My laboratory is focused on understanding the molecular events that determine a neuron’s fate following injury or during disease, and ultimately, whether they can be modulated therapeutically to promote better outcomes in patients. One strategy for promoting protection and/or regeneration in the nervous system is the inhibition of histone deacetylase (HDAC) enzymes by a class of compounds called HDAC inhibitors. These compounds inhibit the zinc hydrolase function of HDACs and allow the unopposed acetylation of histones, as well as certain transcription factors and cellular proteins. Acetylation of histone proteins within chromatin can enhance gene expression, while the acetylation of transcription factors and cellular proteins can modulate their protein-protein interaction and function.
Excitotoxicity, oxidative stress, and DNA damage are all well-established factors contributing to the degeneration of neurons during disease and injury. One line of investigation in my lab has been to define the mechanism by which HDAC inhibition protects neurons against DNA damage-induced death. We recently published that the neuroprotective efficacy of HDAC inhibitors can be attributed to their ability to modify the specific acetylation pattern of the tumour suppressor protein p53, which is a major regulator of pro-death gene expression during neurodegeneration (Brochier et. al., 2013). These observations have led us to explore the connection between HDAC activity and DNA repair in neurons. Unrepaired DNA damage can lead to cell death, whereas misrepaired damage increases the likelihood of mutagenesis, chromosome rearrangement, and loss of crucial genetic information. In replicating cells, such instability can result in apoptosis or cellular transformation. In the case of neurons residing in the adult brain, which are post-mitotic and terminally differentiated, and cannot be readily replaced after trauma or disease, the prospect of high fidelity repair would seem critical. Our efforts have revealed that in addition to protection from DNA damage-induced death, inhibiting HDAC activity also promotes durable DNA repair in post-mitotic neurons. Our current work explores the precise molecular mechanisms of this repair, both in vitro and in vivo.
Another line of investigation focuses on the prospect that HDAC inhibition in neurons may promote axonal repair. We have found that specifically inhibiting the function of HDAC6 in the axons of neurons can overcome the inhibitory effects of both myelin associated-glycoprotein and chondroitin sulfate proteoglycans, two molecules that impede axon regeneration after injury in the growth-hostile environment of a damaged spinal cord (Rivieccio et. al., 2009). Interestingly, the mechanism is independent of transcription, and involves HDAC6’s function as an α-tubulin deacetylase. We are currently investigating the regulation of HDAC6 following injury, its function and molecular targets in growth-inhibited axons, and the extent to which its activity can be inhibited to promote regeneration, both in vitro and in vivo. We model regeneration in vitro by culturing primary neurons with myelin associated-glycoprotein and/or chondroitin sulfate proteoglycans, while in vivo regeneration is studied in models of optic nerve crush injury (in retina-specific HDAC6 knockout mice) and thoracic contusion spinal cord injury (in pharmacologically HDAC6-inhibited rats).
In addition to HDAC inhibition, we are also interested in the potential for promoting protection and/or regeneration in the nervous system by increasing cellular levels of nicotinamide adenine dinucleotide (NAD+). NAD+ is an important energy substrate and cofactor involved in multiple metabolic reactions, including glycolysis, DNA repair processes, and the function of several NAD+-dependent enzymes, such poly (ADP-ribose) polymerases (PARPs) and sirtuins (a different class of histone deacetylase enzymes). These studies were established through collaboration with Drs Anthony Sauve and Samie Jaffrey in the Department of Pharmacology, and funded by the NY State SCIRB. Through these studies we have shown that by augmenting neuronal NAD+ via a NAD+ precursor, nicotinamide riboside, we can robustly protect neurons against DNA damage- and oxygen and glucose deprivation-induced death, and overcome the inhibitory effects of both myelin associated-glycoprotein and chondroitin sulfate proteoglycans, in vitro. Of immense importance to this project, the lab, and the collaboration, we have just completed thoracic contusion spinal cord injury studies in rats and find that nicotinamide riboside treatment can promote tissue sparing and greater functional recovery. We are continuing to investigate the roles of sirtuins and PARPs in promoting protection and axon growth (Brochier et al., 2015). We have also begun to explore the potential for nicotinamide riboside (as well as HDAC6 inhibition) in ameliorating peripheral neuropathy in a mouse model of Charcot Marie Tooth disease.