Metabolomic and transcriptional profiling reveals bioenergetic stress and activation of cell death and inflammatory pathways in vivo after neuronal deletion of NAMPT

Effect of alternating nicotinamide phosphoribosyltransferase expression levels on mitophagy in Alzheimer's disease mouse models

AD is the abbreviation for Alzheimer's Disease, which is a neurodegenerative disorder that features progressive dysfunction in cognition. Previous research has reported that mitophagy impairment and mitochondrial dysfunction have been crucial factors in the AD's pathogenesis. More recently, literature has emerged which offers findings suggesting that the nicotinamide adenine dinucleotide (short for NAD+) augmentation eliminates the defective mitochondria and restores mitophagy. Meanwhile, as an enzyme which is rate-limiting, the Nicotinamide phosphoribosyltransferase, or NAMPT, is part of the salvage pathway of NAD+ synthesis. Therefore, the aim of the research project has been to produce proof for how the NAMPT-NAD +-silent information-regulated transcription factors1/3 (short for SIRT1/3) axis function in mediating mitophagy in APP/PS1 mice aged six months. The results revealed that the NAMPT-NAD+-SIRT1/3 axis in the APP/PS1 mice's hippocampus was considerably declined. Surprisingly, P7C3 (an NAMPT activator) noticeably promoted the NAD+-SIRT1/3 axis, improved mitochondrial structure and function, enhanced mitophagy activity along with the ability of learning and memory. While FK866 (an NAMPT inhibitor) reversed the decreased NAD+-SIRT1/3 axis, and even exacerbated Aβ plaque deposition level in the APP/PS1 mice's hippocampus. The findings observed in this study indicate two main points: avoiding downregulation of the NAMPT activity can prevent AD-related mitophagy impairment; on the other hand, NAMPT characterizes a potential therapeutic intervention regarding AD pathogenesis.

Dietary NMN supplementation enhances motor and NMJ function in ALS

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease that causes the degeneration of motor neurons in the motor cortex and spinal cord. Patients with ALS experience muscle weakness and atrophy in the limbs which eventually leads to paralysis and death. NAD+ is critical for energy metabolism, such as glycolysis and oxidative phosphorylation, but is also involved in non-metabolic cellular reactions. In the current study, we determined whether the supplementation of nicotinamide mononucleotide (NMN), an NAD+ precursor, in the diet had beneficial impacts on disease progression using a SOD1G93A mouse model of ALS. We found that the ALS mice fed with an NMN-supplemented diet (ALS+NMN mice) had modestly extended lifespan and exhibited delayed motor dysfunction. Using electrophysiology, we studied the effect of NMN on synaptic transmission at neuromuscular junctions (NMJs) in symptomatic of ALS mice (18 weeks old). ALS+NMN mice had larger end-plate potential (EPP) amplitudes and maintained better responses than ALS mice, and also had restored EPP facilitation. While quantal content was not affected by NMN, miniature EPP (mEPP) amplitude and frequency were elevated in ALS+NMN mice. NMN supplementation in diet also improved NMJ morphology, innervation, mitochondrial structure, and reduced reactive astrogliosis in the ventral horn of the lumbar spinal cord. Overall, our results indicate that dietary consumption of NMN can slow motor impairment, enhance NMJ function and improve healthspan of ALS mice.

Drug discovery targeting nicotinamide phosphoribosyltransferase (NAMPT): Updated progress and perspectives

Nicotinamide phosphoribosyltransferase (NAMPT) is a key rate-limiting enzyme in the nicotinamide adenine dinucleotide (NAD+) salvage pathway, primarily catalyzing the synthesis of nicotinamide mononucleotide (NMN) from nicotinamide (NAM), phosphoribosyl pyrophosphate (PRPP), and adenosine triphosphate (ATP). Metabolic diseases, aging-related diseases, inflammation, and cancers can lead to abnormal expression levels of NAMPT due to the pivotal role of NAD+ in redox metabolism, aging, the immune system, and DNA repair. In addition, NAMPT can be secreted by cells as a cytokine that binds to cell membrane receptors to regulate intracellular signaling pathways. Furthermore, NAMPT is able to reduce therapeutic efficacy by enhancing acquired resistance to chemotherapeutic agents. Recently, a few novel activators and inhibitors of NAMPT for neuroprotection and anti-tumor have been reported, respectively. However, NAMPT activators are still in preclinical studies, and only five NAMPT inhibitors have entered the clinical stage, unfortunately, three of which were terminated or withdrawn due to safety concerns. Novel drug design strategies such as proteolytic targeting chimera (PROTAC), antibody-drug conjugate (ADC), and dual-targeted inhibitors also provide new directions for the development of NAMPT inhibitors. In this perspective, we mainly discuss the structure, biological function, and role of NAMPT in diseases and the currently discovered activators and inhibitors. It is our hope that this work will provide some guidance for the future design and optimization of NAMPT activators and inhibitors.

The NAD salvage pathway in mesenchymal cells is indispensable for skeletal development in mice

NAD is an essential co-factor for cellular energy metabolism and multiple other processes. Systemic NAD+ deficiency has been implicated in skeletal deformities during development in both humans and mice. NAD levels are maintained by multiple synthetic pathways but which ones are important in bone forming cells is unknown. Here, we generate mice with deletion of Nicotinamide Phosphoribosyltransferase (Nampt), a critical enzyme in the NAD salvage pathway, in all mesenchymal lineage cells of the limbs. At birth, NamptΔPrx1 exhibit dramatic limb shortening due to death of growth plate chondrocytes. Administration of the NAD precursor nicotinamide riboside during pregnancy prevents the majority of in utero defects. Depletion of NAD post-birth also promotes chondrocyte death, preventing further endochondral ossification and joint development. In contrast, osteoblast formation still occurs in knockout mice, in line with distinctly different microenvironments and reliance on redox reactions between chondrocytes and osteoblasts. These findings define a critical role for cell-autonomous NAD homeostasis during endochondral bone formation.

The Depletion of NAMPT Disturbs Mitochondrial Homeostasis and Causes Neuronal Degeneration in Mouse Hippocampus

Nicotinamide phosphoribosyltransferase (NAMPT) is the key enzyme in the salvaging synthesis pathway of the nicotinamide adenine dinucleotide (NAD). Both NAMPT and NAD progressively decline upon aging and neurodegenerative diseases. The depletion of NAMPT induces mitochondrial dysfunction in motor neurons and causes bioenergetic stress in neurons. However, the roles of NAMPT in hippocampus neurons need to be further studied. Using floxed Nampt (Nampt^flox/flox) mice, we knocked out Nampt specifically in the hippocampus CA1 neurons by injecting rAAV-hSyn-Cre-APRE-pA. The depletion of NAMPT in hippocampus neurons induced cognitive deficiency in mice. Nevertheless, no morphological change of hippocampus neurons was observed with immunofluorescent imaging. Under the transmission electron microscope, we observed mitochondrial swollen and mitochondrial number decreasing in the cell body and the neurites of hippocampus neurons. In addition, we found the intracellular Aβ (6E10) increased in the hippocampus CA1 region. The intensity of Aβ42 remained unchanged, but it tended to aggregate. The GFAP level, an astrocyte marker, and the Iba1 level, a microglia marker, significantly increased in the mouse hippocampus. In the primary cultured rat neurons, NAMPT inhibition by FK866 decreased the NAD level of neurons at > 10^-9 M. FK866 dropped the mitochondrial membrane potential in the cell body of neurons at > 10^-9 M and in the dendrite of neurons at > 10^-8 M. FK866 decreased the number and shortened the length of branches of neurons at > 10^-7 M. Together, likely due to the injury of mitochondria, the decline of NAMPT level can be a critical risk factor for neurodegeneration.

Metabolic Activation of PARP as a SARS-CoV-2 Therapeutic Target-Is It a Bait for the Virus or the Best Deal We Could Ever Make with the Virus? Is AMBICA the Potential Cure?

The COVID-19 pandemic has had a great impact on global health and is an economic burden. Even with vaccines and anti-viral medications we are still scrambling to get a balance. In this perspective, we have shed light upon an extremely feasible approach by which we can control the SARS-CoV-2 infection and the associated complications, bringing some solace to this ongoing turmoil. We are providing some insights regarding an ideal agent which could prevent SARS-CoV-2 multiplication. If we could identify an agent which is an activator of metabolism and is also bioactive, we could prevent corona activation (AMBICA). Some naturally occurring lipid molecules best fit this identity as an agent which has the capacity to replenish our host cells, specifically immune cells, with ATP. It could also act as a source for providing a substrate for host cell PARP family members for MARylation and PARylation processes, leading to manipulation of the viral macro domain function, resulting in curbing the virulence and propagation of SARS-CoV-2. Identification of the right lipid molecule or combination of lipid molecules will fulfill the criteria. This perspective has focused on a unique angle of host-pathogen interaction and will open up a new dimension in treating COVID-19 infection.

NAMPT inhibition reduces macrophage inflammation through the NAD+/PARP1 pathway to attenuate liver ischemia-reperfusion injury

BACKGROUND

Liver ischemia-reperfusion injury (IRI) is a major complication in the perioperative period and often leads to liver failure and even systemic inflammation. Previous studies have suggested that the inflammatory response participated in the liver damage during liver IRI. Nicotinamide phosphoribosyl transferase (NAMPT) is required for the maintenance of cellular nicotinamide adenine dinucleotide (NAD+) levels, catalyzing the rate-limiting step in the NAD + salvage pathway. NAMPT is strongly upregulated during inflammation and constitutes an important mechanistic link between inflammatory, metabolic, and transcriptional pathways. The aim of our study was to investigate the role of NAMPT in liver IRI.

METHODS

We investigated the effect of pharmacological inhibition of NAMPT with FK866 in models of liver IRI. Liver damage was assessed by HE staining, serum ALT/AST, and TUNEL staining. To examine the mechanism, primary hepatocytes, liver macrophages and RAW264.7 cells were treated with or without NAMPT inhibitors before hypoxia-reoxygenation. Liver macrophages and RAW 264.7 cells activation in vitro was evaluated by western blotting, flow cytometry, and ELISA.

RESULT

We found that NAMPT was upregulated in liver IRI. Treatment with the NAMPT inhibitor FK866 ameliorated liver IRI and suppressed inflammation in mice. Although NAMPT plays an important role both in hepatocytes and liver macrophages, we focused on the impact of NAMPT on liver macrophages. The mechanism revealed that FK866 potently inhibited NAMPT activity, as demonstrated by reduced liver NAD+ and intracellular NAD+, resulting in reduced abundance and activity of NAD + -dependent enzymes, including poly (ADP-ribose) polymerase 1 (PARP1), thus inhibiting macrophage M1 polarization by reducing CD86, iNOS, TNF-α, and interleukin (IL)-1β. Taken together, our data suggested that NAMPT can regulate macrophage polarization through NAD+/PARP1 to ameliorate liver injury, and that FK866-mediated NAMPT blockade may be a therapeutic approach in liver IRI.

Emerging evidence for compromised axonal bioenergetics and axoglial metabolic coupling as drivers of neurodegeneration

Impaired bioenergetic capacity of the nervous system is thought to contribute to the pathogenesis of many neurodegenerative diseases (NDD). Since neuronal synapses are believed to be the major energy consumers in the nervous system, synaptic derangements resulting from energy deficits have been suggested to play a central role for the development of many of these disorders. However, long axons constitute the largest compartment of the neuronal network, require large amounts of energy, are metabolically and structurally highly vulnerable, and undergo early injurious stresses in many NDD. These stresses likely impose additional energy demands for continuous adaptations and repair processes, and may eventually overwhelm axonal maintenance mechanisms. Indeed, pathological axon degeneration (pAxD) is now recognized as an etiological focus in a wide array of NDD associated with bioenergetic abnormalities. In this paper I first discuss the recognition that a simple experimental model for pAxD is regulated by an auto-destruction program that exhausts distressed axons energetically. Provision of the energy substrate pyruvate robustly counteracts this axonal breakdown. Importantly, energy decline in axons is not only a consequence but also an initiator of this program. This opens the intriguing possibility that axon dysfunction and pAxD can be suppressed by preemptively energizing distressed axons. Second, I focus on the emerging concept that axons communicate energetically with their flanking glia. This axoglial metabolic coupling can help offset the axonal energy decline that activates the pAxD program but also jeopardize axon integrity as a result of perturbed glial metabolism. Third, I present compelling evidence that abnormal axonal energetics and compromised axoglial metabolic coupling accompany the activation of the pAxD auto-destruction pathway in models of glaucoma, a widespread neurodegenerative condition with pathogenic overlap to other common NDD. In conclusion, I propose a novel conceptual framework suggesting that therapeutic interventions focused on bioenergetic support of the nervous system should also address axons and their metabolic interactions with glia.

Nuclear NAD+-biosynthetic enzyme NMNAT1 facilitates development and early survival of retinal neurons

Despite mounting evidence that the mammalian retina is exceptionally reliant on proper NAD+ homeostasis for health and function, the specific roles of subcellular NAD+ pools in retinal development, maintenance, and disease remain obscure. Here, we show that deletion of the nuclear-localized NAD+ synthase nicotinamide mononucleotide adenylyltransferase-1 (NMNAT1) in the developing murine retina causes early and severe degeneration of photoreceptors and select inner retinal neurons via multiple distinct cell death pathways. This severe phenotype is associated with disruptions to retinal central carbon metabolism, purine nucleotide synthesis, and amino acid pathways. Furthermore, transcriptomic and immunostaining approaches reveal dysregulation of a collection of photoreceptor and synapse-specific genes in NMNAT1 knockout retinas prior to detectable morphological or metabolic alterations. Collectively, our study reveals previously unrecognized complexity in NMNAT1-associated retinal degeneration and suggests a yet-undescribed role for NMNAT1 in gene regulation during photoreceptor terminal differentiation.

Adipokines as Immune Cell Modulators in Multiple Sclerosis

Multiple sclerosis (MS), a chronic inflammatory and demyelinating disease of the central nervous system (CNS), is a major clinical and societal problem, which has a tremendous impact on the life of patients and their proxies. Current immunomodulatory and anti-inflammatory therapies prove to be relatively effective; however, they fail to concomitantly stop ongoing neurological deterioration and do not reverse acquired disability. The proportion to which genetic and environmental factors contribute to the etiology of MS is still incompletely understood; however, a recent association between MS etiology and obesity was shown, with obesity greatly increasing the risk of developing MS. An altered balance of adipokines, which are white adipose tissue (WAT) hormones, plays an important role in the low-grade chronic inflammation during obesity by their pervasive modification of local and systemic inflammation. Vice versa, inflammatory factors secreted by immune cells affect adipokine function. To explore the role of adipokines in MS pathology, we will here review the reciprocal effects of adipokines and immune cells and summarize alterations in adipokine levels in MS patient cohorts. Finally, we will discuss proof-of-concept studies demonstrating the therapeutic potential of adipokines to target both neuroinflammation and neurodegeneration processes in MS.