Live imaging of mitochondrial dynamics in CNS dopaminergic neurons in vivo demonstrates early reversal of mitochondrial transport following MPP+ exposure

• We generated transgenic zebrafish expressing florescent proteins in mitochondria.
• Mitochondrial transport was imaged in the axons of dopaminergic neurons in vivo.
• Developmental changes in transport correlated with axonal growth and synaptogenesis.
• MPP+ exposure caused a profound increase in retrograde transport.
• These changes in transport preceded all other signs of pathogenesis.

Extensive convergent evidence collectively suggests that mitochondrial dysfunction is central to the pathogenesis of Parkinson's disease (PD). Recently, changes in the dynamic properties of mitochondria have been increasingly implicated as a key proximate mechanism underlying neurodegeneration. However, studies have been limited by the lack of a model in which mitochondria can be imaged directly and dynamically in dopaminergic neurons of the intact vertebrate CNS. We generated transgenic zebrafish in which mitochondria of dopaminergic neurons are labeled with a fluorescent reporter, and optimized methods allowing direct intravital imaging of CNS dopaminergic axons and measurement of mitochondrial transport in vivo. The proportion of mitochondria undergoing axonal transport in dopaminergic neurons decreased overall during development between 2 days post-fertilization (dpf) and 5 dpf, at which point the major period of growth and synaptogenesis of the relevant axonal projections is complete. Exposure to 0.5–1.0 mM MPP+ between 4 and 5 dpf did not compromise zebrafish viability or cause detectable changes in the number or morphology of dopaminergic neurons, motor function or monoaminergic neurochemistry. However, 0.5 mM MPP+ caused a 300% increase in retrograde mitochondrial transport and a 30% decrease in anterograde transport. In contrast, exposure to higher concentrations of MPP+ caused an overall reduction in mitochondrial transport. This is the first time mitochondrial transport has been observed directly in CNS dopaminergic neurons of a living vertebrate and quantified in a PD model in vivo. Our findings are compatible with a model in which damage at presynaptic dopaminergic terminals causes an early compensatory increase in retrograde transport of compromised mitochondria for degradation in the cell body. These data are important because manipulation of early pathogenic mechanisms might be a valid therapeutic approach to PD. The novel transgenic lines and methods we developed will be useful for future studies on mitochondrial dynamics in health and disease.