Kazumasa Moriwaki

Claudin-2–deficient mice are defective in the leaky and cation-selective paracellular permeability properties of renal proximal tubules

Claudin-2 is highly expressed in tight junctions of mouse renal proximal tubules, which possess a leaky epithelium whose unique permeability properties underlie their high rate of NaCl reabsorption. To investigate the role of claudin-2 in paracellular NaCl transport in this nephron segment, we generated knockout mice lacking claudin-2 (Cldn2−/−). The Cldn2−/− mice displayed normal appearance, activity, growth, and behavior. Light microscopy revealed no gross histological abnormalities in the Cldn2−/− kidney. Ultrathin section and freeze-fracture replica electron microscopy revealed that, similar to those of wild types, the proximal tubules of Cldn2−/− mice were characterized by poorly developed tight junctions with one or two continuous tight junction strands. In contrast, studies in isolated, perfused S2 segments of proximal tubules showed that net transepithelial reabsorption of Na+, Cl–, and water was significantly decreased in Cldn2−/− mice and that there was an increase in paracellular shunt resistance without affecting the apical or basolateral membrane resistances. Moreover, deletion of claudin-2 caused a loss of cation (Na+) selectivity and therefore relative anion (Cl–) selectivity in the proximal tubule paracellular pathway. With free access to water and food, fractional Na+ and Cl– excretions in Cldn2−/− mice were similar to those in wild types, but both were greater in Cldn2−/− mice after i.v. administration of 2% NaCl. We conclude that claudin-2 constitutes leaky and cation (Na+)–selective paracellular channels within tight junctions of mouse proximal tubules.

Megaintestine in Claudin-15–Deficient Mice

BACKGROUND & AIMS Claudins, the major components of tight junction (TJ) strands, which form paracellular barriers, consist of 24 family members, the combination of which determines the properties of TJ-based paracellular barriers. Here, we generated claudin-15-deficient (Cldn15(-/-)) mice to examine the ubiquitously expressed functions of claudin-15. METHODS We generated Cldn15(-/-) mice by the conventional gene-targeting strategy. Because the upper small intestine was enlarged in Cldn15(-/-) mice, we analyzed the phenotype from various angles regarding histology, physiology, and cell biology. RESULTS Cldn15(-/-) mice were born and grew normally with an enlarged upper small intestinal phenotype, megaintestine. Deficiency of claudin-15 did not cause a compensatory increase in the background expression of other types of claudins, claudin-1, -2, -3, -4, -7, -12, -18, -20, and -23, in the small intestine. Cldn15(-/-) mice showed enhanced proliferation of normal cryptic cells after weaning without diseased states such as polyps or cancer, resulting in megaintestine, in which the upper small intestine was approximately 2 times larger than normal in length and diameter. The number of transit-amplifying cells in crypts increased approximately 2-fold. Freeze-fracture electron microscopy revealed that deficiency of claudin-15 decreased the number of TJ strands, although the electric conductance was decreased in distal segments in Cldn15(-/-) jejunum, as compared with Cldn15(+/+) littermates. CONCLUSIONS Based on the specific roles of claudins in paracellular barrier formation without any direct role in cell proliferation, as previously shown in cultured epithelial cells, we propose that claudin-15-based formation of TJs to organize the microenvironment including ion conductance is important for normal-sized morphogenesis of the small intestine.

A PLANETESIMAL ACCRETION ZONE IN A CIRCUMBINARY DISK

We investigate the conditions for planetesimal accretion in a circumbinary disk. Until recently, it had been believed that only single solar-type stars might harbor planetary systems. On the other hand, circumbinary disks have been detected by infrared or radio observation. Planets may be formed in such disks. Binary systems give stronger gravitational perturbation against planetesimals orbiting nearer to the binary. Therefore, the relative velocities between planetesimals will be larger and when they exceed the escape velocity it is impossible for the planetesimals to accumulate into planets. We performed long-term numerical integrations of binary and planetesimal orbital motions in the framework of the coplanar elliptic restricted three-body problem and have found the upper limit of the orbital radius inside of which the relative velocity between the planetesimals exceeds the escape velocity. One of our results is that planetesimal accretion cannot occur in a zone within 13 AU from the barycenter of the binary system when the binary semimajor axis is 1 AU, the binary eccentricity 0.1, the total mass m1 + m2 = 1 M☉, and the mass ratio m2/(m1 + m2) = 0.2. In regions farther out than 13 AU, planetesimals can accrete. We also derive analytic expressions of the eccentricity of a planetesimal pumped up by the gravitational perturbation of the binary and the inner boundary radius of the planetesimal accretion zone according to the secular perturbation theory.

The role of claudin-based tight junctions in morphogenesis.

In morphogenetic events during developmental processes, the hydrostatic pressure caused by fluid accumulation is thought to be an important factor determining the shape of epithelial structures. Recent studies using loss of function of claudins have demonstrated that tight junctions, by their control of fluid accumulation, have crucial roles in morphogenesis. Interestingly, both the barrier and the channel functions of tight junctions appear to contribute to morphogenesis.

Stability of a Planet in a Binary System: MACHO 97-BLG-41

An extrasolar planet was detected in a binary system by gravitational microlensing. This is considered to be the first one detected orbiting both components of a binary star system. The event is now known as MACHO 97-BLG-41. Whether such a planetary system can last on a large timescale is a major concern to celestial mechanics. We investigate the stability of the planetary orbits with the observed mass ratios of the three bodies by taking the binary and planetary eccentricities as parameters. The eccentricities could hardly be determined by gravitational microlensing but may be estimated by long-term numerical integrations of the binary and planetary orbital motions. We performed such long-term numerical integrations of the coplanar elliptic restricted three-body problem with various initial conditions in order to see what initial conditions produce stable planetary orbits during the integration for 106 binary periods (2.8 × 106 yr). The results of our numerical integrations permit us to estimate the upper limit of binary eccentricity, which ensures stable planetary orbital motion to be about 0.5 in the cases of circular initial orbits of the planet. In the cases of elliptic initial orbits of the planet, the planetary orbital motion is found to be less stable; hence, the upper limit of the binary eccentricity is estimated to be smaller than that in the cases of circular initial orbits of the planet. The upper limit of the initial planetary eccentricity is estimated to be about 0.4 for stable planetary orbital motion. The results of similar integrations for retrograde orbits indicate that the planetary retrograde orbits are more stable than the prograde ones.

TRKB tyrosine kinase receptor is a potential therapeutic target for poorly differentiated oral squamous cell carcinoma

It has been reported that one of the neurotrophin receptors, tropomyosin receptor kinase B (TRKB), is frequently overexpressed in various tumor tissues including oral squamous cell carcinoma (OSCC), and that its upregulation promotes tumor progression in human cancers. However, the correlation between TRKB overexpression and clinicopathological characteristics is not fully elucidated. Here, we present the correlation between the expression levels of TRKB and/or its secreted ligand, brain-derived neurotrophic factor (BDNF), and clinicopathological characteristics, especially regarding tumor differentiation, tissue invasion, and disease-free survival in patients with OSCC. The results obtained through immunohistochemical analysis of human OSCC tumor specimens showed that the expression levels of TRKB and/or BDNF, were significantly higher in moderately and poorly differentiated OSCC (MD/PD-OSCC) tumor cells than in well differentiated cells (WD-OSCC). Moreover, the OSCC tumors highly expressing TRKB and/or BDNF exhibited promotion in tissue invasion and reduction in disease-free survival in the patients. In an orthotopic transplantation mouse model of human OSCC cell lines, administration of a TRKB-specific inhibitor significantly suppressed the tumor growth and invasion in PD-OSCC-derived tumor cells, but not in WD-OSCC-derived tumor cells. Moreover, the TRKB inhibitor selectively blocked BDNF-induced tumor cell proliferation and migration accompanied with the suppression of TRKB phosphorylation in PD-OSCC but not in WD-OSCC in vitro. Taken together, these data suggest that the BDNF/TRKB signaling pathway may regulate tumor progression in poorly differentiated OSCC. Expression levels of signal molecules may be an accurate prognosis marker for tumor aggressiveness, and the molecules may be an attractive target for new OSCC therapies.