A dual leucine kinase–dependent axon self-destruction program promotes Wallerian degeneration

Abstract

Axon degeneration underlies many common neurological disorders, but the signaling pathways that orchestrate axon degeneration are unknown. We demonstrate that the dual leucine kinase (DLK) promotes degeneration of severed axons in Drosophila and mice, and its target JNK promotes degeneration locally in axons as they commit to degenerate. This pathway also promotes degeneration after chemotherapy exposure, and thus may be a component of a general axon self-destruction program.Axons degenerate in a range of neurological disorders including mechanical injury, chemotherapy-induced neuropathy, hereditary neuropathies, glaucoma, diabetes, and neurodegenerative diseases such as Alzheimer's Disease and Parkinson's Disease 1 . Degenerating axons follow a stereotyped pathological progression that was first described in the 1850's for the breakdown of the distal segments of severed axons and termed Wallerian degeneration 2 . This degeneration likely results from an active self-destruction program rather than passive deterioration 2 since it is delayed by over-expression of the chimeric Wallerian degeneration slow 3 (Wld s ) protein and its component nicotinamide mononucleotide adenylyltransferase 4, 5 , by preventing Ca 2+ influx 6 , by blocking protein degradation 7 , and by disrupting the phagocytic clearance of axon fragments 8 . A variety of insults trigger axon degeneration 2, 9 , so identifying the components of the hypothesized axon self-destruction program could have broad clinical value. To date, however, no loss-of-function mutations have been identified that disrupt the internal axon breakdown machinery, and the internal signaling pathways that orchestrate axon breakdown in injury and disease remain unknown.Candidate components of axon breakdown pathways should be present in axons and activated by diverse cellular insults. One such candidate is DLK, a mitogen-activated protein kinase kinase kinase (MAP3K) 10 . One of DLK's downstream targets, the mitogen-activated protein kinase (MAPK) c-Jun N-terminal kinase (JNK), is activated following axonal injury 11 . We tested the hypothesis that DLK promotes axon degeneration using a well-established Drosophila olfactory receptor neuron (ORN) axotomy model 12, 8 . We expressed green fluorescent protein (GFP) in ORNs to visualize their axons, which extend from cell bodies in the antennae into the antennal lobes of the brain and across a midline commissure (Fig. 1a). To severe ORN axons and induce degeneration, we removed the antennae from wildtype flies and mutants lacking the Drosophila ortholog of DLK, wallenda (wnd) 13 . Most wildtype axons (Fig. 1b), while wnd mutant axons were significantly preserved (Fig. 1c). Wnd is therefore required for normal axon degeneration in Drosophila.Wnd could act within neurons to promote breakdown after injury or within surrounding cells to promote axon clearance. To distinguish between these possibilities, we expressed Wnd in the GFP-expressing subpopulation of ORNs in wnd mutant flies. Such Wnd expression was not sufficient t...

Dual leucine zipper kinase is required for excitotoxicity induced neuronal degeneration

“…This represents a departure from previous studies that have demonstrated neuroprotective phenotypes resulting from DLK deletion, all of which are dependent on insults that originate in the axon (Chen et al, 2008;Miller et al, 2009;Ghosh et al, 2011;Shin et al, 2012). The divergent DLK localization and function described here argues that DLK may regulate neuronal degeneration in contexts such as cerebral ischemia, where excitotoxicity is a predominant driver of neuronal cell death.…”

Functional genomic screening identifies dual leucine zipper kinase as a key mediator of retinal ganglion cell death

“…Moreover we demonstrate, using a conditional knockout approach, that inhibition of DLK signaling can promote RGC survival in vivo and identify a small molecule kinase inhibitor that protects RGC somata and axons in a rodent model of glaucoma. Our findings, which implicate DLK as being an important player in pathologic neuronal cell death, fit in well with the growing literature demonstrating DLK's role as a key signaling molecule in synapse formation, neuronal development, axonal injury and regeneration, and developmental neuronal degeneration (16)(17)(18)(19)(20)(21)(22)(23).…”

“…DLK has been shown to mediate developmental apoptosis in peripheral motor and sensory neurons (17,18). In adult peripheral neurons, it has been implicated as an important mediator of distal axonal degeneration (22) and proximal axonal regeneration (16,19,20) after axonal injury. It is required for the retrograde transmission of injury mediators such as the JNK scaffold protein JNK interacting protein 3 (JIP3) and phosphorylated STAT3 and plays a role in the induction of expression of proregenerative genes (21).…”

DLK initiates a transcriptional program that couples apoptotic and regenerative responses to axonal injury

“…DLK protein is present in axons, and protein levels are increased in response to axonal injury (5). Loss of DLK has been shown to protect distal axons from Wallerian degeneration (6) and to abrogate stress-induced retrograde c-Jun N-terminal kinase (JNK) signaling through interaction with the scaffolding protein JNK-interacting protein 3 (JIP3) (7)(8)(9). In many instances, injury-induced JNK activation in neurons results in apoptosis through phosphorylation of activator protein 1 (AP-1) transcription factors such as c-Jun, which initiates a proapoptotic gene expression program (10,11).…”