Impairment of cell division in tolA mutants of Escherichia coli at low and high medium osmolarities

Mutations in the tolA gene of Escherichia coli cause the cell to become sensitive to detergents and to some antibiotics, to release periplasmic enzymes and to be resistant to group A colicins; tolA mutations also lead to mucoid phenotype. TolA is a three-domain protein anchored in the inner membrane by its N-terminal domain. The second domain is proposed to span the periplasmic space and to interact with trimeric porins of the outer membrane. TolA proteins are considered to be located in the adhesion zones between inner and outer membranes. Our observations by confocal and electron microscopy have revealed that tolA mutants show modified morphology and produce DNA-free cells. Increasing or decreasing medium osmolarity amplifies these defects; mutants become essentially unable to locate the division site properly so that cells of highly unequal lengths are produced. Moreover, septation is impaired with asymmetric constrictions and oblique septa. These results suggest that TolA could play a role in positioning the division sites via the organisation of either the outer membrane or the possible adhesion zones.

A yeast t-SNARE involved in endocytosis

The ORF YOL018c (TLG2) of Saccharomyces cerevisiae encodes a protein that belongs to the syntaxin protein family. The proteins of this family, t-SNAREs, are present on target organelles and are thought to participate in the specific interaction between vesicles and acceptor membranes in intracellular membrane trafficking. TLG2 is not an essential gene, and its deletion does not cause defects in the secretory pathway. However, its deletion in cells lacking the vacuolar ATPase subunit Vma2p leads to loss of viability, suggesting that Tlg2p is involved in endocytosis. In tlg2Delta cells, internalization was normal for two endocytic markers, the pheromone alpha-factor and the plasma membrane uracil permease. In contrast, degradation of alpha-factor and uracil permease was delayed in tlg2Delta cells. Internalization of positively charged Nanogold shows that the endocytic pathway is perturbed in the mutant, which accumulates Nanogold in primary endocytic vesicles and shows a greatly reduced complement of early endosomes. These results strongly suggest that Tlg2p is a t-SNARE involved in early endosome biogenesis.

The yeast actin-related protein Arp2p is required for the internalization step of endocytosis

The Saccharomyces cerevisiae actin-related protein Arp2p is an essential component of the actin cytoskeleton. We have tested its potential role in the endocytic and exocytic pathways by using a temperature-sensitive allele, arp2-1. The fate of the plasma membrane transporter uracil permease was followed to determine whether Arp2p plays a role in the endocytic pathway. Inhibition of normal endocytosis as revealed by maintenance of active uracil permease at the plasma membrane and strong protection against subsequent vacuolar degradation of the protein were observed in the mutant at the restrictive temperature. Furthermore, arp2-1 cells accumulated ubiquitin-permease conjugates, formed prior to internalization. These effects were also visible at permissive temperature, whereas the actin cytoskeleton appeared to be normally polarized. The soluble hydrolase carboxypeptidase Y and the lipophilic dye FM 4-64 were targeted normally to the vacuole in arp2-1 cells. Thus, Arp2p is required for internalization but does not play a major role in later steps of endocytosis. Synthetic lethality was demonstrated between arp2-1 and the endocytic mutant end3-1, suggesting participation of Arp2p and End3p in the same process. Finally, no evidence for a major defect in secretion was apparent; invertase secretion and delivery of uracil permease to the plasma membrane were unaffected in arp2-1 cells.

Membrane translocation of diphtheria toxin fragment A exploits early to late endosome trafficking machinery

After reaching early endosomes by receptor-mediated endocytosis, diphtheria toxin (DT) molecules have two possible fates. A large pool enters the degradative pathway whereas a few molecules become cytotoxic by translocating their catalytic fragment A (DTA) into the cytosol. Impairment of DT degradation by microtubule depolymerization does not block DT cytotoxicity. Therefore, DTA membrane translocation into the cytosol occurs from an endocytic compartment located upstream of late endosomes. Comparisons between early endosomes and endocytic carrier vesicles in a cell-free translocation assay have demonstrated that early endosomes are the earliest endocytic compartment from which DTA translocates. DTA translocation is ATP-dependent, requires early endosomal acidification, and is increased by the addition of cytosol. Cytosol-dependent DTA translocation is GTP gamma S-insensitive but is blocked by anti-beta COP antibodies.

The putative immunity protein of the gram-positive bacteria Leuconostoc mesenteroides is preferentially located in the cytoplasm compartment

Immunity proteins are though to protect bacteriocin-producing bacterial strains against the bactericidal effects of their own bacteriocin. The immunity protein which protects the lactic acid bacterium Leuconostoc mesenteroides against mesentericin Y105(37) bacteriocin was detected and localized by immunofluorescence and electron microscopy, using antibodies directed against the C-terminal end of the predicted immunity protein. The antibodies recognized the immunity proteins of various strains of Leuconostoc, including Leuconostoc mesenteroides and Leuconostoc gelidum. This study demonstrated that immunity proteins produced by Leuconostoc mesenteroides accumulated in the cytoplasmic compartment of the bacteria. This is in contrast with other known immunity proteins, such as the colicin immunity proteins, which are integral membrane proteins possessing three to four transmembrane domains.

"In vitro" phosphorylation of annexin 2 heterotetramer by protein kinase C. Comparative properties of the unphosphorylated and phosphorylated annexin 2 on the aggregation and fusion of chromaffin granule membranes

Heterotetrameric annexin 2 phosphorylated "in vitro" by rat brain protein kinase C is purified and obtained devoid of unphosphorylated protein; it contains 2 mol of phosphate/mol of heterotetramer. The aggregative and binding properties of the phosphorylated annexin 2 toward purified chromaffin granules are compared with those of the unphosphorylated annexin 2. Annexin 2 binds to chromaffin granules with high affinity. Phosphorylation of annexin 2 decreases the affinity of this binding without affecting the maximum binding capacity. The binding curves are strongly cooperative. It is suggested that a surface oligomerization of the proteins may take place upon binding. Besides, phosphorylation of annexin 2 is followed by a dissociation of the light chains from the heavy chains in the heterotetramer. Whereas annexin 2 induces the aggregation of chromaffin granules at microM calcium concentration, the phosphorylated annexin 2 does not induce aggregation at any concentration of calcium either at pH 6 or 7. The phosphorylation of annexin 2 by protein kinase C, MgATP, and 12-O-tetradecanoylphorbol-13-acetate on chromaffin granules induces a fusion of chromaffin granules membranes observed in electron microscopy. The fusion requires the activation of protein kinase C by 12-O-tetradecanoylphorbol-13-acetate. Given these results and since annexin 2 is phosphorylated by protein kinase C under stimulation of chromaffin cells, it is suggested that phosphorylated annexin 2 may be implicated in the fusion step during exocytosis of chromaffin granules.

Abnormal incisor-tooth differentiation in transgenic mice expressing the muscle-specific desmin gene

Immunocytochemistry and electron microscopic observations on the incisor-tooth organ of transgenic mice expressing the muscle-specific desmin gene under the direction of the vimentin promoter, reveal that the expression of the hybrid transgene occurs both in mesenchymal cells and differentiating odontoblasts. The muscle-specific desmin, as estimated by fluorescence intensity, is more expressed in immature mesenchymal cells than in postmitotic differentiated odontoblasts. The expression of the transgene generates alteration of the odontoblast-intermediate filament network and interferes with the secretory activity of both odontoblasts and ameloblasts. Our results are consistent with the hypothesis that odontoblasts have inductive properties on the differentiation of ameloblasts and that intermediate filaments among other factors play the role of cell and tissue organizer.

GEP31, a new gastric epithelial protein, early expressed during ontogenesis

A set of monoclonal antibodies directed against various gastric markers was produced in order to study the developmental expression of the gastric mucosa. A previously described monoclonal antibody, mab 146.14, labeled the proton pump (H+, K+) ATPase specifically located in gastric parietal cells. Mabs 15.1 and 121.17 revealed the mucus of mucous neck cells and mucous surface cells, respectively. By addition, mab 81.1 was directed against a cell surface membrane protein antigen composed of a major 31 kDa component (called GEP31). Our study mainly focused on the characterization of GEP31. This protein was typically located in the gastrointestinal epithelial tract (stomach, small intestine, colon). Moreover, interesting features were observed during the study of the early ontogenesis of the gastric mucosa. The 31 kDa protein was detected at the onset of gland formation (day 18), and a gradual increase in expression of the protein could be observed between day 18 and day 19. Furthermore, a comparative study of the expression of different terminal differentiation markers of gastric epithelial cells ((H+, K+) ATPase, mucins) during the early period of ontogenesis revealed that GEP31 could be detected well before the appearance of these markers. To our knowledge, GEP31 thus appears as the earliest expressed specific cell surface epithelial gastric antigen described to date. Furthermore, the partial N-terminal amino acid sequence of GEP31 was determined and revealed that it is not a known protein.

Upregulation of V1a vasopressin receptors by glucocorticoids

WRK1 cells (a rat mammary tumor cell line) exhibit a vasopressinergic receptor of V1a subtype tightly coupled to phospholipase C. Addition of dexamethasone to the culture medium principally potentiated the vasopressin-sensitive accumulation of inositol phosphates and to a lesser extent the NaF-sensitive phospholipase C activity. On the opposite, such treatment was without effect on the basal level of intracellular inositol phosphates or on bradykinin- or serotonin-sensitive phosphoinositide metabolisms. Glucocorticoid receptors were probably involved in these actions since dexamethasone was found to be more potent than aldosterone or corticosterone. Dexamethasone treatment also increased the number of vasopressin binding sites without affecting its affinity for vasopressin or other specific vasopressin analogues. These results strongly suggest that dexamethasone principally acts at the vasopressin receptor level by affecting its synthesis and/or the translation of its mRNA and also affects the G protein that couples the V1a receptor to the phospholipase C. These results explain how glucocorticoids may regulate the transduction mechanisms involved in vasopressin actions on WRK1 cells. They provide explanations for understanding the cross talk between adrenal steroids and hormones, which mobilize intracellular calcium.

Pharmacological characterization of inositol 1,4,5-trisphosphate binding sites: relation to Ca2+ release

Two subcellular fractions, one enriched in plasma membranes and the other in endoplasmic reticulum membranes, were obtained from WRK1 cells using a combination of differential centrifugations and Percoll gradient fractionation. Specific inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) binding sites were detected in these two preparations. Endoplasmic reticulum membranes exhibited a binding capacity which was about 5-fold higher than that of plasma membranes. Dose-dependent Ins(1,4,5)P3 binding was determined. Experimental data obtained with endoplasmic reticulum membranes could be adequately fitted with a two-site model (a high-affinity binding site with Kd and Bmax values of 0.7 +/- 0.15 nM and 12.9 +/- 5 fmol/mg protein and a low-affinity binding site with Kd and Bmax values of 44.2 +/- 14.6 nM and 143 +/- 43 fmol/mg protein). Both the high- and low-affinity binding sites were selective for Ins(1,4,5)P3. Besides Ins(1,4,5)P3, Ins(1,3,4,5)P4 also discriminated between the two populations of sites while heparin interacted with the high- and low-affinity binding sites with the same affinity. Ins(1,4,5)P3-induced calcium release from endoplasmic reticulum vesicles was determined by monitoring the calcium concentration in the extravesicular compartment with fura-2. Under experimental conditions where the degradation of Ins(1,4,5)P3 was reduced (incubation at 0 degrees C), a high-affinity Ins(1,4,5)P3-induced calcium release (apparent Kact around 20 nM) could be demonstrated. These results suggest that in WRK1 cells, the endoplasmic reticulum is a major site for Ins(1,4,5)P3 action and that the high-affinity binding sites located on the endoplasmic reticulum membranes may contribute to the physiological regulation of the cytosolic free calcium concentration.

Long-term expression of the c-fos protein during the in vitro differentiation of cerebellar granule cells induced by potassium or NMDA

Levels of the c-fos protein were assayed in mouse cerebellar granule cells during their in vitro development under different culture conditions. When grown in media favoring both their survival and differentiation, i.e. in the presence of 30 mM K+ or 12.5 mM K+ plus 100 microM N-methyl-D-aspartate (NMDA), the c-fos protein becomes detectable in the nucleus of granule cells on and after 6 days and persists to high levels until the culture begins to decline. The protein c-fos appears therefore after the critical period described for the survival effect of K+ depolarization or NMDA receptor stimulation which corresponds to days 2-5 after plating. The c-fos protein remains however scarcely detectable or undetectable throughout the life-span of cells cultured under conditions providing poor survival and differentiation, i.e. in the presence of low K+ (5 or 12.5 mM) alone or when the effect of NMDA is blocked by the NMDA receptor antagonist MK-801. Interestingly, in cortical and striatal neurons, the survival and differentiation of which being not affected by depolarizing media, no c-fos protein is detected whatever the culture conditions tested at least during the first 18 days in vitro. This suggests that long-term expression of the c-fos gene might be related to some aspect of the late in vitro differentiation process of cerebellar granule cells.

Isolation and functional properties of an arginine-selective endoprotease from rat intestinal mucosa. A putative prosomatostatin convertase

The endoproteolytic activity previously detected in rat intestinal mucosal extracts (Beinfeld M., Bourdais, J., Kuks, P., Morel, A., and Cohen, P. (1989) J. Biol. Chem. 264, 4460-4465), was purified to homogeneity as a 65-kDa molecular species. This putative proprotein-processing enzyme cleaves the peptide bond on the carboxyl side of a single arginine residue in hepta-[Leu62-Gln-Arg-Ser-Ala-Asn-Ser68] or trideca-[Asp56-Glu-Met-Arg-Leu-Glu-Leu-Gln-Arg-Ser-Ala-Asn-+ ++Ser68] peptides, reproducing the prosomatostatin sequence around Arg64, the locus for endoproteolytic release of either somatostatin-28 or its NH2-terminal fragment, somatostatin-28-(1-12), from their common precursor. This enzyme exhibits a strict selectivity for arginyl residues, as demonstrated with related substrates, and did not cleave at lysyl residues. Moreover, only arginyl residues belonging to peptides of the prosomatostatin family were cleaved, since no hydrolysis of peptides from other prohormones was detected. In addition, the arginine residue situated at position -5 on the NH2-terminal side of Arg64 not only did not function as a cleavage locus, but had no effect on the overall cleavage kinetics of the prosomatostatin-(56-68) peptide substrate. This enzyme also cleaved, but with much less efficiency, the peptide bond on the carboxyl side of an arginine in peptides containing either an Arg-Lys or a Lys-Arg doublet corresponding to prohormone cleavage sites. This enzyme was insensitive to divalent cation chelators, was completely inhibited by aprotinin and leupeptin, and was somewhat inhibited by other serine-protease inhibitors. It is concluded that this endoprotease is a serine protease and could be involved in prohormone or proprotein post-translational processing at single arginine cleavage sites.

Prosomatostatin is processed in the Golgi apparatus of rat neural cells

Proteolytic processing of somatostatin precursor produces several peptides including somatostatin-14 (S-14), somatostatin-28 (S-28), and somatostatin-28 (1-12) (S-28(1-12)). The subcellular sites at which these cleavages occur were identified by quantitative evaluation of these products in enriched fractions of the biosynthetic secretory apparatus of rat cortical or hypothalamic cells. Each of the major cellular compartments was obtained by discontinuous gradient centrifugation and was characterized both by specific enzyme markers and electron microscopy. The prosomatostatin-derived fragments were measured by radioimmunoassay after chromatographic separation. Two specific antibodies were used, allowing the identification of either S-28(1-12) or S-14 which results from peptide bond hydrolysis at a monobasic (arginine) and a dibasic (Arg-Lys) cleavage site, respectively. These antibodies also revealed prosomatostatin-derived forms containing at their COOH terminus the corresponding dodeca- and tetradecapeptide sequences. Whereas the reticulum-enriched fractions contained the highest levels of prosomatostatin, the proportion of precursor was significantly lower in the Golgi apparatus. In the latter fraction, other processed forms were also present, i.e. S-14 and S-28(1-12) together with the NH2-terminal domain (1-76) of prosomatostatin (pro-S(1-76). Inhibition of the intracellular transport either by monensin or by preincubation at reduced temperature resulted in an increase of prosomatostatin-derived peptides in the Golgi-enriched fractions. Finally, immunogold labeling using antibodies raised against S-28(1-12) and S-14 epitopes revealed the presence of these forms almost exclusively in the Golgi-enriched fraction mainly at the surface of saccules and vesicles. Together these data demonstrate that in rat neural cells, prosomatostatin proteolytic processing at both monobasic and dibasic sites is initiated at the level of the Golgi apparatus.

Plasma membrane-cytoskeleton damage in eye lenses of transgenic mice expressing desmin

Immunocytochemistry of eye lens cells from transgenic mice coexpressing desmin and vimentin reveals that the transgenic desmin expression is not uniform. In the same lens, some epithelial and fiber cells overexpress desmin, while in others the desmin gene seems to be silent. Conversely, the endogenous vimentin is always expressed. The concomitant expression of vimentin and desmin results in the assembly of hybrid intermediate filaments (IFs). Moreover, the overexpression of the transgene generates pleomorphic IF assembly and leads to intermingled filamentous whorls and to accumulation of amorphous desmin. The abnormalities of IF assembly induced by the genetic manipulation are correlated with perturbation of the enucleation process in the lens fibers, changes in cell shape, fiber fusion and extensive internalization of the general plasma membrane and junctional domains. The alterations of lens cells described in this study were observed in all transgenic mice examined. The level of expression of the transgene was paralleled by the degree of damage. Our results indicate that proper expression, assembly and membrane interaction of IFs play an important role in the terminal differentiation of the lenticular epithelium into fiber cells. We anticipate that alterations during these processes may initiate cataract formation.

Presence of an immunologically-defined class of peri-Golgi vesicles in cultured mammalian cells

Immunsera of mice injected with clathrin-depleted coated vesicle membranes, purified from rat liver, revealed a preferential labeling of some perinuclear structures by immunofluorescence microscopy on NRK cells. Subsequent production of 4 monoclonal antibodies was achieved. The antigen was strictly located in the Golgi area of the cell but was not an intrinsic element of the Golgi complex. The restricted location of the structures excluded these were lysosomes which appeared more dispersed in these cells. After nocodazole treatment the material was found dispersed in the cytoplasm. This provided a means of distinguishing the antigen from clathrin-coated structures and Golgi intrinsic elements. Immunolocalization at the electron microscope level confirmed the data obtained at the light level. Some peroxidase reaction product was rarely associated with Golgi elements, but predominantly stained small neighboring Golgi vesicles (50 nm diameter), as well as tubulo-elongated structures and some large (500 nm) irregular-shaped vesicles. A 32 kDa molecular weight antigen was characterized by immunopurification from NRK cells metabolically labeled with 35S-Met. This 32 kDa antigen appeared as part of a higher multimolecular membrane component of 300 kDa. A 170 kDa and a 70 kDa components were immunodetected in a semi-purified membrane fraction from rat liver, demonstrating that the antigen was a minor but very antigenic contaminant of the coated vesicle preparation used as immunogen. In conclusion, the labeled peri-Golgi structures may be part of the newly characterized trans-Golgi network and/or of the reticular/vesicular endosomal, prelysosomal structure recently described.

Membrane-cytoskeleton dynamics in rat parietal cells: mobilization of actin and spectrin upon stimulation of gastric acid secretion

The gastric parietal (oxyntic) cell is presented as a model for studying the dynamic assembly of the skeletal infrastructure of cell membranes. A monoclonal antibody directed to a 95-kD antigen of acid-secreting membranes of rat parietal cells was characterized as a tracer of the membrane movement occurring under physiological stimuli. The membrane rearrangement was followed by immunocytochemistry both at the light and electron microscopic level on semithin and thin frozen sections from resting and stimulated rat gastric mucosa. Double labeling experiments demonstrated that a specific and massive mobilization of actin, and to a lesser extent of spectrin (fodrin), was involved in this process. In the resting state, actin and spectrin were mostly localized beneath the membranes of all cells of the gastric gland, whereas the bulk of acid-secreting membranes appeared diffusely distributed in the cytoplasmic space of parietal cells without any apparent connection with cytoskeletal proteins. In stimulated cells, both acid-secreting material and actin (or spectrin) extensively colocalized at the secretory apical surface of parietal cells, reflecting that acid-secreting membranes were now exposed at the lumen of the secretory canaliculus and that this insertion was stabilized by cortical proteins. The data are compatible with a model depicting the membrane movement occurring in parietal cells as an apically oriented insertion of activated secretory membranes from an intracellular storage pool. The observed redistribution of actin and spectrin argues for a direct control by gastric acid secretagogues of the dynamic equilibrium existing between nonassembled (or preassembled) and assembled forms of cytoskeletal proteins.

Evidence of two steps in the homologous desensitization of vasopressin-sensitive phospholipase C in WRK1 cells. Uncoupling and loss of vasopressin receptors

The exposure of WRK1 cells to arginine vasopressin (AVP), lysine vasopressin, or oxytocin for 18 h at 37 degrees C induced a homologous desensitization of the vasopressin- (VP) receptors. Dose-response curves of [3H]lysine vasopressin binding to control and desensitized WRK1 cells revealed a decrease in the maximal number of binding sites without any modification of its affinity (Kd values = 4.40 +/- 0.76 nM and 4.65 +/- 0.78 nM for control and desensitized conditions, respectively). The phenomenon was time- and dose-dependent. It was directly related to receptor occupancy, since the concentration of VP analogues leading to a half-maximal occupancy of VP receptors was closely related to the concentration of the corresponding analogue leading to a half-maximal decrease in VP-binding sites. It was also agonist-specific, since the V1 vasopressin antagonist desGly9d(CH2)5[D-Tyr(Et)2]VAVP was unable to affect the number of receptors. These desensitization processes were completely inhibited when the functional coated pits present in WRK1 cells were suppressed, indicating that the loss of VP-binding sites was related to receptor internalization. The exposure of WRK1 cells to a vasopressin agonist for 18 h also led to an inhibition of the vasopressin-sensitive phospholipase C activity. It was time- and agonist-dose-dependent, and occurred without any detectable changes in apparent affinity values (1.40 +/- 0.04 and 1.90 +/- 0.36 nM for control and desensitized cells, respectively). Control experiments showed that these inhibitions could not have been caused by a decrease in the labeling of inositol lipids. It is likely that they were mainly due to receptor internalization since (i) the hormonal treatment did not modify the basal level of phospholipase C; (ii) the maximal loss of VP-binding site was similar to the maximal inhibition of VP-stimulated IP accumulation; (iii) the recoveries of both VP-binding sites and VP-sensitive phospholipase C activity followed exactly the same time course (t1/2 = 4 h). In addition to this homologous desensitization of VP-sensitive phospholipase C activity, AVP also induced heterologous desensitization of bradykinin-sensitive phospholipase C activity. However, this effect was relatively weak (maximal inhibition 17 +/- 3%). The time course of VP-sensitive phospholipase C desensitization was more rapid than that of VP-receptors, indicating that desensitization involved at least two distinct steps, a rapid uncoupling step, and a later loss of vasopressin receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Synapse formation and development of neurotransmitter functions in neuronal cells from chick brain cultured in a serum-free, defined medium

Cells dissociated from cerebral hemispheres of 8-day-old chick embryos were seeded on poly-L-lysine coated Petri dishes in serum-containing medium. After 24 hr the culture medium was switched to a serum-free, chemically defined medium. These cultures contain mainly neuronal cells until day 14, characterized by the presence of acetylcholinesterase activity and neurofilament proteins. After 2 weeks glial cells progressively contaminated the neuronal culture. Cultures were maintained for a period of 4 weeks. From day 6 on numerous synapses with clear vesicles were observed. The activity of choline acetyltransferase remained low throughout the culture period, while GABA levels increased in parallel with synaptogenesis. Our observations indicate that chick cerebral hemisphere neuronal cultures grown in serum-free, chemically defined medium contain GABAergic neurons that undergo maturation.

Vasopressin receptors from cultured mesangial cells resemble V1a type

Mesangial cells respond to vasopressin by contraction and increased prostaglandin production. The purpose of the present study is to characterize vasopressin receptors from these cells. Glomeruli were isolated from rat kidneys and plated for explant growth of mesangial cells. Membranes were prepared from cells grown for 6 wk and tested for their ability to bind [3H]vasopressin (lysine vasopressin). These membranes contained a single class of specific vasopressin binding sites [equilibrium dissociation constant (Kd) = 10 +/- 1 nM, maximal binding capacity (Bmax) = 270 +/- 7 fmol/mg protein for 5 determinations]. Vasopressin induced a dose-dependent (apparent Kact value = 2 nM) accumulation of labeled inositol phosphates in myo[3H]inositol-prelabeled mesangial cells incubated in the presence of 10 mM of Li. Conversely, vasopressin failed to alter the adenylate cyclase activity of mesangial cell membranes. Competition experiments with a series of vasopressin structural analogues that have different degrees of affinity for V2-(renal), V1a- (vascular and hepatic), and V1b- (adenohypophyseal) receptors, indicated that vasopressin receptors from rat glomerular mesangial cells resemble the V1a- receptor subtype.

Vasopressin, angiotensin and adrenergic receptors of rat liver Golgi fractions--molecular weight of the angiotensin-receptor irreversible complex after in vitro and in vivo labelling

The binding of vasopressin, angiotensin II and prazosin (alpha 1-adrenergic antagonist) to purified heavy (GH) and (intermediate + light) (GI + L) rat liver Golgi fractions was studied. The three types of ligands showed a saturable and specific binding in Golgi fractions; the maximal specific binding of [3H]vasopressin, [3H]prazosin and [125I]Sar-N3-Phe-angiotensin II was respectively 5-10%, 20-30% and 30-40% of that detected in purified plasma membranes. The apparent binding affinities of the three ligands were the same whether determined in Golgi fractions or plasma membranes. The presence of vasopressin, alpha 1-adrenergic and angiotensin receptors in very different proportions, as compared to the amount of receptor detected in plasma membranes, in GH and GI + L Golgi fractions was not compatible with the idea that a plasma membrane impurity accounted for the detection of receptor in the purified intracellular particulate fractions. In vivo injection of [125I]Sar-N3-Phe-angiotensin II resulted in a receptor-mediated endocytosis of the iodo-angiotensin analog into the GH and GI + L Golgi fractions. The apparent molecular weight of the irreversible complex, [125I]angiotensin-receptor, was estimated in subcellular fractions using SDS-PAGE electrophoresis. This value was identical after either in vivo or in vitro labelling (MW = 63,000) and was indistinguishable from the molecular weight of the irreversible hormone receptor complex present in the plasma membranes.

Activation of membrane phospholipase C by vasopressin. A requirement for guanyl nucleotides

Vasopressin stimulates the liberation of labelled inositol phosphate in partially purified plasma membranes prepared from myo-[3H]inositol prelabelled WRK1 cells. This stimulatory effect was very rapid (165% stimulation of inositol trisphosphate accumulation after a 10 s incubation period in the presence of 1 microM vasopressin), concentration dependent (EC50 = 12 nM) and was abolished by an antagonist of the vasopressor response to vasopressin. GTP, even at high concentrations (0.1 mM), did not increase inositol phosphate release: it was found to be absolutely necessary for hormonal stimulation of phospholipase C activity. Non-hydrolysable analogues of GTP may also stimulate this enzyme activity.

Effects of chick brain extract on the proliferation of chick neuroblasts cultured in media supplemented with low and high serum concentrations

The proliferative activity of chick neuroblasts cultured in a medium containing a low (5%) or a high (20%) concentration of fetal calf serum (FCS) was analyzed and the influence of a chick brain extract was investigated. Morphological observations and tritiated thymidine incorporation measurements have shown that neuroblasts from 6 day-old chick embryo cerebral hemispheres proliferate more actively in the medium with 5% FCS compared to the medium with 20% FCS. The medium containing 5% FCS favoured the maintenance of neuronal cells in a neuroblast stage as shown by electron microscopy. The stimulatory effect of brain extract on the proliferation of neuroblasts is stronger in the low serum culture condition. These findings indicate that a low serum-containing medium is an adequate condition to study neuronal proliferation and effects of growth factors on these cells.

Development of cholinergic properties in nerve cell cultures in the presence of brain extract

Cells dissociated from cerebral hemispheres of 6-day-old chick embryos were cultured either in standard nutrient medium or in the presence of a brain extract from 8-day-old chick embryo. Morphological observations showed the development of bipolar and multipolar neurons in both culture conditions and acetylcholinesterase activity was found in all neuronal cells. Brain extract stimulated the morphological maturation of neurons, expressed by the formation of fiber bundles, fine structural maturation and development of synapses rich in clear vesicles. Furthermore, acetylcholinesterase and choline acetyltransferase activities were higher in the cultures treated with brain extract. In these cultures, the values of choline acetyltransferase activity reached a peak at 10 days and then decreased. These observations are discussed with particular reference to proliferation, maturation and degeneration of cholinergic neurons.

Arguments in favour of endocytosis of glycoprotein components of the membranes of parallel fibers by Purkinje cells during the development of the rat cerebellum

Chloroquine (a drug known to induce a dysfunction of lysosomes) was used to study the behavior of Concanavalin A binding glycoproteins located on the axolemma of parallel fibers in young rat cerebella, and abundant on these membranes at a period preceding synaptogenesis with the dendrites of Purkinje cells. Chloroquine induces in Purkinje cells a large accumulation of grains consisting of membrane whorls in lysosomes. These grains stain for Concanavalin A, and do not stain either for a mitochondrial marker (aspartate aminotransferase mitochondrial isoenzyme) or for a marker of the Purkinje cell internal membrane (PSG). It is suggested that the material accumulating in the Purkinje cells under the effect of chloroquine comes from the parallel fibers. Together with the observation that alpha-D-mannosidase (involved in the degradation of these glycoproteins) is exclusively located inside Purkinje cells, these results provide a firm indication that this material enters the Purkinje cells through pinocytosis. The absence of ATPase activity (ATPase is a glycoprotein plasma membrane marker highly concentrated on parallel fibers) within these grains suggested that not all the components of these membranes are pinocytosed, but that the process is specific for certain molecules. These results are compatible with the ultrastructural observations of others, and support the arguments in favour of the pinocytosis phenomenon being one of the first steps of synapse formation. The observed specificity of pinocytosis for certain molecules suggests that a receptor-mediated recognition of some glycans of glycoproteins is the preliminary event in the establishment of synapses.

Long-term effects of brain trypsinization before cell seeding on cell morphology and surface composition

The relation between the pattern of proteins localized in the surface of astroglial cells and cell differentiation was investigated in primary cultures derived from neonatal rat brains, dissociated either mechanically (MDC) or by 3 (TDC3) and 30 minutes (TDC30) trypsinization. Morphological and ultrastructural studies revealed a bed layer composed of flat, polygonal young and differentiated astrocytes in all types of cultures and a surface layer composed of small, ovoide undifferentiated cells which were more numerous in TDC30 than in TDC3 and MDC. The enrichment in undifferentiated cells, induced by prolonged brain trypsinization prior cell seeding, was observed during two weeks in culture; latter, by day 20, the cell population in all cultures was that of differentiated astrocytes. The presence of structural and enzymatic cell markers indicated that the cell population in MDC and TDC3 as well as in TDC3, including the small cells, was of astroglial origin. Concomitant with the morphological changes, cells in TDC30 were less accessible to surface labeling than those composing MDC. Subsequent electrophoresis of the labeled surface proteins demonstrated that a 140-130 K complex was the most "sensible" to brain trypsinization and that their accessibility to the surface probing was maximal during the differentiation of astrocytes in MDC or of small cells in TDC30. By day 20, these components were not significantly labeled in both, MDC, and TDC30, cultures. The use of two types of astrocytes primary culture which were different in the ratio of differentiated to undifferentiated cells and their surface labeling at different growth stages showed a variation in the composition of surface proteins during the cell maturation. The increased accessibility of some surface proteins to external probing when the cells developed to differentiated astrocytes might suggest their involvement in cell differentiation.

Isolation of cell surface membranes from cultured C6 glioblastoma cells

Plasma membranes were isolated from C6 glioblastoma cells by two methods. In the first method cells were treated with concanavalin A and lysed in hypotonic medium. After partial separation of plasma membranes from other cell material, the lectin was displaced with alpha-methyl-D-mannoside. In the second method untreated cells or cells iodinated in a lactoperoxidase-catalyzed reaction were homogenized in isotonic medium. Membrane fractions obtained by either homogenization procedure were further purified by rate zonal and equilibrium centrifugations into linear density gradients. Disruption of the glioblastoma cell membrane gives rise to heterogeneous assemblies of membrane fragments. Two populations of plasma membranes were isolated from untreated and from iodinated cells: a "lighter" membrane fraction characterized by relatively lower sedimentation velocity and buoyant density, and a "heavier" membrane fraction of relatively faster sedimentation velocity and higher buoyant density. Both fractions showed electrophoretic patterns similar to those of 125I-labeled cell surface proteins. Their specific (Na+ + K+)-ATPase activity was seven- to eightfold the homogenate activity (recovery, 13.1%). Both fractions were, however, still contaminated by smooth endoplasmic reticulum, as judged from the activity of NADPH-dependent cytochrome c reductase (recovery, 2.4%). It is suggested that plasma membrane fragments present in the two fractions might differ in the organization of their structures, e.g., membrane vesicle intactness and membrane orientation.

Isolation of intermediate filaments from rat astrocytes in culture

Intermediate filaments from rat astrocytes in culture were isolated by subcellular fractionation. The fractionation was monitored by electron microscopy and by quantitative immunoelectrophoresis using rabbit antibody directed against human glial fibrillary acidic protein (GFA). Morphologically intermediate filaments appeared helical with a mean diameter of 10 nm. Isolated filaments were disassembled at highly alkaline pH. After lowering of pH to slightly acidic values reassembled filaments, approximately 17 nm in diameter, were observed. As revealed by sodium dodecyl sulfate polyacrylamide gel electrophoresis the filament preparation was composed predominantly of a 51,000 molecular weight protein, corresponding to GFA. At high loads additional proteins of molecular weights 43,000 and 58,000 were detected. These latter proteins may represent residual actin and vimentin, respectively.

Effects of brain extracts from chick embryo on the development of astroblasts in culture

Different soluble extracts were prepared from chick brains and tested for their effect on the development of glial cells prepared from the cerebral hemispheres of newborn rats and grown in primary culture. Every brain extract tested, from animals between 8 days embryonic and adult ages, stimulated astroblast proliferation and accelerated their maturation as shown by morphological changes, the appearance of gliofilament bundles and by the increase of the amount of the glial-specific protein S-100 between 10 and 40 days in culture. These effects were observed in cultures continuously treated with extracts beginning with the 5th day after seeding. In cultures in which treatment was stopped 25 days after seeding, S-100 level continued to increase like in continuously treated cells. An identical increase of S-100 protein level was induced by treatments started as early as 24 h or as late as 25 days after seeding. In contrast, when the extracts were added at time 0 a toxic effect was observed and no stimulation of the maturation occurred.

Biochemical and ultrastructural studies of cultured rat astroglial cells: effect of brain extract and dibutyryl cyclic AMP on glial fibrillary acidic protein and glial filaments

The growth of astroglial cells in primary cultures derived from newborn rat cerebral hemispheres was investigated in the absence and in the presence of newborn rat brain extract or dBcAMP. The parameters chosen were the content of DNA, total protein, and glial fibrillary acidic protein (GFA) as well as the morphologic development of gliofilaments. During the entire culture period the DNA content increased in control culture indicating a continuous cell division, whereas the cells stopped dividing after 14 or 4 days of treatment with either brain extract or dBcAMP respectively. In contrast, a constant increase of total protein was found in both control and treated cultures. Since cell divisions had stopped in treated cultures, the increase in total protein in these cultures indicates growth of the individual cells. The GFA levels increased progressively and similarly in control cultures and in cultures treated with brain extract. The values in the treated cultures remained slightly higher than those in controls. Conversely, immediately after the addition of dBcAMP a sudden increase in GFA protein occurred and the amounts were statistically significantly different from those of the controls. The GFA levels were expressed relative to total protein indicating that GFA constitutes an increasing amount of the total protein of the individual cells during culture. The changes in the amount of GFA was shown to parallel the morphologic development of gliofilaments. Indeed, when the level of GFA increased a progressive accumulation of gliofilaments was observed. The results obtained were discussed in relation to the astrocytic maturation.

Immunohistochemical and immunocytochemical localization of myosin, chromogranin A and dopamine-beta-hydroxylase in nerve cells in culture and in adrenal glands

Dopamine-beta-hydroxylase, chromogranin A and myosin were purified from bovine adrenal medulla and antibodies prepared against these proteins. Indirect immunocytochemical methods were used to localize dopamine-beta-hydroxylase, chromogranin A and myosin in bovine adrenal medulla and myosin in rat adrenal glands and cells from rat C.N.S. maintained in primary culture. Dopamine-beta-hydroxylase and chromogranin A were found in chromaffin granules, in agreement with biochemical data and, using electron microscopy, dopamine-beta-hydroxylase was found within the matrix and in the surrounding membrane of the storage granule, whereas chromogranin A was confined to the granule matrix. Myosin was localized in the vascular system irrigating adrenal glands, fibroblasts lining the vessels and chromaffin cells. In chromaffin cells, staining was found at the cell boundaries and electron microscopy showed myosin to be associated with the plasma membrane. Faint immunocytochemical staining by antimyosin antibodies was observed around certain exocytotic profiles but particular association with such structures was not demonstrable. Myosin localization was also studied in bovine adrenal cortex, where it was found in vascular channels and faintly in adrenal cortical cells, as in rat adrenal cortex and medulla, where identical patterns were obtained. In neuronal and glial cells dissociated from 13 day rat embryo cerebral hemispheres and cultured for 48 h, localization of myosin was studied using immunohistochemistry. The neuritic expansions and growth cones of neurons were fluorescent, whereas in glial cells, filamentous networks were visualized enclosing the nucleus and as long fibres traversing the entire cytoplasm.

Freeze-fracture study of the chromaffin cell during exocytosis: evidence for connections between the plasma membrane and secretory granules and for movements of plasma membrane-associated particles

Exocytosis was studied in acetylcholine-stimulated bovine adrenal medulla. During a pre-exocytotic stage, chromaffin granules are found in juxtaposition to the plasma membrane and separated from it by an electron dense space 25--27 A in width. Freeze-fracture studies show this stage to be characterized by connections between the granules and the plasma membrane. These connections are apparently cytoplasmic but bridge both membranes; they are presumably proteinaceous, but their exact nature remains to be elucidated. Later stages of exocytosis were also studied by the freeze-fracture technique; a typical feature is the lack of intramembrane particles around the fusion site. Both connections and membrane particle movement are discussed in terms of recent biochemical findings.

Size heterogeneity of the structural units of brain nuclear ribonucleoprotein particles

Nuclear ribonucleoprotein particles were examined by electron microscopy after negative staining. The results confirmed the polymeric nature of the particles. The constitutive units were distributed into 4 size classes (diameters varying from 100 to 300 A, approximately) indicating that the monoparticle population was heterogenous. When examined on ultrathin sections, the particles had a fibrillar appearance.