Liver Protein Could Be Behind Exercise’s “Brain Benefits”

Whether you opt for a stroll in the park, a few lengths of the swimming pool or perhaps an all-out HIIT session that makes you question how your legs will possibly carry you around for the rest of the day – exercise is often either a passion or a chore.

We know that it is good for us – science says so. Exercise can dramatically reduce your risk of developing a major illness, such as Type 2 diabetes, heart disease or cancer, by up to 50%, in addition to helping you live longer. It's also good for the aging brain – exercise is correlated with a reduced risk for mild cognitive impairment and also improves cognition in populations that are at-risk for Alzheimer's disease.

Alana Horowitz, a graduate student in the Villeda lab at the University of California San Francisco (UCSF), told Technology Networks: "In interventional studies, where participants will engage in supervised exercise, researchers can more specifically observe the state of memory before and after exercise. They’ve shown that improvements in fitness level from these exercise regimens correlate with improvements in cognitive tasks, such as delayed word recall."

What's remarkable about these data, Horowitz says, is that exercise is effective regardless of when it's implemented. You can start exercising at mid-life or later in life, in your 50s, 60s or 70s and still reap the cognitive benefits.

The health benefits associated with exercise are often sufficient to inspire individuals to exercise, even if it isn't their favourite task in the world. However, due to physical limitations such as age or fragility, not everyone is able to easily hop on a treadmill. 

In a new study, published in the journal Science, Horowitz and colleagues at UCSF make some interesting discoveries surrounding a little-studied liver protein. Their findings suggest that it might be responsible for the neurological benefits associated with exercise in the aging brain. Their work is in mice, but it's possible that with extended research their findings could one day be helpful for individuals that are physically unable to exercise. 

Exercise induces changes in the aging mouse brain

The research follows on from previous work conducted in the Villeda lab, where the team demonstrated that biological factors present in the blood of young mice can effectively "rejuvenate" an aging mouse brain, and, on the flip-side, biological factors in the blood of older mice could induce premature age-related cognitive decline in younger mice.

Firstly, Horowitz and colleagues assessed the brains of aged mice who had been exercising for several weeks: "Our mice underwent what we call a 'voluntary running' paradigm for six weeks. We added a running wheel to each of their cages, so they could run as much or as little as they wished," said Horowitz. Age-matched mice were provided with nesting material as a control measure.

The scientists found that exercise induced adult neurogenesis, increased expression of brain-derived neurotrophic factor and improved hippocampal-dependent learning and memory when compared to controls.

The blood from active mice was then transferred to sedentary mice over a treatment period of four weeks, which resulted in improvements in learning and memory tasks and increased neurogenesis that mirrored the earlier results found in the active mice. What components of the active mice's blood could have caused these changes in the brain?

A protein puzzle

To explore this, the scientists analyzed and measured the soluble proteins present in the blood of active mice compared to the sedentary mice. They identified 30 different candidate proteins before focusing on a single protein: Gpld1.

"We first narrowed our candidate pool to 12 factors that changed in both mature and aged mice. This gave us confidence that we were looking at a robust phenomenon. From those 12 candidates, we did a deep dive in the literature to look for any hints that the proteins might be linked to cognition or neurogenesis. We chose to pursue Gpld1 because, although it had not been previously linked to cognition, it was implicated in the widest range of processes – from hormonal responses to metabolic processes," said Horowitz.

"We figured that if the protein had already been investigated thoroughly, someone would have stumbled upon this effect," Saula Villeda, assistant professor in the Department of Anatomy at UCSF, added in a press release. "I like to say -- if you're going to take a risk by exploring something new, you might as well go big!"

The scientists discovered that, post-exercise, Gpld1 – a protein produced in the liver – was increasingly expressed in the blood circulation of the mice, and this correlated with improvements in the animal's cognitive performance assessed by a radial arm water maze test (RAWM) and contextual fear conditioning behavioral tests. Interestingly, elevated Gpld1 has also been found in the blood of active elderly adults when compared to sedentary adults in human research.

Overexpressing Gpld1 in aged mice

To further assess whether Gpld1 could be responsible for the apparent "brain benefits" observed, Horowitz and colleagues adopted genetic engineering to evoke overproduction of Gpld1 in the livers of aged mice. When asked about this choice of method, Horowitz said: "We chose our approach to increase levels of Gpld1 in blood because it most closely mimicked the effects of exercise. Exercise increases Gpld1 expression in the liver, which dumps it into circulation, so we used genetic approaches to mimic that in aged sedentary mice."

Once overproduction of Gpld1 had been triggered, the mice were subject to a multitude of tests including RAWM, forced alternation (a Y maze test) and novel object recognition tests (NOR). "Our cognitive tests are designed to be sensitive to age-related changes. They can detect cognitive deficits associated with age and if we’re able to reverse that decline with our interventions."

But can assessing the cognition of mice be compared to an assessment in humans? "Some of the tests are actually pretty similar to cognitive tests used in humans. The limitation to them is what motivates the mouse to participate in the task. For example, if you wanted to test my memory, you can just ask me 'Look at these two objects, which one have I shown you before?'. We can’t ask that to a mouse, so we have to take advantage of their primal instincts, such as their natural tendency to explore an object they haven’t seen before, to measure their memory," said Horowitz.  

Overexpression of Gpld1 associated with significant cognitive improvement

The aged animals overexpressing Gpld1 were found to commit significantly fewer errors when locating the target platform in RAWM training compared to controls, and spent significantly more time in the novel arm and with the novel object in Y maze and NOR testing when compared to control groups that did not overexpress Gpld1. In parallel, analyses of the mice's brains revealed increased neurogenesis in the hippocampus.

Taken together, these results imply that Gpld1 could be a molecule implicated in the cognitive improvements associated with exercise. When asked for how long the improvements were observed in the mice overexpressing Gpld1, Horowitz said: "The short answer is, we don’t know the limit of these effects yet. We measured as far out as 60 days after treatment and saw benefits at that time point. We haven’t tracked further out to see if or when the effects fade."

“To be honest, I didn’t expect to succeed in finding a single molecule that could account for so much of the benefits of exercise on the brain. It seemed more likely that exercise would exert many small, subtle effects that add up to a large benefit, but which would be hard to isolate.” Villeda said. “When I saw these data, I was completely floored.”

But is Gpld1 the only molecule responsible for inducing these effects? Horowitz suspects not.

Whilst the scientists did test another candidate from their pool of 30 proteins – Pon1, a liver-derived circulating factor – it was not sufficient to significantly enhance cognition. "I fully expect there to be other candidates that can have similar effects or synergies with Gpld1 to benefit other aspects of brain function," Horowitz commented.

How is Gpld1 producing improvements in cognition and increasing neurogenesis?

Interestingly, the team were able to show through a suite of laboratory experiments that liver-derived systemic Gpld1 does not readily enter the brain – it seems it can't cross the blood-brain barrier, which begs the question: what underlying mechanisms underpin the improvements in cognition and neurogenesis that were observed in the study?

The physiological role of Gpld1 is somewhat elusive. Nevertheless, cleavages of its substrates are implicated in signaling cascades linked to blood coagulation and inflammation. The team therefore propose that, considering inflammation and blood coagulation are elevated in aging individuals and are linked to cognitive decline, Gpld1 might be eliciting some form of reversal effect on these processes. 

Analyzing the protein's physiological role will be their next step, according to Horowitz: "While we tested changes in neurogenesis and cognition, effects on other cell types or brain processes are still unknown. We also need to figure out how the targets of Gpld1 mediate these effects. We’re actively working to tease apart these complex signaling networks and are really excited to see where it leads."

Hypothetically speaking, might there one day be a pill that can produce the beneficial brain effects of exercise, without actually exercising? Whilst this could improve the lives of individuals that are physically limited, it's very early days, and there are several factors that need further exploration and clarification.

For example, neurogenesis was one of the measured variables in this research. However, debate over whether neurogenesis even occurs in the adult human brain is still ongoing, which could be a potential caveat to translating the findings from animal models to humans. But Horowitz believes that neurogenesis is just one piece of the puzzle, stating that she would expect changes in other cells and processes, such as mature neurons and neuroinflammation, that would more fully account for the cognitive effects that were seen in the study.

Reference: Horowitz et al. (2020). Blood factors transfer beneficial effects of exercise on neurogenesis and cognition to the aged brain. Science. DOI: 10.1126/science.aaw2622.

Alana Horowitz was speaking to Molly Campbell, Science Writer, Technology Networks.