Role of Barrier Integrity and Dysfunctions in Maintaining the Healthy Gut and Their Health Outcomes

Mucosal surface layers are the critical borders throughout epithelial membranes. These epithelial cells segregate luminal material from external environments. However, mucosal linings are also accountable for absorbing nutrients and requiring specific barrier permeability. These functional acts positioned the mucosal epithelium at the epicenter of communications concerning the mucosal immune coordination and foreign materials, such as dietary antigens and microbial metabolites. Current innovations have revealed that external stimuli can trigger several mechanisms regulated by intestinal mucosal barrier system. Crucial constituents of this epithelial boundary are physical intercellular structures known as tight junctions (TJs). TJs are composed of different types transmembrane proteins linked with cytoplasmic adaptors which helps in attachment to the adjacent cells. Disruption of this barrier has direct influence on healthy or diseased condition, as barrier dysfunctions have been interrelated with the initiation of inflammation, and pathogenic effects following metabolic complications. In this review we focus and overview the TJs structure, function and the diseases which are able to influence TJs during onset of disease. We also highlighted and discuss the role of phytochemicals evidenced to enhance the membrane permeability and integrity through restoring TJs levels.

Structural snapshots of human PepT1 and PepT2 reveal mechanistic insights into substrate and drug transport across epithelial membranes

The uptake of peptides in mammals plays a crucial role in nutrition and inflammatory diseases. This process is mediated by promiscuous transporters of the Solute Carrier Family 15, which form part of the Major Facilitator superfamily. Besides the uptake of short peptides, Peptide transporter 1 (PepT1) is a highly abundant drug transporter in the intestine and represents a major route for oral drug delivery. Peptide transporter 2 (PepT2) allows in addition renal drug reabsorption from ultrafiltration and brain-to-blood efflux of neurotoxic compounds. Here we present cryo-EM structures of human PepT1 in an outward open state and of human PepT2 in an inward facing partially occluded state with a bound substrate. The structures reveal the architecture of human peptide transporters and provide mechanistic insights into substrate recognition and conformational transitions during transport. Importantly, this may support future drug design efforts to increase the bioavailability of different drugs in the human body.

Investigating Lipid Headgroup Composition within Epithelial Membranes: A Systematic Review

Membrane lipid composition is often quoted within the literature, but with very little insight into how or why these compositions vary when compared to other biological membranes. One prominent area...

Spreading of a mycobacterial cell-surface lipid into host epithelial membranes promotes infectivity

Several virulence lipids populate the outer cell wall of pathogenic mycobacteria. Phthiocerol dimycocerosate (PDIM), one of the most abundant outer membrane lipids, plays important roles in both defending against host antimicrobial programs and in evading these programs altogether. Immediately following infection, mycobacteria rely on PDIM to evade Myd88-dependent recruitment of microbicidal monocytes which can clear infection. To circumvent the limitations in using genetics to understand virulence lipids, we developed a chemical approach to track PDIM during Mycobacterium marinum infection of zebrafish. We found that PDIM's methyl-branched lipid tails enabled it to spread into host epithelial membranes to prevent immune activation. Additionally, PDIM’s affinity for cholesterol promoted this phenotype; treatment of zebrafish with statins, cholesterol synthesis inhibitors, decreased spreading and provided protection from infection. This work establishes that interactions between host and pathogen lipids influence mycobacterial infectivity and suggests the use of statins as tuberculosis preventive therapy by inhibiting PDIM spread.

Spreading of a mycobacterial cell surface lipid into host epithelial membranes promotes infectivity

ABSTRACTSeveral virulence lipids populate the outer cell wall of pathogenic mycobacteria (Jackson, 2014). Phthiocerol dimycocerosate (PDIM), one of the most abundant outer membrane lipids (Anderson, 1929), plays important roles in both defending against host antimicrobial programs (Camacho et al., 2001; Cox et al., 1999; Murry et al., 2009) and in evading these programs altogether (Cambier et al., 2014a; Rousseau et al., 2004). Immediately following infection, mycobacteria rely on PDIM to evade toll-like receptor (TLR)-dependent recruitment of bactericidal monocytes which can clear infection (Cambier et al., 2014b). To circumvent the limitations in using genetics to understand virulence lipid function, we developed a chemical approach to introduce a clickable, semi-synthetic PDIM onto the cell wall of Mycobacterium marinum. Upon infection of zebrafish, we found that PDIM rapidly spreads into host epithelial membranes, and that this spreading inhibits TLR activation. PDIM’s ability to spread into epithelial membranes correlated with its enhanced fluidity afforded by its methyl-branched mycocerosic acids. Additionally, PDIM’s affinity for cholesterol promoted its occupation of epithelial membranes; treatment of zebrafish with statins, cholesterol synthesis inhibitors, decreased spreading and provided protection from infection. This work establishes that interactions between host and pathogen lipids influence mycobacterial infectivity and suggests the use of statins as tuberculosis preventive therapy by inhibiting PDIM spread.

AFM based elasticity of intestinal epithelium correlate with barrier function under drug action

ABSTRACTOver the past few years, atomic force microscopy (AFM) has developed as a mature research tool for measuring the nanomechanical properties of tissue, cells and biological structures. The force spectroscopy mode of AFM allows the local elasticity of biological samples to be measured. The mechanical properties of cells are highly affected by homeostatic changes observed during disease. In the case of the intestine, the aetiology for some conditions is still unclear. To improve the clinical translation of pre-clinical models, a new and different approach could be to study cellular behaviour in health and disease from a mechanical point of view. Specifically, knowledge of changes in epithelial membranes in response to drugs is useful for interpreting both drug action and disease development. Here, we used human intestinal Caco-2 cells as a first step to record epithelial membrane elasticity measurements at the nanoscale using AFM. Three different drugs were selected to influence intestinal epithelium integrity by specifically targeting different functional aspects of the membrane, such as permeability and support. Results indicate a relationship between measured cell elasticity and cell viability markers, such as cellular toxicity and membrane barrier functions. Our work represents a proof-of-concept that cells suffer a particular change in elastic properties depending upon the mechanism of action of an applied drug. The following may provide an efficient approach for diagnosing intestinal pathologies and testing drugs for clinical use.STATEMENT OF SIGNIFICANCEWe present evidence that epithelial membrane suffers a particular change in elastic properties depending upon the mechanism of action of an applied drug. These changes can be monitored over time using AFM technology and may provide an alternative and efficient approach for diagnosing intestinal pathologies and testing drugs for clinical use.