Poly-ADP-ribose polymerase (PARP) superfamily members covalently link either a single ADP-ribose (ADPR) or a chain of ADPR units to proteins using nicotinamide adenine dinucleotide (NAD) as the source of ADPR. While the well-known poly-ADP-ribosylating (PARylating) PARPs primarily function in the DNA damage response, many non-canonical mono-ADP-ribosylating (MARylating) PARPs are associated with cellular antiviral responses. We recently demonstrated robust upregulation of several PARPs following infection with Murine Hepatitis Virus (MHV), a model coronavirus. Here we show that SARS-CoV-2 infection strikingly upregulates MARylating PARPs and induces the expression of genes encoding enzymes for salvage NAD synthesis from nicotinamide (NAM) and nicotinamide riboside (NR), while downregulating other NAD biosynthetic pathways. We show that overexpression of PARP10 is sufficient to depress cellular NAD and that the activities of the transcriptionally induced enzymes PARP7, PARP10, PARP12 and PARP14 are limited by cellular NAD and can be enhanced by pharmacological activation of NAD synthesis. We further demonstrate that infection with MHV induces a severe attack on host cell NAD+ and NADP+. Finally, we show that NAMPT activation, NAM and NR dramatically decrease the replication of an MHV virus that is sensitive to PARP activity. These data suggest that the antiviral activities of noncanonical PARP isozyme activities are limited by the availability of NAD, and that nutritional and pharmacological interventions to enhance NAD levels may boost innate immunity to coronaviruses.

ABSTRACTInnate immune responses are critical to control of viruses. Coronaviruses (CoVs) are positive strand RNA viruses with double-stranded RNA replication intermediates that are recognized by the innate immune system. Upon recognition, infected cells secrete interferon, which induces a set of interferon-stimulated genes that inhibit viral replication. Here we show that SARS-CoV-2 infection strikingly dysregulates the set of genes involved in consumption and biosynthesis of nicotinamide adenine dinucleotide (NAD). Highly induced genes include those encoding noncanonical poly(ADP-ribose) polymerase (PARP) family members known to function as mono ADP-ribosyltransferases but not known to deplete cellular NAD and genes encoding enzymes for salvage NAD synthesis from nicotinamide (NAM) and nicotinamide riboside (NR). We demonstrate that overexpression of PARP10 is sufficient to depress NAD levels and that the enzymatic activities of PARP10, PARP12 and PARP14 are limited by and can be enhanced by pharmacological activation of NAM salvage. We further showed that infection with the β-coronavirus murine hepatitis virus (MHV) induces a severe attack on host cell NAD+ and NADP+. Finally we show that NAMPT activation, NAM and NR dramatically decrease replication in a MHV infection model that is sensitive to PARP activity. The data show that the antiviral activities of noncanonical PARP isozyme activities are limited by their own consumption of cellular NAD and that nutritional and pharmacological interventions to enhance NAD-based defenses may boost innate immunity.

Mammalian cells use a complex network of redox-dependent processes necessary to maintain cellular integrity during oxidative metabolism, as well as to protect against and/or adapt to stress. The disruption of these redox-dependent processes, including those in the mitochondria, creates a cellular environment permissive for progression to a malignant phenotype and the development of resistance to commonly used anticancer agents. An extension of this paradigm is that when these mitochondrial functions are altered by the events leading to transformation and ensuing downstream metabolic processes, they can be used as molecular biomarkers or targets in the development of new therapeutic interventions to selectively kill and/or sensitize cancer versus normal cells. In this Review we propose that mitochondrial oxidative metabolism is altered in tumor cells, and the central theme of this dysregulation is electron transport chain activity, folate metabolism, NADH/NADPH metabolism, thiol-mediated detoxification pathways, and redox-active metal ion metabolism. It is proposed that specific subgroups of human malignancies display distinct mitochondrial transformative and/or tumor signatures that may benefit from agents that target these pathways.

To date no models exist to study MnSOD deficiency in human cells. To address this deficiency, we created a SOD2-null human cell line that is completely devoid of detectable MnSOD protein expression and enzyme activity. We utilized the CRISPR/Cas9 system to generate biallelic SOD2 disruption in HEK293T cells. These SOD2-null cells exhibit impaired clonogenic activity, which was rescued by either treatment with GC4419, a pharmacological small-molecule mimic of SOD, or growth in hypoxia. The phenotype of these cells is primarily characterized by impaired mitochondrial bioenergetics. The SOD2-null cells displayed perturbations in their mitochondrial ultrastructure and preferred glycolysis as opposed to oxidative phosphorylation to generate ATP. The activities of mitochondrial complex I and II were both significantly impaired by the absence of MnSOD activity, presumably from disruption of the Fe/S centers in NADH dehydrogenase and succinate dehydrogenase subunit B by the aberrant redox state in the mitochondrial matrix of SOD2-null cells. By creating this model we provide a novel tool with which to study the consequences of lack of MnSOD activity in human cells.

Neuroinflammation is initiated by activation of the brain's innate immune system in response to an inflammatory challenge. Insufficient control of neuroinflammation leads to enhanced or prolonged pathology in various neurological conditions including multiple sclerosis and Alzheimer's disease. Nicotinamide adenine dinucleotide (NAD + ) plays critical roles in cellular energy metabolism and calcium homeostasis. Our previous study demonstrated that deletion of CD38, which consumes NAD + , suppressed cuprizone-induced demyelination, neuroinflammation, and glial activation. However, it is still unknown whether CD38 directly affects neuroinflammation through regulating brain NAD + level. In this study, we investigated the effect of CD38 deletion and inhibition and supplementation of NAD + on lipopolysaccharide (LPS)-induced neuroinflammation in mice. Intracerebroventricular injection of LPS significantly increased CD38 expression especially in the hippocampus. Deletion of CD38 decreased LPSinduced inflammatory responses and glial activation. Pre-administration of apigenin, a flavonoid with CD38 inhibitory activity, or nicotinamide riboside (NR), an NAD + precursor, increased NAD + level, and significantly suppressed induction of cytokines and chemokines, glial activation and subsequent neurodegeneration after LPS administration. In cell culture, LPS-induced inflammatory responses were suppressed by treatment of primary astrocytes or microglia with apigenin, NAD + , NR or 78c, the latter