a molecular approach to determine the biological age of individual canines.
Disruption of Proteostasis
Chaperone proteins play an important role in the post-translational maturation of nascent proteins by facilitating their folding. They also function as protectors of mature proteins under various stressful conditions, by helping to maintain their natural conformation and by preventing aggregation. In dogs, the few studies that investigated chaperone proteins in relation to aging reported similar age-related changes as in humans. For example, blood levels of the Hsp70 chaperone were shown to decrease with age in dogs, similarly to what had been previously reported in humans.
Deregulation of Nutrient Sensing
Cellular metabolism, protein synthesis, and autophagy are strictly regulated by various signaling pathways. Most of these have evolved to synchronize cell growth and metabolism with nutrient availability; hence, they are often referred to as nutrient sensing pathways. Many of them converge on the target of rapamycin (TOR) kinase. Importantly, the function of mTOR can be efficiently inhibited by rapamycin, which is an already approved immunosuppressant in human medicine, and therefore has been proposed as a promising anti-aging compound to be used in humans. However, it was reported to cause severe side effects in medical dosages. Therefore, optimal dosages, which do not cause undesirable syndromes, yet still exert longevity promoting effects should be carefully determined in preclinical studies. Actually, pharmaceutical studies have already been initiated to investigate the effects of rapamycin on the lifespan of dogs.
Nutrient sensing pathways converge on the regulation of mitochondrial activity, as these organelles are the main sources of energy (in the form of adenosine triphosphate, ATP) in eukaryotic cells under normal circumstances, when enough oxygen is present. The availability of nutrients determines the rate of mitochondrial respiration, which, however, generates not only ATP but also chemical by-products, including reactive oxygen species. The oxidative burden created by mitochondria may be especially high in neurons, which solely depend on aerobic mitochondrial respiration as energy source. The role of mitochondrial dysfunction and increased oxidative burden in neural aging has been investigated in dogs. In general, dog brains were shown to accumulate oxidative damage with age. Several mitochondrial diseases are known in dogs, which have human homologs, such as the sensory ataxic neuropathy found in Golden Retriever dogs or the familial dilated cardiomyopathy in Doberman Pinschers. As several promising anti-aging drugs are likely to be tested in dogs in preclinical studies, looking into their effects on mitochondrial function and testing their possible interactions with mitochondrial genotypes can be highly relevant for humans.
A marked elevation of senescent cell numbers was reported in old mice, although not in all tissues. Importantly, this accumulation process can result from both the increased generation of senescent cells and a decreased activity of macrophages that are able to eliminate aged or apoptotic cells from tissues. Little is known about the accumulation of senescent cells in canine tissues, although this phenomenon is also likely to show fundamental similarities with other mammalian species. As there is a growing interest toward pharmacological approaches to deplete senescent cells in tissues by specific apoptosis inducing agents (senolytic drugs), dogs may eventually be involved in testing these types of anti-aging interventions.
Stem Cell Exhaustion
Tissue renewal depends on the abundance and replicative capacity of tissue-specific stem cells. Hematopoietic stem cells (HSCs) were reported to have reduced replicative capacity in both aged mice and humans, mainly because of accumulating DNA damage. This reduction can explain the old age anemia of elderly people. Importantly, similar forms of age-associated changes in blood parameters, including anemia, were reported in dogs. Besides pharmacological interventions, stem cell therapy has also been suggested as a possible anti-aging intervention, with highlighted promises to treat certain forms of neurodegeneration. In this regard, stem cell therapy trials conducted on dogs affected by forms of neurodegeneration could represent a crucial step before progressing to human trials. In the case of the Golden Retriever model for Duchenne muscular dystrophy, successful stem cell-based interventions had actually preceded human clinical trials
Altered Intercellular Communication
In addition to hormones and metabolites, extracellular vesicles released by cells into the blood, called exosomes and ectosomes, have emerged as important transducers of various cellular signals. Consequently, exosomes may also modulate aging and neurodegeneration. Exosome research in dogs have been limited until recently. However, blood miRNA levels - which were hypothesized to be mainly found in exosomes - were reported to correlate with disease phenotypes in canine Duchenne muscular dystrophy. Similarly, miRNA content in circulating exosomes was shown to correlate with progression of secondary heart failure in cases of myxomatous mitral valve disease in dogs. Altogether, investigations about the connections between exosome content and aging or age-related pathologies in dogs may lead to the identification of diagnostic markers with potential translational prospects into human studies.