Metabolic function for human renalase: oxidation of isomeric forms of β-NAD(P)H that are inhibitory to primary metabolism

Product Distribution of Steady-State and Pulsed Electrochemical Regeneration of 1,4-NADH and Integration with Enzymatic Reaction

The direct electrochemical reduction of nicotinamide adenine dinucleotide (NAD+) results in various products, complicating the regeneration of the crucial 1,4-NADH cofactor for enzymatic reactions. Previous research primarily focused on steady-state polarization to examine potential impacts on product selectivity. However, this study explores the influence of dynamic conditions on the selectivity of NAD+ reduction products by comparing two dynamic profiles with steady-state conditions. Our findings reveal that the main products, including 1,4-NADH, several dimers, and ADP-ribose, remained consistent across all conditions. A minor by-product, 1,6-NADH, was also identified. The product distribution varied depending on the experimental conditions (steady state vs. dynamic) and the concentration of NAD+, with higher concentrations and overpotentials promoting dimerization. The optimal yield of 1,4-NADH was achieved under steady-state conditions with low overpotential and NAD+ concentrations. While dynamic conditions enhanced the 1,4-NADH yield at shorter reaction times, they also resulted in a significant amount of unidentified products. Furthermore, this study assessed the potential of using pulsed electrochemical regeneration of 1,4-NADH with enoate reductase (XenB) for cyclohexenone reduction.

Renalase inhibition regulates β cell metabolism to defend against acute and chronic stress

Renalase (Rnls), annotated as an oxidase enzyme, is a GWAS gene associated with Type 1 Diabetes (T1D) risk. We previously discovered that Rnls inhibition delays diabetes onset in mouse models of T1D in vivo , and protects pancreatic β cells against autoimmune killing, ER and oxidative stress in vitro . The molecular biochemistry and functions of Rnls are entirely uncharted. Here we find that Rnls inhibition defends against loss of β cell mass and islet dysfunction in chronically stressed Akita mice in vivo . We used RNA sequencing, untargeted and targeted metabolomics and metabolic function experiments in mouse and human β cells and discovered a robust and conserved metabolic shift towards glycolysis, amino acid abundance and GSH synthesis to counter protein misfolding stress, in vitro . Our work illustrates a function for Rnls in mammalian cells, and suggests an axis by which manipulating intrinsic properties of β cells can rewire metabolism to protect against diabetogenic stress.

Proteomic profiling of renal tissue of normo- and hypertensive rats with the renalase peptide RP220 as an affinity ligand

Renalase (RNLS) is a recently discovered protein that plays an important role in the regulation of blood pressure by acting inside and outside cells. Intracellular RNLS is a FAD-dependent oxidoreductase that oxidizes isomeric forms of β-NAD(P)H. Extracellular renalase lacking its N-terminal peptide and cofactor FAD exerts various protective effects via non-catalytic mechanisms. Certain experimental evidence exists in the literature that the RP220 peptide (a 20-mer peptide corresponding to the amino acid sequence RNLS 220-239) reproduces a number of non-catalytic effects of this protein, acting on receptor proteins of the plasma membrane. The possibility of interaction of this peptide with intracellular proteins has not been studied. Taking into consideration the known role of RNLS as a possible antihypertensive factor, the aim of this study was to perform proteomic profiling of the kidneys of normotensive and hypertensive rats using RP220 as an affinity ligand. Proteomic (semi-quantitative) identification revealed changes in the relative content of about 200 individual proteins in the kidneys of hypertensive rats bound to the affinity sorbent as compared to the kidneys of normotensive animals. Increased binding of SHR renal proteins to RP220 over the normotensive control was found for proteins involved in the development of cardiovascular pathology. Decreased binding of the kidney proteins from hypertensive animals to RP220 was noted for components of the ubiquitin-proteasome system, ribosomes, and cytoskeleton.

Selenoprotein P, peroxiredoxin-5, renalase, and total antioxidant status in patients with suspected obstructive sleep apnea

PURPOSE

The aim of this study was to investigate the relationship between selenoprotein P, peroxiredoxin-5, renalase, total antioxidant status (TAS), mean blood pressure (mBP), and apnea-hypopnea index (AHI).

METHODS

The study group consisted of 112 patients hospitalized to verify the diagnosis of obstructive sleep apnea (OSA). The inclusion criteria were consent to participate in the study and age ≥ 18 years. Patients with active proliferative disease, severe systemic diseases, or mental diseases were excluded from the study. Each patient underwent full polysomnography and had blood pressure measured. Blood samples were collected and laboratory test was performed.

RESULTS

Among 112 patients enrolled, there was a statistically significant negative linear correlation between blood pressure values (sBP, dBP, mBP) and selenoprotein P, renalase, and TAS levels. Similarly, there was a negative linear correlation between AHI and selenoprotein P, renalase, and TAS levels, but none between AHI and peroxiredoxin-5. Based on the obtained regression models, higher selenoprotein P, peroxiredoxin-5, and renalase levels were independently associated with higher TAS. Lower mBP values were independently associated with the use of antihypertensive drugs, higher TAS, and younger age. Male gender, higher BMI, and higher mBP were independently associated with higher AHI.

CONCLUSIONS

Higher concentrations of selenoprotein P, peroxiredoxin-5, and renalase were associated with higher TAS, which confirms their antioxidant properties. There was an indirect connection between tested antioxidants and blood pressure values.

The search for potential hypotensive peptides in the amino acid sequence of human renalase and their identification in proteolytic fragments of this protein

Renalase (RNLS) is a secretory protein discovered in 2005. It plays an important role in the regulation of blood pressure. Studies by two independent laboratories have shown that administration of purified recombinant RNLS reduced blood pressure in experimental animals. However, the mechanisms of the antihypertensive effect of RNLS still remain unclear, especially in the context of the shift in the catalytic paradigm of this protein. In addition, there is growing evidence that endogenous plasma/serum RNLS, detected by enzyme immunoassay, is not an intact protein secreted into the extracellular space, and exogenous recombinant RNLS is effectively cleaved during short-term incubation with human plasma samples. This suggests that the antihypertensive effect of RNLS may be due to peptides formed during proteolytic processing. Based on the results of a bioinformatics analysis of potential RNLS cleavage sites (Fedchenko et al., Medical Hypotheses, 2022; DOI: 10.1016/j.mehy.2022.110895), a number of short peptides have been identified in the RNLS sequence that show similarity to fragments of known peptide inhibitors of angiotensin-converting enzyme. Some of them were found as a part of larger RNLS peptides, formed during RNLS cleavage by chymotrypsin and, and to a lesser extent, by trypsin.

Coexistence of Cardiovascular Risk Factors and Blood Renalase Concentration

Cardiovascular diseases (CVDs) are one of the biggest health challenges facing health systems around the world. There are certain risk factors (CVRFs) that contribute to CVD. Risk factors associated with lifestyle such as tobacco consumption are particularly essential. Renalase is a recently discovered flavoprotein that may be involved in the progression of cardiometabolic diseases. The aim of the study was to investigate the relation between CVRFs and blood renalase concentration (BRC). The study group consisted of 96 people (51% women) who were hospitalized in the internal medicine department. CVRFs were measured using the AHA Life 7 scale. The E3109Hu ELISA kit was used to assess BRC. We found higher BRC in groups with a lower number of CVRFs (p < 0.05). We found a negative correlation between BRC and the number of CVRFs (r = -0.41). With the regression analysis, obesity, smoking, and a lack of physical activity (LoPE) were independently associated with lower blood renalase concentration. ROC analysis indicated the highest accuracy of BRC < 38.98 ng/mL in patients with ≥5 CVRFs. In conclusion, patients with a higher number of CVRFs had lower BRCs. The CVRFs particularly associated with a lower BRC were obesity, smoking, and LoPE.

Protective Renalase Deficiency in β-Cells Shapes Immune Metabolism and Function in Autoimmune Diabetes

Type 1 diabetes (T1D) is caused by the immune-mediated loss of pancreatic β-cells that produce insulin. The latest advances in stem cell (SC) β-cell differentiation methods have made a cell replacement therapy for T1D feasible. However, recurring autoimmunity would rapidly destroy transplanted SC β-cells. A promising strategy to overcome immune rejection is to genetically engineer SC β-cells. We previously identified Renalase (Rnls) as a novel target for β-cell protection. Here we show that Rnls deletion endows β-cells with the capacity to modulate the metabolism and function of immune cells within the local graft microenvironment. We used flow cytometry and single-cell RNA sequencing to characterize β-cell graft-infiltrating immune cells in a mouse model for T1D. Loss of Rnls within transplanted β-cells affected both the composition and the transcriptional profile of infiltrating immune cells in favor of an anti-inflammatory profile with decreased antigen-presenting capacity. We propose that changes in β-cell metabolism mediate local immune regulation and that this feature could be exploited for therapeutic goals.

ARTICLE HIGHLIGHTS

Protective Renalase (Rnls) deficiency impacts β-cell metabolism. Rnls-deficient β-cell grafts do not exclude immune infiltration. Rnls deficiency in transplanted β-cells broadly modifies local immune function. Immune cell in Rnls mutant β-cell grafts adopt a noninflammatory phenotype.

The Multi-Faceted Nature of Renalase for Mitochondrial Dysfunction Improvement in Cardiac Disease

The cellular mechanisms and signaling network that guide the cardiac disease pathophysiology are inextricably intertwined, which explains the current scarcity of effective therapy and to date remains the greatest challenge in state-of-the-art cardiovascular medicine. Accordingly, a novel concept has emerged in which cardiomyocytes are the centerpiece of therapeutic targeting, with dysregulated mitochondria as a critical point of intervention. Mitochondrial dysfunction pluralism seeks a multi-faceted molecule, such as renalase, to simultaneously combat the pathophysiologic heterogeneity of mitochondria-induced cardiomyocyte injury. This review provides some original perspectives and, for the first time, discusses the functionality spectrum of renalase for mitochondrial dysfunction improvement within cardiac disease, including its ability to preserve mitochondrial integrity and dynamics by suppressing mitochondrial ΔΨm collapse; overall ATP content amelioration; a rise of mtDNA copy numbers; upregulation of mitochondrial genes involved in oxidative phosphorylation and cellular vitality promotion; mitochondrial fission inhibition; NAD⁺ supplementation; sirtuin upregulation; and anti-oxidant, anti-apoptotic, and anti-inflammatory traits. If verified that renalase, due to its multi-faceted nature, behaves like the "guardian of mitochondria" by thwarting pernicious mitochondrial dysfunction effects and exerting therapeutic potential to target mitochondrial abnormalities in failing hearts, it may provide large-scale benefits for cardiac disease patients, regardless of the underlying causes.

Cyclic-AMP response element binding protein (CREB) and microRNA miR-29b regulate renalase gene expression under catecholamine excess conditions

AIMS

Renalase, a key mediator of cross-talk between kidneys and sympathetic nervous system, exerts protective roles in various cardiovascular/renal disease states. However, molecular mechanisms underpinning renalase gene expression remain incompletely understood. Here, we sought to identify the key molecular regulators of renalase under basal/catecholamine-excess conditions.

MATERIALS AND METHODS

Identification of the core promoter domain of renalase was carried out by promoter-reporter assays in N2a/HEK-293/H9c2 cells. Computational analysis of the renalase core promoter domain, over-expression of cyclic-AMP-response-element-binding-protein (CREB)/dominant negative mutant of CREB, ChIP assays were performed to determine the role of CREB in transcription regulation. Role of the miR-29b-mediated-suppression of renalase was validated in-vivo by using locked-nucleic-acid-inhibitors of miR-29. qRT-PCR and Western-blot analyses measured the expression of renalase, CREB, miR-29b and normalization controls in cell lysates/ tissue samples under basal/epinephrine-treated conditions.

KEY FINDINGS

CREB, a downstream effector in epinephrine signaling, activated renalase expression via its binding to the renalase-promoter. Physiological doses of epinephrine and isoproteronol enhanced renalase-promoter activity and endogenous renalase protein level while propranolol diminished the promoter activity and endogenous renalase protein level indicating a potential role of beta-adrenergic receptor in renalase gene regulation. Multiple animal models (acute exercise, genetically hypertensive/stroke-prone mice/rat) displayed directionally-concordant expression of CREB and renalase. Administration of miR-29b inhibitor in mice upregulated endogenous renalase expression. Moreover, epinephrine treatment down-regulated miR-29b promoter-activity/transcript levels.

SIGNIFICANCE

This study provides evidence for renalase gene regulation by concomitant transcriptional activation via CREB and post-transcriptional attenuation via miR-29b under excess epinephrine conditions. These findings have implications for disease states with dysregulated catecholamines.

The effect of renalase-derived peptides on viability of HepG₂ and PC3 cells

Renalase (RNLS) is a recently discovered protein, which plays different roles inside and outside cells. Intracellular RNLS is a FAD-dependent oxidoreductase (EC 1.6.3.5), while extracellular RNLS lacks its N-terminal peptide, FAD cofactor, and exhibits various protective effects in a non-catalytic manner. Certain evidence exists, that plasma/serum RNLS is not an intact protein secreted into the extracellular space, and exogenous recombinant RNLS is effectively degraded during short-term incubation with human plasma samples. Some synthetic analogues of the RNLS sequence (e.g. the Desir's peptide RP-220, a 20-mer peptide corresponding to the RNLS sequence 220-239) have effects on cell survival. This suggests that RNLS-derived peptides, formed during proteolytic processing, may have own biological activity. Based on results of a recent bioinformatics analysis of potential cleavage sites of RNLS (Fedchenko et al., Medical Hypotheses, 2022) we have investigated the effect of four RNLS-derived peptides as well as RP-220 and its fragment (RP-224) on the viability of two cancer cell lines: HepG₂ (human hepatoma) and PC3 (prostate cancer). Two RNLS-derived peptides (RP-207 and RP-220) decreased the viability of HepG₂ cells in a concentration dependent manner. The most pronounced and statistically significant effect (30-40% inhibition of cell growth) was observed at 50 μM concentration of each peptide. In the experiments with PC3 cells five of six RNLS-derived peptides had a significant impact on the cell viability. RP-220 and RP-224 decreased cell viability; however, no concentration dependence of this effect was observed in the range of concentrations studied (1-50 μM). Three other RNLS-derived peptides (RP-207, RP-233, and RP-265) increased viability of PC3 cells by 20-30%, but no concentration-dependence of this effect was found. Data obtained suggest that some RNLS-derived peptides may influence the viability of various cells and manifestation and direction of the effect (increase of decrease of the cell viability) is cell-type-specific.

Selenoprotein P, Peroxiredoxin-5, Renalase and Selected Cardiovascular Consequences Tested in Ambulatory Blood Pressure Monitoring and Echocardiography

This study aimed to assess the relationship between chosen antioxidants, namely selenoprotein P (SELENOP), peroxiredoxin-5 (Prdx-5), renalase and selected cardiovascular consequences tested in ambulatory blood pressure monitoring (ABPM) and echocardiography (ECHO). In our work, cardiovascular consequences refer to higher mean blood pressure (MBP) and pulse pressure (PP) on ABPM, as well as to left atrial enlargement (LAE), left ventricular hypertrophy (LVH) and lower left ventricular ejection fraction (LVEF%) on ECHO. The study group consisted of 101 consecutive patients admitted to the Department of Internal Medicine, Occupational Diseases and Hypertension to verify the diagnosis of Obstructive Sleep Apnoea (OSA). Each patient underwent full polysomnography, blood tests, ABPM and ECHO. Both selenoprotein-P and renalase levels correlated with different ABPM and ECHO parameters. We found no correlation between the peroxiredoxin-5 level and none of the tested parameters. We point to the possible application of SELENOP plasma-level testing in the initial selection of high cardiovascular-risk patients, especially if access to more advanced examinations is limited. We further suggest SELENOP measurement as a possible indicator of patients at increased left ventricular hypertrophy risk who should be of particular interest and may benefit from ECHO testing.

Cardiac-specific renalase overexpression alleviates CKD-induced pathological cardiac remodeling in mice

Introduction

CKD-induced pathological cardiac remodeling is characterized by myocardial hypertrophy and cardiac fibrosis. The available therapeutic options are limited, it is thus urgently needed to identify novel therapeutic targets. Renalase (RNLS) is a newly discovered protein secreted by the kidney and was found beneficial in many renal diseases. But whether it exerts protective effects on cardiac remodeling in CKD remains unclear.

Methods

RNLS knockout (KO) and wild-type (WT) mice were both used to build CKD models and the adeno-associated virus (AAV9) system was used to overexpress RNLS cardiac specifically. Echocardiography was performed to detect cardiac structural changes every 6 weeks until 18 weeks post-surgery. High throughput sequencing was performed to understand the underlying mechanisms and the effects of RNLS on cardiac fibroblasts were validated in vitro.

Results

Knockout of RNLS aggravated cardiac remodeling in CKD, while RNLS cardiac-specific overexpression significantly reduced left ventricular hypertrophy and cardiac fibrosis induced by CKD. The following RNA-sequencing analysis revealed that RNLS significantly downregulated the extracellular matrix (ECM) receptor interaction pathway, ECM organization, and several ECM-related proteins. GSEA results showed RNLS significantly downregulated several profibrotic biological processes of cardiac fibroblasts which were upregulated by CKD, including fibroblast proliferation, leukocyte migration, antigen presentation, cytokine production, and epithelial-mesenchymal transition (EMT). In vitro, we validated that RNLS reduced the primary cardiac fibroblast proliferation and α-SMA expression stimulated by TGF-β.

Conclusion

In this study, we examined the cardioprotective role of RNLS in CKD-induced cardiac remodeling. RNLS may be a potential therapeutic factor that exerts an anti-fibrotic effect in pathological cardiac remodeling.

Electrocatalytic NAD+ reduction via hydrogen atom-coupled electron transfer

Nicotinamide adenine dinucleotide cofactor (NAD(P)H) is regarded as an important energy carrier and charge transfer mediator. Enzyme-catalyzed NADPH production in natural photosynthesis proceeds via a hydride transfer mechanism. Selective and effective regeneration of NAD(P)H from its oxidized form by artificial catalysts remains challenging due to the formation of byproducts. Herein, electrocatalytic NADH regeneration and the reaction mechanism on metal and carbon electrodes are studied. We find that the selectivity of bioactive 1,4-NADH is relatively high on Cu, Fe, and Co electrodes without forming commonly reported NAD2 byproducts. In contrast, more NAD2 side product is formed with the carbon electrode. ADP-ribose is confirmed to be a side product caused by the fragmentation reaction of NAD+. Based on H/D isotope effects and electron paramagnetic resonance analysis, it is proposed that the formation of NADH on these metal electrodes proceeds via a hydrogen atom-coupled electron transfer (HadCET) mechanism, in contrast to the direct electron-transfer and NAD˙ radical pathway on carbon electrodes, which leads to more by-product, NAD2. This work sheds light on the mechanism of electrocatalytic NADH regeneration, which is different from biocatalysis.

Renalase: a novel regulator of cardiometabolic and renal diseases

Renalase is a ~38 kDa flavin-adenine dinucleotide (FAD) domain-containing protein that can function as a cytokine and an anomerase. It is emerging as a novel regulator of cardiometabolic diseases. Expressed mainly in the kidneys, renalase has been reported to have a hypotensive effect and may control blood pressure through regulation of sympathetic tone. Furthermore, genetic variations in the renalase gene, such as a functional missense polymorphism (Glu37Asp), have implications in the cardiovascular and renal systems and can potentially increase the risk of cardiometabolic disorders. Research on the physiological functions and biochemical actions of renalase over the years has indicated a role for renalase as one of the key proteins involved in various disease states, such as diabetes, impaired lipid metabolism, and cancer. Recent studies have identified three transcription factors (viz., Sp1, STAT3, and ZBP89) as key positive regulators in modulating the expression of the human renalase gene. Moreover, renalase is under the post-transcriptional regulation of two microRNAs (viz., miR-29b, and miR-146a), which downregulate renalase expression. While renalase supplementation may be useful for treating hypertension, inhibition of renalase signaling may be beneficial to patients with cancerous tumors. However, more incisive investigations are required to unravel the potential therapeutic applications of renalase. Based on the literature pertaining to the function and physiology of renalase, this review attempts to consolidate and comprehend the role of renalase in regulating cardiometabolic and renal disorders.

The Scientific Rationale for the Introduction of Renalase in the Concept of Cardiac Fibrosis

Cardiac fibrosis represents a redundant accumulation of extracellular matrix proteins, resulting from a cascade of pathophysiological events involved in an ineffective healing response, that eventually leads to heart failure. The pathophysiology of cardiac fibrosis involves various cellular effectors (neutrophils, macrophages, cardiomyocytes, fibroblasts), up-regulation of profibrotic mediators (cytokines, chemokines, and growth factors), and processes where epithelial and endothelial cells undergo mesenchymal transition. Activated fibroblasts and myofibroblasts are the central cellular effectors in cardiac fibrosis, serving as the main source of matrix proteins. The most effective anti-fibrotic strategy will have to incorporate the specific targeting of the diverse cells, pathways, and their cross-talk in the pathogenesis of cardiac fibroproliferation. Additionally, renalase, a novel protein secreted by the kidneys, is identified. Evidence demonstrates its cytoprotective properties, establishing it as a survival element in various organ injuries (heart, kidney, liver, intestines), and as a significant anti-fibrotic factor, owing to its, in vitro and in vivo demonstrated pleiotropy to alleviate inflammation, oxidative stress, apoptosis, necrosis, and fibrotic responses. Effective anti-fibrotic therapy may seek to exploit renalase’s compound effects such as: lessening of the inflammatory cell infiltrate (neutrophils and macrophages), and macrophage polarization (M1 to M2), a decrease in the proinflammatory cytokines/chemokines/reactive species/growth factor release (TNF-α, IL-6, MCP-1, MIP-2, ROS, TGF-β1), an increase in anti-apoptotic factors (Bcl2), and prevention of caspase activation, inflammasome silencing, sirtuins (1 and 3) activation, and mitochondrial protection, suppression of epithelial to mesenchymal transition, a decrease in the pro-fibrotic markers expression (’α-SMA, collagen I, and III, TIMP-1, and fibronectin), and interference with MAPKs signaling network, most likely as a coordinator of pro-fibrotic signals. This review provides the scientific rationale for renalase’s scrutiny regarding cardiac fibrosis, and there is great anticipation that these newly identified pathways are set to progress one step further. Although substantial progress has been made, indicating renalase’s therapeutic promise, more profound experimental work is required to resolve the accurate underlying mechanisms of renalase, concerning cardiac fibrosis, before any potential translation to clinical investigation.

KATP channel dependent heart multiome atlas

Plasmalemmal ATP sensitive potassium (K_ATP) channels are recognized metabolic sensors, yet their cellular reach is less well understood. Here, transgenic Kir6.2 null hearts devoid of the K_ATP channel pore underwent multiomics surveillance and systems interrogation versus wildtype counterparts. Despite maintained organ performance, the knockout proteome deviated beyond a discrete loss of constitutive K_ATP channel subunits. Multidimensional nano-flow liquid chromatography tandem mass spectrometry resolved 111 differentially expressed proteins and their expanded network neighborhood, dominated by metabolic process engagement. Independent multimodal chemometric gas and liquid chromatography mass spectrometry unveiled differential expression of over one quarter of measured metabolites discriminating the Kir6.2 deficient heart metabolome. Supervised class analogy ranking and unsupervised enrichment analysis prioritized nicotinamide adenine dinucleotide (NAD⁺), affirmed by extensive overrepresentation of NAD⁺ associated circuitry. The remodeled metabolome and proteome revealed functional convergence and an integrated signature of disease susceptibility. Deciphered cardiac patterns were traceable in the corresponding plasma metabolome, with tissue concordant plasma changes offering surrogate metabolite markers of myocardial latent vulnerability. Thus, Kir6.2 deficit precipitates multiome reorganization, mapping a comprehensive atlas of the K_ATP channel dependent landscape.

Association of renalase with clinical outcomes in hospitalized patients with COVID-19

Renalase is a secreted flavoprotein with anti-inflammatory and pro-cell survival properties. COVID-19 is associated with disordered inflammation and apoptosis. We hypothesized that blood renalase levels would correspond to severe COVID-19 and survival. In this retrospective cohort study, clinicopathologic data and blood samples were collected from hospitalized COVID-19 subjects (March—June 2020) at a single institution tertiary hospital. Plasma renalase and cytokine levels were measured and clinical data abstracted from health records. Of 3,450 COVID-19 patients, 458 patients were enrolled. Patients were excluded if <18 years, or opted out of research. The primary composite outcome was intubation or death within 180 days. Secondary outcomes included mortality alone, intensive care unit admission, use of vasopressors, and CPR. Enrolled patients had mean age 64 years (SD±17), were 53% males, and 48% non-whites. Mean renalase levels was 14,108·4 ng/ml (SD±8,137 ng/ml). Compared to patients with high renalase, those with low renalase (< 8,922 ng/ml) were more likely to present with hypoxia, increased ICU admission (54% vs. 33%, p < 0.001), and cardiopulmonary resuscitation (10% vs. 4%, p = 0·023). In Cox proportional hazard model, every 1000 ng/ml increase in renalase decreased the risk of death or intubation by 5% (HR 0·95; 95% CI 0·91–0·98) and increased survival alone by 6% (HR 0·95; CI 0·90–0·98), after adjusting for socio-demographics, initial disease severity, comorbidities and inflammation. Patients with high renalase-low IL-6 levels had the best survival compared to other groups (p = 0·04). Renalase was independently associated with reduced intubation and mortality in hospitalized COVID-19 patients. Future studies should assess the pathophysiological relevance of renalase in COVID-19 disease.

Bioelectrosynthesis systems

Bioelectrosynthesis (BES) systems exploit extracellular electron transport pathways to augment cellular metabolism. This strategy can be used to improve the economic viability of bio-based syntheses versus conventional methods, most notably petrochemical-based syntheses. It also has the potential to reduce the carbon footprint of biomanufacturing processes. Efficient channeling of cathode-derived electrons towards biosynthesis requires a better understanding of the biological mechanisms of electron transport as well as detailed evaluation of all aspects of process performance. More advanced solutions may deploy cell free systems that use ex situ generated reducing equivalents to improve economic performance.

Renalase-A new understanding of its enzymatic and non-enzymatic activity and its implications for future research

Renalase was first described in 2005 and since then it became an object of scientific interest because of its proposed ability to catalyse circulating neurotransmitters and its promising antihypertensive effects. However, further research on the enzymatic activity of renalase did not confirm these initial findings and yielded that renalase serves to oxidize isomeric forms of β-NAD(P)H and recycle them by forming β-NAD(P)+. Moreover, in contrast to initial assumptions, it is indicated that renalase's enzymatic activity is confined to the cell and that extracellular renalase loses its enzymatic properties. These new reports led scientists to question as to whether renalase, as an enzyme, still has the potential to influence various systemic physiological responses (e.g. blood pressure). It was also put into question whether many physiological discoveries published based on the notion that renalase is secreted into the blood and acts by oxidation of catecholamines can still be considered valid. In this article, we attempt to review the literature to confront these doubts and find further possible directions of research on the importance of renalase. Our aim was to evaluate recent reports of non-enzymatic activity for renalase.

Renalase: A Multi-Functional Signaling Molecule with Roles in Gastrointestinal Disease

The survival factor renalase (RNLS) is a recently discovered secretory protein with potent prosurvival and anti-inflammatory effects. Several evolutionarily conserved RNLS domains are critical to its function. These include a 20 aa site that encodes for its prosurvival effects. Its prosurvival effects are shown in GI disease models including acute cerulein pancreatitis. In rodent models of pancreatic cancer and human cancer tissues, increased RNLS expression promotes cancer cell survival but shortens life expectancy. This 37 kD protein can regulate cell signaling as an extracellular molecule and probably also at intracellular sites. Extracellular RNLS signals through a specific plasma membrane calcium export transporter; this interaction appears most relevant to acute injury and cancer. Preliminary studies using RNLS agonists and antagonists, as well as various preclinical disease models, suggest that the immunologic and prosurvival effects of RNLS will be relevant to diverse pathologies that include acute organ injuries and select cancers. Future studies should define the roles of RNLS in intestinal diseases, characterizing the RNLS-activated pathways linked to cell survival and developing therapeutic agents that can increase or decrease RNLS in relevant clinical settings.

Advances in electrochemical cofactor regeneration: enzymatic and non-enzymatic approaches

Nicotinamide adenine dinucleotide(NAD(P)H) is a metabolically interconnected redox cofactor serving as a hydride source for the majority of oxidoreductases, and consequently constituting a significant cost factor for bioprocessing. Much research has been devoted to the development of efficient, affordable, and sustainable methods for the regeneration of these cofactors through chemical, electrochemical, and photochemical approaches. However, the enzymatic approach using formate dehydrogenase is still the most abundantly employed in industrial applications, even though it suffers from system complexity and product purity issues. In this review, we summarize non-enzymatic and enzymatic electrochemical approaches for cofactor regeneration, then discuss recent developments to solve major issues. Issues discussed include Rh-catalyst mediated enzyme mutual inactivation, electron-transfer rates, catalyst sustainability, product selectivity and simplifying product purification. Recently reported remedies are discussed, such as heterogeneous metal catalysts generating H⁺ as the sole byproduct or high activity and stability redox-polymer immobilized enzymatic systems for sustainable organic synthesis.

Chemical and Biochemical Reactivity of the Reduced Forms of Nicotinamide Riboside

All life forms require nicotinamide adenine dinucleotide, NAD⁺, and its reduced form NADH. They are redox partners in hundreds of cellular enzymatic reactions. Changes in the intracellular levels of total NAD (NAD⁺ + NADH) and the (NAD⁺/NADH) ratio can cause cellular dysfunction. When not present in protein complexes, NADH and its phosphorylated form NADPH degrade through intricate mechanisms. Replenishment of a declining total NAD pool can be achieved with biosynthetic precursors that include one of the reduced forms of nicotinamide riboside (NR⁺), NRH. NRH, like NADH and NADPH, is prone to degradation via oxidation, hydration, and isomerization and, as such, is an excellent model compound to rationalize the nonenzymatic metabolism of NAD(P)H in a biological context. Here, we report on the stability of NRH and its propensity to isomerize and irreversibly degrade. We also report the preparation of two of its naturally occurring isomers, their chemical stability, their reactivity toward NRH-processing enzymes, and their cell-specific cytotoxicity. Furthermore, we identify a mechanism by which NRH degradation causes covalent peptide modifications, a process that could expose a novel type of NADH-protein modifications and correlate NADH accumulation with "protein aging." This work highlights the current limitations in detecting NADH's endogenous catabolites and in establishing the capacity for inducing cellular dysfunction.

The Interaction of Porcine Dihydropyrimidine Dehydrogenase with the Chemotherapy Sensitizer: 5-Ethynyluracil

Dihydropyrimidine dehydrogenase (DPD) is a complex enzyme that reduces the 5,6-vinylic bond of pyrimidines, uracil, and thymine. 5-Fluorouracil (5FU) is also a substrate for DPD and a common chemotherapeutic agent used to treat numerous cancers. The reduction of 5FU to 5-fluoro-5,6-dihydrouracil negates its toxicity and efficacy. Patients with high DPD activity levels typically have poor outcomes when treated with 5FU. DPD is thus a central mitigating factor in the treatment of a variety of cancers. 5-Ethynyluracil (5EU) covalently inactivates DPD by cross-linking with the active-site general acid cysteine in the pyrimidine binding site. This reaction is dependent on the simultaneous binding of 5EU and nicotinamide adenine dinucleotide phosphate (NADPH). This ternary complex induces DPD to become activated by taking up two electrons from the NADPH. The covalent inactivation of DPD by 5EU occurs concomitantly with this reductive activation with a rate constant of ∼0.2 s-1. This kinact value is correlated with the rate of reduction of one of the two flavin cofactors and the localization of a mobile loop in the pyrimidine active site that places the cysteine that serves as the general acid in catalysis proximal to the 5EU ethynyl group. Efficient cross-linking is reliant on enzyme activation, but this process appears to also have a conformational aspect in that nonreductive NADPH analogues can also induce a partial inactivation. Cross-linking then renders DPD inactive by severing the proton-coupled electron transfer mechanism that transmits electrons 56 Å across the protein.

Improved soluble expression and use of recombinant human renalase

Electrochemical bioreactor systems have enjoyed significant attention in the past few decades, particularly because of their applications to biobatteries, artificial photosynthetic systems, and microbial electrosynthesis. A key opportunity with electrochemical bioreactors is the ability to employ cofactor regeneration strategies critical in oxidative and reductive enzymatic and cell-based biotransformations. Electrochemical cofactor regeneration presents several advantages over other current cofactor regeneration systems, such as chemoenzymatic multi-enzyme reactions, because there is no need for a sacrificial substrate and a recycling enzyme. Additionally, process monitoring is simpler and downstream processing is less costly. However, the direct electrochemical reduction of NAD(P)⁺ on a cathode may produce adventitious side products, including isomers of NAD(P)H that can act as potent competitive inhibitors to NAD(P)H-requiring enzymes such as dehydrogenases. To overcome this limitation, we examined how nature addresses the adventitious formation of isomers of NAD(P)H. Specifically, renalases are enzymes that catalyze the oxidation of 1,2- and 1,6-NAD(P)H to NAD(P)⁺, yielding an effective recycling of unproductive NAD(P)H isomers. We designed several mutants of recombinant human renalase isoform 1 (rhRen1), expressed them in E. coli BL21(DE3) to enhance protein solubility, and evaluated the activity profiles of the renalase variants against NAD(P)H isomers. The potential for rhRen1 to be employed in engineering applications was then assessed in view of the enzyme’s stability upon immobilization. Finally, comparative modeling was performed to assess the underlying reasons for the enhanced solubility and activity of the mutant enzymes.

Genome-scale in vivo CRISPR screen identifies RNLS as a target for beta cell protection in type 1 diabetes

Type 1 diabetes (T1D) is caused by the autoimmune destruction of pancreatic beta cells. Pluripotent stem cells can now be differentiated into beta cells, thus raising the prospect of a cell replacement therapy for T1D. However, autoimmunity would rapidly destroy newly transplanted beta cells. Using a genome-scale CRISPR screen in a mouse model for T1D, we show that deleting RNLS, a genome-wide association study candidate gene for T1D, made beta cells resistant to autoimmune killing. Structure-based modelling identified the U.S. Food and Drug Administration-approved drug pargyline as a potential RNLS inhibitor. Oral pargyline treatment protected transplanted beta cells in diabetic mice, thus leading to disease reversal. Furthermore, pargyline prevented or delayed diabetes onset in several mouse models for T1D. Our results identify RNLS as a modifier of beta cell vulnerability and as a potential therapeutic target to avert beta cell loss in T1D.

Pyridoxamine-phosphate oxidases and pyridoxamine-phosphate oxidase-related proteins catalyze the oxidation of 6-NAD(P)H to NAD(P)

6-NADH and 6-NADPH are strong inhibitors of several dehydrogenases that may form spontaneously from NAD(P)H. They are known to be oxidized to NAD(P)+ by mammalian renalase, an FAD-linked enzyme mainly present in heart and kidney, and by related bacterial enzymes. We partially purified an enzyme oxidizing 6-NADPH from rat liver, and, surprisingly, identified it as pyridoxamine-phosphate oxidase (PNPO). This was confirmed by the finding that recombinant mouse PNPO oxidized 6-NADH and 6-NADPH with catalytic efficiencies comparable to those observed with pyridoxine- and pyridoxamine-5'-phosphate. PNPOs from Escherichia coli, Saccharomyces cerevisiae and Arabidopsis thaliana also displayed 6-NAD(P)H oxidase activity, indicating that this 'side-activity' is conserved. Remarkably, 'pyridoxamine-phosphate oxidase-related proteins' (PNPO-RP) from Nostoc punctiforme, A. thaliana and the yeast S. cerevisiae (Ygr017w) were not detectably active on pyridox(am)ine-5'-P, but oxidized 6-NADH, 6-NADPH and 2-NADH suggesting that this may be their main catalytic function. Their specificity profiles were therefore similar to that of renalase. Inactivation of renalase and of PNPO in mammalian cells and of Ygr017w in yeasts led to the accumulation of a reduced form of 6-NADH, tentatively identified as 4,5,6-NADH3, which can also be produced in vitro by reduction of 6-NADH by glyceraldehyde-3-phosphate dehydrogenase or glucose-6-phosphate dehydrogenase. As 4,5,6-NADH3 is not a substrate for renalase, PNPO or PNPO-RP, its accumulation presumably reflects the block in the oxidation of 6-NADH. These findings indicate that two different classes of enzymes using either FAD (renalase) or FMN (PNPOs and PNPO-RPs) as a cofactor play an as yet unsuspected role in removing damaged forms of NAD(P).

A facile analytical method for reliable selectivity examination in cofactor NADH regeneration

This study demonstrates a novel method to quantify selective (1,4-NADH) and unselective products (1,2- and 1,6-NADH) in NADH regeneration using combined UV-Vis spectroscopy and biological assays.

Renalase in children with chronic kidney disease

Background: Renalase is kidney-derived molecule initially considered as catecholamine-inactivating enzyme. However, recent studies suggest that renalase exerts potent cardio- and nephroprotective actions, not related to its enzymatic activity. Purpose: To assess renalase level in children with chronic kidney disease (CKD). Material and methods: Serum renalase, BMI, arterial stiffness, peripheral and central blood pressure, intima-media thickness (IMT), medications, and biochemical parameters were analyzed in 38 children with CKD (12.23 ± 4.19 years) (stage G2-5). Control group consisted of 38 healthy children. Results: In the study group, GFR was 25.74 ± 8.94 mL/min/1.73 m²; 6 children were dialyzed; 26 had arterial hypertension. Renalase level was higher in the study group compared to control group (p < 0.001). In CKD children renalase correlated (p < 0.05) with BMI Z-score (r = -0.36), alfacalcidol dose (r = 0.41), GFR (r = -0.69), hemoglobin (r = -0.48), total cholesterol (r = 0.35), LDL-cholesterol (r = 0.36), triglycerides (r = 0.52), phosphate (r = 0.35), calcium-phosphorus product (r = 0.35), parathormone (r = 0.58), and pulse wave velocity Z-score (r = 0.42). In multivariate analysis GFR (β = -0.63, p < 0.001), triglycerides (β = 0.59, p = 0.002), and alfacalcidol dose (β = -0.49, p = 0.010) were determinants of renalase. Conclusions: In children with CKD there is a strong correlation between renalase level and CKD stage. Furthermore, in these patients renalase does not correlate with blood pressure but may be a marker of arterial stiffness.

Syntheses and chemical properties of β-nicotinamide riboside and its analogues and derivatives

The β-anomeric form of nicotinamide riboside (NR+) is a precursor for nicotinamide adenine dinucleotide (NAD+), a redox cofactor playing a critical role in cell metabolism. Recently, it has been demonstrated that its chloride salt (NR+Cl-) has beneficial effects, and now NR+Cl- is available as a dietary supplement. Syntheses and studies of analogues and derivatives of NR+ are of high importance to unravel the role of NR+ in biochemical processes in living cells and to elaborate the next generation of NR+ derivatives and conjugates with the view of developing novel drug and food supplement candidates. This review provides an overview of the synthetic approaches, the chemical properties, and the structural and functional modifications which have been undertaken on the nicotinoyl riboside scaffold.

Functional polymorphism of the renalase gene is associated with cardiac hypertrophy in female patients with aortic stenosis

Renalase decreases circulating catecholamines concentration and is important in maintaining primary cellular metabolism. Renalase acts through the plasma membrane calcium ATPase 4b in the heart, which affects pressure overload but not exercise induced heart hypertrophy. The aim of this study was to test the association between a functional polymorphism Glu37Asp (rs2296545) of the renalase gene and left ventricular hypertrophy in a large cohort of patients with aortic stenosis. The study group consisted of 657 patients with aortic stenosis referred for aortic valve replacement. Preoperative echocardiographic assessment was performed to obtain cardiac phenotypes. Generalized-linear models were implemented to analyze data using crude or full model adjusted for selected clinical factors. In females, the Asp37 variant of the Glu37Asp polymorphism was associated with higher left ventricular mass (p = 0.0021 and p = 0.055 crude and full model respectively), intraventricular septal thickness (p = 0.0003 and p = 0.0143) and posterior wall thickness (p = 0.0005 and p = 0.0219) all indexed to body surface area, as well as relative wall thickness (p = 0.001 and p = 0.0097). No significant associations were found among the male patients. In conclusion, we have found the association of the renalase Glu37Asp polymorphism with left ventricle hypertrophy in large group of females with aortic stenosis. The Glu37Asp polymorphism causes not only amino-acid substitution in FAD binding domain but may also change binding affinity of the hypoxia- and hypertrophy-related transcription factors and influence renalase gene expression. Our data suggest that renalase might play a role in hypertrophic response to pressure overload, but the exact mechanism requires further investigation.

Improved strategies for electrochemical 1,4-NAD(P)H2 regeneration: A new era of bioreactors for industrial biocatalysis

Industrial enzymatic reactions requiring 1,4-NAD(P)H₂ to perform redox transformations often require convoluted coupled enzyme regeneration systems to regenerate 1,4-NAD(P)H₂ from NAD(P) and recycle the cofactor for as many turnovers as possible. Renewed interest in recycling the cofactor via electrochemical means is motivated by the low cost of performing electrochemical reactions, easy monitoring of the reaction progress, and straightforward product recovery. However, electrochemical cofactor regeneration methods invariably produce adventitious reduced cofactor side products which result in unproductive loss of input NAD(P). We review various literature strategies for mitigating adventitious product formation by electrochemical cofactor regeneration systems, and offer insight as to how a successful electrochemical bioreactor system could be constructed to engineer efficient 1,4-NAD(P)H₂-dependent enzyme reactions of interest to the industrial biocatalysis community.

The enzyme: Renalase

Within the last two years catalytic substrates for renalase have been identified, some 10 years after its initial discovery. 2- and 6-dihydronicotinamide (2- and 6-DHNAD) isomers of β-NAD(P)H (4-dihydroNAD(P)) are rapidly oxidized by renalase to form β-NAD(P)⁺. The two electrons liberated are then passed to molecular oxygen by the renalase FAD cofactor forming hydrogen peroxide. This activity would appear to serve an intracellular detoxification/metabolite repair function that alleviates inhibition of primary metabolism dehydrogenases by 2- and 6-DHNAD molecules. This activity is supported by the complete structural assignment of the substrates, comprehensive kinetic analyses, defined species specific substrate specificity profiles and X-ray crystal structures that reveal ligand complexation consistent with this activity. This apparently intracellular function for the renalase enzyme is not allied with the majority of the renalase research that holds renalase to be a secreted mammalian protein that functions in blood to elicit a broad array of profound physiological changes. In this review a description of renalase as an enzyme is presented and an argument is offered that its enzymatic function can now reasonably be assumed to be uncoupled from whole organism physiological influences.

Comparative analysis of expression of genes encoding enzymes of catecholamine catabolism and renalase in tissues of normotensive and hypertensive rats

Comparative analysis of expression of genes encoding enzymes of catecholamine catabolism (monoaminbe oxidases A and B (MAO A and MAO B) and catechol-O-methyl transferase (COMT)) and renalase has been carried out in tissues of normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). Among investigated tissues the highest level of mRNA of genes encoding key enzymes of catecholamine catabolism (MAO A, MAO B, COMT) was found in the heart of WKY rats. In SHR the mRNA levels of these genes were lower (p<0.05-0.01), however, no similar changes were observed in the tissues studied in dependence of hypertension. The relative mRNA levels of the studied genes normalized versus actin mRNA significantly varied. In heart and kidney the relative level of COMT mRNA significantly exceeded the relative levels of both MAO A mRNA and MAO B mRNA. In the brain differences in mRNAs of MAOA, MAOB, and COMT were less pronounced. However, in all examined tissue the renalase mRNA level was much (at least 10-20-fold) lower than any other mRNA studied. Taking into consideration known correlations between mRNAs and corresponding protein products reported in the literature for many genes these results suggest that in the case of any catalytic scenarios proposed or even proved for renalase this protein cannot contribute to catecholamine degradation. It is also unlikely that the products of renalase reaction, b-NAD(P)+ and hydrogen peroxide, can exhibit a hypotensive effect due to low expression of the renalase encoding gene.

Ligand binding phenomena that pertain to the metabolic function of renalase

Renalase catalyzes the oxidation of isomers of β-NAD(P)H that carry the hydride in the 2 or 6 positions of the nicotinamide base to form β-NAD(P)⁺. This activity is thought to alleviate inhibition of multiple β-NAD(P)-dependent enzymes of primary and secondary metabolism by these isomers. Here we present evidence for a variety of ligand binding phenomena relevant to the function of renalase. We offer evidence of the potential for primary metabolism inhibition with structures of malate dehydrogenase and lactate dehydrogenase bound to the 6-dihydroNAD isomer. The previously observed preference of renalase from Pseudomonas for NAD-derived substrates over those derived from NADP is accounted for by the structure of the enzyme in complex with NADPH. We also show that nicotinamide nucleosides and mononucleotides reduced in the 2- and 6-positions are renalase substrates, but bind weakly. A seven-fold enhancement of acquisition (k_red/K_d) for 6-dihydronicotinamide riboside was observed for human renalase in the presence of ADP. However, generally the addition of complement ligands, AMP for mononucleotide or ADP for nucleoside substrates, did not enhance the reductive half-reaction. Non-substrate nicotinamide nucleosides or nucleotides bind weakly suggesting that only β-NADH and β-NADPH compete with dinucleotide substrates for access to the active site.

NAD+ Is a Food Component That Promotes Exit from Dauer Diapause in Caenorhabditis elegans

The free-living soil nematode Caenorhabditis elegans adapts its development to the availability of food. When food is scarce and population density is high, worms enter a developmentally arrested non-feeding diapause stage specialized for long-term survival called the dauer larva. When food becomes available, they exit from the dauer stage, resume growth and reproduction. It has been postulated that compound(s) present in food, referred to as the “food signal”, promote exit from the dauer stage. In this study, we have identified NAD⁺ as a component of bacterial extract that promotes dauer exit. NAD⁺, when dissolved in alkaline medium, causes opening of the mouth and ingestion of food. We also show that to initiate exit from the dauer stage in response to NAD⁺ worms require production of serotonin. Thus, C. elegans can use redox cofactors produced by dietary organisms to sense food.

[The history of renalase from amine oxidase to a a-NAD(P)H-oxidase/anomerase]

Renalase is a recently discovered secretory protein, which plays a certain (still poorly understood) role in regulation of blood pressure. The review summarizes own and literature data accumulated since the first publication on relanase (2005). Initial reports on FAD-dependent amine oxidase activity of this protein were not confirmed in independent experiments performed in different laboratories. In addition, proposed amine oxidase activity of circulating extracellular renalase requires the presence of FAD, which has not been detected either in blood or urinary renalase. Moreover, renalase excreted into urine lacks its N-terminal peptide, which is ultimately needed for accommodation of the FAD cofactor. Results of the Aliverti's group on NAD(P)H binding by renalase and weak diaphorase activity of this protein stimulated further studies of renalase as NAD(P)H oxidase catalyzing reaction of catecholamine co-oxidation. However, physiological importance of such extracellular catecholamine-metabolizing activity (demonstrated in one laboratory and not detected in another laboratory) remains unclear due to existence of much more active enzymatic systems (e.g. neutrophil NAD(P)H oxidase, xanthine oxidase/xanthine) in circulation, which can perform such co-oxidation reactions. Recently a-NAD(P)H oxidase/anomerase activity of renalase, which also pomotes oxidative conversion of b-NADH isomers inhibiting activity of NAD-dependent dehydrogenases, has been described. However, its possible contribution to the antihypertensive effect of renalase remains unclear. Thus, the antihypertensive effect of renalase still remains a phenomenon with unclear biochemical mechanim(s) and functions of intracellular and extracellular (circulating) renalases obviously differ.

Inhibition of renalase expression and signaling has antitumor activity in pancreatic cancer

An essential feature of cancer is dysregulation of cell senescence and death. Renalase, a recently discovered secreted flavoprotein, provides cytoprotection against ischemic and toxic cellular injury by signaling through the PI3K-AKT and MAPK pathways. Here we show that renalase expression is increased in pancreatic cancer tissue and that it functions as a growth factor. In a cohort of patients with pancreatic ductal adenocarcinoma, overall survival was inversely correlated with renalase expression in the tumor mass, suggesting a pathogenic role for renalase. Inhibition of renalase signaling using siRNA or inhibitory anti-renalase antibodies decreased the viability of cultured pancreatic ductal adenocarcinoma cells. In two xenograft mouse models, either the renalase monoclonal antibody m28-RNLS or shRNA knockdown of renalase inhibited pancreatic ductal adenocarcinoma growth. Inhibition of renalase caused tumor cell apoptosis and cell cycle arrest. These results reveal a previously unrecognized role for the renalase in cancer: its expression may serve as a prognostic maker and its inhibition may provide an attractive therapeutic target in pancreatic cancer.

Bacterial Renalase: Structure and Kinetics of an Enzyme with 2- and 6-Dihydro-β-NAD(P) Oxidase Activity from Pseudomonas phaseolicola

Despite a lack of convincing in vitro evidence and a number of sound refutations, it is widely accepted that renalase is an enzyme unique to animals that catalyzes the oxidative degradation of catecholamines in blood in order to lower vascular tone. Very recently, we identified isomers of β-NAD(P)H as substrates for renalase (Beaupre, B. A. et al. (2015) Biochemistry, 54, 795-806). These molecules carry the hydride equivalent on the 2 or 6 position of the nicotinamide base and presumably arise in nonspecific redox reactions of nicotinamide dinucleotides. Renalase serves to rapidly oxidize these isomers to form β-NAD(P)⁺ and then pass the electrons to dioxygen, forming H₂O₂. We have also shown that these substrate molecules are highly inhibitory to dehydrogenase enzymes and thus have proposed an intracellular metabolic role for this enzyme. Here, we identify a renalase from an organism without a circulatory system. This bacterial form of renalase has the same substrate specificity profile as that of human renalase but, in terms of binding constant (K(d)), shows a marked preference for substrates derived from β-NAD⁺. 2-dihydroNAD(P) substrates reduce the enzyme with rate constants (k(red)) that greatly exceed those for 6-dihydroNAD(P) substrates. Taken together, k(red)/K(d) values indicate a minimum 20-fold preference for 2DHNAD. We also offer the first structures of a renalase in complex with catalytically relevant ligands β-NAD⁺ and β-NADH (the latter being an analogue of the substrate(s)). These structures show potential electrostatic repulsion interactions with the product and a unique binding orientation for the substrate nicotinamide base that is consistent with the identified activity.

Renalase does not catalyze the oxidation of catecholamines

It is widely accepted that the function of human renalase is to oxidize catecholamines in blood. However, this belief is based on experiments that did not account for slow, facile catecholamine autoxidation reactions. Recent evidence has shown that renalase has substrates with which it reacts rapidly. The reaction catalyzed defines renalase as an oxidase, one that harvests two electrons from either 2-dihydroNAD(P) or 6-dihydroNAD(P) to form β-NAD(P)(+) and hydrogen peroxide. The apparent metabolic purpose of such a reaction is to avoid inhibition of primary dehydrogenase enzymes by these β-NAD(P)H isomers. This article demonstrates that renalase does not catalyze the oxidation of neurotransmitter catecholamines. Using high-performance liquid chromatography we show that there is no evidence of consumption of epinephrine by renalase. Using time-dependent spectrophotometry we show that the renalase FAD cofactor spectrum is unresponsive to added catecholamines, that adrenochromes are not observed to accumulate in the presence of renalase and that the kinetics of single turnover reactions with 6-dihydroNAD are unaltered by the addition of catecholamines. Lastly we show using an oxygen electrode assay that plasma renalase activity is below the level of detection and only when exogenous renalase and 6-dihydroNAD are added can dioxygen be observed to be consumed.

Identification of a receptor for extracellular renalase

Background

An increased risk for developing essential hypertension, stroke and diabetes is associated with single nucleotide gene polymorphisms in renalase, a newly described secreted flavoprotein with oxidoreductase activity. Gene deletion causes hypertension, and aggravates acute ischemic kidney (AKI) and cardiac injury. Independent of its intrinsic enzymatic activities, extracellular renalase activates MAPK signaling and prevents acute kidney injury (AKI) in wild type (WT) mice. Therefore, we sought to identity the receptor for extracellular renalase.

Methods and Results

RP-220 is a previously identified, 20 amino acids long renalase peptide that is devoid of any intrinsic enzymatic activity, but it is equally effective as full-length recombinant renalase at protecting against toxic and ischemic injury. Using biotin transfer studies with RP-220 in the human proximal tubular cell line HK-2 and protein identification by mass spectrometry, we identified PMCA4b as a renalase binding protein. This previously characterized plasma membrane ATPase is involved in cell signaling and cardiac hypertrophy. Co-immunoprecipitation and co-immunolocalization confirmed protein-protein interaction between endogenous renalase and PMCA4b. Down-regulation of endogenous PMCA4b expression by siRNA transfection, or inhibition of its enzymatic activity by the specific peptide inhibitor caloxin1b each abrogated RP-220 dependent MAPK signaling and cytoprotection. In control studies, these maneuvers had no effect on epidermal growth factor mediated signaling, confirming specificity of the interaction between PMCA4b and renalase.

Conclusions

PMCA4b functions as a renalase receptor, and a key mediator of renalase dependent MAPK signaling.

Human urinary renalase lacks the N-terminal signal peptide crucial for accommodation of its FAD cofactor

Renalase is a recently discovered secretory protein involved in the regulation of blood pressure. Cells synthesize all known isoforms of human renalase (1 and 2) as flavoproteins. Accommodation of FAD in the renalase protein requires the presence of its N-terminal peptide. However, in secretory proteins, such peptides are usually cleaved during their export from the cell. In the present study, we have isolated human renalase from urinary samples of healthy volunteers and human recombinant renalases 1 and 2 expressed in Escherichia coli cells. In these proteins, we investigated the presence of the renalase N-terminal peptide and the FAD cofactor and performed computer-aided molecular analysis of the renalase crystal structure to evaluate possible consequences of removal of the N-terminal peptide. In contrast to human recombinant renalase isoforms 1 and 2 containing non-covalently bound FAD and clearly detectable N-terminal peptide, renalase purified from human urine lacks both the N-terminal signal peptide and FAD. The computer-aided analysis indicates that the removal of this peptide results in inability of the truncated renalase to bind the FAD cofactor. Thus, our results indicate that human renalase secreted in urine lacks its N-terminal peptide, and therefore catalytic activities of urinary renalase reported in the literature cannot be attributed to FAD-dependent mechanisms. We suggest that FAD-dependent catalytic functions are intrinsic properties of intracellular renalases, whereas extracellular renalases act in FAD- and possibly catalytic-independent manner.

The catalytic function of renalase: A decade of phantoms

Ten years after the initial identification of human renalase the first genuinely catalytic substrates have been identified. Throughout the prior decade a consensus belief that renalase is produced predominantly by the kidney and catalytically oxidizes catecholamines in order to lower blood pressure and slow the heart has prevailed. This belief was, however, based on fundamentally flawed scientific observations that did not include control reactions to account for the well-known autoxidation of catecholamines in oxygenated solutions. Nonetheless, the initial claims have served as the kernel for a rapidly expanding body of research largely predicated on the belief that catecholamines are substrates for this enzyme. The proliferation of scientific studies pertaining to renalase as a hormone has proceeded unabated despite well-reasoned expressions of dissent that have indicated the deficiencies of the initial observations and other inconsistencies. Our group has very recently identified isomeric forms of β-NAD(P)H as substrates for renalase. These substrates arise from non-specific reduction of β-NAD(P)(+) that forms β-4-dihydroNAD(P) (β-NAD(P)H), β-2-dihydroNAD(P) and β-6-dihydroNAD(P); the latter two being substrates for renalase. Renalase oxidizes these substrates with rate constants that are up to 10(4)-fold faster than any claimed for catecholamines. The electrons harvested are delivered to dioxygen via the enzyme's FAD cofactor forming both H2O2 and β-NAD(P)(+) as products. It would appear that the metabolic purpose of this chemistry is to alleviate the inhibitory effect of β-2-dihydroNAD(P) and β-6-dihydroNAD(P) on primary metabolism dehydrogenase enzymes. The identification of this genuinely catalytic activity for renalase calls for re-evaluation of much of the research of this enzyme, in which definitive links between renalase catecholamine consumption and physiological responses were reported. This article is part of a Special Issue entitled: Physiological enzymology and protein functions.