Ecto-nucleotidases play a pivotal role in purinergic signal transmission. They hydrolyze extracellular nucleotides and thus can control their availability at purinergic P2 receptors. They generate extracellular nucleosides for cellular reuptake and salvage via nucleoside transporters of the plasma membrane. The extracellular adenosine formed acts as an agonist of purinergic P1 receptors. They also can produce and hydrolyze extracellular inorganic pyrophosphate that is of major relevance in the control of bone mineralization. This review discusses and compares four major groups of ecto-nucleotidases: the ecto-nucleoside triphosphate diphosphohydrolases, ecto-5′-nucleotidase, ecto-nucleotide pyrophosphatase/phosphodiesterases, and alkaline phosphatases. Only recently and based on crystal structures, detailed information regarding the spatial structures and catalytic mechanisms has become available for members of these four ecto-nucleotidase families. This permits detailed predictions of their catalytic mechanisms and a comparison between the individual enzyme groups. The review focuses on the principal biochemical, cell biological, catalytic, and structural properties of the enzymes and provides brief reference to tissue distribution, and physiological and pathophysiological functions.

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Cellular function and molecular structure of ecto-nucleotidases

Intestinal alkaline phosphatase detoxifies lipopolysaccharide and prevents inflammation in zebrafish in response to the gut microbiota.

Lipopolysaccharide (LPS) may play an important role in chronic diseases through the activation of inflammatory responses. The type of diet consumed is of major concern for the prevention and treatment of these diseases. Evidence from animal and human studies has shown that LPS can diffuse from the gut to the circulatory system in response to the intake of high amounts of fat. The method by which LPS move into the circulatory system is either through direct diffusion due to intestinal paracellular permeability or through absorption by enterocytes during chylomicron secretion. Considering the impact of metabolic diseases on public health and the association between these diseases and the levels of LPS in the circulatory system, this review will mainly discuss the current knowledge about high-fat diets and subclinical inflammation. It will also describe the new evidence that correlates gut microbiota, intestinal permeability and alkaline phosphatase activity with increased blood LPS levels and the biological effects of this increase, such as insulin resistance. Although the majority of the studies published so far have assessed the effects of dietary fat, additional studies are necessary to deepen the understanding of how the amount, the quality and the structure of the fat may affect endotoxaemia. The potential of food combinations to reduce the negative effects of fat intake should also be considered in future studies. In these studies, the effects of flavonoids, prebiotics and probiotics on endotoxaemia should be investigated. Thus, it is essential to identify dietetic strategies capable of minimising endotoxaemia and its postprandial inflammatory effects.

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Influence of a high-fat diet on gut microbiota, intestinal permeability and metabolic endotoxaemia

Gram-negative bacteria decorate their outermost surface structure, lipopolysaccharide, with elaborate chemical moieties, which effectively disguises them from immune surveillance and protects them from the onslaught of host defences. Many of these changes occur on the lipid A moiety of lipopolysaccharide, a component that is crucial for host recognition of Gram-negative infection. In this Review, we describe the regulatory mechanisms controlling lipid A modification and discuss the impact of modifications on pathogenesis, bacterial physiology and bacterial interactions with the host immune system.

/pdf/fortifying-the-barrier-the-impact-of-lipid-a-remodelling-on-1afepv2udh.pdf

Fortifying the barrier: the impact of lipid A remodelling on bacterial pathogenesis

To determine whether the provision of early standard enteral nutrition (EN) confers treatment benefits to critically ill patients. Medline and EMBASE were searched. Hand citation review of retrieved guidelines and systematic reviews were undertaken, and academic and industry experts were contacted. Methodologically sound randomised controlled trials (RCTs) conducted in critically ill patient populations that compared the delivery of standard EN, provided within 24 h of intensive care unit (ICU) admission or injury, to standard care were included. The primary analysis was conducted on clinically meaningful patient-oriented outcomes. Secondary analyses considered vomiting/regurgitation, pneumonia, bacteraemia, sepsis and multiple organ dysfunction syndrome. Meta-analyses were conducted using the odds ratio (OR) metric and a fixed effects model. The impact of heterogeneity was assessed using the I
 2 metric. Six RCTs with 234 participants were analysed. The provision of early EN was associated with a significant reduction in mortality [OR = 0.34, 95% confidence interval (CI) 0.14–0.85] and pneumonia (OR = 0.31, 95% CI 0.12–0.78). There were no other significant differences in outcomes. A sensitivity analysis and a simulation exercise confirmed the presence of a mortality reduction. Although the detection of a statistically significant reduction in mortality is promising, overall trial quality was low, trial size was small, and the findings may be restricted to the patient groups enrolled into included trials. The results of this meta-analysis should be confirmed by the conduct of a large multi-centre trial enrolling diverse critically ill patient groups.

Early enteral nutrition, provided within 24 h of injury or intensive care unit admission, significantly reduces mortality in critically ill patients: a meta-analysis of randomised controlled trials.

Background & Aims: Crohn9s disease (CD) and ulcerative colitis (UC) are chronic multifactorial inflammatory bowel diseases with unknown etiology, but a deregulated mucosal immune response to gut-derived bacterial antigens is thought to be involved. Toll-like receptor ligands, especially lipopolysaccharide (LPS), contribute to the maintenance of the disease. We have previously shown that the enzyme alkaline phosphatase (AP) is able to detoxify LPS and the aim of this study was to examine a possible role in inflammatory bowel diseases. 	Methods: Intestinal AP (iAP) mRNA expression and LPS-dephosphorylation in intestinal biopsies of control persons and IBD patients were examined, and we subsequently studied the effect of orally administered iAP-tablets on the progression of dextran sodium sulphate-induced colitis in rats. Results: In healthy persons, iAP mRNA and enzyme activity was high in the ileum relative to the colon. In UC and Crohn9s patients iAP mRNA expression was found markedly reduced when inflamed tissue was compared to non-inflamed tissue. Oral administration of iAP-tablets to colitic rats resulted in a significant attenuation of colonic inflammation as reflected by reduced mRNA levels for TNFα, IL-1β, IL-6 and iNOS, a reduced iNOS-staining and inflammatory cell influx, and a significantly improved morphology of the intestinal wall. Conclusions: The present study shows that epithelial iAP mRNA expression is reduced in both UC and Crohn9s patients. The rat model demonstrates that oral administration of active iAP-enzymes in the intestinal tract, results in a significant reduction of inflammation. This provides new insight on IBD pathology and a novel treatment approach to this severe inflammatory disease.

https://gut.bmj.com/content/gutjnl/58/3/379.full.pdf

Role of alkaline phosphatase in colitis in man and rats

Gut-derived lipopolysaccharide (LPS) plays a role in the pathogenesis of liver diseases like fibrosis. The enzyme alkaline phosphatase (AP) is present in, among others, the intestinal wall and liver and has been previously shown to dephosphorylate LPS. Therefore, we investigated the effect of LPS on hepatic AP expression and the effect of AP on LPS-induced hepatocyte responses. LPS-dephosphorylating activity was expressed at the hepatocyte canalicular membrane in normal and fibrotic animals. In addition to this, fibrotic animals also displayed high LPS-dephosphorylating activity around bile ducts. The enzyme was shown to dephosphorylate LPS from several bacterial species. LPS itself rapidly enhanced the intrahepatic mRNA levels for this enzyme within 2 h by a factor of seven. Furthermore, in vitro and in vivo studies showed that exogenous intestinal AP quickly bound to the asialoglycoprotein receptor on hepatocytes. This intestinal isoform significantly attenuated LPS-induced hepatic tumor necrosis factor-alpha and nitric oxide (nitrite and nitrate) responses in vitro. The enzyme also reduced LPS-induced hepatic glycogenolysis in vivo. This study shows that LPS enhances AP expression in hepatocytes and that intestinal AP is rapidly taken up by these same cells, leading to an attenuation of LPS-induced responses in vivo. Gut-derived LPS-dephosphorylating activity or enzyme upregulation within hepatocytes by LPS may therefore be a protective mechanism within the liver.

A Tuin

Papers

Role of alkaline phosphatase in colitis in man and rats

On the role and fate of LPS-dephosphorylating activity in the rat liver.

A new technique for preparing precision-cut slices from small intestine and colon for drug biotransformation studies.

Oral administration of alkaline phosphatase ameliorates colitis

It may not be intestinal, but tissue non-specific alkaline phosphatase (Response)