Underlying mechanisms of hyperuricemia-induced renal damage

Background and Aims Asymptomatic hyperuricemia (HU) is common among patients with chronic kidney disease (CKD) but the causative role of HU on CKD progression remains controversial. Two large multi-center randomized controlled trials (RCTs), the CKD-FIX and the PERL study, have now disproven a causal relation. On the other hand, a causative role of persistent HU exists with gouty arthritis but a rapid correction of HU with urate lowering therapy can also elicit acute gout attacks. This suggests a more complex role of HU in this context. Hence, we hypothesized that soluble uric acid (sUA) has immunomodulatory effects on leukocyte function during the immune response to monosodium urate (MSU) crystals. Method Alb-creERT2;Glut9lox/lox and Glut9lox/lox control mice were injected with tamoxifen and placed either on a chow or high-fat diet with inosine to induce HU with or without CKD. After two weeks, MSU crystals or vehicle were injected into air pouches or postcapillary venules in the cremaster muscle of transgenic mice. Leukocyte infiltration and the extent of inflammation were assessed by flow cytometry, intravital microscopy, ELISA, and colorimetric assays. Human blood neutrophils and CD14+ monocytes were isolated from CKD stage G2-4 (n=10) and CKD stage G5 (n=18) patients or healthy individuals (n=15). Neutrophil transwell migration assays were performed and the number of migrated human neutrophils towards chemoattractants determined by flow cytometry. Human blood CD14+ monocytes were treated with MSU crystals or lipopolysaccharide ex vivo, and the expression of inflammatory mediators determined by RT-PCR and flow cytometry. Results We found that the presence of HU impaired leukocyte recruitment into MSU crystal-injected air pouches of mice with or without CKD. Intravital microscopy revealed that HU specifically reduced leukocyte adhesion, extravasation, and leukocyte-related tissue inflammation. The CKD-mediated attenuation of MSU crystal-induced inflammation was fully reversible by treating HU with urate lowering therapy such as rasburicase. In blood neutrophils isolated from healthy individuals, sUA diminished β2 integrin activation and expression, and hence impaired neutrophil migration ex vivo. An impaired migratory capability was also observed in neutrophils from CKD stage G2-4 or G5D patients. Moreover, sUA attenuated Toll-like and TNFα receptor-induced human monocyte activation, a process depending on the intracellular uptake of sUA via the urate transporter SLC2A9 (GLUT9), the only urate transporter expressed by these immune cells. Conclusion We identify sUA as an endogenous modulator of innate immunity. HU modulates neutrophil migration by altering efficient β2 integrin activation and suppresses pattern recognition receptor-driven monocyte activation via SLC2A9 in gouty arthritis related or unrelated to CKD (see Figure). This process provides a molecular explanation for several previously unexplained clinical phenomena in the context of gouty arthritis and renal failure.

BackgroundEpidemiological studies conducted in patients (pts) from the general population and pts with chronic renal failure have shown that uric acid is an independent risk factor for the development and...

CD44‐Targeted Monoclonal Antibody is Anti‐Inflammatory in an <i>In‐Vivo</i> Model of Monosodium Urate Crystal Induced Peritonitis

TET2-mutant clonal hematopoiesis and risk of gout

Uric acid (UA), the product of purine catabolism, is released from injured or infected cells. Beyond maximal solubility, UA precipitates into monosodium urate (MSU) crystals which cause inflammation by activating the innate immune system. UA levels are increased in several respiratory diseases associated with hypoxemia including ARDS, pulmonary hypertension or COPD. The aim of this study was to analyse the effects of MSU crystals on vascular function in isolated pulmonary arteries (PA). PA smooth muscle cells (SMC) were incubated for 24h in the absence (Ctrl) or presence of MSU crystals (250μg/ml). IL-6 levels were determined by ELISA and contractile responses analysed in human PA mounted in a wire myograph. Exposure to MSU crystals for 24h significantly increased IL-6 release by rat PASMCs (Ctrl=0.4±0.1; MSU=2.2±0.7 ng/ml). Induction of IL-6 by MSU crystals was inhibited by the TAK1 inhibitor 5Z-7-oxozeaenol but was not affected by the TLR4 antagonist TAK242 or the pan-caspase inhibitor Z-VAD-FMK. Furthermore, MSU crystals also stimulated human PASMCs to release IL-6 (Ctrl=2±1; MSU=7±2 ng/ml), markedly impaired hypoxic pulmonary vasoconstriction (HPV) and reduced the relaxant responses to acetylcholine while preserving the contractile responses to endothelin-1 in human PA. In summary, MSU crystals induce endothelial dysfunction, impair HPV and stimulate the release of IL-6 by PASMCs through a mechanism which involves TAK-1 activation. These findings suggest that MSU crystals might contribute to the development of pulmonary vascular dysfunction and arterial hypoxemia in conditions associated with high levels of UA. Supported by Spanish MINECO 2011-28150 and ISCIII CP12/03304.

Gout is a common of inflammatory arthritis and is caused by the deposition of monosodium urate (MSU) crystals as a result of hyperuricemia (HUA). Although HUA is considered to be the main risk factor for gout, only approximately 10% of the individuals with HUA will eventually experience a gout attack. In this review, we first briefly introduce the development of gout and then summarize several possible reasons for its development. Genetic factors play a more prominent role in gout than in other diseases; functional mutations related to urate control and innate immunity components have been found to be associated with gout. Here, we list some of the most prominent genes involved in the pathogenesis of gout. In joints with MSU deposition, mature macrophages may uptake MSU crystals without causing inflammation, and this helps to maintain joints in an asymptomatic state. As an auxiliary inflammation pathway, the ATP-P2X7R-NLRP3 axis may contribute to the amplification of MSU-induced inflammation to affect the development of gout. Finally, this review summarizes the research progress on natural products that can be used in the treatment of HUA and gout.

Clec12a: Quieting the Dead

Gout is an acute rheumatic disorder that occurs in connection with the deposition of monosodium urate (MSU) crystals in the joints. This disease is characterized by intermittent episodes of severe pain and inflammatory joint swelling which are seemingly driven by prostaglandins. In this study we investigated the effect of MSU crystals on arachidonic acid (AA) metabolism in the mouse. We have demonstrated that prostaglandins and other AA metabolites were transiently formed after MSU crystal injection with peak levels occurring after 10 min. In contrast, free AA levels remained high for 2-4 hours after MSU crystal injection. By contrast, when exogenous AA was administered instead of MSU crystals, both the eicosanoids and AA diminished at the same high rates. The metabolism of exogenously administered AA to eicosanoids was inhibited by pretreatment with MSU crystals. No inhibition of AA metabolism was observed when mice were pretreated with AA itself, Ca2+ ionophore (A23187), or zymosan. We conclude that the MSU crystal treatment of mice results in a transient eicosanoid production which is followed by attenuated AA metabolism. It could be that MSU crystals similarly inhibit AA metabolism in gout and thereby limit the duration of gout attacks.

IntroductionGout is a chronic inflammatory disease due to monosodium urate (MSU) crystal deposits in articular joints. Phagocytosis of MSU crystals by monocytes/macrophages results in downstream expression and production of interleukin‐1 beta (IL‐1β). The two‐step process of gout inflammation involves 1) priming of monocytes with danger signals e.g. lipopolysaccharide (LPS) or soluble uric acid (UA) and 2) MSU phagocytosis and activation of the NLRP3 inflammasome in monocytes. Protein‐phosphatase‐2A (PP2A) is a serine/threonine phosphatase, and a reduction in intracellular macrophage PP2A activity was shown to aggravate LPS‐induced proinflammatory cytokine production. The prodrug Fingolimod (FTY720) and its phosphorylated active metabolite (p‐FTY720) are efficacious in multiple sclerosis and their mechanism of action involves activation of intracellular PP2A. The objective of this study is to investigate the role of PP2A in regulating monocyte priming and activation by MSU crystals and whether intracellular PP2A activation exerts an anti‐inflammatory effect in MSU‐stimulated monocytes.MethodsHuman THP‐1 monocytes (ATCC; 500,000 cells per well) were treated with UA (50 mg/dL) and LPS (10ng/mL) for 24 hours. Subsequently, cells were treated with MSU crystals (100ug/mL) and MSU phagocytosis was determined at 4 hours using flow cytometry and the percentage of cells displaying side‐scattering above a threshold value, indicative of crystal phagocytosis, were determined. Intracellular PP2A activity was determined at 6 hours following MSU stimulation using the PP2A Immunoprecipitation Phosphatase Assay Kit (Fisher Scientific). IL‐1β expression and protein levels at 6 hours were determined using quantitative polymerase chain reaction and ELISA, respectively. THP‐1 monocytes were treated with FTY720 or p‐FTY720 (2.5mM for both treatments; Cayman Chemicals) and PP2A activity was determined at 1, 3, 6 and 24 hours. THP‐1 monocytes were treated with UA+LPS followed by MSU ± p‐FTY720 (2.5mM for 6 hours) and IL‐1β expression and protein levels were determined. Statistical analyses were performed using analysis of variance (ANOVA) with Tukey’s post‐hoc test.ResultsUA+LPS pre‐treatment increased MSU phagocytosis by THP‐1 monocytes (fig. 1A and 1B; p&lt;0.001) and IL‐1β expression (fig. 1C; p&lt;0.001) and production (fig. 1D; p&lt;0.001). This effect was associated with a reduction in intracellular PP2A activity (fig. 1E; p&lt;0.001). FTY720 and p‐FTY720 increased intracellular PP2A activity in monocytes (fig. 2A; p&lt;0.001) with a higher peak effect achieved at 1 hour for p‐FTY720 (p&lt;0.01). p‐FTY720 treatment reduced IL‐1β expression (fig. 2B; p&lt;0.001) and mature IL‐1β production (fig. 2C; p&lt;0.001) in UA+LPS pre‐treated human monocytes following MSU stimulation.ConclusionUA and LPS enhanced MSU phagocytosis, expression and production of IL‐1β, potentially via a reduction in intracellular PP2A activity. FTY720 and p‐FTY720 activated PP2A in monocytes with an onset of effect of 1 hour and correspondingly reduced IL‐1β expression and production. PP2A is a novel therapeutic target for gout treatment.Impact of soluble uric acid (UA) and lipopolysaccharide (LPS) pre‐treatments on THP‐1 monocyte phagocytic activity towards monosodium urate (MSU) crystals, interleukin‐1 beta (IL‐1β) expression and production and association with intracellular protein phosphatase‐2A (PP2A) activity. A) Representative flow cytometry scatter plots showing enhanced MSU crystal phagocytosis (as shown by more cells with increased side‐scatter in P2 region of interest) by THP‐1 monocytes following UA+LPS pre‐treatment. B) UA and UA+LPS pre‐treatment increased the percentage of THP‐1 monocytes positive for MSU phagocytosis. C) UA+LPS pre‐treatment increased IL‐1β expression by THP‐1 monocytes following MSU challenge. D) UA+LPS pre‐treatment increased IL‐1β production by THP‐1 monocytes following MSU challenge. E) Intracellular PP2A activity was reduced in UA or UA+LPS pre‐treated THP‐1 monocytes. *p&lt;0.001; **p&lt;0.01; n.s.: non significant.Figure 1Impact of FTY720 (Fingolimod; prodrug) and phosphorylated FTY720 (p‐FTY‐720; active metabolite) treatments on protein phosphatase‐2A (PP2A) activity in human THP‐1 monocytes and impact of p‐FTY720 treatment on interleukin‐1 beta (IL‐1β) expression and production by monosodium urate (MSU) crystal stimulated THP‐1 monocytes ± soluble uric acid (UA) and lipopolysaccharide (LPS) pre‐treatment. Treatments were conducted using 2.5μM concentrations. A) FTY720 and p‐FTY720 increased intracellular PP2A activity in THP‐1 monocytes. B) p‐ FTY720 reduced IL‐1β expression in THP‐1 monocytes. C) p‐FTY720 reduced IL‐1β production in THP‐1 monocytes. *p&lt;0.001; **p&lt;0.01; n.s.: non significant.Figure 2

Soluble monosodium urate, but not its crystal, induces toll like receptor 4-dependent immune activation in renal mesangial cells

IntroductionGout results from an innate immune response to monosodium urate (MSU) crystals deposited in joints. Increased very low-density lipoprotein (VLDL) has been associated with gout. The apolipoprotein B (apo B), which is present on VLDL, regulates neutrophil response to MSU crystals and has been positively associated with gout. Furthermore, the gene (A1CF) encoding the complementation factor for the APOB mRNA-editing enzyme is associated with urate levels. However, the relationship of apo B and VLDL with gout and hyperuricaemia (HU) is still unclear. Therefore, we tested the association of VLDL and apo B with HU and with gout compared to HU.MethodsNew Zealand European (n = 90) and Māori and Pacific Island (Polynesian) (n = 90) male gout case and control sample sets were divided into normouricaemia (NU), asymptomatic HU and gout groups. Size exclusion chromatography and enzyme-linked immunosorbant assay was used to measure VLDL and apo B. Multivariate logistic regression was used to assess the risk of gout and HU per unit change in VLDL and apo B.ResultsIncreased levels of VLDL triglycerides (Tg) were observed in the gout sample set compared to NU and HU in Europeans (P = 1.8 × 10-6 and 1 × 10-3, respectively), but only compared to NU in Polynesians (P = 0.023). This increase was driven by increased number of VLDL particles in the European participants and by the Tg-enrichment of existing VLDL particles in the Polynesian participants. Each mmol/L increase in VLDL Tg was significantly associated with gout in the presence of HU in Europeans, with a similar trend in Polynesians (OR = 7.61, P = 0.011 and 2.84, P = 0.069, respectively). Each μmol/L increase in total apo B trended towards decreased risk of HU (OR = 0.47; P = 0.062) and, conversely, with increased risk of gout compared to HU (OR = 5.60; P = 0.004).ConclusionsIncreased VLDL Tg is associated with the risk of gout compared to HU. A genetic approach should be taken to investigate the possibility for causality of VLDL in gout. Apolipoprotein B may have pleiotropic effects in determining HU and gout.

Monosodium urate (MSU) crystals are the cause of gout and are formed from uric acid and sodium. Because they can be rotated by the torque induced by an applied magnetic field, we have characterized their detailed magnetic orientational properties to improve the detection sensitivity from outside the body using the magnetic orientation. The MSU crystals were precipitated by dissolving uric acid powder in aqueous sodium hydroxide. In serum, the MSU crystals were small and uniformly sized, similar to those obtained from gout patients. The MSU crystals were not completely oriented perpendicular to the magnetic-field direction, but rather the c-axis of the crystal was inclined with respect to the field direction. Specifically, they were inclined at a mean of 69.3° under a magnetic field of 500 mT. The MSU crystals were not perfectly oriented in magnetic fields <200 mT. The inclination results from the molecular structure of the MSU crystal.

Gout is a common crystal-induced arthritis, in which monosodium urate (MSU) crystals precipitate within joints and soft tissues and elicit an inflammatory response. The causes of elevated serum urate and the inflammatory pathways activated by MSU crystals have been well studied, but less is known about the processes leading to crystal formation and growth. Uric acid, the final product of purine metabolism, is a weak acid that circulates as the deprotonated urate anion under physiologic conditions, and combines with sodium ions to form MSU. MSU crystals are known to have a triclinic structure, in which stacked sheets of purine rings form the needle-shaped crystals that are observed microscopically. Exposed, charged crystal surfaces are thought to allow for interaction with phospholipid membranes and serum factors, playing a role in the crystal-mediated inflammatory response. While hyperuricemia is a clear risk factor for gout, local factors have been hypothesized to play a role in crystal formation, such as temperature, pH, mechanical stress, cartilage components, and other synovial and serum factors. Interestingly, several studies suggest that MSU crystals may drive the generation of crystal-specific antibodies that facilitate future MSU crystallization. Here, we review MSU crystal biology, including a discussion of crystal structure, effector function, and factors thought to play a role in crystal formation. We also briefly compare MSU biology to that of uric acid stones causing nephrolithasis, and consider the potential treatment implications of MSU crystal biology.

The pyroptotic cell death effector gasdermin D (GSDMD) is required for murine models of hereditary inflammasome-driven, IL-1β-dependent, autoinflammatory disease, making it an attractive therapeutic target. However, the importance of GSDMD for more common conditions mediated by pathological IL-1β activation, such as gout, remain unclear. In this study, we address whether GSDMD and the recently described GSDMD inhibitor necrosulfonamide (NSA) contribute to monosodium urate (MSU) crystal-induced cell death, IL-1β release, and autoinflammation. We demonstrate that MSU crystals, the etiological agent of gout, rapidly activate GSDMD in murine macrophages. Despite this, the genetic deletion of GSDMD or the other lytic effector implicated in MSU crystal killing, mixed lineage kinase domain-like (MLKL), did not prevent MSU crystal-induced cell death. Consequently, GSDMD or MLKL loss did not hinder MSU crystal-mediated release of bioactive IL-1β. Consistent with in vitro findings, IL-1β induction and autoinflammation in MSU crystal-induced peritonitis was not reduced in GSDMD-deficient mice. Moreover, we show that the reported GSDMD inhibitor, NSA, blocks inflammasome priming and caspase-1 activation, thereby preventing pyroptosis independent of GSDMD targeting. The inhibition of cathepsins, widely implicated in particle-induced macrophage killing, also failed to prevent MSU crystal-mediated cell death. These findings 1) demonstrate that not all IL-1β-driven autoinflammatory conditions will benefit from the therapeutic targeting of GSDMD, 2) document a unique mechanism of MSU crystal-induced macrophage cell death not rescued by pan-cathepsin inhibition, and 3) show that NSA inhibits inflammasomes upstream of GSDMD to prevent pyroptotic cell death and IL-1β release.

To investigate the correlation between the constitution of traditional Chinese medicine (TCM) and hyperuricemia (HUA) and gout. Databases including China National Knowledge Infrastructure (CNKI), WanFang Data, China Science and Technology Journal Database (VIP), China Biology Medicine Disc (CBMdisc), PubMed, The Cochrane Library, Web of Science, and Excerpta Medical Database (Embase) were searched to collect observational studies about TCM constitution in HUA and gout from inception to November 21, 2021. The distribution of TCM constitution types in HUA and gout patients was presented by proportion, while the correlation was presented by odds ratio (OR) and 95% CI. Meta-analysis was performed using StataCorp Stata (STATA) version 16.0 software. Twenty-one cross-sectional studies and 10 case-control studies involving 38028 samples were included, among which 27526 patients were diagnosed with HUA and 2048 patients with gout. Phlegm-dampness constitution (PDC), damp-heat constitution (DHC), and qi-deficiency constitution (QDC) are the most common types, accounting for 24% (20%-27%), 22% (16%-27%), and 15% (12%-18%), respectively, in HUA patients, while DHC, PDC, and blood stasis constitution (BSC) accounted for 28% (18%-39%), 23% (17%-29%), and 11% (8%-15%), respectively, in gout patients. PDC and DHC were the main constitution types in patients with HUA or gout in south China, east China, north China, southwest China, northwest China, and northeast China. There was no difference in the distribution of PDC and QDC in male or female patients with HUA, while males with DHC in HUA were more common than females. The proportion of PDC or DHC among HUA patients was 1.93 times and 2.14 times higher than that in the general population (OR and 95% CI: 1.93 (1.27, 2.93), 2.14 (1.47, 3.13)), while the proportions of PDC, DHC, and BSC were 3.59 times, 4.85 times, and 4.35 times higher than that of the general groups (OR and 95% CI: 3.59 (1.65, 7.80), 4.85 (1.62, 14.57), and 4.35(2.33, 8.11)). PDC, DHC, and QDC are the main constitution types of patients with HUA, while PDC and QDC may be the risk factors for HUA. DHC, PDC, and BSC are the main constitution types of patients with gout, and they may be the risk factors for gout. In clinical and scientific research, more attention should be paid to the relationship between the above-mentioned TCM constitution in HUA or gout. Nevertheless, because the quality of the included observational studies is low, more prospective cohort studies related to TCM constitution and HUA or gout can be carried out to verify the causality between TCM constitution and HUA or gout.