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Enzyme replacement therapy has been a very effective treatment for several lysosomal storage diseases. However, correcting central nervous system (CNS) storage has been challenging due to the presence of the blood-brain barrier (BBB), which hampers the entry of circulating lysosomal enzymes into the brain. In our previous studies, we discovered that luminally expressed cation-independent mannose 6-phosphate (M6P) receptor is a universal transporter for lysosomal enzymes that contain M6P moieties on the enzyme molecule. This receptor-mediated transport of lysosomal enzymes showed developmental down-regulation that resulted in a failure of delivery of lysosomal enzymes across the BBB in the adult brain. Conceptually, if one can re-induce M6P receptor-mediated transport of lysosomal enzymes in adult BBB, this could provide a novel brain targeting approach for treating abnormal storage in the CNS, regardless of the age of subjects. We found that systemic adrenergic stimuli restored functional transport of β-glucuronidase across the adult BBB. The concept of manipulating BBB transport activity by endogenous characteristics has also been demonstrated by another group who showed effective treatment in a Pompe disease model animal in vivo. It is intriguing that lysosomal enzymes utilize multiple mechanisms for their transport across the BBB. This review explores pharmacological manipulations for the delivery of lysosomal enzymes into the CNS, and the mechanisms of their transport across the BBB, based on existing evidence from studies of β-glucuronidase, sulfamidase, acid α-glucosidase, and arylsulfatase A.

G(M1)-gangliosidosis and Morquio B are rare lysosomal storage diseases associated with a neurodegenerative disorder or dwarfism and skeletal abnormalities, respectively. These diseases are caused by deficiencies in the lysosomal enzyme human β-D-galactosidase (h-β-GAL), which lead to accumulations of the h-β-GAL substrates, G(M1) ganglioside and keratan sulfate due to mutations in the h-β-GAL gene. H-β-GAL is an exoglycosidase that catalyzes the hydrolysis of terminal β-linked galactose residues. Here, we present the crystal structures of h-β-GAL in complex with its catalytic product galactose or with its inhibitor 1-deoxygalactonojirimycin. H-β-GAL showed a novel homodimer structure; each monomer was comprised of a catalytic TIM barrel domain followed by β-domain 1 and β-domain 2. The long loop region connecting the TIM barrel domain with β-domain 1 was responsible for the dimerization. To gain structural insight into the molecular defects of h-β-GAL in the above diseases, the disease-causing mutations were mapped onto the three-dimensional structure. Finally, the possible causes of the diseases are discussed.

The combined use of immunofluorescence method with oligosaccharide-specific monoclonal antibodies against glycolipids and a confocal laser scanning microscopic system was successful to detect the storage of glycolipids in cultured cells derived from patients with lysosomal diseases. The accumulated glycolipids were observed stereoscopically as granular inclusions in the cells. An immunofluorometric semiquantative determination was achieved of globotriaosylceramide in cultured fibroblasts from Fabry disease patients or GM2-ganglioside in cultured amniocytes from Tay-Sachs disease. Heterozygote identification of the former disease or prenatal diagnosis of the latter one were confirmed by counting immunoreactive cells or by digital imaging analysis. The present immunofluorescence method is applicable to the diagnosis of the other lipidosis with accumulation of the specific glycolipids.

The lysosomes of most cells function principally in intracellular digestion. They contain several dozens of enzymes, mostly acid hydrolyases. Vacuolar-ATPase on lysosomal membrane establishes low-pH environment required for efficient expression of hydrolyase activity. Of several dozen disorders of human and various animals known to be due to lysosomal dysfunctions, most of the lysosomal storage diseases are of genetic origin. Metabolic pathways of intracellular macromolecules within lysosomes have been established from studies on the molecular and biochemical defects on lysosomal enzymes. In the lysosomal storage diseases most of all, the metabolic pathways of breakdown of lipids, glycosaminoglycans or oligosaccharides are severely affected. The degradation pathways of glycoproteins, proteoglycans, and glycolipids by lysosomal enzymes are described in this brief review.

A case of Farber's disease (disseminated lipogranulomatosis) is reported. A Japanese boy presented with progressive joint deformity, a hoarse voice, mental retardation and skin granulomatous lesions. He died of bronchopneumonia at the age of 2 years. Biopsied specimen from the perianal granulomatous lesion showed many macrophage contained lipids, and acidic oligo-, or polysaccharides, or, glycoconjugates. Electron microscopic study revealed, in addition to curvilinear tubular bodies (Farber's bodies), numerous granular structures accumulated in the cytoplasm of the macrophage. Biochemical analysis demonstrated a storage of free ceramide in the patient's liver, and confirmed the patient's diagnosis. This is the 5th case reported in Japan.