Viruses could be partly to blame for a neurodegenerative disease.

Avindra Nath at the National Institute of Neurological Disorders and Stroke in Bethesda, Maryland, and his colleagues studied post-mortem brain tissue from 11 people who had amyotrophic lateral sclerosis (ALS; also known as motor neuron disease), which leads to muscle weakness and paralysis. They found that neurons expressed a key protein from the human endogenous retrovirus-K, the DNA of which has been incorporated into human genomes over millions of years of evolution.

When the team introduced the gene encoding this protein into cultured human neurons, the cells decreased in number and retracted their neurites — the projections that connect to other cells. Mice expressing this protein showed a loss of muscle-controlling neurons and had muscle dysfunction.

Preventing this virus from being activated could slow the course of ALS, the authors say.

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