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.”

In this image of single striatal neurons, the neuron in the center (yellow) contains an abnormal intracellular accumulation of huntingtin protein (orange). Credit: Dr. Steven Finkbeiner, Gladstone Institute of Neurological Disease, The Taube-Koret Center for Huntington’s Disease Research, and UCSF.

“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 Huntington case courtesy of Dr Bruno Di Muzio, Radiopaedia.org, rID: 57653. The most striking MRI feature is that of caudate head atrophy resulting in enlargement of the frontal horns. Patients typically present with progressive rigidity, involuntary movements, dementia, psychosis and emotional instability.

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?