Alpha-glucosidase deficiency (acid maltase deficiency-AMD) is a clinically and biochemically heterogeneous lysosomal storage disease existing in a generalized infantile and late-onset neuromuscular froms. Both clinical states are characterized by a catalytically active microsomal Alpha-glucosidase of unknown function. Studies of intracellular transport of lysosomal hydrolases and evolving models of glycoprotein biosynthesis suggest a dual role for the microsomal enzyme: as a biosynthetic precursor of the lysosomal enzyme and as a glycoprotein-processing enzyme in the post-translational assembly of macromolecular species. Recently two human neutral Alpha-glucosidases, with diverse biochemical properties, have been characterized and mapped to separate chromosomal loci distinct from the lysosomal Alpha-glucosidase. Similarly, two microsomal """"""""processing"""""""" Alpha-glucosidases were subsequently identified and found to exhibit partial catalytic overlap with the neutral Alpha-glucosidase activity. In addition, we have identified a tissue-specific Alpha-glucosidase activity in renal tissue, which has partial electrophoretic identity with the lysosomal enzyme and is present in normal amounts in AMD renal samples. We had previously demonstrated reduced levels of catalytically-active lysosomal Alpha-glucosidase only in tissues from late-onset AMD patients, while recent studies have shown reduced rates of synthesis of a biosynthetic form of the lysosomal enzyme in these mutant cells, with lack of immunoprecipitable material in the infantile AMD cells. The finding of heterogeneous morphological alterations in AMD tissues coupled with the evolving complexity of lysosomal enzyme biosynthesis and pathology suggested to us that graded cellular alterations could occur as a consequence of a selective mutant locus. As such, we conducted preliminary studies which revealed major alterations in whole cell glycoprotein biosynthesis and membrane assembly. We believe a program combining study of glycoprotein biochemistry and molecular biology will best allow us to: characterize the complex interrelationships between lysosomal and microsomal Alpha-glucosidases, determine the molecular basis of functional differences between infantile and late-onset AMD, and study the consequences of a selective mutant locus on post-translational cellular processing events. These studies will involve: comparative hydrolase assays using putative natural saccharide substrates on purified neutral Alpha-glucosidases, detailed structural analysis of glycoprotein biosynthetic intermediates and mature membrane glycoproteins, in vitro cell-free translation and biosynthetic labeling, and recombinant DNA techniques. These studies are crucial to our understanding of molecular mechanisms in lysosomal and general cellular pathology and to refinement in techniques important to gene and enzyme replacement therapy in neurodegenerative diseases.
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