Traumatic axonopathy in spinal tracts after impact acceleration head injury: Ultrastructural observations and evidence of SARM1-dependent axonal degeneration

White matter damage and degeneration in traumatic brain injury

Traumatic brain injury (TBI) is a complex condition that can resolve over time but all too often leads to persistent symptoms, and the risk of poor patient outcomes increases with aging. TBI damages neurons and long axons within white matter tracts that are critical for communication between brain regions; this causes slowed information processing and neuronal circuit dysfunction. This review focuses on white matter injury after TBI and the multifactorial processes that underlie white matter damage, potential for recovery, and progression of degeneration. A multiscale perspective across clinical and preclinical advances is presented to encourage interdisciplinary insights from whole-brain neuroimaging of white matter tracts down to cellular and molecular responses of axons, myelin, and glial cells within white matter tissue.

Programmed axon death: a promising target for treating retinal and optic nerve disorders

Programmed axon death is a druggable pathway of axon degeneration that has garnered considerable interest from pharmaceutical companies as a promising therapeutic target for various neurodegenerative disorders. In this review, we highlight mechanisms through which this pathway is activated in the retina and optic nerve, and discuss its potential significance for developing therapies for eye disorders and beyond. At the core of programmed axon death are two enzymes, NMNAT2 and SARM1, with pivotal roles in NAD metabolism. Extensive preclinical data in disease models consistently demonstrate remarkable, and in some instances, complete and enduring neuroprotection when this mechanism is targeted. Findings from animal studies are now being substantiated by genetic human data, propelling the field rapidly toward clinical translation. As we approach the clinical phase, the selection of suitable disorders for initial clinical trials targeting programmed axon death becomes crucial for their success. We delve into the multifaceted roles of programmed axon death and NAD metabolism in retinal and optic nerve disorders. We discuss the role of SARM1 beyond axon degeneration, including its potential involvement in neuronal soma death and photoreceptor degeneration. We also discuss genetic human data and environmental triggers of programmed axon death. Lastly, we touch upon potential therapeutic approaches targeting NMNATs and SARM1, as well as the nicotinamide trials for glaucoma. The extensive literature linking programmed axon death to eye disorders, along with the eye's suitability for drug delivery and visual assessments, makes retinal and optic nerve disorders strong contenders for early clinical trials targeting programmed axon death.

The transcriptional response of cortical neurons to concussion reveals divergent fates after injury

Traumatic brain injury (TBI) is a risk factor for neurodegeneration, however little is known about how different neuron types respond to this kind of injury. In this study, we follow neuronal populations over several months after a single mild TBI (mTBI) to assess long ranging consequences of injury at the level of single, transcriptionally defined neuronal classes. We find that the stress responsive Activating Transcription Factor 3 (ATF3) defines a population of cortical neurons after mTBI. We show that neurons that activate ATF3 upregulate stress-related genes while repressing many genes, including commonly used markers for these cell types. Using an inducible reporter linked to ATF3, we genetically mark damaged cells to track them over time. Notably, we find that a population in layer V undergoes cell death acutely after injury, while another in layer II/III survives long term and retains the ability to fire action potentials. To investigate the mechanism controlling layer V neuron death, we genetically silenced candidate stress response pathways. We found that the axon injury responsive kinase MAP3K12, also known as dual leucine zipper kinase (DLK), is required for the layer V neuron death. This work provides a rationale for targeting the DLK signaling pathway as a therapeutic intervention for traumatic brain injury. Beyond this, our novel approach to track neurons after a mild, subclinical injury can inform our understanding of neuronal susceptibility to repeated impacts.

Genetic ablation of Sarm1 attenuates expression and mislocalization of phosphorylated TDP-43 after mouse repetitive traumatic brain injury

Traumatic brain injury (TBI), particularly when moderate-to-severe and repetitive, is a strong environmental risk factor for several progressive neurodegenerative disorders. Mislocalization and deposition of transactive response DNA binding protein 43 (TDP-43) has been reported in both TBI and TBI-associated neurodegenerative diseases. It has been hypothesized that axonal pathology, an early event after TBI, may promote TDP-43 dysregulation and serve as a trigger for neurodegenerative processes. We sought to determine whether blocking the prodegenerative Sarm1 (sterile alpha and TIR motif containing 1) axon death pathway attenuates TDP-43 pathology after TBI. We subjected 111 male Sarm1 wild type, hemizygous, and knockout mice to moderate-to-severe repetitive TBI (rTBI) using a previously established injury paradigm. We conducted serial neurological assessments followed by histological analyses (NeuN, MBP, Iba-1, GFAP, pTDP-43, and AT8) at 1 month after rTBI. Genetic ablation of the Sarm1 gene attenuated the expression and mislocalization of phosphorylated TDP-43 (pTDP-43) and accumulation of pTau. In addition, Sarm1 knockout mice had significantly improved cortical neuronal and axonal integrity, functional deficits, and improved overall survival after rTBI. In contrast, removal of one Sarm1 allele delayed, but did not prevent, neurological deficits and neuroaxonal loss. Nevertheless, Sarm1 haploinsufficient mice showed significantly less microgliosis, pTDP-43 pathology, and pTau accumulation when compared to wild type mice. These data indicate that the Sarm1-mediated prodegenerative pathway contributes to pathogenesis in rTBI including the pathological accumulation of pTDP-43. This suggests that anti-Sarm1 therapeutics are a viable approach for preserving neurological function after moderate-to-severe rTBI.

NAD+, Axonal Maintenance, and Neurological Disease

Significance: The remarkable geometry of the axon exposes it to unique challenges for survival and maintenance. Axonal degeneration is a feature of peripheral neuropathies, glaucoma, and traumatic brain injury, and an early event in neurodegenerative diseases. Since the discovery of Wallerian degeneration (WD), a molecular program that hijacks nicotinamide adenine dinucleotide (NAD+) metabolism for axonal self-destruction, the complex roles of NAD+ in axonal viability and disease have become research priority. Recent Advances: The discoveries of the protective Wallerian degeneration slow (WldS) and of sterile alpha and TIR motif containing 1 (SARM1) activation as the main instructive signal for WD have shed new light on the regulatory role of NAD+ in axonal degeneration in a growing number of neurological diseases. SARM1 has been characterized as a NAD+ hydrolase and sensor of NAD+ metabolism. The discovery of regulators of nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) proteostasis in axons, the allosteric regulation of SARM1 by NAD+ and NMN, and the existence of clinically relevant windows of action of these signals has opened new opportunities for therapeutic interventions, including SARM1 inhibitors and modulators of NAD+ metabolism. Critical Issues: Events upstream and downstream of SARM1 remain unclear. Furthermore, manipulating NAD+ metabolism, an overdetermined process crucial in cell survival, for preventing the degeneration of the injured axon may be difficult and potentially toxic. Future Directions: There is a need for clarification of the distinct roles of NAD+ metabolism in axonal maintenance as contrasted to WD. There is also a need to better understand the role of NAD+ metabolism in axonal endangerment in neuropathies, diseases of the white matter, and the early stages of neurodegenerative diseases of the central nervous system. Antioxid. Redox Signal. 39, 1167-1184.

Mouse models of human CNTNAP1-associated congenital hypomyelinating neuropathy and genetic restoration of murine neurological deficits

The Contactin-associated protein 1 (Cntnap1) mouse mutants fail to establish proper axonal domains in myelinated axons. Human CNTNAP1 mutations are linked to hypomyelinating neuropathy-3, which causes severe neurological deficits. To understand the human neuropathology and to model human CNTNAP1C323R and CNTNAP1R764C mutations, we generated Cntnap1C324R and Cntnap1R765C mouse mutants, respectively. Both Cntnap1 mutants show weight loss, reduced nerve conduction, and progressive motor dysfunction. The paranodal ultrastructure shows everted myelin loops and the absence of axo-glial junctions. Biochemical analysis reveals that these Cntnap1 mutant proteins are nearly undetectable in the paranodes, have reduced surface expression and stability, and are retained in the neuronal soma. Postnatal transgenic expression of Cntnap1 in the mutant backgrounds rescues the phenotypes and restores the organization of axonal domains with improved motor function. This study uncovers the mechanistic impact of two human CNTNAP1 mutations in a mouse model and provides proof of concept for gene therapy for CNTNAP1 patients.

Application of P7C3 Compounds to Investigating and Treating Acute and Chronic Traumatic Brain Injury

Traumatic brain injury (TBI) is a leading worldwide cause of disability, and there are currently no medicines that prevent, reduce, or reverse acute or chronic neurodegeneration in TBI patients. Here, we review the target-agnostic discovery of nicotinamide adenine dinucleotide (NAD+)/NADH-stabilizing P7C3 compounds through a phenotypic screen in mice and describe how P7C3 compounds have been applied to advance understanding of the pathophysiology and potential treatment of TBI. We summarize how P7C3 compounds have been shown across multiple laboratories to mitigate disease progression safely and effectively in a broad range of preclinical models of disease related to impaired NAD+/NADH metabolism, including acute and chronic TBI, and note the reported safety and neuroprotective efficacy of P7C3 compounds in nonhuman primates. We also describe how P7C3 compounds facilitated the recent first demonstration that chronic neurodegeneration 1 year after TBI in mice, the equivalent of many decades in people, can be reversed to restore normal neuropsychiatric function. We additionally review how P7C3 compounds have facilitated discovery of new pathophysiologic mechanisms of neurodegeneration after TBI. This includes the role of rapid TBI-induced tau acetylation that drives axonal degeneration, and the discovery of brain-derived acetylated tau as the first blood-based biomarker of neurodegeneration after TBI that directly correlates with the abundance of a therapeutic target in the brain. We additionally review the identification of TBI-induced tau acetylation as a potential mechanistic link between TBI and increased risk of Alzheimer's disease. Lastly, we summarize historical accounts of other successful phenotypic-based drug discoveries that advanced medical care without prior recognition of the specific molecular target needed to achieve the desired therapeutic effect.