Protection for nerve cells - eating up defective proteins

Nerve cells in the Central Nervous System can defend against and recover from damage by proteinlike clusters which are associated with neurodegenerative diseases such as Huntington's Chorea.

The neurons are able to "eat up" and "digest" defective proteins by specialized processes within each cell.

Dr. Ron R. Kopito, professor of biological sciences from Stanford University, California, USA, and his co-workers have elucidated the principal mechanisms which enables nerve cells to protect themselves. He reported their findings at an international conference on "Neurodegenerative Diseases: Molecular Mechanisms in a Functional Genomics Framework" at the Max-Delbrück Communications Center (MDC.C) in Berlin, Germany.

Researchers assume that the aggregation of insoluble proteins in Huntington's Chorea, the mutant protein huntingtin, causes this genetic disease, for which no cure exists. It is characterized by jerky, uncontrolled movements of the body and face and, therefore, is called Chorea (Greek for "dance") Huntington. Its scientific name goes back to the New York physician George Huntington who was the first to rigorously describe the deadly disease in 1872.

Ataxin-1 is another pathogenic protein which causes a disease called spinocerebellar ataxia, which is also an incurable hereditary nervous disorder. Symptoms are poor balance when walking, impaired coordination of hand and leg movements, and unclear, slurred speech.

In both diseases as in other neurodegenerative disorders such as Alzheimer disease nerve cells slowly degenerate and eventually are destroyed.

Dr. Kopito said in Berlin: "Our data suggest that the ubiquitin-proteasome system is used to destroy mutant proteins before they can accumulate and damage cells. However, in some cases, this system is inadequate.

Through a second process known as autophagy, or "self-eating," nerve cells are able to defend against the toxic effects of aggregated forms of mutant huntingtin and ataxin-1 that have escaped the surveillance of the ubiquitin proteasome system.

"Autophagy is induced in response to impaired ubiquitin proteasome system activity". This cellular system normally controls the quality of proteins produced by the cell machinery and ensures that defective proteins are sorted out and destroyed.

"However, autophagy does not work for defective proteins aggregated in the compartments of the nucleus of the nerve cells", Dr. Kopito pointed out. "This may help to explain why protein aggregates are more toxic when directed to the nucleus of the nerve cells."