Time-Restricted Feeding Clears Mutant Huntingtin Protein From the Brains of Mice
Eating for brain health may look like intermittent fasting, or time-restricted feeding for patients who can’t afford to lose weight.
Dagmar Ehrnhoefer studies the brain. She wouldn’t call herself a metabolic researcher, although her studies of Huntington disease, a neurodegenerative disease that is strictly inherited, have led her to study autophagy — the process of cellular self-eating — and therefore metabolic processes. As you may have learned in reading this blog, metabolism and what and when we eat are intricately linked with autophagy. They are also linked with the diseases of aging that come as cellular self-eating fails to keep up with the buildup of dysfunctional cells over time.
“We are used to thinking of the brain as a metabolically protected organ, but there are certainly metabolic signals that reach the brain,” Dagmar said.
As a postdoctoral researcher at the UBC in Vancouver, Dagmar conducted research on the accumulation of mutant Huntingtin protein (HTT) in mice. She explored how we might reverse this accumulation through both lifestyle interventions and medication. She recently helped publish results of this research in Acta Neuropathologica Communications, in a paper titled “Preventing mutant huntingtin proteolysis and intermittent fasting promote autophagy in models of Huntington disease.”
“If your parents have Huntington disease, you have a 50% chance of also inheriting the disease,” Dagmar said. “The cause of the disease is a protein mutation. If you have a mutation in Huntingtin protein, you will invariably inherit the disease. One of the currently popular therapeutic strategies is an attempt to remove the mutant protein by any means possible, either at the RNA stage or the protein stage.”
One potential way to clear the mutant Huntingtin protein (HTT) is to leverage the body’s natural protein clearing and breakdown process, called autophagy.
“We’ve been trying to accelerate the clearance of mutant Huntingtin through autophagy,” Dagmar said. “Autophagy literally means ‘self-eating’ — it happens naturally when cells are faced with starvation. In this state, a cell has to generate energy by digesting its own proteins.”
The tricky thing about enhancing the clearance of mutant HTT through autophagy is that the Huntingtin protein itself is an important player in this process of cellular self-eating. In turns out that HTT both regulates autophagy and, in turn, is degraded during autophagy so that it doesn’t accumulate in the brain. In its wild type form, HTT promotes autophagosome formation, an important step in the process of a cell self-destructing by eating itself. Autophagosomes are vesicles that form inside of the cell to help it “eat itself.” The mutant Huntingtin protein, on the other hand, accumulates in brain cells and prevents them from functioning normally while also preventing them from “cleaning house” naturally via autophagy.
“Autophagy is an important biological process that is essential for the removal of damaged organelles and toxic or aggregated proteins by delivering them to the lysosome for degradation. Consequently, autophagy has become a primary target for the treatment of neurodegenerative diseases that involve aggregating proteins. Huntington disease (HD) […] is unique among the neurodegenerative proteinopathies in that autophagy is not only dysfunctional but wild type (wt) HTT also appears to play several roles in regulating the dynamics of autophagy.” — Martin et al. 2015
“We think this protein accumulation is part of what causes Huntington disease symptoms,” Dagmar said. “If we could accelerate and boost the body’s clearance of this protein, we assume that the affected cells would function better and symptoms would improve.”
A Complicated Mouse Paves the Way to Understanding Huntington Protein and Autophagy
One mouse model of Huntington disease that Dagmar and colleagues investigated in their study displays an odd phenotype — no disease symptoms. These mice still have mutations in their Huntingtin protein genes (HTT). However, they also have another point mutation that prevents their mutant proteins from being cleaved in two and made dysfunctional for autophagy, like the mutant proteins in the typical mouse model for the disease are. In other words, a mouse with a cleavage-resistant form of mutant Huntingtin protein doesn’t display the typical disease symptoms because while these mutant proteins accumulate in the brain, they are still functional in their role as autophagy regulators.
“We think that this weird mouse model with mutant Huntingtin but no disease symptoms is the result of a fixed autophagy process,” Dagmar said. “If mutant Huntingtin doesn’t accumulate, it doesn’t cause all the problems associated with the disease state. Of course, this doesn’t really help us with a therapy in humans, because you can’t mutate the mutant Huntingtin protein in people! But if we could develop a drug that would prevent the cleavage that happens in the dysfunctional Huntingtin protein, it might also help clear the mutant protein from the body through autophagy.”
The Path to Intermittent Fasting as a Mutant Huntingtin Clearing Process
Studying the role of autophagy in Huntingtin disease naturally lead Dagmar and her colleagues to consider how caloric restriction and even intermittent fasting might improve this process in a diseased brain. A dietary intervention for Huntington disease isn’t necessarily self-evident, however, as patients with this disease naturally struggle to maintain weight. Malnutrition is a big concern for patients struggling with Huntington disease, essentially ruling out permanent caloric restriction despite the known connection between caloric restriction and autophagy. However, intermittent fasting without long-term caloric restriction remains a potential option for triggering autophagy and protein clearance without weight loss.
“We started at first with the idea of a 24-hour fast,” Dagmar said. The research group conducted their study of the effects of fasting on autophagy and mutant HTT clearance in a Huntington disease mouse model. “We saw that in the liver, which is one of the first organs to be affected by fasting, the mutant Huntingtin protein actually disappeared. We thought, OK, there is something to this. But we didn’t know if this effect could reach the brain within such a short time frame.”
Dagmar and colleagues decided that if fasts longer than 24 hours were contraindicated in patients with Huntington disease due to issues of weight loss, a daily time-restricted feeding schedule might be a better potential trigger for autophagy. They weren’t sure, but they suspected that a strict daily feeding schedule could promote autophagy and mutant protein clearance over time without the need for strict caloric restriction. Time-restricted feeding, in either humans or mice, consists of fasting for 16–18 hours per day, or eating within a 6–8 hour window during daylight hours.
“We really don’t want to encourage anyone with Huntington disease to fast,” Dagmar said. “These patients already struggle with weight loss and malnutrition.”
An alternative approach to long-term fasting involving strictly regulating food intake to certain hours of the day, however, could be a safe approach to promoting proper autophagy and mutant protein clearance.
“We gave our mice as much food as they wanted, just during certain times of the day,” Dagmar said.
Amazingly, when mice with the equivalent of Huntington disease were placed on a strict time-restricted feeding schedule, involving an 8-hour feeding window, Dagmar and colleagues started to notice an effect in the brains of the animals. Mutant HTT was getting cleared, even from brain tissue.
But how do the impacts of a time-restricted feeding schedule reach the brain, specifically in terms of inducing clearance of mutant proteins?
“Our findings imply that mutant HTT clearance could be enhanced by a regulated dietary schedule that promotes autophagy.” — Dagmar
Cleaning House: Eating by the Body’s Clock
“We know that the mouse model that we used in our study but also patients with Huntington disease suffer from circadian dysfunction,” Dagmar said. “This of course plays into metabolism and the regulation of autophagy — it all works together. It you have a more regulated food intake schedule, whether you are a mouse or a human, you’ll likely have a more regular circadian rhythm.”
We know that prolonged overnight fasting can cause nutrient deprivation that kicks the body into a more energy-efficient mode, in which it prefers to burn fats and break down various cellular components, especially damaged ones. However, Dagmar and her fellow researchers were surprised to discover that prolonged overnight fasting in mice actually triggered autophagy in the brain. The researchers confirmed this through electron and fluorescence microscopy of brain tissue, in which they found visual evidence of autophagic vacuoles and positive fluorescence labeling for LC3, a microtubule-associated protein that has become a reliable marker for autophagy.
“I was surprised to find these effects in the brain.” — Dagmar
“We’ve always thought of the brain as being metabolically protected,” Dagmar said. “Based on current scientific literature, most researchers would say that short-term fluctuations in glucose have no effect on the brain. But nevertheless, if you extend fasting periods long enough, and repeat them regularly, the brain adjusts.”
Time-restricted feeding acts as both a broad signal to the body to start “cleaning house,” clearing out unneeded cellular components and aggregated proteins, as well as a specific trigger for the body to break down misfolded proteins. This turns out to be a double whammy against mutant Huntingtin protein. Mutant HTT is more abundant in the brain during later disease stages, but it is also a misfolded protein, making it a preferential autophagy substrate. In other words, when your body senses potential starvation and starts cleaning house, it’s going to start with aggregated, dysfunctional, misfolded proteins — just like mutant Huntingtin.
Dagmar says that future studies should investigate the potential role of circadian rhythm regulation in the effect that time-restricted feeding has on autophagy and clearance of mutant proteins. For example, are clock genes impacted by a time-restricted feeding schedule? Does time-restricted feeding promote measurably better sleep quality, with associated neuroprotective effects?
Summary: The Missing Puzzle Piece Linking
Fasting as a treatment for Huntington disease is counterintuitive. Traditionally, doctors don’t want Huntington disease patients to practice any type of fasting due to concerns of excessive weight loss. However, several behavioral studies in mouse models for Huntington disease have revealed that improving the animals’ circadian rhythms through a strict feeding and activity schedule results in an improvement in disease symptoms. But until Dagmar’s study, researchers hadn’t made the connection between circadian regulation, autophagy and the clearance of mutant Huntingtin protein. The puzzle piece that could potentially link intermittent fasting or specifically time-restricted feeding with mutant protein clearance was missing.
Throw in the circadian rhythm benefits of time-restricted feeding, and you start to see a clearer picture emerge for how lifestyle interventions such as regular overnight fasting could impact Huntington disease symptoms by way of autophagy and clearance of mutant HTT from the brain.
Could a time-restricted feeding intervention have similar effects for other neurodegenerative diseases characterized by abnormal protein accumulation? We eagerly await human research studies on this topic — let us know if you have an interest in studying the impacts of intermittent fasting on various disease symptoms and outcomes via our LIFE apps!
