Regeneration of dopaminergic neurons and other neuronal cell types in zebrafish

Abstract

Unlike mammals, zebrafish have a remarkable capacity to regenerate the central nervous system. Following neuronal loss by physical injury or chemical ablation zebrafish are capable of replacing neurons through increased neurogenesis, resulting in functional recovery. In the adult zebrafish brain, certain populations of dopaminergic and noradrenergic neurons, identified by immunohistochemistry for tyrosine hydroxylase (Th+), are regenerated after ablation with 6-hydroxydopamine (6OHDA), an analogue of dopamine commonly used to specifically ablate these neurons. Here I ask where these newly formed Th+ neurons originate and which signals are involved in their regeneration. In the adult zebrafish new neurons are derived from progenitor cells, the soma of which form part of the ependyma and which have radial processes extending to the pial surface, termed ependymo-radial glial cells (ERGs). In this thesis I show that ERGs lining the diencephalic ventricle are a heterogeneous population in terms of expression of her4, gfap, and olig2. Using genetic lineage tracing and proliferation analysis I demonstrate that regenerated Th+ neurons are derived from specific ERGs at the diencephalic ventricle. In contrast to mammals, Th+ neurons are constantly generated in the adult zebrafish brain. Here I show that injection of 6OHDA elicits an immune response, and that inhibiting this immune response with the artificial glucocorticoid dexamethasone attenuates proliferation of ERGs and neurogenesis of Th+ neurons to control levels. Although stimulating an immune response increases proliferation of ERGs, an immune response is not sufficient to increase Th+ neurogenesis. This demonstrates that an immune response is necessary but not sufficient for the regeneration of Th+ neurons in the adult zebrafish brain. Following a spinal cord lesion, both larvae and adult zebrafish are capable of functional regeneration. Spinal cord regeneration has been shown to involve increased neurogenesis of motor neurons; however, the extent to which other neuronal populations are regenerated has not been fully elucidated. Here I show that glutamatergic neurons are regenerated after a spinal cord lesion in larvae, and GABAergic neurons are regenerated in both larvae and adults. Taken together, these results provide new insights into the regeneration of the central nervous system in zebrafish. I identify populations of neurons which are regenerated, progenitor cells that give rise to regenerated neurons, and I demonstrate the pivotal role of the immune response in modulating regeneration. These results could ultimately inform future attempts to promote neuroregeneration in mammals.