Neuronal storage diseases are inborn errors of metabolism that result from deficient activity of lysosomal hydrolases. The resulting catabolic deficiency leads to an accumulation of undergraded substrates in the digestive-vacuolar (lysosomal) apparatus of cells, and to an expanding cascade of events that eventually compromises cell function. Although individuals with these diseases often appear normal at birth, neurodegenerative changes inevitable ensue. Psychomotor deficiencies can be severe and may include mental retardation, motor system dysfunction, sensory deficits, and seizures. Although intracellular storage in non-neuropathic forms of lysosomal disorders has been successfully ameliorated by bone marrow transplantation (BMT), the application of BMT to storage diseases with neuronal involvement (e.g, Hurler's disease) has been highly controversial. Working with an inherited model of lysosomal alpha-D-mannosidase deficiency (alpha- mannosidosis), we have unequivocally demonstrated not only that this enzyme increases in activity in the CNS post-BMT, but that intraneuronal storage is reversed and/or prevented. Most importantly, we have used an indigogenic histochemical substrate to demonstrate that acidic alpha- mannosidase is present with neurons and other cells of the CNS. This remarkable finding has established the principle of therapeutic efficacy for BMT in neuronal storage diseases and has led us to evaluate treatment in a different type of storage disorder - GM2 gangliosidosis - using an animal model of BETA-D-N-acetylhexosaminidase deficiency. Our preliminary studies reveal a dramatically different result: In spite of significant elevations of Beta-hexosaminidase activity in brain (30% of normal), substrate reduction was not evident and histochemical staining demonstrated that the enzyme was limited to brain microglia/macrophages. We believe that differences in efficacy in the above models can be exploited in the testing of hypotheses on the mechanism underlying successful treatment. The most commonly stated rationale for use of BMT in children is that donor blood monocytes enter brain, differentiate as microglia, and provide a source of enzyme to enzyme deficient brain cells. Alternative hypotheses include uptake of 'free' enzyme derived from the circulation, and 'metabolic filtration' which depends on substrate diffusion out of diseased cells with uptake and degradation by donor cells. None of these hypotheses is proven and we propose to test them using multidisciplinary in vivo and in vitro studies. Transplants will be carried out in out models at different ages to assess the importance of early treatment, and the dynamics of monocyte invasion of brain in the early weeks post-BMT will be critically examined. Therapeutic effectiveness for these studies will be assessed by clinical, biochemical, histochemical, immunocytochemical and histopathologic criteria. Using cell culture, we will determine whether putative bone marrow-derived cells from normal animals have the capacity to transfer lysosomal enzyme to brain cells from affected animals and whether differential secretion or uptake of alpha-mannosidase and Beta- hexosaminidase occurs. Alternative mechanisms leading to substrate depletion also will be tested. Taken together, these multidisciplinary studies will provide valuable insight into mechanism(s) underlying metabolic correction in neurons following BMT and into pragmatic issues related to BMT as therapy for neuronal storage diseases in children.