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Originally defined as a molecular fraction (“cozymase”) that accelerated fermentation in yeast
extracts
The medical importance of NAD+ was established early with the discovery of pellagra, a disease
characterized by four “Ds”: dermatitis, diarrhea, dementia, and death. A heat-stable dietary factor
(known as pellagra-preventing factor) that cured pellagra was determined to be a NAD+
precursor called niacin. This provided the first evidence of a therapeutic role for what is now
vitamin B3 (1).
decreased cellular NAD+ concentrations occur
under defined conditions, including aging, and
supplementation with NAD+ precursors may be
useful against aging and its chronic diseases.
NAD+ biosynthesis, degradation, and salvage
NAD+ is an important cosubstrate for three classes of enzymes: (i) the sirtuins (SIRTs), (ii)
the adenosine diphosphate (ADP)–ribose transferases (ARTs) and poly(ADP-ribose)
polymerases (PARPs), and (iii) the cyclic ADP-ribose (cADPR) synthases (CD38 and CD157).
NAD+ biosynthetic pathways
nicotinic acid
phosphoribosyltransf
erase (NAPRT)
NAMN transferase (NMNAT)
NAD+ synthase (NADS)
A key enzyme in this pathway is
nicotinamide mononucleotide
adenylyl transferase (NMNAT), which
transforms nicotinic acid
mononucleotide (NAMN) into
nicotinic acid adenine dinucleotide
(NAAD) in the presence of adenosine
triphosphate (ATP). Three forms of
the enzyme have distinct subcellular
localizations: NMNAT1 in the nucleus,
NMNAT2 in the cytosol and Golgi,
and NMNAT3 in the cytosol and
mitochondria
NAD+ as an enzyme cosubstrate
Nicotinamide adenine dinucleotide is a critical cofactor for other enzymes, including the
sirtuin protein deacetylases, the ADP-ribose transferases and PARP, and the cADPR
synthases (CD38 and CD157).
The sirtuin protein deacylases
These proteins are
conserved from bacteria to
humans. They remove acyl
groups from lysine
residues on proteins in a
NAD-dependent manner.
NAD+ is cleaved between
nicotinamide and ADPribose, and the latter
serves as an acyl acceptor,
generating acyl-ADP-ribose
Requirement of NAD and SIR2 for Life-Span Extension by Calorie Restriction in Saccharomyces
cerevisiae
Calorie restriction extends life-span in a wide variety of
organisms. Although it has been suggested that calorie
restriction may work by reducing the levels of reactive
oxygen species produced during respiration, the
mechanism by which this regimen slows aging is
uncertain. Here, we mimicked calorie restriction in yeast
by physiological or genetic means and showed a
substantial extension in life-span. This extension was not
observed in strains mutant forSIR2 (which encodes the
silencing protein Sir2p) orNPT1 (a gene in a pathway in
the synthesis of NAD, the oxidized form of nicotinamide
adenine dinucleotide). These findings suggest that the
increased longevity induced by calorie restriction
requires the activation of Sir2p by NAD.
Mitochondrial sirtuins: regulators of protein acylation
and metabolism
Yeast sirtuin Sir2 was identified as a gene controlling aging in yeast, and this function was confirmed
in worms and Drosophila melanogaster
The sirtuin SIRT6 regulates lifespan in male mice, nature 2012
Alterations in the IGF1–AKT pathway in Sirt6transgenic males.
Sirt1 extends life span and delays aging in mice through the regulation of Nk2 homeobox 1
in the DMH and LH.
There are seven sirtuins in mammals in
different subcellular compartments: nuclear
for SIRT1, SIRT6, and SIRT7; cytoplasmic for
SIRT2; and mitochondrial for SIRT3, SIRT4,
and SIRT5
The protein deacetylase of SIRT1 functions
as an epigenetic regulator by targeting
specific histone-acetylated residues (e.g.,
H3K9, H3K14, and H4K16) but also regulates
transcription by deacetylating transcription
factors (such as TP53, NF-κB, PGC-1α, and
FOXO3a)
SIRT6, which is also localized in the nucleus, is linked to aging by regulating telomere stability
and inflammation through NF-κB signaling. Deacetylation of histone H3K9 appears to be the
modification connecting SIRT6 activity with these aging pathways. Loss of SIRT6 leads to progeria,
whereas gain of function extends life span in male mice by 15%
Genomic Instability and Aging-like Phenotype in the Absence of Mammalian SIRT6
we demonstrate that SIRT6 is a nuclear, chromatin-associated
protein that promotes resistance to DNA damage and suppresses
genomic instability in mouse cells, in association with a role in
base excision repair (BER).
SIRT6 links histone H3 lysine 9 deacetylation to NFkappaB-dependent gene expression and organismal
life span.
SIRT6 attenuates NF-kappaB signaling via H3K9
deacetylation at chromatin, and hyperactive NF-kappaB
signaling may contribute to premature and normal
aging.
Sirtuin 3 is the major mitochondrial protein deacetylase (28). Its expression is enhanced by
fasting and calorie restriction and is decreased during aging and by a high-fat diet
Circadian Clock NAD+ Cycle Drives Mitochondrial Oxidative Metabolism in Mice
Poly(ADP-ribose) polymerases
Activated PARP1 and PARP2 catalyze the transfer of
multiple ADP-ribose moieties from NAD+ to protein
acceptors, generating long poly(ADP-ribose) (PAR)
chains
This DNA-dependent nuclear PARP is strongly activated by DNA damage, leading to
consumption of a large amount of cellular NAD+. In fact, DNA damage leads to a decrease
(up to 80%) in cellular NAD+ concentrations. PARP1 is important in DNA damage detection
and repair, as well as in a cell’s decision to repair itself or die after a genotoxic insult
RNA Regulation by Poly(ADP-Ribose) Polymerases
RNA Regulation by PARPs during Non-stress Conditions
(A) PARP1-mediated poly(ADP-ribosyl)ation of histones
results in chromatin relaxation and increased accessibility
for transcription. During maturation of pre-mRNA,
components of the splicing machinery are ADPribosylated. The nucleolus, a nuclear structure mainly
composed of RNA and RNA binding proteins, is held
together by a dense meshwork of poly(ADP-ribose)
generated by PARP1. This keeps the components involved
in ribosome biogenesis in close proximity to one another
and facilitates assembly. In addition, poly(ADPribosyl)ation is required for the shuttling of protein
components between the nucleolus and Cajal bodies.
(B) Cytoplasmic RNA regulation by PARPs. PARP7, PARP10,
PARP12, and PARP13 are involved in translation inhibition,
e.g., by ADP-ribosylation of the elongation factor EF2 and
ribosomal proteins. PARP13 and PARP14 promote
degradation of specific transcripts by targeting these
transcripts to the cellular RNA decay machinery. PARP13
can also inhibit microRNA-mediated mRNA silencing.
RNA Regulation by PARPs during Stress Conditions
(A) Cytoplasmic stress induces stress granule assembly
mediated by PARP5a, PARP12, PARP13, and PARP14.
MicroRNA-mediated silencing is relieved upon
cytoplasmic stress or viral infection, a process which
requires PARP13 function. PARP7, PARP12, PARP13, and
PARP14 directly inhibit translation in response to stress
or upon viral infection. Viral RNA is additionally
targeted to the RNA decay machinery by PARP13.
(B) During heat shock PARP1 regulates splicing by
recruiting hnRNPs to poly(ADP-ribosyl)ated proteins.
This results in the dissociation of the hnRNPs from their
target mRNAs. Heat shock-activated PARP1 also
mediates poly(ADP-ribosyl)ation of poly(A)polymerase
(PAP), resulting in decreased polyadenylation activity of
the protein. RNAs lacking poly(A) tails fail to be
exported to the cytoplasm and are degraded. During
DNA damage, PARP2 binds to accumulated rRNA
through its SAP domain, activating its enzymatic activity.
Chromatin to Clinic: The Molecular Rationale for PARP1 Inhibitor Function
NAD+ as an enzyme cofactor
Why do concentrations of NAD+ decrease during the aging process?
The fact that supplementation with NMN (a product of NAMPT) corrects defects
associated with aging may indicate that the NAD salvage pathway is deficient in aging.
Decreased NAMPT expression occurs in several tissues (e.g., pancreas, white adipose
tissue, and skeletal muscle) during aging (89), resulting from defective circadian rhythm
regulation by CLOCK and BMAL (22) or from the oxidative stress and chronic
inflammation associated with aging (7)