Identifying FDA-approved molecules to treat SCA6

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.

Colorful pile of medicines in blister packs which color are White, Yellow, Black and Pink pills.
By using drugs already approved by the FDA, the team hopes to reduce the time frame for pre-clinical therapy development. Photo used under license by Wanchana Phuangwan/Shutterstock.com.
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Snapshot: What is Gait Analysis in Ataxia Mouse Models?

A key role of the cerebellum is to control and fine-tune coordinated movement such as walking. Although walking is an unconscious behaviour, it is actually very complex and requires many systems to work together. The specific mannerisms and patterns of coordinated movement that make up how an individual walks are called gait.

Since ataxia affects cells in the cerebellum, many ataxia patients exhibit a change in their gait. This change can reduce their mobility and be disruptive to daily life. Analyzing gait using behavioural experiments in ataxia mouse models helps researchers to better understand the disease. But how exactly does one study gait patterns in a mouse?

Black rat walking in front of white background
Photo used under license by Eric Isselee/Shutterstock.com.

Traditionally, a researcher measures gait by performing a footprint analysis that uses non-toxic water-coloured paint and a long strip of white paper. The front and back feet of the mouse are dipped into two different colours of paint. Then the mouse then runs across the paper leaving a trail of coloured footprints to be analyzed. This allows for several gait measurements to be taken. From this, the researcher can then determine how ataxia changed the mouse’s gait. For example, a researcher can look at if the mouse takes shorter strides than a healthy control or whether the mouse tends to walk in a more crooked manner.

Although easy to perform, footprint analysis is time-consuming and highly prone to error as the experimenter does all measurements with a ruler by hand after the mouse has run. Since gait is a complex action with many variables, some subtle differences may be difficult to detect this way. Luckily, researchers have developed several digital gait analysis systems, such as the DigiGait, CatWalk, and TreadScan systems. These digital gait devices make use of transparent corridors with cameras underneath that allow the researcher to record the running behaviour of the animal. Researchers then use software to automatically detect and analyze the footprints.

One lab has taken gait analysis even further. They developed a method to detect extremely subtle differences in gait that the human eye cannot detect. This technique, called LocoMouse, was developed by the Carey lab to analyze patterns of limb movements, rather than simply footprints. LocoMouse utilizes artificial intelligence to recognize and analyze the movement of limb, tail, and head position in a walking mouse.

Using this, the Carey lab has shown a significant difference between a healthy mouse and one with ataxia. Most importantly, the method also detects differences between different ataxia mouse models. By uncovering subtle differences in gait, researchers may better understand the different underlying physiological changes in the cerebellum in different ataxias.

It should be noted that a key pitfall of studying gait in mice is that they are four-legged while humans walk on two legs. This is important, and means that the variables that affect gait in a mouse will be different than those of a human. There may not be a direct correlation between gait changes in ataxia patients and gait changes in mice. That being said, gait analysis remains an important tool in the ataxia researchers’ toolbox. It will continue to provide critical insight into how ataxic physiology affects behaviour.

If you would like to learn more about gait analysis in mice, take a look at these resources by the Noldus Information Technology and Mouse Specifics Inc.

Snapshot written by Eviatar Fields and edited by Dr. Chandana Kondapalli.

El BDNF puede revertir la ataxia en ratones SCA1

Escrito por Anna Cook Editado por Dr. David Bushart. Publicado inicialmente en el 19 de Marzo de 2021. Traducción al español fueron hechas por FEDAES y Carlos Barba.

El factor neurotrófico derivado del cerebro -BDNF- puede prevenir la ataxia en ratones SCA1. Una nueva investigación muestra que el tratamiento funciona incluso si se inicia después de que los ratones desarrollan signos de ataxia.

SCA1 es una enfermedad neurodegenerativa causada por una mutación en el gen Ataxin1 . Las personas con SCA1 a menudo desarrollan síntomas alrededor de los 30-40 años, aunque esto puede variar. Los síntomas más comunes incluyen ataxia o problemas de movimiento que dificultan moverse y caminar. Estos síntomas empeoran progresivamente y eventualmente provocan problemas para tragar o hablar. Actualmente no existe cura para SCA1, por lo que es importante que se realicen investigaciones sobre posibles tratamientos.

El laboratorio de la Dra. Marija Cvetanovic de la Universidad de Minnesota ha estado utilizando un modelo de ratón de SCA1 para tratar de identificar nuevos tratamientos. En el pasado, estos investigadores han demostrado que una molécula llamada factor neurotrófico derivado del cerebro (BDNF) podría retrasar la aparición de ataxia en un modelo de ratón de SCA1.

A laboratory mouse sitting on a researcher's hand.
La investigación con ratones SCA1 muestra que el tratamiento con BDNF puede tener un impacto, incluso después de que comienzan a aparecer los síntomas de la ataxia.. Foto utilizada bajo licencia por unoL/Shutterstock.com.

El BDNF es una molécula que se encuentra en el cerebro y es muy importante para el desarrollo saludable del cerebro. Es necesario para que muchos procesos del cerebro funcionen con normalidad. Los investigadores demostraron que los niveles de BDNF se redujeron en los cerebros de los ratones SCA1. Los investigadores inyectaron BDNF en los cerebros de estos ratones para intentar compensar el BDNF perdido. Este tratamiento, antes de que los ratones comenzaran a desarrollar síntomas de ataxia, previno la aparición de problemas motores y la muerte de las células de Purkinje.

Este trabajo anterior fue muy prometedor, pero había un problema. En este estudio, el tratamiento solo se probó antes de que los ratones SCA1 desarrollaran signos de problemas motores o cambios en sus cerebros. En el mundo real, si queremos ayudar a los pacientes con SCA1, necesitamos tratamientos que funcionen incluso una vez que la enfermedad haya comenzado a progresar. Por lo tanto, era importante que los investigadores averiguaran si este tratamiento funcionaría más adelante en la progresión de la enfermedad. Eso es exactamente lo que hicieron a continuación: en diciembre de 2020, el laboratorio de Cvetanovic publicó los resultados de su estudio que probaba el BDNF como tratamiento después de que los ratones habían comenzado a desarrollar signos de SCA1.

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Snapshot: What is the Morris Water Maze Test?

Spinocerebellar ataxias (SCAs) are well known for worsening motor coordination symptoms caused by the degeneration of the cerebellum. Yet, increasing reports indicate that broader changes are occurring in the brains of some SCA patients. This includes changes in the hippocampus, a brain region critical for learning and memory. One way to test learning and memory in mice is the Morris Water Maze Test. Researchers use this test on SCA mouse models to investigate how and when learning and memory symptoms arise. More importantly, we can also use this test to evaluate the effect of potential treatments on learning and memory.

white mouse swimming with its head poking up above the water
Although mice can swim quite well, they don’t like swimming. The Morris Water Maze takes advantage of this to test the learning and memory of mice. Photo used under license by Aleksandar Risteski/Shutterstock.com.

The Morris Water Maze consists of a large circular pool of opaque water. A platform is placed in the pool just under the surface of the water so that the mouse won’t be able to see it. Though mice are good swimmers, they don’t particularly enjoy swimming. Mice will always attempt to find the platform as quickly as possible. Shapes on the walls around the pool help the mice orient themselves within the pool (first panel in the figure below).

The first time a mouse swims in the pool (second panel in the figure), the mouse tends to swim aimlessly around until they eventually find the hidden platform. Each subsequent time the mouse swims in the pool, the mouse will get better and better. Using the shapes on the wall to help identify where they are in the pool, the mouse will eventually learn and memorize the platform’s location.

First day, mouse does not know wehere the plaform is an swims a lot. Second day, the mouse still swims a while but remembers where the platform is. On the last day, the mouse knows where the platform is and goes right there.
The three steps in the Morris Water Maze. Image made by Larissa Nitschke use BioRender.

As that happens, they will be better and better at the task. Eventually, the mice will swim immediately to the platform when placed in the pool (third panel in the figure). Researchers can measure this improvement by measuring how much time it takes the mouse to reach the platform and the length of its path to the platform. Additionally, to assess the strength of the memory, researchers can take out the platform from the pool in what is called a “probe trial”. Mice that spend more time in the area where the platform used to be are considered to have built the strongest memories of that location.

As is the case for some SCA mouse models, mice with impaired learning and memory have more difficulty learning and remembering the correct location of the platform. As a result, they spend a longer time searching for and swim longer distances to the platform. Overall, they display a poorer improvement over time. By using the Morris Water Maze Test on SCA models that receive different treatments, scientists can then further test which therapy could improve their learning and memory symptoms. Therefore, the Morris Water Maze Test may help identify new therapeutic strategies to treat learning and memory problems in patients.

If you would like to learn more about the Morris Water Maze, take a look at these resources by the Scholarpedia and JOVE.

Snapshot written by Carrie Sheeler and edited by Dr. Larissa Nitschke.

Snapshot: What is the balance beam test?

When you think of a balance beam, you might think of gymnastics. For humans, a balance beam is a surface where we perform jumps, flips, and other athletic feats. Whether it’s a child taking their first class, or an Olympic athlete going for gold, the balance beam requires both balance and coordination. When a scientist puts a mouse through the balance beam test, they don’t ask them to do this kind of complicated routine, but they are testing those same abilities.

Little Black Mouse on a White Background
Little Black Mouse on a White Background. Photo used under license by Michiel de Wit/Shutterstock.com.

The equipment setup for the balance beam test is simple: two platforms with a beam running between them plus lots of padding underneath so the mouse doesn’t get hurt if it falls off. Over multiple days, the scientist will train the mouse to run across the beam from one platform to another. Once the mouse has been trained, it will go through multiple official test runs. In these tests, the scientist will measure the time it takes for the mouse to cross the beam. They will also count the number of times one of its paws slips off the beam during the crossing. You can see some videos of mice doing the test here.

Mice that have problems with balance and coordination usually take longer to cross the balance beam and have more paw slips during the crossing. The mice might take longer to cross because they are clinging to the beam to try to stay on. Their paws might slip more because they cannot coordinate their movements properly. The scientist can also compare the measurements from the first day of training with the measures taken during the official runs. This shows how well the mouse learned to stay on the beam. This is useful because learning how to do a task and performing the task are two different things. Some parts of the brain are more important for learning, while others are more important for doing the task. Thus, telling those two aspects apart can be useful.

Mouse cossing a balance beam connecting two platforms

A typical balance beam setup, with two platforms and a beam between them. Image by Amy Smith-Dijak.

The balance beam test has been used to understand balance and coordination in both healthy mice and mouse models of disease. In healthy mice, scientists studying the basic biology of balance and coordination use this assay to test if changing the way particular parts of the brain work changes the mouse’s performance. For diseases in which lack of balance and coordination are major features, such as spinocerebellar ataxias, this test is a simple way to check how fast the disease progresses in mouse models. The assay can further be used to test possible treatments for these diseases: better scores after the treatment indicate that the therapy helped the mice improve their balance and coordination.

To sum it up, the balance beam test is a simple and effective assay to measure a mouse’s balance and coordination. Its use helps scientists to understand the basic biology of balance and coordination, as well as uncover why they are impaired in some diseases. Using the balance beam test on mouse models of disease that underwent different treatments, scientists can further measure if the therapy would improve the mouse’s balance and coordination. Therefore, the balance beam test might even help to find new treatments for motor coordination diseases.

If you would like to learn more about the balance beam test, take a look at these resources by the Maze Engineers and Creative Biolabs.

Snapshot written by Dr. Amy Smith-Dijak and edited by Dr.Larissa Nitschke.