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.
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.Continue reading “The Cognitive Deficits of Mice and Men: How the cerebellum contributes to the cognitive symptoms of SCA1”