Self-organized and directed branching results in optimal coverage in developing dermal lymphatic networks.

Branching morphogenesis is a ubiquitous process that gives rise to high exchange surfaces in the vasculature and epithelial organs. Lymphatic capillaries form branched networks, which play a key role in the circulation of tissue fluid and immune cells. Although mouse models and correlative patient data indicate that the lymphatic capillary density directly correlates with functional output, i.e., tissue fluid drainage and trafficking efficiency of dendritic cells, the mechanisms ensuring efficient tissue coverage remain poorly understood. Here, we use the mouse ear pinna lymphatic vessel network as a model system and combine lineage-tracing, genetic perturbations, whole-organ reconstructions and theoretical modeling to show that the dermal lymphatic capillaries tile space in an optimal, space-filling manner. This coverage is achieved by two complementary mechanisms: initial tissue invasion provides a non-optimal global scaffold via self-organized branching morphogenesis, while VEGF-C dependent side-branching from existing capillaries rapidly optimizes local coverage by directionally targeting low-density regions. With these two ingredients, we show that a minimal biophysical model can reproduce quantitatively whole-network reconstructions, across development and perturbations. Our results show that lymphatic capillary networks can exploit local self-organizing mechanisms to achieve tissue-scale optimization.

Glucomannan and beta-glucan degradation by Mytilus edulis Cel45A: Crystal structure and activity comparison with GH45 subfamily A, B and C.

The enzymatic hydrolysis of barley beta-glucan, konjac glucomannan and carboxymethyl cellulose by a ß-1,4-D-endoglucanase MeCel45A from blue mussel, Mytilus edulis, which belongs to subfamily B of glycoside hydrolase family 45 (GH45), was compared with GH45 members of subfamilies A (Humicola insolens HiCel45A), B (Trichoderma reesei TrCel45A) and C (Phanerochaete chrysosporium PcCel45A). Furthermore, the crystal structure of MeCel45A is reported. Initial rates and hydrolysis yields were determined by reducing sugar assays and product formation was characterized using NMR spectroscopy. The subfamily B and C enzymes exhibited mannanase activity, whereas the subfamily A member was uniquely able to produce monomeric glucose. All enzymes were confirmed to be inverting glycoside hydrolases. MeCel45A appears to be cold adapted by evolution, as it maintained 70% activity on cellohexaose at 4 °C relative to 30 °C, compared to 35% for TrCel45A. Both enzymes produced cellobiose and cellotetraose from cellohexaose, but TrCel45A additionally produced cellotriose.

IFNγ Regulates Activated Vδ2+ T Cells through a Feedback Mechanism Mediated by Mesenchymal Stem Cells.

γδ T cells play a role in a wide range of diseases such as autoimmunity and cancer. The majority of circulating human γδ T lymphocytes express a Vγ9Vδ2+ (Vδ2+) T cell receptor (TCR) and following activation release pro-inflammatory cytokines. In this study, we show that IFNγ, produced by Vδ2+ cells, activates mesenchymal stem cell (MSC)-mediated immunosupression, which in turn exerts a negative feedback mechanism on γδ T cell function ranging from cytokine production to proliferation. Importantly, this modulatory effect is limited to a short period of time (<24 hours) post-T cell activation, after which MSCs can no longer exert their immunoregulatory capacity. Using genetically modified MSCs with the IFNγ receptor 1 constitutively silenced, we demonstrate that IFNγ is essential to this process. Activated γδ T cells induce expression of several factors by MSCs that participate in the depletion of amino acids. In particular, we show that indolamine 2,3-dioxygenase (IDO), an enzyme involved in L-tryptophan degradation, is responsible for MSC-mediated immunosuppression of Vδ2+ T cells. Thus, our data demonstrate that γδ T cell responses can be immuno-modulated by different signals derived from MSC.

Human mesenchymal stromal cells modulate T-cell responses through TNF-α-mediated activation of NF-κB.

Although mesenchymal stromal cells (MSCs) possess the capacity to modulate immune responses, little is known about the mechanisms that underpin these processes. In this study, we show that immunosupression is mediated by activation of nuclear factor kappa B (NF-κB) in human MSCs. This pathway is activated by TNF-α that is generated following TCR stimulation of T cells. Inhibition of NF-κB through silencing of IκB kinase ß or the TNF-α receptor abolishes the immunosuppressive capacity of MSCs. Our data also indicate that MSC-associated NF-κB activation primarily leads to inhibition of T-cell proliferation with little effect on expression of the activation markers CD69 and CD25. Thus, our data support the hypothesis that the TNF-α/NF-κB signalling pathway is required for the initial priming of immunosuppressive function in human MSCs. Interestingly, drugs that interfere with NF-κB activation significantly antagonise the immunoregulatory effect of MSCs, which could have important implications for immunosuppression regimens in the clinic.

Human mesenchymal stromal cell-mediated immunoregulation: mechanisms of action and clinical applications.

Mesenchymal stromal cells (MSCs) are multipotent cells found in connective tissues that can differentiate into bone, cartilage, and adipose tissue. Interestingly, they can regulate immune responses in a paracrine way and allogeneic MSCs do not elicit immune response. These properties have encouraged a number of clinical trials in a broad range of regenerative therapies. Although these trials were first focused on their differentiation properties, in the last years, the immunosuppressive features have gained most of the attention. In this review, we will summarize the up-to-date knowledge about the immunosuppressive mechanisms of MSCs in vivo and in vitro and the most promising approaches in clinical investigation.

Histone deacetylase 3 modulates the expansion of human hematopoietic stem cells.

Epigenetic changes are regarded as emerging major players for hematopoietic stem cell (HSC) biology. Although some histone deacetylase (HDAC) inhibitors, such as valproic acid (VA), induce differentiation and apoptosis in a variety of leukemic cells in vitro, they produce a favorable effect on the expansion of normal HSCs. In this study, we have identified the VA target HDAC3 as a negative regulator of umbilical cord blood HSC expansion. We demonstrate that knockdown of the transcript dramatically improves CD34+ cell expansion, which correlates with a higher potential to generate colony-forming units in functional assays. We show that this effect is mediated at the level of primitive hematopoietic cells and that it is not due to negative effects on specific cell commitment or alterations in the cell cycle. HDAC3 inhibition does not block commitment to the monocytic lineage and the maturation of monocyte precursors, which are the main inhibited pathways in the presence of VA. Therefore, our results identify HDAC3 as a promising target for therapies aiming to expand HSCs.

Structure of the methyltransferase domain from the Modoc virus, a flavivirus with no known vector.

The Modoc virus (MODV) is a flavivirus with no known vector (NKV). Evolutionary studies have shown that the viruses in the MODV group have evolved in association with mammals (bats, rodents) without transmission by an arthropod vector. MODV methyltransferase is the first enzyme from this evolutionary branch to be structurally characterized. The high-resolution structure of the methyltransferase domain of the MODV NS5 protein (MTase(MODV)) was determined. The protein structure was solved in the apo form and in complex with its cofactor S-adenosyl-L-methionine (SAM). Although it belongs to a separate evolutionary branch, MTase(MODV) shares structural characteristics with flaviviral MTases from the other branches. Its capping machinery is a relatively new target in flaviviral drug development and the observed structural conservation between the three flaviviral branches indicates that it may be possible to identify a drug that targets a range of flaviviruses. The structural conservation also supports the choice of MODV as a possible model for flavivirus studies.

New crystal structures of human glutathione transferase A1-1 shed light on glutathione binding and the conformation of the C-terminal helix.

Human glutathione transferase A1-1 is a well studied enzyme, but despite a wealth of structural and biochemical data a number of aspects of its catalytic function are still poorly understood. Here, five new crystal structures of this enzyme are described that provide several insights. Firstly, the structure of a complex of the wild-type human enzyme with glutathione was determined for the first time at 2.0 angstroms resolution. This reveals that glutathione binds in the G site in a very similar fashion as the glutathione portion of substrate analogues in other structures and also that glutathione binding alone is sufficient to stabilize the C-terminal helix of the protein. Secondly, we have studied the complex with a decarboxylated glutathione conjugate that is known to dramatically decrease the activity of the enzyme. The T68E mutant of human glutathione transferase A1-1 recovers some of the activity that is lost with the decarboxylated glutathione, but our structures of this mutant show that none of the earlier explanations of this phenomenon are likely to be correct. Thirdly, and serendipitously, the apo structures also reveal the conformation of the crucial C-terminal region that is disordered in all previous apo structures. The C-terminal region can adopt an ordered helix-like structure even in the apo state, but shows a strong tendency to unwind. Different conformations of the C-terminal regions were observed in the apo states of the two monomers, which suggests that cooperativity could play a role in the activity of the enzyme.

Production of a truncated soluble human semicarbazide-sensitive amine oxidase mediated by a GST-fusion protein secreted from HEK293 cells.

Elevated levels of semicarbazide-sensitive amine oxidase (SSAO) activity have been observed in several human conditions such as congestive heart failure, diabetes mellitus, and inflammation. The reactive aldehydes and hydrogen peroxide produced by SSAO have been suggested to contribute to the progression of vascular complications associated with these conditions. In addition, SSAO activity has been shown to be involved in the leukocyte extravasation process at sites of inflammation. To facilitate characterization and development of specific and selective inhibitors of SSAO, we have developed a method for production of recombinant human SSAO. The extracellular region (residues 29-763) of human SSAO was expressed in HEK293 cells in fusion with a mutated Schistosoma japonicum glutathione S-transferase (GST) and secreted to the culture medium. The mutGST-SSAO fusion protein was purified in a single step by glutathione-affinity chromatography followed by site-specific cleavage using a GST-3C protease fusion protein to remove the mutGST fusion partner. A second glutathione-affinity chromatography step was then used to capture both the mutGST fusion partner and the GST-3C protease, resulting in milligram quantities of pure, enzymatically active, and soluble recombinant human SSAO.

Structure of human semicarbazide-sensitive amine oxidase/vascular adhesion protein-1.

Semicarbazide-sensitive amine oxidase (SSAO) belongs to a ubiquitous family of copper-containing amine oxidases (CuAOs). SSAO is also known as vascular adhesion protein-1 (VAP-1) and has been identified as one of the adhesion molecules involved in the leukocyte-extravasation process. The structure of a truncated soluble form of human SSAO has been solved and refined to 2.5 A. As expected, SSAO is a homodimer with a fold typical of the CuAO family. The topaquinone (TPQ) cofactor and a copper ion characteristic of CuAOs are present in the active site, with the TPQ in the active ;off-copper' conformation. The structure reveals that a leucine residue (Leu469) located adjacent to the active site could function as a gate controlling its accessibility. An RGD motif is displayed on the surface, where it could be involved in integrin binding and possibly play a role in the shedding of SSAO from the membrane. Carbohydrate moieties are observed at five of six potential N-glycosylation sites. Carbohydrates attached to Asn232 flank the active-site entrance and might influence substrate specificity. The structure of an adduct of SSAO and the irreversible inhibitor 2-hydrazinopyridine has been solved and refined to 2.9 A resolution. Together, these structures will aid efforts to identify natural substrates, provide valuable information for the design of specific inhibitors and direct further studies.

Crystallization of a truncated soluble human semicarbazide-sensitive amine oxidase.

Human semicarbazide-sensitive amine oxidase (SSAO) is a homodimeric copper-containing monoamine oxidase that occurs in both a membrane-bound and a soluble form. SSAO is also known as vascular adhesion protein-1 (VAP-1). A truncated soluble form of human SSAO (comprising residues 29-763) was expressed in human embryonic kidney 293 cells and purified to homogeneity. Tetragonal crystals were obtained and a data set extending to 2.5 A was collected. The crystals are merohedrally twinned and the estimation of the twinning fraction was complicated by pseudo-symmetry and the anisotropic character of the crystals. Using a recently developed method for twinning detection that is insensitive to phenomena such as anisotropy or pseudo-symmetry [Padilla & Yeates (2003), Acta Cryst. D59, 1124-1130], the twinning fraction was estimated to be 0.3. The structure was eventually solved by molecular replacement in space group P4(3).

Incorporation of a single His residue by rational design enables thiol-ester hydrolysis by human glutathione transferase A1-1.

A strategy for rational enzyme design is reported and illustrated by the engineering of a protein catalyst for thiol-ester hydrolysis. Five mutants of human glutathione (GSH; gamma-Glu-Cys-Gly) transferase A1-1 were designed in the search for a catalyst and to provide a set of proteins from which the reaction mechanism could be elucidated. The single mutant A216H catalyzed the hydrolysis of the S-benzoyl ester of GSH under turnover conditions with a k(cat)/K(M) of 156 M(-1) x min(-1), and a catalytic proficiency of >10(7) M(-1) when compared with the first-order rate constant of the uncatalyzed reaction. The wild-type enzyme did not hydrolyze the substrate, and thus, the introduction of a single histidine residue transformed the wild-type enzyme into a turnover system for thiol-ester hydrolysis. By kinetic analysis of single, double, and triple mutants, as well as from studies of reaction products, it was established that the enzyme A216H catalyzes the hydrolysis of the thiol-ester substrate by a mechanism that includes an acyl intermediate at the side chain of Y9. Kinetic measurements and the crystal structure of the A216H GSH complex provided compelling evidence that H216 acts as a general-base catalyst. The introduction of a single His residue into human GSH transferase A1-1 created an unprecedented enzymatic function, suggesting a strategy that may be of broad applicability in the design of new enzymes. The protein catalyst has the hallmarks of a native enzyme and is expected to catalyze various hydrolytic, as well as transesterification, reactions.

The crystal structure of Echinococcus granulosus fatty-acid-binding protein 1.

We describe the 1.6 A crystal structure of the fatty-acid-binding protein EgFABP1 from the parasitic platyhelminth Echinococcus granulosus. E. granulosus causes hydatid disease, which is a major zoonosis. EgFABP1 has been implicated in the acquisition, storage, and transport of lipids, and may be important to the organism since it is incapable of synthesising most of its lipids de novo. Moreover, EgFABP1 is a promising candidate for a vaccine against hydatid disease. The crystal structure reveals that EgFABP1 has the expected 10-stranded beta-barrel fold typical of the family of intracellular lipid-binding proteins, and that it is structurally most similar to P2 myelin protein. We describe the comparison of the crystal structure of EgFABP1 with these proteins and with an older homology model for EgFABP1. The electron density reveals the presence of a bound ligand inside the cavity, which we have interpreted as palmitic acid. The carboxylate group of the fatty acid interacts with the protein's P2 motif, consisting of a conserved triad R em leader R-x-Y. The hydrophobic tail of the ligand assumes a fairly flat, U-shaped conformation and has relatively few interactions with the protein.We discuss some of the structural implications of the crystal structure of EgFABP1 for related platyhelminthic FABPs.