WldS and PGC-1α Regulate Mitochondrial Transport and Oxidation State after Axonal Injury

Abstract

Mitochondria carry out many of the processes implicated in maintaining axon health or causing axon degeneration, including ATP and reactive oxygen species (ROS) generation, as well as calcium buffering and protease activation. Defects in mitochondrial function and transport are common in axon degeneration, but how changes in specific mitochondrial properties relate to degeneration is not well understood. Using cutaneous sensory neurons of living larval zebrafish as a model, we examined the role of mitochondria in axon degeneration by monitoring mitochondrial morphology, transport, and redox state before and after laser axotomy. Mitochondrial transport terminated locally after injury in wild-type axons, an effect that was moderately attenuated by expressing the axon-protective fusion protein Wallerian degeneration slow (WldS). However, mitochondrial transport arrest eventually occurred in WldS-protected axons, indicating that later in the lag phase, mitochondrial transport is not required for axon protection. By contrast, the redox-sensitive biosensor roGFP2 was rapidly oxidized in the mitochondrial matrix after injury, and WldS expression prevented this effect, suggesting that stabilization of ROS production may mediate axon protection. Overexpression of PGC-1␣, a transcriptional coactivator with roles in both mitochondrial biogenesis and ROS detoxification, dramatically increased mitochondrial density, attenuated roGFP2 oxidation, and delayed Wallerian degeneration. Collectively, these results indicate that mitochondrial oxidation state is a more reliable indicator of axon vulnerability to degeneration than mitochondrial motility.

Mitostasis in Neurons: Maintaining Mitochondria in an Extended Cellular Architecture

“…Thus, how neuronal mitochondria behave in vivo has also required attention. The last decade has seen substantial progress to address this question, based on experimental paradigms that allow monitoring mitochondrial dynamics in vivo in the peripheral and central nervous system of a wide range of species including nematodes (Fatouros et al, 2012; Rawson et al, 2014; Williams et al, 2013), fruit flies (Babic et al, 2015; Pilling et al, 2006; Vagnoni and Bullock, 2016; Wang and Schwarz, 2009a), zebrafish (O’Donnell et al, 2013; Plucinska et al, 2012), and mice (Chandrasekaran et al, 2006; Misgeld et al, 2007). Reassuringly, the basic tenets of mitochondrial dynamics derived from in vitro work have been confirmed in vivo.…”

Redox imaging using genetically encoded redox indicators in zebrafish and mice

“…For mosaic sensor expression, the method of choice is injection of DNA. Given that DNA molecules are distributed randomly within the cells of a developing embryo, only individual cells will express the transgene (O'Donnell et al, 2013). Meanwhile, a few stable transgenic strains were established, which allow redox imaging at later stages and even in adults (Table 2).…”

“…O'Donnell et al applied roGFP2 targeted to the mitochondrial matrix to measure the role of mitochondrial ROS production upon axonal injury. Their observations elegantly demonstrated not only changes in the oxidation state of axonal mitochondria upon injury, but also that effects on mitochondrial oxidation state were functionally relevant to axon degeneration and protection (O'Donnell et al, 2013). One study measured epithelial ROS production using a newly generated transgenic line expressing the enzyme-coupled roGFP2 sensor Grx1-roGFP2 under the control of a β-actin promoter (Seiler et al, 2012).…”

Signaling mechanisms regulating Wallerian degeneration

“…Neuronal expression of Wld S is sufficient to suppress the granular disintegration of both motor and sensory axons, and the axons of multiple types of CNS neurons [4]. Somewhat surprisingly, expression of mouse Wld S was also shown to robustly suppress Wallerian degeneration in the fruit fly Drosophila [7•], and more recently in zebrafish [8], indicating the mechanistic action of Wld S axon protection is evolutionarily conserved.…”

“…A systematic use of biomarkers for very specific signaling events (e.g. Ca 2+ signaling, or reactive oxygen species production) [8,19] or changes in cell biology (e.g. axon trafficking, cytoskeletal breakdown) [22] should go a long way toward determining how distinct genetic pathways might drive the step-wise disassembly of axons in different contexts, and where any genetic or chemical manipulation impinges on these processes.…”