Disruption of protein folding causes neurodegeneration, mental retardation

Excess accumulation in brain cells of a fat molecule called GM1-ganglioside (GM1) disrupts the folding of newly assembled proteins into their proper shapes, triggering nerve degeneration and mental retardation in children.

This finding, from investigators at St. Jude Children's Research Hospital, is published in the Sept. 10 issue of Molecular Cell.

The disease, called GM1 gangliosidosis, disrupts the normal function of brain cells and causes them to self-destruct. The St. Jude discovery offers strong evidence for the cause of GM1 gangliosidosis in children.

GM1 gangliosidosis is a lysosomal storage disorders, an inherited disease in which one or more enzymes in the lysosomes are defective. Lysosomes are the cell's recycling centers, where proteins, fats and other molecules are broken down into their basic building blocks, which are then reused to make new molecules.

Lysosomal storage diseases occur when lysosomes lack the enzymes they need to perform their recycling tasks, leading to abnormal accumulation of the molecules the lysosome is supposed to break down. These diseases are responsible for most severe cases of nerve degeneration and mental retardation among children.

The discovery identifies for the first time the endoplasmic reticulum (ER) -- the cell's protein processing factory -- as the location of biochemical reactions leading to brain cell death in children with this disease.

"Our finding is exciting because children with GM1 gangliosidosis are severely affected and their outlooks are dismal," said Alessandra d'Azzo, Ph.D., member of the St. Jude department of Genetics and Tumor Cell Biology. "Now that we have a better understanding of what causes the damage, we may be able to design treatments that specifically remedy this problem. If this finding holds true for other lysosomal storage diseases, the impact could be especially fruitful."

GM1, a type of fatty molecule called a lipid, is a critical component of normal neurons. But when the cell lacks beta-galactosidase -- the enzyme needed for normal breakdown and recycling of GM1 -- this lipid accumulates and causes gangliosidosis.

Until now, the mechanism by which excess GM1 in lysosomes causes GM1 gangliosidosis was unknown. The St. Jude team demonstrated that the buildup of GM1 in the lysosomes causes a "back-up" of this lipid in the endoplasmic reticulum (ER) site where proteins are folded into their proper shape. This excess GM1 causes depletion of calcium, whose concentration in the ER is critical for proper protein folding. In turn, the accumulation of unfolded or improperly folded proteins triggers the "unfolded protein response" (UPR) in the cell. The UPR is the cell's emergency response to the accumulation of faulty proteins. This response attempts to slow or halt new protein production and increase efforts to correct the folding of misfolded proteins. Over time, however, this emergency response may fail to cope with the stress, and the cell activates a self-destruct program and dies. As more and more brain cells die, the child suffers the symptoms of GM1 gangliosidosis.

"No one knew that the UPR cascade could be triggered by GM1 accumulation in the ER," said Alessandra Tessitore, Ph.D., formerly a senior research technician at St. Jude. "This finding was unexpected and an important step in the search for an effective treatment for GM1 gangliosidosis."

In the study, d'Azzo's team examined the spinal cords of mice that lacked the gene for beta-galactosidase, the enzyme also missing in children with GM1 gangliosidosis and responsible for breaking down GM1. The investigators found significantly more dying cells in spinal cords of mice missing the gene than in normal mice.

In addition, the St. Jude investigators studied the effect of adding large amounts of GM1 to normal cells called murine (mouse) embryonic fibroblasts (MEFs), as well as groups of nerve-like cells called neurospheres. The researchers then compared the behavior of those cells with that of cells that were similar, except that they lacked beta-galactosidase, which caused them to accumulate GM1. Regardless of whether the researchers added GM1 or the GM1 accumulated because of loss of beta-galactosiadase, the excess GM1 caused depletion of calcium from this compartment. Following loss of calcium, the genes responsible for the UPR become activated and eventually cause cell death.

Other authors of this work are Maria del P. Martin, Renata Sano, Yanjun Ma, Linda Mann, Angela Ingrassia and Linda M. Hendershot (St. Jude); and Eric D. Laywell and Dennis A. Steindler (University of Florida, Gainesville).

This work was supported in part by the National Institutes of Health, the National Cancer Institute, Phillip and Elizabeth Gross, the Assisi Foundation of Memphis, and ALSAC. Alessandra d'Azzo holds an endowed chair in genetics and gene therapy from the Jewelers Charity Fund; and Renata Sano was sponsored by the Brazilian Government Agency, CAPES.