The bioactive lysophospholipid mediator sphingosine-1-phosphate (S1P) promotes the egress of newly formed T cells from the thymus and the release of immature B cells from the bone marrow. It has remained unclear, however, where and how S1P is released. Here, we show that in mice, the S1P transporter spinster homolog 2 (Spns2) is responsible for the egress of mature T cells and immature B cells from the thymus and bone marrow, respectively. Global Spns2-KO mice exhibited marked accumulation of mature T cells in thymi and decreased numbers of peripheral T cells in blood and secondary lymphoid organs. Mature recirculating B cells were reduced in frequency in the bone marrow as well as in blood and secondary lymphoid organs. Bone marrow reconstitution studies revealed that Spns2 was not involved in S1P release from blood cells and suggested a role for Spns2 in other cells. Consistent with these data, endothelia-specific deletion of Spns2 resulted in defects of lymphocyte egress similar to those observed in the global Spns2-KO mice. These data suggest that Spns2 functions in ECs to establish the S1P gradient required for T and B cells to egress from their respective primary lymphoid organs. Furthermore, Spns2 could be a therapeutic target for a broad array of inflammatory and autoimmune diseases.

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 freezefracture 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. mouse proximal tubule | tight junction | paracellular transport | Na/Cl transport | water transport T ight junctions (TJs) are circumferential seals around cells that selectively modulate paracellular permeability between extracellular compartments (1-3). On ultrathin-section electron microscopy, TJs appear as foci where the plasma membranes of neighboring cells make complete contact (4). On freeze-fracture electron microscopy, TJs appear as a continuous and anastomosing network of intramembranous particle strands (TJ strands) (5). These strands are mainly composed of linearly polymerized integral membrane proteins called claudins with molecular masses of ∼23 kDa (2, 3, 6). The claudin gene family contains more than 20 members in humans and in mice (2, 3, 7). The expression pattern of claudins varies considerably; most cell types express more than two claudins in various combinations to constitute mosaic TJ strands.Through the formation of TJ strands, claudins are directly involved in creating a primary barrier to the paracellular diffusion of solutes and water across epithelia (8). However, TJs are not a simple barrier: the barrier varies in tightness, measured by the transepithelial electrical resistance (R T ), and charge selectivity. Furuse et al. (9) reported that, when canine claudin-2 cDNA was transfected into high-resistance Madin-Darby canine kidney (MDCK) I cells primarily expressing claudins-1 and -4, the R T decreas...

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The trophectoderm (TE) is the first epithelium to be generated during mammalian early development. The TE works as a barrier that isolates the inner cell mass from the uterine environment and provides the turgidity of the blastocyst through elevated hydrostatic pressure. In this study, we investigated the role of tight junctions (TJs) in the barrier function of the TE during mouse blastocyst formation. RT-PCR and immunostaining revealed that the mouse TE expressed at least claudin 4, 6, and 7 among the 24 members of the claudin gene family, which encode structural and functional constituents of TJs. When embryos were cultured in the presence of a GST-fused C-terminal half of Clostridium perfringens enterotoxin (GST-C-CPE), a polypeptide with inhibitory activity to claudin 4 and 6, normal blastocyst formation was remarkably inhibited; the embryos had no or an immature blastocoel cavity without expansion, and blastomeres showed a rounded shape. In these embryos, claudin 4 and 6 proteins were absent from TJs and the barrier function of the TE was disrupted; however the basolateral localization of the Na+/K+-ATPase alpha1 subunit and aquaporin 3, which are thought to be involved in blastocyst formation, appeared normal. These results clearly demonstrate that the barrier function of TJs in the TE is required for normal blastocyst formation.