Pierre Hubert

Peptide-based interference of the transmembrane domain of neuropilin-1 inhibits glioma growth in vivo.

Angiogenesis in glioblastoma is largely dependent on vascular endothelial growth factor (VEGF) signalling. Consistently, the VEGF coreceptor NRP1 promotes angiogenesis and tumour growth in gliomas. Here, we provide data showing that an innovative peptidic tool targeting the transmembrane domain of NRP1 efficiently blocks rat and human glioma growth in vivo. We show both in vivo and in vitro that the antitumour effect results from the anti-proliferative, anti-migratory and anti-angiogenic properties of the compound. The proposed NRP1 antagonizing peptide is therefore a promising novel class of anti-angiogenic drugs that might prolong glioma patient survival. Our results finally show for the first time that the transmembrane domain of important signalling receptors can be antagonized in vivo thereby providing a new avenue towards the development of atypical antagonists with strong therapeutic potential.

Single-spanning transmembrane domains in cell growth and cell-cell interactions More than meets the eye?

As a whole, integral membrane proteins represent about one third of sequenced genomes, and more than 50% of currently available drugs target membrane proteins, often cell surface receptors. Some membrane protein classes, with a defined number of transmembrane (TM) helices, are receiving much attention because of their great functional and pharmacological importance, such as G protein-coupled receptors possessing 7 TM segments. Although they represent roughly half of all membrane proteins, bitopic proteins (with only 1 TM helix) have so far been less well characterized. Though they include many essential families of receptors, such as adhesion molecules and receptor tyrosine kinases, many of which are excellent targets for biopharmaceuticals (peptides, antibodies, et al.). A growing body of evidence suggests a major role for interactions between TM domains of these receptors in signaling, through homo and heteromeric associations, conformational changes, assembly of signaling platforms, etc. Significantly, mutations within single domains are frequent in human disease, such as cancer or developmental disorders. This review attempts to give an overview of current knowledge about these interactions, from structural data to therapeutic perspectives, focusing on bitopic proteins involved in cell signaling.

The architecture of Rhodobacter sphaeroides chromatophores

The chromatophores of Rhodobacter (Rb.) sphaeroides represent a minimal bio-energetic system, which efficiently converts light energy into usable chemical energy. Despite extensive studies, several issues pertaining to the morphology and molecular architecture of this elemental energy conversion system remain controversial or unknown. To tackle these issues, we combined electron microscope tomography, immuno-electron microscopy and atomic force microscopy. We found that the intracellular Rb. sphaeroides chromatophores form a continuous reticulum rather than existing as discrete vesicles. We also found that the cytochrome bc1 complex localizes to fragile chromatophore regions, which most likely constitute the tubular structures that interconnect the vesicles in the reticulum. In contrast, the peripheral light-harvesting complex 2 (LH2) is preferentially hexagonally packed within the convex vesicular regions of the membrane network. Based on these observations, we propose that the bc1 complexes are in the inter-vesicular regions and surrounded by reaction center (RC) core complexes, which in turn are bounded by arrays of peripheral antenna complexes. This arrangement affords rapid cycling of electrons between the core and bc1 complexes while maintaining efficient excitation energy transfer from LH2 domains to the RCs.

Watching intracellular lipolysis in mycobacteria using time lapse fluorescence microscopy

The fact that Mycobacterium tuberculosis mobilizes lipid bodies (LB) located in the cytosol during infection process has been proposed for decades. However, the mechanisms and dynamics of mobilization of these lipid droplets within mycobacteria are still not completely characterized. Evidence in favour of this characterization was obtained here using a combined fluorescent microscopy and computational image processing approach. The decrease in lipid storage levels observed under nutrient depletion conditions was correlated with a significant increase in the size of the bacteria. LB fragmentation/condensation cycles were monitored in real time. The exact contribution of lipases in this process was confirmed using the lipase inhibitor tetrahydrolipstatin, which was found to prevent LB degradation and to limit the bacterial cell growth. The method presented here provides a powerful tool for monitoring in vivo lipolysis in mycobacteria and for obtaining new insights on the growth of cells and their entry into the dormant or reactivation phase. It should be particularly useful for studying the effects of chemical inhibitors and activators on cells as well as investigating other metabolic pathways.

Encapsulation of insulin for oral administration preserves interaction of the hormone with its receptor in vitro

It has been shown that insulin associated with nanocapsules of isobutylcyanoacrylate retains biological activity after oral administration to diabetic rats from 6 to 21 days. Because part of this action is unexplained, we focused on the interaction of encapsulated insulin with the insulin receptor in vitro. We have shown that encapsulated insulin is able 1) to bind to insulin receptors both in rat liver plasma membranes and after solubilization from Chinese hamster ovary (CHO) cells transfected with the gene of human insulin receptor, 2) to accelerate 125I-labeled insulin dissociation from its receptor, and 3) to ensure transduction of a signal leading to stimulation of the β-subunit phosphorylation, with parameters similar to those of native insulin. In addition, encapsulated 125I-insulin was rapidly internalized in transfected CHO cells. Analysis of cell-associated radioactivity showed that encapsulated insulin remained largely intact (> 80%) after 3 h, whereas native insulin was mostly degraded. These data indicate that encapsulated insulin fulfills all the earliest events at the receptor level leading to biological actions and suggests that encapsulation protects insulin against insulin degradation inside the cells.

Molecular mechanisms of Tau binding to microtubules and its role in microtubule dynamics in live cells.

Summary Despite extensive studies, the molecular mechanisms of Tau binding to microtubules (MTs) and its consequences on MT stability still remain unclear. It is especially true in cells where the spatiotemporal distribution of Tau–MT interactions is unknown. Using Förster resonance energy transfer (FRET), we showed that the Tau–MT interaction was distributed along MTs in periodic hotspots of high and low FRET intensities. Fluorescence recovery after photobleaching (FRAP) revealed a two-phase exchange of Tau with MTs as a rapid diffusion followed by a slower binding phase. A real-time FRET assay showed that high FRET occurred simultaneously with rescue and pause transitions at MT ends. To further explore the functional interaction of Tau with MTs, the binding of paclitaxel (PTX), tubulin acetylation induced by trichostatin A (TSA), and the expression of non-acetylatable tubulin were used. With PTX and TSA, FRAP curves best fitted a single phase with a long time constant, whereas with non-acetylatable &agr;-tubulin, curves best fitted a two phase recovery. Upon incubation with PTX and TSA, the number of high and low FRET hotspots decreased by up to 50% and no hotspot was observed during rescue and pause transitions. In the presence of non-acetylatable &agr;-tubulin, a 34% increase in low FRET hotspots occurred, and our real-time FRET assay revealed that low FRET hotspots appeared with MTs recovering growth. In conclusion, we have identified, by FRET and FRAP, a discrete Tau–MT interaction, in which Tau could induce conformational changes of MTs, favoring recovery of MT self-assembly.

Lipid composition in liver and brain of genetically obese (ob/ob), heterozygote (ob/+) and normal (+/+) mice

Lipid composition was studied in liver and brain of normal (+/+), heterozygote (ob/+) and obese (ob/ob) mice. It was found that this genetic defect is expressed differently in the lipid composition of these organs. Cholesterol is increased in liver but strongly decreased in brain of obese animals. Phosphatide fatty acid composition is modified in liver and not in brain. In contrast, phospholipids and total ganglioside sialic acid are affected similarly in both organs. Although clinically normal, heterozygote (ob/+) mice already show an abnormal lipid composition in liver and brain. The potential importance of these results is presented.

Transmembrane recognition of the semaphorin co-receptors neuropilin 1 and plexin A1: coarse-grained simulations.

The cancer associated class 3 semaphorins require direct binding to neuropilins and association to plexins to trigger cell signaling. Here, we address the role of the transmembrane domains of neuropilin 1 and plexin A1 for the dimerization of the two receptors by characterizing the assembly in lipid bilayers using coarse-grained molecular dynamics simulations. From experimental evidence using a two-hybrid system showing the biochemical association of the two receptors transmembrane domains, we performed molecular simulations in DOPC and POPC demonstrating spontaneously assembly to form homodimers and heterodimers with a very high propensity for right-handed packing of the helices. Inversely, left-handed packing was observed with a very low propensity. This mode of packing was observed uniquely when the plexin A1 transmembrane domain was involved in association. Potential of mean force calculations were used to predict a hierarchy of self-association for the monomers: the two neuropilin 1 transmembrane domains strongly associated, neuropilin 1 and plexin A1 transmembrane domains associated less and the two plexin A1 transmembrane domains weakly but significantly associated. We demonstrated that homodimerization and heterodimerization are driven by GxxxG motifs, and that the sequence context modulates the packing mode of the plexin A1 transmembrane domains. This work presents major advances towards our understanding of membrane signaling platforms assembly through membrane domains and provides exquisite information for the design of antagonist drugs defining a novel class of therapeutic agents.

Transmembrane domain targeting peptide antagonizing ErbB2/Neu inhibits breast tumor growth and metastasis.

Breast cancer is still a deadly disease despite major achievements in targeted therapies designed to block ligands or ligand-binding subunits of major tyrosine kinase receptors. Relapse is significant and metastases deleterious, which demands novel strategies for fighting this disease. Here, we report a proof-of-concept experiment demonstrating that small peptides interfering with the transmembrane domain of the tyrosine kinase epidermal growth factor receptor ErbB2 exhibit anticancer properties when used at micromolar dosages in a genetically engineered mouse model of breast cancer. Different assays demonstrate the specificity of the ErbB2-targeting peptide, which induces long-term reduction of ErbB2 phosphorylation and Akt signaling consistent with reduced tumor cell proliferation and increased survival. Microcomputed tomography analysis established the antimetastatic activity of the peptide and its impact on primary tumor growth. This reveals the interior of the cell membrane as an unexplored dimension for drug design.

Substitution of the insulin receptor transmembrane domain with that of glycophorin A inhibits insulin action.

To study the role of transmembrane (TM) domains interactions in the activation of the insulin receptor, we have replaced the insulin receptor TM domain with that of glycophorin A (GpA), an erythrocyte protein that spontaneously forms detergent‐resistant dimers through TM–TM interactions. Insulin receptor cDNA sequences with the TM domain replaced by that of GpA were constructed and stably transfected in CHO cells. Insulin binding to cells and solubilized receptors was not modified. Electrophoresis after partial reduction of disulfide bonds revealed an altered structure for the soluble chimeric receptors, seen as an altered mobility apparently due to increased interactions between the β subunits of the receptor. Insulin signaling was markedly decreased for cells transfected with chimeric receptors compared with cells transfected with normal receptors. A decrease in insulin‐induced receptor kinase activity was observed for solubilized chimeric receptors. In conclusion, substitution by the native GpA TM domain of the insulin receptor results in structurally modified chimeric receptors that are unable to transmit the insulin signal properly. It is hypothesized that this substitution may impose structural constraints that prevent the proper changes in conformation necessary for activation of the receptor kinase. Other mutants modifying the structure or the membrane orientation of the glycophorin A TM domain are required to better understand these constraints.—Gardin, A., Auzan, C., Clauser, E., Malherbe, T., Aunis, D., Crémel, G., Hubert, P. Substitution of the insulin receptor transmembrane domain with that of glycophorin A inhibits insulin action. FASEB J. 13, 1347–1357 (1999)