Live Imaging Reveals the Cellular Events Downstream of SARM1 Activation

Abstract

SARM1 is an inducible NAD+ hydrolase that triggers axon loss and neuronal cell death in the injured and diseased nervous system. While SARM1 activation and enzyme function are well defined, the cellular events downstream of SARM1 activity but prior to axonal demise are much less well understood. Defects in calcium, mitochondria, ATP, and membrane homeostasis occur in injured axons, but the relationships among these events have been difficult to disentangle because prior studies analyzed large collections of axons in which cellular events occur asynchronously. Here we used live imaging with single axon resolution to investigate the cellular events downstream of SARM1 activity. Our studies support a model in which SARM1 NADase activity leads to an ordered sequence of events from loss of cellular ATP, to defects in mitochondrial movement and depolarization, followed by calcium influx, externalization of phosphatidylserine, and loss of membrane permeability prior to catastrophic axonal self destruction.

A SARM1-mitochondrial feedback loop drives neuropathogenesis in a Charcot-Marie-Tooth disease type 2A rat model

“…Brain lysates were prepared as described (9). Antibodies used: Rabbit anti-SARM1 (1:1000, 13022 Cell signaling), Mouse anti-β-actin (1:4000, A2228 Sigma Aldrich), HRP-conjugated anti-rabbit (1:5000, AB 2307391 Jackson Immuno Research), and HRP-conjugated anti-mouse (1:5000, 115-035-003 Jackson Immuno Research).…”

“…SARM1 is the central executioner of the programmed axon destruction pathway (6,7), and so is a candidate to mediate axon loss in CMT2A. SARM1 is an enzyme that, when activated, cleaves the essential metabolic co-factor NAD + (8), inducing a stereotyped local metabolic collapse with loss of ATP, defects in mitochondrial motility and depolarization, followed by calcium influx and ultimately axon fragmentation (9). Loss of SARM1 blocks axon degeneration in mouse models of axotomy (10,11), traumatic brain injury (12), and glaucoma (13).…”

Macrophage depletion blocks congenital SARM1-dependent neuropathy

“…Our data demonstrate that SARM1 activation is necessary but not sufficient for macrophage activation, as SARM1 activation in motor but not sensory axons induces macrophage activation. SARM1 activity drives calcium influx in damaged axons, which is followed by exposure of the phagocytic "eat-me" signal phosphatidylserine (PS) on the extracellular leaflet of the plasma membrane, and later, frank axon degeneration (82). SARM1 is required for PS exposure in response to axotomy-and vincristine-induced axon degeneration (83), and PS exposure is a driver of axon phagocytosis (84).…”

Disruption of lactate metabolism in the peripheral nervous system leads to motor-selective deficits

“…Conversely, when LDH is deleted in SCs, we see oxygen consumption increase and hypermyelination in the form of tomacula is detected at an early stage, with demyelination occurring subsequently. In addition to reduced lactate, nerves of LDH SCKO mice have reduced ATP, which may explain some of the axonal deficits and NMJ abnormalities 59 .…”