Axonal degeneration is mediated by necroptosis activation

Abstract

Axonal degeneration contributes to functional impairment in several disorders of the nervous system, constituting an important target for neuroprotection. Several individual factors and subcellular events have been implicated in axonal degeneration, but the identification of an integrative signaling pathway activating this self-destructive process has remained elusive. Through pharmacological and genetic approaches, we tested whether necroptosis, a regulated cell death mechanism, implicated in the pathogenesis of several neurodegenerative diseases, is involved in axonal degeneration. Pharmacological inhibition of the necroptotic kinase RIPK1 using necrostatin-1 strongly delayed axonal degeneration in the peripheral and central nervous system of wild-type mice of either sex and protected in vitro sensory axons from degeneration after mechanical and toxic insults. These effects were also observed after genetic knock down of RIPK3, a second key regulator of necroptosis, and the downstream effector, MLKL. RIPK1 inhibition prevented mitochondrial fragmentation in vitro and in vivo, a typical feature of necrotic death, and inhibition of mitochondrial fission by Mdivi also resulted in reduced axonal loss in damaged nerves. Furthermore, electrophysiological analysis demonstrated that inhibition of necroptosis delays not only the morphological degeneration of axons but also the loss of their electrophysiological function after nerve injury. Activation of the necroptotic pathway early during injury-induced axonal degeneration was evidenced by increased phosphorylation of the downstream effector MLKL. Our results demonstrate that axonal degeneration proceeds by necroptosis, defining a novel mechanistic framework in the axonal degenerative cascade for therapeutic interventions in a wide variety of conditions that lead to neuronal loss and functional impairment.

SIGNIFICANCE STATEMENT

We show that axonal degeneration triggered by diverse stimuli is mediated by the activation of the necroptotic programmed cell death program by a cell-autonomous mechanism. We believe that this work represents a critical advance for the field since it identifies a defined degenerative pathway involved in axonal degeneration in both PNS and CNS, a process that has been proposed as an early event in several neurodegenerative conditions and a major contributor of neuronal death. The identification of necroptosis as a key mechanism for axonal degeneration, is an important step to develop novel therapeutic strategies for nervous system disorders, particularly those related to chemotherapy-induced peripheral neuropathies or CNS diseases in which axonal degeneration is a common factor.