The human brain is made up of many different types of cells. Each of them has slightly different roles in a healthy brain. The goal of our research is to understand how SCA1 makes these different cells sick in different ways. We want to check if different parts of the brain show distinct or unique changes because of SCA1.
We are also interested in identifying which physical changes in the brain lead to specific SCA1 symptoms. We do a lot of our research on a specific type of brain cell called glial cells.
Why do you do this research?
Most brain research focus on neurons. But 50% of the cells in your brain aren’t neurons, they are glial cells! Glial cells help support and regulate neuronal activity, but they often get overlooked. But more scientists like us are researching glial cells. They do a lot for your brain.
If we want to develop successful therapies for SCA1, we need to understand how glial cells are impacted. Without that knowledge, we will not have the full picture. That’s why we do this work.
We have a number of fluffy companions in our lab. Please check the Creative Catalysts page of our Lab Website for pictures!
Written by Dr. Ambika Tewari Edited by Dr. Hayley McLoughlin
Lentiviral expression of an shRNA against ataxin-3 was well-tolerated and produced no measurable adverse effects in wild-type mice.
Evaluating the safety profile is a necessary and crucial step in qualifying a therapy for use in patients. Gene therapy is an experimental technique that has demonstrated tremendous progress in the treatment or reversal of a disease, specifically monogenic disorders. Carefully investigating the safety and tolerance of gene therapy is important to gauge its suitability for clinical trials. Gene therapy tools can be used in different ways to achieve the same therapeutic effect: the faulty gene can be replaced with a healthy copy, the mutated gene can be repaired, or the mutant copy of the gene can be silenced. You can learn more about gene therapy in this pat SCAsource Snapshot.
Spinocerebellar ataxia type 3 (SCA3) or Machado-Joseph disease (MJD) causes progressive loss of neurons in the spinal cord, and several regions of the brain. This includes the cerebellum, brainstem, striatum and substantia nigra. These neurons have crucial functions. Without these neurons, patients experience motor incoordination, loss of balance, and in severe cases, premature death. While great progress continues to be made in understanding how a mutation in a single gene, Ataxin-3, causes the symptoms of SCA3, there is still no treatment to stop the disease progression. As a monogenic disorder, SCA3, like other Spinocerebellar ataxias (SCA), is a promising candidate for gene therapy. While there are no approved gene therapies for SCA yet, there any several research labs and companies working towards achieving this goal.
The researchers in this study have been working on gene therapy for SCA3 since 2008. They have researched how gene therapy could offer protection against further decline, in several cell and mouse models of SCA3. They used an approach where they decreased the levels of the mutant Ataxin-3 gene while leaving the normal Ataxin-3 gene intact. This is known as allele-specific targeting. They demonstrated that using this technique, they could significantly reduce the behavioral and neuropathological changes that occur in SCA3 mice. Mice treated with the gene therapy showed improvements in their balance and motor coordination.
Gene therapy in its most basic form involves two components, the gene that will replace or remove the diseased gene and a vector that will transport this new gene to its site of action. The most commonly used vectors today are adeno-associated virus (AAVs) followed by retrovirus. These viruses have been specifically engineered to deliver their passenger to the specified location. While both vectors have been through several years of preclinical and clinical testing for numerous gene therapy candidates, there are questions that remain regarding their safety. (1) Does the gene therapy product continue to be expressed in the targeted area long-term; (2) If there is long-term expression does it cause any adverse measurable effects to the targeted area; (3) Does the long-term expression affect the normal functioning of the targeted cells/organ.
Written by Dr. Hannah Shorrock Edited by Dr. Hayley McLoughlin
Nitschke and colleagues identify a microRNA that regulates ataxin-1 levels and rescues motor deficits in a mouse model of SCA1
What if you could use systems already in place in the cell to regulate levels of toxic proteins in disease? This is the approach that Nitschke and colleagues took to identify the cellular pathways that regulate ataxin-1 levels. Through this strategy, the group found a microRNA, a small single-stranded RNA, called miR760, that regulates levels of ataxin-1 by directly binding to its mRNA and inhibiting expression. By increasing levels of miR760 in a mouse model of SCA1, ataxin-1 protein levels decreased and motor function improved. This approach has the potential to identify possible therapies for SCA1. It may also help identify disease-causing mutations in ataxia patients with unknown genetic causes.
Spinocerebellar Ataxia type 1 (SCA1) is an autosomal dominant disease characterized by a loss of coordination and balance. SCA1 is caused by a CAG repeat expansion in the ATXN1 gene. This results in the ataxin-1 protein containing an expanded polyglutamine tract. With the expanded polyglutamine tract, ataxin-1 is toxic to cells in the brain and leads to dysfunction and death of neurons in the cerebellum and brainstem.
As with all protein-coding genes, surrounding the protein coding region of ATXN1 gene are the 5’ (before the coding sequence) and 3’ (after the coding sequence) untranslated regions (UTRs). These regions are not translated into the final ataxin-1 protein product but are important for the regulation of this process. Important regulation factors called enhancers and repressors of translation located in 5’ and 3’ UTRs. ATXN1 has a long 5’ UTR. Genes that require fine regulation, such as growth factors, are often found to have long 5’ UTRs: the longer a 5’ UTR, the more opportunity for regulation of gene expression. The group, therefore, tested the hypothesis that the 5’ UTR is involved in regulating the expression of ataxin-1.
In their initial studies, Nitschke and colleagues identified that the ATXN1 5’UTR is capable of reducing both protein and RNA levels when placed in front of (5’ to) a reporter coding sequence. One common mechanism through which this regulation of gene expression could be occurring is the binding of microRNAs, or miRNAs, to the ATXN1 5’UTR. miRNAs are short single-stranded RNAs that form base pairs with a specific sequence to which the miRNA has a complementary sequence; this leads to regulation of expression of the mRNA to which the miRNA is bound.
Using an online microRNA target prediction database called miRDB, the group identified two microRNAs that could be responsible for these changes in gene expression through binding to the ATXN1 5’ UTR. By increasing the expression of one of these microRNAs, called miR760, ataxin-1 protein levels were reduced in cell culture. Conversely, using a miR760 inhibitor so that the miRNA could not perform its normal functions led to increased levels of ataxin-1. Together this shows that miR760 negatively regulates ataxin-1 expression.
Written by Dr Hannah Shorrock Edited by Dr. Larissa Nitschke
Pastor and colleagues identify FDA-approved small molecules that selectively reduce the toxic polyglutamine-expanded protein in SCA6.
Selectively targeting disease-causing genes without disrupting cellular functions is essential for successful therapy development. In spinocerebellar ataxia type 6 (SCA6), achieving this selectivity is particularly complicated as the disease-causing gene produces two proteins that contain an expanded polyglutamine tract. In this study, Pastor and colleagues identified several Food and Drug Administration (FDA) approved small molecules that selectively reduce the levels of one of these polyglutamine-containing proteins without affecting the levels of the other protein, which is essential for normal brain function. By using drugs already approved by the United States Food and Drug Administration to treat other diseases, referred to as FDA-approved drugs, the team hopes to reduce the time frame for pre-clinical therapy development.
SCA6 is an autosomal dominant ataxia that causes progressive impairment of movement and coordination. This is due to the dysfunction and death of brain cells, including Purkinje neurons in the cerebellum. SCA6 is caused by a CAG repeat expansion in the CACNA1A gene. CACNA1A encodes two proteins: the a1A subunit, the main pore-forming subunit of the P/Q type voltage-gated calcium ion channel, as well as a transcription factor named a1ACT.
The a1A subunit is essential for life. Its function is less affected by the presence of the expanded polyglutamine tract than that of a1ACT. The transcription factor, a1ACT, controls the expression of various genes involved in the development of Purkinje cells. Expressing a1ACT protein containing an expanded polyglutamine tract in mice causes cerebellar atrophy and ataxia. While reducing levels of the a1A subunit may have little effect on SCA6 disease but impact normal brain cell function, reducing levels of a1ACT may improve disease in SCA6. Therefore, Pastor and colleagues decided to test the hypothesis that selectively reducing levels of the a1ACT protein without affecting levels of the a1A protein may be a viable therapeutic approach for SCA6.
Written by Stephanie Coffin Edited by Dr. Brenda Toscano
Ataxin-1 may not be the only protein important in driving neurodegeneration in SCA1
Why does a protein that cause disease only cause toxicity in specific regions of the brain, despite being in all cells of the body? This is the question authors attempt to answer in this article, with a focus on spinocerebellar ataxia type 1 (SCA1) and the disease causing protein, Ataxin-1. SCA1 is a polyglutamine expansion disorder, meaning patients with the disease have a CAG repeat in the ATXN1 gene that is larger than that of the healthy population. This mutant allele is then translated into a mutant protein, causing SCA1. Ataxin-1 protein is expressed throughout the entire brain, however, toxicity (cell death and problems) is mainly restricted to neurons of the cerebellum and brainstem. This phenomenon is called “selective vulnerability” and refers to disorders in which a restricted group of neurons degenerate, despite widespread expression of the disease protein. Selective vulnerability occurs in many diseases, including Alzheimer’s, Huntington’s, and Parkinson’s disease and is currently under investigation by many scientists in the field of neurodegeneration.
In SCA1, this selective vulnerability can be narrowed further in the cerebellum. The cerebellum is broken down into lobules (I-X), with lobules II-V described as the anterior region and lobules IX-X as the nodular zone. Studies have previously shown cerebellar Purkinje cells to be particularly sensitive to mutant ataxin-1, and within the cerebellum, neurons in the anterior region degenerate faster than those in the nodular zone. This paper wanted to understand the mechanism of this interesting biology, hypothesizing that there are genes whose are expressed mainly in these zones could correlate with the pattern of Purkinje cell degeneration. To this end, the authors used the mouse model ataxin-1 [82Q], which overexpresses human ataxin-1 with 82 CAG repeats specifically in cerebellar Purkinje cells.
First, the authors confirmed the finding that neurons from the anterior region of the cerebellum degenerate earlier than those in the nodular zone. They did this by assessing the health and number of Purkinje cells, which indeed appeared to be better in the cells located in the nodular zone. Next, techniques assessing expression of RNA in SCA1 and control cerebellum, showed that there are a number of genes which are uniquely dysregulated in the anterior cerebellum of SCA1 mice. Neurons function and communicate with each other via ion channels, and interestingly, the genes found to be dysregulated in the anterior cerebellum of SCA1 mice were related to ion channel signaling.