SARM1 Suppresses Axon Branching Through Attenuation of Axonal Cytoskeletal Dynamics

Abstract

Axon branching is a fundamental aspect of neuronal morphogenesis, neuronal circuit formation, and response of the nervous system to injury. Sterile alpha and TIR motif containing 1 (SARM1) was initially identified as promoting Wallerian degeneration of axons. We now report a novel function of SARM1 in postnatal sensory neurons; the suppression of axon branching. Axon collateral branches develop from axonal filopodia precursors through the coordination of the actin and microtubule cytoskeleton. In vitro analysis revealed that cultured P0-2 dorsal root ganglion sensory neurons from a SARM1 knockout (KO) mouse exhibit increased numbers of collateral branches and axonal filopodia relative to wild-type neurons. In SARM1 KO mice, cutaneous sensory endings exhibit increased branching in the skin in vivo with normal density of innervation. Transient axonal actin patches serve as cytoskeletal platforms from which axonal filopodia emerge. Live imaging analysis of axonal actin dynamics showed that SARM1 KO neurons exhibit increased rates of axonal actin patch formation and increased probability that individual patches will give rise to a filopodium before dissipating. SARM1 KO axons contain elevated levels of drebrin and cortactin, two actin regulatory proteins that are positive regulators of actin patches, filopodia formation, and branching. Live imaging of microtubule plus tip dynamics revealed an increase in the rate of formation and velocity of polymerizing tips along the axons of SARM1 KO neurons. Stationary mitochondria define sites along the axon where branches may arise, and the axons of SARM1 KO sensory neurons exhibit an increase in stationary mitochondria. These data reveal SARM1 to be a negative regulator of axonal cytoskeletal dynamics and collateral branching.

NMNAT2 supports vesicular glycolysis via NAD homeostasis to fuel fast axonal transport

“…It remains unknown why axonal extension is insensitive to NMNAT2 loss before P5/DIV8, when glycolytic ATP supply is already needed [ 105 ]. Similarly, eliminating the NAD + consumer, SARM1, in embryonic sensory neurons does not affect axonal branching, while its postnatal removal promotes axonal branching [ 106 ]. We speculate that other mechanisms, such as the newly discovered Wnk kinases [ 107 ], exist in the early developmental stage to ensure NAD homeostatic balance in the absence of NMNAT2, allowing a sufficient level of glycolytic flux to fuel cargo transport, cytoskeleton dynamics, and axon outgrowth.…”

Sensory Schwann cells set perceptual thresholds for touch and selectively regulate mechanical nociception

“…Sections were then washed 3 times with PBS and mounted using fluorescent mounting medium for imaging (Dako, #S3023). The anti-PGP9.5 antibody was verified by relative expression and has been validated in various studies 52 – 54 . The S100β antibody has been validated in many studies including 19 , 21 .…”

Ubiquitin Proteasome System and Microtubules Are Master Regulators of Central and Peripheral Nervous System Axon Degeneration

“…Other studies support the hypothesis of a shared mechanism between WD and CIPN, since both vincristine (VCR) and bortezomib (BTZ) activate a common SARM1-dependent axon degeneration pathway [73]. Interestingly, a recent paper proposed a novel function for SARM1 in sensory neurons, as a negative regulator of axonal cytoskeleton dynamics and collateral branching; indeed, SARM1 depletion leads to increase of MT polymerization dynamics in cultured neurons and to an increased branching of cutaneous sensory innervation in skin of SARM1 ko mice [74]. Modifications of cytoskeletal proteins are described in several peripheral neuropathies and they might interfere with cytoskeleton assembly [75] and thus axonal transport [76].…”