Intestinal tight junctions and their importance in health and disease: role of dietary lipids

MicroRNA in gastrointestinal cell signalling

Our gut forms an important organ and its formation, functioning and homeostasis are maintained by several factors including cell signalling pathways and commensal microflora. These factors affect pathological, physiological and immunological parameters to maintain gut health and prevent its inflammation. Among these, different intracellular signalling pathways play an important role in regulating gut homeostasis. These pathways are in turn regulated by various microRNAs that play a key role in maintaining the balance between tolerance and inflammation. This review highlights the importance of various cell signalling pathways in modulating gut homeostasis and the role specific miRNAs play in their regulation.

Parathyroid hormone-related protein regulates intestinal calcium transport in sea bream (Sparus auratus)

Parathyroid hormone-related protein (PTHrP) is a factor associated with normal development and physiology of the nervous, cardiovascular, immune, reproductive, and musculoskeletal systems in higher vertebrates. It also stimulates whole body calcium uptake in sea bream ( Sparus auratus) larvae with an estimated 60% coming from intestinal uptake in seawater. The present study investigated the role of PTHrP in the intestinal calcium transport in the sea bream in vitro. Unidirectional mucosal-to-serosal and serosal-to-mucosal45Ca fluxes were measured in vitro in duodenum, hindgut, and rectum mounted in Ussing chambers. In symmetric conditions with the same saline, bathing apical and basolateral sides of the preparation addition of piscine PTHrP 1–34 (6 nM) to the serosal surface resulted in an increase in mucosal to serosal calcium fluxes in duodenum and hindgut and a reduction in serosal to mucosal in the rectum, indicating that different mechanisms are responsive to PTHrP along the intestine. In control asymmetric conditions, with serosal normal and mucosal bathed with a saline similar in composition to the intestinal fluid, there was a net increase in calcium uptake in all regions. The addition of 6 nM PTHrP 1–34 increased net calcium uptake two- to threefold in all regions. The stimulatory effect of PTHrP on net intestinal calcium absorption is consistent with a hypercalcemic role for the hormone. The results support the view that PTHrP, alone or in conjunction with recently identified PTH-like peptides, counteracts in vivo the hypocalcemic effects of stanniocalcin.

Intestinal transport during fasting and malnutrition

Fasting or malnutrition (FM) has dramatic effects on small intestinal mucosal structure and transport function. Intestinal secretion of ions and fluid is increased by FM both under basal conditions and in response to secretory agonists. Intestinal permeability to ions and macromolecules may also be elevated by FM, which increases the potential for fluid and electrolyte losses and for anaphylactic responses to luminal antigens. Mucosal atrophy induced by FM reduces total intestinal absorption of nutrients, but nutrient absorption normalized to mucosal mass may actually be enhanced by a variety of mechanisms, including increased transporter gene expression, electrochemical gradients, and ratio of mature to immature cells. These observations underscore the value of enteral feeding during health and disease.

Stanniocalcin: a novel protein regulating calcium and phosphate transport across mammalian intestine

Stanniocalcin (STC) is an anti-hypercalcemic glycoprotein hormone previously identified in the corpuscles of Stannius in bony fish and recently in the human genome. This study undertook to express human STC in Chinese hamster ovary (CHO) cells and to determine its effects on calcium and phosphate absorption in swine and rat intestine. Unidirectional mucosal-to-serosal (Jm-->s) and serosal-to-mucosal (Js-->m) 45Ca and 32P fluxes were measured in vitro in duodenal tissue in voltage-clamped Ussing chambers. Addition of STC (10-100 ng/ml) to the serosal surface of the duodenum resulted in a simultaneous increase in calcium Jm-->s and Js-->m fluxes, with a subsequent reduction in net calcium absorption. This was coupled with an STC-stimulated increase in phosphate absorption. Intestinal conductance was increased at the highest dose of STC (100 ng/ml) in swine tissue. The addition of STC to the mucosal surface had no effect on calcium and phosphate fluxes. STC at doses of 10-1,000 ng/ml had no effect on short-circuit current in any region of the rat intestine. In conclusion, human recombinant STC decreases the absorption of calcium and stimulates the absorption of phosphate in both swine and rat duodenum. STC is a novel regulatory protein that regulates mammalian intestinal calcium and phosphate transport.

Calcium site specificity. Early Ca2+-related tight junction events

The molecular mechanisms by which Ca2+ and metal ions interact with the binding sites that modulate the tight junctions (TJs) have not been fully described. Metal ions were used as probes of these sites in the frog urinary bladder. Basolateral Ca2+ withdrawal induces the opening of the TJs, a process that is abruptly terminated when Ca2+ is readmitted, and is followed by a complete recovery of the TJ seal. Mg2+ and Ba2+ were incapable of keeping the TJ sealed or of inducing TJ recovery. In addition, Mg2+ causes a reversible concentration-dependent inhibition of the Ca2+-induced TJ recovery. The effects of extracellular Ca2+ manipulation on the TJs apparently is not mediated by changes of cytosolic Ca2+ concentration. The transition elements, Mn2+ and Cd2+, act as Ca2+ agonists. In the absence of Ca2+, they prevent TJ opening and almost immediately halt the process of TJ opening caused by Ca2+ withdrawal. In addition, Mn2+ promotes an almost complete recovery of the TJ seal. Cd2+, in spite of stabilizing the TJs in the closed state and halting TJ opening, does not promote TJ recovery, an effect that apparently results from a superimposed toxic effect that is markedly attenuated by the presence of Ca2+. The interruption of TJ opening caused by Ca2+, Cd2+, or Mn2+, and the stability they confer to the closed TJs, might result from the interaction of these ions with E-cadherin. Addition of La3+ (2 microM) to the basolateral Ca2+-containing solution causes an increase of TJ permeability that fully reverses when La3+ is removed. This effect of La3+, observed in the presence of Ca2+ (1 mM), indicates a high La3+ affinity for the Ca2+-binding sites. This ability of La3+ to open TJs in the presence of Ca2+ is a relevant aspect that must be considered when using La3+ in the evaluation of TJ permeability of epithelial and endothelial membranes, particularly when used during in vivo perfusion or in the absence of fixatives.