Mitogenic activity of high molecular weight forms of insulin-like growth factor-II in amniotic fluid

Amniotic fluid (AF) collected from ewes and goats at mid gestation displayed mitogenic activity in mouse fibroblasts. Upon fractionation of this material by size exclusion chromatography, the mitogenic activity was resolved into two peaks, whose activity was inhibited by an anti-IGF type 1 receptor blocking antibody. One of the peaks contained IGF-I and IGF-II (mature form), whereas the other contained high M(r) precursor forms of IGF-II. The presence in this latter fraction of IGF-binding proteins (IGFBP) suggests that the AF IGFBPs do not efficiently inhibit the mitogenic activity of the high M(r) forms of IGF-II. In agreement with this conclusion, exogenous IGFBP-1 failed to affect this activity. Analysis of IGF-II in sheep AF showed that the AF concentrations of both forms of IGF-II increased dramatically from mid pregnancy until 106-120 days of gestation, and fell thereafter. The amniotic IGFBPs followed a similar evolution. High M(r) forms of IGF-II were also found in human AF, with a pattern of electrophoretic migration different from that of sheep. We suggest that the precursor forms of IGF-II may play an important role in foetal development.

Requirement of the VanY and VanX D,D-peptidases for glycopeptide resistance in enterococci

Transposon Tn 1546 confers resistance to glycopeptide antibiotics in enterococci and encodes two D,D-peptidases (VanX and VanY) in addition to the enzymes for the synthesis of D-alanyl-D-lactate (D-Ala-D-Lac). VanY was produced in the baculovirus expression system and purified as a proteolytic fragment that lacked the putative N-terminal membrane anchor of the protein. The enzyme was a Zn2+-dependent D,D-carboxypeptidase that cleaved the C-terminal residue of peptidoglycan precursors ending in R-D-Ala-D-Ala or R-D-Ala-D-Lac but not the dipeptide D-Ala-D-Ala. The specificity constants kcat/Km were 17- to 67-fold higher for substrates ending in the R-D-Ala-D-Ala target of glycopeptides. In Enterococcus faecalis, VanY was present in membrane and cytoplasmic fractions, produced UDP-MurNAc-tetrapeptide from cytoplasmic peptidoglycan precursors and was required for high-level glycopeptide resistance in a medium supplemented with D-Ala. The enzyme could not replace the VanX D,D-dipeptidase for the expression of glycopeptide resistance but a G237D substitution in the host D-Ala:D-Ala ligase restored resistance in a vanX null mutant. Deletion of the membrane anchor of VanY led to an active D,D-carboxypeptidase exclusively located in the cytoplasmic fraction that did not contribute to glycopeptide resistance in a D-Ala-containing medium. Thus, VanX and VanY had non-overlapping functions involving the hydrolysis of D-Ala-D-Ala and the removal of D-Ala from membrane-bound lipid intermediates respectively.

Initial characterization of anosmin-1, a putative extracellular matrix protein synthesized by definite neuronal cell populations in the central nervous system

The KAL gene is responsible for the X-chromosome linked form of Kallmann's syndrome in humans. Upon transfection of CHO cells with a human KAL cDNA, the corresponding encoded protein, KALc, was produced. This protein is N-glycosylated, secreted in the cell culture medium, and is localized at the cell surface. Several lines of evidence indicate that heparan-sulfate chains of proteoglycan(s) are involved in the binding of KALc to the cell membrane. Polyclonal and monoclonal antibodies to the purified KALc were generated. They allowed us to detect and characterize the protein encoded by the KAL gene in the chicken central nervous system at late stages of embryonic development. This protein is synthesized by definite neuronal cell populations including Purkinje cells in the cerebellum, mitral cells in the olfactory bulbs and several subpopulations in the optic tectum and the striatum. The protein, with an approximate molecular mass of 100 kDa, was named anosmin-1 in reference to the deficiency of the sense of smell which characterizes the human disease. Anosmin-1 is likely to be an extracellular matrix component. Since heparin treatment of cell membrane fractions from cerebellum and tectum resulted in the release of the protein, we suggest that one or several heparan-sulfate proteoglycans are involved in the binding of anosmin-1 to the membranes in vivo.

Analysis of C-terminally truncated dengue 2 and dengue 3 virus envelope glycoproteins: processing in insect cells and immunogenic properties in mice

We constructed two recombinant Autographa californica nuclear polyhedrosis baculoviruses. Spodoptera frugiperda (Sf9) cells containing these constructs produce carboxy-terminally truncated envelope E proteins representing dengue (DEN) virus serotypes 2 and 3. The two recombinant proteins contained their homologous signal sequences at the N terminus and were truncated by 71 and 74 amino acids at the C terminus, respectively. This allowed the translocation of the recombinant proteins to the endoplasmic reticulum followed by glycosylation processing and secretion into the extracellular medium. An additional unglycosylated form which was not secreted was detected inside the infected Sf9 cells. Sera from Swiss mice immunized with the infected Sf9 cell lysates gave a DEN cross-reactive response in ELISA and substantial amounts of neutralizing antibodies to the homologous virus. Similar antibody titres were obtained when the two recombinant proteins were inoculated concomitantly. BALB/c mice were vaccinated with three doses of the recombinant E proteins, taken as monovalent or bivalent immunogens, and challenged with mouse-adapted DEN-2 virus. DEN-2 E protein induced a good protection (90%) against lethal encephalitis and recombinant DEN-3 E protein gave a substantial cross-protection (54%). Eighty-two percent of the mice immunized with a mixture of both recombinant E proteins survived the DEN-2 virus challenge.

Comparison of the biochemical properties of unprocessed and processed forms of the small GTP-binding protein, rab6p

The rab6 protein (rab6p) belongs to a large family of ras-like low-molecular-mass GTP-binding proteins thought to be involved in the regulation of intracellular transport in mammalian cells. When expressed in the baculovirus/insect cell system, two major forms of rab6p are obtained; a 24-kDa cytosolic unprocessed form and a 23-kDa membrane-bound form which represents the processed lipid-modified protein. Here, we have purified both forms to homogeneity and we have studied and compared their biochemical properties. Unprocessed and processed rab6p display similar binding-rate constants (kon) for GDP and GTP (1-1.9 microM-1 min-1). However, significant differences exist in the dissociation constants of bound guanine nucleotides. Processed rab6p in low and high magnesium solutions displays similar koff values for GTP and GDP. However, unprocessed rab6p has a koff value higher for GDP than for GTP in both low and high magnesium solutions. Their intrinsic GTPase activities also differ; unprocessed rab6p has an almost undetectable GTPase activity, whereas that of processed rab6p is in the same range as that reported for other ras and ras-like GTP-binding proteins (0.012 +/- 0.002 min-1). These results suggest that post-translational modifications of rab6p might induce subtle changes in the three-dimensional structure of the protein which affect the guanine-nucleotide-binding/hydrolysis activity.

Induction of phagocytic behaviour in human epithelial cells by Escherichia coli cytotoxic necrotizing factor type 1

Cytotoxic necrotizing factor type 1 (CNF1) from strains of pathogenic Escherichia coli induces in human epithelial HEp-2 cells, a profound reorganization of the actin cytoskeleton into prominent stress fibres and membrane ruffles. We report here that this process is associated with induction of phagocytic-like activity. CNF1-treated cells acquired the ability to ingest latex beads as well as non-invasive bacteria such as Listeria innocua, which were taken as a model system. Uptake of bacteria was similar to pathogen-induced phagocytosis, since L. innocua transformed with DNA coding for the pore-forming toxin listeriolysin O behaved, with respect to intracellular growth, like the invasive, pathogenic species L. monocytogenes. Our results raise the possibility that, in vivo, pathogenic CNF1-producing E. coli may invade epithelia by this novel induced phagocytic-like mechanism.

High level functional expression of human beta 1-adrenergic receptor in baculovirus-infected cells screened by a rapid in situ procedure

A novel screening assay for the identification of baculovirus infected cells expressing membrane receptors was developed by using a replica transfer technique. Sf9 cells were cotransfected with wild type baculoviral DNA and the transfer vector pVL941-beta 1 containing the coding region of the human beta 1-adrenergic receptor gene. Infected cells embedded in agarose were incubated with [125I]-iodocyanopindolol and transferred onto filters that were subsequently autoradiographed. This procedure resulted in the isolation of recombinant baculoviruses that expressed beta 1-adrenergic receptors. Binding assays carried out with [125I]-ICYP indicated that more than 600,000 receptors were expressed per cell, the highest level noted so far for this receptor in genetically engineered cells. Sf9 cells expressing the beta 1-AR were analysed by ligand binding, competition experiments, adenylyl cyclase stimulation and photoaffinity labeling. These cells express a homogenous population of receptors and display the known pharmacological properties of beta 1-AR in human tissues.

Batch production and secretion kinetics of hybridomas: Pulse-chase experiments

Pulse chase experiments of two mouse hybridoma lines were conducted in order to elucidate the kinetics of monoclonal antibody (mAb) production and secretion during different stages of batch cultures. The results indicate that a stock of cytoplasmic IgG exists in hybridoma cells and that the concentration of this stored IgG depends on the cell line used and the stage of the culture. This stored IgG can be released by dying cells, and a certain quantity of the secreted IgG is derived from this source. However, only between 0.3 and 9.3% of the released IgG of U0208 (average: 2.08%) and between 2.08 and 25.8% of the IgG, released from I.13.17 (average: 6.95%), were of storage origin, calculated on culture viability and intracellular IgG-stock. Comparing the accumulation of radio-labelled IgG (IgG*) in the supernatant with the reduction of cytoplasmic IgG* during the chase experiments, the percentages range between 14 and 50%, somewhat higher values probably caused by changes in the culture conditions. These changes led to a release of IgG during the chase experiments, which accounts for about 20-25% of the totally secreted IgG. It could be established that during the logarithmic growth phase of batch cultures a certain percentage of synthesized IgG was not released but stored within the cells: for U0208: 0.3-4.5%, for I.13.17: 1-7.6%. During the stationary and death phase, this percentage ranged between 1.5 and 20% for U0208 and between 0.5 and 8.1% for I.13.17. Finally, the chase experiments also revealed that the time of synthesis, assembly, and secretion of mAbs does not vary much during the different phases of batch cultures, and is within the range of 1.5 and 3 hrs.