Cellular communication between neurons within our brain is complex and busy, much like a USPS mailroom.
To keep things running smoothly, the brain uses specialized molecules, termed alpha-2-delta (α2δ) proteins, to coordinate the sending and receival of signals between nerve cells in the brain.
Genetic variations in these types of proteins can impact important brain messaging and function, resulting in chronic pain, autism spectrum disorders, epilepsy, migraines, and other conditions.
New research led by Samuel Young, Jr., PhD, professor and Roper Investigator in the Department of Pediatrics and the Department of Pharmacology at the UNC School of Medicine, has revealed that these tiny molecules play more complex roles than previously thought, shifting paradigms and changing the way that researchers understand communication in the brain.
The latest findings, which were published in Neuron, demonstrate that these proteins are essential for controlling the overall strength of signals and their ability to shapeshift neurons in response to new memories or stressors. Most notably, the findings could lead to the development of new drug targets for neurological disorders resulting from altered α2δ proteins.
"Our main findings basically ran counter to all the paradigms in the field," said Young, who is also Director of the UNC Gene Therapy Center. "Through the creation of a first-of-its-kind knockout mouse model, we know much more about these complex proteins. Like what happens without them, implicate unknown mechanisms by which current drugs that target α2δ isoforms regulate neuronal circuit function, and how we can better treat a plethora of neurological disorders."
The Unconfirmed Roles of Specialized Proteins
Neural communication occurs through synapses, the point-to-point contact of information transfer between neurons. Synapses contain a presynaptic compartment which contains synaptic vesicles which release neurotransmitters and a post-synaptic compartment which contains receptors that bind neurotransmitters. Within synapses, the highly complex collection of proteins tightly regulates this process.
α2δ proteins also serve as important targets for potent anticonvulsant therapeutics, such as gabapentin and pregabalin, making them key proteins of interest by the Illuminating Druggable Genome (IDG) project by the National Institutes of Health in the United States.
Despite their importance, the precise roles that these proteins carry out at the synapse remains largely unresolved. To address these issues, first author and former graduate student and now current post-doctoral fellow in the Young lab, William Milanick, PhD, set out to elucidate the roles of three different types of α2δ proteins: α2δ1, α2δ2, and α2δ3 in controlling brain function synaptic transmission.
Creating a Model
Milanick and team needed to create a conditional knockout mammalian model which would allow them to investigate the functions of these proteins over time and in specific neurons and their location in the synapses of the brain, with the goal of overcoming the current limitations of models used in the field to study these proteins.
Researchers created a single mouse line that would allow for the spatial and temporal ablation of all three major types of α2δ proteins: α2δ1, α2δ2, and α2δ3 in the brain.
Using this model, they could determine which synaptic functions would continue to occur-or would not happen at all-without these proteins
The Findings and More Unknowns
Young and his team first confirmed that the proteins play several key roles in ensuring that synaptic vesicles with contain neurotransmitters that transmit chemical signals between neurons are delivered at the right place and time to successfully transfer signals from one cell to another.
Their model was then used to test previous conclusions about the proteins' roles in creating synapses, synaptic development over time, and the proper organization of neurotransmitter-releasing channels. Researchers found that all of these functions were still being carried out in models that did not have these proteins.
"We wanted to know what these α2δ proteins were necessary for," said Young. "By using a triple conditional knockout mouse model to specifically delete these proteins at the presynaptic compartment of the synapse, we were able to confirm that the α2δ proteins are not essential components of s are in synapse development, the clustering of calcium channels which control neurotransmitter release, or the alignment of neurotransmitter release and receiving sites on the postsynaptic side of the synapse."
Although researchers ruled out many of the proteins' previously thought roles, they also discovered a new one. They found that loss of these proteins led to reduced levels of another essential protein, Munc13. Munc13 plays a critical role in the release of transmitters and the synapse's ability to strengthen or weaken over time, in response to activity.
Mutations in Munc13 proteins have been implicated in frontal temporal lobe dementia and amyotrophic lateral sclerosis, or AML, a terminal progressive neurodegenerative disorder that leads to muscle weakness and paralysis.
Young says more research is needed to explore the individual roles of α2δ1, α2δ2, and α2δ3, their regulation of brain function as well as their implications in neurological disorders and the development of therapeutics.
This work was supported by the National Institutes of Deafness and Other Communication Disorders (R01 DC014093), the National Institute of Neurological Disorders and Stroke (R01 NS110742), the National Center for Advancing Translational Sciences (R03TR004161), the University of Iowa Distinguished Scholar Award, and start-up funds from the University of North Carolina-Chapel Hill.
