Transcription factor Pebbled/RREB1 regulates injury-induced axon degeneration

Jonathan E Farley,Thomas C Burdett,Romina Barria,Lukas J Neukomm,Kevin P Kenna,John E Landers,Marc R Freeman

doi:10.1073/pnas.1715837115

Abstract

Genetic studies of Wallerian degeneration have led to the identification of signaling molecules (e.g., dSarm/Sarm1, Axundead, and Highwire) that function locally in axons to drive degeneration. Here we identify a role for the Drosophila C2H2 zinc finger transcription factor Pebbled [Peb, Ras-responsive element binding protein 1 (RREB1) in mammals] in axon death. Loss of Peb in Drosophila glutamatergic sensory neurons results in either complete preservation of severed axons, or an axon death phenotype where axons fragment into large, continuous segments, rather than completely disintegrate. Peb is expressed in developing and mature sensory neurons, suggesting it is required to establish or maintain their competence to undergo axon death. peb mutant phenotypes can be rescued by human RREB1, and they exhibit dominant genetic interactions with dsarm mutants, linking peb/RREB1 to the axon death signaling cascade. Surprisingly, Peb is only able to fully block axon death signaling in glutamatergic, but not cholinergic sensory neurons, arguing for genetic diversity in axon death signaling programs in different neuronal subtypes. Our findings identify a transcription factor that regulates axon death signaling, and peb mutant phenotypes of partial fragmentation reveal a genetically accessible step in axon death signaling.

Highlights

Genetic studies of Wallerian degeneration have led to the identification of signaling molecules that function locally in axons to drive degeneration
We present the identification and characterization of a role for Pebbled (Peb), a transcription factor, in axon degeneration. peb mutants show two predominant axon deathdefective phenotypes: (i) severed distal axons are fully preserved morphologically, or (ii) the axon shaft breaks into large fragments that fail to disintegrate
Animals were aged for 7 d postaxotomy, after which wings were dissected from the animal and axon death was quantified and imaged in the proximal wing vein

Summary

Results

Isolation of a Mutant That Suppresses WD. To identify novel endogenous regulators of WD, we performed an X chromosomebased F2 forward genetic screen in glutamatergic sensory neurons in the adult Drosophila wing using mosaic analysis using a repressible cell marker (MARCM) [26]. To explore how Peb regulates axon death, we drove expression of cDNA constructs encoding full-length Peb, truncated Peb containing zinc fingers 1–8 or 10–14, and the human homolog, Rasresponsive element binding protein 1, (Peb, Peb−, Peb−, and hRREB1, respectively) in control and peb345x clones, and quantified axon degeneration 7 dpa (Fig. 3A). Expression of either Peb or hRREB1 in control animals led to the production of PFAs, which are only ever rarely observed in the earliest phase of normal axon degeneration (Fig. 3 B and C) These data suggest Peb expression levels may need to be fine-tuned for proper rescue of execution of axon death and destruction of PFAs, with too much or too little Peb resulting in PFA formation.

D Severed Intact

Discussion

Methods