Temporal changes in key developmental transcription factors in dopamine neurons during MPP+ induced injury and recovery in zebrafish brain

Abstract

Poor regeneration of functional dopaminergic neurons (DA) is one of the most common healthcare challenges for Parkinson’s Disease (PD) patients. In contrast to mammals, zebrafish has an amazing potential to repair their dopaminergic neurons (DA) after injury. However, the molecular mechanisms that regulate these reparative events remain to be determined. To address this, we compared the temporal changes in key transcription factors (TFs), which regulate the developmental trajectories of DA neurons during injury and regeneration in zebrafish. MPP⁺ was exposed to zebrafish embryos between 18 Hpf and 96 Hpf to create a model of DA injury and regeneration after injury (1 day, 2 days, 3 days, 4 days, 5 days, 6 days, 7 days and 87 days post DA injury). During time series of MPP⁺ exposure, we found temporal alterations expression patterns of TFs; Nurr1, Foxa1, Lmx1a/b and En1/2 using WISH and RT-PCR. This turning point in expression dynamics coincided with a DA phenotypic turning point, as shown by 50% decline in TH⁺/DAT⁺ neurons and locomotor activity in the days following the MPP⁺ exposure. Using this model, we demonstrate for the first time that zebrafish are capable of regenerating a functional DA phenotype after 50% DA ablation. Following acute DA injury, mRNA levels of most TFs started to increase between 3-8 days after injury which was significantly elevated to normal levels in adult zebrafish brain i.e. at 87^th day after injury. Remarkably, the changes in mRNA expression of TFs temporally correlate with corresponding increase in TH/DAT expression and functional recovery. Taken together, this study showed a highly relevant role of TFs for dopamine producing neurons during regeneration of DA neurons following ablation with restoration of normal behavior. This study implies that TFs as potential therapeutic targets for enhancing regeneration of DA neurons in mammals observed over.