Primary Traumatic Axonopathy in Mice Subjected to Impact Acceleration: A Reappraisal of Pathology and Mechanisms with High-Resolution Anatomical Methods

Abstract

Traumatic axonal injury (TAI) is a common neuropathology in traumatic brain injury and is featured by primary injury to axons. Here, we generated TAI with impact acceleration of the head in male transgenic mice in which specific populations of neurons and their axons are labeled with yellow fluorescent protein. This model results in axonal lesions in multiple axonal tracts along with blood-brain barrier disruption and neuroinflammation. The corticospinal tract, a prototypical long tract, is severely affected and is the focus of this study. Using optimized CLARITY at single-axon resolution, we visualized the entire corticospinal tract volume from the pons to the cervical spinal cord in 3D and counted the total number of axonal lesions and their progression over time. Our results divulged the presence of progressive traumatic axonopathy that was maximal at the pyramidal decussation. The perikarya of injured corticospinal neurons atrophied, but there was no evidence of neuronal cell death. We also used CLARITY at single-axon resolution to explore the role of the NMNAT2-SARM1 axonal self-destruction pathway in traumatic axonopathy. When we interfered with this pathway by genetically ablating SARM1 or by pharmacological strategies designed to increase levels of Nicotinamide (Nam), a feedback inhibitor of SARM1, we found a significant reduction in the number of axonal lesions early after injury. Our findings show that high-resolution neuroanatomical strategies reveal important features of TAI with biological implications, especially the progressive axonopathic nature of TAI and the role of the NMNAT2-SARM1 pathway in the early stages of axonopathy. In the first systematic application of novel high-resolution neuroanatomical tools in neuropathology, we combined CLARITY with 2-photon microscopy, optimized for detection of single axonal lesions, to reconstruct the injured mouse brainstem in a model of traumatic axonal injury (TAI) that is a common pathology associated with traumatic brain injury. The 3D reconstruction of the corticospinal tract at single-axon resolution allowed for a more advanced level of qualitative and quantitative understanding of TAI. Using this model, we showed that TAI is an axonopathy with a prominent role of the NMNAT2-SARM1 molecular pathway, that is also implicated in peripheral neuropathy. Our results indicate that high-resolution anatomical models of TAI afford a level of detail and sensitivity that is ideal for testing novel molecular and biomechanical hypotheses.

Cell Death and Neurodegeneration

“…In Wallerian degeneration paradigms, in which the axon is lesioned and disconnected from the cell body, sterile α and TIR motif-containing protein 1 (SARM1) is necessary and sufficient for axon degeneration, and axons of SARM1 knockout mice are protected in several in vivo neurodegeneration models (Osterloh et al 2012;Gerdts et al 2013Gerdts et al , 2016Henninger et al 2016;Turkiew et al 2017;Ziogas and Koliatsos 2018). Upon axonal damage or stress, SARM1, which harbors intrinsic NADase activity, becomes activated and hydrolyses NAD, resulting in reduction of axonal ATP levels and ultimately degradation of neurofilaments and axonal fragmentation via the calpain family of proteases (Gerdts et al 2013(Gerdts et al , 2016.…”

Systemic loss of Sarm1 protects Schwann cells from chemotoxicity by delaying axon degeneration

“…he peripheral nerves that communicate skin, muscle, and sensory organs with the brain must maintain functionality throughout life despite frequent stress and trauma [1][2][3][4][5][6] . Loss of integrity of peripheral neurons and associated cells, including glia, is a common occurrence in severe neurological dysfunctions that include weakness, pain, and loss of sensation 7 .…”

Palmitoylation enables MAPK-dependent proteostasis of axon survival factors

“…Genetic screens for modifiers of Wallerian degeneration identified the Toll/Interleukin-1 receptor domain-containing protein SARM1 as a central executioner of pathological axon degeneration (6,7). Genetic inactivation of SARM1 confers strong preservation after traumatic nerve injury, vincristine-induced peripheral neuropathy, oxidative stress, and other common neuronal insults, such that inhibiting SARM1 activity has extensive therapeutic potential (6)(7)(8)(9)(10)(11)(12).…”