Mutant Nmnat1 leads to a retina-specific decrease of NAD+ accompanied by increased poly(ADP-ribose) in a mouse model of NMNAT1-associated retinal degeneration

Abstract

Nicotinamide mononucleotide adenylyltransferase 1 (NMNAT1) is required for nuclear NAD+ biosynthesis in all nucleated cells, and despite its functional ubiquity, mutations in this gene lead to an isolated retinal degeneration. The mechanisms underlying how mutant NMNAT1 causes disease are not well understood, nor is the reason why the pathology is confined to the retina. Using a mouse model of NMNAT1-associated retinal degeneration that harbors the p.Val9Met mutation, we tested the hypothesis that decreased function of mutant NMNAT1 has a greater effect on the levels of NAD+ in the retina than elsewhere in the body. Measurements by liquid chromatography with tandem mass spectrometry (LC–MS/MS) showed an early and sustained decrease of NAD+ in mutant retinas that was not observed in other tissues. To understand how consumers of nuclear NAD+ are affected by the reduced availability of NAD+ in mutant retinas, poly(ADP-ribose) polymerase (PARP) and nuclear sirtuin activity were evaluated. PARP activity was elevated during disease progression, as evidenced by overproduction of poly(ADP-ribose) (PAR) in photoreceptors, whereas histone deacetylation (HDAC) activity of nuclear sirtuins was not altered. We hypothesized that PARP could be activated because of elevated levels of oxidative stress; however, we did not observe oxidative DNA damage, lipid peroxidation, or a low glutathione (GSH) to oxidized glutathione (GSSG) ratio. TUNEL staining revealed that photoreceptors appear to ultimately die by apoptosis, although the low NAD+ levels and overproduction of PAR suggests that cell death may include aspects of the parthanatos cell death pathway.

Nuclear NAD+-biosynthetic enzyme NMNAT1 facilitates development and early survival of retinal neurons

“…Combined with our data confirming a lack of transcriptional upregulation of Nmnat2 or Nmnat3 in NMNAT1 knockout retinas (Figure 3-Supplement 1), one possible explanation for the cell-type specific degeneration we observe is that relatively higher levels of NMNAT2/3 in inner retinal neurons can partially compensate for the NAD + deficit caused by loss of NMNAT1. Recent results in a mutant NMNAT1 mouse model indicating relative depletion of NAD + in the retina but not other tissues (Greenwald et al, 2021) are consistent with such a hypothesis. Overall, our results demonstrate that NMNAT1 deficiency during retinal development affects multiple cell types beyond photoreceptors and suggest retinal cell-type specific requirements of NAD + metabolism to be further investigated.…”

“…However, recent results indicate that NLRP3 is, under certain circumstances, capable of being activated independently of gasdermin D (Gutierrez et al, 2017). Considering a recent report suggesting involvement of the PARP1-associated 'parthanatos' cell death pathway in NMNAT1 mutant retinas (Greenwald et al, 2021), we did not detect accumulation of poly ADP-ribose (PAR) in AC3pyknotic nuclei (data not shown), arguing against involvement of this pathway under these conditions.…”

Nuclear NAD+-biosynthetic enzyme NMNAT1 facilitates survival of developing retinal neurons

“…However, recent results indicate that NLRP3 is, under certain circumstances, capable of being activated independently of gasdermin D (Gutierrez et al, 2017). Considering a recent report suggesting involvement of the PARP1-associated ‘parthanatos’ cell death pathway in NMNAT1 mutant retinas (Greenwald et al, 2021), we did not detect accumulation of poly ADP-ribose (PAR) in AC3 — pyknotic nuclei (data not shown), arguing against involvement of this pathway under these conditions.…”

The key role of the NAD biosynthetic enzyme nicotinamide mononucleotide adenylyltransferase in regulating cell functions

“…Recent studies in a mice model of NMNAT1 associated retinal degeneration have highlighted an overactivation of PARP1 and a consequent drop in NAD levels specifically in retina and not in other tissues. 82 A homozygous missense mutation in the NMNAT2 gene has been reported to be associated with a childhood-onset polyneuropathy with erythromelalgia. 83 Notably, the mutation impairs both the activity and thermal stability of the enzyme, and increases its turnover rate in cells.…”

“…The reason why NMNAT1 mutations causes a pathology confined to the retina is still matter of investigation. Recent studies in a mice model of NMNAT1 associated retinal degeneration have highlighted an overactivation of PARP1 and a consequent drop in NAD levels specifically in retina and not in other tissues 82 …”