Brain barriers: Crosstalk between complex tight junctions and adherens junctions

Unique intercellular junctional complexes between the central nervous system (CNS) microvascular endothelial cells and the choroid plexus epithelial cells form the endothelial blood-brain barrier (BBB) and the epithelial blood-cerebrospinal fluid barrier (BCSFB), respectively. These barriers inhibit paracellular diffusion, thereby protecting the CNS from fluctuations in the blood. Studies of brain barrier integrity during development, normal physiology, and disease have focused on BBB and BCSFB tight junctions but not the corresponding endothelial and epithelial adherens junctions. The crosstalk between adherens junctions and tight junctions in maintaining barrier integrity is an understudied area that may represent a promising target for influencing brain barrier function.

Intrinsic characteristics of the proton pump in the luminal membrane of a tight urinary epithelium. The relation between transport rate and delta mu H.

A number of tight urinary epithelia, as exemplified by the turtle bladder, acidify the luminal solution by active transport of H+ across the luminal cell membrane. The rate of active H+ transport (JH) decreases as the electrochemical potential difference for H+ [delta mu H = mu H(lumen) - mu H(serosa)] across the epithelium is increased. The luminal cell membrane has a low permeability for H+ equivalents and a high electrical resistance compared with the basolateral cell membrane. Changes in JH thus reflect changes in active H+ transport across the luminal membrane. To examine the control of JH by delta mu H in the turtle bladder, transepithelial electrical potential differences (delta psi) were imposed at constant acid-base conditions or the luminal pH was varied at delta psi = 0 and constant serosal PCO2 and pH. When the luminal compartment was acidified from pH 7 to 4 or was made electrically positive, JH decreased as a linear function of delta mu H as previously described. When the luminal compartment was made alkaline from pH 7 to 9 or was made electrically negative, JH reached a maximal value, which was the same whether the delta mu H was imposed as a delta pH or a delta psi. The nonlinear JH vs. delta mu H relation does not result from changes in the number of pumps in the luminal membrane or from changes in the intracellular pH, but is a characteristic of the H+ pumps themselves. We propose a general scheme, which, because of its structural features, can account for the nonlinearity of the JH vs. delta mu H relations and, more specifically, for the kinetic equivalence of the effects of the chemical and electrical components of delta mu H. According to this model, the pump complex consists of two components: a catalytic unit at the cytoplasmic side of the luminal membrane, which mediates the ATP-driven H+ translocation, and a transmembrane channel, which mediates the transfer of H+ from the catalytic unit to the luminal solution. These two components may be linked through a buffer compartment for H+ (an antechamber).

Chemical composition of basalts and minerals from basalts of the Bouvet triple junction

This study focuses on mafic volcanic rocks from the Bouvet triple junction, which fall into six geochemically distinct groups: (1) N-MORB, the most widespread type, encountered throughout the study area. (2) Subalkaline volcanics, hawaiites and mugearites strongly enriched in lithophile elements and radiogenic isotopes and composing the Bouvet volcanic rise, and compositionally similar basalts and basaltic andesites from the Spiess Ridge, generated in a deeper, fertile mantle region. (3) Relatively weakly enriched basalts, T-MORB derived by the mixing of Type 1 and 2 melts and exposed near the axes of the Mid-Atlantic, Southwest Indian, and America-Antarctic Ridges. (4) Basalts with a degree of trace lithophile element enrichment similar to the Spiess Ridge and Bouvet Island rocks, but higher in K, P, Ti, and Cr. These occur within extensional structures: the rift valley of the Southwest Indian Ridge, grabens of the East Dislocation Zone, and the linear rise between the Spiess Ridge and Bouvet volcano. Their parental melts presumably separated from plume material that spread from the main channels and underwent fluid-involving differentiation in the mantle. (5) A volcanic suite ranging from basalt to rhyolite, characterized by low concentrations of lithophile elements, particularly TiO2, and occurring on the Shona Seamount and other compressional features within the Antarctic and South American plates near the Bouvet triple junction. Unlike Types 1 to 4, which display tholeiitic differentiation trends, this suite is calc-alkaline. Its parental melts were presumably related to the plume material as well but, subsequently, they underwent a profound differentiation involving fluids and assimilated surrounding rocks in closed magma chambers in the upper mantle. Alternatively, the Shona Seamount might be a fragment of an ancient oceanic island arc. (6) Enriched basalts, distinguished from the other enriched rock types in very high P and radiogenic isotope abundances and composing a tectonic uplift near the junction of the three rifts. It thus follows that the main factors responsible for the compositional diversity of volcanic rocks in this region include (i) mantle source heterogeneity, (ii) plume activity, (iii) an intricate geodynamic setup at the triple junction giving rise to stresses in adjacent plate areas, and (iv) the geological prehistory. The slow spreading rate and ensuing inefficient mixing of the heterogeneous mantle material result in strong spatial variations in basaltic compositions.