Spin labeling studies of the interactions of potentially useful therapeutic agents for the treatment of Alzheimer’s disease with cytoskeletal proteins in erythrocyte ghosts and brain synaptosomal membranes

Alzheimer’s disease (AD) is the major dementing disorder of the elderly with four million victims of this disease in the United States. The molecular basis of AD remains unknown, but the biochemical and biophysical state of cytoskeletal proteins is reportedly altered in AD cortical neurons. No approved effective therapy for AD is available. We have used electron spin resonance (ESR) and a protein-specific spin label to investigate the interactions of three potential therapeutic agents in AD: tacrine, velnacrine, and acetylcarnitine, with cytoskeletal proteins in erythrocyte membranes. Further, we report for the first time the effects of tacrine on the physical state of membrane proteins in brain neocortex synaptosomal membranes. All three agents lead to decreased segmental motion and increased cytoskeletal protein-protein interactions in erythrocyte membranes in order of effectiveness: tacrine > velnacrine > acetylcarnitine. In addition, we have synthesized N-methylacridinium methosulfate which has a positive charge on the opposite side of the molecule relative to tacrine. This former agent gave a less pronounced diminution of the relevant ESR parameter than that caused by tacrine, implying that the orientation of the molecule with its interaction site is important in the increased cytoskeletal protein-protein interactions induced by tacrine. With synaptosomal membranes tacrine also significantly decreased segmental motion of membrane proteins. Our ESR results on erythrocyte and brain membranes suggest that in addition to their biochemical effects, these potential AD therapeutic agents function to strengthen cytoskeletal protein-protein interactions. These results are discussed with reference to possible molecular mechanisms involving cytoskeletal proteins in AD.