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S Calvet

Publications and source records attributed to S Calvet.

14 recordsLinked to original sources
PubMed1998

Identification of a signaling pathway activated specifically in the somatodendritic compartment by a heparan sulfate that regulates dendrite growth.

In two earlier reports we demonstrated that natural heparan sulfate, but not dermatan or chondroitin sulfate glycosaminoglycans, stimulate axonal elongation and inhibit dendrite growth in vitro (Lafont et al., 1992). The latter specific effect on dendrite elongation was reproduced by chemically synthesized heparan sulfates and by SR 80037A, a purified sulfated and hexanoylated heparin fragment (Lafont et al., 1994). Adding radioactive SR 80037A to purified neurons demonstrated the existence, at the neuronal surface, of heparan sulfate-specific and saturable binding sites, suggesting that SR 80037A activates specific signal transduction pathways. In the present study, using rat or mouse neurons from the embryonic cortex, we show that SR 80037A signaling involves one or several G-coupled receptor or receptors, small GTPases rhoA and/or rhoC, and one or several PKCs. We also demonstrate that the rapid soma rounding elicited by SR 80037A does not require protein synthesis but that the long-term effect on dendrite initiation requires protein synthesis in a short period after the addition of the heparan sulfate. Finally, by preparing membranes from the somatodendritic or axonal compartments we demonstrate that the identified signaling pathway is activated by SR 80037A primarily in the somatodendritic compartment and is not sensitive to the addition of a dermatan sulfate glycosaminoglycan that does not induce the axonal phenotype by impairing dendrite initiation and elongation.

Adenosine Diphosphate↗
PubMed1997

Association of Engrailed homeoproteins with vesicles presenting caveolae-like properties.

We report here that the homeoproteins Engrailed-1 and Engrailed-2 are present in specific non-nuclear subcellular compartments. Using electron microscopy, we observed that chick-Engrailed-2 expressed in COS-7 cells associates with membrane fractions that are characterized as caveolae. This characterization is based on morphological, biochemical and immunological criteria such as, in particular, the absence of clathrin coat and the presence of caveolin and cholera toxin-binding sites. These data are fully confirmed by subcellular fractionation experiments, which demonstrate that transfected chick-Engrailed-2 is present in low density membrane fractions that are resistant to Triton X-100, enriched in caveolin and solubilized by the addition of a cholesterol-binding detergent, a set of properties highly characteristic of caveolae. The association of Engrailed-2 with specific membrane fractions observed after transfection in COS-7 cells is also observed for endogenous Engrailed-1 and Engrailed-2 expressed at late embryonic stages in the cerebellum and posterior mesencephalon of the rodent. Indeed, the two proteins are present in membrane fractions that bear all the characteristics of microdomains or caveolae-like domains, i.e. Triton X-100 resistance, saponin solubilization, low density on sucrose gradients, enrichment in glycosphingolipid GM1, absence of transmembrane Neural Cell Adhesion Molecule, presence of the glypiated (GPI-anchored) glycoprotein F3/F11 and of the acylated growth-associated protein GAP-43. Finally we demonstrate that part of the membrane-associated Engrailed, either expressed in COS-7 cells or endogenously present in neural tissues, is not accessible to proteolytic enzymes unless the membranes have been permeabilized with detergent. This study suggests that, in addition to their well-known presence in the nucleus, Engrailed proteins are also associated with caveolae-like vesicles that are primarily transported anterogradely into the axon, and that they can get access to a compartment compatible with secretion.

Animals↗
PubMed1996

Cell internalization of the third helix of the Antennapedia homeodomain is receptor-independent.

We have recently reported that a 16-amino acid long polypeptide corresponding to the third helix of the DNA binding domain (homeodomain) of Antennapedia, a Drosophila transcription factor, is internalized by cells in culture (Derossi, D., Joliot, A. H., Chassaing, G., and Prochiantz, A.(1994) J. Biol. Chem. 269, 10444-10450). The capture of the homeodomain and of its third helix at temperatures below 10 degrees C raised the problem of the mechanism of internalization. The present demonstration, that a reverse helix and a helix composed of D-enantiomers still translocate across biological membranes at 4 and 37 degrees C strongly suggests that the third helix of the homeodomain is internalized by a receptor-independent mechanism. The finding that introducing 1 or 3 prolines in the structure does not hamper internalization also demonstrates that the alpha-helical structure is not necessary. The data presented are compatible with a translocation process based on the establishment of direct interactions with the membrane phospholipids. The third helix of the homeodomain has been used successfully to address biologically active substances to the cytoplasm and nucleus of cells in culture (Théodore, L., Derossi, D., Chassaing, G., Llirbat, B., Kubes, M., Jordan, P., Chneiweiss, H., Godement, P., and Prochiantz, A.(1995) J. Neurosci. 15, 7158-7167). Therefore, in addition to their physiological implications (Prochiantz, A., and Théodore, L.(1995) BioEssays 17, 39-45), the present results open the way to the molecular design of cellular vectors.

Amino Acid Sequence↗
PubMed1993

Enzymatic synthesis of X-Phe-Leu-NH2 in low water content systems: influence of the N-alpha protecting group and the reaction medium composition.

The influence of eight different N-terminal protecting groups (For, Ac, Boc, Fmoc, Mal, Pheac, Aloc and Z) on the alpha-chymotrypsin-catalyzed synthesis of the dipeptide derivative X-Phe-Leu-NH2 in organic media was studied. Groups such as Ac, For, Boc, Z, Mal, Pheac and Alloc always rendered good peptide yields (92% to 99%) either in acetonitrile or in ethyl acetate. Good correlations were found between molecular and physico-chemical characteristics of the N-alpha moiety such as the hydrophobicity (log P), ovality and dipole moment and the global reaction rate parameter k'. High k' values were obtained with the less hydrophobic groups, Ac, For and Mal, that have ovality values close to one and the highest dipole moments. Furthermore, it was found that the relative rate of hydrolysis and aminolysis of the acyl-enzyme intermediate expressed as the partition parameter p is affected by the N-alpha moiety of the acyl donor. Correlations between this parameter and the dipole moment of the protecting group were observed.

Acetates↗
PubMed1992

Racemization free coupling of peptide segments. Synthesis of an insect neuropeptide.

The total synthesis of the insect neuropeptide derivative Z-Gly-Gly-Ser-Leu-Tyr-Ser-Phe-Gly-Leu-NH2 has been carried out by a convergent solid phase strategy. For the coupling of the N-terminal pentapeptide to the C-terminal tetrapeptide, three different methods were assayed. Racemization of the acyl activated amino acid during the fragment condensation reaction was monitored by HPLC. Best results were obtained by enzymatic coupling in a low water containing media using adsorbed alpha-chymotrypsin. An optically pure product was obtained in 82% yield after 1 h of reaction. Chemical methods such as DIC/HOBt and BOP/HOBt/NMM always rendered highly optically impure products containing 10-20% of the D-epimer.

Amino Acid Sequence↗
PubMed1989

Effects of L-asparaginase on rat embryonic development and yolk sac membrane in vitro.

A comparative analysis of the teratogenic effects of L-asparaginase on 10.5- and 11.5-day rat embryos after 24 and 48 hours of exposure in vitro, respectively, were performed. Several medium concentrations of L-asparaginase (0.05, 0.25, and 1.5 IU/ml) were tested in both embryo series. Resulting embryos were submitted to morphological studies in a search for a specific route of pathogenesis. Morphological alterations of the visceral yolk sac were also studied to investigate its contribution to L-asparaginase teratogenicity in rats. Main embryonic malformations (open truncal neural tube, open encephalic vesicles, anophthalmia, lack of inversion, abnormal frontolateral protrusions, great vascular dilations at the cephalic level) and developmental retardation were already generated after the first 24 hours of culture (embryos of 10.5 days) and presented a dose-response relationship. Vascular dilations and neurulation disturbances seemed to be related to an early mesenchyme deficiency. Reduced number of mesenchymal cells was more evident in embryos of 10.5 days than those of 11.5 days, suggesting the existence of a later compensatory mechanism of cellular proliferation in the older embryo. Visceral yolk-sac endodermal cells at both embryonic stages were greatly deformed and enlarged by an increase of the high electron-dense vacuolar system. Therefore, both a blockage of the processes of lysosomal digestion and derived trophic deficiencies probably existed. A double teratogenic mechanism for L-asparaginase is postulated: a direct action mainly in younger embryos (before invagination of the embryo into the yolk sac) and a yolk sac-mediated one.

Analysis of Variance↗
PubMed1986

Neuronal types of the cerebral cortex of the adult chicken (Gallus gallus). A Golgi study.

The telencephalic cortex of adult chicken (Gallus gallus) consists of two anatomical regions: the medial cortex (or hippocampal cortex) and the dorsal cortex (or parahippocampal cortex) which we have denominated "dorsomedial cortex" (DMC) as a whole. The dorsomedial cortex shows, with Nissl's technique, two main layers of lamination, besides the ependymal layer: plexiform layer and granular layer. In the plexiform layer, there is a sub-layer called periventricular. In Golgi impregnations, the plexiform layer of the DMC is filled with neurons of horizontal type and fibers. The granular layer of the medial region is mainly occupied with pyramidal and bipyramidal neurons, as well as some short-axon and long-axon multipolar neurons. In the granular layer of the dorsal region basically long-axon multipolar neurons are found and, to a lesser extend, short-axon multipolar ones are also seen. In the periventricular sub-layer of all the DMC are located the periventricular neurons. In general, we can say that the chicken DMC is a poorly developed cortical structure.

Animals↗
PubMedDate not supplied

[Methodological considerations concerning peripheral nerve morphometry].

Morphometric studies of the peripheral nerve often present widely varying results which, in part at least, may be attributed to the different methodologies used. Two questions may be of importance with regard to the reliability of such results: the preselection of fibres according to their morphology and the method used in quantifying observations. In this work a morphological study was carried out on the myelinated fibres of the sciatic nerve of a rat in order to evaluate fibre selection criteria. A morphometric analysis was also performed using manual measurements, image digitalisation and surveying, and automatic image analysis. It was shown that morphological variability of transverse section fibres is considerable and that, really, the proportion of circular fibres with homogeneous compact myelin is only 50 to 70%, from which we can conclude that the selection of fibres carried out in some studies wishing to eliminate abnormal fibres is somewhat exaggerated. By analysing the fibres using various methods significant differences appear, some due to the fact that distinct methods may be used to calculate the same parameters in a different way. The most reliable parameters would appear to be those which do not depend on the shape of the fibres and those which automatic or semiautomatic methods can calculate directly such as areas and perimeters. In any case quantifying methods seem hardly discriminatory and the differences between methods disappear if analyses are carried out using random samples. Preselection of fibres appears unnecessary in this context as in no case are the results altered. Finally we suggest finishing quantitative analyses with qualitative studies which would permit getting more information especially useful in cases of ageing, regeneration or pathological studies.

Aging↗