Genetic Lassos May Steer Neurons Toward Survival During Lou Gehrig's Disease
For release: Wednesday, April 10, 2013
RNA lariats (green) accumulate in the cytoplasm, and not the nucleus (blue), of yeast expressing mutant DBR1. Lariats may reduce TDP-43 toxicity by clustering with, or “tying-up”, TDP-43 (yellow spots) suggesting they could potential targets for treating ALS. Courtesy of the Dr. Aaron Gitler, Stanford University School of Medicine, Palo Alto, CA.
“Perhaps these lariats act as decoys,” said Aaron Gitler, Ph.D., an associate professor of genetics at the Stanford University School of Medicine, Palo Alto, CA, and a lead author on the study published in Nature Genetics.
Amyotrophic lateral sclerosis (ALS), or Lou Gehrig’s disease, is a degenerative disease which degrades motor neurons, the nerve cells in the brain and spinal cord that control muscle movements. Usually occurring in people over 40, ALS initially disrupts muscle strength and coordination. As the disease progresses, everyday activities, such as walking, swallowing and breathing, worsen.
About 10 percent of patients have the inherited or familial forms of ALS (FALS). About one third of FALS cases are caused by mutations in a gene called chromosome 9 open reading frame 72 (C9orf72) and about 20 percent are cause by mutations in another gene called superoxide dismutase 1 (SOD1). In contrast, the cause for the majority of cases, called sporadic ALS (SALS), remains a mystery.
During ALS, proteins in degenerating motor neurons often aggregate in the cell’s cytoplasm to form structures called inclusion bodies which are thought to be toxic. Inclusion bodies contain multiple proteins, including one called transactive response DNA-binding protein 43 (TDP-43). Mutations in TDP-43 have been found to be linked to some FALS and SALS cases, suggesting that it may be an effective target for treating ALS.
Normally TDP-43 helps process genetic information by binding to different forms of RNA molecules in either the nucleus or the cytoplasm of a cell. Some studies suggest that TDP-43 clusters destroy neurons by binding to certain RNA molecules at abnormally high levels and disrupting processing. Dr. Gitler and his colleagues used yeast to find that circular RNA lariats, or loops, may prevent toxicity by binding to, or “tying-up”, TDP-43 clusters.
Yeast cells have many of the same molecules as human nerve cells and reproduce rapidly, making them a convenient system for studying the basic principles of human diseases, including ALS.
“It seems a little odd to use yeast to study a human brain disease but it’s a powerful system for studying disease genetics,” said Dr. Gitler. Previously, Dr. Gitler and his colleagues showed that overexpressing TDP-43 in yeast increased cell death and caused the surviving cells to have protein aggregates reminiscent of inclusion bodies.
The researchers worked with a team at the Gladstone Institute, San Francisco, CA, including Drs. Robert Farese Jr. and Steve Finkbeiner, to test how overexpressing TDP-43 affected a variety of mutant yeast in which each yeast strain lacked a gene that is not necessary for survival. This experiment, called a loss-of-function screen, showed that some mutations suppressed TDP-43 toxicity whereas others enhanced it.
Dr. Gitler and his colleagues focused on a protein called DBR1 which suppressed TDP-43 toxicity. DBR1 is an enzyme that normally cuts messenger RNA loops, or lariats, into straight strings. Messenger RNA (mRNA) carries transcribed genetic information from a cell’s nucleus into the cytoplasm. Lariats form as other enzymes edit genetic information by cutting off parts of mRNA.
The researchers provided evidence that lariats may act as decoys by binding to TDP-43 clumps and preventing the clumps from interfering with other important RNAs in the cell. Eliminating DBR1 in rat and human neurons also prevented TDP-43 toxicity confirming it may be a target for treating ALS.
“Developing small molecules that block DBR1 activity or mimic lariats may be a new strategy for treating ALS”, said Dr. Gitler.
- By Christopher G. Thomas, Ph.D.
Reference: Armakola et al., “Inhibition of RNA lariat debranching enzyme suppresses TDP-43 toxicity in ALS disease models.” Nature Genetics, December 2012, Vol. 12, pp. 1303-1311. DOI: 10.1038/ng.2434
Last Modified April 10, 2013