Adenosinergic signaling inhibits oxalate transport by human intestinal Caco2-BBE cells through the A2B adenosine receptor

A Direct Link Implicating Loss of SLC26A6 to Gut Microbial Dysbiosis, Compromised Barrier Integrity, and Inflammation

BACKGROUND & AIMS

Putative anion transporter-1 (PAT1, SLC26A6) plays a key role in intestinal oxalate and bicarbonate secretion. PAT1 knockout (PKO) mice exhibit hyperoxaluria and nephrolithiasis. Notably, diseases such as inflammatory bowel disease are also associated with higher risk of hyperoxaluria and nephrolithiasis. However, the potential role of PAT1 deficiency in gut-barrier integrity and susceptibility to colitis is currently elusive.

METHODS

Age-matched PKO and wild-type littermates were administered 3.5% dextran sulfate sodium in drinking water for 6 days. Ileum and colon of control and treated mice were harvested. Messenger RNA and protein expression of tight junction proteins were determined by reverse transcription polymerase chain reaction and western blotting. Severity of inflammation was assessed by measuring diarrheal phenotype, cytokine expression, and hematoxylin and eosin staining. Gut microbiome and associated metabolome were analyzed by 16S ribosomal RNA sequencing and mass spectrometry, respectively.

RESULTS

PKO mice exhibited significantly higher loss of body weight, gut permeability, colonic inflammation, and diarrhea in response to dextran sulfate sodium treatment. In addition, PKO mice showed microbial dysbiosis and significantly reduced levels of butyrate and butyrate-producing microbes compared with controls. Co-housing wild-type and PKO mice for 4 weeks resulted in PKO-like signatures on the expression of tight junction proteins in the colons of wild-type mice.

CONCLUSIONS

Our data demonstrate that loss of PAT1 disrupts gut microbiome and related metabolites, decreases gut-barrier integrity, and increases host susceptibility to intestinal inflammation. These findings, thus, highlight a novel role of the oxalate transporter PAT1 in promoting gut-barrier integrity, and its deficiency appears to contribute to the pathogenesis of inflammatory bowel diseases.

SLC26 family: a new insight for kidney stone disease

The solute-linked carrier 26 (SLC26) protein family is comprised of multifunctional transporters of substrates that include oxalate, sulphate, and chloride. Disorders of oxalate homeostasis cause hyperoxalemia and hyperoxaluria, leading to urinary calcium oxalate precipitation and urolithogenesis. SLC26 proteins are aberrantly expressed during kidney stone formation, and consequently may present therapeutic targets. SLC26 protein inhibitors are in preclinical development. In this review, we integrate the findings of recent reports with clinical data to highlight the role of SLC26 proteins in oxalate metabolism during urolithogenesis, and discuss limitations of current studies and potential directions for future research.

Neutrophil-Epithelial Crosstalk During Intestinal Inflammation

Neutrophils are the most abundant leukocyte population in the human circulatory system and are rapidly recruited to sites of inflammation. Neutrophils play a multifaceted role in intestinal inflammation, as they contribute to the elimination of invading pathogens. Recently, their role in epithelial restitution has been widely recognized; however, they are also associated with bystander tissue damage. The intestinal epithelium provides a physical barrier to prevent direct contact of luminal contents with subepithelial tissues, which is extremely important for the maintenance of intestinal homeostasis. Numerous studies have demonstrated that transepithelial migration of neutrophils is closely related to disease symptoms and disruption of crypt architecture in inflammatory bowel disease and experimental colitis. There has been growing interest in how neutrophils interact with the epithelium under inflammatory conditions. Most studies focus on the effects of neutrophils on intestinal epithelial cells; however, the effects of intestinal epithelial cells on neutrophils during intestinal inflammation need to be well-established. Based on these data, we have summarized recent articles on the role of neutrophil-epithelial interactions in intestinal inflammation, particularly highlighting the epithelium-derived molecular regulators that mediate neutrophil recruitment, transepithelial migration, and detachment from the epithelium, as well as the functional consequences of their crosstalk. A better understanding of these molecular events may help develop novel therapeutic targets for mitigating the deleterious effects of neutrophils in inflammatory bowel disease.

Pathophysiology and Management of Hyperoxaluria and Oxalate Nephropathy: A Review

Hyperoxaluria results from either inherited disorders of glyoxylate metabolism leading to hepatic oxalate overproduction (primary hyperoxaluria), or increased intestinal oxalate absorption (secondary hyperoxaluria). Hyperoxaluria may lead to urinary supersaturation of calcium oxalate and crystal formation, causing urolithiasis and deposition of calcium oxalate crystals in the kidney parenchyma, a condition termed oxalate nephropathy. Considerable progress has been made in the understanding of pathophysiological mechanisms leading to hyperoxaluria and oxalate nephropathy, whose diagnosis is frequently delayed and prognosis too often poor. Fortunately, novel promising targeted therapeutic approaches are on the horizon in patients with primary hyperoxaluria. Patients with secondary hyperoxaluria frequently have long-standing hyperoxaluria-enabling conditions, a fact suggesting the role of triggers of acute kidney injury such as dehydration. Current standard of care in these patients includes management of the underlying cause, high fluid intake, and use of calcium supplements. Overall, prompt recognition of hyperoxaluria and associated oxalate nephropathy is crucial because optimal management may improve outcomes.

Oxalate Flux Across the Intestine: Contributions from Membrane Transporters

Epithelial oxalate transport is fundamental to the role occupied by the gastrointestinal (GI) tract in oxalate homeostasis. The absorption of dietary oxalate, together with its secretion into the intestine, and degradation by the gut microbiota, can all influence the excretion of this nonfunctional terminal metabolite in the urine. Knowledge of the transport mechanisms is relevant to understanding the pathophysiology of hyperoxaluria, a risk factor in kidney stone formation, for which the intestine also offers a potential means of treatment. The following discussion presents an expansive review of intestinal oxalate transport. We begin with an overview of the fate of oxalate, focusing on the sources, rates, and locations of absorption and secretion along the GI tract. We then consider the mechanisms and pathways of transport across the epithelial barrier, discussing the transcellular, and paracellular components. There is an emphasis on the membrane-bound anion transporters, in particular, those belonging to the large multifunctional Slc26 gene family, many of which are expressed throughout the GI tract, and we summarize what is currently known about their participation in oxalate transport. In the final section, we examine the physiological stimuli proposed to be involved in regulating some of these pathways, encompassing intestinal adaptations in response to chronic kidney disease, metabolic acid-base disorders, obesity, and following gastric bypass surgery. There is also an update on research into the probiotic, Oxalobacter formigenes, and the basis of its unique interaction with the gut epithelium. © 2021 American Physiological Society. Compr Physiol 11:1-41, 2021.

Genetic Prioritization, Therapeutic Repositioning and Cross-Disease Comparisons Reveal Inflammatory Targets Tractable for Kidney Stone Disease

Background

Formation of kidney stones resulting in urological disorders remains a major cause of morbidity in renal diseases and many others. Innate immunity, mainly inflammasome, has demonstrated a key role in the development of kidney stone disease (or "nephrolithiasis"), but a molecular rationale for therapeutic intervention targeting immunity is far from clear. We reason that identifying inflammatory gene networks underlying disease risk would inform immunotherapeutic targets for candidate drug discovery.

Results

We generated an atlas of genetic target prioritization, with the top targets highly enriched for genes involved in the NF-kB regulation, including interaction neighbors of inflammasome genes. We identified a network of highly ranked and interconnecting genes that are of functional relevance to nephrolithiasis and mediate crosstalk between inflammatory pathways. Crosstalk genes can be utilized for therapeutic repositioning, as highlighted by identification of ulixertinib and losmapimod that are both under clinical investigation as inhibitors of inflammatory mediators. Finally, we performed cross-disease comparisons and druggable pocket predictions, identifying inflammatory targets that are specific to and tractable for nephrolithiasis.

Conclusion

Genetic targets and candidate drugs, in silico identified in this study, provide the rich information of how to target innate immune pathways, with the potential of advancing immunotherapeutic strategies for nephrolithiasis.

Extracellular ATP hydrolysis in Caco-2 human intestinal cell line

Extracellular nucleotides and nucleosides activate signaling pathways that play major roles in the physiology and pathophysiology of the gastrointestinal tract. Ectonucleotidases hydrolyze extracellular nucleotides and thus regulate ligand exposure to purinergic receptors. In this study, we investigated the expression, localization and activities of ectonucleotidases using Caco-2 cells, a model of human intestinal epithelial cells. In addition, by studying ATP release and the rates of extracellular ATP (eATP) hydrolysis, we analyzed the contribution of these processes to the regulation of eATP in these cells. Results show that Caco-2 cells regulate the metabolism of eATP and by-products by ecto-nucleoside triphosphate diphosphohydrolase-1 and -2, a neutral ecto-phosphatase and ecto-5'-nucleotidase. All these ectoenzymes were kinetically characterized using intact cells, and their presence confirmed by denatured and native gels, western blot and cytoimmunofluorescence techniques. In addition, regulation of eATP was studied by monitoring the dynamic balance between intracellular ATP release and ectoATPase activity. Following mechanical and hypotonic stimuli, Caco-2 cells triggered a strong but transient release of intracellular ATP, with almost no energy cost, leading to a steep increase of eATP concentration, which was later reduced by ectoATPase activity. A data-driven algorithm allowed quantifying and predicting the rates of ATP release and ATP consumption contributing to the dynamic accumulation of ATP at the cell surface.

Protectin DX restores Treg/Th17 cell balance in rheumatoid arthritis by inhibiting NLRP3 inflammasome via miR-20a

Regulatory T-cell (Treg)/T-helper 17 (Th17) cell balance plays an important role in the progression of rheumatoid arthritis (RA). Our study explored the protective effect of protectin DX (PDX), which restored Treg/Th17 cell balance in RA, and the role of the nucleotide-binding domain (NOD)-like receptor protein 3 (NLRP3) inflammasome pathway in this process. Using mass spectrometry, we discovered that level of PDX decreased in active-RA patients and increased in inactive-RA patients compared with HCs, and serum PDX was a potential biomarker in RA activity detection (area under the curve [AUC] = 0.86). In addition, a collagen-induced arthritis (CIA) mice model was constructed and PDX obviously delayed RA progression in the CIA model, upregulating Tregs and anti-inflammatory cytokines while downregulating Th17 cells and pro-inflammatory cytokines. Moreover, NLRP3 knockout and rescue experiments demonstrated that NLRP3 participated in PDX-mediated Treg/Th17 cell balance restoration, joint injury amelioration and inflammatory-response attenuation using Nlrp3-/- mice. Furthermore, microarray and verified experiments confirmed that PDX reduced NLRP3 expression via miRNA-20a (miR-20a). In summary, we confirmed for the first time that PDX could effectively ameliorate CIA progression by restoring Treg/Th17 cell balance, which was mediated by inhibition of the NLRP3 inflammasome pathway via miR-20a.

Gut-kidney axis in oxalate homeostasis

PURPOSE OF REVIEW

The gut-kidney axis plays a critical role in oxalate homeostasis, and better understanding of oxalate transport regulatory mechanisms is essential for developing novel therapies.

RECENT FINDINGS

Oxalate potentially contributes to chronic kidney disease (CKD) progression, CKD - and end stage renal disease (ESRD)-associated cardiovascular diseases, polycystic kidney disease (PKD) progression, and/or poor renal allograft survival, emphasizing the need for plasma and urinary oxalate lowering therapies. One promising strategy would be to enhance the bowel's ability to secrete oxalate, which might be facilitated by the following findings. Oxalobacter formigenes (O. formigenes)-derived factors recapitulate O. formigenes colonization effects by reducing urinary oxalate excretion in hyperoxaluric mice by inducing colonic oxalate secretion. Protein kinase A activation stimulates intestinal oxalate transport by enhancing the surface expression of the oxalate transporter SLC26A6 (A6). Glycosylation also stimulates A6-mediated oxalate transport. The colon adapts to chronic acidosis in rats through increased colonic oxalate secretion as previously reported in CKD rats, and A6-mediated enteric oxalate secretion is critical in reducing the body oxalate burden in CKD mice. Intestinal oxalate transport is negatively regulated by proinflammatory cytokines and cholinergic, purinergic, and adenosinergic signaling.

SUMMARY

These findings could facilitate the development of novel therapeutics for hyperoxalemia, hyperoxaluria, and related disorders if similar regulatory mechanisms are confirmed in humans.

Activation of the PKA signaling pathway stimulates oxalate transport by human intestinal Caco2-BBE cells

Most kidney stones are composed of calcium oxalate, and small increases in urine oxalate enhance the stone risk. The mammalian intestine plays a crucial role in oxalate homeostasis, and we had recently reported that Oxalobacter-derived factors stimulate oxalate transport by human intestinal Caco2-BBE (C2) cells through PKA activation. We therefore evaluated whether intestinal oxalate transport is directly regulated by activation of the PKA signaling pathway. To this end, PKA was activated with forskolin and IBMX (F/I). F/I significantly stimulated (3.7-fold) [14C]oxalate transport by C2 cells [≥49% of which is mediated by the oxalate transporter SLC26A6 (A6)], an effect completely blocked by the PKA inhibitor H89, indicating that it is PKA dependent. PKA stimulation of intestinal oxalate transport is not cell line specific, since F/I similarly stimulated oxalate transport by the human intestinal T84 cells. F/I significantly increased (2.5-fold) A6 surface protein expression by use of immunocytochemistry. Assessing [14C]oxalate transport as a function of increasing [14C]oxalate concentration in the flux medium showed that the observed stimulation is due to a F/I-induced increase (1.8-fold) in Vmax and reduction (2-fold) in Km. siRNA knockdown studies showed that significant components of the observed stimulation are mediated by A6 and SLC26A2 (A2). Besides enhancing A6 surface protein expression, it is also possible that the observed stimulation is due to PKA-induced enhanced A6 and/or A2 transport activity in view of the reduced Km. We conclude that PKA activation positively regulates oxalate transport by intestinal epithelial cells and that PKA agonists might therapeutically impact hyperoxalemia, hyperoxaluria, and related kidney stones.