Oxidative damage is implicated in the pathogenesis of neurodegenerative disorders, including Alzheimer's, Parkinson's, and Huntington's diseases, and in normal aging. Here, we model oxidative stress in neurons using photogenerated radicals in a simplified membrane-encapsulated microtubule system. Using fluorescence and differential interference contrast microscopies, we monitor photochemically induced microtubule breakdown on the supported region of membrane in encapsulating synthetic liposomes as a function of lipid composition and environment. Degradation of vesicle-encapsulated microtubules is caused by attack from free radicals formed upon UV excitation of the lipid-soluble fluorescent probe, 6-(9-anthroyloxy)stearic acid. Probe concentration was typically limited to a regime in which microtubule degradation was slow, and microtubule degradation was monitored by changes in the observed protrusion of the membrane surface. The kinetics of microtubule degradation are influenced by lipid saturation level, fluorescent probe concentration, and the presence of free-radical scavengers. This system is sufficient to reproduce some degenerative morphologies found in vivo.O xidative modifications to proteins in neurons have been implicated in a number of degenerative disorders and have become a controversial topic in elucidating the progression of Parkinson's, Huntington's, and Alzheimer's diseases (1-7). Microtubules (MTs) are key cytoskeletal proteins in nerve cell axons. These protein polymers are responsible for many disparate functions of cells, including structural support and imparting polarity, and they are involved in motility and transport of intracellular cargo (8). Fundamental studies of MTs in vivo are complicated by the presence of the full complement of biomolecules that are required for cell survival. Thus, a substantial effort has been devoted to characterizing MT dynamics in vitro, and these studies have led to increased understanding of issues such as dynamic instability (9) and the involvement of molecular motors in producing motility (10). To mimic the confinement of a bilayer, MT assemblies (asters) have previously been examined in rigid, microfabricated wells (11). The drawback of such studies is that interactions between MTs and components of the elastic lipid membrane are neglected. It is important to be able to isolate and to quantify specific factors that impact the structural integrity of the cytoskeleton, while maintaining a simplified cell-like environment.Toward this aim, we have developed a minimal physical system with which to investigate the effects of oxidative stress on the cytoskeleton. Previous studies have demonstrated that incorporating actin and tubulin into liposomes and inducing these proteins to polymerize results in a morphological change in the liposome; a protrusion of the membrane is observed at one or both ends of the protein polymer (12-15). The asymmetry occurs frequently, when the MT wraps around within the encapsulating liposome (16). Here, we employ the membrane as a...