Of Mice and Men: NAD+ Boosting with Niacin Provides Hope for Mitochondrial Myopathy Patients

A clinical approach to diagnosis and management of mitochondrial myopathies

This paper provides an overview of the different types of mitochondrial myopathies (MM), associated phenotypes, genotypes as well as a practical clinical approach towards disease diagnosis, surveillance, and management. nDNA-related MM are more common in pediatric-onset disease whilst mtDNA-related MMs are more frequent in adults. Genotype-phenotype correlation in MM is challenging due to clinical and genetic heterogeneity. The multisystemic nature of many MMs adds to the diagnostic challenge. Diagnostic approaches utilizing genetic sequencing with next generation sequencing approaches such as gene panel, exome and genome sequencing are available. This aids molecular diagnosis, heteroplasmy detection in MM patients and furthers knowledge of known mitochondrial genes. Precise disease diagnosis can end the diagnostic odyssey for patients, avoid unnecessary testing, provide prognosis, facilitate anticipatory management, and enable access to available therapies or clinical trials. Adjunctive tests such as functional and exercise testing could aid surveillance of MM patients. Management requires a multi-disciplinary approach, systemic screening for comorbidities, cofactor supplementation, avoidance of substances that inhibit the respiratory chain and exercise training. This update of the current understanding on MMs provides practical perspectives on current diagnostic and management approaches for this complex group of disorders.

Longitudinal metabolomics analysis reveals the acute effect of cysteine and NAC included in the combined metabolic activators

Growing evidence suggests that the depletion of plasma NAD⁺ and glutathione (GSH) may play an important role in the development of metabolic disorders. The administration of Combined Metabolic Activators (CMA), consisting of GSH and NAD⁺ precursors, has been explored as a promising therapeutic strategy to target multiple altered pathways associated with the pathogenesis of the diseases. Although studies have examined the therapeutic effect of CMA that contains N-acetyl-l-cysteine (NAC) as a metabolic activator, a system-wide comparison of the metabolic response to the administration of CMA with NAC and cysteine remains lacking. In this placebo-controlled study, we studied the acute effect of the CMA administration with different metabolic activators, including NAC or cysteine with/without nicotinamide or flush free niacin, and performed longitudinal untargeted-metabolomics profiling of plasma obtained from 70 well-characterized healthy volunteers. The time-series metabolomics data revealed the metabolic pathways affected after the administration of CMAs showed high similarity between CMA containing nicotinamide and NAC or cysteine as metabolic activators. Our analysis also showed that CMA with cysteine is well-tolerated and safe in healthy individuals throughout the study. Last, our study systematically provided insights into a complex and dynamics landscape involved in amino acid, lipid and nicotinamide metabolism, reflecting the metabolic responses to CMA administration containing different metabolic activators.

Ratiometric NAD+ Sensors Reveal Subcellular NAD+ Modulators

Mapping NAD⁺ dynamics in live cells and human is essential for translating NAD⁺ interventions into effective therapies. Yet, genetically encoded NAD⁺ sensors with better specificity and pH resistance are still needed for the cost-effective monitoring of NAD⁺ in both subcellular compartments and clinical samples. Here, we introduce multicolor, resonance energy transfer-based NAD⁺ sensors covering nano- to millimolar concentration ranges for clinical NAD⁺ measurement and subcellular NAD⁺ visualization. The sensors captured the blood NAD⁺ increase induced by NMN supplementation and revealed the distinct subcellular effects of NAD⁺ precursors and modulators. The sensors then enabled high-throughput screenings for mitochondrial and nuclear NAD⁺ modulators and identified α-GPC, a cognition-related metabolite that induces NAD⁺ redistribution from mitochondria to the nucleus relative to the total adenine nucleotides, which was further confirmed by NAD⁺ FRET microscopy.

The CD38 glycohydrolase and the NAD sink: implications for pathological conditions

Nicotinamide adenine dinucleotide (NAD) acts as a cofactor in several oxidation-reduction (redox) reactions and is a substrate for a number of nonredox enzymes. NAD is fundamental to a variety of cellular processes including energy metabolism, cell signaling, and epigenetics. NAD homeostasis appears to be of paramount importance to health span and longevity, and its dysregulation is associated with multiple diseases. NAD metabolism is dynamic and maintained by synthesis and degradation. The enzyme CD38, one of the main NAD-consuming enzymes, is a key component of NAD homeostasis. The majority of CD38 is localized in the plasma membrane with its catalytic domain facing the extracellular environment, likely for the purpose of controlling systemic levels of NAD. Several cell types express CD38, but its expression predominates on endothelial cells and immune cells capable of infiltrating organs and tissues. Here we review potential roles of CD38 in health and disease and postulate ways in which CD38 dysregulation causes changes in NAD homeostasis and contributes to the pathophysiology of multiple conditions. Indeed, in animal models the development of infectious diseases, autoimmune disorders, fibrosis, metabolic diseases, and age-associated diseases including cancer, heart disease, and neurodegeneration are associated with altered CD38 enzymatic activity. Many of these conditions are modified in CD38-deficient mice or by blocking CD38 NADase activity. In diseases in which CD38 appears to play a role, CD38-dependent NAD decline is often a common denominator of pathophysiology. Thus, understanding dysregulation of NAD homeostasis by CD38 may open new avenues for the treatment of human diseases.

Evolving concepts in NAD+ metabolism

NAD(H) and NADP(H) have traditionally been viewed as co-factors (or co-enzymes) involved in a myriad of oxidation-reduction reactions including the electron transport in the mitochondria. However, NAD pathway metabolites have many other important functions, including roles in signaling pathways, post-translational modifications, epigenetic changes, and regulation of RNA stability and function via NAD-capping of RNA. Non-oxidative reactions ultimately lead to the net catabolism of these nucleotides, indicating that NAD metabolism is an extremely dynamic process. In fact, recent studies have clearly demonstrated that NAD has a half-life in the order of minutes in some tissues. Several evolving concepts on the metabolism, transport, and roles of these NAD pathway metabolites in disease states such as cancer, neurodegeneration, and aging have emerged in just the last few years. In this perspective, we discuss key recent discoveries and changing concepts in NAD metabolism and biology that are reshaping the field. In addition, we will pose some open questions in NAD biology, including why NAD metabolism is so fast and dynamic in some tissues, how NAD and its precursors are transported to cells and organelles, and how NAD metabolism is integrated with inflammation and senescence. Resolving these questions will lead to significant advancements in the field.

The NADase enzyme CD38: an emerging pharmacological target for systemic sclerosis, systemic lupus erythematosus and rheumatoid arthritis

PURPOSE OF REVIEW

Here we review recent literature on the emerging role of nicotinamide adenine dinucleotide (NAD) metabolism and its dysfunction via the enzyme CD38 in the pathogenesis of rheumatologic diseases. We evaluate the potential of targeting CD38 to ameliorate NAD-related metabolic imbalance and tissue dysfunction in the treatment of systemic sclerosis (SSc), systemic lupus erythematous (SLE), and rheumatoid arthritis (RA).

RECENT FINDINGS

In this review, we will discuss emerging basic, preclinical, and human data that point to the novel role of CD38 in dysregulated NAD-homeostasis in SSc, SLE, and RA. In particular, recent studies implicate increased activity of CD38, one of the main enzymes in NAD catabolism, in the pathogenesis of persistent systemic fibrosis in SSc, and increased susceptibility of SLE patients to infections. We will also discuss recent studies that demonstrate that a cytotoxic CD38 antibody can promote clearance of plasma cells involved in the generation of RA antibodies.

SUMMARY

Recent studies identify potential therapeutic approaches for boosting NAD to treat rheumatologic diseases including SSc, RA, and SLE, with particular attention to inhibition of CD38 enzymatic activity as a target. Key future directions in the field include the determination of the cell-type specificity and role of CD38 enzymatic activity versus CD38 structural roles in human diseases, as well as the indicators and potential side effects of CD38-targeted treatments.