Carlos A. Landa

Regulated expression of a 16-kd galectin-like protein in activated rat macrophages.

We investigated the presence of a galectin‐like protein in rat mononuclear cells using a polyclonal antibody raised against a soluble lactose‐binding lectin purified from adult chicken liver that immunoreacted strongly with a broad protein band of about 16 kd in Western blot assays. Immunochemical studies revealed a constitutive expression of this protein in mononuclear cells mainly in the macrophage (Mφ) population. Subcellular localization was assessed by Western blot assays of the cytosolic and membrane fractions of different cell populations studied: (1) spleen mononuclear cells, (2) T cell‐enriched, (3) B cell‐ and Mφ‐enriched populations, and (4) peritoneal cells, processed in the presence of lactose. In broad agreement with immunocytochemical studies of nonpermeabilized and permeabilized cells, Western blot assays suggest that this protein is localized mainly in the cytoplasmic compartment but also associated with the cell surface. By flow cytometric analyses we detected about a 14% of ED1 double‐positive cells corresponding to Mφs that constitutively express this galectin‐like protein associated with their cell surface. The cytosolic fraction obtained from the Mφ‐enriched cell population showed hemagglutinating activity specifically inhibited by β‐galactoside–related sugars. Moreover, this galectin‐like protein was retained in a lactosyl‐Sepharose matrix and specifically eluted with lactose. In this work, evidence is also provided to show that different stimuli are able to modulate the expression of the galectin‐like protein. Expression was upregulated in inflammatory and activated Mφs, revealing a significant increase in phorbol ester– and formylmethionine oligopeptide–treated cells. Both stimuli involving protein kinase C activation pathway have been able not only to up‐regulate the total expression of this protein but also to modulate its subcellular localization.

THE SITE OF SYNTHESIS OF GANGLIOSIDES IN THE CHICK OPTIC SYSTEM

Abstract– In the retinas of 1‐day‐old chickens that received an intraocular injection of N‐[3H]acetylmannosamine the labelling of N‐acetylneuraminic acid and CMP‐N‐acetylneuraminic acid increased for at least 8 h and that of gangliosides for at least 24 h after injection. In the optic tectum contralateral to the injected eye at 8 h after the intraocular injection, the labelling of gangliosides exceeded the labelling of gangliosides in the ipsilateral tectum by approx 20‐fold. In the contralateral tectum the highest concentration of labelled gangliosides was in subfractions enriched in synaptosomes and synaptic plasma membranes. No significant contralateral ipsilateral differences were found in the acid soluble substances of the tectum. In the optic tectum, labelled gangliosides appeared earlier in the neuronal perikarya than in synaptosomes when the injection was intracranial. Conversely, when the injection was intraocular the labelling appeared earlier in the synaptosomes than in the neuronal perikarya. The radioactivity pattern of the optic tectum gangliosides resembled the pattern of retina gangliosides when N‐[3H]acetylmannosamine was injected intraocularly, but when N‐[3H]acetylmannosamine was given intracerebrally the radioactivity pattern resembled that of optic tectum gangliosides. Intraocular injection of colchicine or vinblastine did not affect the labelling of retinal gangliosides from N‐[3H]acetylmannosamine injected into the same eye but prevented the appearance of labelled gangliosides in the optic tectum. In vitro the ganglioside glycosylating activity of optic tectum synaptosomes and synaptic plasma membranes was between 6 and 10‐fold lower than that found in the optic tectum neuronal perikarya. These findings support the notion that the main subcellular site of synthesis of neuronal gangliosides is in the neuronal perikarya, from which they are translocated to the nerve endings.

Disposition of Gangliosides and Sialosylglycoproteins in Neuronal Membranes

Abstract Labeled gangliosides and glycoproteins were obtained by incubation of homogenized neuronal perikarya from rat brain with CMP‐[3H]N‐acetyl neuraminic acid. The highest degree of labelling was observed in a subcellular fraction that also showed the highest specific activities for several ganglioside glycosyltransferases. The [3H] sialosylglycoconjugates of this fraction remained associated with the membranes after treatment with 1 m‐KCl, 125 mm‐EDTA, repeated freezing and thawing, or controlled sonication, but were solubilized by sodium deoxycholate (DOC) at a concentration high enough to solubilize the choline phospholipids. About 75% of the neuraminidase‐labile sialosyl residues of these labeled endogenous gangliosides and glycoproteins were protected from the action of added neuraminidase or pronase or both enzymes added together. The protection was not abolished by pretreatment of the membranes with high ionic strength or with EDTA but was abolished by sonication or low concentration of DOC. Between 50 and 80% of the neuraminidase‐labile sialosyl residues of the gangliosides of the neuronal perikaryon membrane fraction labeled in vivo by an intracerebral injection of N‐[3H]acetylmannos‐amine were, at 3 h after the injection, also protected from the action of added neuraminidase. The protection was abolished by the addition of DOC. In contrast with the behavior of the labeled glycoconjugates of this neuronal perikaryon fraction, the gangliosides and sialosylglycoproteins from intact synaptosomes were accessible to neuraminidase. It is suggested that most gangliosides and sialosylglycoproteins are sialosylated as intrinsic components of the neuronal perikaryon membrane fraction and that at some stage of the process of transport through the axon and incorporation into the synaptic plasma membrane they change their accessibility to added enzymes.

Regulation of Ganglioside Composition and Synthesis Is Different in Developing Chick Retinal Pigment Epithelium and Neural Retina

Abstract: We examined the immunocytochemical expression of GM3 and QD3 in 3‐day‐old chick embryo retinal pigment epithelium (RPE) and neural retina (NR). We also compared the composition of gangliosides and the activities of key ganglioside glycosyltransferases of the RPE and NR of 8‐, 12‐, and 15‐day old embryos. The immunocytochemical studies in 3‐day‐old embryos showed heavy expression of GM3 and GD3 at the inner and outer layers of the optic vesicle that are the precursors of the RPE and NR, respectively. The compositional and enzymatic studies showed pronounced differences between RPE and NR of 8‐day and older embryos. HPTLC showed that at 8 days the major species were GM3 and GD3 in RPE and GD3 and GT3 in NR. As development proceeded, GD3 decreased in both tissues, GM3 became the major ganglioside in RPE, and ganglio‐series gangliosides (mainly GD1a) became the major species in NR. At 15 days the major species were GD1 a in NR and GM3 in RPE. Enzyme determinations showed that whereas in RPE from 12‐day‐old embryos GM2 synthase was under the limit of detection and GD3 synthase activity was about sixfold lower than GM3 synthase, in NR the activities of GM3 and GD3 synthases were similar and both six‐to ninefold lower than GM2 synthase. These results evidence a markedly different modulation of the ganglioside glycosylating system in cells of a common origin that through distinct differentiation pathways originate two closely related tissues of the optic system. In addition, they reinforce the relevance of the relative activities of key transferases in determining the pattern of gangliosides in different cell types.

Chitosan-induced phospholipase A2 activation and arachidonic acid mobilization in P388D1 macrophages

We have found that chitosan, a polysaccharide present in fungal cell walls, is able to activate macrophages for enhanced mobilization of arachidonic acid in a dose‐ and time‐dependent manner. Studies aimed at identifying the intracellular effector(s) implicated in chitosan‐induced arachidonate release revealed the involvement of the cytosolic Group IV phospholipase A2 (PLA2), as judged by the inhibitory effect of methyl arachidonoyl fluorophosphonate but not of bromoenol lactone. Interestingly, priming of the macrophages with lipopolysaccharide renders the cells more sensitive to a subsequent stimulation with chitosan, and this enhancement is totally blocked by the secretory PLA2 inhibitor 3‐(3‐acetamide)‐1‐benzyl‐2‐ethylindolyl‐5‐oxy‐propanesulfonic acid (LY311727). Collectively, the results of this work establish chitosan as a novel macrophage‐activating factor that elicits AA mobilization in P388D1 macrophages by a mechanism involving the participation of two distinct phospholipases A2.

Galectins: a key intersection between glycobiology and immunology

Galectins are a family of evolutionarily conserved animal lectins, widely distributed from lower invertebrates to mammals. They share sequence and structure similarities in the carbohydrate recognition domain and specificity for polylactosamine-enriched glycoconjugates. In the last few years significant experimental data have been accumulated concerning their participation in different biological processes requiring carbohydrate recognition such as cell adhesion, cell growth regulation, inflammation, immunomodulation, apoptosis and metastasis. In the present review we will discuss some exciting questions and advances in galectin research, highlighting the significance of these proteins in immunological processes and their implications in biomedical research, disease diagnosis and clinical intervention. Designing novel therapeutic strategies based on carbohydrate recognition will provide answers for the treatment of autoimmune disorders, inflammatory processes, allergic reactions and tumor spreading.

Adult Rat Retina Interneurons Synthesize GD3: GD3 Expression by These Cells Is Regulated by Cell‐Cell Interactions

GD3, a ganglioside of the lactosyl series, is prevalent in rat retina neuronal cells. We studied here whether rat retina neurons synthesize their own surface GD3 or if they acquire it from Müller glia cells. We analyzed the activity of GD3 synthase and the in vivo labeling of gangliosides from N‐[3H]acetylmannosamine in adult rat retinas after selective destruction of Müller glia cells with the gliotoxic α‐D,L‐aminoadipate (AAA). Immunostaining of rat retina sections and western blot analysis with an antivimentin antibody confirmed the gliotoxic effect of AAA. Neither GD3 synthase activity nor the in vivo labeling of GD3 and other gangliosides was significantly affected by AAA, indicating that neuronal cells synthesize their own GD3. We next analyzed the regulation of the expression of GD3 by these neurons in culture. About 80% of freshly dissociated cells from retina of 4‐day‐old rats (R4) immunoexpress surface GD3. After 3 days in dispersed cell culture conditions, GD3 expression was under the limit of detection in 80% of neuronal cells, indicating a failure of these cells to maintain the expression of surface GD3 in these experimental conditions. Most flat Miiller glia‐derived cells present in these cultures were GD3 positive. Surface GD3 was detected in ∼60% of neuronal cells dissociated from R4 tissue that was developed in vitro as an organ culture for 3 days. Likewise, ~50% of neurites that had grown out from R4 retinal ex‐plants within 3 days in culture and whose neuronal character was indicated by immunoexpression of growth‐associated protein GAP‐43 were GD3 positive. These findings suggest that the tissue organization and/or specific interactions modulate GD3 expression in neuronal cells. Under dispersed‐cell culture conditions, c‐pathway gangliosides (GQ1c and GT1c), which are built up from the sialylation of GD3 and later completion of the oligosaccharide backbone, were detected in ~60% of neuronal cells, suggesting a maintenance of production of GD3 as an intermediate for gangliotetraosyl gangliosides.

Biosynthesis of gangliosides in cultured retina from chick embryos

Retina tissue from 7-day chick embryos was maintained in culture for up to 10 days. After 5 days in culture the incorporation of [3H]leucine into proteins and of [3H]glucosamine into gangliosides was similar to that found in retinas from 12-day embryos. The incorporation of [3H]thymidine into DNA decreased steadily with time in culture; after 5 days it was about 20% of the initial value and approximately twice that determined in retinas from 12-day embryos. The radioactivity pattern of gangliosides labeled with [3H]glucosamine showed a predominance of the label in disialosyllactosylceramide (GD3); up to the 3rd day of culture. From then on, there was a progressive increase in the labeling of disialosylgangliotetraosylceramide (GD1a); by day 7 of culture, labeling of GD1a predominated and the labeling pattern was indistinguishable from that found in retinas from 12-day-old embryos. The specific activities of the CMP-NeuAc:GM3 sialosyl- and UDP-GalNAc:GM3 N-acetylgalactosaminyl-transferases decreased to 15% and increased to 400%, respectively, of the values determined in the retinas of 7-day embryos. The cultured retinas progressed in their organization into layers with culture time. The labeling transition from GD3 to GD1a was also detected after inhibition of the histotypic organization by addition of 5-bromo 2-deoxyuridine to the culture medium. Results suggest that high activity of GM3:sialosyl transferase and high labeling of GD3 are associated with the proliferative state of retina cells, while high activity of GM3:N-acetylgalactosaminyltransferase and high labeling of GD1a are associated with the non-proliferative, differentiated state of these cells.

Effect of light on the labeling of optic tectum gangliosides after an intraocular injection of N-[3H]acetylmannosamine.

Abstract Axonally transported gangliosides from retina were more labeled in the optic tectum of chickens exposed to light compared to those maintained in the dark. No differences were observed between the labeling of retinal gangliosides from the two groups. These results indicate that light modifies either the labeling of ganglion cell gangliosides or their axonal transport.

The Biosynthesis of Brain Gangliosides — Evidence for a “Transient Pool” and an “End Product Pool” of Gangliosides

Incubation of sugar nucleotides with brain membranes in which endogenous, incomplete gangliosides are glycosylated, and addition of sugars to exogenous acceptors indicated the same pathway of synthesis of gangliosides (for a review, see Caputto et al. 1974; 1976). An exception was the pathway of completion of GDlb, for which a different route was worked out using endogenous (Arce et al., 1971) or exogenous acceptors (Cumar et al., 1971; 1972). The pathway of synthesis found for GTlb (Arce et al., 1971) was recently confirmed by using exogenous acceptors (Mestrallet et al., 1974).