Basolateral sorting signals regulating tissue-specific polarity of heteromeric monocarboxylate transporters in epithelia

Many solute transporters are heterodimers composed of non-glycosylated catalytic and glycosylated accessory subunits. These transporters are specifically polarized to the apical or basolateral membranes of epithelia, but this polarity may vary to fulfill tissue-specific functions. To date, the mechanisms regulating the tissue-specific polarity of heteromeric transporters remain largely unknown. Here, we investigated the sorting signals that determine the polarity of three members of the proton-coupled monocarboxylate transporter (MCT) family, MCT1, MCT3 and MCT4, and their accessory subunit CD147. We show that MCT3 and MCT4 harbor strong redundant basolateral sorting signals (BLSS) in their C-terminal cytoplasmic tails that can direct fusion proteins with the apical marker p75 to the basolateral membrane. In contrast, MCT1 lacks a BLSS and its polarity is dictated by CD147, which contains a weak BLSS that can direct Tac, but not p75 to the basolateral membrane. Knockdown experiments in MDCK cells indicated that basolateral sorting of MCTs was clathrin-dependent but clathrin adaptor AP1B-independent. Our results explain the consistently basolateral localization of MCT3 and MCT4 and the variable localization of MCT1 in different epithelia. They introduce a new paradigm for the sorting of heterodimeric transporters in which a hierarchy of apical and BLSS in the catalytic and/or accessory subunits regulates their tissue-specific polarity.

AP1B sorts basolateral proteins in recycling and biosynthetic routes of MDCK cells

The epithelial-specific adaptor AP1B sorts basolateral proteins, but the trafficking routes where it performs its sorting role remain controversial. Here, we used an RNAi approach to knock down the medium subunit of AP1B (mu1B) in the prototype epithelial cell line Madin-Darby canine kidney (MDCK). Mu1B-knocked down MDCK cells displayed loss of polarity of several endogenous and exogenous basolateral markers transduced via adenovirus vectors, but exhibited normal polarity of apical markers. We chose two well characterized basolateral protein markers, the transferrin receptor (TfR) and the vesicular stomatitis virus G protein, to study the sorting role of AP1B. A surface-capture assay introduced here showed that mu1B-knocked down MDCK cells plated on filters at confluency and cultured for 4.5 d, sorted TfR correctly in the biosynthetic route but incorrectly in the recycling route. In contrast, these same cells missorted vesicular stomatitis virus G apically in the biosynthetic route. Strikingly, recently confluent MDCK cells (1-3 d) displayed AP1B-dependence in the biosynthetic route of TfR, which decreased with additional days in culture. Sucrose density gradient analysis detected AP1B predominantly in TfR-rich endosomal fractions in MDCK cells confluent for 1 and 4 d. Our results are consistent with the following model: AP1B sorts basolateral proteins in both biosynthetic and recycling routes of MDCK cells, as a result of its predominant functional localization in recycling endosomes, which constitute a post-Golgi station in the biosynthetic route of some plasma membrane proteins. TfR utilizes a direct route from Golgi to basolateral membrane that is established as the epithelial monolayer matures.

Viral protein U (Vpu)-mediated enhancement of human immunodeficiency virus type 1 particle release depends on the rate of cellular proliferation

Viral protein U (Vpu) is a 17-kDa phosphoprotein that enhances the release of viral particles from human immunodeficiency virus type 1-infected cells. This study shows that the effect of Vpu on efficient particle release depends on the rate of cell proliferation. Cells arrested by contact inhibition, chemical arresting agents, or terminal differentiation (i.e., macrophages) all exhibited a striking dependence on Vpu for efficient particle release, as shown by examination of particle production from transfections with full-length clones, infections, and the vaccinia virus expression system. In contrast, actively proliferating cells did not exhibit enhanced particle release with Vpu expression. This study demonstrates the necessity of Vpu for efficient viral particle release from quiescent cells.

New concepts in fibrinolysis and angiogenesis

Angiogenesis, the process by which new blood vessels form from preexisting vasculature, underlies a number of biologic processes including embryologic development, inflammation, wound healing, hypoxic retinal vascular proliferation, tumor growth, and atherosclerosis. The fibrinolytic system represents a cascade of serine protease activation events that culminate in the generation of plasmin. Although in-vitro studies suggest several possible roles that plasmin might play in angiogenesis, angiogenesis and fibrinolytic activity do not always correlate in in-vivo systems. During cutaneous and corneal wound healing, for example, angiogenesis proceeds normally in plasminogen-deficient animals. Similarly, the growth of most neoplasms is unimpaired in the absence of plasminogen. On the other hand, hypoxia-driven vascular proliferation may require plasmin-like activity, and angiogenesis within the atherosclerotic plaque seems to be associated with increased expression of fibrinolytic proteins. Recently, several nonplasmin fibrinolysins that may support the invasive phenotype of endothelial cells under specific circumstances have been identified. Thus, the contribution of individual fibrinolysins appears to be context-specific, just as the profile of endothelial cell gene expression depends upon the surrounding tissue milieu.

The N-terminal matrix domain of HIV-1 Gag is sufficient but not necessary for viral protein U-mediated enhancement of particle release through a membrane-targeting mechanism

Viral protein U (Vpu) is an 81 amino acid phosphoprotein found in human immunodeficiency virus type 1 (HIV-1)-infected cells. One function of Vpu is to enhance the release of virus particles from the plasma membrane in infected cells. Using subcellular fractionation, we observed that Vpu promotes the targeting of Pr55 Gag to the plasma membrane, the site of viral assembly. Deletions of Pr55, which removed most of the N-terminal matrix domain (p39) or the C-terminal domains of nucleocapsid and p6 (p41), still allowed for virus-like particle production. Moreover, the release of these particles remained Vpu-responsive. The N-terminal matrix (MA) domain of Gag, which contains its membrane-binding domain, is sufficient for Vpu-mediated enhanced release into the supernatant. Furthermore, a MA-GFP fusion protein showed enhanced membrane binding in the presence of Vpu. This demonstrates that Vpu action may be mediated by allowing Gag, specifically the N-terminal matrix domain, to efficiently associate with the plasma membrane. Thus MA appears sufficient but not necessary for Vpu-mediated enhanced particle release.

An ICAM-1 like cell adhesion molecule is responsible for CD34 positive haemopoietic stem cells adhesion to bone-marrow stroma

The microenvironment in the haematopoietic organs plays an important role in regulating and sustaining differentiation and self-renewal of haematopoietic stem cells. Although crucial for stem cell maintenance and homing, the stromal cell-stem cell interactions are poorly understood. Here we show that an ICAM-like molecule is responsible for stem cell adhesion to stromal cells in vitro. The molecule was characterized by a monoclonal antibody 3E10. Immunoblotting results indicated that the molecule had an electrophoretic mobility equal to that of intercellular cell adhesion molecule-1 (ICAM-1). Binding inhibition assays, however, showed that inhibition of binding of enriched CD34 cells by 3E10 was more prominent in comparison with that of ICAM-1.

Umbilical cord blood as a source of CD34 positive haematopoietic stem cells

In this paper we have used a monoclonal antibody to CD34 an antigen expressed solely on stem cells, and stem cell colony assays to show that umbilical cord blood has nearly the same number of functional stem cells as compared to normal bone-marrow. The number of CD34+ve cells in cord blood being 2 to 2.7 per cent, whereas bone-marrow had 3 to 3.5 per cent. The multi-potent colony forming cells (CFU-GEMM) were 60 +/- 18 in cord blood per 2 x 10(5) mononuclear cells (MNCs), whereas normal bone-marrow had 70 +/- 10 per 2 x 10(5) MNCs. Enrichment of these stem cells on Percoll gradients was successful for normal bone-marrow but not for cord blood.