Young Investigator and Young Investigator SCA Awards

Manu Ben-Johny, PhD
Johns Hopkins University, Baltimore, MD

Aberrant Regulation of Voltage-gated Na channels in the Pathophysiology of Spinocerebellar Ataxia 27

Spinocerebellar ataxia 27 (SCA27) is a recently identified type of ataxia. Patients with SCA27 experience tremors and difficulties with gait and often perform poorly on cognitive tasks. The genetic basis of this disorder has been localized to mutations within a small protein called Fibroblast growth factor homologous factor 4 (FHF4), which is found within neurons. FHF4 interacts with voltage-gated sodium channels—molecules critical for generation of electrical impulses in neurons. Despite FHF4’s important role in the development of SCA27, the effect of FHF4 on the function sodium channels is not yet fully understood. This prominent gap in the defining the basic mechanisms that govern FHF4 action on sodium channels has clouded our understanding of SCA27, including how this debilitating disorder develops and progresses.

Curiously, the sodium channel interface that docks FHF4 also harbors calmodulin, a protein that is found within all eukaryotic organisms, ranging from one-celled organisms to human beings. Calmodulin senses changes in the level of intracellular calcium ions to coordinate the activity of numerous proteins. Thus, it allows cells to attain calcium homeostasis. Our recent work has shown that calmodulin also dynamically reduces the activity of sodium channels in response to increases in cellular calcium levels. This feedback control mechanism allows the cell to reduce generation of electrical impulses in neurons after periods of excess activity. Does FHF4 then regulate sodium channels by overriding calmodulin regulation? Could changes in this interaction then cause changes in electrical properties of neurons as observed with SCA27? To answer these questions, our team will study the interplay between FHF and calmodulin in tuning sodium channel activity within cerebellar Purkinje neurons. We will also explore whether altered dynamics of intracellular calcium ions may play a role in the development of ataxia.

Overall, this project promises to advance our fundamental understanding of the molecular interactions that are essential to fine-tune neuronal function and to ultimately evoke coordination of movement. We also hope that these studies will help identify important molecular targets for the development of new therapies for ataxia.

 

Marija Cvetanovic, PhD
University of Minnesota, Minneapolis, MN

Role of astrocyte calcium signaling in the pathogenesis of SCA1
 

Astrocytes are brain cells that neurons need to work normally. Astrocytes help keep neurons alive by giving the neurons nutrients and oxygen and by helping keep the neurons’ environment stable and healthy. The level of calcium within astrocytes affects how well they work.

In spinocerebellar ataxia (SCA1), astrocytes undergo a process called astrogliosis, In astrogliosis, astrocytes increase to above-normal levels as a way to protect the nervous system from further damage.

In this study, my team and I will be using mouse models to study how astrogliosis affects the level of calcium in the astrocytes. Using astrogliosis as a guide, we will learn whether calcium in astrocytes is changed in the presence of SCA1. We will also test whether changing the levels of calcium in the astrocytes helps prevent or lessen the severity of SCA1 symptoms. If we discover that calcium in astrocytes plays a significant role in the development of SCA1, scientists could one day develop therapies that can change those levels of calcium—and hopefully delay development of the disease.


Vincenzo Gennarino, PhD
Baylor College of Medicine, Houston, TX

PUMILIO1 deficiency: understanding a new ataxia gene and its role in cerebellar dysfunction in mice and humans
 

The molecular genetic revolution of the 1990s brought us tremendous knowledge of the genetic mutations that cause many neurological diseases, including many ataxias. Further research into the proteins produced by these genes has revealed that there is another way for a protein to cause havoc in the brain besides being mutated: it might be expressed at levels too low or too high. In the case of several neurodegenerative diseases, including spinocerebellar ataxia type 1 (SCA1) and more common diseases such as Alzheimer’s and Parkinson’s, we have discovered that too much of even the normal version of the disease-driving protein can cause disease. What if we could find a way to lower the levels of these proteins in the brain? Could we slow disease progression? These questions led me to search for the factors that regulate the levels of ataxin1, the protein that is involved in SCA1. I discovered that ataxin1 levels are controlled by an RNA-binding protein called Pumilio1 (PUM1). More importantly, we showed that if you take away PUM1 in a mouse model of SCA1, and ataxin1 returns to normal levels, the SCA1 mice no longer have ataxia.

We also noticed, however, that mice lacking Pum1 (the mouse version of the protein is written in lower-case letters) were quite sick: they developed seizures, and they developed ataxia earlier than the SCA1 mice. This led us to suspect that loss of PUM1 function might be the culprit behind some childhood ataxias. In collaboration with medical geneticists around the world, we have identified seven patients with deletion of PUM1 and two patients with mutations in PUM1 who show symptoms similar to the Pum1 mutant mice. This finding confirms our hypothesis that PUM1 deficiency is a genetic cause of an early-onset ataxia syndrome.

In this study, we aim to characterize the phenotype of Pum1 mutant mice as precisely as possible and study the effects of loss of function of PUM1 in patient-derived cell lines. This research is necessary to understand how PUM1 deficiency causes childhood ataxias and neurological dysfunction. Our research will also help us understand all the targets of PUM1’s activities in neurons so we can learn whether altering PUM1 levels in the brain could help patients with SCA1 or those with other neurodegenerative diseases.

 

Vikram Khurana, MD, PhD
Brigham and Women’s Hospital and Harvard Stem Cell Institute
Boston, MA

Systematic edgotyping of ataxin proteins in cellular systems from yeast to patient neurons.
 

The identification of gene mutations that cause neurodegeneration offers tremendous hope for understanding and reversing the way neurodegenerative diseases develop. However, researchers have yet to convert genetic insights into real preventive or disease-slowing therapies for patients. Genes code for proteins—the building blocks, signaling molecules and enzymes of our cells. Ultimately, a very significant result of gene mutations is that they lead to abnormal protein changes. We know that gene mutations involved in neurodegenerative diseases lead to some important alterations in protein folding and function, and we know that the mutated proteins collect in affected brain cells. But a global and systematic understanding of these alterations in living cells—and different types of cells—is lacking. To address this lack of base knowledge, we use an unbiased, efficient method, called edgotyping, to systematically look at how gene mutations alter the interactions that take place between affected proteins and the cells they occupy. The edgotyping method can be applied to simple cells (such as yeast) and complex cells (such as brain cells, or neurons—even neurons made directly from patients)—and everything in between.

In this proposal, we will be applying edgotyping methods to examine the ataxin protein mutations that lead to spinocerebellar ataxias. We willl systematically define how ataxin mutations (so-called polyglutamine expansions) change the way proteins interact with living cells. The changes in the protein map that result from ataxin mutations have never been examined using edgotyping. But we do know from previous work that some of these changes are important and can be taken advantage of when developing treatments. We have every expectation that the data and platforms we generate in this project will lead to major insights into the biology and treatment of spinocerebellar ataxias, with important implications for neurodegenerative disease in general.

 

Sathiji Nageshwaran, MD
Harvard University, Boston, MA

Transcriptional activation using CRISPR/Cas mutant proteins as a novel therapy for Frataxin gene silencing
 

The underlying cause of Friedreich’s ataxia is the insufficient production of frataxin. The faulty gene that produces insufficient amounts of frataxin contains a GAA mutation. Interestingly, those who have one copy (carriers) of this faulty gene also show up to a 50% reduction in the amount of frataxin protein, but they do not develop any symptoms of Friedreich’s ataxia. Furthermore, the frataxin produced from a faulty copy of the frataxin gene functions as normal.

These facts suggest that by increasing levels of frataxin protein to the amount that carriers produce, we may be able to hinder the progression of the disease.

This project sets out to increase the production of frataxin by addressing the problem at its source. Because of the presence of the GAA mutation, a cell struggles to access and read the information within the gene. This is because the environment (called the epigenome) in which the gene is now found does not permit the “readers” of the DNA to pass along it. This project will investigate the possibility of improving a cell’s ability to access and read the gene without affecting other genes.

The project will make use of the new genome-engineering tool CRISPR, which allows a protein to be targeted to specific genes, but in a manner in which only the environment surrounding the gene is altered and not the gene sequence itself. We hope that changing the silencing environment around the frataxin gene will provide a radical new approach to treating Friedreich’s ataxia.

 

Jana Schmidt, PhD
University of Tuebingen, Germany

Alleviation of proteasomal inhibition as a therapeutic approach for SCA3
 

Spinocerebellar ataxia type 3 (SCA3), also known as Machado-Joseph disease (MJD), is caused by the expansion of a repetitive structure within the protein ataxin-3.

Proteins that are unwanted or no longer required for brain function are marked in our cells by a certain label called ubiquitin. The cell then degrades the ubiquitin-labelled proteins in a shredder-like process. In SCA3 patients, this degradation process is disturbed, leading to the accumulation of the expanded ataxin-3 and other proteins. Studies of brain tissue have identified collections of protein as a hallmark of the disease in SCA3 patients and also in other neurological disorders.

In order to identify the mechanisms leading to SCA3 and to identify ways to prevent the onset of symptoms, we generated mouse models of the disease. Our models allow us to study disease processes in a time-lapse fashion. That means that processes needing decades to take place in man can be studied in a mouse model within months. In our research, we recently studied the process by which ataxin-3 degrades and then applied it to specific mouse models. When we prevented a certain type of ubiquitin label from existing in the mice, we observed that these mice had much less aggregated ataxin-3, did not get the disease and did not develop impaired movement. This means that modulating this ubiquitin label may be a promising therapeutic strategy in SCA3. However, such approaches cannot be directly translated from mouse to man.

In our project, we aim to further understand what’s happening inside the cell to lessen the disease in our specific mice. We also hope to figure out whether and how our approach could be translated as a therapy to human SCA3 patients. A successful therapy would help remove the toxic, disease-causing ataxin-3—preventing its aggregation and, consequently, the start of the disease and its symptoms.

 

Jill Sergesketter Butler, PhD
University of Alabama at Birmingham
Birmingham, AL

Reduced expression of mitochondrial aldehyde dehydrogenases contributes to metabolic stress in Friedreich’s ataxia
 

Friedreich’s ataxia (FRDA) is a severe form of ataxia caused by decreased production of a protein called frataxin. In patients who have Friedreich’s ataxia, the loss of frataxin protein eventually damages multiple organs, including the heart and pancreas. Cells in the heart and brain are the most sensitive to changes in frataxin levels. Even though frataxin is just a single protein, lower levels of it can cause other cell proteins and genes to stop working properly. The goal of this study is to get new information about how and when frataxin causes those cell and gene changes. As we learn more about those changes, we hope to narrow down which particular signs, or biomarkers, show that Friedreich’s ataxia may progressing in the body.

Results from previous research my team and I have conducted show that Friedreich’s ataxia cells have decreased levels of particular enzymes called mitochondrial aldehyde dehydrogenases. The job of these enzymes is to rid cells of certain toxins that can damage heart and brain cells. In this study, we will be further evaluating the levels and activities of mitochondrial aldehyde dehydrogenases in Friedreich’s ataxia cells. In a controlled setting, we will be changing the level and activity of these enzymes to determine how they affect the health and growth of Friedreich’s ataxia cells. In doing this work, we hope to better understand what level of mitochondrial aldehyde dehydrogenase activity is needed to prevent or reverse damage in Friedreich’s ataxia cells. Our findings could help pave the way for new therapies that deliver the right amounts of these protective enzymes into the Friedreich’s ataxia cells—so the ataxia can be treated effectively.

 

Bing Yao, PhD
Emory University, Atlanta, GA

Epigenetic Modulation Mediated by RNA-Binding Proteins in Neurodegeneration
 

Ribonucleic acid (RNA) molecules, like DNA, are essential information-carriers in all organisms. RNA is particularly important for making proteins in the body. Proteins that bind to RNA play fundamental roles in controlling different aspects of RNA functions, which are important for neurons to work properly. Dysfunction with RNA-binding proteins often leads to neurodegenerative disorders that cause ataxia. Fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disorder that usually develops in the late stage of adulthood. It is caused by a 55 to 200 copies of nucleotide “CGG” repeats in the fragile X mental retardation 1 (FMR1) gene. In FXTAS, these extra CGG repeats can restrict the RNA-binding proteins hnRNP A2/B1. This, in turn, leads to the death of critical motor neurons called Purkinje cells, which leads to ataxia.

In this NAF Young investigator award application, I hypothesize that RNA-binding protein hnRNP A2/B1 has a novel role to directly bind to DNA and control critical gene expression to influence the life course of Purkinje cells. In the proposed specific aims, I will first study whether and how hnRNP A2/B1 binds to DNA regions of the genome and whether and how restriction of hnRNP A2/B1 leads to Purkinje cell death. I will then use a cutting-edge method called next-generation sequencing to isolate the genuine roles of hnRNP A2/B1 in controlling the expression of genes related to FXTAS. This study will provide novel insights into how RNA-binding proteins help direct the function of normal neuron cells and, conversely, how their restriction can influence the development of ataxia-related disease.