Lysophosphatidic acid stimulates the intestinal brush border Na(+)/H(+) exchanger 3 and fluid absorption via LPA(5) and NHERF2

Modeling the cell biology of monogenetic intestinal epithelial disorders

Monogenetic variants are responsible for a range of congenital human diseases. Variants in genes that are important for intestinal epithelial function cause a group of disorders characterized by severe diarrhea and loss of nutrient absorption called congenital diarrheas and enteropathies (CODEs). CODE-causing genes include nutrient transporters, enzymes, structural proteins, and vesicular trafficking proteins in intestinal epithelial cells. Several severe CODE disorders result from the loss-of-function in key regulators of polarized endocytic trafficking such as the motor protein, Myosin VB (MYO5B), as well as STX3, STXBP2, and UNC45A. Investigations of the cell biology and pathophysiology following loss-of-function in these genes have led to an increased understanding of both homeostatic and pathological vesicular trafficking in intestinal epithelial cells. Modeling different CODEs through investigation of changes in patient tissues, coupled with the development of animal models and patient-derived enteroids, has provided critical insights into the enterocyte differentiation and function. Linking basic knowledge of cell biology with the phenotype of specific patient variants is a key step in developing effective treatments for rare monogenetic diseases. This knowledge can also be applied more broadly to our understanding of common epithelial disorders.

LPA5-Dependent signaling regulates regeneration of the intestinal epithelium following irradiation

Lysophosphatidic acid (LPA) is a bioactive lipid molecule that regulates a wide array of cellular functions, including proliferation, differentiation, and survival, via activation of cognate receptors. The LPA5 receptor is highly expressed in the intestinal epithelium, but its function in restoring intestinal epithelial integrity following injury has not been examined. Here, we use a radiation-induced injury model to study the role of LPA5 in regulating intestinal epithelial regeneration. Control mice (Lpar5f/f) and mice with an inducible, epithelial cell-specific deletion of Lpar5 in the small intestine (Lpar5IECKO) were subjected to 10 Gy total body X-ray irradiation and analyzed during recovery. Repair of the intestinal mucosa was delayed in Lpar5IECKO mice with reduced epithelial proliferation and increased crypt cell apoptosis. These effects were accompanied by reduced numbers of OLFM4+ intestinal stem cells (ISCs). The effects of LPA5 on ISCs were corroborated by studies using organoids derived from Lgr5-lineage tracking reporter mice with deletion of Lpar5 in Lgr5+-stem cells (Lgr5Cont or Lgr5ΔLpar5). Irradiation of organoids resulted in fewer numbers of Lgr5ΔLpar5 organoids retaining Lgr5+-derived progenitor cells compared with Lgr5Cont organoids. Finally, we observed that impaired regeneration in Lpar5IECKO mice was associated with reduced numbers of Paneth cells and decreased expression of Yes-associated protein (YAP), a critical factor for intestinal epithelial repair. Our study highlights a novel role for LPA5 in regeneration of the intestinal epithelium following irradiation and its effect on the maintenance of Paneth cells that support the stem cell niche.NEW & NOTEWORTHY We used mice lacking expression of the lysophosphatidic acid receptor 5 (LPA5) in intestinal epithelial cells and intestinal organoids to show that the LPA5 receptor protects intestinal stem cells and progenitors from radiation-induced injury. We show that LPA5 induces YAP signaling and regulates Paneth cells.

cGMP-dependent kinase 2, Na+/H+ exchanger NHE3, and PDZ-adaptor NHERF2 co-assemble in apical membrane microdomains

AIM

Trafficking, membrane retention, and signal-specific regulation of the Na+/H+ exchanger 3 (NHE3) are modulated by the Na+/H+ Exchanger Regulatory Factor (NHERF) family of PDZ-adapter proteins. This study explored the assembly of NHE3 and NHERF2 with the cGMP-dependent kinase II (cGKII) within detergent-resistant membrane microdomains (DRMs, "lipid rafts") during in vivo guanylate cycle C receptor (Gucy2c) activation in murine small intestine.

METHODS

Small intestinal brush border membranes (siBBMs) were isolated from wild type, NHE3-deficient, cGMP-kinase II-deficient, and NHERF2-deficient mice, after oral application of the heat-stable Escherichia coli toxin (STa) analog linaclotide. Lipid raft and non-raft fractions were separated by Optiprep density gradient centrifugation of Triton X-solubilized siBBMs. Confocal microscopy was performed to study NHE3 redistribution after linaclotide application in vivo.

RESULTS

In the WT siBBM, NHE3, NHERF2, and cGKII were strongly raft associated. The raft association of NHE3, but not of cGKII, was NHERF2 dependent. After linaclotide application to WT mice, lipid raft association of NHE3 decreased, that of cGKII increased, while that of NHERF2 did not change. NHE3 expression in the BBM shifted from a microvillar to a terminal web region. The linaclotide-induced decrease in NHE3 raft association and in microvillar abundance was abolished in cGKII-deficient mice, and strongly reduced in NHERF2-deficient mice.

CONCLUSION

NHE3, cGKII, and NHERF2 form a lipid raft-associated signal complex in the siBBM, which mediates the inhibition of salt and water absorption by Gucy2c activation. NHERF2 enhances the raft association of NHE3, which is essential for its close interaction with the exclusively raft-associated activated cGKII.

A dual role of lysophosphatidic acid type 2 receptor (LPAR2) in nonsteroidal anti-inflammatory drug-induced mouse enteropathy

Lysophosphatidic acid (LPA) is a bioactive phospholipid mediator that has been found to ameliorate nonsteroidal anti-inflammatory drug (NSAID)-induced gastric injury by acting on lysophosphatidic acid type 2 receptor (LPAR2). In this study, we investigated whether LPAR2 signaling was implicated in the development of NSAID-induced small intestinal injury (enteropathy), another major complication of NSAID use. Wild-type (WT) and Lpar2 deficient (Lpar2-/-) mice were treated with a single, large dose (20 or 30 mg/kg, i.g.) of indomethacin (IND). The mice were euthanized at 6 or 24 h after IND treatment. We showed that IND-induced mucosal enteropathy and neutrophil recruitment occurred much earlier (at 6 h after IND treatment) in Lpar2-/- mice compared to WT mice, but the tissue levels of inflammatory mediators (IL-1β, TNF-α, inducible COX-2, CAMP) remained at much lower levels. Administration of a selective LPAR2 agonist DBIBB (1, 10 mg/kg, i.g., twice at 24 h and 30 min before IND treatment) dose-dependently reduced mucosal injury and neutrophil activation in enteropathy, but it also enhanced IND-induced elevation of several proinflammatory chemokines and cytokines. By assessing caspase-3 activation, we found significantly increased intestinal apoptosis in IND-treated Lpar2-/- mice, but it was attenuated after DBIBB administration, especially in non-obese diabetic/severe combined immunodeficiency (NOD/SCID) mice. Finally, we showed that IND treatment reduced the plasma activity and expression of autotaxin (ATX), the main LPA-producing enzyme, and also reduced the intestinal expression of Lpar2 mRNA, which preceded the development of mucosal damage. We conclude that LPAR2 has a dual role in NSAID enteropathy, as it contributes to the maintenance of mucosal integrity after NSAID exposure, but also orchestrates the inflammatory responses associated with ulceration. Our study suggests that IND-induced inhibition of the ATX-LPAR2 axis is an early event in the pathogenesis of enteropathy.

Regulation of the intestinal Na+/H+ exchanger NHE3 by AMP-activated kinase is dependent on phosphorylation of NHE3 at S555 and S563

Electroneutral NaCl transport by Na+/H+ exchanger 3 (NHE3, SLC9A3) is the major Na+ absorptive mechanism in the intestine and decreased NHE3 activity contributes to diarrhea. Patients with diabetes often experience gastrointestinal adverse effects and medications are often a culprit for chronic diarrhea in type 2 diabetes (T2D). We have shown previously that metformin, the most widely prescribed drug for the treatment of T2D, induces diarrhea by inhibition of Na+/H+ exchanger 3 (NHE3) in rodent models of T2D. Metformin was shown to activate AMP-activated protein kinase (AMPK), but AMPK-independent glycemic effects of metformin are also known. The current study is undertaken to determine whether metformin inhibits NHE3 by activation of AMPK and the mechanism by which NHE3 is inhibited by AMPK. Inhibition of NHE3 by metformin was abolished by knockdown of AMPK-α1 or AMPK-α2. AMPK activation by 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) phosphorylated NHE3 at S555. S555 is the primary site of phosphorylation by protein kinase A (PKA), but AMPK phosphorylated S555 independently of PKA. Using Mass spectrometry, we found S563 as a newly recognized phosphorylation site in NHE3. Altering either S555 or S563 to Ala was sufficient to block the inhibition of NHE3 activity by AMPK. NHE3 inhibition is dependent on ubiquitination by the E3 ubiquitin ligase Nedd4-2 and metformin was shown to induce NHE3 internalization via Nedd4-2-mediated ubiquitination. AICAR did not increase NHE3 ubiquitination when S555 or S563 was mutated. We conclude that AMPK activation inhibits NHE3 activity and NHE3 inhibition is associated with phosphorylation of NHE3 at S555 and S563.NEW & NOTEWORTHY We show that AMP-activated protein kinase (AMPK) phosphorylates NHE3 at S555 and S563 to inhibit NHE3 activity in intestinal epithelial cells. Phosphorylation of NHE3 by AMPK is necessary for ubiquitination of NHE3.

Inhibition of protein kinase C-α and activation of ezrin by Lactobacillus acidophilus restore Na+/H+ exchange activity and fluid absorption in db/db mice

Gastrointestinal (GI) complications, including diarrhea, constipation, and gastroparesis, are common in patients with diabetes. Dysregulation of the Na+/H+ exchanger NHE3 in the intestine is linked to diarrhea and constipation, and recent studies showed that NHE3 expression is reduced in type 1 diabetes and metformin causes diarrhea in the db/db mouse model of type 2 diabetes (T2D) via inhibition of NHE3. In this study, we investigated whether NHE3 expression is altered in type 2 diabetic intestine and the underlying mechanism that dysregulates NHE3. NHE3 expression in the brush border membrane (BBM) of the intestine of diabetic mice and humans was decreased. Protein kinase C (PKC) activation is associated with pathologies of diabetes, and immunofluorescence (IF) analysis revealed increased BBM PKCα abundance. Inhibition of PKCα increased NHE3 BBM abundance and NHE3-mediated intestinal fluid absorption in db/db mice. Previous studies have shown that Lactobacillus acidophilus (LA) stimulates intestinal ion transporters. LA increased NHE3 BBM expression and mitigated metformin-mediated inhibition of NHE3 in vitro and in vivo. To understand the underlying mechanism of LA-mediated stimulation of NHE3, we used Caco-2bbe cells overexpressing PKCα that mimic the elevated state of PKCα in T2D. LA diminished PKCα BBM expression, increased phosphorylation of ezrin, and the interaction of NHE3 with NHE regulatory factor 2 (NHERF2). In addition, inhibition of PKCι blocked phosphorylation of ezrin and activation of NHE3 by LA. These findings demonstrate that NHE3 is downregulated in T2D, and LA restores NHE3 expression via regulation of PKCα, PKCι, and ezrin.NEW & NOTEWORTHY We used mouse models of type 2 diabetes (T2D) and human patient-derived samples to show that Na+/H+ exchanger 3 (NHE3) expression is decreased in T2D. We show that protein kinase C-α (PKCα) is activated in diabetes and inhibition of PKCα increased NHE3 expression and mitigates diarrhea. We show that Lactobacillus acidophilus (LA) stimulates NHE3 via inhibition of PKCα and phosphorylation of ezrin.

Emerging Roles of Lysophosphatidic Acid in Macrophages and Inflammatory Diseases

Lysophosphatidic acid (LPA) is a bioactive phospholipid that regulates physiological and pathological processes in numerous cell biological functions, including cell migration, apoptosis, and proliferation. Macrophages are found in most human tissues and have multiple physiological and pathological functions. There is growing evidence that LPA signaling plays a significant role in the physiological function of macrophages and accelerates the development of diseases caused by macrophage dysfunction and inflammation, such as inflammation-related diseases, cancer, atherosclerosis, and fibrosis. In this review, we summarize the roles of LPA in macrophages, analyze numerous macrophage- and inflammation-associated diseases triggered by LPA, and discuss LPA-targeting therapeutic strategies.

Lysophosphatidic acid, a simple phospholipid with myriad functions

Lysophosphatidic acid (LPA) is a simple phospholipid consisting of a phosphate group, glycerol moiety, and only one hydrocarbon chain. Despite its simple chemical structure, LPA plays an important role as an essential bioactive signaling molecule via its specific six G protein-coupled receptors, LPA_1-6. Recent studies, especially those using genetic tools, have revealed diverse physiological and pathological roles of LPA and LPA receptors in almost every organ system. Furthermore, many studies are illuminating detailed mechanisms to orchestrate multiple LPA receptor signaling pathways and to facilitate their coordinated function. Importantly, these extensive "bench" works are now translated into the "bedside" as exemplified by approaches targeting LPA₁ signaling to combat fibrotic diseases. In this review, we discuss the physiological and pathological roles of LPA signaling and their implications for clinical application by focusing on findings revealed by in vivo studies utilizing genetic tools targeting LPA receptors.

Comparative Analyses Reveal the Genetic Mechanism of Ambergris Production in the Sperm Whale Based on the Chromosome-Level Genome

Sperm whales are a marine mammal famous for the aromatic substance, the ambergris, produced from its colon. Little is known about the biological processes of ambergris production, and this study aims to investigate the genetic mechanism of ambergris production in the sperm whale based on its chromosome-level genome. Comparative genomics analyses found 1207 expanded gene families and 321 positive selected genes (PSGs) in the sperm whale, and functional enrichment analyses suggested revelatory pathways and terms related to the metabolism of steroids, terpenoids, and aldosterone, as well as microbiota interaction and immune network in the intestine. Furthermore, two sperm-whale-specific missense mutations (Tyr393His and Leu567Val) were detected in the PSG LIPE, which has been reported to play vital roles in lipid and cholesterol metabolism. In total, 46 CYP genes and 22 HSD genes were annotated, and then mapped to sperm whale chromosomes. Furthermore, phylogenetic analysis of CYP genes in six mammals found that CYP2E1, CYP51A and CYP8 subfamilies exhibited relative expansion in the sperm whale. Our results could help understand the genetic mechanism of ambergris production, and further reveal the convergent evolution pattern among animals that produce similar odorants.

Biological functions of bacterial lysophospholipids

Lysophospholipids (LPLs) are lipid-derived metabolic intermediates in the cell membrane. The biological functions of LPLs are distinct from their corresponding phospholipids. In eukaryotic cells LPLs are important bioactive signaling molecules that regulate many important biological processes, but in bacteria the function of LPLs is still not fully defined. Bacterial LPLs are usually present in cells in very small amounts, but can strongly increase under certain environmental conditions. In addition to their basic function as precursors in membrane lipid metabolism, the formation of distinct LPLs contributes to the proliferation of bacteria under harsh circumstances or may act as signaling molecules in bacterial pathogenesis. This review provides an overview of the current knowledge of the biological functions of bacterial LPLs including lysoPE, lysoPA, lysoPC, lysoPG, lysoPS and lysoPI in bacterial adaptation, survival, and host-microbe interactions.

Altered MYO5B Function Underlies Microvillus Inclusion Disease: Opportunities for Intervention at a Cellular Level

Microvillus inclusion disease (MVID) is a congenital diarrheal disorder resulting in life-threatening secretory diarrhea in newborns. Inactivating and nonsense mutations in myosin Vb (MYO5B) have been identified in MVID patients. Work using patient tissues, cell lines, mice, and pigs has led to critical insights into the pathology of MVID and a better understanding of both apical trafficking in intestinal enterocytes and intestinal stem cell differentiation. These studies have demonstrated that loss of MYO5B or inactivating mutations lead to loss of apical sodium and water transporters, without loss of apical CFTR, accounting for the major pathology of the disease. In addition, loss of MYO5B expression induces the formation of microvillus inclusions through apical bulk endocytosis that utilizes dynamin and PACSIN2 and recruits tight junction proteins to the sites of bulk endosome formation. Importantly, formation of microvillus inclusions is not required for the induction of diarrhea. Recent investigations have demonstrated that administration of lysophosphatidic acid (LPA) can partially reestablish apical ion transporters in enterocytes of MYO5B KO mice. In addition, further studies have shown that MYO5B loss induces an imbalance in Wnt/Notch signaling pathways that can lead to alterations in enterocyte maturation and tuft cell lineage differentiation. Inhibition of Notch signaling leads to improvements in those cell differentiation deficits. These studies demonstrate that directed strategies through LPA receptor activation and Notch inhibition can bypass the inhibitory effects of MYO5B loss. Thus, effective strategies may be successful in MVID patients and other congenital diarrhea syndromes to reestablish proper apical membrane absorption of sodium and water in enterocytes and ameliorate life-threatening congenital diarrhea.

The Role of Plasma Membrane Sodium/Hydrogen Exchangers in Gastrointestinal Functions: Proliferation and Differentiation, Fluid/Electrolyte Transport and Barrier Integrity

The five plasma membrane Na⁺/H⁺ exchanger (NHE) isoforms in the gastrointestinal tract are characterized by distinct cellular localization, tissue distribution, inhibitor sensitivities, and physiological regulation. NHE1 (Slc9a1) is ubiquitously expressed along the gastrointestinal tract in the basolateral membrane of enterocytes, but so far, an exclusive role for NHE1 in enterocyte physiology has remained elusive. NHE2 (Slc9a2) and NHE8 (Slc9a8) are apically expressed isoforms with ubiquitous distribution along the colonic crypt axis. They are involved in pH_i regulation of intestinal epithelial cells. Combined use of a knockout mouse model, intestinal organoid technology, and specific inhibitors revealed previously unrecognized actions of NHE2 and NHE8 in enterocyte proliferation and differentiation. NHE3 (Slc9a3), expressed in the apical membrane of differentiated intestinal epithelial cells, functions as the predominant nutrient-independent Na⁺ absorptive mechanism in the gut. The new selective NHE3 inhibitor (Tenapanor) allowed discovery of novel pathophysiological and drug-targetable NHE3 functions in cystic-fibrosis associated intestinal obstructions. NHE4, expressed in the basolateral membrane of parietal cells, is essential for parietal cell integrity and acid secretory function, through its role in cell volume regulation. This review focuses on the expression, regulation and activity of the five plasma membrane Na⁺/H⁺ exchangers in the gastrointestinal tract, emphasizing their role in maintaining intestinal homeostasis, or their impact on disease pathogenesis. We point to major open questions in identifying NHE interacting partners in central cellular pathways and processes and the necessity of determining their physiological role in a system where their endogenous expression/activity is maintained, such as organoids derived from different parts of the gastrointestinal tract.

Metformin Inhibits Na+/H+ Exchanger NHE3 Resulting in Intestinal Water Loss

Glycemic control is the key to the management of type 2 diabetes. Metformin is an effective, widely used drug for controlling plasma glucose levels in diabetes, but it is often the culprit of gastrointestinal adverse effects such as abdominal pain, nausea, indigestion, vomiting, and diarrhea. Diarrhea is a complex disease and altered intestinal transport of electrolytes and fluid is a common cause of diarrhea. Na+/H+ exchanger 3 (NHE3, SLC9A3) is the major Na+ absorptive mechanism in the intestine and our previous study has demonstrated that decreased NHE3 contributes to diarrhea associated with type 1 diabetes. The goal of this study is to investigate whether metformin regulates NHE3 and inhibition of NHE3 contributes to metformin-induced diarrhea. We first determined whether metformin alters intestinal water loss, the hallmark of diarrhea, in type 2 diabetic db/db mice. We found that metformin decreased intestinal water absorption mediated by NHE3. Metformin increased fecal water content although mice did not develop watery diarrhea. To determine the mechanism of metformin-mediated regulation of NHE3, we used intestinal epithelial cells. Metformin inhibited NHE3 activity and the effect of metformin on NHE3 was mimicked by a 5'-AMP-activated protein kinase (AMPK) activator and blocked by pharmacological inhibition of AMPK. Metformin increased phosphorylation and ubiquitination of NHE3, resulting in retrieval of NHE3 from the plasma membrane. Previous studies have demonstrated the role of neural precursor cell expressed, developmentally down-regulated 4-2 (Nedd4-2) in regulation of human NHE3. Silencing of Nedd4-2 mitigated NHE3 inhibition and ubiquitination by metformin. Our findings suggest that metformin-induced diarrhea in type 2 diabetes is in part caused by reduced Na+ and water absorption that is associated with NHE3 inhibition, probably by AMPK.

Survival of Stem Cells and Progenitors in the Intestine Is Regulated by LPA5-Dependent Signaling

BACKGROUND & AIMS

Regeneration of the epithelium by stem cells in the intestine is supported by intrinsic and extrinsic factors. Lysophosphatidic acid (LPA), a bioactive lipid mediator, regulates many cellular functions, including cell proliferation, survival, and cytokine secretion. Here, we identify LPA₅ receptor as a potent regulator of the survival of stem cells and transit-amplifying cells in the intestine.

METHODS

We have used genetic mouse models of conditional deletion of Lpar5, Lpar5^f/f;Rosa-Cre^ERT (Lpar5^KO), and intestinal epithelial cell-specific Lpar5^f/f;AhCre (Lpar5^IECKO) mice. Mice were treated with tamoxifen or β-naphthoflavone to delete Lpar5 expression. Enteroids derived from these mice were used to determine the effect of Lpar5 loss on the apoptosis and proliferation of crypt epithelial cells.

RESULTS

Conditional loss of Lpar5 induced ablation of the intestinal mucosa, which increased morbidity of Lpar5^KO mice. Epithelial regeneration was compromised with increased apoptosis and decreased proliferation of crypt epithelial cells by Lpar5 loss. Interestingly, intestinal epithelial cell-specific Lpar5 loss did not cause similar phenotypic defects in vivo. Lpar5 loss reduced intestinal stem cell marker gene expression and reduced lineage tracing from Lgr5⁺ ISCs. Lpar5 loss induced CXCL10 expression which exerts cytotoxic effects on intestinal stem cells and progenitors in the intestinal crypts. By co-culturing Lpar5^KO enteroids with wild-type or Lpar5^KO splenocytes, we demonstrated that lymphocytes protect the intestinal crypts via a LPA₅-dependent suppression of CXCL10.

CONCLUSIONS

LPA₅ is essential for the regeneration of intestinal epithelium. Our findings reveal a new finding that LPA₅ regulates survival of stem cells and transit-amplifying cells in the intestine.

Nedd4-2-dependent Ubiquitination Potentiates the Inhibition of Human NHE3 by Cholera Toxin and Enteropathogenic Escherichia coli

BACKGROUND & AIMS

Diarrhea is one of the most common illnesses and is often caused by bacterial infection. Recently, we have shown that human Na⁺/H⁺ exchanger NHE3 (hNHE3), but not non-human NHE3s, interacts with the E3 ubiquitin ligase Nedd4-2. We hypothesize that this property of hNHE3 contributes to the increased severity of diarrhea in humans.

METHODS

We used humanized mice expressing hNHE3 in the intestine (hNHE3^int) to compare the contribution of hNHE3 and mouse NHE3 to diarrhea induced by cholera toxin (CTX) and enteropathogenic Escherichia coli (EPEC). We measured Na⁺/H⁺ exchange activity and fluid absorption. The role of Nedd4-2 on hNHE3 activity and ubiquitination was determined by knockdown in Caco-2bbe cells. The effects of protein kinase A (PKA), the primary mediator of CTX-induced diarrhea, on Nedd4-2 and hNHE3 phosphorylation and their interaction were determined.

RESULTS

The effects of CTX and EPEC were greater in hNHE3^int mice than in control wild-type (WT) mice, resulting in greater inhibition of NHE3 activity and increased fluid accumulation in the intestine, the hallmark of diarrhea. Activation of PKA increased ubiquitination of hNHE3 and enhanced interaction of Nedd4-2 with hNHE3 via phosphorylation of Nedd4-2 at S342. S342A mutation mitigated the Nedd4-2-hNHE3 interaction and blocked PKA-induced inhibition of hNHE3. Unlike non-human NHE3s, inhibition of hNHE3 by PKA is independent of NHE3 phosphorylation, suggesting a distinct mechanism of hNHE3 regulation.

CONCLUSIONS

The effects of CTX and EPEC on hNHE3 are amplified, and the unique properties of hNHE3 may contribute to diarrheal symptoms occurring in humans.

Clostridioides difficile: innovations in target discovery and potential for therapeutic success

INTRODUCTION

Clostridioides difficile infection (CDI) remains a worldwide clinical problem. Increased incidence of primary infection, occurrence of hypertoxigenic ribotypes, and more frequent occurrence of drug resistant, recurrent, and non-hospital CDI, emphasizes the urgent unmet need of discovering new therapeutic targets.

AREAS COVERED

We searched PubMed and Web of Science databases for articles identifying novel therapeutic targets or treatments for C. difficile from 2001 to 2021. We present an updated review on current preclinical efforts on designing inhibitory compounds against these drug targets and indicate how these could become the focus of future therapeutic approaches. We also evaluate the increasing exploitability of gut microbial-derived metabolites and host-derived therapeutics targeting VEGF-A, immune targets and pathways, ion transporters, and microRNAs as anti-C. difficile therapeutics, which have yet to reach clinical trials. Our review also highlights the therapeutic potential of re-purposing currently available agents . We conclude by considering translational hurdles and possible strategies to mitigate these problems.

EXPERT OPINION

Considerable progress has been made in the development of new anti-CDI drug candidates. Nevertheless, a greater comprehension of CDI pathogenesis and host-microbe interactions is beginning to uncover potential novel therapeutic targets, which can be exploited to plug gaps in the CDI drug discovery pipeline.

Luminal Chemosensory Cells in the Small Intestine

In addition to the small intestine's well-known function of nutrient absorption, the small intestine also plays a major role in nutrient sensing. Similar to taste sensors seen on the tongue, GPCR-coupled nutrient sensors are expressed throughout the intestinal epithelium and respond to nutrients found in the lumen. These taste receptors respond to specific ligands, such as digested carbohydrates, fats, and proteins. The activation of nutrient sensors in the intestine allows for the induction of signaling pathways needed for the digestive system to process an influx of nutrients. Such processes include those related to glucose homeostasis and satiety. Defects in intestinal nutrient sensing have been linked to a variety of metabolic disorders, such as type 2 diabetes and obesity. Here, we review recent updates in the mechanisms related to intestinal nutrient sensors, particularly in enteroendocrine cells, and their pathological roles in disease. Additionally, we highlight the emerging nutrient sensing role of tuft cells and recent work using enteroids as a sensory organ model.

Lysophosphatidic acid (LPA) receptor modulators: Structural features and recent development

Lysophosphatidic acid (LPA) activates six LPA receptors (LPAR_1-6) and regulates various cellular activities such as cell proliferation, cytoprotection, and wound healing. Many studies elucidated the pathological outcomes of LPA are due to the alteration in signaling pathways, which include migration and invasion of cancer cells, fibrosis, atherosclerosis, and inflammation. Current pathophysiological research on LPA and its receptors provides a means that LPA receptors are new therapeutic targets for disorders associated with LPA. Various chemical modulators are developed and are under investigation to treat a wide range of pathological complications. This review summarizes the physiological and pathological roles of LPA signaling, development of various LPA modulators, their structural features, patents, and their clinical outcomes.

Control of Intestinal Epithelial Permeability by Lysophosphatidic Acid Receptor 5

BACKGROUND & AIMS

Epithelial cells form a monolayer at mucosal surface that functions as a highly selective barrier. Lysophosphatidic acid (LPA) is a bioactive lipid that elicits a broad range of biological effects via cognate G protein-coupled receptors. LPA receptor 5 (LPA₅) is highly expressed in intestinal epithelial cells, but its role in the intestine is not well-known. Here we determined the role of LPA₅ in regulation of intestinal epithelial barrier.

METHODS

Epithelial barrier integrity was determined in mice with intestinal epithelial cell (IEC)-specific LPA₅ deletion, Lpar5^ΔIEC. LPA was orally administered to mice, and intestinal permeability was measured. Dextran sulfate sodium (DSS) was used to induce colitis. Human colonic epithelial cell lines were used to determine the LPA₅-mediated signaling pathways that regulate epithelial barrier.

RESULTS

We observed increased epithelial permeability in Lpar5^ΔIEC mice with reduced claudin-4 expression. Oral administration of LPA decreased intestinal permeability in wild-type mice, but the effect was greatly mitigated in Lpar5^ΔIEC mice. Serum lipopolysaccharide level and bacterial loads in the intestine and liver were elevated in Lpar5^ΔIEC mice. Lpar5^ΔIEC mice developed more severe colitis induced with DSS. LPA₅ transcriptionally regulated claudin-4, and this regulation was dependent on transactivation of the epidermal growth factor receptor, which induced localization of Rac1 at the cell membrane. LPA induced the translocation of Stat3 to the cell membrane and promoted the interaction between Rac1 and Stat3. Inhibition of Stat3 ablated LPA-mediated regulation of claudin-4.

CONCLUSIONS

This study identifies LPA₅ as a regulator of the intestinal barrier. LPA₅ promotes claudin-4 expression in IECs through activation of Rac1 and Stat3.

Lysophosphatidic acid (LPA) as a modulator of plasma membrane Ca2+-ATPase from basolateral membranes of kidney proximal tubules

Lysophosphatidic acid (LPA) acts through the activation of G protein-coupled receptors, in a Ca²⁺-dependent manner. We show the effects of LPA on the plasma membrane Ca²⁺-ATPase (PMCA) from kidney proximal tubule cells. The Ca²⁺-ATPase activity was inhibited by nanomolar concentrations of LPA, with maximal inhibition (~50%) obtained with 20 nM LPA. This inhibitory action on PMCA activity was blocked by Ki16425, an antagonist for LPA receptors, indicating that this lipid acts via LPA1 and/or LPA3 receptor. This effect is PKC-dependent, since it is abolished by calphostin C and U73122, PKC, and PLC inhibitors, respectively. Furthermore, the addition of 10^-8 M PMA, a well-known PKC activator, mimicked PMCA modulation by LPA. We also demonstrated that the PKC activation leads to an increase in PMCA phosphorylation. These results indicate that LPA triggers LPA1 and/or LPA3 receptors at the BLM, inducing PKC-dependent phosphorylation with further inhibition of PMCA. Thus, LPA is part of the regulatory lipid network present at the BLM and plays an important role in the regulation of intracellular Ca²⁺ concentration that may result in significant physiological alterations in other Ca²⁺-dependent events ascribed to the renal tissue.

Pharmacological and Parenteral Nutrition-Based Interventions in Microvillus Inclusion Disease

Microvillus inclusion disease (MVID) is a rare inherited and invariably fatal enteropathy, characterized by severe intractable secretory diarrhea and nutrient malabsorption. No cure exists, and patients typically die during infancy because of treatment-related complications. The need for alternative treatment strategies is evident. Several pharmacological interventions with variable successes have been tried and reported for individual patients as part of their clinical care. Unfortunately, these interventions and their outcomes have remained hidden in case reports and have not been reviewed. Further, recent advances regarding MVID pathogenesis have shed new light on the outcomes of these pharmacological interventions and offer suggestions for future clinical research and trials. Hence, an inventory of reported pharmacological interventions in MVID, their rationales and outcomes, and a discussion of these in the light of current knowledge is opportune. Together with a discussion on MVID-specific pharmacokinetic, -dynamic, and -genetic concerns that pose unique challenges regarding pharmacological strategies, we envision that this paper will aid researchers and clinicians in their efforts to develop pharmacological interventions to combat this devastating disease.

Bioactive lipids in inflammatory bowel diseases - From pathophysiological alterations to therapeutic opportunities

Inflammatory bowel diseases (IBDs), such as Crohn's disease and ulcerative colitis, are lifelong diseases that remain challenging to treat. IBDs are characterized by alterations in intestinal barrier function and dysregulation of the innate and adaptive immunity. An increasing number of lipids are found to be important regulators of inflammation and immunity as well as gut physiology. Therefore, the study of lipid mediators in IBDs is expected to improve our understanding of disease pathogenesis and lead to novel therapeutic opportunities. Here, through selected examples - such as fatty acids, specialized proresolving mediators, lysophospholipids, endocannabinoids, and oxysterols - we discuss how lipid signaling is involved in IBD physiopathology and how modulating lipid signaling pathways could affect IBDs.

Ubiquitin-specific peptidase 7 (USP7) and USP10 mediate deubiquitination of human NHE3 regulating its expression and activity

Na⁺ /H⁺ exchanger NHE3 of human or primates differs from NHE3 of other animals by having a PY motif, which mediates interaction with the E3 ubiquitin (Ub) ligase Nedd4-2. Ub-conjugation of human NHE3 by Nedd4-2 regulates endocytosis of NHE3 but does not affect its cellular expression. Because Ub-conjugation is a reversible process, the aim of this study is to identify deubiquitinating enzyme (DUB) regulating the post-endosomal fate of human NHE3. Using an activity-based chemical screening, we identified ubiquitin specific protease-7 (USP7) and USP10 that bind NHE3. The roles of DUBs in regulation of NHE3 were studied by determining the effects of silencing of USP7 and USP10. Knockdown of USP7 or USP10 resulted in increased NHE3 ubiquitination and decreased NHE3 expression at the surface membrane and cellular level. The endocytic retrieval of NHE3 was promoted by depletion of USP7 or USP10, with increased association of NHE3 with Rab5a and Rab7. Inhibition of USP7 and USP10 by chemical inhibitors or knockdown had an additive effect on NHE3. In addition, NHE3 half-life was reduced accounting for decreased NHE3 protein abundance. NHE3 is inhibited by protein kinase A. Activation of PKA by forskolin decreased the binding of USP7 and USP10 to NHE3, while increasing ubiquitination of NHE3. Knockdown of USP10 had an additive effect on PKA-dependent inhibition of NHE3. These findings demonstrate that USP7 and USP10 are DUBs that regulate NHE3 ubiquitination and expression, and reveal a new mechanism of NHE3 inhibition involving DUBs.

Lysophosphatidic Acid Increases Maturation of Brush Borders and SGLT1 Activity in MYO5B-deficient Mice, a Model of Microvillus Inclusion Disease

BACKGROUND & AIM

Myosin VB (MYO5B) is an essential trafficking protein for membrane recycling in gastrointestinal epithelial cells. The inactivating mutations of MYO5B cause the congenital diarrheal disease, microvillus inclusion disease (MVID). MYO5B deficiency in mice causes mislocalization of SGLT1 and NHE3, but retained apical function of CFTR, resulting in malabsorption and secretory diarrhea. Activation of lysophosphatidic acid (LPA) receptors can improve diarrhea, but the effect of LPA on MVID symptoms is unclear. We investigated whether LPA administration can reduce the epithelial deficits in MYO5B-knockout mice.

METHODS

Studies were conducted with tamoxifen-induced, intestine-specific knockout of MYO5B (VilCre^ERT2;Myo5b^flox/flox) and littermate controls. Mice were given LPA, an LPAR2 agonist (GRI977143), or vehicle for 4 days after a single injection of tamoxifen. Apical SGLT1 and CFTR activities were measured in Üssing chambers. Intestinal tissues were collected, and localization of membrane transporters was evaluated by immunofluorescence analysis in tissue sections and enteroids. RNA sequencing and enrichment analysis were performed with isolated jejunal epithelial cells.

RESULTS

Daily administration of LPA reduced villus blunting, frequency of multivesicular bodies, and levels of cathepsins in intestinal tissues of MYO5B-knockout mice compared with vehicle administration. LPA partially restored the brush border height and the localization of SGLT1 and NHE3 in small intestine of MYO5B-knockout mice and enteroids. The SGLT1-dependent short-circuit current was increased and abnormal CFTR activities were decreased in jejunum from MYO5B-knockout mice given LPA compared with vehicle.

CONCLUSIONS

LPA may regulate a MYO5B-independent trafficking mechanism and brush border maturation, and therefore be developed for treatment of MVID.

LPAR2 receptor activation attenuates radiation-induced disruption of apical junctional complexes and mucosal barrier dysfunction in mouse colon

The tight junction (TJ) and barrier function of colonic epithelium is highly sensitive to ionizing radiation. We evaluated the effect of lysophosphatidic acid (LPA) and its analog, Radioprotein-1, on γ-radiation-induced colonic epithelial barrier dysfunction using Caco-2 and m-IC_C12 cell monolayers in vitro and mice in vivo. Mice were subjected to either total body irradiation (TBI) or partial body irradiation (PBI-BM5). Intestinal barrier function was assessed by analyzing immunofluorescence localization of TJ proteins, mucosal inulin permeability, and plasma lipopolysaccharide (LPS) levels. Oxidative stress was analyzed by measuring protein thiol oxidation and antioxidant mRNA. In Caco-2 and m-IC_C12 cell monolayers, LPA attenuated radiation-induced redistribution of TJ proteins, which was blocked by a Rho-kinase inhibitor. In mice, TBI and PBI-BM5 disrupted colonic epithelial tight junction and adherens junction, increased mucosal permeability, and elevated plasma LPS; TJ disruption by TBI was more severe in Lpar2^-/- mice compared to wild-type mice. RP1, administered before or after irradiation, alleviated TBI and PBI-BM5-induced TJ disruption, barrier dysfunction, and endotoxemia accompanied by protein thiol oxidation and downregulation of antioxidant gene expression, cofilin activation, and remodeling of the actin cytoskeleton. These data demonstrate that LPAR2 receptor activation prevents and mitigates γ-irradiation-induced colonic mucosal barrier dysfunction and endotoxemia.

Lysophosphatidic Acid Receptors: Biochemical and Clinical Implications in Different Diseases

Lysophosphatidic acid (LPA, 1-acyl-2-hemolytic-sn-glycerol-3-phosphate) extracted from membrane phospholipid is a kind of simple bioactive glycophospholipid, which has many biological functions such as stimulating cell multiplication, cytoskeleton recombination, cell survival, drug-fast, synthesis of DNA and ion transport. Current studies have shown that six G-coupled protein receptors (LPAR1-6) can be activated by LPA. They stimulate a variety of signal transduction pathways through heterotrimeric G-proteins (such as Gα12/13, Gαq/11, Gαi/o and GαS). LPA and its receptors play vital roles in cancers, nervous system diseases, cardiovascular diseases, liver diseases, metabolic diseases, etc. In this article, we discussed the structure of LPA receptors and elucidated their functions in various diseases, in order to better understand them and point out new therapeutic schemes for them.

Pulsatilla Decoction Can Treat the Dampness-Heat Diarrhea Rat Model by Regulating Glycerinphospholipid Metabolism Based Lipidomics Approach

Ethnopharmacological Relevance

Diarrhea is a major medical problem in clinical practice. According to the theory of traditional Chinese medicine (TCM), different types of diarrhea should be treated with different TCM formulations based on the targeted medical condition. Dampness-heat diarrhea (DHD) is a serious diarrheal disease and Pulsatilla decoction (PD), a TCM, has been found effective against DHD.

Objective

The aim of this study was to clarify the mechanism of action of PD in DHD using an untargeted lipidomics strategy.

Materials and Methods

Wistar rats were randomized to four groups, including the control group, model group, PD groups and self-healing group. The PD groups were given a daily intragastric gavage of PD at doses of 3.76 g/kg. The rat model of DHD established by such complex factors as high-sugar and high-fat diet, improper diet, high temperature and humidity environment, drinking and intraperitoneal injection of Escherichia coli., which imitated the inducing conditions of DHD. Then the clinical symptoms and signs, blood routine, serum inflammatory cytokines levels and the histopathological changes of main organs were detected and observed to evaluate DHD model and therapeutic effect of PD. Lipid biomarkers of DHD were selected by comparing the control and model groups with the colon lipidomics technology and an ultra-high performance liquid chromatography (UHPLC) coupled with Q Exactive plus mass analyzer. Multivariate statistical analysis and pattern recognition were employed to examine different lipids within the colon of PD-treated rats.

Results

The clinical symptoms and signs of the model rats were consistent with the diagnostic criteria of DHD. After treatment with PD, the clinical symptoms and signs of the rats with DHD were improved; the indexes of blood routine and inflammatory cytokines levels tended to be normal. The lipidomics profile of the model group were evidently disordered when compared to the control group. A total of 42 significantly altered lipids between the model-control groups were identified by multivariate statistical analysis. DHD may result from such lipid disorders which are related to glycerophospholipid metabolism, arachidonic acid (AA) metabolism, and sphingolipid metabolism. After PD treatment, the lipidomic profiles of the disorders tended to recover when compared to the model group. Twenty lipid molecules were identified and some glycerophospholipids and AA levels returned close to the normal level.

Conclusion

Glycerophospholipid metabolism may play an important role in the treatment of dampness-heat induced diarrhea using PD.

A SNP of bacterial blc disturbs gut lysophospholipid homeostasis and induces inflammation through epithelial barrier disruption

BACKGROUND

Alteration of commensal bacterial composition is associated with many inflammatory diseases. However, few studies have pinpointed the specific bacterial genes that may suppress host immune responses against microbes and maintain homeostasis in the host intestine.

METHODS

High-throughput screening was performed in Caenorhabditis elegans with a single gene knockout ut screening was performed in Caenorhabditis elegans with a single gene knockout Escherichia coli (E. coli) library and identified the immune suppression gene blc. The coding sequences of blc among different kinds of E. coli strains were aligned to identify the single nucleotide polymorphisms (SNPs). Physiological and biochemical experiments were performed in C. elegans and mice to explore the function of the blc variant.

FINDINGS

By screening 3983 E. coli mutants, we discovered that 9 bacterial genes, when deleted, activate innate immunity in the host C. elegans. Among these 9 genes, the gene encoding blc showed a distinctive SNP in many clinically pathogenic bacteria. We found that bacteria with this SNP, which converts Blc G84 to Blc E84, are highly enriched in the faeces of patients with inflammatory bowel disease (IBD). Exposure to BlcE84-encoding bacteria resulted in epithelial barrier disruption and immune activation in both worms and mice. Detailed analysis indicated that infection with BlcE84-encoding bacteria causes a significant decrease in LPE levels in the intestine and subsequently disrupts gut epithelial integrity in mice. Consistently, the levels of LPE in patients with IBD are significantly lower than those in healthy people. Finally, supplementation with LPE, which activates LPA1/PLCβ/PKC signaling, reversed the defects induced by BlcE84-encoding bacteria.

INTERPRETATION

Our results identified a novel bacterial gene, blc, in E. coli that regulates host gut integrity and immunity. FUND: The Ministry of Science and Technology of China; the National Natural Science Foundation of China; and the Natural Science Foundation of Jiangsu Province.

The Druggability of Solute Carriers

The transport of materials across membranes is a vital process for all aspects of cellular function, including growth, metabolism, and communication. Protein transporters are the molecular gates that control this movement and serve as key points of regulation for these processes, thus representing an attractive class of therapeutic targets. With more than 400 members, the solute carrier (SLC) membrane transport proteins are the largest family of transporters, yet, they are pharmacologically underexploited relative to other protein families and many of the available chemical tools possess suboptimal selectivity and efficacy. Fortuitously, there is increased interest in elucidating the physiological roles of SLCs as well as growing recognition of their therapeutic potential. This Perspective provides an overview of the SLC superfamily, including their biochemical and functional features, as well as their roles in various human diseases. In particular, we explore efforts and associated challenges toward drugging SLCs, as well as highlight opportunities for future drug discovery.

Lysophosphatidic Acid and Autotaxin-associated Effects on the Initiation and Progression of Colorectal Cancer

The intestinal epithelium interacts dynamically with the immune system to maintain its barrier function to protect the host, while performing the physiological roles in absorption of nutrients, electrolytes, water and minerals. The importance of lysophosphatidic acid (LPA) and its receptors in the gut has been progressively appreciated. LPA signaling modulates cell proliferation, invasion, adhesion, angiogenesis, and survival that can promote cancer growth and metastasis. These effects are equally important for the maintenance of the epithelial barrier in the gut, which forms the first line of defense against the milieu of potentially pathogenic stimuli. This review focuses on the LPA-mediated signaling that potentially contributes to inflammation and tumor formation in the gastrointestinal tract.

Physiology of Electrolyte Transport in the Gut: Implications for Disease

We now have an increased understanding of the genetics, cell biology, and physiology of electrolyte transport processes in the mammalian intestine, due to the availability of sophisticated methodologies ranging from genome wide association studies to CRISPR-CAS technology, stem cell-derived organoids, 3D microscopy, electron cryomicroscopy, single cell RNA sequencing, transgenic methodologies, and tools to manipulate cellular processes at a molecular level. This knowledge has simultaneously underscored the complexity of biological systems and the interdependence of multiple regulatory systems. In addition to the plethora of mammalian neurohumoral factors and their cross talk, advances in pyrosequencing and metagenomic analyses have highlighted the relevance of the microbiome to intestinal regulation. This article provides an overview of our current understanding of electrolyte transport processes in the small and large intestine, their regulation in health and how dysregulation at multiple levels can result in disease. Intestinal electrolyte transport is a balance of ion secretory and ion absorptive processes, all exquisitely dependent on the basolateral Na⁺ /K⁺ ATPase; when this balance goes awry, it can result in diarrhea or in constipation. The key transporters involved in secretion are the apical membrane Cl^- channels and the basolateral Na⁺ -K⁺ -2Cl^- cotransporter, NKCC1 and K⁺ channels. Absorption chiefly involves apical membrane Na⁺ /H⁺ exchangers and Cl^- /HCO₃ ^- exchangers in the small intestine and proximal colon and Na⁺ channels in the distal colon. Key examples of our current understanding of infectious, inflammatory, and genetic diarrheal diseases and of constipation are provided. © 2019 American Physiological Society. Compr Physiol 9:947-1023, 2019.

Autotaxin determines colitis severity in mice and is secreted by B cells in the colon

Autotaxin (ATX or ENPP2) is a secreted lysophospholipase D that produces lysophosphatidic acid (LPA), a pleiotropic lipid mediator acting on specific GPCRs. ATX and LPA have been implicated in key (patho)physiologic processes, including embryonic development, lymphocyte homing, inflammation, and cancer progression. Using LPA receptor knockout mice, we previously uncovered a role for LPA signaling in promoting colitis and colorectal cancer. Here, we examined the role of ATX in experimental colitis through inducible deletion of Enpp2 in adult mice. ATX expression was increased upon induction of colitis, whereas ATX deletion reduced the severity of inflammation in both acute and chronic colitis, accompanied by transient weight loss. ATX expression in lymphocytes was strongly reduced in Rag1-/- and μMT mice, suggesting B cells as a major ATX-producing source, which was validated by immunofluorescence and biochemical analyses. ATX secretion by B cells from control, but not Enpp2 knockout, mice led to ERK activation in colorectal cancer cells and promoted T cell migration. We conclude that ATX deletion suppresses experimental colitis and that B cells are a major source of ATX in the colon. Our study suggests that pharmacological inhibition of ATX could be a therapeutic strategy in colitis.-Lin, S., Haque, A., Raeman, R., Guo, L., He, P., Denning, T. L., El-Rayes, B., Moolenaar, W. H., Yun, C. C. Autotaxin determines colitis severity in mice and is secreted by B cells in the colon.

EGFR as a Negative Regulatory Protein Adjusts the Activity and Mobility of NHE3 in the Cell Membrane of IPEC-J2 Cells With TGEV Infection

Transmissible gastroenteritis (TGE) has caused devastating economic losses to the swine industry worldwide, despite extensive research focusing on the pathogenesis of virus infection. The molecular pathogenic mechanism of TGEV-induced diarrhea in piglets is unknown. Intestinal diarrhea is closely related to the function of the Na⁺/H⁺ exchanger protein NHE3 in the brush border membrane of small intestine epithelial cells. The epidermal growth factor receptor (EGFR) may act to regulate NHE3 expression. In addition, EGFR may promote viral invasion of host cells. The present study aimed to determine whether NHE3 activity is regulated by altering EGFR expression to affect Na⁺ absorption in TGEV-infected intestinal epithelial cells. Porcine intestinal epithelial cells were used as models for TGEV infection. The results showed that Na⁺ absorption and NHE3 expression levels decreased in TGEV-infected cells. Proliferation of TGEV within IPEC-J2 cells could be inhibited by treatment with the EGFR inhibitor AG1478 and knockdown; resulting in recovery of Na⁺ absorption in TGEV infected cells and increasing the activity and expression of NHE3. Moreover, we demonstrated that NHE3 activity was regulated through the EGFR/ERK pathway. Importantly, NHE3 mobility on the plasma membrane of TGEV infected cells was significantly weaker than that in normal cells, and EGFR inhibition and knockdown recovered this mobility. Our research indicated that NHE3 activity was negatively regulated by EGFR in TGEV-infected intestinal epithelial cells.

Inhibition of autotaxin alleviates inflammation and increases the expression of sodium-dependent glucose cotransporter 1 and Na+/H+ exchanger 3 in SAMP1/Fc mice

Crohn's disease (CD) is a chronic, relapsing, inflammatory disease that is often associated with malnutrition because of inflammation in the small intestine. Autotaxin (ATX) is a secreted enzyme that produces extracellular lysophosphatidic acid. Increasing evidence suggests that ATX is upregulated during inflammation, and inhibition of ATX has been effective in attenuating chronic inflammatory conditions, such as arthritis and pulmonary fibrosis. This study aims to determine whether inhibition of ATX alleviates CD-associated inflammation and malnutrition by using SAMP1/Fc mice, a model of CD-like ileitis. SAMP1/Fc mice were treated the ATX inhibitor PF-8380 for 4 wk. Inhibition of ATX led to increased weight gain in SAMP1/Fc mice, decreased T helper 2 cytokine expression, including IL-4, IL-5, and IL-13, and attenuated immune cell migration. SAMP1/Fc mice have low expression of Na⁺-dependent glucose transporter 1 (SGLT1), suggesting impaired nutrient absorption associated with ileitis. PF-8380 treatment significantly enhanced SGLT1 expression in SAMP1/Fc mice, which could reflect the increased weight changes. However, IL-4 or IL-13 did not alter SGLT1 expression in Caco-2 cells, ruling out their direct effects on SGLT1 expression. Immunofluorescence analysis showed that the expression of sucrase-isomaltase, a marker for intestinal epithelial cell (IEC) differentiation, was decreased in inflamed regions of SAMP1/Fc mice, which was partially restored by PF-8380. Moreover, expression of Na⁺/H⁺ exchanger 3 was also improved by PF-8380, suggesting that suppression of inflammation by PF-8380 enhanced IEC differentiation. Our study therefore suggests that ATX is a potential target for treating intestinal inflammation and restoration of the absorptive function of the intestine. NEW & NOTEWORTHY This study is the first, to our knowledge, to determine whether autotoxin (ATX) inhibition improves inflammation and body weights in SAMP1/Fc mice, a mouse model of ileitis. ATX inhibition increased body weights of SAMP1/Fc mice and increased Na⁺-dependent glucose transporter 1 (SGLT1) expression. Increased SGLT1 expression in the inflamed regions was not a direct effect of cytokines but an indirect effect of increased epithelial cell differentiation upon ATX inhibition.

Expression of lysophosphatidic acid receptor 5 is necessary for the regulation of intestinal Na+/H+ exchanger 3 by lysophosphatidic acid in vivo

Lysophosphatidic acid (LPA) is a bioactive lipid molecule, which regulates a broad range of pathophysiological processes. Recent studies have demonstrated that LPA modulates electrolyte flux in the intestine, and its potential as an antidiarrheal agent has been suggested. Of six LPA receptors, LPA₅ is highly expressed in the intestine. Recent studies by our group have demonstrated activation of Na⁺/H⁺ exchanger 3 (NHE3) by LPA₅. However, much of what has been elucidated was achieved using colonic cell lines that were transfected to express LPA₅. In the current study, we engineered a mouse that lacks LPA₅ in intestinal epithelial cells, Lpar5^ΔIEC, and investigated the role of LPA₅ in NHE3 regulation and fluid absorption in vivo. The intestine of Lpar5^ΔIEC mice appeared morphologically normal, and the stool frequency and fecal water content were unchanged compared with wild-type mice. Basal rates of NHE3 activity and fluid absorption and total NHE3 expression were not changed in Lpar5^ΔIEC mice. However, LPA did not activate NHE3 activity or fluid absorption in Lpar5^ΔIEC mice, providing direct evidence for the regulatory role of LPA₅. NHE3 activation involves trafficking of NHE3 from the terminal web to microvilli, and this mobilization of NHE3 by LPA was abolished in Lpar5^ΔIEC mice. Dysregulation of NHE3 was specific to LPA, and insulin and cholera toxin were able to stimulate and inhibit NHE3, respectively, in both wild-type and Lpar5^ΔIEC mice. The current study for the first time demonstrates the necessity of LPA₅ in LPA-mediated stimulation of NHE3 in vivo. NEW & NOTEWORTHY This study is the first to assess the role of LPA₅ in NHE3 regulation and fluid absorption in vivo using a mouse that lacks LPA₅ in intestinal epithelial cells, Lpar5^ΔIEC. Basal rates of NHE3 activity and fluid absorption, and total NHE3 expression were not changed in Lpar5^ΔIEC mice. However, LPA did not activate NHE3 activity or fluid absorption in Lpar5^ΔIEC mice, providing direct evidence for the regulatory role of LPA₅.

Production of extracellular lysophosphatidic acid in the regulation of adipocyte functions and liver fibrosis

Lysophosphatidic acid (LPA), a glycerophospholipid, consists of a glycerol backbone connected to a phosphate head group and an acyl chain linked to sn-1 or sn-2 position. In the circulation, LPA is in sub-millimolar range and mainly derived from hydrolysis of lysophosphatidylcholine, a process mediated by lysophospholipase D activity in proteins such as autotaxin (ATX). Intracellular and extracellular LPAs act as bioactive lipid mediators with diverse functions in almost every mammalian cell type. The binding of LPA to its receptors LPA_1-6 activates multiple cellular processes such as migration, proliferation and survival. The production of LPA and activation of LPA receptor signaling pathways in the events of physiology and pathophysiology have attracted the interest of researchers. Results from studies using transgenic and gene knockout animals with alterations of ATX and LPA receptors genes, have revealed the roles of LPA signaling pathways in metabolic active tissues and organs. The present review was aimed to summarize recent progresses in the studies of extracellular and intracellular LPA production pathways. This includes the functional, structural and biochemical properties of ATX and LPA receptors. The potential roles of LPA production and LPA receptor signaling pathways in obesity, insulin resistance and liver fibrosis are also discussed.

Prevention and treatment of secretory diarrhea by the lysophosphatidic acid analog Rx100

IMPACT STATEMENT

A critical barrier in treating diarrheal disease is easy-to-use effective treatments. Rx100 is a first in class, novel small molecule that has shown efficacy after both subcutaneous and oral administration in a mouse cholera-toxin- and Citrobacter rodentium infection-induced diarrhea models. Our findings indicate that Rx100 a metabolically stable analog of the lipid mediator lysophosphatidic acid blocks activation of CFTR-mediated secretion responsible for fluid discharge in secretory diarrhea. Rx100 represents a new treatment modality which does not directly block CFTR but attenuates its activation by bacterial toxins. Our results provide proof-of-principle that Rx100 can be developed for use as an effective oral or injectable easy-to-use drug for secretory diarrhea which could significantly improve care by eliminating the need for severely ill patients to regularly consume large quantities of oral rehydration therapies and offering options for pediatric patients.

SLC9 Gene Family: Function, Expression, and Regulation

The Slc9 family of Na⁺ /H⁺ exchangers (NHEs) plays a critical role in electroneutral exchange of Na⁺ and H⁺ in the mammalian intestine as well as other absorptive and secretory epithelia of digestive organs. These transport proteins contribute to the transepithelial Na⁺ and water absorption, intracellular pH and cellular volume regulation as well as the electrolyte, acid-base, and fluid volume homeostasis at the systemic level. They also influence the function of other membrane transport mechanisms, affect cellular proliferation and apoptosis as well as cell migration, adherence to the extracellular matrix, and tissue repair. Additionally, they modulate the extracellular milieu to facilitate other nutrient absorption and to regulate the intestinal microbial microenvironment. Na⁺ /H⁺ exchange is inhibited in selected gastrointestinal diseases, either by intrinsic factors (e.g., bile acids, inflammatory mediators) or infectious agents and associated bacterial toxins. Disrupted NHE activity may contribute not only to local and systemic electrolyte imbalance but also to the disease severity via multiple mechanisms. In this review, we describe the cation proton antiporter superfamily of Na⁺ /H⁺ exchangers with a particular emphasis on the eight SLC9A isoforms found in the digestive tract, followed by a more integrative description in their roles in each of the digestive organs. We discuss regulatory mechanisms that determine the function of Na⁺ /H⁺ exchangers as pertinent to the digestive tract, their regulation in pathological states of the digestive organs, and reciprocally, the contribution of dysregulated Na⁺ /H⁺ exchange to the disease pathogenesis and progression. © 2018 American Physiological Society. Compr Physiol 8:555-583, 2018.

The Role of Ion Transporters in the Pathophysiology of Infectious Diarrhea

Every year, enteric infections and associated diarrhea kill millions of people. The situation is compounded by increases in the number of enteric pathogens that are acquiring resistance to antibiotics, as well as (hitherto) a relative paucity of information on host molecular targets that may contribute to diarrhea. Many forms of diarrheal disease depend on the dysregulation of intestinal ion transporters, and an associated imbalance between secretory and absorptive functions of the intestinal epithelium. A number of major transporters have been implicated in the pathogenesis of diarrheal diseases and thus an understanding of their expression, localization, and regulation after infection with various bacteria, viruses, and protozoa likely will prove critical in designing new therapies. This article surveys our understanding of transporters that are modulated by specific pathogens and the mechanism(s) involved, thereby illuminating targets that might be exploited for new therapeutic approaches.

Both NHERF3 and NHERF2 are necessary for multiple aspects of acute regulation of NHE3 by elevated Ca2+, cGMP, and lysophosphatidic acid

The intestinal epithelial brush border Na⁺/H⁺ exchanger NHE3 accounts for a large component of intestinal Na absorption. NHE3 is regulated during digestion by signaling complexes on its COOH terminus that include the four multi-PDZ domain-containing NHERF family proteins. All bind to NHE3 and take part in different aspects of NHE3 regulation. Because the roles of each NHERF appear to vary on the basis of the cell model or intestinal segment studied and because of our recent finding that a NHERF3-NHERF2 heterodimer appears important for NHE3 regulation in Caco-2 cells, we examined the role of NHERF3 and NHERF2 in C57BL/6 mouse jejunum using homozygous NHERF2 and NHERF3 knockout mice. NHE3 activity was determined with two-photon microscopy and the dual-emission pH-sensitive dye SNARF-4F. The jejunal apical membrane of NHERF3-null mice appeared similar to wild-type (WT) mice in surface area, microvillus number, and height, which is similar to results previously reported for jejunum of NHERF2-null mice. NHE3 basal activity was not different from WT in either NHERF2- or NHERF3-null jejunum, while d-glucose-stimulated NHE3 activity was reduced in NHERF2, but similar to WT in NHERF3 KO. LPA stimulation and UTP (elevated Ca²⁺) and cGMP inhibition of NHE3 were markedly reduced in both NHERF2- and NHERF3-null jejunum. Forskolin inhibited NHE3 in NHERF3-null jejunum, but the extent of inhibition was reduced compared with WT. The forskolin inhibition of NHE3 in NHERF2-null mice was too inconsistent to determine whether there was an effect and whether it was altered compared with the WT response. These results demonstrate similar requirement for NHERF2 and NHERF3 in mouse jejunal NHE3 regulation by LPA, Ca²⁺, and cGMP. The explanation for the similarity is not known but is consistent with involvement of a brush-border NHERF3-NHERF2 heterodimer or sequential NHERF-dependent effects in these aspects of NHE3 regulation. NEW & NOTEWORTHY NHERF2 and NHERF3 are apical membrane multi-PDZ domain-containing proteins that are involved in regulation of intestinal NHE3. This study demonstrates that NHERF2 and NHERF3 have overlapping roles in NHE3 stimulation by LPA and inhibition by elevated Ca²⁺ and cGMP. These results are consistent with their role being as a NHERF3-NHERF2 heterodimer or via sequential NHERF-dependent signaling steps, and they begin to clarify a role for multiple NHERF proteins in NHE3 regulation.

Lysophosphatidic Acid Receptor 1 Is Important for Intestinal Epithelial Barrier Function and Susceptibility to Colitis

Intestinal epithelial cells form a barrier that is critical in protecting the host from the hostile luminal environment. Previously, we showed that lysophosphatidic acid (LPA) receptor 1 regulates proliferation of intestinal epithelial cells, such that the absence of LPA1 mitigates the epithelial wound healing process. This study provides evidence that LPA1 is important for the maintenance of epithelial barrier integrity. The epithelial permeability, determined by fluorescently labeled dextran flux and transepithelial resistance, is increased in the intestine of mice with global deletion of Lpar1, Lpar1^-/- (Lpa1^-/-). Serum liposaccharide level and bacteria loads in the intestinal mucosa and peripheral organs were elevated in Lpa1^-/- mice. Decreased claudin-4, caudin-7, and E-cadherin expression in Lpa1^-/- mice further suggested defective apical junction integrity in these mice. Regulation of LPA1 expression in Caco-2 cells modulated epithelial permeability and the expression levels of junctional proteins. The increased epithelial permeability in Lpa1^-/- mice correlated with increased susceptibility to an experimental model of colitis. This resulted in more severe inflammation and increased mortality compared with control mice. Treatment of Caco-2 cells with tumor necrosis factor-α and interferon-γ significantly increased paracellular permeability, which was blocked by cotreatment with LPA, but not LPA1 knockdown cells. Similarly, orally given LPA blocked tumor necrosis factor-mediated intestinal barrier defect in mice. LPA1 plays a significant role in maintenance of epithelial barrier in the intestine via regulation of apical junction integrity.

CFTR-NHERF2-LPA₂ Complex in the Airway and Gut Epithelia

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP- and cGMP-regulated chloride (Cl⁻) and bicarbonate (HCO₃⁻) channel localized primarily at the apical plasma membrane of epithelial cells lining the airway, gut and exocrine glands, where it is responsible for transepithelial salt and water transport. Several human diseases are associated with altered CFTR channel function. Cystic fibrosis (CF) is caused by the absence or dysfunction of CFTR channel activity, resulting from mutations in the gene. Secretory diarrhea is caused by the hyperactivation of CFTR channel activity in the gastrointestinal tract. CFTR is a validated target for drug development to treat CF, and extensive research has been conducted to develop CFTR inhibitors for therapeutic interventions of secretory diarrhea. The intracellular processing, trafficking, apical membrane localization, and channel function of CFTR are regulated by dynamic protein–protein interactions in a complex network. In this paper, we review the current knowledge of a macromolecular complex of CFTR, Na⁺/H⁺ exchanger regulatory factor 2 (NHERF2), and lysophosphatidic acids (LPA) receptor 2 (LPA₂) at the apical plasma membrane of airway and gut epithelial cells, and discuss its relevance in human physiology and diseases. We also explore the possibilities of targeting this complex to fine tune CFTR channel activity, with a hope to open up new avenues to develop novel therapies for CF and secretory diarrhea.

Phosphorylation of NHE3-S719 regulates NHE3 activity through the formation of multiple signaling complexes

CK2 regulates NHE3 by phosphorylating a single C-terminal amino acid, which, when mutated, reduces basal NHE3 activity and its acute stimulation and inhibition. It also is necessary for binding of proteins throughout the C-terminus, which means that it determines the C-terminal structure.

Casein kinase 2 (CK2) binds to the NHE3 C-terminus and constitutively phosphorylates a downstream site (S719) that accounts for 40% of basal NHE3 activity. The role of CK2 in regulation of NHE3 activity in polarized Caco-2/bbe cells was further examined by mutation of NHE3-S⁷¹⁹ to A (not phosphorylated) or D (phosphomimetic). NHE3-S719A but not -S719D had multiple changes in NHE3 activity: 1) reduced basal NHE3 activity—specifically, inhibition of the PI3K/AKT-dependent component; 2) reduced acute stimulation of NHE3 activity by LPA/LPA₅R stimulation; and 3) reduced acute inhibition of NHE3 activity—specifically, elevated Ca²⁺ related (carbachol/Ca²⁺ ionophore), but there was normal inhibition by forskolin and hyperosmolarity. The S719A mutant had reduced NHE3 complex size, reduced expression in lipid rafts, increased BB mobile fraction, and reduced binding to multiple proteins that bind throughout the NHE3 intracellular C-terminus, including calcineurin homologous protein, the NHERF family and SNX27 (related PDZ domains). These studies show that phosphorylation of the NHE3 at a single amino acid in the distal part of the C-terminus affects multiple aspects of NHE3 complex formation and changes the NHE3 lipid raft distribution, which cause changes in specific aspects of basal as well as acutely stimulated and inhibited Na⁺/H⁺ exchange activity.

Daily Intake of High-Fat Diet with Lysophosphatidic Acid-Rich Soybean Phospholipids Augments Colon Tumorigenesis in Kyoto Apc Delta Rats

BACKGROUND

Oral administration of lysophosphatidic acid (LPA) was shown to attenuate gastric ulceration in rats and mice but aggravate intestinal tumorigenesis in mice.

AIMS

The present study examined whether dietary LPA induces or prevents development of colorectal tumor in rats.

METHODS

Kyoto Apc Delta rats fed high-fat diet with or without an LPA-rich soybean phospholipid mixture (LSP, 0.1 or 1%) were treated with azoxymethane and dextran sodium sulfate to induce colorectal tumorigenesis. Rats were killed 15 weeks after azoxymethane treatment, and size, total number, location, and severity of colorectal tumors were assessed. Expression of mRNA of LPA receptors in rat colon tissue was assayed.

RESULTS

Rats fed the diet supplemented with 1% LSP had a higher number of tumors 2-4 mm long compared than those with or without 0.1% LSP. The mean distance of tumors >4 mm long from the anus was significantly higher than those of tumors <2 and 2-4 mm long in rats fed 1% LSP-supplemented diet. Supplementation of the diet with 0.1% LSP decreased mRNA expression of LPA5 in colon tumors of rats.

CONCLUSIONS

Dietary supplementation of LPA-rich phospholipids dose-dependently augmented colorectal tumorigenesis. Decreased expression of LPA5 in colon tumors may be relevant to augmented tumorigenesis.

Gintonin absorption in intestinal model systems

Background

Recently, we identified a novel ginseng-derived lysophosphatidic acid receptor ligand, called gintonin. We showed that gintonin induces [Ca²⁺]i transient-mediated morphological changes, proliferation, and migration in cells expressing lysophosphatidic acid receptors and that oral administration of gintonin exhibits anti-Alzheimer disease effects in model mice. However, little is known about the intestinal absorption of gintonin. The aim of this study was to investigate gintonin absorption using two model systems.

Methods

Gintonin membrane permeation was examined using a parallel artificial membrane permeation assay, and gintonin absorption was evaluated in a mouse everted intestinal sac model.

Results

The parallel artificial membrane permeation assay showed that gintonin could permeate an artificial membrane in a dose-dependent manner. In the everted sac model, gintonin absorption increased with incubation time (from 0 min to 60 min), followed by a decrease in absorption. Gintonin absorption into everted sacs was also dose dependent, with a nonlinear correlation between gintonin absorption and concentration at 0.1–3 mg/mL and saturation at 3–5 mg/mL. Gintonin absorption was inhibited by the Rho kinase inhibitor Y-27632 and the sodium–glucose transporter inhibitor phloridzin. Moreover, lipid extraction with methanol also attenuated gintonin absorption, suggesting the importance of the lipid portion of gintonin in absorption. This result shows that gintonin might be absorbed through passive diffusion, paracellular, and active transport pathways.

Conclusion

The present study shows that gintonin could be absorbed in the intestine through transcellular and paracellular diffusion, and active transport. In addition, the lipid component of gintonin might play a key role in its intestinal absorption.

Deletion of Na+/H+ exchanger regulatory factor 2 represses colon cancer progress by suppression of Stat3 and CD24

The Na(+)/H(+) exchanger regulatory factor (NHERF) family of proteins is scaffolds that orchestrate interaction of receptors and cellular proteins. Previous studies have shown that NHERF1 functions as a tumor suppressor. The goal of this study is to determine whether the loss of NHERF2 alters colorectal cancer (CRC) progress. We found that NHERF2 expression is elevated in advanced-stage CRC. Knockdown of NHERF2 decreased cancer cell proliferation in vitro and in a mouse xenograft tumor model. In addition, deletion of NHERF2 in Apc(Min/+) mice resulted in decreased tumor growth in Apc(Min/+) mice and increased lifespan. Blocking NHERF2 interaction with a small peptide designed to bind the second PDZ domain of NHERF2 attenuated cancer cell proliferation. Although NHERF2 is known to facilitate the effects of lysophosphatidic acid receptor 2 (LPA2), transcriptome analysis of xenograft tumors revealed that NHERF2-dependent genes largely differ from LPA2-regulated genes. Activation of β-catenin and ERK1/2 was mitigated in Apc(Min/+);Nherf2(-/-) adenomas. Moreover, Stat3 phosphorylation and CD24 expression levels were suppressed in Apc(Min/+);Nherf2(-/-) adenomas. Consistently, NHERF2 knockdown attenuated Stat3 activation and CD24 expression in colon cancer cells. Interestingly, CD24 was important in the maintenance of Stat3 phosphorylation, whereas NHERF2-dependent increase in CD24 expression was blocked by inhibition of Stat3, suggesting that NHERF2 regulates Stat3 phosphorylation through a positive feedback mechanism between Stat3 and CD24. In summary, this study identifies NHERF2 as a novel oncogenic protein and a potential target for cancer treatment. NHERF2 potentiates the oncogenic effects in part by regulation of Stat3 and CD24.

Role of epithelial ion transports in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic inflammatory disorder with a complex pathogenesis. Diarrhea is a highly prevalent and often debilitating symptom of IBD patients that results, at least in part, from an intestinal hydroelectrolytic imbalance. Evidence suggests that reduced electrolyte absorption is more relevant than increased secretion to this disequilibrium. This systematic review analyses and integrates the current evidence on the roles of epithelial Na(+)-K(+)-ATPase (NKA), Na(+)/H(+) exchangers (NHEs), epithelial Na(+) channels (ENaC), and K(+) channels (KC) in IBD-associated diarrhea. NKA is the key driving force of the transepithelial ionic transport and its activity is decreased in IBD. In addition, the downregulation of apical NHE and ENaC and the upregulation of apical large-conductance KC all contribute to the IBD-associated diarrhea by lowering sodium absorption and/or increasing potassium secretion.

Autotaxin-LPA receptor axis in the pathogenesis of lung diseases

Lysophosphatidic acid (LPA) is a small lipid which mediates a variety of cellular functions via the activation of LPA receptors. LPA is generated from lysophosphatidylcholine by the extracellular enzyme, autotaxin (ATX). Elevated ATX expression, LPA production and their signaling pathways have been reported in multiple pathological conditions of lung tissue, including inflammation, fibrosis and cancer. Emerging evidence has highlighted the importance of ATX and LPA receptors in the pathogenesis of lung diseases. Here, we briefly review the current knowledge of different roles of the ATX-LPA receptor axis in lung diseases focusing on inflammation, fibrosis and cancer.