Mitochondrial impairments identified in SCA7 mouse model and patient cells

Written by Dr. Colleen A. Stoyas Edited by Dr. Monica Banez 

Duke University researchers have found that altered cellular metabolism and mitochondrial dysfunction play a central role in spinocerebellar ataxia type 7 (SCA7), a result that has therapeutic implications for this disease.

Spinocerebellar ataxia type 7 (SCA7) is a dominantly-inherited ataxia characterized by retinal degeneration and cerebellar atrophy. As retinal degeneration advances, patients experience progressive central vision loss. Atrophy (i.e., cell loss) in the cerebellum causes a progressive loss of balance, as the cerebellum is the region of the brain that controls coordinated movement and motor learning. SCA7 patients also experience difficulty speaking and swallowing in later stages of the disease. Symptoms can manifest at any age, though the disease is particularly aggressive when found in infants and children. SCA7 is caused by an expansion mutation in the Ataxin-7 (ATXN7) gene, which produces a protein containing extra repeats of the amino acid glutamine. These additional glutamines make the protein fold in an incorrect shape. Much like an umbrella turned inside-out, this protein, once it loses its shape, does not work in the way it’s meant to. Dr. Albert La Spada has previously shown that the ataxin-7 protein is necessary for the expression of genes that are central to the normal function of the eye – particularly, the retina. Now, his group has provided evidence that abnormal cellular metabolism underlies the brain changes observed in SCA7.

Mice whose brains carry the SCA7 mutation model the juvenile forms of this disease. Using this mouse model, the La Spada group observed changes in the network and physical size of the brain’s mitochondria. Mitochondria are the cell’s “power plants,” and are responsible for the chemical reactions (known as cellular metabolism) that generate the energy our cells need to function. Cellular metabolism is assessed by measuring metabolites, which are the products of these chemical reactions. The La Spada group’s researchers identified dysfunction in the mitochondria in SCA7 due to an underlying decrease in one specific metabolite: NAD+.

microscope and sample slide
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Short for nicotinamide adenine dinucleotide, NAD+ is necessary for proper mitochondrial function. A general reduction of NAD+ occurs as humans age, as well as in a host of other neurodegenerative disorders (many of which exhibit mitochondrial dysfunction). This recent recent by Dr. La Spada and his team has shown that NAD+ is also reduced in mitochondria in SCA7.

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Snapshot: How do clinicians measure the severity of ataxia in patients?

Coordination of smooth and effective movements is essential in daily tasks, such as speaking or walking. The ability to successfully orchestrate these movements is commonly referred to as “motor coordination”. While SCA patients can generally initiate movements with their bodies, their ability to execute these in a smooth and precise fashion is impaired. For instance, motor incoordination can be seen in a patient with ataxia’s inability to walk in a straight line, or in the difficulty they experience when swallowing. These and other motor function problems can greatly impact daily life. Assessing how well a patient can perform these movements provides an indication of how affected they are by the disease.

Black pencil lying on top of paper that has scoring chart on it
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Unlike what’s measured with more familiar medical tests, such as blood pressure or levels of blood sugar, human movement cannot be quantified easily with clear numbers. To address this, multiple rates scales have been developed to help measure standardize motor coordination examinations. One of these scales is the Scale for the Assessment and Rating of Ataxia (SARA). An experienced clinician (typically a neurologist) evaluates a patient’s ability to perform a series of tasks (such as standing and walking) and then, using the SARA, assigns a score for each task. The process takes about 15-20 minutes, and typically involves the following tests:

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VEGF-mimicking nanoparticles improve SCA1 disease phenotype in mice

Written by Dr. Chandrakanth Edamakanti Edited by Dr. David Bushart

VEGF nanoparticles offer a new avenue for developing treatments for SCA1 and other neurodegenerative disorders

Spinocerebellar ataxia type 1 (SCA1) is a neurogenerative disorder with symptoms that typically begin in the third or fourth decade of life. The disease is characterized mainly by motor incoordination that becomes progressively worse with age. Eventually, patients succumb to the disease about fifteen years after onset due to breathing problems. SCA1 is known as a “polyglutamine expansion” disorder, which means it is caused by a glutamine-rich region of a protein becomes abnormally large due to a genetic mutation. In SCA1, the polyglutamine expansion occurs due to a mutation in the ataxin-1 gene (ATXN1), causing the subsequent ataxin-1 protein to have abnormal functions.

Previously, a research team led by Dr. Puneet Opal found that the levels of a protein called VEGF (vascular endothelial growth factor) is reduced in cerebellum of a mouse model of SCA1. The team was able to improve disease symptoms in these mice by restoring VEGF protein levels using two different methods: i) by crossing the SCA1 mice with another strain of mouse that expressed high levels of VEGF, and ii) delivering recombinant protein (rVEGF) into the brains of SCA1 mice (Cvetanovic M et al 2011). However, the researchers noted that it would be challenging to implement the rVEGF delivery strategy for clinical therapy, since one would need to overcome the extreme financial cost and difficulty that comes with using recombinant proteins.

stethoscope and blood pressure cuff
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VEGF is crucial for maintaining the microvasculature (small arteries and veins) in the brain and also supports neuronal health and regeneration. Current evidence suggests that VEGF therapy could be beneficial for several neurodegenerative conditions such as stroke, Alzheimer’s disease, Parkinson’s disease, and ALS. Unfortunately, significant impediments have prevented the translation of recombinant VEGF therapy to the clinic. In a recently published ‘Brain’ research article, Dr. Opal and his team sought to address this obstacle by exploring a potential low-cost VEGF treatment strategy known as VEGF peptide mimetics. These peptide mimetics are smaller and simpler molecules that mimic biological compounds; in this case, VEGF. Peptide mimetics are typically smaller than the original molecule (small enough to be considered “nanoparticles”), which helps limit side effects and makes delivery to the target much easier than using recombinant proteins like rVEGF.

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Snapshot: What is the Cerebellum?

The cerebellum, often referred to as the “little brain”, is part of the brain that is located behind the cerebrum (forebrain). The cerebellum accounts for about 10% of the brain’s volume. Despite occupying a small volume, the cerebellum contains more than half of the neurons in the brain. Most of the evolutionary research with respect to the brain has been focused on the forebrain; however, recent evidence suggests that the expansion of the size of the cerebellum might have given humans an edge with respect to higher behavioral functions, such as the use of tools. Therefore, the cerebellum has played a vital role during evolution, and this suggests an indispensable function for the human cerebellum.

cartoon diagram of the human brain, with the cerebelum coloured in pink
Diagram of the human brain, with the cerebellum highlighted in pink. Picture courtesy of Wikimedia Commons.

What does the cerebellum do?

For several decades, scientists believed that the main role of the cerebellum was to maintain posture and balance, to fine-tune motor movements, and to enforce motor learning. If you think about performing a certain movement (these thoughts happen in the forebrain), the cerebellum compares these “movement plans” with what movements were actually made and corrects for errors if there were any. This fine-tuning makes movements precise and is critical for making voluntary movements such as walking, running, or speaking. Therefore, it is with the help of the cerebellum that we learn to get better at throwing a curveball, riding a bike, or learning any other complex motor tasks.

Is that all the cerebellum does?

Well, scientists used to think so. Over the past two decades, new evidence has made scientists to re-evaluate their thoughts about the cerebellum. Scientists now believe that the role of the cerebellum extends beyond fine-tuning motor movements, and likely includes cognitive functioning and certain reward-seeking behaviors. However, this aspect of cerebellar function is still being studied and there is a lot for scientists to uncover.

What happens when the cerebellum is damaged?

The cerebellum is one of the primary culprits in many types of cerebellar ataxia, where the damaged cerebellum forces the affected individuals to gradually lose their ability to walk. Therefore, it is imperative to better understand how the cerebellum contributes to ataxia to provide better treatment for patients. Apart from ataxia, the cerebellum may also contribute to other disorders such as dystonia, Parkinson’s disease, Huntington’s disease, Alzheimer’s disease, and autism spectrum disorders. Therefore, understanding what happens when the cerebellum goes awry is critical for improving the quality-of-life for patients all over the globe.

If you would like to learn more about the cerebellum, take a look at these resources by the Khan Academy and BrainFacts.org.

Snapshot written by Dr. Sriram Jayabal and edited by Dr. David Bushart.

A Creatine-rich Diet Delays Disease in SCA3 Mice

Written by Dr. Lauren R. Moore Edited by Larissa Nitschke

Creatine, a common dietary supplement taken by athletes, delays symptoms and improves balance and strength in SCA3 mice.

Could a common nutritional supplement used by athletes to boost performance also provide benefits to ataxia patients? This was the main question addressed by a recent study of Spinocerebellar Ataxia Type 3 (SCA3), the most common dominantly-inherited ataxia in the world. The study, published in March 2018, was led by Dr. Sara Duarte-Silva at the University of Minho in Portugal. Dr. Duarte-Silva and her team investigated whether feeding SCA3 mice a diet enriched with creatine – a popular dietary supplement – improves the symptoms and brain changes that are associated with SCA3. Researchers found that a high-creatine diet delayed disease and slowed the worsening of symptoms in SCA3 mice. This study provides promising evidence that increasing or adding creatine in daily consumption may have protective benefits for SCA3 patients.

SCA3 is one of six hereditary ataxias caused by a unique type of genetic mutation known as a CAG trinucleotide repeat expansion. This occurs when a repeating sequence of three DNA nucleotides – Cytosine-Adenine-Guanine or “CAG” for short – is expanded, creating an abnormally high number of repeats. In SCA3, mutation occurs in a gene encoding the protein ATXN3 and produces an abnormally long “sticky” region in the disease protein. This sticky region, called a polyglutamine expansion, impairs ATXN3’s normal functions and causes it to build up in brain cells as toxic protein clumps. As a result, the brain’s ability to make and store energy is often impaired in SCA3 patients (a deficit that is also seen in many other brain disorders). Thus, drugs or compounds that improve overall energy production and use in brain cells could be beneficial in SCA3 and other ataxias.

man with white pill in his hand
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One such compound that may increase energy efficiency – particularly in the brain and muscles – is creatine. Creatine is made naturally by the body, but can also be consumed through foods like red meats and seafood. In addition, creatine is a common ingredient in many commercially-available dietary supplements that claim to improve athletic performance by boosting energy and building muscle. Creatine has recently been shown to have some benefits in mouse models of other brain diseases with similarities to SCA3. However, whether creatine could benefit SCA3 patients hadn’t been investigated prior to this study.

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