Endothelial cells that make up brain capillaries and constitute the blood-brain barrier become different from peripheral endothelial cells in response to inductive factors found in the nervous system. We have established a cell culture model of the blood-brain barrier by treating brain endothelial cells with a combination of astrocyte-conditioned medium and agents that elevate intracellular cAMP. These cells form high resistance tight junctions and exhibit low rates of paracellular leakage and fluid-phase endocytosis. They also undergo a dramatic structural reorganization as they form tight junctions. Results from these studies suggest modes of manipulating the permeability of the blood-brain barrier, potentially providing the basis for increasing the penetration of drugs into the central nervous system.

/pdf/a-cell-culture-model-of-the-blood-brain-barrier-o49u0mvc9n.pdf

A cell culture model of the blood-brain barrier.

Tissue barriers that restrict passage of liquids, ions, and larger solutes are essential for the development of multicellular organisms. In simple organisms this allows distinct cell types to interface with the external environment. In more complex species, the diversity of cell types capable of forming barriers increases dramatically. Although the plasma membranes of these barrier-forming cells prevent flux of most hydrophilic solutes, the paracellular, or shunt, pathway between cells must also be sealed. This function is accomplished in vertebrates by the zonula occludens, or tight junction. The tight junction barrier is not absolute but is selectively permeable and is able to discriminate between solutes on the basis of size and charge. Many tight junction components have been identified over the past 20 years, and recent progress has provided new insights into the proteins and interactions that regulate structure and function. This review presents these data in a historical context and proposes an integrated model in which dynamic regulation of tight junction protein interactions determines barrier function.

Tight Junction Pore and Leak Pathways: A Dynamic Duo

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a fe...

Electrolyte Transport in the Mammalian Colon: Mechanisms and Implications for Disease

https://www.gastrojournal.org/article/S0016-5085(99)70126-5/pdf

Altered tight junction structure contributes to the impaired epithelial barrier function in ulcerative colitis

The intestinal epithelium is faced with the complex task of providing a barrier while also allowing nutrient and water absorption. The frequency with which these processes are disrupted in disease can be taken as evidence of their importance. It is therefore of interest to define the mechanisms of altered intestinal barrier and transport function and develop means to correct disease-associated defects. Over the past 10 years, some of the molecular events underlying physiological epithelial barrier regulation have been described. Remarkably, recent advances have shown that activation of the same mechanisms is central to barrier dysfunction in both in vitro and in vivo models of disease. Although the contribution of barrier dysfunction to pathogenesis of chronic disease remains incompletely understood, it is now clear that cytoskeletal regulation of barrier function is both an important pathogenic process and that targeted inhibition of myosin light chain kinase, which affects this cytoskeleton-dependent tight junction dysfunction, is an attractive candidate for therapeutic intervention.

Molecular basis of epithelial barrier regulation: from basic mechanisms to clinical application.

In ‘leaky‘ epithelia, ions move through both a transcellular and a paracellular (serial alignment of tight junction and intercellular space) path. The efficiency of transepithelial transport could therefore be regulated if the cell was able to alter reversibly the permeability of tight junctions. (These are specialized regions of the apical cell membranes common to all epithelia.) We now report that such a mechanism indeed exists in the Necturus gallbladder. It is effected by cyclic AMP, which is already known to mediate surface membrane phenomena in a variety of cell systems through its interaction with the cytoskeletal system1,2. In gallbladders mounted and perfused with electrolyte solutions in an Ussing-type chamber, exposure of the mucosal surface to cyclic AMP analogues increased transepithelial electrical resistance, potential difference and short-circuit current and decreased NaCl dilution potentials in a rapid and reversible manner. We also observed rapid depolarization of cell membrane electrical potentials and a slow decline in intracellular K+ activity. Freeze-fracture electron microscopy of tissues fixed with glutaraldehyde during the peak electrical response showed a reorientation of intramembranous junctional fibrils, suggesting that cyclic AMP reduces the ionic permeability of the paracellular pathway in this epithelium by altering the structure of tight junctions.

B. Hainau

Papers

Regulation of epithelial tight junction permeability by cyclic AMP.