Angiogenesis promoted by vascular endothelial growth factor: Regulation through α1β1 and α2β1 integrins

Vascular endothelial growth factor (VEGF), also known as vascular permeability factor, is a cytokine of central importance for the angiogenesis associated with cancers and other pathologies. Because angiogenesis often involves endothelial cell (EC) migration and proliferation within a collagen-rich extracellular matrix, we investigated the possibility that VEGF promotes neovascularization through regulation of collagen receptor expression. VEGF induced a 5- to 7-fold increase in dermal microvascular EC surface protein expression of two collagen receptors—the α₁β₁ and α₂β₁ integrins—through induction of mRNAs encoding the α₁ and α₂ subunits. In contrast, VEGF did not induce increased expression of the α₃β₁ integrin, which also has been implicated in collagen binding. Integrin α₁-blocking and α₂-blocking antibodies (Ab) each partially inhibited attachment of microvascular EC to collagen I, and α₁-blocking Ab also inhibited attachment to collagen IV and laminin-1. Induction of α₁β₁ and α₂β₁ expression by VEGF promoted cell spreading on collagen I gels which was abolished by a combination of α₁-blocking and α₂-blocking Abs. In vivo, a combination of α₁-blocking and α₂-blocking Abs markedly inhibited VEGF-driven angiogenesis; average cross-sectional area of individual new blood vessels was reduced 90% and average total new vascular area was reduced 82% without detectable effects on the pre-existing vasculature. These data indicate that induction of α₁β₁ and α₂β₁ expression by EC is an important mechanism by which VEGF promotes angiogenesis and that α₁β₁ and α₂β₁ antagonists may prove effective in inhibiting VEGF-driven angiogenesis in cancers and other important pathologies.     

Reciprocal regulation of Gsα by palmitate and the βγ subunit

Hormonal activation of G_s, the stimulatory regulator of adenylyl cyclase, promotes dissociation of α_s from Gβγ, accelerates removal of covalently attached palmitate from the Gα subunit, and triggers release of a fraction of α_s from the plasma membrane into the cytosol. To elucidate relations among these three events, we assessed biochemical effects in vitro of attached palmitate on recombinant α_s prepared from Sf9 cells. In comparison to the unpalmitoylated protein (obtained from cytosol of Sf9 cells, treated with a palmitoyl esterase, or expressed as a mutant protein lacking the site for palmitoylation), palmitoylated α_s (from Sf9 membranes, 50% palmitoylated) was more hydrophobic, as indicated by partitioning into TX-114, and bound βγ with 5-fold higher affinity. βγ protected GDP-bound α_s, but not α_s· GTP[γS], from depalmitoylation by a recombinant esterase. We conclude that βγ binding and palmitoylation reciprocally potentiate each other in promoting membrane attachment of α_s and that dissociation of α_s·GTP from βγ is likely to mediate receptor-induced α_s depalmitoylation and translocation of the protein to cytosol in intact cells.     

α- and β-adrenergic stimulation of arachidonic acid metabolism in cells in culture

Madin-Darby canine kidney cells (MDCK) synthesize prostaglandin (PG) F(2alpha), PGI(2) (measured as 6-keto-PGE(1alpha)), PGE(2), PGD(2), and thromboxane A(2) (measured as thromboxane B(2)). When incubated in the presence of norepinephrine (6 muM), the syntheses of these arachidonic acid metabolites are stimulated 3-fold. Norepinephrine's effect can be antagonized by the addition of alpha-adrenergic receptor blocking agents (phenoxybenzamine>phentolamine>yohimbine>dibenamine>tolazoline) but not by the beta-adrenergic blocking drug propranolol. Norepinephrine's stimulation is also inhibited by low concentrations of dihydroergotamine, bromocryptine, ergocryptine, and ergotamine. The stimulation of PG synthesis by norepinephrine is reversible, continues during the 24 hr of incubation, and requires the presence of norepinephrine at the receptor site but it is not blocked by the addition of colchicine, cytochalasin B, or cycloheximide. Neither phenoxybenzamine nor ergotamine at concentrations that block norepinephrine's stimulation of PG biosynthesis suppresses the increase in PG synthesis induced by exogenous arachidonic acid, suggesting that the alpha-adrenergic regulation is not occurring primarily at the cyclooxygenase step in the metabolism of arachidonic acid. In mouse lymphoma cells (WEHI-5), low concentrations of isoproterenol or norepinephrine stimulate the synthesis of thromboxane, an effect that can be blocked by the addition of propranolol but not by relatively high concentrations of phenoxybenzamine or ergotamine. Taken together, these results suggest that alpha-adrenergic receptor stimulation promotes the deacylation of phospholipids by MDCK cells whereas beta-adrenergic mechanisms lead to activation of similar pathways in WEHI-5 cells.     

Optimal strategies for carrier screening and prenatal diagnosis of α- and β-thalassemia

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Novel α + β Zr Alloys with Enhanced Strength

Low-alloyed zirconium alloys are widely used in nuclear applications due to their low neutron absorption cross-section. These alloys, however, suffer from limited strength. Well-established guidelines for the development of Ti alloys were applied to design new two-phase ternary Zr alloys with improved mechanical properties. Zr-4Sn-4Nb and Zr-8Sn-4Nb alloys have been manufactured by vacuum arc melting, thermo-mechanically processed by annealing, forging, and aging to various microstructural conditions and thoroughly characterized. Detailed Scanning electron microscopy (SEM) analysis showed that the microstructural response of the alloys is rather similar to alpha + beta Ti alloys. Duplex microstructure containing primary alpha phase particles surrounded by lamellar alpha + beta microstructure can be achieved by thermal processing. Mechanical properties strongly depend on the previous treatment. Ultimate tensile strength exceeding 700 MPa was achieved exceeding the strength of commercial Zr alloys for nuclear applications by more than 50%. Such an improvement in strength more than compensates for the increased neutron absorption cross-section. This study aims to exploit the potential of alpha + beta Zr alloys for nuclear applications.     

Multivalent structure of an αβT cell receptor

Whether there is one or multiple αβT cell antigen receptor (TCR) recognition modules in a given TCR/CD3 complex is a long-standing controversy in immunology. We show that T cells from transgenic mice that coexpress comparable amounts of two distinct TCRβ chains incorporate at least two αβTCRs in a single TCR/CD3 complex. Evidence for bispecific αβTCRs was obtained by immunoprecipitation and immunoblotting and confirmed on the surface of living cells both by fluorescence resonance energy transfer and comodulation assays by using antibodies specific for TCRβ-variable regions. Such (αβ)₂TCR/CD3 or higher-order complexes were evident in T cells studied either ex vivo or after expansion in vitro. T cell activation is thought by many, but not all, to require TCR cross-linking by its antigen/major histocompatibility complex ligand. The implications of a multivalent (αβ)₂TCR/CD3 complex stoichiometry for the ordered docking of specific antigen/major histocompatibility complex, CD4, or CD8 coreceptors and additional TCRs are discussed.     

Probing of β-adrenergic receptors by novel fluorescent β-adrenergic blockers

The synthesis of two high-affinity fluorescent beta-adrenergic blockers is described: dl-N(1)-[2-hydroxy-3-(1-naphthyloxy)propyl]-N(2)-(9-acridyl)-1,2-propanediamine (9-aminoacridylpropanolol, 9-AAP) and dl-N-[2-hydroxy-3-(1-naphthyloxy)propyl]-N'-dansylethylenediamine (dansyl analogue of propranolol, DAPN). Both 9-AAP and DAPN inhibit competitively the l-epinephrine-dependent adenylate cyclase activity [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] in turkey erythrocyte membranes without affecting the fluoride-stimulated adenylate cyclase activity. Similarly, 9-AAP and DAPN inhibit in a competitive manner the binding of [(125)I]-iodohydroxybenzylpindolol to these beta-adrenergic receptors. The two fluorescent beta-adrenergic blockers 9-AAP and DAPN probe specifically beta-adrenergic receptors in the central nervous system as well as in other organs when injected into rats. The fluorescence pattern can be monitored by fluorescence microscopy performed on cryostat slices of these organs. The appearance of the characteristic fluorescence pattern can be blocked in a stereospecific fashion by a prior injection of l-propranolol and not by a prior injection of d-propranolol. These compounds therefore offer a powerful means to map beta-adrenergic receptors in vivo. The stereospecific displacement of 9-AAP from the beta-adrenergic receptors of turkey erythrocyte membranes by l-propranolol and by l-epinephrine can be detected in vitro using front-face fluorescence. The potential use of these compounds to probe beta-receptors in vitro and in vivo is discussed.     

β-Trcp couples β-catenin phosphorylation-degradation and regulates Xenopus axis formation

Regulation of beta-catenin stability is essential for Wnt signal transduction during development and tumorigenesis. It is well known that serine-phosphorylation of beta-catenin by the Axin-glycogen synthase kinase (GSK)-3beta complex targets beta-catenin for ubiquitination-degradation, and mutations at critical phosphoserine residues stabilize beta-catenin and cause human cancers. How beta-catenin phosphorylation results in its degradation is undefined. Here we show that phosphorylated beta-catenin is specifically recognized by beta-Trcp, an F-box/WD40-repeat protein that also associates with Skp1, an essential component of the ubiquitination apparatus. beta-catenin harboring mutations at the critical phosphoserine residues escapes recognition by beta-Trcp, thus providing a molecular explanation for why these mutations cause beta-catenin accumulation that leads to cancer. Inhibition of endogenous beta-Trcp function by a dominant negative mutant stabilizes beta-catenin, activates Wnt/beta-catenin signaling, and induces axis formation in Xenopus embryos. Therefore, beta-Trcp plays a central role in recruiting phosphorylated beta-catenin for degradation and in dorsoventral patterning of the Xenopus embryo.     

The α chain of laminin-1 is independently secreted and drives secretion of its β- and γ-chain partners

A mammalian recombinant strategy was established to dissect rules of basement membrane laminin assembly and secretion. The α-, β-, and γ-chain subunits of laminin-1 were expressed in all combinations, transiently and/or stably, in a near-null background. In the absence of its normal partners, the α chain was secreted as intact protein and protein that had been cleaved in the coiled-coil domain. In contrast, the β and γ chains, expressed separately or together, remained intracellular with formation of ββ or βγ, but not γγ, disulfide-linked dimers. Secretion of the β and γ chains required simultaneous expression of all three chains and their assembly into αβγ heterotrimers. Epitope-tagged recombinant α subunit and recombinant laminin were affinity-purified from the conditioned medium of αγ and αβγ clones. Rotary-shadow electron microscopy revealed that the free α subunit is a linear structure containing N-terminal and included globules with a foreshortened long arm, while the trimeric species has the typical four-arm morphology of native laminin. We conclude that the α chain can be delivered to the extracellular environment as a single subunit, whereas the β and γ chains cannot, and that the α chain drives the secretion of the trimeric molecule. Such an α-chain-dependent mechanism could allow for the regulation of laminin export into a nascent basement membrane, and might serve an important role in controlling basement membrane formation.     

Cross-α/β polymorphism of PSMα3 fibrils

The formation of ordered cross-β amyloid protein aggregates is associated with a variety of human disorders. While conventional infrared methods serve as sensitive reporters of the presence of these amyloids, the recently discovered amyloid secondary structure of cross-α fibrils presents new questions and challenges. Herein, we report results using Fourier transform infrared spectroscopy and two-dimensional infrared spectroscopy to monitor the aggregation of one such cross-α–forming peptide, phenol soluble modulin alpha 3 (PSMα3). Phenol soluble modulins (PSMs) are involved in the formation and stabilization of Staphylococcus aureus biofilms, making sensitive methods of detecting and characterizing these fibrils a pressing need. Our experimental data coupled with spectroscopic simulations reveals the simultaneous presence of cross-α and cross-β polymorphs within samples of PSMα3 fibrils. We also report a new spectroscopic feature indicative of cross-α fibrils.

Amyloids are elongated fibers of proteins or peptides typically composed of stacked cross β-sheets (1, 2). Self-assembling amyloids are notorious for their involvement in human neurodegenerative diseases such as Alzheimer’s and Parkinson’s diseases (1, 2). Phenol soluble modulins (PSMs) are amyloid peptides secreted by the bacteria Staphylococcus aureus (S. aureus) (3–5). Of the PSM family, PSMα3 is of recent interest due to its unique secondary structure upon fibrillation. Whereas other PSM variants undergo conformational changes with aggregation, the α-helical PSMα3 peptide retains its secondary structure while stacking in a manner reminiscent of β-sheets, forming what has been termed cross-α fibrils (3, 4, 6). Although “α-sheet” amyloid fibrils have been previously observed in two-dimensional infrared (2DIR) (7) and associated with PSMs (8), the novel cross-α fibril is distinct from that class of structures. To avoid confusion between these two similarly named but distinct secondary structures, a comparison between the α-sheet domain in cytosolic phosphatase A2 (9) (Protein Data Bank [PDB] identification:1rlw) (10) and cross-α fibrils adopted by PSMα3 (PDB ID:5i55) (3) has been highlighted in SI Appendix, Fig. S1. Interestingly, shorter terminations of PSMα3 have been shown to exhibit β-sheet polymorphs (11). The proposed cross-α fibril structure of the full-length PSMα3 peptide has been confirmed with X-ray diffraction and circular dichroism (4). The present study aims to further characterize these fibrils with linear and nonlinear infrared spectroscopies.S. aureus is an infectious human pathogen with the ability to form communities of microorganisms called biofilms that hinder traditional treatment methods (12–14). PSMs contribute to inflammatory response and play a crucial role in structuring and detaching biofilms (11, 12, 14). While biofilm growth requires the presence of multiple PSMs (14, 15), Andreasen and Zaman have demonstrated that PSMα3 acts as a scaffold, seeding the amyloid formation of other PSMs (5). To effectively inhibit S. aureus biofilm growth, a better understanding of PSMα3 aggregation is needed.The α-helical structure of PSMα3 (12) presents a challenge for probing the vibrational modes and secondary structure of both the monomer and the fibrils. While IR spectroscopy has been used extensively to characterize β-sheets (16–19), the spectral features associated with α-helices are difficult to distinguish from those of the random coil secondary structure (20, 21). This limitation has left researchers to date with an incomplete picture of the spectroscopic features unique to cross-α fibers. The present work combines a variety of 2DIR methods to remove these barriers and probe the active infrared vibrational modes of cross-α fibers.The full-length, 22-residue PSMα3 peptide was synthesized and prepared for aggregation studies following reported methods (3, 4, 11). A total of 10 mM PSMα3 was incubated in D₂O at room temperature over 7 d. These data were compared to the monomer treated under similar conditions. Monomeric samples were prepared at a significantly lower concentration of 0.5 mM to prevent aggregation. Fiber formation was confirmed by transmission electron microscopy (see SI Appendix, Fig. S2 for details). Fourier transform infrared (FTIR) spectra were taken for both the fibrils in solution as well as the low concentration monomers. Spectroscopic simulations of the PSMα3 monomer and fibers were performed on previously reported PDB structures (PDB identification: 5i55) (3) (Fig. 1).Open in a separate windowFig. 1.PDB structures of PSMα3 (A) monomers and (B) cross-α fibers extended along the screw axis. (C) FTIR spectra of 0.5 mM monomeric PSMα3 (blue) compared to the 10 mM PSMα3 fibril (red) in D₂O upon aggregation.     

β1- and α6-integrin are surface markers on mouse spermatogonial stem cells

Although spermatogenesis is essential for reproduction, little is known about spermatogonial stem cells. These cells provide the basis for spermatogenesis throughout adult life by undergoing self-renewal and by providing progeny that differentiate into spermatozoa. A major impediment to our understanding of the biology of these stem cells is the inability to distinguish them from spermatogonia that are committed to differentiation. We made use of the known association of stem cells with basement membranes and our spermatogonial transplantation assay system to identify specific molecular markers on the stem cell surface. Selection of mouse testis cells with anti-beta1- or anti-alpha6-integrin antibody, but not anti-c-kit antibody, produced cell populations with a significantly enhanced ability to colonize recipient testes and generate donor cell-derived spermatogenesis. We demonstrate spermatogonial stem cell-associated antigens by using an assay system based on biological function. Furthermore, the presence of surface integrins on spermatogonial stem cells suggests that these cells share elements of a common molecular machinery with stem cells in other tissues.     

Streaming potential measurements in αβγ-rat epithelial Na+ channel in planar lipid bilayers

Streaming potentials across cloned epithelial Na⁺ channels (ENaC) incorporated into planar lipid bilayers were measured. We found that the establishment of an osmotic pressure gradient (Δπ) across a channel-containing membrane mimicked the activation effects of a hydrostatic pressure differential (ΔP) on αβγ-rENaC, although with a quantitative difference in the magnitude of the driving forces. Moreover, the imposition of a Δπ negates channel activation by ΔP when the Δπ was directed against ΔP. A streaming potential of 2.0 ± 0.7 mV was measured across αβγ-rat ENaC (rENaC)-containing bilayers at 100 mM symmetrical [Na⁺] in the presence of a 2 Osmol/kg sucrose gradient. Assuming single file movement of ions and water within the conduction pathway, we conclude that between two and three water molecules are translocated together with a single Na⁺ ion. A minimal effective pore diameter of 3 Å that could accommodate two water molecules even in single file is in contrast with the 2-Å diameter predicted from the selectivity properties of αβγ-rENaC. The fact that activation of αβγ-rENaC by ΔP can be reproduced by the imposition of Δπ suggests that water movement through the channel is also an important determinant of channel activity.     

α2-macroglobulin associates with β-amyloid peptide and prevents fibril formation

We have used the yeast two-hybrid system to isolate cDNAs encoding proteins that specifically interact with the 42-aa β-amyloid peptide (Aβ), a major constituent of senile plaques in Alzheimer’s disease. The carboxy terminus of α₂-macroglobulin (α2M), a proteinase inhibitor released in response to inflammatory stimuli, was identified as a strong and specific interactor of Aβ, utilizing this system. Direct evidence for this interaction was obtained by co-immunoprecipitation of α2M with Aβ from the yeast cell, and by formation of SDS-resistant Aβ complexes in polyacrylamide gels by using synthetic Aβ and purified α2M. The association of Aβ with α2M and various purified amyloid binding proteins was assessed by employing a method measuring protein–protein interactions in liquid phase. The dissociation constant by this technique for the α2M–Aβ association using labeled purified proteins was measured (K_d = 350 nM). Electron microscopy showed that a 1:8 ratio of α2M to Aβ prevented fibril formation in solution; the same ratio to Aβ of another acute phase protein, α₁-antichymotrypsin, was not active in preventing fibril formation in vitro. These results were corroborated by data obtained from an in vitro aggregation assay employing Thioflavine T. The interaction of α2M with Aβ suggests new pathway(s) for the clearance of the soluble amyloid peptide.     

Interferon γ induces differential upregulation of α and β chemokine secretion in colonic epithelial cell lines

Background—Production of chemoattractant factorsby the intestinal epithelium may contribute to mucosal infiltration byinflammatory cells in inflammatory bowel disease. Secretion of the α chemokine interleukin 8 (IL-8), a neutrophil chemoattractant, has beenwidely studied, but little is known about epithelial secretion of β chemokines, which are preferentially involved in recruiting monocytes.
Aims—To investigate the profiles of α and β chemokine secretion in colonic cell lines and their differentialmodulation by interferon γ (IFN-γ), a product of activated Tlymphocytes and natural killer cells.
Methods and results—HT29-19A, a model of theCl secretory crypt cell, exhibited a parallel secretionof the α chemokines IL-8 and GROα, which could be markedlyupregulated by tumour necrosis factor α (TNF-α) and IL-1β. Thesecells showed no significant expression of the β chemokines RANTES(regulated upon activation T cell expressed and secreted), MIP-1α(macrophage inflammatory protein 1α), and MCP-1 (monocyte chemotacticprotein 1) under these conditions, but IFN-γ in combination withTNF-α caused a dose dependent induction of RANTES and MCP-1secretion. This was accompanied by a marked increase of RANTES mRNA. Incontrast, IFN-γ had no significant effect on TNF-α stimulated IL-8secretion. Caco-2 cells, with features more typical of villusabsorptive cells, were relatively poor secretors of α chemokines butsecreted high levels of MCP-1 in response to IL-1β. IFN-γ did notinfluence α or β chemokine secretion in these cells.
Conclusions—These studies suggest that intestinalepithelial cells may produce chemokines capable of attracting bothneutrophils and monocytes. The ability of IFN-γ to activate theexpression of β chemokines preferentially could facilitate thedevelopment of chronic inflammatory infiltrates.

Keywords:inflammatory bowel disease; RANTES; interferongamma; chemokine

     

Anti-β2 glycoprotein I (β2GPI) autoantibodies recognize an epitope on the first domain of β2GPI

Anticardiolipin (aCL) autoantibodies are associated with thrombosis, recurrent fetal loss, and thrombocytopenia. Only aCL found in autoimmune disease require the participation of the phospholipid binding plasma protein β2 glycoprotein I (β2GPI) for antibody binding and now are called anti-β2GPI. The antigenic specificity of aCL affinity purified from 11 patients with high titers was evaluated in an effort to better understand the pathophysiology associated with aCL. Seven different recombinant domain-deleted mutants of human β2GPI, and full length human β2GPI (wild-type), were used in competition assays to inhibit the autoantibodies from binding to immobilized wild-type β2GPI. Only those domain-deleted mutants that contained domain 1 inhibited the binding to immobilized wild-type β2GPI from all of the patients. The domain-deleted mutants that contained domain 1 inhibited all aCL in a similar but not identical pattern, suggesting that these aCL recognize a similar, but distinguishable, epitope(s) present on domain 1.