An Automated Microscope System to Monitor Dynamic Stress Responses in Neurons
Despite years of incremental progress in our understanding of diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's Disease (HD), and amyotrophic lateral sclerosis (ALS), there are still no disease-modifying therapeutics. The discrepancy between the number of lead compounds and approved drugs may partially be a result of the methods used to generate the leads and highlights the need for new technology to obtain more detailed and physiologically relevant information on cellular processes in normal and diseased states. We developed a high-throughput automated microscope system and primary neuron model of HD that allows us to monitor dynamic stress responses in intact, fully differentiated neurons. We are able to assay thousands of conditions, including millions of neurons, in a short period of time, which can reveal completely new aspects of biology and identify lead therapeutics in the span of a few months when conventional methods could take years or fail all together. We use this system to understand how neurons, a long-lived, postmitotic cell type, differ in their acute responses to proteotoxic insults including those associated with malformed protein. We show that neurons have a deficient acute stress response to multiple known stimuli, including thermal stress, Hsp90 inhibition, and proteasome inhibition. We find that neurons have low expression of the two major stress-responsive transcriptional activators in mammals, HSF1 and HSF2. We also find that neurons have high constitutive chaperoning capability through relatively high levels of Hsp90 and Hsc70. High Hsp90, however, decreases the acute stress response through negative regulation of HSF1. By increasing HSF1 levels, we were able to restore the response in neurons and protect them from the toxicity associated with malformed protein. We propose a mechanism for the attenuated acute stress response in neurons that implicates a high Hsp90 to HSF1 ratio, which results in an increased ability to cope with chronic stresses, but decreased ability to respond acutely when basal homeostatic responses are overwhelmed. We conclude that targeted therapies to bolster acute stress responses in neurons through increasing HSF1 expression or activation will therefore benefit HD and other neurodegenerative disorders of protein conformation.