Microtubule dynamics imbalance leads to axonal transport impairment in MPP+-treated PC12 cells

Many neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease and Amyotrophic lateral sclerosis, show axonal pathologies with abnormal accumulation of proteins and organelles. This can be easily explained with the impairment of axonal transport, that, in turn, can lead to the dying-back degeneration of neurons in many of the disorders mentioned above. At the present days the question is: which is the trigger event leading to the block of axonal transport? Disruption to transport can occur via damage to cargoes, reduction of the ATP supply, dysfunction of motor proteins and alteration of microtubules (De Vos et al., 2008). Recently, it has been also reported that in isolated squid axoplasm, the administration of MPP+, a well known parkinsonism-inducing neurotoxin, leads to the alterations in fast axonal transport through the activation of caspase and protein kinase C (Morfini et al., 2007). Starting from the notion that microtubules are a common target of toxins and mutated proteins inducing Parkinson’s disease (Feng, 2006), and knowing that MPP+ alters microtubule dynamics in vitro (Cappelletti et al., 2005), we have investigated if the imbalance of microtubule dynamics could be the triggering event that leads to axonal transport impairment in PC12 cells treated with MPP+. Here we show that accumulations of mitochondria in axonal swellings, a typical marker of impaired axonal transport, are noticeable in a condition where the only alteration that could induce disruption of transport is microtubule dynamics imbalance. Using low dose of MPP+ we observed the increase of stable microtubules (acetylated and detyrosinated tubulin) without any sign of dysfunction of motor proteins (kinesin and dynein), damage to mitochondria or reduction of ATP supply. Further, in our model, the inhibition of caspase and PKC does not protect the cells from the neurotoxin. However, all the alterations that are supposed to be causative for the block of axonal transport are observed with higher dose of MPP+, suggesting that they are secondary events. These data clearly suggest the pivotal role of microtubule dynamics alteration in the chain of event that through the impairment of axonal transport leads to the degeneration of neurons in Parkinson’s disease.