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The Cognitive Deficits of Mice and Men: How the cerebellum contributes to the cognitive symptoms of SCA1

Written by Kim M. Gruver Edited by David Bushart

What’s cognition got to do with ataxia? Could the cerebellum mediate both cognitive and motor symptoms in the same disease? And how can scientists use mice to find out?

Spinocerebellar ataxia type 1, or SCA1, is a progressive neurodegenerative disease that has no cure. In SCA1, an expanded CAG repeat sequence in the ATXN1 gene increases the chain length of the amino acid glutamine (Q), so SCA1 is called a “polyQ” disease. As suggested by its name, the cerebellum is a heavily affected brain region in SCA1. Since the cerebellum is involved in motor coordination, it is no surprise that dysregulated control of movement, or ataxia, is a major symptom of SCA1.

However, what may come as a surprise is that some SCA1 patients also experience changes in cognition in addition to ataxia. Since the mutated ATXN1 gene is found throughout the brain, it has been difficult to tease apart whether the cerebellum contributes to the cognitive symptoms of SCA1 in addition to the motor symptoms. It is possible that cognitive symptoms of SCA1 might be exclusively caused by brain regions other than the cerebellum. For example, ATXN1 is also highly expressed in the prefrontal cortex, a region known for mediating many cognitive processes. But before we discount the possibility that the cerebellum plays a role in the cognitive symptoms experienced by some SCA1 patients, it is important to note an interesting observation in neuroscience research that has emerged in recent decades. Scientists have described a surprising role of the cerebellum in a host of neurological disorders like autism and schizophrenia. In light of these findings, that the cerebellum could be implicated in both the motor and cognitive symptoms of SCA1 may not be so far-fetched.

two borwn lab mice held in the hand of a researcher wearing plastic gloves — Two lab mice from the National Institutes of Health, image courtesy of WikiMedia.

A powerful tool on the researcher’s lab bench to study diseases like SCA1 is the laboratory mouse. Since 1902, mice have played an indispensable role in disease research. Scientists can breed mice that express human genes, such as a mutated form of ATXN1, to figure out what goes awry in diseases like SCA1. Animal models of disease help researchers to identify potential treatment strategies that may be useful to humans. Since such in-depth analysis and careful experimental manipulation is impossible in human patients, animal models are an invaluable tool to study diseases like SCA1.

In the SCA1 field, scientists use multiple animal models to study SCA1. Researchers have harnessed the differences between these mouse models to address different questions, such as:

“How does the number of CAG repeats affect SCA1 symptoms in mice?”
“What happens if the ATXN1 gene is removed altogether?”
“Do SCA1 symptoms still occur if the mutant ATXN1 gene is restricted to cerebellar Purkinje cells?

In mice and in humans, we know that the length of the polyQ expansion in the ATXN1 gene correlates with both the severity and the age of symptom onset of SCA1. Mice that express more CAG repeats (a longer polyQ expansion) in their ATXN1 gene experience more severe symptoms that start earlier in life than mice with a shorter polyQ expansion. When mutant ATXN1 expression is restricted to Purkinje cells in the cerebellum, mice display motor impairments similar to what is observed in mice with mutant ATXN1 expression everywhere in the brain. This tells us that disrupting healthy ATXN1 expression in Purkinje cells alone is sufficient to cause motor symptoms that stem from SCA1. To put it plainly, mouse models of SCA1 have been a crucial component of SCA1 research.

Since human SCA1 patients experience behavioral symptoms, scientists also use behavioral tools to evaluate the symptoms of SCA1 mice. Motor coordination tests are essential in ataxia research. These tests allow scientists to determine whether a potential intervention improves or worsens symptoms in mice. This is the first step to evaluate whether an intervention could be promising for human patients. However, as we discussed earlier, motor impairments are not the only symptom faced by SCA1 patients: many exhibit cognitive deficits as well. But could mice be used to evaluate something as complex as cognition? Can laboratory mice really help scientists uncover whether the cerebellum contributes to the cognitive impairments observed in SCA1? Researchers at the University of Minnesota say yes.

Snapshot: What is Omaveloxolone?

A new therapeutic compound shows promise to treat Friedrich’s ataxia.

What is Friedrich’s ataxia (FA)?

Friedrich’s ataxia is a genetic neurodegenerative disease that affects many organs, most notably nerves, muscles, and heart. FA is a recessive ataxia. Symptoms typically present in childhood and result in significant physical disability. Cognition (thinking, memory) remains intact.

Some of the symptoms a person with FA may experience include ataxia (loss of movement coordination), fatigue, muscle weakness, cardiomyopathy (heart issues), scoliosis (curvature of the spine) and sensory impairments (vision, hearing). Life expectancy is reduced as a result of the disease.

The genetic change that is present in FA affects the production of a protein called frataxin. Frataxin deficiency leads to abnormal iron accumulation in mitochondria. As mitochondria are critical for energy metabolism and other important functions in cells, their dysfunction causes faulty energy production and undesirable toxicity in the form of reactive oxygen species.

There is currently no treatment available to patients with FA.

white medical pills in the shape of a question mark — What is Omaveloxolone? How could it help people with Friedrich’s Ataxia? Photo by Anna Shvets on Pexels.com

How does Omaveloxolone work?

Omevaloxolone is a synthetic compound. It works by counteracting deficits seen in disease at the cellular level. Omevaloxolone promotes Nrf2, which works to activate a series of defence mechanisms that help cells handle oxidative stress (mentioned above). Nrf2 is also important for improving the energy production machinery mitochondria require to function efficiently. Thus, by activating Nrf2, Omevaloxolone is thought to mitigate oxidative damage, improve energy production, and promote neuroprotection. Additionally, Omevaloxolone and similar compounds exhibit anti-inflammatory action.

What exactly has been validated?

In the MOXIe clinical trial, study participants with FA from several countries were randomized to either daily omaveloxolone (drug) or placebo (control). Their neurological function, activities of daily living, and ataxia were assessed at baseline (at the beginning) and after 48 months of receiving treatment. At the end of this period, the data showed statistically significant improvement in each of these measures. Participants who received omaveloxolone fared better than those who did not (placebo). Additionally, participants who received omaveloxolone saw improvements after treatment compared to their own baseline at the beginning of the study.

What is happening next?

The next step in testing omaveloxolone is to have a long-term study to examine its safety (and any side effects) over the course of a few years. Instead of having a control group in this type of study, called an open-label extension, now everyone enrolled received the same amount of omaveloxolone. This study is already underway and is expected to be completed by 2022. There have been some modifications to the long-term safety study in response to COVID-19, but Reata doesn’t expect there to be a significant delay in their timelines.

If you would like to learn more about omaveloxolone, take a look at these resources by the Reata Pharmaceuticals and ClinicalTrials.gov. To learn more about Friedrich’s Ataxia, visit the Friedrich’s Ataxia Research Alliance website.

Snapshot written by Dr. Judit M. Pérez Ortiz and edited by Larissa Nitschke.

New molecule can reverse the Huntington’s disease mutation in lab models

Written by Dr. Michael Flower Edited by Dr. Rachel Harding

Editor’s Note: This article was initially published by HDBuzz on March 13, 2020. They have graciously allowed us to build on their work and add a section on how this research may be relevant to ataxia. This additional writing was done by Celeste Suart and edited by David Bushart.

A collaborative team of scientists from Canada and Japan have identified a small molecule which can change the CAG-repeat length in different lab models of Huntington’s disease.

CAG repeats are unstable

Huntington’s disease is caused by a stretch of C, A and G chemical letters in the Huntingtin gene, which are repeated over and over again until the number of repeats passes a critical limit; at least 36 CAG-repeats are needed to result in HD.

In fact, these repeats can be unstable, and carry on getting bigger throughout HD patients’ lives, but the rate of change of the repeat varies in different tissues of the body.

In the blood, the CAG repeat is quite stable, so an HD genetic blood test result remains reliable. But the CAG repeat can expand particularly fast in some deep structures of the brain that are involved in movement, where they can grow to over 1000 CAG repeats. Scientists think that there could be a correlation between repeat expansion and brain cell degeneration, which might explain why certain brain structures are more vulnerable in HD.

a print out of genetic information show as a list of A,T, C, and G letters — The CAG repeat of the huntingtin gene sequence can be changed to include more and more repeats, in a process called repeat expansion. This can also happens in some ataxia related genes. Image credit: “Gattaca?” by IRGlover is licensed under CC BY-NC 2.0

But why?

This raises the question, what is it that’s causing the CAG repeat to get bigger? It seems to be something to do with DNA repair.

We’re all exposed continually to an onslaught of DNA damage every day, from sunlight and passive smoking, to ageing and what we eat. Over millions of years, we’ve evolved a complex web of DNA repair systems to rapidly repair damage done to our genomes before it can kill our cells or cause cancer. Like all cellular machines, that DNA repair machinery is made by following instructions in certain genes. In effect, our DNA contains the instructions for repairing itself, which is quite trippy but also fairly cool.

“What is it that’s causing the CAG repeat to get bigger? ”

We’ve known for several years that certain mouse models of HD have less efficient systems to repair their DNA, and those mice have more stable CAG repeats. What’s more, deleting certain DNA repair genes altogether can prevent repeat expansion entirely.

But hang on, isn’t our DNA repair system meant to protect against mutations like these?? Well normally, yes. However, it appears a specific DNA repair system, called mismatch repair, sees the CAG repeat in the huntingtin gene as an error, and tries to repair it, but does a shoddy job and introduces extra repeats.

Why does this matter?

There’s been an explosion of interest in this field recently, largely because huge genetic studies in HD patients have found that several DNA repair genes can affect the age HD symptoms start and the speed at which they progress. One hypothesis to explain these findings is that slowing down repeat expansion slows down the disease. What if we could make a drug that stops, or even reverses repeat expansion? Maybe we could slow down or even prevent HD.

Continue reading “New molecule can reverse the Huntington’s disease mutation in lab models”

Byproducts of canola oil production show therapeutic potential for MJD and Parkinson’s Disease

Written by Dr. Maria do Carmo Costa, Edited by Dr. Hayley McLoughlin

Collaboration between researchers in Portugal and the United Kingdom discover that a canola oil by-product shows promise, corrects MJD/SCA3 and Parkinson’s Disease symptoms in animal models.

Isolated compounds or extracts (containing a mixture of compounds) from certain plants are showing promise as potential anti-aging drugs or as therapeutics for neurodegenerative diseases. Some of these plant compounds or extracts can improve the capacity of cells to fight oxidative stress that is defective in aging and in some neurodegenerative diseases. Machado-Joseph disease, also known as Spinocerebellar ataxia type 3, and Parkinson’s disease are two neurodegenerative diseases in which cells inability to defend against oxidative stress contributes to neuronal death. In this study, the groups of Dr. Thoo Lin and Dr. Maciel partnered to test the therapeutic potential of an extract from the canola plant rapeseed pomace (RSP) with antioxidant properties in Machado-Joseph disease and Parkinson’s disease worm (Caenorhabditis elegans) models.

Canola field with snowcapped mountains in the background, July 1990 — Canola field with snowcapped mountains in the background, image courtesy of USDA NRCS Montana on Flickr.

Machado-Joseph disease is a dominant neurodegenerative ataxia caused by an expansion of CAG nucleotides in the ATXN3 gene resulting in a mutant protein (ATXN3). While in unaffected individuals this CAG repeat harbors 12 to 51 trinucleotides, in patients with Machado-Joseph disease contains 55 to 88 CAG repeats. As each CAG trinucleotide in the ATXN3 gene encodes one amino acid glutamine (Q), the disease protein harbors a stretch of continuous Qs, also known as polyglutamine (polyQ) tract.

Parkinson’s disease that is characterized by loss of dopaminergic neurons can be caused either by genetic mutations or by environmental factors. Mutations in the genes encoding the protein a-synuclein and the enzyme tyrosine hydroxylase (a crucial enzyme for the production of dopamine) are amongst the genetic causes of patients with Parkinson’s disease.

Continue reading “Byproducts of canola oil production show therapeutic potential for MJD and Parkinson’s Disease”

Snapshot: What are Preprints?

Scientific research takes a long time: experiments are performed, clinical trials are run, and the data that’s generated has to be analysed and understood before it can be published. Together, this process does not happen quickly. Though people may not realise it, one step that takes a lot of time between generating data and publishing a paper is the publishing process itself.

Publishing a scientific paper can take anywhere from a few months to years. If we look at statistics from 2018 for the journal PLOS ONE, for instance, we see that the median time it took a paper to go through the publication process was 6 months. That means that half of the papers took less than 6 months to process, while the other half took longer. In addition, it often takes multiple submissions to different journals before a paper is accepted, and researcher teams can only submit a paper to one journal at a time. Considering all this, it is no surprise that the process of publishing a scientific article can take a substantial amount of time.

Illustration of a scientist working at a laptop computer, sharing ideas with colleagues — A scientist working on writing a scientific paper. Image courtesy of Piqsels.

However, there are many important things that happen during this process: manuscripts are screened by a journal editor, appropriate experts in the field are asked to perform peer review, and reviews are submitted to the editor. This editor then makes a decision on how to move forward (usually asking the authors to update their work to meet the reviewers’ requests). Papers can be rejected at any of these stages, and this process may occur two or three times at any one journal before a final decision to accept or reject is made.

Due to the time this process takes, there is a delay in getting the results of a study to fellow scientists – information that could drastically influence the experiments being conducted in research laboratories right now. Thankfully, this is where preprints come in.

Preprints are final drafts of papers that research teams share on public servers before/as they start the publication process. This means that other researchers can see the draft manuscripts long before the “official” paper is published. One of the most popular sites for unpublished preprints in the life sciences is bioRxiv (pronounced “bio-archive”), which, as of March 2020, has had over 77,000 preprint papers uploaded to its servers.

What are the benefits of preprint papers?

One of the main benefits of preprints is the rapid spread of information. Instead of the months- or years-long delay to share papers, the scientific community can read preprints to learn about some of the latest findings in the field.

The authors also benefit from uploading a preprint because it acts as a time stamp for when they revealed their results. Establishing a priority of research findings can be important to authors because of the competitive nature of science. Likewise, because of the collaborative nature of science, multiple different research groups may choose to upload preprints on similar results at the same time to ensure priority is not assigned to one study while the other studies are held up in the publishing process.

It is also much easier for researchers to share information on “negative data” through preprints, as well as replication studies. Negative data are the results you get when an experiment reveals that your initial prediction – known as your “hypothesis” – is likely incorrect. Replication studies are repeats of experiments that have previously been published by other groups, and they serve to double-check that the work of those groups was done correctly. Although replication studies and negative data are very important to the scientific process, it can be hard to publish these studies, as some journals do not view such studies as new or interesting. Preprints are an alternative to share this information.

Do we still need traditional publishing?

Despite the extensive time it takes, traditional publishing will always have one advantage over preprints: peer review. Peer review can catch little mistakes and improve the overall quality of papers in many ways, including suggesting additional experiments and/or alterations to the writing style. In rare cases where fraud or plagiarism occurs, peer review can also prevent the publication of such studies.

This difference between preprints and published papers has recently been highlighted by bioRxiv. During the COVID-19 pandemic, bioRxiv published the following statement:

“bioRxiv is receiving many new papers on coronavirus 2019-nCoV. A reminder: these are preliminary reports that have not been peer-reviewed. They should not be regarded as conclusive, guide clinical practice/health-related behavior, or be reported in news media as established information”

This statement is a response to some media outlets not completely understanding the difference between preprints and published papers. As bioRxiv says, they are different and should not be treated the same.

This does not mean, though, that we cannot trust the information we find in preprint papers – it just means that we need to critically analyse the information in preprints. When reading preprints, it is important to understand that they are a preview of a work in progress, not the final product.

If you would like to learn more about preprints, take a look at this article by Science Magazine, this video by iBiology, or this definition by bioRxiv.

Snapshot written by Celeste Suart and edited by Dr. Hannah Shorrock.