Enzyme replacement therapies, allogeneic
bone marrow transplantation and gene
therapies are treatment options for
lysosomal storage diseases caused by inherited deficiencies of soluble lysosomal
enzymes. Independent from the approach, the
enzyme must be delivered to lysosomes of deficient patient cells. Little is known about the dissemination of
enzyme within a tissue where cells compete for uptake via different receptor systems, binding affinities and endocytic rates. To evaluate dissemination and lysosomal targeting of a lysosomal
enzyme in the CNS, we analysed receptor-mediated endocytosis of
arylsulfatase A (ASA) by different types of brain-derived cell lines and primary murine brain cells. For ASA expressed by chinese hamster ovary cells for
enzyme replacement therapy of
metachromatic leukodystrophy, endocytic rates decline from microglia to neurons and astrocytes and to oligodendrocytes. Only immature oligodendrocytes endocytose significant amounts of
enzyme. Uptake by non-microglial cells is due to
mannose 6-phosphate receptors, whereas several receptor systems participate in endocytosis by microglial cells. Interestingly, ASA expressed by microglial cells cannot be taken up in a
mannose 6-phosphate dependent manner. The resulting failure to correct non-microglial cells corroborates in vivo data and indicates that
therapeutic effects of allogeneic
bone marrow transplantation and hematopoietic stem cell gene therapy on
metachromatic leukodystrophy are independent of metabolic cross-correction of neurons, astrocytes and oligodendrocytes by receptor-mediated endocytosis.