Mechanisms of Guided Axon Repair and Molecular Damage in Neurons

Abstract

The nervous system processes information through complex networks of neurons and damage to these cells can be enormously debilitating. Morphologically, neurons are connected together by biological wires called axons, which establish the fundamental topology of these networks. During development, this axonal connectivity is built through highly orchestrated processes of axon extension and guidance. In the human central nervous system, following injury, these axons are unable to precisely re-connect to their long-distance targets and overcoming these intrinsic limitations is necessary to re-establish functionality. Using zebrafish motor neurons engineered to express a photoactivatable Rac1 protein as a model of axon guidance, I show that the endogenous guidance machinery of the growth cone can be co-opted to precisely and noninvasively direct axon growth trajectories using light. Axons can be extended directionally, over large distances, and within the complex environment of the whole, intact organism. Notably, I find that competing endogenous signals can be overridden to redirect axon growth across typically repulsive barriers and construct novel circuitry. In addition, optogenetic Rac1 stimulation can rescue genetic axon guidance defects in mutant zebrafish. These data demonstrate that a ubiquitous central component of the intrinsic growth cone guidance machinery can be co-opted to non-invasively shape connectivity within neuronal networks in living intact organisms. In addition to axonal injury, neurons can suffer molecular damage to their fundamental genomic blueprints. Chromosomal loss of small DNA fragments can result in the formation of extrachromosomal circular DNAs (eccDNAs), which are present even in embryonic brains. To investigate eccDNA formation in neurons, eccDNAs were sequenced from two subtypes of murine cortical neurons. These eccDNAs arose stochastically, but were enriched in dynamically regulated, gene coding regions and in activity-dependent genes. These results suggest that eccDNA formation may reflect a novel mechanism of DNA damage, with important consequences for neuronal function, aging, and disease. Overall, damage to neurons can have devastating consequences and as our understanding of these cells deepens, new therapeutic strategies will be needed to repair this damage.