The interplay between NAD+, SIRT1 and circadian rhythms in cellular ageing and fibrosis

Henderson, John David (2021) The interplay between NAD+, SIRT1 and circadian rhythms in cellular ageing and fibrosis. Doctoral thesis, Northumbria University.

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Abstract

Recently, the redox cofactor NAD+ has been implicated as a potentially key biomolecule in the quest for healthy ageing, with its function as a rate-limiting cofactor for the deacetylase SIRT1 widely believed to be key.

Consequently, there is a heightened interest in better understanding the regulation of the NAD+/SIRT1 axis and identifying mechanisms that increase NAD+ levels. Within this work, the circadian regulation of NAD+ in human PBMCs (peripheral blood monocytes) in vivo and NHDFs (normal human dermal fibroblasts) in vitro is characterised, with NAD+ levels found to be temporally regulated by an endogenous mechanism dependent on the ‘master circadian regulator’ BMAL1 (brain and muscle ARNT-like 1). Importantly, this introduces the prospect of co-ordinating NAD+ targeting interventions with the diurnal oscillations of NAD+ - whereby timepoints associated with a daily nadir are preferentially targeted for NAD+ enhancement.

The demonstrated efficacy of a ‘multi-targeted’ approach at enhancing NAD+ titres was another important feature of this work, with the use of multiple dietary compounds targeting various steps in NAD+ synthesis and degradation pathways shown to be effective at increasing NAD+ both in vitro and within a small human cohort (n=2). At present supplementation with the NAD+ precursor NR (nicotinamide riboside) is the most widely used intervention for the purpose of enhancing NAD+, however the results presented here suggest a multi-targeted approach merits further investigation and may eventually offer a superior alternative to NR.

Although widely regarded as beneficial for physiological health, there are scenarios whereby increased NAD+ may be undesirable, for example pathologies fuelled by upregulated energy metabolism pathways. It is shown herein that TGF-β (transforming growth factor beta) induced fibrotic signalling in vitro is accompanied by increased glycolysis, which is indispensable for the enhanced expression of the pro-fibrotic marker collagen I. Furthermore, inhibition of NAD+ synthesis by blocking NAMPT activity via FK866 attenuated TGF-β mediated collagen I upregulation, highlighting that reducing NAD+ levels can prevent fibrotic events in vitro.

Thus, this work contributes to the biogerontology field by identifying mechanisms and interventions that modulate NAD+ levels, potentially providing the basis for new strategies with the purpose of enhancing human healthspan. Such strategies should be used with caution in diseases associated enhanced energy metabolism however, in which the role of NAD+ requires further investigation.

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