Drug Discovery Opportunities and Challenges at G Protein Coupled Receptors for Long Chain Free Fatty Acids

Discovery of G protein coupled receptors for long chain free fatty acids (FFAs), FFA1 (GPR40) and GPR120, has expanded our understanding of these nutrients as signaling molecules. These receptors have emerged as important sensors for FFA levels in the circulation or the gut lumen, based on evidence from in vitro and rodent models, and an increasing number of human studies. Here we consider their promise as therapeutic targets for metabolic disease, including type 2 diabetes and obesity. FFA1 directly mediates acute FFA-induced glucose-stimulated insulin secretion in pancreatic beta-cells, while GPR120 and FFA1 trigger release of incretins from intestinal endocrine cells, and so indirectly enhance insulin secretion and promote satiety. GPR120 signaling in adipocytes and macrophages also results in insulin sensitizing and beneficial anti-inflammatory effects. Drug discovery has focused on agonists to replicate acute benefits of FFA receptor signaling, with promising early results for FFA1 agonists in man. Controversy surrounding chronic effects of FFA1 on beta-cells illustrates that long term benefits of antagonists also need exploring. It has proved challenging to generate highly selective potent ligands for FFA1 or GPR120 subtypes, given that both receptors have hydrophobic orthosteric binding sites, which are not completely defined and have modest ligand affinity. Structure activity relationships are also reliant on functional read outs, in the absence of robust binding assays to provide direct affinity estimates. Nevertheless synthetic ligands have already helped dissect specific contributions of FFA1 and GPR120 signaling from the many possible cellular effects of FFAs. Approaches including use of fluorescent ligand binding assays, and targeting allosteric receptor sites, may improve further pre-clinical ligand development at these receptors, to exploit their unique potential to target multiple facets of diabetes

Analysis of epithelial cell shedding and gaps in the intestinal epithelium

The intestinal barrier is formed by a monolayer of columnar epithelial cells. This barrier is effectively maintained despite the high turnover of epithelial cells in the gut. Defects in the mechanism by which barrier function is maintained are believed to play a central role in the pathogenesis of inflammatory bowel disease (IBD). Proinflammatory cytokines such as TNF-α and IFN-γ are often elevated in inflamed tissue of patients with IBD. In fact, anti-TNF-α therapy is routinely administered to patients with Crohn's disease. We have previously demonstrated that intestinal epithelial cells are shed from the intestine leaving a 'gap' in the epithelium that is able to maintain barrier function. The rate of cell shedding and barrier permeability is substantially increased by the administration of TNF-α. Loss of barrier function at the site of a gap may provide a site of entry for disease-causing bacteria

Clostridium difficile: a healthcare associated infection of unknown significance in adults in sub-Saharan Africa

Background: Clostridium difficile infection (CDI) causes a high burden of disease in high-resource healthcare systems, with significant morbidity, mortality and financial implications. CDI is a healthcare-associated infection for which the primary risk factor is antibiotic usage and it is the leading cause of bacterial diarrhoea in HIV infected patients in USA. Little is known about the disease burden of CDI in sub-Saharan Africa, where HIV and healthcare associated infection have a higher prevalence and antibiotic usage is less restricted. Aim: To review published literature on CDI in sub-Saharan Africa, highlighting areas for future research. Methods: English language publications since 1995 were identified from online databases (PubMed, Medline, Google Scholar, SCOPUS) and personal collections of articles, using combinations of keywords to include C. difficile, Africa and HIV. Results: Ten relevant studies were identified. There is considerable variation in methodology to assess for carriage of toxigenic C. difficile and its associations. Eight studies report carriage of toxigenic C. difficile. Three (of four) studies found an association with antibiotic usage. One (of four) studies showed an association with HIV infection. One study showed no association with degree of immunosuppression in HIV. Two (of three) studies showed an association between carriage of toxigenic C. difficile and diarrhoeal illness. Conclusion: Whilst the carriage of toxigenic C. difficile is well described in sub-Saharan Africa, the impact of CDI in the Region remains poorly l understood and warrants high quality research

TNF-α-induced intestinal epithelial cell shedding:implications for intestinal barrier function

Although epithelial cells are continuously shed from the surface of the intestine, the intestinal epithelium maintains the integrity of the epithelial barrier. This is achieved through a highly dynamic process involving reorganization of tight junction and adherens junction proteins. This process both ejects the cell from the epithelial monolayer and plugs the gap left after the cell is shed. Inflammatory insults can trigger a disturbance of these barrier functions by increasing rates of cell shedding. Epithelial cell shedding and loss of barrier can be visualized by confocal laser endomicroscopy in humans. A simple grading system of confocal laser endomicroscopic images can stratify inflammatory bowel disease (IBD) patients in remission into those who will relapse over the subsequent six months and those who will not. Here, we review the mechanisms governing maintenance of these barrier functions and the implications of inflammation-induced barrier dysfunction in IBD