Extracellular NAD(+) induces calcium signaling and apoptosis in human osteoblastic cells

Intermittent Exposure to a 16 Hz Extremely Low Frequency Pulsed Electromagnetic Field Promotes Osteogenesis In Vitro through Activating Piezo 1-Induced Ca2+ Influx in Osteoprogenitor Cells

Exposure to extremely low frequency pulsed electromagnetic fields (ELF-PEMF) is supposed to simulate local EMF generated during mechanical stimulation of bone and may therefore be used to improve bone regeneration. This study aimed at optimizing the exposure strategy and investigating the underlying mechanisms of a 16 Hz ELF-PEMF, previously reported to boost osteoblast function. Comparing influences of daily continuous (30 min every 24 h) and intermittent (10 min every 8 h) exposure to the 16 Hz ELF-PEMF on osteoprogenitor cells revealed that the intermittent exposure strategy enhanced the 16 Hz ELF-PEMF effects regarding cell numbers and osteogenic function. Gene expression of piezo 1 and related Ca²⁺ influx were significantly increased in SCP-1 cells with the daily intermittent exposure. Pharmacological inhibition of piezo 1 with Dooku 1 largely abolished the positive effect of the 16 Hz ELF-PEMF exposure on osteogenic maturation of SCP-1 cells. In summary, the intermittent exposure strategy enhanced the positive effects of 16 Hz continuous ELF-PEMF exposure in terms of cell viability and osteogenesis. This effect was shown to be mediated by an increased expression of piezo 1 and related Ca²⁺ influx. Thus, the intermittent exposure strategy is a promising way to further optimize the therapeutic effects of the 16 Hz ELF-PEMF regarding fracture healing or osteoporosis.

Dynamic Aging: Channeled Through Microenvironment

Aging process is a complicated process that involves deteriorated performance at multiple levels from cellular dysfunction to organ degeneration. For many years research has been focused on how aging changes things within cell. However, new findings suggest that microenvironments, circulating factors or inter-tissue communications could also play important roles in the dynamic progression of aging. These out-of-cell mechanisms pass on the signals from the damaged aging cells to other healthy cells or tissues to promote systematic aging phenotypes. This review discusses the mechanisms of how senescence and their secretome, NAD⁺ metabolism or circulating factors change microenvironments to regulate systematic aging, as well as the potential therapeutic strategies based on these findings for anti-aging interventions.

Expression of CD38 in myeloma bone niche: A rational basis for the use of anti-CD38 immunotherapy to inhibit osteoclast formation

It is known that multiple myeloma (MM) cells express CD38 and that a recently developed human anti-CD38 monoclonal antibody Daratumumab mediates myeloma killing. However, the expression of CD38 and other functionally related ectoenzymes within the MM bone niche and the potential effects of Daratumumab on bone cells are still unknown. This study firstly defines by flow cytometry and immunohistochemistry the expression of CD38 by bone marrow cells in a cohort of patients with MM and indolent monoclonal gammopathies. Results indicate that only plasma cells expressed CD38 at high level within the bone niche. In addition, the flow cytometry analysis shows that CD38 was also expressed by monocytes and early osteoclast progenitors but not by osteoblasts and mature osteoclasts. Indeed, CD38 was lost during in vitro osteoclastogenesis. Consistently, we found that Daratumumab reacted with CD38 expressed on monocytes and its binding inhibited in vitro osteoclastogenesis and bone resorption activity from bone marrow total mononuclear cells of MM patients, targeting early osteoclast progenitors. The inhibitory effect was not observed from purified CD14⁺ cells, suggesting an indirect inhibitory effect of Daratumumab. Interestingly, all-trans retinoic acid treatment increased the inhibitory effect of Daratumumab on osteoclast formation.

These observations provide a rationale for the use of an anti-CD38 antibody-based approach as treatment for multiple myeloma-induced osteoclastogenesis.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Importance of Glutamate Dehydrogenase (GDH) in Clostridium difficile Colonization In Vivo

Clostridium difficile is the principal cause of antibiotic-associated diarrhea. Major metabolic requirements for colonization and expansion of C. difficile after microbiota disturbance have not been fully determined. In this study, we show that glutamate utilization is important for C. difficile to establish itself in the animal gut. When the gluD gene, which codes for glutamate dehydrogenase (GDH), was disrupted, the mutant C. difficile was unable to colonize and cause disease in a hamster model. Further, from the complementation experiment it appears that extracellular GDH may be playing a role in promoting C. difficile colonization and disease progression. Quantification of free amino acids in the hamster gut during C. difficile infection showed that glutamate is among preferred amino acids utilized by C. difficile during its expansion. This study provides evidence of the importance of glutamate metabolism for C. difficile pathogenesis.

Mitochondrial dysfunction causing cardiac sodium channel downregulation in cardiomyopathy

Cardiomyopathy is associated with cardiac Na(+) channel downregulation that may contribute to arrhythmias. Previously, we have shown that elevated intracellular NADH causes a decrease in cardiac Na(+) current (I(Na)) signaled by an increase in mitochondrial reactive oxygen species (ROS). In this study, we tested whether the NADH-mitochondria ROS pathway was involved in the reduction of I(Na) in a nonischemic cardiomyopathic model and correlated the findings with myopathic human hearts. Nonischemic cardiomyopathy was induced in C57BL/6 mice by hypertension after unilateral nephrectomy, deoxycorticosterone acetate (DOCA) pellet implantation, and salt water substitution. Sham operated mice were used as controls. After six weeks, heart tissue and ventricular myocytes isolated from mice were utilized for whole cell patch clamp recording, NADH/NAD(+) level measurements, and mitochondrial ROS monitoring with confocal microscopy. Human explanted hearts were studied using optical mapping. Compared to the sham mice, the arterial blood pressure was higher, the left ventricular volume was significantly enlarged (104.7±3.9 vs. 87.9±6.1 μL, P<0.05), and the ejection fraction was reduced (37.1±1.8% vs. 49.4±3.7%, P<0.05) in DOCA mice. Both the whole cell and cytosolic NADH level were increased (279±70% and 123±2% of sham, respectively, P<0.01), I(Na) was decreased (60±10% of sham, P<0.01), and mitochondrial ROS overproduction was observed (2.9±0.3-fold of sham, P<0.01) in heart tissue and myocytes of myopathic mice vs. sham. Treatment of myocytes with NAD(+) (500 μM), mitoTEMPO (10 μM), chelerythrine (50 μM), or forskolin (5 μM) restored I(Na) back to the level of sham. Injection of NAD(+) (100mg/kg) or mitoTEMPO (0.7 mg/kg) twice (at 24h and 1h before myocyte isolation) to animals also restored I(Na). All treatments simultaneously reduced mitochondrial ROS levels to that of controls. CD38 was found to transduce the extracellular NAD(+) signal. Correlating with the mouse model, failing human hearts showed a reduction in conduction velocity that improved with NAD(+). Nonischemic cardiomyopathy was associated with elevated NADH level, PKC activation, mitochondrial ROS overproduction, and a concomitant decrease in I(Na). Reducing mitochondrial ROS by application of NAD(+), mitoTEMPO, PKC inhibitors, or PKA activators, restored I(Na). NAD(+) improved conduction velocity in human myopathic hearts.

Identification of a novel pathway of transforming growth factor-β1 regulation by extracellular NAD+ in mouse macrophages: in vitro and in silico studies

Extracellular β-nicotinamide adenine dinucleotide (NAD(+)) is anti-inflammatory. We hypothesized that NAD(+) would modulate the anti-inflammatory cytokine Transforming Growth Factor (TGF)-β1. Indeed, NAD(+) led to increases in both active and latent cell-associated TGF-β1 in RAW 264.7 mouse macrophages as well as in primary peritoneal macrophages isolated from both C3H/HeJ (TLR4-mutant) and C3H/HeOuJ (wild-type controls for C3H/HeJ) mice. NAD(+) acts partially via cyclic ADP-ribose (cADPR) and subsequent release of Ca(2+). Treatment of macrophages with the cADPR analog 3-deaza-cADPR or Ca(2+) ionophores recapitulated the effects of NAD(+) on TGF-β1, whereas the cADPR antagonist 8-Br-cADPR, Ca(2+) chelation, and antagonism of L-type Ca(2+) channels suppressed these effects. The time and dose effects of NAD(+) on TGF-β1 were complex and could be modeled both statistically and mathematically. Model-predicted levels of TGF-β1 protein and mRNA were largely confirmed experimentally but also suggested the presence of other mechanisms of regulation of TGF-β1 by NAD(+). Thus, in vitro and in silico evidence points to NAD(+) as a novel modulator of TGF-β1.

Metabolic stress, reactive oxygen species, and arrhythmia

Cardiac arrhythmias can cause sudden cardiac death (SCD) and add to the current heart failure (HF) health crisis. Nevertheless, the pathological processes underlying arrhythmias are unclear. Arrhythmic conditions are associated with systemic and cardiac oxidative stress caused by reactive oxygen species (ROS). In excitable cardiac cells, ROS regulate both cellular metabolism and ion homeostasis. Increasing evidence suggests that elevated cellular ROS can cause alterations of the cardiac sodium channel (Na(v)1.5), abnormal Ca(2+) handling, changes of mitochondrial function, and gap junction remodeling, leading to arrhythmogenesis. This review summarizes our knowledge of the mechanisms by which ROS may cause arrhythmias and discusses potential therapeutic strategies to prevent arrhythmias by targeting ROS and its consequences. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

Calcium and bone disease

Calcium transport and calcium signaling are of basic importance in bone cells. Bone is the major store of calcium and a key regulatory organ for calcium homeostasis. Bone, in major part, responds to calcium-dependent signals from the parathyroids and via vitamin D metabolites, although bone retains direct response to extracellular calcium if parathyroid regulation is lost. Improved understanding of calcium transporters and calcium-regulated cellular processes has resulted from analysis of genetic defects, including several defects with low or high bone mass. Osteoblasts deposit calcium by mechanisms including phosphate and calcium transport with alkalinization to absorb acid created by mineral deposition; cartilage calcium mineralization occurs by passive diffusion and phosphate production. Calcium mobilization by osteoclasts is mediated by acid secretion. Both bone forming and bone resorbing cells use calcium signals as regulators of differentiation and activity. This has been studied in more detail in osteoclasts, where both osteoclast differentiation and motility are regulated by calcium.

Cardiac Na+ current regulation by pyridine nucleotides

RATIONALE

Mutations in glycerol-3-phosphate dehydrogenase 1-like (GPD1-L) protein reduce cardiac Na+ current (I(Na)) and cause Brugada Syndrome (BrS). GPD1-L has >80% amino acid homology with glycerol-3-phosphate dehydrogenase, which is involved in NAD-dependent energy metabolism.

OBJECTIVE

Therefore, we tested whether NAD(H) could regulate human cardiac sodium channels (Na(v)1.5).

METHODS AND RESULTS

HEK293 cells stably expressing Na(v)1.5 and rat neonatal cardiomyocytes were used. The influence of NADH/NAD+ on arrhythmic risk was evaluated in wild-type or SCN5A(+/-) mouse heart. A280V GPD1-L caused a 2.48+/-0.17-fold increase in intracellular NADH level (P<0.001). NADH application or cotransfection with A280V GPD1-L resulted in decreased I(Na) (0.48+/-0.09 or 0.19+/-0.04 of control group, respectively; P<0.01), which was reversed by NAD+, chelerythrine, or superoxide dismutase. NAD+ antagonism of the Na+ channel downregulation by A280V GPD1-L or NADH was prevented by a protein kinase (PK)A inhibitor, PKAI(6-22). The effects of NADH and NAD+ were mimicked by a phorbol ester and forskolin, respectively. Increasing intracellular NADH was associated with an increased risk of ventricular tachycardia in wild-type mouse hearts. Extracellular application of NAD+ to SCN5A(+/-) mouse hearts ameliorated the risk of ventricular tachycardia.

CONCLUSIONS

Our results show that Na(v)1.5 is regulated by pyridine nucleotides, suggesting a link between metabolism and I(Na). This effect required protein kinase C activation and was mediated by oxidative stress. NAD+ could prevent this effect by activating PKA. Mutations of GPD1-L may downregulate Na(v)1.5 by altering the oxidized to reduced NAD(H) balance.

N-type and P/Q-type calcium channels regulate differentially the release of noradrenaline, ATP and beta-NAD in blood vessels

Using HPLC techniques we evaluated the electrical field stimulation-evoked overflow of noradrenaline (NA), adenosine 5'-triphosphate (ATP), and beta-nicotinamide adenine dinucleotide (beta-NAD) in the presence of low nanomolar concentrations of omega-conotoxin GVIA or omega-agatoxin IVA in the canine mesenteric arteries and veins. omega-conotoxin GVIA abolished the evoked overflow of NA and beta-NAD in artery and vein, whereas the evoked overflow of ATP remained unchanged in the presence of omega-conotoxin GVIA. omega-agatoxin IVA significantly reduced the evoked overflow of ATP and beta-NAD. The overflow of NA remained largely unaffected by omega-agatoxin IVA, except at 16Hz in the vein where the overflow of NA was reduced by about 50%. Artery and vein exhibited similar expression levels of the alpha(1B) (CaV2.2, N-type) subunit, whereas the vein showed greater levels of the alpha(1A) (CaV2.1, P/Q-type) subunit than artery. Therefore, there are at least two release sites for NA, beta-NAD and ATP in the canine mesenteric artery and vein: an N-type-associated site releasing primarily NA, beta-NAD and some ATP, and a P/Q-type-associated site releasing ATP, beta-NAD and some NA. The N-type-mediated mechanisms are equally expressed in artery and vein, whereas the P/Q-type-mediated mechanisms are more pronounced in the vein and may ensure additional neurotransmitter release at higher levels of neural activity. In artery, beta-NAD caused a dual effect consisting of vasodilatation or vasoconstriction depending on concentrations, whereas vein responded with vasodilatation only. In contrast, ATP caused vasoconstriction in both vessels. beta-NAD and ATP may mediate disparate functions in the canine mesenteric resistive and capacitative circulations.

Beta-nicotinamide adenine dinucleotide is an inhibitory neurotransmitter in visceral smooth muscle

Peripheral inhibitory nerves are physiological regulators of the contractile behavior of visceral smooth muscles. One of the transmitters responsible for inhibitory neurotransmission has been reputed to be a purine, possibly ATP. However, the exact identity of this substance has never been verified. Here we show that beta-nicotinamide adenine dinucleotide (beta-NAD), an inhibitory neurotransmitter candidate, is released by stimulation of enteric nerves in gastrointestinal muscles, and the pharmacological profile of beta-NAD mimics the endogenous neurotransmitter better than ATP. Levels of beta-NAD in superfusates of muscles after nerve stimulation exceed ATP by at least 30-fold; unlike ATP, the release of beta-NAD depends on the frequency of nerve stimulation. beta-NAD is released from enteric neurons, and release was blocked by tetrodotoxin or omega-conotoxin GVIA. beta-NAD is an agonist for P2Y1 receptors, as demonstrated by receptor-mediated responses in HEK293 cells expressing P2Y1 receptors. Exogenous beta-NAD mimics the effects of the enteric inhibitory neurotransmitter. Responses to beta-NAD and inhibitory junction potentials are blocked by the P2Y1-selective antagonist, MRS2179, and the nonselective P2 receptor antagonists, pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid and suramin. Responses to ATP are not blocked by these P2Y receptor inhibitors. The expression of CD38 in gastrointestinal muscles, and specifically in interstitial cells of Cajal, provides a means of transmitter disposal after stimulation. beta-NAD meets the traditional criteria for a neurotransmitter that contributes to enteric inhibitory regulation of visceral smooth muscles.

Calcium signalling and calcium transport in bone disease

Calcium transport and calcium signalling mechanisms in bone cells have, in many cases, been discovered by study of diseases with disordered bone metabolism. Calcium matrix deposition is driven primarily by phosphate production, and disorders in bone deposition include abnormalities in membrane phosphate transport such as in chondrocalcinosis, and defects in phosphate-producing enzymes such as in hypophosphatasia. Matrix removal is driven by acidification, which dissolves the mineral. Disorders in calcium removal from bone matrix by osteoclasts cause osteopetrosis. On the other hand, although bone is central to management of extracellular calcium, bone is not a major calcium sensing organ, although calcium sensing proteins are expressed in both osteoblasts and osteoclasts. Intracellular calcium signals are involved in secondary control including cellular motility and survival, but the relationship of these findings to specific diseases is not clear. Intracellular calcium signals may regulate the balance of cell survival versus proliferation or anabolic functional response as part of signalling cascades that integrate the response to primary signals via cell stretch, estrogen, tyrosine kinase, and tumor necrosis factor receptors.

NAD+ ameliorates inflammation-induced epithelial barrier dysfunction in cultured enterocytes and mouse ileal mucosa

In the course of other experiments, we serendipitously observed that extracellular nicotinamide adenine dinucleotide (NAD+) ameliorated the development of epithelial hyperpermeability when monolayers of Caco-2 enterocyte-like cells were incubated with cytomix, a mixture containing interferon-gamma, interleukin-1beta, and tumor necrosis factor-alpha. We sought to characterize the effects of NAD+ on inflammation-induced epithelial barrier dysfunction using Caco-2 monolayers that were exposed to cytomix in the absence or presence of NAD+ or other purine-containing molecules. Paracellular barrier function measured as the apical-to-basolateral passage of fluorescein isothiocyanate-conjugated dextran (mol. wt. approximately 4000) was preserved in a concentration-dependent manner when immunostimulated Caco-2 cells were exposed to extracellular NAD+. Incubation with NAD+ prevented cytomix-induced derangements in the expression and localization of the tight junction proteins occludin and zonula occludens-1 in Caco-2 cells. Treatment of cytomix-stimulated cells with NAD+ also blocked nuclear factor-kappaB (NF-kappaB) activation, inducible nitric-oxide synthase induction, and increased production of nitric oxide (NO.). Ileal mucosal permeability to fluorescein isothiocyanate-dextran mol. wt. approximately 4000 was increased in mice 18 h after lipopolysaccharide (endotoxin) injection, but treatment of endotoxemic mice with NAD+ ameliorated the development of gut mucosal hyperpermeability. Thus, extracellular NAD+ seems to ameliorate inflammation-induced intestinal epithelial barrier dysfunction by inhibiting NF-kappaB activation and increased NO. production.

Proline oxidase induces apoptosis in tumor cells, and its expression is frequently absent or reduced in renal carcinomas

Proline oxidase is a p53-induced gene that can mediate apoptosis in lung carcinoma cells. Here, we provide evidence implicating a role for proline oxidase in renal carcinoma. We observed absent or reduced expression of proline oxidase in 8 of 12 primary renal cell carcinomas, with respect to their normal tissue counterparts. Two renal cell carcinomas, which displayed little or no expression of proline oxidase, expressed p53s that were less capable of inducing proline oxidase than p53 isolated from normal renal tissue. One of those tumor-derived p53s contained a double transition mutation at amino acid residues 125 (Ala to Thr) and 193 (Arg to His), and the other exhibited a single transition mutation at amino acid 149 (Ser to Phe). Forced up-regulation of proline oxidase induced the formation of reactive oxygen species and mediated apoptosis in the 786-0 renal cell carcinoma cell line. A proline oxidase antisense vector repressed p53-induced up-regulation of proline oxidase, release of cytochrome c from mitochondria, and apoptosis in 786-0 renal carcinoma cells. Taken together, these findings support a role for proline oxidase as a downstream effector in p53-mediated apoptosis. We hypothesize that its altered expression can contribute to the development of renal carcinomas. The presence of proline oxidase in mitochondria, a primary organelle that regulates apoptosis, places this molecule in a subcellular localization that can directly influence the apoptotic pathway and thus tumorigenesis.

Disordered osteoclast formation and function in a CD38 (ADP-ribosyl cyclase)-deficient mouse establishes an essential role for CD38 in bone resorption

We have evaluated the role of the ADP-ribosyl cyclase, CD38, in bone remodeling, a process by which the skeleton is being renewed constantly through the coordinated activity of osteoclasts and osteoblasts. CD38 catalyzes the cyclization of its substrate, NAD+, to the Ca2+-releasing second messenger, cyclic ADP-ribose (cADPr). We have shown previously that CD38 is expressed both in osteoblasts and osteoclasts. Its activation in the osteoclast triggers Ca2+ release through ryanodine receptors (RyRs), stimulation of interleukin-6 (IL-6), and an inhibition of bone resorption. Here, we have examined the consequences of deleting the CD38 gene in mice on skeletal remodeling. We report that CD38-/- mice displayed a markedly reduced bone mineral density (BMD) at the femur, tibia, and lumbar spine at 3 months and at the lumbar spine at 4 months, with full normalization of the BMD at all sites at 5 months. The osteoporosis at 3 months was accompanied by a reduction in primary spongiosa and increased osteoclast surfaces on histomorphometric analysis. Hematopoetic stem cells isolated ex vivo from CD38-/- mice showed a dramatic approximately fourfold increase in osteoclast formation in response to incubation for 6 days with RANK-L and M-CSF. The osteoclasts so formed in these cultures showed a approximately 2.5-fold increase in resorptive activity compared with wild-type cells. However, when adherent bone marrow stromal cells were allowed to mature into alkaline phosphatase-positive colony-forming units (CFU-Fs), those derived from CD38-/- mice showed a significant reduction in differentiation compared with wild-type cells. Real-time RT-PCR on mRNA isolated from osteoclasts at day 6 showed a significant reduction in IL-6 and IL-6 receptor mRNA, together with significant decreases in the expression of all calcineurin A isoforms, alpha, beta, and gamma. These findings establish a critical role for CD38 in osteoclast formation and bone resorption. We speculate that CD38 functions as a cellular NAD+ "sensor," particularly during periods of active motility and secretion.

Extracellular NAD+: a novel autocrine/paracrine signal in osteoblast physiology

Intercellular communication allows co-ordination of cell metabolism and sensitivity to extracellular stimuli. In bone cells, paracrine stimulation and cell-to-cell coupling through gap junctions induce the formation of complex intercellular networks, which favours the intercellular exchange of nutrients and second messengers, ultimately controlling the process of bone remodelling. The importance of local factors in bone remodelling is known since many years. Bone cells secrete and respond to a variety signals, among which include prostaglandins, cytokines, growth factors, and ATP. We here report evidence that extracellular NAD(+) is a novel extracellular signal stimulating osteoblast differentiation. We found that HOBIT human osteoblastic cells, which are known to express ADP-ribosyl cyclase/CD38 activity, respond to micromolar concentrations of extracellular NAD(+) with oscillatory increases of the cytosolic Ca(2+) concentration. The initial Ca(2+) response was followed by a time-dependent inhibition of cell growth, the appearance of an epithelial morphology, and by an increase of alkaline phosphatase and osteocalcin expression. Under resting condition HOBIT cells release NAD(+) in the extracellular medium and the release is significantly potentiated by mechanical stimulation. Taken together these results point to NAD(+) as a novel autocrine/paracrine factor involved in stimulation and maintenance of the osteoblast differentiated phenotype.