Galactosylceramidase (GALC) deficiency in Krabbe disease (KD) causes toxic accumulation of galactosylsphingosine (psychosine) in myelin-forming cells, leading to demyelination of the nervous system. To reduce demyelination, current therapies seek to provide the missing enzyme to myelinating glia via infiltrating macrophages after the transplantation of bone marrow cells (BMT) from healthy donors into affected patients. Although the experience gained from this approach supports the use of BMT, KD patients suffer neurological sequelae. This suggests that the pathogenic mechanisms in KD are more complex than previously thought and that new therapeutic strategies are needed to cure KD. Experiments in our laboratory using the Twitcher mouse, a natural model for KD, indicate: 1) BMT- treated mice show neuronal and axonal damage by the time sufficient therapeutic enzyme accumulates in the nervous system ;2) psychosine is also produced and accumulates in neurons in the absence of mutant glia, causing the blockage of fast axonal transport via the activity of protein phosphatase 1 (PP1);and 3) mutant neurons show abnormal intracellular levels of Ca linked to deregulated expression of the Na+ Ca exchanger (NCX1). These observations suggest that GALC-deficient neurons mount a stress response that contributes to the pathology and that PP1 and NCX1 are two potential key components in the mechanism that mediates axonal defects in KD. Thus, we hypothesize that the deficiency of GALC in KD not only affects myelination but also triggers intrinsic and contemporaneous defects in neurons. To test this hypothesis we propose specific experiments to modulate PP1 and NCX1 activities in Twitcher neurons. These experiments will provide proof-of-concept that neuroprotective strategies can synergize with/improve the therapeutic benefits of traditional BMT-based treatments. Specifically, we will: 1) determine whether controlled and specific reduction of neuronal PP1 activity using siRNA specific silencing protects axonal transport in mutant neurons;2) determine whether flecainide, an antiarrhythmic drug with a proven ability to reduce sodium channel firing and NCX1 activity, improves NCX1-mediated influx of calcium in axons;and 3) determine whether these neuroprotective strategies combined with metabolic correction after BMT in newborn Twitcher mice improve clinical outcome. Results from these experiments will shed light on the molecular role of PP1 and NCX1 activity mediating neuronal dysfunction in KD and will provide a unique opportunity to improve the current BMT-based metabolic corrective strategies used to treat this leukodystrophy. The insight obtained will be relevant to other lysosomal storage disorders, which like KD are associated with aggressive neurological deterioration and for which there are no available cures.
Krabbe disease is a lysosomal storage disease that results in demyelination of the brain and nerves in affected individuals. Some Krabbe patients are treated with hematogenous cell replacement, which delays the onset of symptoms. However, a definitive and complete cure for this disease has not been achieved and treated patients continue to undergo deterioration and neurological deficits. The role of neuronal loss in Krabbe disease is not completely understood, but a consensus is emerging that dysfunction of axons and neurons leads to permanent neurological deficits in several neurodegenerative disorders, including multiple sclerosis, Alzheimer disease, Parkinson disease and others. Our preliminary studies provide evidence that Krabbe disease is compounded by axonal defects. In addition to the loss of myelin, neurodegeneration is likely a limiting factor in reducing the efficiency of traditional therapies. Thus, a combined therapy that provides not only enzyme replacement but also neuroprotection is likely to synergize or enhance the therapeutic benefits. Our objective is to examine whether two novel neuroprotective strategies targeting specific aspects of neurodegeneration in Krabbe disease can be combined with traditional bone marrow transplantation to fully prevent development of the disease. Results of the proposed experiments will provide proof-of-concept for the design of combined neuroprotective therapies to treat human Krabbe patients and the rational basis for studies of other leukodystrophies that involve degeneration of axons and myelin.
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