Investigating mechanisms of oxidative-stress induced BDNF axonal transport deficits in basal forebrain cholinergic neurons

Abstract

Aging and Alzheimer’s disease (AD) are associated with decreased cognitive function and neural degeneration. The basal forebrain is one of the first areas of the brain to degenerate in AD and depends on the neurotrophin brain-derived neurotrophic factor (BDNF) for survival. Loss of BDNF transport from target neurons may contribute to basal forebrain cholinergic neuron (BFCN) vulnerability in AD and aging. Oxidative stress is associated with cholinergic dysfunction and cognitive decline in aging and AD, and it is possible that oxidative stress may contribute to BDNF transport deficits in BFCNs. BFCNs are grown in microfluidic chambers that allow isolation of BFCN soma and axon terminals so transport of biotinylated and fluorescently labelled BDNF can be quantified. The objective of my research was to determine if oxidative stress induces BDNF retrograde transport deficits in BFCNs, and the mechanism behind this effect. I found that oxidative stress does reduce BDNF retrograde transport in BFCNs. Because it has previously been shown that aged BFCNs have decreased BDNF transport and downregulate the BDNF receptor TrkB, expression of both TrkB and p75NTR receptors was tested following oxidative stress using immunocytochemistry (ICC) and western blotting. This experiment showed that oxidative stress does not affect p75NTR or TrkB receptor levels. A likely alternative is that oxidative stress may lead to alterations in the transport machinery responsible for retrograde BDNF transport. I hypothesized that oxidative stress decreases retrograde axonal transport of BDNF via increased insulin-like growth factor 1 receptor (IGF1R) activity, which decreases the protein expression of the adaptor proteins BICD1 and Hook1 by inhibiting GSK3β activity via the PI3K-Akt pathway. ICC and western blotting showed that oxidative stress has no effect on either BICD1 or Hook1 levels. Future directions of this work involve further studying the involvement of the IGF1R pathway in oxidative stress, and the effect on other proteins involved in BDNF transport, including htt and DISC1.