Molecular Mechanism behind Purkinje Cell Toxicity in SCA1 Uncovered

Written by Dr. Chandrakanth Edamakanti   Edited by Dr. Hayley McLoughlin

Recent study decodes the protein signature of toxic Purkinje cells, finding that Purkinje cell mTORC1 signaling is impaired in SCA1.

Spinocerebellar ataxia type 1 (SCA1) is a late onset cerebellar neurodegenerative disorder caused by a mutation (in this case, an abnormal polyglutamine stretch) in the Ataxin-1 gene. People with this condition experience problems with coordination and balance, a set of symptoms known as ataxia. The protein produced by this faulty gene, ATXN1, is particularly toxic to the Purkinje cells, the sole output neurons of the cerebellum. However, the reason behind the selective toxicity of Purkinje cells in SCA1 is unknown.

The main focus of this article is to address this question. It is the first study to find the protein signature of toxic Purkinje cells in SCA1 mice. In the end, the authors identified widespread protein changes that are associated with Purkinje cell toxicity.

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The authors used mass spectrometry, a technique that allows researchers to detect specific proteins based on their mass, to identify the proteins that are associated with the disease. Purkinje cells from the cerebella of adolescent (35 day-old) SCA1 mice were used as samples for the mass spectrometry analysis. This age was chosen because 35 day-old SCA1 mice show a behavioral and pathological phenotype (i.e., motor behavior and Purkinje cell toxicity) that is similar to SCA1 patients. Purkinje cells were isolated from the cerebellum using laser capture microdissection (LCM), where laser beams are used to carefully separate out the Purkinje cells from adjacent tissue.

After analyzing the mass spectrometry results, the authors found a list of proteins that were altered in SCA1 mouse Purkinje cells that are associated with neuronal functions and signaling to other cells. One of the most interesting of these results was that the levels of Homer-3 protein were reduced. Homer-3 is a dendritic protein known to regulate glutamate signaling between neurons. This glutamate signaling normally functions as an activator of neighboring neurons. From previous studies, we know that the toxic mutant ATXN1 protein impairs the expression of genes that are associated with glutamate signaling, and that the major source of glutamate input to Purkinje cells comes from climbing fibers. Because of this, the authors predicted that the reduction in Homer-3 levels is a consequence of abnormal glutamate signaling in SCA1.

As the authors found out, this was the case. They discovered that the connections between climbing fibers and Purkinje cells in their SCA1 mice were lost very early (i.e., before disease onset). As a proof of principle, authors chemically destroyed the inputs from climbing fibers and found that the Homer-3 levels were further reduced. In contrast, by activating glutamate signaling, the authors were able to restore Homer-3 levels.

Activation of neurons by glutamate signaling is associated with activation of critical downstream signaling pathway such as mTORC1.  So, the authors next investigated the mTORC1 signaling pathway in SCA1 Purkinje cells. They found a great reduction of mTORC1 signaling in these cells. In addition, when the authors suppressed mTORC1 signaling in either the cerebellum or in Purkinje cells alone, the levels of Homer-3 were further reduced and disease severity was enhanced. Finally, by restoring the levels of Homer-3, the authors were able to improve climbing fiber connections, increase mTORC1 signaling in Purkinje cells, and reduce disease symptoms in SCA1 mice.

In conclusion, the authors demonstrated here that the levels of mTORC1/Homer-3 signaling was reduced to below normal levels in SCA1 mouse Purkinje cells. Furthermore, they found that glutamatergic transmission from climbing fibers (which determines the levels of mTORC1/Homer-3 in Purkinje cells) was compromised, and that restoration of levels of Homer-3 protein is enough to diminish SCA1 symptoms in mice.

This discovery of a novel mechanism of degeneration (mTORC1 dysfunction) in the Purkinje cells of the cerebellum leads us closer to understanding SCA1. These results might open up new therapeutic avenues to treat SCA1, as well as other neurodegenerative movement disorders that are associated with the cerebellum.

Key Terms

Purkinje cells: A type of neuron in the cerebellum. They are some of the largest cells in the brain. Purkinje cell loss/pathology is a common feature in cerebellar ataxia, and determining why these neurons are so important for movement will help us better understand SCAs.

Spinocerebellar ataxia type 1 (SCA1): A dominantly-inherited and fatal neurodegenerative disease caused by the abnormal expansion of CAG repeats in the gene ATXN1.

Climbing Fiber (CF): A type of neuronal fiber which connects two types of neurons: inferior olive neurons and Purkinje cells.

Laser Capture Microdissection (LCM): A technique used to separate out a certain cell type of interest from tissue samples. It is dissection on a microscopic level.

Conflict of Interest Statement

The author and editor declare no conflicts of interests.

Citation of Article Reviewed

Ruegsegger C, Stucki DM, Steiner S, Angliker N, Radecke J, Keller E, Zuber B, Rüegg MA, Saxena S. Impaired mTORC1-Dependent Expression of Homer-3 Influences SCA1 Pathophysiology. Neuron. 2016 Jan 6;89(1):129-46. doi: 10.1016/j.neuron.2015.11.033. https://www.ncbi.nlm.nih.gov/pubmed/26748090