Huntington’s Aggregates Disrupted by Protein-Like Polymer, Rescuing Neurons in Mouse Model

Scientists at Northwestern, and Case Western Reserve universities have developed the first polymer-based therapeutic for the genetic disorder Huntington’s disease, an incurable neurodegenerative disease that causes nerve cells to break down in the brain.

The new treatment leverages peptide-brush polymers, which act as a shield to prevent proteins from binding to one another. In studies in mice the treatment successfully rescued neurons to reverse symptoms. Treated mice also experienced no significant side effects, indicating that the therapy was nontoxic and well tolerated.

Although further tests will be needed, the researchers can envisage that in their future the treatment could be administered as a once-weekly injection to delay disease onset or reduce symptoms in patients with the genetic mutation. Northwestern’s Nathan Gianneschi, PhD, who led the polymer therapeutic development, and colleagues reported on their work in Science Advances, in a paper titled “Proteomimetic polymer blocks mitochondrial damage, rescues Huntington’s neurons, and slows onset of neuropathology in vivo.” In their paper the team noted, “Excitingly, these studies support the idea of “polymers as therapeutics,” rather than polymers as carriers of drug molecules in the typical sense of serving as a payload delivery system.”

Gianneschi is the Jacob and Rosaline Cohn Professor of Chemistry at Northwestern’s Weinberg College of Arts and Sciences and professor of materials science and engineering and biomedical engineering at Northwestern’s McCormick School of Engineering as well as in Pharmacology at Feinberg School of Medicine. He also is a member of the International Institute of Nanotechnology. Gianneschi co-led the study with Xin Qi, PhD, the Jeanette M. and Joseph S. Silber Professor of Brain Sciences and co-director of the Center for Mitochondrial Research and Therapeutics, at Case Western Reserve University.

Huntington’s disease (HD) is an autosomal dominant, fatal neurodegenerative disease that affects anywhere from four to thirteen individuals per 100,000 in Western populations, the authors explained. Individuals with Huntington’s disease have a genetic mutation that triggers proteins to misfold and clump together in the brain. These clumps interfere with cell function and eventually lead to cell death. “HD is characterized by motor dysfunction, involuntary movements, dystonia, cognitive decline, intellectual impairment, and emotional disturbances,” the investigators continued. “Symptoms are usually adult onset, beginning between 30 and 50 years of age ….” As the disease progresses, patients lose the ability to talk, walk, swallow and concentrate. Most patients die within 10 to 20 years after symptoms first appear.

“Huntington’s is a horrific, insidious disease,” said Gianneschi. “If you have this genetic mutation, you will get Huntington’s disease. It’s unavoidable; there’s no way out. There is no real treatment for stopping or reversing the disease, and there is no cure. These patients really need help. So, we started thinking about a new way to address this disease. The misfolded proteins interact and aggregate. We’ve developed a polymer that can fight those interactions.”

The new study builds on previous work from Qi’s laboratory at Case Western Reserve. In 2016, Qi and her team identified a protein (valosin-containing protein or VCP) that abnormally binds to the mutant Huntington (mtHtt) protein—an intracellular protein-protein interaction (PPI)—causing protein aggregates. These aggregates accumulate within a cell’s mitochondria, an organelle that generates the energy needed to power a cell’s biochemical reactions. Without functioning mitochondria, the cells become dysfunctional and then self-destruct. “mtHtt on the mitochondria binds to valosin-containing protein (VCP) erroneously, inducing aberrant mitochondrial VCP accumulation, resulting in excessive mitophagy and subsequent neuronal cell death,” the team continued in their newly published work.

As part of that 2016 study, Qi also uncovered a naturally occurring peptide that disrupts the interaction between the VCP and the mutant Huntington protein. In cells exposed to the peptide, both the VCP and mutant Huntington protein bound to the peptide—instead of each other.

“Qi’s team identified a peptide that comes from the mutant protein itself and basically controls the protein-protein interface,” Gianneschi said. “That peptide inhibited mitochondrial death, so it showed promise.” But the peptide, by itself, faced several limitations. Because they are easily broken down by enzymes, peptides have a short lifespan in the body and often have difficulty effectively entering cells.

“A major drawback for most peptide-based drugs is poor innate cell membrane permeability, such that many peptide-based drugs fail to reach their putative intracellular targets,” the team wrote. Peptides also have an inherently low molecular weight, which leads to rapid renal clearance, they continued. “This rapid renal clearance is coupled with decreased stability and high susceptibility to rapid degradation by proteolytic enzymes in vivo, leading to overall poor performance for peptide-based drugs.”

For the peptide to inhibit Huntington’s disease, it needs to cross the blood-brain barrier in large enough quantities to prevent large-scale protein aggregation. “The peptide has a very small footprint with respect to the protein interfaces,” Gianneschi explained. “The proteins stick to each other like Velcro. In this analogy, one protein has hooks and the other has loops. The peptide, on its own, is like trying to undo a patch of Velcro by pulling apart one hook and loop at a time. By the time you get to the bottom of the patch, the top has already come back together and resealed. We needed something big enough to disrupt the entire interface.”

To overcome these obstacles, Gianneschi and his team developed a biocompatible polymer that displays multiple copies of the active peptide. The new structure, termed a protein-like polymer (PLP), has a polymer backbone with peptides attached like branches. Not only does the structure protect the peptides from destructive enzymes, it also helps them cross the blood-brain barrier and enter cells. “In contrast to the classical, biological, and linear polypeptide configuration, we have developed a new class of “polypeptide” wherein peptides form side chains originating from a polymer backbone scaffold,” the team further stated.

In laboratory experiments, Gianneschi and his team injected the protein-like polymer into a mouse model of Huntington’s disease. The polymers stayed in the body 2,000 times longer than traditional peptides. In biochemical and neuropathological examinations, the researchers found the treatment prevented mitochondrial fragmentation to preserve the health of brain cells. According to Gianneschi, the mice with Huntington’s disease also lived longer and behaved more like normal mice. The collective results of their experiments, the authors noted, demonstrated that the treatment “reduced neuropathology and behavioral phenotypes in HD animal models.”

“In one study, the mice are examined in an open field test,” Gianneschi said. “In the animals with Huntington’s, as the disease progresses, they stay along the edges of the box. Whereas normal animals cross back and forth to explore the space. The treated animals with Huntington’s disease started to do the same thing. It’s quite compelling when you see animals behave more normally than they would otherwise.”

Next, Gianneschi will continue to optimize the polymer, with plans to explore its use in other neurodegenerative diseases. “My childhood friend was diagnosed with Huntington’s at age 18 through a genetic test,” Gianneschi said. “He’s now in an assisted living facility because he needs 24-hour, full-time care. I remain highly motivated—both personally and scientifically—to continue traveling down the path.”

In their paper the authors concluded, “If generalizable, the PLP then serves as a platform technology for taking lead peptides into the cell, with promise as tools for engaging difficult and currently undruggable protein targets, in particular, the troublesome collection of PPIs that exhibit featureless yet disease driving interfaces.”