Deletion of CD38 Suppresses Glial Activation and Neuroinflammation in a Mouse Model of Demyelination

Uric acid in health and disease: From physiological functions to pathogenic mechanisms

Owing to renal reabsorption and the loss of uricase activity, uric acid (UA) is strictly maintained at a higher physiological level in humans than in other mammals, which provides a survival advantage during evolution but increases susceptibility to certain diseases such as gout. Although monosodium urate (MSU) crystal precipitation has been detected in different tissues of patients as a trigger for disease, the pathological role of soluble UA remains controversial due to the lack of causality in the clinical setting. Abnormal elevation or reduction of UA levels has been linked to some of pathological status, also known as U-shaped association, implying that the physiological levels of UA regulated by multiple enzymes and transporters are crucial for the maintenance of health. In addition, the protective potential of UA has also been proposed in aging and some diseases. Therefore, the role of UA as a double-edged sword in humans is determined by its physiological or non-physiological levels. In this review, we summarize biosynthesis, membrane transport, and physiological functions of UA. Then, we discuss the pathological involvement of hyperuricemia and hypouricemia as well as the underlying mechanisms by which UA at abnormal levels regulates the onset and progression of diseases. Finally, pharmacological strategies for urate-lowering therapy (ULT) are introduced, and current challenges in UA study and future perspectives are also described.

Hypothalamic astrocyte NAD+ salvage pathway mediates the coupling of dietary fat overconsumption in a mouse model of obesity

Nicotinamide adenine dinucleotide (NAD)+ serves as a crucial coenzyme in numerous essential biological reactions, and its cellular availability relies on the activity of the nicotinamide phosphoribosyltransferase (NAMPT)-catalyzed salvage pathway. Here we show that treatment with saturated fatty acids activates the NAD+ salvage pathway in hypothalamic astrocytes. Furthermore, inhibition of this pathway mitigates hypothalamic inflammation and attenuates the development of obesity in male mice fed a high-fat diet (HFD). Mechanistically, CD38 functions downstream of the NAD+ salvage pathway in hypothalamic astrocytes burdened with excess fat. The activation of the astrocytic NAMPT-NAD+-CD38 axis in response to fat overload induces proinflammatory responses in the hypothalamus. It also leads to aberrantly activated basal Ca2+ signals and compromised Ca2+ responses to metabolic hormones such as insulin, leptin, and glucagon-like peptide 1, ultimately resulting in dysfunctional hypothalamic astrocytes. Our findings highlight the significant contribution of the hypothalamic astrocytic NAD+ salvage pathway, along with its downstream CD38, to HFD-induced obesity.

The Roles of Caloric Restriction Mimetics in Central Nervous System Demyelination and Remyelination

The dysfunction of myelinating glial cells, the oligodendrocytes, within the central nervous system (CNS) can result in the disruption of myelin, the lipid-rich multi-layered membrane structure that surrounds most vertebrate axons. This leads to axonal degeneration and motor/cognitive impairments. In response to demyelination in the CNS, the formation of new myelin sheaths occurs through the homeostatic process of remyelination, facilitated by the differentiation of newly formed oligodendrocytes. Apart from oligodendrocytes, the two other main glial cell types of the CNS, microglia and astrocytes, play a pivotal role in remyelination. Following a demyelination insult, microglia can phagocytose myelin debris, thus permitting remyelination, while the developing neuroinflammation in the demyelinated region triggers the activation of astrocytes. Modulating the profile of glial cells can enhance the likelihood of successful remyelination. In this context, recent studies have implicated autophagy as a pivotal pathway in glial cells, playing a significant role in both their maturation and the maintenance of myelin. In this Review, we examine the role of substances capable of modulating the autophagic machinery within the myelinating glial cells of the CNS. Such substances, called caloric restriction mimetics, have been shown to decelerate the aging process by mitigating age-related ailments, with their mechanisms of action intricately linked to the induction of autophagic processes.

Astrocytes: Lessons Learned from the Cuprizone Model

A diverse array of neurological and psychiatric disorders, including multiple sclerosis, Alzheimer's disease, and schizophrenia, exhibit distinct myelin abnormalities at both the molecular and histological levels. These aberrations are closely linked to dysfunction of oligodendrocytes and alterations in myelin structure, which may be pivotal factors contributing to the disconnection of brain regions and the resulting characteristic clinical impairments observed in these conditions. Astrocytes, which significantly outnumber neurons in the central nervous system by a five-to-one ratio, play indispensable roles in the development, maintenance, and overall well-being of neurons and oligodendrocytes. Consequently, they emerge as potential key players in the onset and progression of a myriad of neurological and psychiatric disorders. Furthermore, targeting astrocytes represents a promising avenue for therapeutic intervention in such disorders. To gain deeper insights into the functions of astrocytes in the context of myelin-related disorders, it is imperative to employ appropriate in vivo models that faithfully recapitulate specific aspects of complex human diseases in a reliable and reproducible manner. One such model is the cuprizone model, wherein metabolic dysfunction in oligodendrocytes initiates an early response involving microglia and astrocyte activation, culminating in multifocal demyelination. Remarkably, following the cessation of cuprizone intoxication, a spontaneous process of endogenous remyelination occurs. In this review article, we provide a historical overview of studies investigating the responses and putative functions of astrocytes in the cuprizone model. Following that, we list previously published works that illuminate various aspects of the biology and function of astrocytes in this multiple sclerosis model. Some of the studies are discussed in more detail in the context of astrocyte biology and pathology. Our objective is twofold: to provide an invaluable overview of this burgeoning field, and, more importantly, to inspire fellow researchers to embark on experimental investigations to elucidate the multifaceted functions of this pivotal glial cell subpopulation.

Hyaluronan hydrates and compartmentalises the CNS/PNS extracellular matrix and provides niche environments conducive to the optimisation of neuronal activity

The central nervous system/peripheral nervous system (CNS/PNS) extracellular matrix is a dynamic and highly interactive space-filling, cell-supportive, matrix-stabilising, hydrating entity that creates and maintains tissue compartments to facilitate regional ionic micro-environments and micro-gradients that promote optimal neural cellular activity. The CNS/PNS does not contain large supportive collagenous and elastic fibrillar networks but is dominated by a high glycosaminoglycan content, predominantly hyaluronan (HA) and collagen is restricted to the brain microvasculature, blood-brain barrier, neuromuscular junction and meninges dura, arachnoid and pia mater. Chondroitin sulphate-rich proteoglycans (lecticans) interactive with HA have stabilising roles in perineuronal nets and contribute to neural plasticity, memory and cognitive processes. Hyaluronan also interacts with sialoproteoglycan associated with cones and rods (SPACRCAN) to stabilise the interphotoreceptor matrix and has protective properties that ensure photoreceptor viability and function is maintained. HA also regulates myelination/re-myelination in neural networks. HA fragmentation has been observed in white matter injury, multiple sclerosis, and traumatic brain injury. HA fragments (2 × 105  Da) regulate oligodendrocyte precursor cell maturation, myelination/remyelination, and interact with TLR4 to initiate signalling cascades that mediate myelin basic protein transcription. HA and its fragments have regulatory roles over myelination which ensure high axonal neurotransduction rates are maintained in neural networks. Glioma is a particularly invasive brain tumour with extremely high mortality rates. HA, CD44 and RHAMM (receptor for HA-mediated motility) HA receptors are highly expressed in this tumour. Conventional anti-glioma drug treatments have been largely ineffective and surgical removal is normally not an option. CD44 and RHAMM glioma HA receptors can potentially be used to target gliomas with PEP-1, a cell-penetrating HA-binding peptide. PEP-1 can be conjugated to a therapeutic drug; such drug conjugates have successfully treated dense non-operative tumours in other tissues, therefore similar applications warrant exploration as potential anti-glioma treatments.

Neurodegeneration and inflammation crosstalk: Therapeutic targets and perspectives

Glia, which was formerly considered to exist just to connect neurons, now plays a key function in a wide range of physiological events, including formation of memory, learning, neuroplasticity, synaptic plasticity, energy consumption, and homeostasis of ions. Glial cells regulate the brain's immune responses and confers nutritional and structural aid to neurons, making them an important player in a broad range of neurological disorders. Alzheimer's, ALS, Parkinson's, frontotemporal dementia (FTD), and epilepsy are a few of the neurodegenerative diseases that have been linked to microglia and astroglia cells, in particular. Synapse growth is aided by glial cell activity, and this activity has an effect on neuronal signalling. Each glial malfunction in diverse neurodegenerative diseases is distinct, and we will discuss its significance in the progression of the illness, as well as its potential for future treatment.

Acquisition of Immune Privilege in GBM Tumors: Role of Prostaglandins and Bile Salts

Based on the postulate that glioblastoma (GBM) tumors generate anti-inflammatory prostaglandins and bile salts to gain immune privilege, we analyzed 712 tumors in-silico from three GBM transcriptome databases for prostaglandin and bile synthesis/signaling enzyme-transcript markers. A pan-database correlation analysis was performed to identify cell-specific signal generation and downstream effects. The tumors were stratified by their ability to generate prostaglandins, their competency in bile salt synthesis, and the presence of bile acid receptors nuclear receptor subfamily 1, group H, member 4 (NR1H4) and G protein-coupled bile acid receptor 1 (GPBAR1). The survival analysis indicates that tumors capable of prostaglandin and/or bile salt synthesis are linked to poor outcomes. Tumor prostaglandin D₂ and F₂ syntheses are derived from infiltrating microglia, whereas prostaglandin E₂ synthesis is derived from neutrophils. GBMs drive the microglial synthesis of PGD₂/F₂ by releasing/activating complement system component C3a. GBM expression of sperm-associated heat-shock proteins appears to stimulate neutrophilic PGE₂ synthesis. The tumors that generate bile and express high levels of bile receptor NR1H4 have a fetal liver phenotype and a RORC-Treg infiltration signature. The bile-generating tumors that express high levels of GPBAR1 are infiltrated with immunosuppressive microglia/macrophage/myeloid-derived suppressor cells. These findings provide insight into how GBMs generate immune privilege and may explain the failure of checkpoint inhibitor therapy and provide novel targets for treatment.

Advanced theragnostics for the central nervous system (CNS) and neurological disorders using functional inorganic nanomaterials

Various types of inorganic nanomaterials are capable of diagnostic biomarker detection and the therapeutic delivery of a disease or inflammatory modulating agent. Those multi-functional nanomaterials have been utilized to treat neurodegenerative diseases and central nervous system (CNS) injuries in an effective and personalized manner. Even though many nanomaterials can deliver a payload and detect a biomarker of interest, only a few studies have yet to fully utilize this combined strategy to its full potential. Combining a nanomaterial's ability to facilitate targeted delivery, promote cellular proliferation and differentiation, and carry a large amount of material with various sensing approaches makes it possible to diagnose a patient selectively and sensitively while offering preventative measures or early disease-modifying strategies. By tuning the properties of an inorganic nanomaterial, the dimensionality, hydrophilicity, size, charge, shape, surface chemistry, and many other chemical and physical parameters, different types of cells in the central nervous system can be monitored, modulated, or further studies to elucidate underlying disease mechanisms. Scientists and clinicians have better understood the underlying processes of pathologies for many neurologically related diseases and injuries by implementing multi-dimensional 0D, 1D, and 2D theragnostic nanomaterials. The incorporation of nanomaterials has allowed scientists to better understand how to detect and treat these conditions at an early stage. To this end, having the multi-modal ability to both sense and treat ailments of the central nervous system can lead to favorable outcomes for patients suffering from such injuries and diseases.

Uncompetitive, adduct-forming SARM1 inhibitors are neuroprotective in preclinical models of nerve injury and disease

Axon degeneration is an early pathological event in many neurological diseases. The identification of the nicotinamide adenine dinucleotide (NAD) hydrolase SARM1 as a central metabolic sensor and axon executioner presents an exciting opportunity to develop novel neuroprotective therapies that can prevent or halt the degenerative process, yet limited progress has been made on advancing efficacious inhibitors. We describe a class of NAD-dependent active-site SARM1 inhibitors that function by intercepting NAD hydrolysis and undergoing covalent conjugation with the reaction product adenosine diphosphate ribose (ADPR). The resulting small-molecule ADPR adducts are highly potent and confer compelling neuroprotection in preclinical models of neurological injury and disease, validating this mode of inhibition as a viable therapeutic strategy. Additionally, we show that the most potent inhibitor of CD38, a related NAD hydrolase, also functions by the same mechanism, further underscoring the broader applicability of this mechanism in developing therapies against this class of enzymes.

Cladribine Reduces Trans-Endothelial Migration of Memory T Cells across an In Vitro Blood–Brain Barrier

Multiple sclerosis (MS) is a chronic, demyelinating disease of the central nervous system (CNS) induced by immune dysregulation. Cladribine has been championed for its clinical efficacy with relatively minor side effects in treating MS. Although it is proposed that cladribine exerts an anti-migratory effect on lymphocytes at the blood–brain barrier (BBB) in addition to its lymphocyte-depleting and modulating effects, this has not been properly studied. Here, we aimed to determine if cladribine treatment influences trans-endothelial migration of T cell subsets across an inflamed BBB. Human brain endothelial cells stimulated with pro-inflammatory cytokines were used to mimic the BBB. Peripheral blood mononuclear cells were obtained from healthy controls, untreated and cladribine-treated MS patients. The trans-endothelial migration of CD4⁺ effector memory T (T^EM) and CD8⁺ central memory T (T^CM) cells was reduced in cladribine-treated MS patients. CD28 expression was decreased on both CD4⁺ T^EM and CD8⁺ T^CM cells, suggesting lowered peripheral activation of these cells thereby maintaining the integrity of the BBB. In addition, these cells have likely reconstituted following cladribine treatment, revealing a long-term anti-migratory effect. These results highlight new mechanisms by which cladribine acts to control MS pathogenesis.

Leukoencephalopathy During Daratumumab-Based Therapy: A Case Series of Two Patients with Multiple Myeloma

Leukoencephalopathy in the setting of multiple myeloma (MM) is a rare demyelinating condition, with few reported cases in literature. Daratumumab is a CD38 targeted monoclonal antibody that has been widely used for the management of MM. In the absence of central nervous system (CNS) disease, many medication-induced leukoencephalopathy cases reported with MM, including daratumumab-induced, are associated with progressive multifocal leukoencephalopathy (PML) and John Cunningham (JC) virus. Currently, there are no reported cases of daratumumab-induced leukoencephalopathy among patients without CNS involvement or PML. We discuss 2 patients who developed leukoencephalopathy while receiving daratumumab-based therapy without evidence of PML or CNS disease. Both patients had baseline MRIs without significant white matter changes before daratumumab-based therapy. Patients began experiencing neurological deficits about 6 to 8 months after daratumumab-based therapy initiation. One patient passed away before being assessed for improvement of symptoms with daratumumab cessation. The second patient had some stabilization of symptoms after cessation; however, the leukoencephalopathy remained irreversible. As the class of anti-CD38 monoclonal antibodies expands in MM therapy, we highlight a potential treatment complication and the importance of detecting leukoencephalopathy early among patients receiving anti-CD38 therapy. We recommend vigilant monitoring of any new or worsening neurological symptoms to avoid serious complications of irreversible leukoencephalopathy.

NAD+ metabolism drives astrocyte proinflammatory reprogramming in central nervous system autoimmunity

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS). Astrocytes are the most abundant glial cells in the CNS, and their dysfunction contributes to the pathogenesis of MS and its animal model, experimental autoimmune encephalomyelitis (EAE). Recent advances highlight the pivotal role of cellular metabolism in programming immune responses. However, the underlying immunometabolic mechanisms that drive astrocyte pathogenicity remain elusive. Nicotinamide adenine dinucleotide (NAD⁺) is a vital coenzyme involved in cellular redox reactions and a substrate for NAD⁺-dependent enzymes. Cellular NAD⁺ levels are dynamically controlled by synthesis and degradation, and dysregulation of this balance has been associated with inflammation and disease. Here, we demonstrate that cell-autonomous generation of NAD⁺ via the salvage pathway regulates astrocyte immune function. Inhibition of nicotinamide phosphoribosyltransferase (NAMPT), a key enzyme in the salvage pathway, results in depletion of NAD⁺, inhibits oxidative phosphorylation, and limits astrocyte inflammatory potential. We identified CD38 as the main NADase up-regulated in reactive mouse and human astrocytes in models of neuroinflammation and MS. Genetic or pharmacological blockade of astrocyte CD38 activity augmented NAD⁺ levels, suppressed proinflammatory transcriptional reprogramming, impaired chemotactic potential to inflammatory monocytes, and ameliorated EAE. We found that CD38 activity is mediated via calcineurin/NFAT signaling in mouse and human reactive astrocytes. Thus, NAMPT-NAD⁺-CD38 circuitry in astrocytes controls their ability to meet their energy demands and drives the expression of proinflammatory transcriptional modules, contributing to CNS pathology in EAE and, potentially, MS. Our results identify candidate therapeutic targets in MS.

Targeting CD38 in Neoplasms and Non-Cancer Diseases

Simple Summary

CD38 remains an interesting target for anticancer therapy. Its relatively high abundance in neoplasms and crucial impact on NAD+/cADPR metabolism and the activity of T cells allows for changing the immune response in autoimmune diseases, neoplasms, and finally the induction of cell death. Antibody-dependent cell cytotoxicity is responsible for cell death induced by targeting the tumor with anti-CD38 antibodies, such as daratumumab. A wide range of laboratory experiments and clinical trials show an especially promising role of anti-CD38 therapy against multiple myeloma, NK cell lymphomas, and CD19- B-cell malignancies. More studies are required to include more diseases in the therapeutic protocols involving the modulation of CD38 activity.

Abstract

CD38 is a myeloid antigen present both on the cell membrane and in the intracellular compartment of the cell. Its occurrence is often enhanced in cancer cells, thus making it a potential target in anticancer therapy. Daratumumab and isatuximab already received FDA approval, and novel agents such as MOR202, TAK079 and TNB-738 undergo clinical trials. Also, novel therapeutics such as SAR442085 aim to outrank the older antibodies against CD38. Multiple myeloma and immunoglobulin light-chain amyloidosis may be effectively treated with anti-CD38 immunotherapy. Its role in other hematological malignancies is also important concerning both diagnostic process and potential treatment in the future. Aside from the hematological malignancies, CD38 remains a potential target in gastrointestinal, neurological and pulmonary system disorders. Due to the strong interaction of CD38 with TCR and CD16 on T cells, it may also serve as the biomarker in transplant rejection in renal transplant patients. Besides, CD38 finds its role outside oncology in systemic lupus erythematosus and collagen-induced arthritis. CD38 plays an important role in viral infections, including AIDS and COVID-19. Most of the undergoing clinical trials focus on the use of anti-CD38 antibodies in the therapy of multiple myeloma, CD19- B-cell malignancies, and NK cell lymphomas. This review focuses on targeting CD38 in cancer and non-cancerous diseases using antibodies, cell-based therapies and CD38 inhibitors. We also provide a summary of current clinical trials targeting CD38.

Characterizing CD38 Expression and Enzymatic Activity in the Brain of Spontaneously Hypertensive Stroke-Prone Rats

Background: CD38 is a transmembrane glycoprotein that catabolizes nicotinamide adenine dinucleotide (NAD+) and is the main source for the age-dependent decrease in NAD+ levels. Increased CD38 enzymatic activity has been implicated in several neurological diseases. However, its role in the pathogenesis of cerebral small vessel disease (CSVD) remains unknown. We aimed to characterize CD38 expression and enzymatic activity in the brain of spontaneously hypertensive stroke-prone rats (SHRSP), a genetic model for hypertension and human CSVD, in comparison to age-matched normotensive Wistar Kyoto rats (WKY). Materials and Methods: Age-matched male 7- and 24-week-old WKY and SHRSP were studied. CD38 enzymatic activity was determined in the brain homogenate. Immunohistochemistry and Western Blotting (WB) were used to characterize CD38 expression and localize it in the different cell types within the brain. In addition, expression of nitric oxide synthase (NOS) isoforms and the levels of nitric oxide (NO), superoxide, nicotinamide dinucleotide (phosphate) NAD(P)H were measured the brain of in WKY and SHRSP. Results: CD38 expression and enzymatic activity were increased in SHRSP brains compared to age matched WKY starting at 7 weeks of age. CD38 expression was localized to the endothelial cells, astrocytes, and microglia. We also identified increased CD38 expression using WB with age in SHRSP and WKY. CD38 enzymatic activity was also increased in 24-week SHRSP compared to 7-week SHRSP. In association, we identified evidence of oxidative stress, reduced NO level, reduced NAD(P)H level and endothelial NOS expression in SHRSP compared to age matched WKY. NAD(P)H also decreased with age in WKY and SHRSP. Additionally, activation of astrocytes and microglia were present in SHRSP compared to WKY. Conclusions: CD38 is overexpressed, and its enzymatic activity is increased in SHRSP, a genetic model for marked hypertension and human CSVD. Our results suggest a potential role for CD38 enzymatic activation in the pathogenesis of CSVD and points to the need for future mechanistic and pharmacological studies.

CD38 Enhances TLR9 Expression and Activates NLRP3 Inflammasome after Porcine Parvovirus Infection

(1) Background: Porcine Parvovirus (PPV) is a single-stranded DNA virus without envelope which causes great harm in relation to porcine reproductive disorders in clinic. Cluster of Differentiation 38 (CD38) is a transmembrane protein widely existing in mammals. Its various functions make it a very popular research object, including in the viral infection field. (2) Methods: Western blotting and an EdU Cell Proliferation Kit were used to evaluate the effect of CD38-deficient cells. Relative quantitative real-time RT-PCR was used to detect the transcription levels of cytokines after PPV infection. The renilla luciferase reporter gene assay was used to verify the activation function of CD38 on downstream factors. The fluorescence probe method was used to detect the level of intracellular reactive oxygen species (ROS). (3) Results: This study found that the loss of CD38 function inhibited the up-regulated state of Toll-like Receptor 9 (TLR9), Interferon-α (IFN-α), and Myxovirus Resistance 1 (Mx1) after PPV infection. The luminescence of the group transfected with both CD38 expression plasmid and TLR9 promoter renilla luciferase reporter plasmid was significantly up-regulated compared with the control, suggesting that CD38 may activate the promoter of TLR9. In addition, CD38 deficiency not only activated the transcription of Sirtuin-1 (SIRT1), but also inhibited ROS level and the transcription of NLR Family Pyrin Domain Containing 3 (NLRP3). (4) Conclusion: (i) CD38 may participate in the TLR9/IFN-α/Mx1 pathway by activating the expression of TLR9 after PPV infected PK-15 cells; (ii) CD38 may activate the NLRP3/CASP1 pathway by increasing ROS level; (iii) CD38 deficiency activates the expression of SIRT1 and can prevent the normal proliferation of PPV.

CD38 deficiency protects the retina from ischaemia/reperfusion injury partly via suppression of TLR4/MyD88/NF-κB signalling

PURPOSE

This study aimed to explore cellular localisation of CD38 in the retina and evaluate the role and potential mechanism of CD38 deficiency in retinal ischaemia/reperfusion (I/R) injury.

METHODS

Six-to eight-week-old male CD38 knockout (KO) and wild-type mice in C57BL/6 background were used. Immunostaining was performed to determine the cellular localisation of CD38 in the retina. Haematoxylin and eosin staining and immunostaining of Brn3a were used to evaluate the retinal I/R injury. Western blotting was performed to detect toll-like receptor 4 (TLR4), myeloid differentiation primary response 88 (MyD88), p-p65, ionised calcium-binding adapter molecule 1, Sirtuin1 (Sirt1), Ac-p65, and pro-inflammatory cytokines protein expression.

RESULTS

CD38 was highly expressed in mouse retinal microglia and astrocytes/Müller cells. CD38 deficiency reduced I/R-induced retinal damage and retinal ganglion cell death. Following retinal I/R injury, TLR4, MyD88, nuclear factor-κB p-p65 (NF-κB p-p65), pro-inflammatory cytokines and CD38 protein levels were also upregulated. After I/R injury, retinal inflammation factors IL-1β, IL-6, and TNF-α mRNA and protein levels were increased. IL-1β, IL-6, and TNF-α were reduced in CD38 KO mice after I/R injury. Retinal I/R injury induced the activation of microglia, but this effect was also suppressed by KO of CD38. Additionally, retinal I/R induced a significant increase in Ac-p65 protein levels and decrease in Sirt1 protein levels, while this effect was greatly attenuated by KO of CD38.

CONCLUSION

CD38 deficiency protects the retina from I/R injury by suppressing microglial activation partly via activating Sirt1-mediated suppression of TLR4/MyD88/NF-κB signalling.

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.

Critical Role of Astrocyte NAD+ Glycohydrolase in Myelin Injury and Regeneration

Western-style diets cause disruptions in myelinating cells and astrocytes within the mouse CNS. Increased CD38 expression is present in the cuprizone and experimental autoimmune encephalomyelitis models of demyelination and CD38 is the main nicotinamide adenine dinucleotide (NAD⁺)-depleting enzyme in the CNS. Altered NAD⁺ metabolism is linked to both high fat consumption and multiple sclerosis (MS). Here, we identify increased CD38 expression in the male mouse spinal cord following chronic high fat consumption, after focal toxin [lysolecithin (LL)]-mediated demyelinating injury, and in reactive astrocytes within active MS lesions. We demonstrate that CD38 catalytically inactive mice are substantially protected from high fat-induced NAD⁺ depletion, oligodendrocyte loss, oxidative damage, and astrogliosis. A CD38 inhibitor, 78c, increased NAD⁺ and attenuated neuroinflammatory changes induced by saturated fat applied to astrocyte cultures. Conditioned media from saturated fat-exposed astrocytes applied to oligodendrocyte cultures impaired myelin protein production, suggesting astrocyte-driven indirect mechanisms of oligodendrogliopathy. In cerebellar organotypic slice cultures subject to LL-demyelination, saturated fat impaired signs of remyelination effects that were mitigated by concomitant 78c treatment. Significantly, oral 78c increased counts of oligodendrocytes and remyelinated axons after focal LL-induced spinal cord demyelination. Using a RiboTag approach, we identified a unique in vivo brain astrocyte translatome profile induced by 78c-mediated CD38 inhibition in mice, including decreased expression of proinflammatory astrocyte markers and increased growth factors. Our findings suggest that a high-fat diet impairs oligodendrocyte survival and differentiation through astrocyte-linked mechanisms mediated by the NAD⁺ase CD38 and highlights CD38 inhibitors as potential therapeutic candidates to improve myelin regeneration.SIGNIFICANCE STATEMENT Myelin disturbances and oligodendrocyte loss can leave axons vulnerable, leading to permanent neurologic deficits. The results of this study suggest that metabolic disturbances, triggered by consumption of a diet high in fat, promote oligodendrogliopathy and impair myelin regeneration through astrocyte-linked indirect nicotinamide adenine dinucleotide (NAD⁺)-dependent mechanisms. We demonstrate that restoring NAD⁺ levels via genetic inactivation of CD38 can overcome these effects. Moreover, we show that therapeutic inactivation of CD38 can enhance myelin regeneration. Together, these findings point to a new metabolic targeting strategy positioned to improve disease course in multiple sclerosis and other conditions in which the integrity of myelin is a key concern.

Nicotinamide Riboside Neutralizes Hypothalamic Inflammation and Increases Weight Loss Without Altering Muscle Mass in Obese Rats Under Calorie Restriction: A Preliminary Investigation

Obesity treatments, such as calorie restriction (CR), eventually lead to muscle wasting and higher rates of neuroinflammation, whereas hypothalamic inflammatory conditions impair body weight (BW) control. Nicotinamide riboside (NR) has been proposed against obesity but with little evidence on skeletal muscle tissue (SMT) and neuroinflammation. Therefore, we aimed to investigate the effects of CR on SMT and on hypothalamic inflammatory biomarkers in obese adult male Wistar rats, and whether NR supplementation alone or in combination with CR affects these parameters. Obesity was induced in rats through a cafeteria diet for 6 weeks. After that, a group of obese rats was exposed to CR, associated or not associated with NR supplementation (400 mg/kg), for another 4 weeks. As a result, obese rats, with or without CR, presented lower relative weight of SMT when compared with eutrophic rats. Rats under CR presented lower absolute SMT weight compared with obese and eutrophic rats, in addition to presenting elevated hypothalamic levels of TNF-α. NR supplementation, in all groups, enhanced weight loss and increased relative weight of the SMT. Furthermore, in animals under CR, NR reversed increases TNF-α levels in the hypothalamus. In this study, these data, although succinct, are the first to evidence the effects of NR on SMT and neuroinflammation when associated with CR, especially in obesity conditions. Therefore, this provides preliminary support for future studies in this investigative field. Furthermore, NR emerges as a potential adjuvant for preventing muscle mass loss in the weight loss processes.

Risks and rewards of targeting NAD+ homeostasis in the brain

Strategies to correct declining nicotinamide adenine dinucleotide (NAD⁺) levels in neurological disease and biological ageing are promising therapeutic candidates. These strategies include supplementing with NAD⁺ precursors, small molecule activation of NAD⁺ biosynthetic enzymes, and treatment with small molecule inhibitors of NAD⁺ consuming enzymes such as CD38, SARM1 or members of the PARP family. While these strategies have shown efficacy in animal models of neurological disease, each of these has the mechanistic potential for adverse events that could preclude their preclinical use. Here, we discuss the implications of these strategies for treating neurological diseases, including potential off-target effects that may be unique to the brain.

Inhibition of CD38 and supplementation of nicotinamide riboside ameliorate lipopolysaccharide-induced microglial and astrocytic neuroinflammation by increasing NAD. J Neurochem 2021;158:311-327. [PMID: 33871064 DOI: 10.1111/jnc.15367]

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 LPS-induced 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 a specific CD38 inhibitor. Finally, all these compounds suppressed NF-κB signaling pathway in microglia. These results suggest that CD38-mediated neuroinflammation is linked to NAD⁺ consumption and that boosting NAD⁺ by CD38 inhibition and NR supplementation directly suppress neuroinflammation in the brain.

Nicotinamide adenine dinucleotide metabolism in the immune response, autoimmunity and inflammageing

Metabolism is dynamically regulated to accompany immune cell function, and altered immunometabolism can result in impaired immune responses. Concomitantly, the pharmacological manipulation of metabolic processes offers an opportunity for therapeutic intervention in inflammatory disorders. The nicotinamide adenine dinucleotide (NAD⁺) is a critical metabolic intermediate that serves as enzyme cofactor in redox reactions, and is also used as a co‐substrate by many enzymes such as sirtuins, adenosine diphosphate ribose transferases and synthases. Through these activities, NAD⁺ metabolism regulates a broad spectrum of cellular functions such as energy metabolism, DNA repair, regulation of the epigenetic landscape and inflammation. Thus, the manipulation of NAD⁺ availability using pharmacological compounds such as NAD⁺ precursors can have immune‐modulatory properties in inflammation. Here, we discuss how the NAD⁺ metabolism contributes to the immune response and inflammatory conditions, with a special focus on multiple sclerosis, inflammatory bowel diseases and inflammageing.

Repositioning of Immunomodulators: A Ray of Hope for Alzheimer's Disease?

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder characterized by cognitive decline and by the presence of amyloid β plaques and neurofibrillary tangles in the brain. Despite recent advances in understanding its pathophysiological mechanisms, to date, there are no disease-modifying therapeutic options, to slow or halt the evolution of neurodegenerative processes in AD. Current pharmacological treatments only transiently mitigate the severity of symptoms, with modest or null overall improvement. Emerging evidence supports the concept that AD is affected by the impaired ability of the immune system to restrain the brain's pathology. Deep understanding of the relationship between the nervous and the immune system may provide a novel arena to develop effective and safe drugs for AD treatment. Considering the crucial role of inflammatory/immune pathways in AD, here we discuss the current status of the immuno-oncological, immunomodulatory and anti-TNF-α drugs which are being used in preclinical studies or in ongoing clinical trials by means of the drug-repositioning approach.

CD38: An Immunomodulatory Molecule in Inflammation and Autoimmunity

CD38 is a molecule that can act as an enzyme, with NAD-depleting and intracellular signaling activity, or as a receptor with adhesive functions. CD38 can be found expressed either on the cell surface, where it may face the extracellular milieu or the cytosol, or in intracellular compartments, such as endoplasmic reticulum, nuclear membrane, and mitochondria. The main expression of CD38 is observed in hematopoietic cells, with some cell-type specific differences between mouse and human. The role of CD38 in immune cells ranges from modulating cell differentiation to effector functions during inflammation, where CD38 may regulate cell recruitment, cytokine release, and NAD availability. In line with a role in inflammation, CD38 appears to also play a critical role in inflammatory processes during autoimmunity, although whether CD38 has pathogenic or regulatory effects varies depending on the disease, immune cell, or animal model analyzed. Given the complexity of the physiology of CD38 it has been difficult to completely understand the biology of this molecule during autoimmune inflammation. In this review, we analyze current knowledge and controversies regarding the role of CD38 during inflammation and autoimmunity and novel molecular tools that may clarify current gaps in the field.

Activation of astrocytic sigma-1 receptor exerts antidepressant-like effect via facilitating CD38-driven mitochondria transfer

Despite sigma-1 receptor (Sig-1R) is a promising therapeutic target in depression, little is known regarding the cellular mechanisms underlying its antidepressant responses. Here, we demonstrated that astrocyte can be a direct cellular target of Sig-1R exerting antidepressant-like effect. In multiple behavioral models including forced swimming test (FST), tail suspension test (TST), open field test (OFT), and chronic unpredictable mild stress (CUMS), inhibition of astrocyte function blocked pharmacological Sig-1R activation-induced antidepressant-like effect, while specific activation of astrocytc Sig-1R by adeno-associated virus (AAV) was sufficient to produce antidepressant-like effect. In depression-related cellular tests, Sig-1R agonist or lentivirus-stimulated astrocyte conditioned medium (ACM) promoted neuronal neurite outgrowth, dendritic branch, and survival. Mechanismly, stimulation of Sig-1R enhanced the expression of CD38 via activation of extracellular regulated protein kinases 1/2 (ERK1/2), resulting in facilitating mitochondrial transfer from astrocyte. Furthermore, blockage of CD38-driven astrocyte transferring mitochondria in vivo and in vitro reversed the antidepressant-like effect of pharmacological Sig-1R activation. Thus, this study sheds light on the cellular mechanism of Sig-1R activation producing antidepressant-like effect. These data present the first evidence that enhancement of Sig-1R action on astrocytes entirely exerts antidepressant-like effect, indicating that specific activation of astrocytic Sig-1R may provide a new approach for antidepressant drug development.

Deletion of CD38 and supplementation of NAD+ attenuate axon degeneration in a mouse facial nerve axotomy model

Following facial nerve axotomy, nerve function is not fully restored even after reconstruction. This may be attributed to axon degeneration/neuronal death and sustained neuroinflammation. CD38 is an enzyme that catalyses the hydrolysis of nicotinamide adenine dinucleotide (NAD+) and is a candidate molecule for regulating neurodegeneration and neuroinflammation. In this study, we analyzed the effect of CD38 deletion and NAD+ supplementation on neuronal death and glial activation in the facial nucleus in the brain stem, and on axon degeneration and immune cell infiltration in the distal portion of the facial nerve after axotomy in mice. Compared with wild-type mice, CD38 knockout (KO) mice showed reduced microglial activation in the facial nucleus, whereas the levels of neuronal death were not significantly different. In contrast, the axon degeneration and demyelination were delayed, and macrophage accumulation was reduced in the facial nerve of CD38 KO mice after axotomy. Supplementation of NAD+ with nicotinamide riboside slowed the axon degeneration and demyelination, although it did not alter the level of macrophage infiltration after axotomy. These results suggest that CD38 deletion and supplementation of NAD+ may protect transected axon cell-autonomously after facial nerve axotomy.

Deletion of CD38 and supplementation of NAD+ attenuate axon degeneration in a mouse facial nerve axotomy model

Following facial nerve axotomy, nerve function is not fully restored even after reconstruction. This may be attributed to axon degeneration/neuronal death and sustained neuroinflammation. CD38 is an enzyme that catalyses the hydrolysis of nicotinamide adenine dinucleotide (NAD⁺) and is a candidate molecule for regulating neurodegeneration and neuroinflammation. In this study, we analyzed the effect of CD38 deletion and NAD⁺ supplementation on neuronal death and glial activation in the facial nucleus in the brain stem, and on axon degeneration and immune cell infiltration in the distal portion of the facial nerve after axotomy in mice. Compared with wild-type mice, CD38 knockout (KO) mice showed reduced microglial activation in the facial nucleus, whereas the levels of neuronal death were not significantly different. In contrast, the axon degeneration and demyelination were delayed, and macrophage accumulation was reduced in the facial nerve of CD38 KO mice after axotomy. Supplementation of NAD⁺ with nicotinamide riboside slowed the axon degeneration and demyelination, although it did not alter the level of macrophage infiltration after axotomy. These results suggest that CD38 deletion and supplementation of NAD⁺ may protect transected axon cell-autonomously after facial nerve axotomy.

Neuromodulation of Glial Function During Neurodegeneration

Glia, a non-excitable cell type once considered merely as the connective tissue between neurons, is nowadays acknowledged for its essential contribution to multiple physiological processes including learning, memory formation, excitability, synaptic plasticity, ion homeostasis, and energy metabolism. Moreover, as glia are key players in the brain immune system and provide structural and nutritional support for neurons, they are intimately involved in multiple neurological disorders. Recent advances have demonstrated that glial cells, specifically microglia and astroglia, are involved in several neurodegenerative diseases including Amyotrophic lateral sclerosis (ALS), Epilepsy, Parkinson's disease (PD), Alzheimer's disease (AD), and frontotemporal dementia (FTD). While there is compelling evidence for glial modulation of synaptic formation and regulation that affect neuronal signal processing and activity, in this manuscript we will review recent findings on neuronal activity that affect glial function, specifically during neurodegenerative disorders. We will discuss the nature of each glial malfunction, its specificity to each disorder, overall contribution to the disease progression and assess its potential as a future therapeutic target.

Dynamic Responses of Microglia in Animal Models of Multiple Sclerosis

Microglia play an essential role in maintaining central nervous system (CNS) homeostasis, as well as responding to injury and disease. Most neurological disorders feature microglial activation, a process whereby microglia undergo profound morphological and transcriptional changes aimed at containing CNS damage and promoting repair, but often resulting in overt inflammation that sustains and propagates the neurodegenerative process. This is especially evident in multiple sclerosis (MS), were microglial activation and microglia-driven neuroinflammation are considered key events in the onset, progression, and resolution of the disease. Our understanding of microglial functions in MS has widened exponentially in the last decade by way of new tools and markers to discriminate microglia from other myeloid populations. Consequently, the complex functional and phenotypical diversity of microglia can now be appreciated. This, in combination with a variety of animal models that mimic specific features and processes of MS, has contributed to filling the gap of knowledge in the cascade of events underlying MS pathophysiology. The purpose of this review is to present the most up to date knowledge of the dynamic responses of microglia in the commonly used animal models of MS, specifically the immune-mediated experimental autoimmune encephalomyelitis (EAE) model, and the chemically-induced cuprizone and lysolecithin models. Elucidating the spectrum of microglial functions in these models, from detrimental to protective, is essential to identify emerging targets for therapy and guide drug discovery efforts.

Astrocytes and Microglia as Major Players of Myelin Production in Normal and Pathological Conditions

Myelination is an essential process that consists of the ensheathment of axons by myelin. In the central nervous system (CNS), myelin is synthesized by oligodendrocytes. The proliferation, migration, and differentiation of oligodendrocyte precursor cells constitute a prerequisite before mature oligodendrocytes extend their processes around the axons and progressively generate a multilamellar lipidic sheath. Although myelination is predominately driven by oligodendrocytes, the other glial cells including astrocytes and microglia, also contribute to this process. The present review is an update of the most recent emerging mechanisms involving astrocyte and microglia in myelin production. The contribution of these cells will be first described during developmental myelination that occurs in the early postnatal period and is critical for the proper development of cognition and behavior. Then, we will report the novel findings regarding the beneficial or deleterious effects of astroglia and microglia, which respectively promote or impair the endogenous capacity of oligodendrocyte progenitor cells (OPCs) to induce spontaneous remyelination after myelin loss. Acute delineation of astrocyte and microglia activities and cross-talk should uncover the way towards novel therapeutic perspectives aimed at recovering proper myelination during development or at breaking down the barriers impeding the regeneration of the damaged myelin that occurs in CNS demyelinating diseases.

CD38 in Neurodegeneration and Neuroinflammation

Neurodegenerative diseases are characterized by neuronal degeneration as well as neuroinflammation. While CD38 is strongly expressed in brain cells including neurons, astrocytes as well as microglial cells, the role played by CD38 in neurodegeneration and neuroinflammation remains elusive. Yet, CD38 expression increases as a consequence of aging which is otherwise the primary risk associated with neurodegenerative diseases, and several experimental data demonstrated that CD38 knockout mice are protected from neurodegenerative and neuroinflammatory insults. Moreover, nicotinamide adenine dinucleotide, whose levels are tightly controlled by CD38, is a recognized and potent neuroprotective agent, and NAD supplementation was found to be beneficial against neurodegenerative diseases. The aims of this review are to summarize the physiological role played by CD38 in the brain, present the arguments indicating the involvement of CD38 in neurodegeneration and neuroinflammation, and to discuss these observations in light of CD38 complex biology.

Deep rTMS Mitigates Behavioral and Neuropathologic Anomalies in Cuprizone-Exposed Mice Through Reducing Microglial Proinflammatory Cytokines

In comparison to conventional repetitive transcranial magnetic stimulation (rTMS), theta burst stimulation is stronger and more effective as a brain stimulation approach within short periods. Although this deep rTMS technique is being applied in treating neuropsychiatric disorders, few animal studies have attempted to clarify the neurobiological mechanisms underlying its beneficial effects. This animal study examined the effects of deep rTMS on the cuprizone-induced neuropathologic and behavioral anomalies and explored the underlying mechanism. Adolescent male C57BL/6 mice were fed a rodent chow without or with cuprizone (CPZ; 0.2% w/w) for 5 weeks. Another two groups of mice were subjected to deep rTMS or sham rTMS once a day during weeks 2-5 of the CPZ-feeding period. The behaviors of all mice were assessed after the withdrawal of CPZ before neuropathological and immunological analyses. Compared to the CNT group, mice in CPZ and CPZ + Sham groups showed deficits in social recognition and spatial working memory as well as anxiety-like behavior, in addition to myelin breakdown and OL loss in the corpus callosum (CC), caudate putamen, cerebral cortex, and hippocampus of the brain. Deep rTMS effectively reduced behavioral anomalies and blocked myelin breakdown and OL loss in CPZ-fed mice. Besides, it also dampened microglia activation at lesion sites and rectified cytokines levels (IL-1β, IL-6, and IL-10) in CPZ-affected regions. The most significant effect was seen in the cerebral cortex where alleviated neuropathology co-existed with less microglia activation and higher IL-10 level. These data provided experimental evidence for the beneficial effects of deep rTMS in CPZ-fed mice and revealed a neurobiological mechanism of the modality.

CD38, CD157, and RAGE as Molecular Determinants for Social Behavior

Recent studies provide evidence to support that cluster of differentiation 38 (CD38) and CD157 meaningfully act in the brain as neuroregulators. They primarily affect social behaviors. Social behaviors are impaired in Cd38 and Cd157 knockout mice. Single-nucleotide polymorphisms of the CD38 and CD157/BST1 genes are associated with multiple neurological and psychiatric conditions, including autism spectrum disorder, Parkinson’s disease, and schizophrenia. In addition, both antigens are related to infectious and immunoregulational processes. The most important clues to demonstrate how these molecules play a role in the brain are oxytocin (OT) and the OT system. OT is axo-dendritically secreted into the brain from OT-containing neurons and causes activation of OT receptors mainly on hypothalamic neurons. Here, we overview the CD38/CD157-dependent OT release mechanism as the initiation step for social behavior. The receptor for advanced glycation end-products (RAGE) is a newly identified molecule as an OT binding protein and serves as a transporter of OT to the brain, crossing over the blood–brain barrier, resulting in the regulation of brain OT levels. We point out new roles of CD38 and CD157 during neuronal development and aging in relation to nicotinamide adenine dinucleotide⁺ levels in embryonic and adult nervous systems. Finally, we discuss how CD38, CD157, and RAGE are crucial for social recognition and behavior in daily life.