BUFFALO, N.Y. — A decade ago, University at Buffalo researchers shed some light on an enduring neuroscience mystery: How exactly does a mutated huntingtin protein (HTT) cause Huntington's disease?
They found that HTT is something like a traffic controller inside neurons , moving different cargo along neuronal highways called axons in concert with other proteins key for cellular function and survival. Reduce the amount of non-mutant HTT and you'll create the neurological equivalent of traffic jams and roadblocks.
Now, the researchers have learned more about what can control the traffic-controlling HTT.
They found that two specific signaling proteins, GSK3ß and ERK1, were expressed more in the neurons of Huntington's disease patients, so they prevented them from functioning in the neurons of fruit fly larvae that have a mutant HTT. This inhibition of GSK-3ß actually led to less defects in the axonal transport process and less neuronal cell death, while inhibiting ERK1 led to more transport problems and more cell death.
"With these findings, we propose that ERK1 may protect neurons in the face of Huntington's disease, while GSK3ß may exacerbate Huntington's disease," says Shermali Gunawardena, PhD, associate professor of biological sciences in UB's College of Arts and Sciences. "Therapeutics may one day be able to target these signaling proteins in different ways — inhibiting GSK3ß and boosting ERK1 — to treat this severe and fatal neurological disorder."
Gunawardena is the corresponding author on a study detailing the research, which was published April 22 in Nature Cell Death & Disease .
Two proteins, two opposing effects
When the HTT gene mutates it repeats the genetic sequences cytosine-adenine-guanine (CAG) too many times. Why this diminishes a person's physical and mental abilities, typically starting around middle age, remains unclear because HTT's purpose and normal function is not fully understood.
In what was a piece of this puzzle, Gunawardena's team previously found that HTT travels along axonal highways by hitching a ride on particular cellular cargo carriers called vesicles. These vesicles are themselves moved by motor proteins known as dyneins and kinesins.
"This time, we focused on the signalers that actually regulate this entire complex transport system: a group of proteins called kinases," said the study's first author, Thomas J. Krzystek, who received a PhD in biological sciences from UB in 2022 and is now a senior scientist at AbbVie. "Kinases modify HTT and other transport components by attaching molecular tags to them known as phosphate groups."
The kinases GSK3ß and ERK1 caught the team's eye because they were upregulated in neurons with Huntington's disease when compared with normal neurons.
To understand this better in a living organism, they turned to the fruit fly. Inhibiting GSK3ß in fruit fly larvae with Huntington's disease decreased their axonal blockages and neuronal cell death. The fruit flies were even able to crawl better.
In a previous study, they found that GSK3ß — short for glycogen synthase kinase-3beta — tells motor proteins whether to stop or go, and that too much GSK-3ß or too little can disrupt the motors and cause traffic blocks by different mechanisms.
"So, while GSK3ß typically plays a positive role in neuronal function, it seems it may actually make a bad situation worse when faced with a mutant HTT," Gunawardena says.
Conversely, inhibiting ERK1 — short for extracellular signal-related kinase — increased axonal blockages and cell death.
"The level of ERK1 is clearly important for Huntington's disease, but whether it's actually modulating the mutant HTT is unclear," Krzystek says. "Either way, the signaling from this ERK1 pathway is neuroprotective in the context of Huntington's disease."
The team also tried elevating the levels of ERK1 and found it decreased traffic blockages and cell death.
"So long as it doesn't affect other processes that ERK1 might be involved in, future treatment could potentially increase a patient's levels of ERK1 to mitigate their neuronal cell death," Gunawardena says. "There's not much that can be done once cells have died, so our whole research is trying to figure out these key, early processes that lead to cell death and whether that can be prevented."
The work was supported by the National Institute of Neurological Disorders and Stroke, part of the National Institutes of Health; the Mark Diamond Research Fund and Stephanie Niciszewska Mucha Fund at UB; and the BrightFocus Foundation.
The study's co-authors – all from UB – include Rasika Rathnayake, a biological sciences PhD student; Gary Iacobucci, PhD, a postdoctoral researcher in the Jacobs School of Medicine and Biomedical Sciences; BS graduates Jia Zeng and Jing Zheng and Michael C. Yu, PhD, associate professor of biological sciences.
