Learning tricks from old dogs

Abstract

A variety of lifespans, faster ageing processes, and shared social spaces with humans mean studies of our canine friends hold great potential for shining light on ageing and cognitive decline.

Domestication of dogs has provided many benefits for humans as companions and valuable working animals, but now there is a new dimension as participants in large‐scale studies of ageing and age‐related diseases. Isolated studies on dogs have been conducted for decades, but now they are becoming better organised and integrated to yield insights into ageing and associated neurological disorders. Such studies would be hard or impossible to conduct using traditional animal models, or on humans themselves for obvious ethical considerations, as well as their long lifespan.

The field is thus highly interested in the Dog Aging Project (DAP), a longitudinal study of more than 45,000 dogs that started in the USA in 2016. DAP has become a focal point for global research in the field, especially for neurodegenerative diseases. It cooperates with the dog owners, who provide ongoing observations of their pets' behaviour, along with permission to collect blood and tissue samples, as well as scanning. Samples are being collected in a biobank, so markers of ageing and neurological decline can be assessed as they change over time in individual animals.

Dogs are better than mice

Traditionally, the most popular mammal for ageing studies has been the laboratory mouse, given its obvious advantages. The animals are often clones or transgenic to allow the study of human genes of interest. This greatly benefits repeatability and allows researchers to assess the role and impact of specific human ageing‐related genes.

But the narrow genetic range is also a disadvantage since it prevents the analysis of individual variation, which is critical for ageing and degenerative diseases. Furthermore, mice are too genetically distant from humans for meaningful studies of many ageing‐related diseases, especially those involving cognitive decline. Dogs are therefore an ideal intermediate, even better in some ways than higher primates that could not be so conveniently studied for ethical and practical reasons.

The Dog Aging Project has become a focal point for global research in the field, especially for neurodegenerative diseases…

The domestication of dogs has made them such good candidates for human ageing studies in a way that their wild ancestors would be much less so. Forced selection for a wide variety of traits has engendered great genetic diversity and a vast array of phenotypes. The smallest breeds such as the Chihuahua weigh around 2 kg while the biggest, like the English Mastiff, can grow up to 95 kg. Domestic dogs also exhibit a big range in average life expectancy, from under 5 years for French Bulldogs to almost 13 years for the Jack Russell Terrier.

The shorter‐lived breeds may be less useful for studies of neurological ageing since they tend to die from conditions associated with inbreeding, such as respiratory failure, skin fold infections and spinal disease, before other disorders occur. But many breeds have outliers that live to much greater ages than the average, with higher deviations than in humans, and this is a potential source of insights into genetic factors. Indeed, within DAP there is a study of such outliers aiming to identify the underlying genetic contributors to their longevity.

Good reasons to study dogs

The reasons for studying ageing in dogs were outlined in a 2019 paper (Sándor & Kubinyi, 2019) co‐authored by Enikö Kubinyi from Eötvös Loránd University in Budapest, Hungary, which had established a dog biobank 2 years earlier (https://ethology.elte.hu/canine_brain_bank). The authors identified seven key advantages of dogs, one of which was the wide variety of lifespans providing opportunities for translational studies on age‐related pathologies. The fact that dogs age much faster than humans, allowing for easier follow‐up studies on ageing‐related changes, is a second factor.

Dogs are socially relevant, with 50% of owners considering their dogs to be part of their family.

Another is the susceptibility of dogs to neurodegenerative disorders and cognitive abnormalities similar to humans. Dogs are also good subjects for pharmacological research because they share metabolic and physiological features with humans, enhanced by domestication. The latter has also contributed to another benefit, cognitive abilities that resemble human social skills. This means they can participate in experiments that involve social interaction and learning.

The sixth canine advantage for ageing studies is that dogs and humans share some genomic sequences with extensive linkage disequilibrium regions. Such sequences are more likely to be inherited together as a block since linkage disequilibrium is the non‐random association of alleles at different loci of the genome. As a consequence, linkage disequilibrium in dogs that is similar to humans makes them suitable for genome‐wide association studies (GWAS) to associate marker alleles and traits of interest with complex inherited age‐related diseases.

The final benefit of using dogs for ageing research is that they are readily available and can be included in large datasets at low costs through citizen science approaches. Indeed, DAP is already exploiting the combination of data derived from tissue in the biobank, with more anecdotal and subjective insights from dog owners. It therefore combines citizen science and almost continuous clinical study with the ability to recruit new animals as old ones die.

Apart from providing valuable information, the involvement of dog owners in the study has an important societal dimension, as noted by Matt Kaeberlein, an American biologist and biogerontologist at the University of Washington in Seattle. “Dogs are socially relevant, with 50% of owners considering their dogs to be part of their family,” he said.

Shared environments

This close bond between dogs and humans also makes the animals particularly relevant for studies of environmental aspects of ageing. “Companion dogs share the human environment, which is something completely neglected in most laboratory studies and impossible to replicate in the lab,” Kaeberlein commented. This has implications for both humans and dogs. For the animals it means they are exposed to the same environment variations that have comparable implications for their health and life expectancies. Indeed, life outcomes for dogs seem correlated with those of their owners and with socio‐economic factors. Daniel Promislow, biogerentologist at University of Washington and DAP's Principal Investigator, highlighted some findings along these lines. “One of our early studies from the Dog Aging Project showed that while all dogs slow down as they age, activity levels tend to differ between urban, suburban and rural neighbourhoods,” he said. “A more recent study, soon to be published, showed that greater household adversity is associated with poorer health outcomes in dogs.”

Companion dogs share the human environment, which is something completely neglected in most laboratory studies and impossible to replicate in the lab.

For humans, this close environmental coupling promises to yield insights about environmental risk factors of disease as the DAP progresses. “Of course, we are especially excited to compare patterns in our own rich dataset with those of the many long‐term longitudinal human studies that have been collected,” Promislow said. “Such comparisons will have unprecedented power to discover risk factors for the diseases that dogs and humans have in common, and to uncover the mechanisms by which one species manages to avoid certain diseases that are present in the other. Looking ahead, we are also very interested in creating a special cohort of Dog Aging Project dogs within a ‘matched cohort’ of people who own dogs, and who themselves are enrolled in a human long‐term longitudinal study. Such a matched cohort study would provide us with a powerful test of the potential of dogs as sentinels of environmental risk factors for human health and disease.”

Old dogs, new tricks

As this suggests, many of the benefits of this large‐scale longitudinal study are yet to come, because the DAP only began recruiting in 2016 and significant numbers of samples spanning multiple years are only just becoming available. But there have been longitudinal findings from other studies, one of which contradicted the long‐standing belief that old dogs cannot be taught new tricks. The study of 119 pet dogs conducted by the Clever Dog Lab at the Messerli Research Institute in Vienna revealed a more nuanced picture (Chapagain et al, 2020). The authors found that problem solving showed a linear decline with age in dogs after 6 years, as did attentiveness, memory, and executive functions. But the results also indicated that older dogs remained trainable provided they were free of specific neurodegenerative diseases, and highlighted how mental stimulation was essential for maintaining quality of life, combined with exercise, play and novel toys, much like humans.

The indication was that older dogs require more motivation and stimulation than young ones but are still capable of learning new tricks.

This was demonstrated in one particular test where dogs had to learn to make eye contact with the trainer after finding and eating a piece of sausage dropped on the floor. If they did this they were rewarded with another piece of sausage, and on this task older dogs performed just as well. The indication was that older dogs require more motivation and stimulation than young ones but are still capable of learning new tricks.

This same study surprised the researchers with the finding that an enriched diet had no impact on retention of cognitive abilities in older age. On the other hand, numerous studies in animals generally have provided strong evidence that diet can extend lifespan and combat some neurodegenerative diseases, but largely through restricting intake. This has now been echoed in DAP with the finding that dogs fed just once a day have lower risk of various diseases than those enjoying two or more meals (Bray et al, 2022). After controlling for sex, age, breed and other potential confounders, dogs fed once daily had lower mean scores on a cognitive dysfunction scale, and reduced odds of having gastrointestinal, dental, orthopaedic, kidney/urinary and liver/pancreas disorders.

Caution is required however after many studies of the effects of calorific restriction in laboratory on disease risk and lifespan. It turned out that the critical factor was average consumption of calories over time, not when they were taken. So it may be that what matters is how much dogs eat, not when. Obesity is known to be a major risk factor for numerous morbities in dogs as in humans, so those pets fed just once a day may tend to eat less overall and therefore are less likely to be overweight. This again points to the value of the ongoing longitudinal study which over time should resolve this issue, for example by controlling for dog weight.

Focus on neurodegenerative diseases

Another recent finding from the DAP – that is not so surprising – is that physical activity is associated with cognitive function in older dogs (Bray et al, 2023). This echoes studies in other animals and humans, in this case combined with the findings on diet that owners have a major role to play in their pets' health and lifespan, if only by ensuring plenty of exercise and avoiding overeating.

Indeed, a major focus of DAP, as well as other canine studies, is risk of neurodegenerative diseases, given the shared pathologies with humans, albeit over timescales roughly seven times shorter. There is already some evidence that dog studies can yield early interventions that could delay cognitive decline, or onset of neurodegenerative diseases in humans, according to Evan MacLean, Director of the Arizona Canine Cognition Centre at the University of Arizona. “We don't know for certain what is causing what, but the generally similar patterns in studies of dogs and humans suggest that there may be common factors at play,” he explained. “Currently there are intervention studies in dogs that, if successful, will likely support a case for evaluating the same treatments in humans.”

MacLean pointed out that dogs naturally develop many of the same age‐related conditions as humans. “As an example, a subset of aging dogs will develop Canine Cognitive Dysfunction, a neurodegenerative disease with many similarities to Alzheimer's disease,” he continued. “This occurs naturally and sporadically, like Alzheimer's disease, which makes it a potentially more realistic model than a transgenic organism that has been engineered to artificially exhibit some particular characteristic. Just as importantly, dogs share our environments, have access to sophisticated healthcare, and have highly variable lifestyles, just like humans. These features make dogs an unusual, and potentially very valuable model for understanding many aspects of human ageing.”

Work on canine models of Alzheimer's has been underway for over a decade by, among others, Elizabeth Head, Director of the Experimental Pathology Program at the University of California, Irvine. She now has high hopes that the DAP will accelerate progress as results come in. “There are several truly unique aspects to this study including studying ageing in pet dogs, who are exposed to environmental conditions much more similar to humans than much of the work I have done in the past,” Head said. “Opportunities to understand the impact of diet, exercise, stress and social interaction on brain ageing will be the highlights, I believe, to the DAP.”

Opportunities to understand the impact of diet, exercise, stress and social interaction on brain ageing will be the highlights, I believe, to the Dog Aging Project.

There has already been significant progress understanding cognitive changes in ageing people through canine studies during the past decade, Head added. “We are learning more about the brain imaging, from non‐invasive neuroimaging using magnetic resonance scans, changes in structure, in how the brain is connected, neurochemistry and blood flow and how similar these changes are also to human brain aging and early Alzheimer disease,” she said. “We are also completing studies on measures of different proteins in the blood that may also be used as biomarkers to tell us about brain changes in dogs. […] Last, we are testing a novel treatment, tacrolimus, which is FDA approved and appears to be reducing the signs of brain aging and may be a great prevention approach to promote healthy brain aging in dogs and humans.”

Drug interventions

Tacrolimus is an immunosuppressive drug approved primarily for reducing risk of organ rejection in transplant patients. The drug works by inhibiting calcineurin, which promotes development and proliferation of T cells involved in the adaptive immune response. But calcineurin can also lead to hyperactivation of astrocytes in the brain and spinal cord, resulting in neuronal death, and inflammation, which increases the risk of Alzheimer's disease, and accelerates general physical and mental decline.

Another drug with immunosuppressive and an even greater interest in the ageing research community is paramycin. From the same family as streptomycin, paramycin was first approved in 1994 by the FDA as an immunosuppressive drug, initially to prevent rejection of transplanted livers.

The stage is set to generate new insights into the complex relationships between genetics and environment that underpin ageing processes.

Around the same time research tried to understand how rapamycin blocks the growth of eukaryote cells as it did in suppressing the adaptive immune system to prevent rejection of transplanted organs. It was discovered that one particular protein was the key target for rapamycin to inhibit cell growth (Selvarani et al, 2021). This was christened Target Of Rapamycin (TOR), and analogues have been found in mammals, called mTOR, which is now known to be a master regulator of eukaryote cell response to nutrients and growth factors, in what has been called the TOR pathway. Targeting this pathway was found to increase average lifespan in diverse organisms, including yeast, worms and mice (Fig 1).

Figure 1. Canis lupus (Lagotto Romagnolo).

More similar to humans than one might think.

There has been accumulating evidence that disruptions to mTOR in mammals is associated with age‐dependent cognitive decline and with the pathogenesis of Alzheimer's disease (Perluigi et al, 2021). Such findings have led to growing interest in testing rapamycin in ageing dogs, and this is being done as part of DAP. “Among the more than 45,000 dogs that have enrolled in the Dog Aging Project, a small proportion of them, about 1 in 100, will be enrolled in the Test of Rapamycin In Aging Dogs (TRIAD) clinical trial,” Promislow said. “It is important to stress that we do not yet know whether rapamycin will improve healthy aging in dogs. The goal of the TRIAD trial is to answer that question. And indeed, the gold standard for safety and efficacy of any drug treatment is a placebo‐controlled, randomized, double‐masked study. Ours is the first to be done testing the ability of a drug to increase lifespan in a free‐living species.”

The stage is set for the DAP and other canine research projects to generate new insights into the complex relationships between genetics and environment that underpin ageing processes. It is also an interesting experiment of combining large‐scale studies with citizen science that can yield not only a better understanding about human ageing and possible interventions but also fascinating insights about one of mankind's faithful companions.

EMBO reports (2023) 24: e57706