Biomolecular organization holds tantalizing clues for neurological disease treatment

The brain is not a static structure. It is adaptive, evolving over time and through lived experience. At the core of the brain’s ability to adapt is a network of tightly regulated organization, from multi-regional coordination between brain structures to the subcellular interactions between proteins and RNA. Biomolecular condensates have captured the interest of scientists for their organizational capabilities at this subcellular level. These droplets within cells are filled with protein, DNA and RNA and have been implicated in numerous biological processes and a swath of neurological diseases. 

However, the fundamental properties of biomolecular condensation and how these membraneless structures contribute to neurological disease are still being unearthed. The study of neurological disease itself lags compared to other organs. This is because parsing the complex and interconnected network within the brain to find the root causes of disease is hugely challenging. The sheer number of genes that can trigger disease amplifies this complexity. 

For J. Paul Taylor, MD, PhD, St. Jude executive vice president, scientific director, Pediatric Translational Neuroscience Initiative director, and Department of Cell and Molecular Biology chair, this led to clinical work early in his career that often felt like taking shots in the dark. 

“I began to see large volumes of patients that had inherited disorders. We would collect blood, and if they didn’t have anything on our gene panels, all we could say was, ‘Sorry, we don’t know what you have,’” Taylor explained. “Most patients were undiagnosed. I got very good at having that conversation and assuring patients that all this would change someday.”

Through his work at St. Jude, Taylor is helping make that prediction a reality, adapting his own work to follow discoveries into new fields with the ultimate goal of translating laboratory findings into clinical impact.  

Order in the disordered

As he began studying genes linked to neurological disease, Taylor hit upon something that would shape the trajectory of his career. The genes implicated in neurological disease were affecting regulatory RNA binding proteins — and a particular part of these proteins called the intrinsically disordered region (IDR). RNA binding proteins are a tale of two halves: one half with very predictable folded “domains” and the other half consisting of a string of amino acids. 

“If the mutations had landed in the folded domains, that would have been the end of the story because that was the conventional place to find mutations,” Taylor said. “IDRs were of little interest at that time. But I was drawn into this world because that’s where the mutations implicated in neurological disease landed.” 

Taylor’s lab set out to understand how these IDRs contributed to neurological disease, but they encountered a problem: Their samples for experiments looked cloudy. Cloudy samples suggested that the proteins were sticking together, or aggregating – but that wasn’t exactly right. “Under the microscope, we weren’t seeing aggregates,” Taylor explained. “We were seeing all these bubbles floating around.”

Further investigation revealed that those “bubbles” were filled to the brim with RNA-binding proteins and RNA. At that time, a type of condensate called stress granules was receiving much attention. Stress granules were making a name for themselves for their apparent link to neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia. Investigating stress granule formation set Taylor on the path to understanding how biomolecular organization through condensates can be instrumental in neurodegenerative disease.

A dense network of biomolecules 

In a seminal publication in Cell, Taylor collaborated with Tanja Mittag, PhD, St. Jude Department of Structural Biology, to tease out the molecular ingredients that trigger stress granule formation. They discovered that the IDR of an RNA-binding protein called hnRNPA1 contains small but frequent amino acid sequences that repeat over and over like a mantra. These repeat sequences cause the IDRs to stick together in fleeting interactions, like a room of people shaking hands. 

At a high enough concentration, these IDRs seemed to spontaneously separate, condensing without a membrane to form a liquid within a liquid. This process is called phase separation and is how biomolecular condensates form. These condensates were the titular bubbles that Taylor first noted under the microscope. While stress granules usually dissolve, they persist in patients with neurological diseases because mutations on the IDR of hnRNPA1 cause the proteins to become unrelentingly interwoven. 

The discovery that sequences of amino acids could drive phase separation opened the floodgates for Taylor. “It became clear that contributions come not just from short motifs in IDRs but also from folded domains and other biomolecules such as RNA,” he said. “We realized there’s a whole zoo of different kinds of interactions that can contribute.” 

Targeting biomolecular organization to treat disease

Much of biology takes place in the context of condensates, with likely undiscovered indirect effects. For neurological disease therapeutic research, this presented an opportunity. Many targets for neurological diseases were considered “undruggable.” However, if the undruggable target existed as part of a condensate, and that condensate was linked to the disease, perhaps dissolving the condensate could affect pathology. 

“We believe that there’s going to be a lot of targetable biology taking place in condensates,” Taylor said. “By understanding how this network interacts, we can create a list of targets for pharmacologic or genetic manipulation.” 

This proof of concept was demonstrated in a Journal of Cell Biology paper targeting the central nodes of stress granules. Hong Joo Kim, PhD, principal scientist in Taylor’s lab and author on the publication, sees how far they have come and how far there still is to go. 

“There are a lot of factors that we need to understand before we actually start to think about using these types of compounds clinically,” Kim said. “But this is going to be helpful for a lot of researchers to take that next step.”

By meticulously teasing out the critical nodes of stress granule formation, Taylor developed an intimate understanding of RNA-binding proteins. This steered him to a recent study involving a rare neurodevelopmental disorder linked to one of them, hnRNPH2. Published in the Journal of Clinical Investigation, he and his team developed a mouse model to help understand the disorder. They presented a story of a different type of biomolecular organization — a finely controlled balance between hnRNPH2 and its functional analog, hnRNPH1, that became disrupted by mutation. The models developed will be invaluable in driving fundamental discoveries into real clinical applications.

“It turns out that was the tip of the iceberg,” Taylor said. “We now know that there’s a large family of RNA binding proteins harboring mutations that lead to neurodevelopmental disorders and epilepsy.” 

Translating discovery to therapy

With his early roots in the clinic, Taylor has always been interested in how discoveries made in the laboratory can affect the lives of children diagnosed with neurological diseases. As researchers have achieved a greater understanding of how biomolecular organization contributes to disease, Taylor’s experiences highlighted the necessity of building a translational program for rare neurological diseases at St. Jude. 

The Pediatric Translational Neuroscience Initiative leverages fundamental research, internal and external partnerships and collaboration, and clinical expertise to create an engine of discovery and application. For ultrarare diseases, which include many neurological diseases affecting children, pharmaceutical funding is not an option. The initiative is poised to help bridge that gap by connecting the lab and the clinic. 

“It’s amazing what we’re able to do now. We can go from the nomination of a mutation to the delivery of a therapy in about two years,” Taylor said. “But I’ve challenged us to get this down to less than 12 months.” With this rapid translational approach, Taylor’s early career prediction is coming true. The undiagnosable and undruggable conditions that baffled patients, clinicians and researchers alike are becoming clear with pathways forward to effective treatments.