Targeting neuronal NAD production through NMNAT2 for neuroprotection

Nicotinamide adenine dinucleotide (NAD) is a redox cofactor and a coenzyme essential for axonal health and neuronal survival. Increasing NAD levels has been suggested to be a promising approach to target neuroprotection in neurodegenerative diseases.

In paper I, we demonstrate that NAD levels decrease in the retina and the optic nerve in a microbead occlusion model in rats with ocular hypertension (OHT). We also demonstrate that nicotinamide (NAM) can rapidly increase NAD levels in neuronal tissues (brain cortex, optic nerve, and retina) in a time-dependent manner.

Alternative approaches to increase NAD levels are to target enzymes involved in the synthesis of NAD. Nicotinamide mononucleotide adenylyl transferase 2 (NMNAT2) is a neuron-specific NAD synthase, which has been implied to be activated by the green tea polyphenol epigallocatechin gallate (EGCG).

In paper II, we study the effects of EGCG on NAD levels and NMNAT2. We demonstrate that EGCG increases NAD levels in a dose- and time-dependent manner. However, due to EGCG's poor stability and bioavailability, we use it as a tool compound to generate more stable analogues to study the structure-activity relationship (SAR) between the compounds and their NAD-elevating effects. We generate 56 compounds and test their NAD-elevating effects and specificity towards neurons and the NAD salvage pathway. We identify multiple compounds that potently increase NAD levels and are neuroprotective in an ex vivo retinal explant model.

Tetrahydroquinoxaline was the most potent core structure identified in paper II. Based on this, we generate tetrahydroquinoxaline derivatives to further study the SAR in paper III. We identify critical moieties for the NAD-boosting effect and assess whether the compounds are dependent on the salvage pathway to produce NAD.

Our findings suggest that several of these novel compounds have significant potential as neuroprotective agents, providing valuable insights for further drug development.

List of scientific papers

I. Tribble JR, Otmani A, Sun S, Ellis SA, Cimaglia G, Vohra R, Jöe M, Lardner E, Venkataraman AP, Domínguez-Vicent A, Kokkali E, Rho S, Jóhannesson G, Burgess RW, Fuerst PG, Brautaset R, Kolko M, Morgan JE, Crowston JG, Votruba M, Williams PA. Nicotinamide provides neuroprotection in glaucoma by protecting against mitochondrial and metabolic dysfunction. Redox Biol. 2021;43:101988. https://doi.org/10.1016/j.redox.2021.101988

II. Tribble JR*, Jöe M*, Varricchio C, Otmani A, Canovai A, Habchi B, Daskalakis E, Chaleckis R, Loreto A, Gilley J, Wheelock CE, Johannesson G, Wong RCB, Coleman MP, Brancale A, Williams PA. NMNAT2 is a druggable target to drive neuronal NAD production. Nature Communications. 2024;15(1):6256. *Co-first authors. https://doi.org/10.1038/s41467-024-50354-5

III. Jöe M, Varricchio C, Cuřínová P, Nicol A, Saleh A, Wheelock CE, Johannesson G, Tribble JR, Brancale A, Williams PA. Novel tetrahydroquinoxaline derivatives increase NAD through the NAD salvage pathway. [Manuscript]