benzyloxycarbonylleucyl-leucyl-leucine-aldehyde and Wallerian-Degeneration

The ubiquitin-proteasome system (UPS), lysosomes, and autophagy are essential protein degradation systems for the regulation of a variety of cellular physiological events including the cellular response to injury. It has recently been reported that the UPS and autophagy mediate the axonal degeneration caused by traumatic insults and the retrieval of nerve growth factors. In the peripheral nerves, axonal degeneration after injury is accompanied by myelin degradation, which is tightly related to the reactive changes of Schwann cells called dedifferentiation. In this study, we examined the role of the UPS, lysosomal proteases, and autophagy in the early phase of Wallerian degeneration of injured peripheral nerves. We found that nerve injury induced an increase in the ubiquitin conjugation and lysosomal-associated membrane protein-1 expression within 1 day without any biochemical evidence for autophagy activation. Using an ex vivo explant culture of the sciatic nerve, we observed that inhibiting proteasomes or lysosomal serine proteases prevented myelin degradation, whereas this was not observed when inhibiting autophagy. Interestingly, proteasome inhibition, but not leupeptin, prevented Schwann cells from inducing dedifferentiation markers such as p75 nerve growth factor receptor and glial fibrillary acidic protein in vitro and in vivo. In addition, proteasome inhibitors induced cell cycle arrest and cellular process formation in cultured Schwann cells. Taken together, these findings indicate that the UPS plays a role in the phenotype changes of Schwann cells in response to nerve injury.

Topics: Animals; Autophagy; Axotomy; Blotting, Western; Cell Cycle; Cell Dedifferentiation; Cell Proliferation; Cells, Cultured; Cysteine Proteinase Inhibitors; Fluorescent Antibody Technique; Image Processing, Computer-Assisted; Leupeptins; Lysosomal-Associated Membrane Protein 1; Lysosomes; Mice; Nerve Fibers, Myelinated; Proteasome Endopeptidase Complex; Schwann Cells; Sciatic Nerve; Ubiquitination; Wallerian Degeneration

Treatment of transected distal axons of rat sympathetic neurons in compartmented cultures with MG132 (5 microM) and other inhibitors of proteasome activity, preserved axonal mitochondrial function, assessed by Mitotracker-Orange and MTT staining, for at least 24 h. MG132 similarly protected axons from undergoing branch elimination (pruning) in response to local NGF deprivation. Axons protected by MG132 displayed persistent phosphorylation of Erk1/2, and pharmacological inhibition of MEK activity with U0126 (50 microM) restored rapid axonal degeneration. Therefore, the proteasome does not appear to be necessary as a general effector of protein degradation during axonal degeneration. Rather, the proteasome functions in the regulation of signaling pathways that control axonal survival and degeneration. Specifically, the down-regulation of the MEK/Erk pathway by the proteasome plays roles in Wallerian degeneration of severed axons and axonal pruning in response to local NGF deprivation. Identification of the pathways that regulate axonal survival and degeneration will provide possible target sites for pharmacological treatments of neurodegenerative diseases and traumatic injury.

Topics: Animals; Animals, Newborn; Axons; Butadienes; Cell Survival; Cells, Cultured; Cysteine Proteinase Inhibitors; Down-Regulation; Enzyme Inhibitors; Leupeptins; MAP Kinase Kinase 1; MAP Kinase Signaling System; Mitogen-Activated Protein Kinase 3; Nerve Growth Factor; Nerve Regeneration; Nitriles; Phosphorylation; Proteasome Endopeptidase Complex; Proteasome Inhibitors; Rats; Rats, Sprague-Dawley; Signal Transduction; Superior Cervical Ganglion; Sympathetic Nervous System; Wallerian Degeneration

Local axon degeneration is a common pathological feature of many neurodegenerative diseases and peripheral neuropathies. While it is believed to operate with an apoptosis-independent molecular program, the underlying molecular mechanisms are largely unknown. In this study, we used the degeneration of transected axons, termed "Wallerian degeneration," as a model to examine the possible involvement of the ubiquitin proteasome system (UPS). Inhibiting UPS activity by both pharmacological and genetic means profoundly delays axon degeneration both in vitro and in vivo. In addition, we found that the fragmentation of microtubules is the earliest detectable change in axons undergoing Wallerian degeneration, which among other degenerative events, can be delayed by proteasome inhibitors. Interestingly, similar to transected axons, degeneration of axons from nerve growth factor (NGF)-deprived sympathetic neurons could also be suppressed by proteasome inhibitors. Our findings suggest a possibility that inhibiting UPS activity may serve to retard axon degeneration in pathological conditions.

Topics: Amino Acids; Animals; Animals, Newborn; Axons; Benzimidazoles; Blotting, Western; Calpain; Cells, Cultured; Chelating Agents; Cysteine Endopeptidases; Cysteine Proteinase Inhibitors; Cytoskeleton; Disease Models, Animal; Drug Interactions; Egtazic Acid; Endopeptidases; Ganglia, Sympathetic; Immunohistochemistry; Leupeptins; Microtubules; Multienzyme Complexes; Nerve Growth Factor; Optic Nerve; Optic Nerve Injuries; Peptide Fragments; Proteasome Endopeptidase Complex; Rats; Time Factors; Tubulin; Ubiquitin; Wallerian Degeneration