Evaluation of type-B RR dimerization in poplar: A mechanism to preserve signaling specificity?

Plants possess specific signaling pathways, such as the MultiStep Phosphorelay (MSP), which is involved in cytokinin and ethylene sensing, and light, drought or osmotic stress sensing. These MSP comprise histidine-aspartate kinases (HKs) as receptors, histidine phosphotransfer (HPts) proteins acting as phosphorelay proteins, and response regulators (RRs), some of which act as transcription factors (type-B RRs). In previous studies, we identified partners of the poplar osmosensing signaling pathway, composed of two HKs, three main HPts, and six type-B RRs. To date, it is unresolved as to how cytokinin or osmotic stress signal specificity is achieved in the MSP in order to generate specific responses. Here, we present a large-scale interaction study of poplar type-B RR dimerization. Using the two-hybrid assay, we were able to show the homodimerization of type-B RRs, the heterodimerization of duplicated type-B RRs, and surprisingly, a lack of interaction between some type-B RRs belonging to different duplicates. The lack of interaction of the duplicates RR12-14 and RR18-19, which are involved in the osmosensing pathway has been confirmed by BiFC experiments. This study reveals, for the first time, an overview of type-B RR dimerization in poplar and makes way for the hypothesis that signal specificity for cytokinin or osmotic stress could be in part due to the fact that it is impossible for specific type-B RRs to heterodimerize.

Highlighting type A RRs as potential regulators of the dkHK1 multi-step phosphorelay pathway in Populus.

In previous studies, we highlighted a multistep phosphorelay (MSP) system in poplars composed of two hybrid-type Histidine aspartate Kinases, dkHK1a and dkHK1b, which interact with three Histidine Phosphotransfer proteins, dkHPt2, 7, and 9, which in turn interact with six type B Response Regulators. These interactions correspond to the dkHK1a-b/dkHPts/dkRRBs MSP. This MSP is putatively involved in an osmosensing pathway, as dkHK1a-b are orthologous to the Arabidopsis osmosensor AHK1, and able to complement a mutant yeast deleted for its osmosensors. Since type A RRs have been characterized as negative regulators in cytokinin MSP signaling due to their interaction with HPt proteins, we decided in this study to characterize poplar type A RRs and their implication in the MSP. For a global view of this MSP, we isolated 10 poplar type A RR cDNAs, and determined their subcellular localization to check the in silico prediction experimentally. For most of them, the in planta subcellular localization was as predicted, except for three RRAs, for which this experimental approach gave a more precise localization. Interaction studies using yeast two-hybrid and in planta BiFC assays, together with transcript expression analysis in poplar organs led to eight dkRRAs being singled out as partners which could interfere the dkHK1a-b/dkHPts/dkRRBs MSP identified in previous studies. Consequently, the results obtained in this study now provide an exhaustive view of dkHK1a-b partners belonging to a poplar MSP.

Enhancement of all-trans-retinoic acid efficiency in granulocytic differentiation of HL-60 cells by incorporation into low density lipoprotein.

All-trans-retinoic acid (ATRA) has been proven to lead to complete remission of acute promyelocytic leukemia by inducing differentiation into granulocytes except when an acquired resistance occurred. High levels of low density lipoprotein (LDL) receptor in cancer cells suggested the use of ATRA incorporated into LDL. 50% of HL-60 cell differentiation were obtained with 5 nmoles/l of ATRA-LDL compared to 150 moles/l of ATRA. Maximal differentiation (80%) was reached at 25 nmoles/l and 1,000 nmoles/l respectively. This higher efficiency suggests the involvement of LDL receptor in ATRA-LDL internalization and/or the protection of the drug, from eventual catabolism, by LDL particles.

Optimization of murine CD8+ cytotoxic T-lymphocyte responses to pseudorabies virus.

The aim of this work was to optimize the procedures used to elicit a cellular immune response to pseudorabies virus (PrV) in mice, using various immunization schedules and routes. An Eu-labeling-based cytotoxic T-lymphocyte (CTL) test was developed to measure the response. This necessitated optimization of numerous steps. In suspension, Eu labeling required high concentrations of dextran-sulfate (DXS) and Eu with a 30-min labeling time at room temperature. For anchored cells, the labeling time was 1 to 48 h, and the labeling efficiency depended strongly on the Eu concentration, but only marginally on the DXS concentration. In vivo and in vitro stimulation protocols were also optimized for the CTL test. For in vitro stimulation, spleen cells were cultured in T-25 flasks at a multiplicity of infection (m.o.i.) of 2. The CTL test was validated by specific depletion of CD8+ CTL, FACS analysis, and by comparing Eu and 51 Cr labeling. Then groups of mice were vaccinated once or twice by various routes (intraperitoneal (i.p.), intravenous (i.v.), subcutaneous (s.c.) and in the rear footpads (FP)) and according to different time schedules. CTL activity was detected only in boosted animals immunized FP, i.p. or i.v. That the cellular immune response contributes to protection was further suggested by the observation that i.p. immunization conferred better protection against challenge than s.c. immunization.

Ultrastructural changes related to multidrug resistance in CEM cells: role of cytoplasmic vesicles in drug exclusion.

The multidrug resistance phenotype is found to be frequently associated with the overexpression of proteins which lead to a decrease of drug accumulation within human tumor cells. A 170 kDa membrane glycoprotein which is related to the overexpression of the mdr1 gene is inserted in the plasma membrane and pumps the cytotoxic drugs out of the cells. The aim of this work was to study the morphological modifications of resistant CEM/VLB 100 cells relative to their parental drug-sensitive ones and the detection of the ultrastructural localization of P-glycoprotein at the cytoplasmic level. Using a scanning electron microscope, CEM resistant cells showed wide smooth protrusions while CEM sensitive cells showed microvilli and fine folds. With transmission electron microscopy, an enhanced secretory system was observed in CEM resistant cells: both electron transparent and electron opaque vesicles were associated with the Golgi system, revealed by wheat germ agglutinin-colloidal gold labelling. These vesicles were the binding site of C 219 and MRK 16 antimembrane glycoprotein antibodies, and some of them were determined to belong to the lysosomal system after PTA staining. These vesicles may be an additional way to decrease the cellular uptake of drugs in multidrug resistant cells. Moreover, some nuclear and nucleolar modifications were also observed. These observations show that MDR has wide morphological features which concern several organelles.

Drug targeting: synthesis and endocytosis of oligonucleotide-neoglycoprotein conjugates.

Inhibition of gene expression by antisense oligonucleotides is limited by their low ability to enter cells. Knowing that sugar binding receptors, also called membrane lectins, efficiently internalize neoglycoproteins bearing the relevant sugar, 6-phosphomannose, for instance, oligonucleotides--substituted on their 5'-end with either a fluorescent probe or a radioactive label on the one hand, and bearing a thiol function on their 3'-end, on the other hand,--were coupled onto 6-phosphomannosylated proteins via a disulfide bridge. The oligonucleotide bound to 6-phosphomannosylated serum albumin is much more efficiently internalized roughly 20 times than the free oligonucleotide. Although most of the oligonucleotides are associated with vesicular compartments, oligonucleotides after releasing from the carrier by reduction of the disulfide bridge may find their way to reach the cytosol and then lead to an increase in the efficiency of the oligonucleotides.

Characterization of an Agrobacterium tumefaciens lectin.

An Agrobacterium tumefaciens suspension induces a strong agglutination of aldehyde-fixed pig erythrocytes at pH 5.0. The agglutination is inhibited by some polysaccharides, such as fucoidin, and also when the pH is raised to 7.0. Lectins (sugar-binding proteins) associated with the bacterial cell wall of A. tumefaciens strain 84.5 were directly evidenced by spectrofluorimetry using fluoresceinylated neoglycoproteins. The specific binding of the fluorescein-labelled neoglycoprotein bearing alpha-L-fucoside residues was also optimal at pH 5.0. A lectin was purified by affinity chromatography on agarose substituted with alpha-L-fucopyranoside. Furthermore, the haemagglutination activity of this lectin was inhibited by polysaccharides isolated from poplar leaves.

Identification of different agrobacterium strains isolated from the same forest nursery.

Several Agrobacterium strains isolated from the same forest nursery from 1982 to 1988 were compared by serological, biochemical, and DNA-DNA hybridization methods. Similarities among strains belonging to biovar 2 were observed by indirect immunofluorescence, whereas biovar 1 strains showed serological heterogeneity. Electrophoretic analysis of bacterial envelope-associated proteins showed that few bands appeared in the strains belonging to biovar 1, whereas many proteins appeared in the case of biovar 2 strains. Chromosomal DNA was analyzed with six random C58 chromosomal fragments. None of the six probes hybridized to the DNA of the two biovar 2 strains. One of the probes gave the same hybridization pattern with all biovar 1 strains, whereas the other probes yielded different patterns. The vir regions were closely related in the different pathogenic strains. The T-DNA and replication regions were less conserved and showed some variations among the strains.

Benzoxazinone kanamycin A conjugate. A new fluorescent probe suitable to detect mycoplasmas in cell culture.

The synthesis of a new benzoxazinone derivative suitable to detect early infection of cultured cells with mycoplasmas is described. p-[beta-(7-dimethylamino 1,4-benzoxazin 2-one 3yl)-vinyl]- phenylpropenoic acid was coupled to kanamycin A, an aminoglycoside leading to a cationic fluorescent probe which fluoresces at 600 nm upon excitation at 490 nm. This fluorescent probe is shown to heavily label the glycocallix of all the mycoplasma strains tested which are found to be associated with contaminated cultured cells and to allow an easy and rapid detection of contamination by fluorescence microscopy and flow cytometry.