Role of epigenetic mechanisms in structural and functional plasticity in the CNS: focus on spinal sensitization

Neurons have a highly specialized morphology whose signal-regulated remodeling is key to their function in neuronal networks. Activity-dependent nuclear calcium signaling is a crucial regulator of gene transcription in hippocampal and spinal cord neurons and mediates gene expression by directly acting on transcription factors or by regulating epigenetic processes, such as the induction of DNA methyltransferases (DNMTs) or the nucleo-cytoplasmic shuttling of class IIa histone deacetylases(HDAC4, -5, -7, and -9). Epigenetic mechanisms regulate several neuroadaptive phenomena in hippocampal neurons including, among others, synaptic plasticity and memory formation. In the first part of this thesis we describe how the subcellular localization of HDAC4 controls the morphology of hippocampal neurons by modulating the expression of a factor critical for dendrite architecture. Epigenetic regulators have also been suggested to mediate central sensitization in spinal cord neurons and the development of chronic pain. The transition from acute to chronic pain is considered a pathological manifestation of neuronal plasticity in nociceptive pathways and shares common molecular pathways with memory formation. However, if and how epigenetic gene regulatory events control also structural remodeling of spinal cord circuits, relevant for central sensitization, remains to be investigated. Here we characterized the impact of synaptic activity and chronic inflammatory pain on the expression and activity of DNMTs and HDACs in spinal cord neurons and found that the de novo methyltransferase Dnmt3a2 and HDAC4 were particularly affected. We demonstrate that activity-induced levels of Dnmt3a2 contribute to spinal sensitization and hypersensitivity in the CFA model of inflammatory pain by regulating the expression of pain- and plasticity-related genes. Moreover, we found that long-lasting, but not acute inflammatory pain, results in a nuclear export of HDAC4 in spinal cord neurons, which is accompanied by increased levels of histone 3 acetylation. Using recombinant adeno-associated virus-mediated expression of a nuclear localized dominant active mutant of HDAC4 in dorsal horn neurons, we demonstrated that nuclear HDAC4 blunts the development of mechanical hypersensitivity without affecting acute nociception. Next generation RNA-sequencing analysis produced a list of HDAC4-regulated candidate genes in the context of chronic inflammatory pain. The identified candidates include both well-known and novel mediators of chronic pain development and have been functionally tested in vivo with gain of function and loss of function experiments. Our results identify HDAC4 and its target genes, as key epigenetic regulators of central sensitization in chronic inflammatory pain and as possible targets for pain therapies