In rare, hereditary storage diseases such as Sandhoff’s disease or Tay-Sachs syndrome, the metabolic waste from accumulating gangliosides cannot be properly disposed of in the nerve cells because key enzymes are missing. The consequences for the patients are serious, ranging from movement restrictions to blindness, mental decline, and early death.
Researchers at the University of Bonn report that they have discovered why these gangliosides also accumulate in patients with other storage diseases and cause a deterioration in them. Their study (“Membrane lipids and their degradation compounds control GM2 catabolism at intralysosomal luminal vesicles”) appears online in the Journal of Lipid Research.
“The catabolism of ganglioside GM2 is dependent on three gene products. Mutations in any of these genes result in a different type of GM2 gangliosidosis (Tay-Sachs disease, B1 variant, Sandhoff disease, and the AB-variant), with GM2 as a major lysosomal storage compound. GM2 is also a secondary storage compound in lysosomal storage diseases like Niemann-Pick disease type A, B, and C with primary storage of SM and cholesterol, respectively,” the investigators wrote. “Reconstitution of GM2 catabolism at liposomal surfaces carrying GM2 revealed that incorporation of lipids into the GM2 carrying membrane like cholesterol, SM, sphingosine, and sphinganine inhibit GM2 hydrolysis by β-hexosaminidase A assisted by GM2 activator protein, while anionic lipids, ceramide, fatty acids, lyso-phosphatidylcholine, and diacylglycerol stimulate GM2 catabolism. In contrast, the hydrolysis of the synthetic, water soluble substrate 4-methylumbelliferyl-6-sulfo-2-acetamido-2-deoxy-β-D-glucopyranoside was hardly affected by membrane lipids such as ceramide or SM, nor was it stimulated by anionic lipids like bis(monoacylglycero)phosphate, either added as liposomes, detergent micelles or lipid aggregates.
“Moreover, we could show that hydrolysis inhibiting lipids had also an inhibiting effect on the solubilization and mobilization of membrane-bound lipids by GM2 activator protein, while the stimulating lipids enhanced lipid mobilization.”
In Tay-Sachs syndrome and Sandhoff’s disease, components of nerve cell membranes cannot be properly degraded, resulting in the ganglioside GM2 being stored in the lysosomes. Gangliosides lipids occur mainly in the ganglion cells of the nervous system. If the GM2-degrading enzyme Hex A is missing or impaired, for example, due to genetic defects, destructive ganglioside storage occurs.
In Sandhoff’s disease, the degradation enzymes Hex A and Hex B are inactive. As with Tay-Sachs syndrome, GM2 storage leads to the destruction of nerve cells. Affected children develop normally in the first months of life; later on, blindness, movement restrictions, and mental decline occur—and eventually an early death.
“Previous therapeutic approaches have not led to any significant successes in these neurodegenerative gangliosidoses,” said Konrad Sandhoff, PhD, senior professor at the LIMES Institute of the University of Bonn. Enzyme replacement therapies have failed due to the impermeability of the blood-brain barrier for these substances.
Sandhoff and his team have deciphered the role of the molecular environment in the lysosome for the successful degradation of GM2. In the test tube, the scientists reconstructed the vesicles on which the GM2 is degraded in the lysosome. Normally, the auxiliary protein GM2AP helps to catch and release the GM2 that sits on the shell of the vesicles. It can then be degraded together with the degradation enzyme Hex A to harmless GM3. However, when the function of Hex A is blocked, it is stored, with fatal consequences for nerve cells.
In the test tube, the scientists investigated the influencing factors that inhibit or improve the degradation of GM2. For example, the smaller the vesicle and the more negatively charged its surface is, the easier the degradation enzyme’s access to the GM2 and the better the “digestion” functions.
The presence of cholesterol and sphingomyelin, on the other hand, significantly reduces GM2 degradation. The investigations showed that the storage of these lipids in Niemann-Pick diseases triggers an additional GM2 accumulation in the lysosome, which significantly exacerbates the type C clinical picture even though the GM2-degrading enzyme Hex A is intact and active. “Genetic disorders of the degradation enzyme evidently trigger a cascade of as yet unknown consequential damages,” added Sandhoff.
In another study, the team showed that this cascade principle also applies to hereditary mucopolysaccharidoses. In these disorders, one of the storage substances, chondroitin sulphate, triggers additional ganglioside storage in the nerve cells by inhibiting GM2 degradation. In addition to existing short stature, coarse facial features, and liver enlargement, it also causes learning difficulties and startle reflexes, which can, however, be alleviated in the animal model by inhibitors of GM2 formation.
“The aim of current therapeutic approaches is to prevent the production of GM2 for these hereditary storage diseases,” said Sandhoff, adding that drugs that are currently on the market only partially fulfill this requirement. “Perhaps gene replacement therapy, which has already been successful in animal models and will soon be used in patients, will be more successful.”
