Molecular chaperones protect cells

Falsely folded proteins and their aggregations in neurons are considered to be the cause of neurodegenerative diseases, among them Chorea Huntington.

Dr. Ulrich Hartl from the Max Planck Institute for Biochemistry in Munich has identified a new family of molecular chaperones that prevent proteins from "misbehaving" and ensure that they fold properly.

This he reported at the International Conference "Neurodegenerative Diseases: Molecular Mechanisms in a Functional Genomics Framework" in the Max Delbrück Communications Center (MDC.C) in Berlin, Germany.

The new family of chaperones, called TriC for short, supports another family of chaperones which Dr. Hartl had discovered a few years ago and which belongs to the family of heat shock proteins (Hsp-70). Both families ensure that proteins fold correctly and that they neither aggregate, nor kill nerve cells.

Proteins, the building material and the machines of life, can only become active when they fold and take on a three-dimensional structure. Scientists assume that insoluble protein aggregations (plaques) in nerve cells trigger Chorea Huntington, an inherited disease.

The disease is characterized by jerky, uncontrolled movements of the body and face and, therefore, is called Chorea (Old Greek for "dance") Huntington. Its scientific name goes back to the New York physician George Huntington who was the first to describe the deadly disease in 1872.

There is no cure for the disease and it can neither be stopped nor reversed. Researchers estimate that 1 in 10,000 persons is effected. So far 30,000 cases are known in the United States, 10, 000 in Canada, and 8,000 in Germany.

In 1993, researchers discovered the gene which produces the mutant protein huntingtin. This protein is considered to be the cause of Chorea Huntington. It is deposited in the nucleus of the nerve cells.

In 1997, Dr. Erich Wanker, then at the Max-Planck-Institute for molecular Genetics, Berlin, now at the Max Delbrück Center for Molecular Medicine (MDC), Berlin-Buch, was able to show that these aggregations consist of falsely folded huntingtin molecules.

The protein production units in nerve cells add too many glutamin building blocks to the amino acid sequence of the huntingtin protein, resulting in polyglutamin chains that are siginificantly longer than normal ones. As a result, the protein loses its normal structure and can no longer be degraded. Scientists assume that these protein aggregations are toxic for nerve cells.

However, it remains unclear as to how and by what mechanism(s) these aggregations effect the nerve cells which lose their their normal function and eventually die. "There are mainly two hypotheses", said Dr. Hartl. "In one model, neurotoxicity results from the ability of polyglutamin-expanded proteins to recruit other important cellular proteins with short polyglutamin stretches into the aggregates." In the other model, aggregating polyglutamin proteins cause a partial inhibition of the garbage disposal of the cells, the ubiquitin-proteasome system.

The heat shock proteins are able to prevent protein aggregation, making them less toxic for the nerve cells, said Dr. Hartl. The TriC-families act together with the heat shock proteins. Both families help the proteins stay in a soluble state and, thus, they do not aggregate. It remains to be seen if these findings can be utilized to develop therapies against neurodegenerative diseases.

The four-day conference, which started on September 6, is organized by the Max Delbrück Center for Molecular Medicine (MDC), the Charité Universitätsmedizin Berlin, and the University of Bonn (all in Germany). 200 clinicians and researchers from Canada, Europe, Japan, and the USA discuss their latest findings there.