Age-Dependent TDP-43-Mediated Motor Neuron Degeneration Requires GSK3, hat-trick, and xmas-2

Abstract

Summary The RNA processing protein TDP-43 is central to the pathogenesis of amyotrophic lateral sclerosis (ALS), the most common adult-onset motor neuron (MN) disease [1–4]. TDP-43 is conserved in Drosophila, where it has been the topic of considerable study, but how TDP-43 mutations lead to age-dependent neurodegeneration is unclear and most approaches have not directly examined changes in MN morphology with age [5]. We used a mosaic approach to study age-dependent MN loss in the adult fly leg where it is possible to resolve single motor axons, NMJs and active zones, and perform rapid forward genetic screens. We show that expression of TDP-43Q331K caused dying-back of NMJs and axons, which could not be suppressed by mutations that block Wallerian degeneration. We report the identification of three genes that suppress TDP-43 toxicity, including Shaggy/GSK3, a known modifier of neurodegeneration [6]. The two additional novel suppressors, Hat-trick and Xmas-2, function in chromatin modeling and RNA export, two processes recently implicated in human ALS [7, 8]. Loss of Shaggy/GSK3, Hat-trick or Xmas-2 does not suppress Wallerian degeneration, arguing TDP-43Q331K-induced and Wallerian degeneration are genetically distinct processes. In addition to delineating genetic factors that modify TDP-43 toxicity, these results establish the Drosophila adult leg as a valuable new tool for the in vivo study of adult MN phenotypes.

Neurodegeneration: A Leg Up on TDP-43

“…Such targets are sorely needed, as there are currently no therapeutic options to halt or significantly slow ALS disease progression. A new study by Sreedharan et al [4] reported in this issue of Current Biology has described an improved system for modeling age-dependent TDP-43 motor neuron toxicity in Drosophila. Restricted expression of ALS-linked mutant TDP-43 in single neurons of the leg in an otherwise normal fly results in a 'dying back' of axons similar to that observed in ALS patients.…”

“…This gross reduction in lifespan has made it difficult to study genes that can modulate the changes in synaptic morphology and function associated with TDP-43 overexpression in adult flies. In this issue, Sreedharan et al describe a new fly model of TDP-43 toxicity that resolves some of the limitations of previous models [4]. Their model is novel in its ability to allow for single motor neurons expressing TDP-43 to be monitored morphologically for degeneration along with their accompanying neuromuscular junctions over several weeks in an otherwise healthy fly (Figure 1).…”

“…This allowed them to discriminate the morphology of single axons in the fly wing in an assay for injuryinduced axonal degeneration [10]. Here, they report a similar MARCM approach that allows for the expression of human TDP-43 in only a few motor or sensory neurons in the fly leg [4]. Axons and neuromuscular junctions can also be marked by expression of membranetethered CD8-GFP, which was used to score TDP-43-associated neurodegeneration.…”

“…Sreedharan et al found that expression of both wild-type TDP-43 and the ALSlinked mutant protein Q331K was particularly toxic to motor neurons over four weeks, with disruption of synaptic morphology and loss of axonal integrity in a 'dying back' fashion [4]. Directing expression of their transgene to motor neurons of the fly leg along with the membrane-tethered GFP facilitated microscopic preparation and analysis and the ability to rapidly perform a mutagenesis screen to identify genetic modifiers of TDP-43 toxicity.…”

“…First, they used genetic manipulations to demonstrate that TDP-43 motor neuron degeneration is distinct from Sarm1-mediated active Wallerian degeneration [4], a process that may occur in ALS patient neurons [11]. Interestingly, this finding appears to be in contrast with a recent report in Caenorhabditis elegans describing that TIR-1, the nematode Sarm1 ortholog, acts downstream of TDP-43 toxicity in neurons (although this may not be a cell autonomous effect) [12], and the finding that fly Wallenda, a potential Wallerian degeneration regulator [13], can modify TDP-43 toxicity [8].…”