Traumatic Axonal Injury in the Optic Nerve: The Selective Role of SARM1 in the Evolution of Distal Axonopathy

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.

Loss of Sarm1 reduces retinal ganglion cell loss in chronic glaucoma

Glaucoma is one of the leading causes of irreversible blindness worldwide and vision loss in the disease results from the deterioration of retinal ganglion cells (RGC) and their axons. Metabolic dysfunction of RGC plays a significant role in the onset and progression of the disease in both human patients and rodent models, highlighting the need to better define the mechanisms regulating cellular energy metabolism in glaucoma. This study sought to determine if Sarm1, a gene involved in axonal degeneration and NAD+ metabolism, contributes to glaucomatous RGC loss in a mouse model with chronic elevated intraocular pressure (IOP). Our data demonstrate that after 16 weeks of elevated IOP, Sarm1 knockout (KO) mice retain significantly more RGC than control animals. Sarm1 KO mice also performed significantly better when compared to control mice during optomotor testing, indicating that visual function is preserved in this group. Our findings also indicate that Sarm1 KO mice display mild ocular developmental abnormalities, including reduced optic nerve axon diameter and lower visual acuity than controls. Finally, we present data to indicate that SARM1 expression in the optic nerve is most prominently associated with oligodendrocytes. Taken together, these data suggest that attenuating Sarm1 activity through gene therapy, pharmacologic inhibition, or NAD+ supplementation, may be a novel therapeutic approach for patients with glaucoma.

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.