Update from the laboratory of Dr. Elizabeth Neufield — November 2011
Our studies of the Sanfilippo syndrome (MPS III) continue along two lines — what is wrong in the brain and how can we treat it? We use the MPS IIIB mouse model for these studies. The primary defect in MPS IIIB is a mutation in the gene (Naglu) encoding ?-N-acetylglucosaminidase, one of the lysosomal enzymes needed for the breakdown of heparan sulfate. As a result, there is a deficiency of α-N-acetylglucosaminidase and heparan sulfate accumulates in lysosomes. But in addition, there is a small area of the brain (the medial entorhinal cortex or MEC) where a number of apparently unrelated substances accumulate; one of these also accumulates in another small area, the dentate gyrus. The medial entorhinal cortex and the dentate gyrus, which are connected to each other, are involved in learning and memory. While our primary study system is the mouse model of MPS IIIB, we have found similar pathology in the brain of the mouse model of MPS IIIA, which is lacking another lysosomal enzyme of heparan sulfate degradation.
The diverse substances that accumulate in the MEC include (among others) proteins modified by ubiquitin, proteins involved in autophagy, and proteins that tend to aggregate. The latter include lysozyme, hyperphosphorylated tau (Ptau) and amyloid beta; the last two are known for their involvement in Alzheimer disease. Most importantly, we have found accumulation of glypican, a heparan sulfate proteoglycan (HSPG) molecule in which the heparan sulfate is attached to a protein backbone. HSPG is the form in which heparan sulfate is made by the cell and in which it functions. After fulfilling its function, the HSPG normally enters lysosomes, where the heparan sulfate part is separated from the protein in order to be broken down. In MPS III, the heparan sulfate cannot be broken down and therefore accumulates inside lysosomes.
Our hypothesis is that neurons can tolerate a certain amount of lysosomal storage of heparan sulfate. But in MEC, the lysosomes are so loaded that they have trouble receiving any more HSPG, which causes a back-up in the cellular trafficking of this substance. The HSPG outside of lysosomes can interact with amyloid beta and Ptau and cause these to form aggregates that are hard to break down as well as toxic to the neuron. We plan to test this hypothesis by various biochemical and molecular methods, as it has implications for developing treatment.
Each step in the cascade of events, from synthesis of HSPG to aggregation of Ptau, can be considered a drug target. We previously showed that the drug lithium chloride could reduce Ptau aggregates in the dentate gyrus. Lithium inhibits an enzyme which puts the phosphate on tau in order to make Ptau. We are now working with a compound that helps (indirectly) to remove the phosphate from Ptau, and will test it in combination with lithium chloride. Another step that may be easy to affect is autophagy. The ideal treatment would be to make HSPG with shorter branches of heparan sulfate so that less would accumulate in lysosomes, a treatment called substrate reduction therapy. We note that by our hypothesis, we would not need to reduce lysosomal storage of heparan sulfate to the normal level, but only to a level that would make it tolerable to MEC neurons by preventing the trafficking back-up of HSPG and the additional accumulations.
The studies mentioned above have been published:
Ohmi K, Zhao HZ and Neufeld EF, PLoS One. 2011;6(11):e27461.