Therapeutic Potential of the Microtubule-Stabilizing Drug, Peloruside A: Implications in Neurodegenerative Diseases

Abstract

Alzheimer’s disease is characterized by the presence of neurofibrillary tangles formed due to hyperphosphorylation of microtubule-associated tau protein in the brain. Tau is localized in the axons of neurons and functions to maintain microtubule integrity by stabilizing the microtubules. Hyperphosphorylated tau dissociates from the microtubule, causing microtubule depolymerization and, thus, affecting microtubule-based neuronal functions. Replacing the loss of tau function by treatment with microtubule-stabilizing drugs could be beneficial in preventing the progressive and extensive loss of neurons seen in Alzheimer’s disease. In this regard, paclitaxel and epothilone, two clinically used microtubule-stabilizing agents, have shown therapeutic potential in transgenic mouse models of tauopathy. Tau binds to the microtubule at or near the paclitaxel binding site and thereby leads to microtubule stabilization, an effect similar to that seen with paclitaxel. Due to a common binding site, paclitaxel and tau can affect binding of each other to the microtubule. Furthermore, overexpression of the longest isoform of human tau (htau40) inhibits axonal transport of cellular organelles and vesicles in the neurons. This effect of tau on axonal transport is due to competition with the motor protein, kinesin-1, for binding to the microtubule. Peloruside A (peloruside) is a microtubule-stabilizing drug isolated from the New Zealand marine sponge Mycale hentscheli. It has a similar mode of action to paclitaxel but binds to a different site on β-tubulin compared to paclitaxel and tau. In the present study, we investigated the effects of peloruside on neuronal function in partial Alzheimer’s-like conditions, using two cellular models of the disease: (1) okadaic acid-induced hyperphosphorylation of tau, and (2) overexpression of htau40 following transfection of rat cortical neurons and neuron-like, differentiated N-2a cells in culture. Peloruside reversed the decrease in axonal outgrowth and branching seen in neuronal cultures treated with okadaic acid. In addition, peloruside rescued neurons from growth cone collapse. Although peloruside had no effect on the hyperphosphorylation of tau caused by okadaic acid, it restored the levels of acetylated-tubulin, a marker of stable microtubules, and reversed the okadaic acid-induced depression of GAP-43, an axonal growth regulator. Furthermore, stabilization by peloruside rescued blocked axonal transport of mitochondria and brain-derived neurotrophic factor in tau-transfected rat cortical neurons and differentiated N-2a cells. Peloruside was effective in stabilizing microtubules independent of tau overexpression; whereas, paclitaxel-induced effects were masked by the overexpression of htau40. The data presented in this thesis are an important addition to the ongoing research on microtubule-stabilizing drugs as a potential clinical tool for slowing down or preventing the development of neurodegenerative diseases. This thesis focused particularly on peloruside as a potential therapeutic agent for Alzheimer’s disease. Although the permeability of peloruside through the blood-brain barrier is yet to be determined, it has the advantages of showing lower toxicity and better tolerance in vivo in animals than paclitaxel.