By Emily Leclerc, Science Writer, Waisman Center
While researchers believe there is no single cause for Autism Spectrum Disorder (ASD), two new studies by Luigi Puglielli, MD, PhD, reveal a new potential genetic connection as a cause of the condition. As our understanding of ASD has developed, several different genetic mutations or gene duplication events have been implicated in the disorder, with some being inherited and others arising spontaneously. Puglielli, a professor in the Department of Medicine and a Waisman Center investigator, has determined that a collection of gene mutations that affect a specific pathway in neurons may be a driver of ASD in some individuals.
Neurons, the brain’s communication cells, rely heavily on delicate balances within their systems. When those balances are disrupted, even slightly, it interferes with the cell’s ability to work as it should. A relatively newly discovered pathway that is reliant on one of these balances, discovered some years ago by Puglielli, appears to play an important role in the development of ASD.
Properly folded proteins are incredibly important for neurons to function correctly, as the neuron’s ability to communicate is reliant on the proteins they release. To ensure the proteins are functioning as they should, neurons have a quality control pathway called Ne-lysine acetylation that occurs in the endoplasmic reticulum (ER) – a part of the cell that manufactures and ships proteins. “The ER makes proteins and some of these proteins will fold correctly,” says Puglielli. “The correctly folded proteins are sorted [by the Ne-lysine acetylation] and told they are good to go while the misfolded ones are then disposed of because they are toxic.”
This pathway relies on the balanced movement of the molecule acetyl-CoA within the cell and then into the endoplasmic reticulum. Acetyl-CoA is an important molecule that is involved in reactions in protein, carbohydrate, and lipid metabolism as well as in the body’s energy production pathways. Disruptions to this movement of acetyl-CoA affects the Ne-lysine acetylation pathway’s function, which then translates into issues with communication between neurons. “Since this pathway has such a strong connection to cellular metabolism and protein quality control, when something goes wrong, the neuron suffers significantly,” says Michael Rigby, PhD, formerly of the Puglielli lab and current medical student at the University of Wisconsin Madison.
It’s like if thousands of gallons of water were suddenly dumped into a river. The river would flood, spilling water across the adjacent lands, its normal flow interrupted. That flooding then has the potential to change the shape of the riverbanks or alter the way the river flows. If the flow of acetyl-CoA in the pathway isn’t just right, the pathway isn’t able to perform its quality control measures as needed.
In 2016, Puglielli observed autistic-like behaviors in mice engineered to have increased levels of the transporter that moves acetyl-CoA into the endoplasmic reticulum as part of the Ne-lysine acetylation pathway. To continue to unravel how disruptions to this pathway may play into the development of ASD, Puglielli and his team began breaking the pathway down into its components to study further.
In two papers published concurrently in the journals Brain and Brain Communications (Increased expression of SLC25A1/CIC causes an autistic-like phenotype with altered neuron morphology and SLC13A5/NaCT overexpression causes disrupted white matter integrity and an autistic-like phenotype), Puglielli reports that overexpression (increased levels) of two different components of the Ne-lysine acetylation pathway also results in mouse models displaying autistic-like behaviors.
The two genes studied in the papers affected transporters that are, in part, responsible for maintaining the flow of acetyl-CoA that the Ne-lysine acetylation pathway relies on. The disturbances to the pathway, which were created by generating novel transgenic mice (a mouse model that has had its genome altered), impacted the pathway’s ability to function effectively. This in turn caused issues within the neurons’ synapses and resulted in the transgenic mice developing autistic-like behaviors. The results of these two papers, and Puglielli’s previous research, further demonstrate that gene mutations or duplications affecting this pathway are mechanistic drivers behind the development of ASD, at least in some individuals.
“Our most recent work has demonstrated that Ne-lysine acetylation plays a critical role in neurodevelopment, and when hyperactive in mice, results in a phenotype that is similar to ASD in humans,” says Rigby, who is the lead author on both studies. “Since the genes we have studied are associated with ASD in humans, when duplicated, we feel that our results – at least in part – provide a mechanistic understanding of why those individuals develop the disorder.”
Puglielli agrees. “We have these three mouse models that we have generated that all target the same pathway, [Ne-lysine acetylation.],” Puglielli says. “And they all lead to autism spectrum disorder. These animals together really prove that this pathway is a very important part of the story.”
For Puglielli, these results reaffirm the need to continue to untangle the intricacies of this pathway to obtain a better understanding of how it contributes to ASD. A small group of people with ASD are known to have mutations or duplications of the genes that contribute to this pathway. The hope is that, down the road, this research will lead to a target for new therapeutics.
“It is possible this mechanism that we have uncovered may play a larger role beyond those with gene duplications,” Rigby says. “Those three new mouse models of ASD provide a launching point for evaluating potential therapeutics that could be translated for possible treatment in humans.”
This research was supported by National Institutes of Health (NIH) R01 NS094154, R01 GM065386, F30 AG066329, RF1 AG052324, U01 CA231081, R01 DK071801 and P41 GM108538 and a core grant to the Waisman Center from NICHD—U54 HD090256. Imaging support was provided by the University of Wisconsin Carbone Cancer Center Support grant P30CA014520 and Waisman Core Grants P30 HD003352-45 and U54 AI117924-03.
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