functional role of a specific ganglioside in neuronal ... · migration mode - tangential migration...

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Braz J Med Biol Res 36(8) 2003

Gangliosides and neuronal motilityBrazilian Journal of Medical and Biological Research (2003) 36: 1003-1013ISSN 0100-879X

Functional role of a specificganglioside in neuronal migrationand neurite outgrowth

Instituto de Biofísica Carlos Chagas Filho,Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brasil

R. Mendez-Oteroand M.F. Santiago

Abstract

Cell migration occurs extensively during mammalian brain develop-ment and persists in a few regions in the adult brain. Defectivemigratory behavior of neurons is thought to be the underlying cause ofseveral congenital disorders. Knowledge of the dynamics and molec-ular mechanisms of neuronal movement could expand our under-standing of the normal development of the nervous system as well ashelp decipher the pathogenesis of neurological developmental disor-ders. In our studies we have identified and characterized a specificganglioside (9-O-acetyl GD3) localized to the membrane of neuronsand glial cells that is expressed in regions of cell migration and neuriteoutgrowth in the developing and adult rat nervous system. In thepresent article we review our findings that demonstrate the functionalrole of this molecule in neuronal motility.

CorrespondenceR. Mendez-Otero

Instituto de Biofísica, CCS, UFRJ

21941-590 Rio de Janeiro, RJ

Brasil

Fax: +55-21-2280-8193

E-mail: [email protected] [email protected]

Presented at SIMEC 2002(International Symposium

on Extracellular Matrix),Angra dos Reis, RJ, Brazil,October 7-10, 2002.

Research supported by PRONEX,CNPq, FAPERJ, and by a grantfrom the Ministry of Science and

Technology (MCT) to the MillenniumInstitute for Tissue Bioengineering,Brazil.

Received February 5, 2003

Accepted May 27, 2003

Key words• Neuronal migration• Gangliosides• Development• 9-O-acetyl GD3• Neurite outgrowth

Introduction

Directional movements occur extensivelyat all stages of morphogenesis of the nervoussystem. In addition to the interkinetic nuclearmigration of the pseudostratified neuroepi-thelium, extensive cell migration occurs inthe developing and adult nervous system andensures that postmitotic immature neuronsgenerated in the primary or secondary germi-native zones acquire their proper positions inthe mature brain. The first mode of migration- radial migration - involves movement ofneuroblasts orthogonally to the pial surface,with most of the cells moving associatedwith a special glia, the radial glia. Guidancealong radial glia is a common mechanism forthe differentiation of projection neurons andthe establishment of laminated structures,

such as the cerebral cortex and the cerebel-lum (for reviews, see Refs. 1,2).

This migration process is a complex de-velopmental program involving a series ofsteps including cell-cell recognition, cell-cell adhesion, cell motility, and finally de-tachment from the glial fibers once neuronsreach their destination (3). The other generalmigration mode - tangential migration - in-volves the displacement of neuronal pro-genitors or postmitotic cells parallel to thepial surface. This is a prominent feature inthe migration of neuronal progenitors fromthe rhombic lip that are destined to brainstemnuclei or to the subpial aspect of the cerebel-lar cortex (4), in the migration of prospectiveluteinizing hormone-releasing hormone fromthe olfactory placode to the hypothalamus(5), of cortical interneurons from the gangli-

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R. Mendez-Otero and M.F. Santiago

onic eminences (6), and of prospective ol-factory interneurons from the subventricularor subependymal zone to the core of theolfactory bulb (7). This mode of migrationdoes not rely on a glial scaffold and severalpossibilities have been hypothesized to ac-count for the migration of neuroblasts undersuch conditions. For example, it has beenproposed that granular neuronal precursorsmigrate associated with axons to form theexternal granular layer of the cerebellum (8),and that some of the cells migrating tangen-tially from the ganglionic eminences to thecortex use the incoming corticofugal fibersas a guide (9). It has also been shown that inthe rostral migratory stream neurons moverapidly along one another in unique chainformations independent of radial glia or ax-onal processes using their migrating neighborsto provide support for their movement (10).

These considerations raise the questionof whether there are common molecularmechanisms underlying neuronal migration(radial or tangential) and other directionalmovements such as axonal pathfinding. Inthis respect, there is already evidence thatsome factors that control directional growthduring tangential migration are similar tothose that control the outgrowth of neuronalgrowth cones (for a review, see Ref. 11).

In our studies we have identified andcharacterized a specific glycolipid localizedto the membrane of neurons and glial cellsthat is expressed in regions of cell migrationand neurite outgrowth in the developing andadult nervous system. This molecule is rec-ognized by a monoclonal antibody namedJones and was identified as the ganglioside9-O-acetyl GD3 (12,13). In several studieswe have shown that the distribution of thisganglioside is temporally and spatially cor-related with neuronal migration and neuriteoutgrowth in the central and peripheral ner-vous system and we thus hypothesize thatthis ganglioside participates in critical stagesof neuronal migration and neurite outgrowth.In this review, we will summarize our stud-

ies that provide evidence for the functionalrole of this glycolipid in neuronal motility.

Gangliosides and neuronal motility

Gangliosides are a large group of sialyl-ated glycosphingolipids widely expressed inmammalian tissues and particularly abun-dant in the nervous system (14). Ganglio-sides possess a remarkable degree of struc-tural diversity, and numerous enzymes areinvolved in their synthesis, recycling, andturnover. The biosynthesis of gangliosidesproceeds in a stepwise manner starting fromlactosyl ceramide and involves several gly-cosyltransferases. Some of the genes codingfor some of the enzymes have been recentlycloned but several questions are still underactive investigation regarding how the levelsof glycosyltransferases are regulated anddetermined (Figure 1A) (for a review, seeRef. 14). An important modification of gan-gliosides is the O-acetylation of sialic acidresidues on C-9, C-7 or C-9 and C-7 ob-served on restricted cells and tissues but theenzymes responsible for these modifications(O-acetyl transferases) have been particu-larly difficult to clone. It has been suggestedthat O-acetylation of gangliosides may serveas a cellular recognition signal and also as atumor marker (12,15). Sialidases are the keyenzymes for ganglioside degradation and theplasma membrane-bound sialidase has beenimplicated not only in the general catabo-lism of gangliosides but also in the modula-tion of cellular function, such as prolifera-tion and differentiation (16).

In the nervous system, the species andamounts of gangliosides undergo profoundchanges during development, suggesting thatthey may play fundamental roles in this pro-cess (17). The pattern of gangliosides changesfrom a relatively simple scheme, with GM3and GD3 gangliosides predominating at ear-lier stages, to a more complex pattern withfour gangliosides of the a- and b-series, GM1,GD1a, GD1b, and GT1b, that constitute the

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major species in the adult (for a review, seeRef. 18). Furthermore, the accumulation ofgangliosides within the neurons in ganglio-side storage diseases results in extensiveneurite outgrowth (19) and exogenously ad-ministered gangliosides accelerate the re-generation of neurons in the central nervoussystem in vivo and stimulate cellular differ-entiation with concomitant neurite sprouting

and extension in vitro (20). Recently, thegenes involved in the biosynthesis of gan-gliosides have been cloned and geneticallyengineered mice or cells lacking most of thegangliosides have been produced (21). Sur-prisingly, the mice and the cells appeared tobe largely normal in their neuronal develop-ment. However, more recent studies haverevealed that mice lacking complex ganglio-

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Figure 1. A, Proposed pathway for the biosynthesis of gangliosides. B, Schematic diagram showing the structure of 9-O-acetyl GD3. This gangliosideis formed by the addition of an acetyl group to the position 9 of the terminal sialic acid of GD3. Cer = ceramide; Gal = galactose; Glc = glucose;SAT-I = GM3 synthase; Sat-II = GD3 synthase; SAT-III = GT3 synthase.

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sides exhibit axonal degeneration and myeli-nation defects (22).

Experiments in several laboratories haveshown that glycosphingolipids, gangliosidesin particular, are important components ofmembrane rafts, where they can mediateimportant physiological functions such ascell adhesion and signal transduction eventsand consequently affect cellular phenotypesand functions (for a review, see Ref. 23).Recently, such microdomains have beentermed “glycosynapses” in analogy to “im-munological synapses” - the membrane as-sembly of immunocyte adhesion and signal-ing (24). Gangliosides are very abundant inglycosynapses and it has also been shownthat anti-ganglioside antibodies can immu-noprecipitate glycosylphosphatidylinositol-anchored proteins (such as TAG-1), Src fam-ily kinases and caveolin (25,26). In addition,several studies have shown the associationof gangliosides with integrins and their abil-ity to modulate integrin function (27).

Our studies have focused on a specificganglioside recognized by the Jones mono-clonal antibody (mAb) (12). In our initialstudies we have shown that the pattern ofstaining with this antibody in the developingrat nervous system correlates temporally andspatially with neuronal migration and neu-rite outgrowth (28,29). The biochemical char-

acterization of the antigen revealed that thismAb recognizes a ganglioside that migratesbetween GM1 and GM2 ganglioside stan-dards. Further characterization using over-lay assays on developed high-performancethin-layer chromatography plates indicatesthat the epitope recognized by the JonesmAb is expressed in several regions of thedeveloping central and peripheral nervoussystem. In most regions examined the epi-tope resides in two bands (29). Further anal-ysis has revealed that the most abundant andfrequent band is 9-O-acetyl GD3, a modifi-cation of GD3 ganglioside in which an acetylester is formed at position 9 of the terminalsialic acid residue (Figure 1B) (13). Theparticular epitope recognized by the JonesmAb depends upon the acetyl group at posi-tion 9 of sialic acid and the immunoreactiv-ity is abolished by mild base treatment sincethis treatment converts 9-O-acetyl GD3 toGD3 that is not recognized by the Jones mAb(13,30,31). This antigen is abundant in thenervous system and in tumor cells derivedfrom the neural crest, and is absent in most ofthe other tissues examined (29). However,the same epitope was described in the im-mune system and was designated CD60b(32). The 9-O-acetyl GD3 is present in theretinas of all mammalian species studied so farbut is not present in extracts of frog retinas(33). At least two other mAbs have been de-scribed (mAb D1.1 and RB13-2) that recog-nize the same epitope in nervous tissue (34,35).The immunoreactivity of the slower migratingband is also abolished by mild base treatmentand it has been suggested that this bandmight correspond to 9-O-acetyl GQ1c (36).

In the developing nervous system, wehave shown that the expression pattern ofthis specific Jones mAb reactive gangliosidecorrelates with times of cell migration in theretina (Figure 2), superior colliculus, cer-ebellum, and telencephalon and in regionsundergoing neurite extension, such as thedeveloping optic tract, the white matter ofthe cerebellum, the dorsal roots (Figure 3),

Figure 2. Immunocytochemicaldistribution of 9-O-acetyl GD3 inthe developing rat retina. Dark-field photomicrograph of animmunogold-stained sectionthrough the eye of an embry-onic day 18 rat embryo. Observethe staining in the central part ofthe retina and the absence oflabeling at the periphery. At thisage the presumptive ganglioncells are formed in the centralpart of the retina and migrate toform the ganglion cell layer.Scale bar: 150 µm.

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the trigeminal system, and olfactory nerve(12,28,29,33,37-39). This has led us to sug-gest that the function of 9-O-acetyl GD3 is tomodulate motility either directly or by modi-fying the efficacy of some other componentof the system (39,40). The association of 9-O-acetylated gangliosides with cell migra-tion has been well characterized in cancerresearch, where it was found that tumorsarising from neural crest-derived tissues ex-press high levels of 9-O-acetyl GD3. Thesegangliosides are concentrated in adhesionplaques and are involved in cell adhesionand migration in tumor cells (14,15). In ad-dition, disialogangliosides co-immunopre-cipitate with αVß3 integrin and GM1 co-immunoprecipitates with the epidermalgrowth factor receptor (for a review, see Ref.24). Furthermore, it was also shown thatcleavage of 9-O-acetyl groups by transgenicexpression of influenza C virus hemaggluti-nin caused abnormalities in the developmentof the retinal layers in the mouse, suggestinga failure in cell migration concomitant withthe absence of 9-O-acetyl GD3 (41). Re-cently, it was shown that down-regulation of9-O-acetyl GD3 by stable transfection of O-acetylesterase cDNA and antisense vectoragainst the GD3-synthase gene results in celldifferentiation in melanoma cell lines (42).Based on our observations and the reportsconcerning other systems, we raised the hy-pothesis that 9-O-acetyl GD3 may play animportant role in neuronal motility in thedeveloping and adult nervous system. In thefollowing sections we will describe the evi-dence showing a functional role for thisganglioside in neuronal migration and neu-rite outgrowth.

Gangliosides in glial-guided radialmigration

In most cases, neuronal radial migrationoccurs on the processes of radial glial cellsand has been studied extensively in the cor-tex and cerebellum although similar arrange-

ments for migrating neurons and specializedglia have been described in a number ofother systems including the retina and thehippocampus (for a review, see Ref. 1). Re-cent studies have begun to provide a frame-work for the molecular mechanisms under-lying this mode of migration (for a review,see Ref. 43).

In the postnatal cerebellum, the postmi-totic neurons in the external granular celllayer also migrate radially but in this caseaway from the pia towards the internal granu-lar cell layer. This migration is also associ-ated with radial glia and this system has beenextensively used as a model for studies ofglial-guided migration (for a review, see Ref.1). We have investigated the possible role of9-O-acetyl GD3 in the glial-guided neuronalmigration using the developing rat cerebel-lum as a model. In previous studies we hadshown that the expression of this gangliosideis developmentally regulated in the develop-ing cerebellum (28). In E14-18 fetuses, im-munocytochemistry labeling with Jones mAbis present, extending from the ventricular tothe pial surface of the cerebellar anlage.Most of the immunoreactivity is distributedin a radially oriented pattern that correspondsto the radial migration of Purkinje cell pre-cursors migrating from the ventricular zonetowards the pial surface. Immunoreactivity,however, is present in the rhombic lip and inthe developing external granular cell layercorresponding to the subpial tangential mi-gration of these cells. During the postnatalperiod in the rat, the cells in the externalgranular cell layer proliferate and migrate

Figure 3. Pattern of 9-O-acetylGD3 expression in the develop-ing dorsal root ganglia. Dark-field photomicrograph of a para-sagittal section through the cer-vical region of an embryonic day18 rat. Observe the intense im-munoreactivity in the gangliaand also in the central and pe-ripheral processes. Scale bar:50 µm.

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radially away from the pia to form the inter-nal granular cell layer. At this stage, immu-nocytochemistry reveals a radially orientedpattern of Jones binding extending from theexternal granular cell layer towards the pre-sumptive granular cell layer (Figure 4). Elec-tron microscopic immunocytochemistry re-vealed that around the peak of cerebellarneuronal migration, 9-O-acetyl GD3 was lo-calized at the contact sites between migrat-ing granule cells and radial glia in the exter-nal granular cell layer and prospective mo-lecular layer (40). We have also observedthat both cultured neurons from the rat cer-ebellum when in contact with glial cells thatsupport cell migration (radial glial cells) andglial cells themselves express 9-O-acetyl GD3(44). In contrast, glial cells with a stellatemorphology, even when in contact with gran-ule neurons, do not express this antigen (44).To test the functional role of 9-O-acetyl GD3in neuronal migration we have used cerebel-lar explants and cerebellar slices. We haveshown that the Jones mAb blocks the migra-tion of neurons in a dose-dependent manner,suggesting that 9-O-acetyl GD3 is involvedin the radial migration of presumptive gran-ule cells in the developing cerebellum (40).These results further support our view thatthis molecule is involved in cell migration.

The immunoreactivity for this ganglio-side is also present in the embryonic telen-cephalon, showing a radial organization cor-related with radial migration in this region(28). Blockage of the ganglioside with Jonesantibody arrests neuronal migration on tis-sue slices of embryonic telencephalon (45).

Gangliosides in tangential migration

Glial-guided neuronal migration has beenrelatively well studied, whereas much less isknown about tangential migration (for a re-view, see Ref. 11). Tangentially migratingneurons are found in the cortex, the cerebel-lum and the rhombencephalon and they mi-grate perpendicularly to the glial scaffolding(6,46,47). Some of the molecular cues thatguide tangential migration have been recentlyidentified and the picture that is emerging isthat the same families of signals involved inthe directional guidance of developing growthcones may also guide the translocation ofcell bodies during tangential migration (9).

The migration of interneurons from theanterior subventricular zone to the olfactorybulb provides a useful system to study thecellular and molecular mechanisms that regu-late tangential neural migration. These neu-ronal precursors migrate through a distinctpathway within the subventricular zone calledthe rostral migratory stream, and this migra-tion occurs even in the adult. Molecularstudies have shown that, in the particularcase of the rostral migratory stream, the mech-anism of migration involves the sialylatedform of the neural cell adhesion molecule(NCAM) and is guided, in part, by negativechemotropism (48).

In view of the abundant evidence for arole of 9-O-acetyl GD3 in directional move-ments of neuronal soma or processes wehave performed an immunohistochemicalanalysis of their expression along the routeof tangential migrations of some olfactorybulb prospective interneurons in the devel-oping and adult rats. We have found that thisganglioside is highly expressed in the lateralventricle subventricular zone and along theroute of tangential migration (rostral migra-tory stream) into the olfactory bulb duringdevelopment (Figure 5). In the adult, wehave found staining around the lateral ven-tricle and in chains of cells in the rostralmigratory stream region (49). In a few sec-

Figure 4. Correlation of 9-O-acetyl GD3 expression with ra-dial cell migration. Fluorescencephotomicrograph of a parasagit-tal section through the cerebel-lum of a newborn rat. Rostral isto the right. Observe the stainingof 9-O-acetyl GD3 throughout thepresumptive cerebellar cortex inthe most anterior folia and theabsence of labeling in the poste-rior folia. Scale bar: 100 µm.

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tions, individual stained cells are observed,with a morphology similar to that of migrat-ing cells. Despite the scarcity of staining,this is one of the few regions where expres-sion of 9-O-acetyl GD3 is found in the adultnervous system. Interestingly, this regionhas been described as one of the regions inwhich neurogenesis persists in the adult brainand from which stem cells can be isolated inthe adult brain. It would be interesting todetermine whether stem cells isolated fromadult brains also express 9-O-acetyl GD3.

To investigate the functional role of 9-O-acetyl gangliosides in tangential migrationwe have used explants of the subventricularzone region as a model. We have found thatmigrating chains similar to the ones formedin vivo are also seen in this in vitro system.The migrating chains from the subventricu-lar zone explant express 9-O-acetyl GD3which is distributed in a punctiform mannerin individual cells and treatment of the cul-tures with the antibody against 9-O-acetylGD3 arrested neuronal migration in thesecultures. These data suggest that 9-O-acetylGD3 participates in neuronophilic as well asgliophilic migration (50,51).

Gangliosides and neurite outgrowth

During development, neuronal growthcones interact with physical and chemicalcues in their environment and these interac-tions guide the growth cones along specificpathways to their appropriate targets (for areview, see Ref. 52). Many factors such assoluble or substrate-bound growth factorsand components of the extracellular matrixprovide favorable environments that allowor promote motility of growth cones andneurite elongation (53). Repulsive and in-hibitory factors have also been describedand may participate in guidance of growthcones (for a review, see Ref. 52). Ganglio-sides, in particular disialogangliosides, havebeen implicated in adhesion systems and incell to substrate adhesion of both melanoma

and neuroblastoma cells (15). Indeed, a num-ber of separate lines of evidence support thehypothesis that these molecules modulatethe activity of the integrin family of cellsurface adhesion molecules (for a review,see Ref. 23) and selectively induce the syn-thesis of MAP2 (high molecular weight mi-crotubule-associated protein 2) (54). Fur-thermore, numerous studies employing avariety of models have reported that ganglio-side treatment will increase the rates of neu-ritogenesis both in vitro and in vivo, andaxonal regeneration and sprouting followingbrain trauma to the central or peripheralnervous system, suggesting a role for gan-gliosides in neurite outgrowth.

In our studies of the expression of 9-O-acetyl GD3 in the developing nervous sys-tem we have found that, in addition to thepattern of staining coinciding with neuronal

Figure 5. Expression of 9-O-acetyl GD3 is correlated withtangential cell migration. A, Pho-tomicrographs of a parasagittalsection through the telencepha-lon of a postnatal day 7 ratstained with the antibodyagainst 9-O-acetyl GD3. Notethe intense staining around thelateral ventricle (LV) extendingtowards the olfactory bulb (tothe left of the figure) and aroundthe caudatum-putamen (CPu).CC, corpus callosum. B, The ros-tral migratory stream shows ahigh expression of 9-O-acetylGD3. Scale bar: 100 µm.

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migration, these gangliosides were also ob-served along pathways of axon extensionssuch as the optic tract, the olfactory nerveand the central and peripheral processes ofdorsal root ganglia. This pathway staining isonly observed while large numbers of growthcones are present at these positions and thestaining disappears from most of the path-ways studied as soon as the axons reach theirtarget regions (28,37). One interesting ex-ception to this generalization, however, isthe olfactory system (37). In this system wehave shown that the expression of 9-O-acetylGD3 begins as early as embryonic day 13,when the olfactory epithelium and the mi-gratory mass were intensely stained. At em-bryonic day 19, the immunoreactivity haddisappeared from the olfactory epitheliumbut remained in a few fascicles and someglomeruli of the olfactory bulb in the new-born and adult. We conclude that the expres-sion of 9-O-acetyl GD3 by olfactory axonsand/or migrating cells may facilitate axonaloutgrowth during development and might beinvolved in the formation of new glomeruliin the mature olfactory bulb (37).

We have recently investigated whether9-O-acetyl gangliosides are also re-expressedduring the regeneration of sciatic nerves thatwere submitted to crushing. We have foundan up-regulation of 9-O-acetyl GD3 expres-sion starting five days after crushing, reach-ing a maximum at day 7, and decreasingafterwards to reach the level of the controlanimals at day 10 (Mendez-Otero R, SilvaAO, Resende VTR and Hedin-Pereira C,unpublished data). These results suggestedto us that 9-O-acetyl GD3 may play a func-tional role in neurite outgrowth during de-velopment and regeneration.

To test the functional role of 9-O-acetylGD3 in neurite outgrowth, we used explantsof dorsal root ganglia. In previous studies wehave shown that embryonic dorsal root gan-glion neurons and their processes either invivo (28), in explant cultures or as dissoci-ated cells present heavy immunoreactivity to

the antibody against the ganglioside. Wethen plated dorsal root ganglion explantsfrom embryonic day 16 onto laminin, col-lagen or fibronectin. Neurites grew out of theexplants and expressed high amounts of 9-O-acetyl GD3 in all substrates. We theninvestigated the functional role of this gan-glioside on neurite extension using embry-onic dorsal root ganglion explants grown onlaminin substrates as a model. In these ex-periments, the behavior of individual growthcones was recorded using a time-lapse video-enhanced imaging system before and afterthe addition of antibodies that recognize spe-cific gangliosides known to be expressed onthese growth cones. Using this system, it waspossible to demonstrate that the advance ofgrowth cones on laminin was halted in thepresence of Jones mAb. The onset of theeffect was rapid and signaled by an immedi-ate cessation of elongation, a loss of lamelli-podia and a retrieval of axoplasm (55). Block-ade of 9-O-acetyl GD3 also induces the ap-pearance of lateral spikes along the neuritesin addition to the effect on the growth cone.We have shown that microfilaments and mi-crotubular rearrangements accompany thesestructural modifications (56). Our resultssuggest that 9-O-acetyl GD3 may play animportant role in the extension of growthcones and consequently influence naviga-tion and pathway finding during develop-ment and regeneration.

Conclusion and future directions

The mechanisms by which gangliosideexpression leads to neuronal migration andneurite outgrowth are still far from beingcompletely identified. However, based onthe data reviewed here and also on severalfindings from different laboratories, it is pos-sible to suggest several mechanisms to ex-plain the role of 9-O-acetyl GD3 in thesephenomena. It is likely that specific ganglio-sides could interact with broadly distributedprotein-based adhesion systems and favored

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subsets of moving cells and/or processes thatexpress these molecules (Figure 6). Examplesof such mechanism are the described inter-actions of gangliosides with integrin recep-tors (57,58) and with TAG-1 (an adhesionmolecule) (25). Another possibility is thatgangliosides may serve to modulate the avail-ability of divalent cations to a number ofdifferent calcium- or magnesium-dependentadhesion systems and consequently regulateligand binding (57). Alternatively, it has beensuggested that gangliosides may regulate thelateral diffusion and assembly of signalingcomplexes in membrane microdomains andtherefore interfere with the functional role ofthese proteins, as proposed for integrin infocal adhesion points and TAG-1 (59). An-other possible model is based on the homo-philic interaction between gangliosides lo-cated on different cell membranes. A similarmodel has been proposed to explain the in-volvement of NCAM in regulating neuralcell adhesion and axon growth. Finally, ithas also been shown in several systems thatselectins and/or galectins can function asreceptors for specific gangliosides (60) andthis interaction could trigger a cascade ofreactions in both cells.

The ganglioside 9-O-acetyl GD3 couldprovide a new cell signaling mechanism inglial-guided neuronal migration in the devel-oping nervous system. Moreover, it is im-portant to note that most neurons migrate byextension of a leading, neurite-like processand that 9-O-acetyl GD3 has been impli-

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cated in both neuronal migration and neuriteoutgrowth. The potential roles for this gan-glioside in identical mechanisms for neu-ronal migration and neurite outgrowth sug-gest an emerging framework in which gly-colipids are involved in cell movement ingeneral.

Acknowledgments

The technical assistance of Felipe Marinsis gratefully acknowledged.

Figure 6. Schematic diagram suggesting possible mechanisms for 9-O-acetyl GD3 actionon neuronal motility. The ganglioside may modulate the integrin receptor through a Ca2+-dependent mechanism and/or by a direct interaction between the ceramide moiety of theganglioside and the integrin transmembrane helix. In a first stage, the integrin receptor isinactive and there is no recognition of extracellular matrix proteins. Later, gangliosides inthe adjacent membrane become laterally packed around the integrin receptor (representedin the scheme by only one ganglioside), providing an optimal Ca2+-enriched microenviron-ment for the activation and recognition of extracellular matrix proteins by this receptor. Inaddition, the gangliosides can interact directly with the integrin receptor through a con-served lysine located inside a 23-amino acid sequence at the carboxy-terminus of theintegrin transmembrane helix. 9-O-GD3 = 9-O-acetyl GD3.

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functional role of a specific ganglioside in neuronal ... · migration mode - tangential migration...

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