Molecular differentiation of the otic vesicle and neural tube in the chick embryo demonstrated by monoclonal antibodies

Tissue-Specific Regulation of HNK-1 Biosynthesis by Bisecting GlcNAc

Human natural killer-1 (HNK-1) is a sulfated glyco-epitope regulating cell adhesion and synaptic functions. HNK-1 and its non-sulfated forms, which are specifically expressed in the brain and the kidney, respectively, are distinctly biosynthesized by two homologous glycosyltransferases: GlcAT-P in the brain and GlcAT-S in the kidney. However, it is largely unclear how the activity of these isozymes is regulated in vivo. We recently found that bisecting GlcNAc, a branching sugar in N-glycan, suppresses both GlcAT-P activity and HNK-1 expression in the brain. Here, we observed that the expression of non-sulfated HNK-1 in the kidney is unexpectedly unaltered in mutant mice lacking bisecting GlcNAc. This suggests that the biosynthesis of HNK-1 in the brain and the kidney are differentially regulated by bisecting GlcNAc. Mechanistically, in vitro activity assays demonstrated that bisecting GlcNAc inhibits the activity of GlcAT-P but not that of GlcAT-S. Furthermore, molecular dynamics simulation showed that GlcAT-P binds poorly to bisected N-glycan substrates, whereas GlcAT-S binds similarly to bisected and non-bisected N-glycans. These findings revealed the difference of the highly homologous isozymes for HNK-1 synthesis, highlighting the novel mechanism of the tissue-specific regulation of HNK-1 synthesis by bisecting GlcNAc.

Functional Analysis of the Glucuronyltransferases GlcAT-P and GlcAT-S of Drosophila melanogaster: Distinct Activities towards the O-linked T-antigen

The Drosophila melanogaster glucuronyltransferases dGlcAT-S and dGlcAT-P were reported to be expressed ubiquitously and results of in vitro activity assays indicate a functional redundancy. We analyzed both transferases in vivo and in vitro and could show significant differences in their activity towards N-and O-glycoproteins in vivo. While GlcAT-P is able to use N-linked N-acetyllactosamine chains and the O-linked T-antigen as a substrate to form non-sulfated HNK1- (GlcAβ1-3Galβ1-4GlcNAcβ1-) and glucuronyl-T-antigens in vivo, GlcAT-S adds glucuronic acid only to N-linked chains, thereby synthesizing only the non-sulfated HNK1-antigen.

Specific expression of an HNK-1 carbohydrate epitope and NCAM on femoral nerve Schwann cells in mice

Schwann cells are glial cells of the peripheral nervous system. There are two known subtypes of Schwann cells: those that are myelin-forming; and those that are non-myelin-forming. In this study, we looked at the expression of cell adhesion molecules in Schwann cells to determine whether other subtypes might exist. We used immunohistological analysis of femoral nerve segments containing sensory and motor fascicles, stained with anti-HNK-1, M6749 and anti-neural cell adhesion molecule (NCAM) monoclonal antibodies. Anti-HNK-1 and M6749 were positive in the motor fascicle, while anti-NCAM was positive in the sensory fascicle. Immunoblot analysis with the anti-HNK-1 and M6749 antibodies showed stronger immunoreactivity in the motor fraction than in the sensory fraction in the 100 kDa band. With the anti-NCAM antibody, the 140 and 120 kDa bands were seen in the sensory fascicle fraction, but not in the motor fascicle fraction. HNK-1-positive-cells were seen in motor fascicles 7 days after transection. However, the level of immunoreactivity diminished at 14 days, and no immunoreactivity was seen at 21 days. NCAM-positive cells were not observed 3 days after transection. In development, HNK-1-positive-cells and NCAM-positive cells were seen after P-21. These results suggest that the Schwann cells from the motor and the sensory fascicles have different subtypes. The motor and sensory Schwann cells may play different roles and function in a different way during peripheral nerve regeneration. In addition, there could be more stages of Schwann cell differentiation than previously thought; it is possible that myelin-forming Schwann cells differentiate into HNK-1-positive-cells (motor myelin-forming Schwann cells) and HNK-1-negative-cells (sensory myelin-forming Schwann cells), and non-myelin-forming Schwann cells differentiate into NCAM-positive cells (sensory non-myelin-forming Schwann cells) and NCAM-negative cells (autonomic non-myelin-forming Schwann cells).

A non-sulfated form of the HNK-1 carbohydrate is expressed in mouse kidney

The HNK-1 carbohydrate, which is recognized by anti-HNK-1 antibody, is well known to be expressed predominantly in the nervous system. The characteristic structural feature of the HNK-1 carbohydrate is 3-sulfo-glucuronyl residues attached to lactosamine structures (Gal beta1-4GlcNAc) on glycoproteins and glycolipids. The biosynthesis of the HNK-1 carbohydrate is regulated mainly by two glucuronyltransferases (GlcAT-P and GlcAT-S) and a sulfotransferase. In this study, we found that GlcAT-S mRNA was expressed at higher levels in the kidney than in the brain, but that both GlcAT-P and HNK-1 sulfotransferase mRNAs, which were expressed at high levels in the brain, were not detected in the kidney. These results suggested that the HNK-1 carbohydrate without sulfate (non-sulfated HNK-1 carbohydrate) is expressed in the kidney. We substantiated this hypothesis using two different monoclonal antibodies: one (anti-HNK-1 antibody) requires sulfate on glucuronyl residues for its binding, and the other (antibody M6749) does not. Western blot analyses of mouse kidney revealed that two major bands (80 and 140 kDa) were detected with antibody M6749, but not with anti-HNK-1 antibody. The 80- and 140-kDa band materials were identified as meprin alpha and CD13/aminopeptidase N, respectively. We also confirmed the presence of the non-sulfated HNK-1 carbohydrate on N-linked oligosaccharides by multistage tandem mass spectrometry. Immunofluorescence staining with antibody M6749 revealed that the non-sulfated HNK-1 carbohydrate was expressed predominantly on the apical membranes of the proximal tubules in the cortex and was also detected in the thin ascending limb in the inner medulla. This is the first study indicating the presence of the non-sulfated HNK-1 carbohydrate being synthesized by GlcAT-S in the kidney. The results presented here constitute novel knowledge concerning the function of the HNK-1 carbohydrate.

HNK-1 sulfotransferase null mice express glucuronyl glycoconjugates and show normal cerebellar granule neuron migration in vivo and in vitro

Sulfoglucuronyl carbohydrate (SGC), reactive with antibody against human natural killer cell antigen, is expressed in several glycolipids, glycoproteins and proteoglycans of the nervous system and has been implicated in cell-cell recognition, neurite outgrowth and neuronal migration during development, through its interaction with SGC-binding protein (SBP) 1. However, sulfotransferase (ST) null mutant mice, which lack SGC, were shown to have normal development with usual gross anatomy of the nervous system and other organs. Failure to observe a severe phenotype in the ST null mice prompted us to determine the compensatory molecular replacement of SGC by analyzing the carbohydrate of glycolipids and glycoproteins of the ST mutant nervous system. In the ST null mice, SGC-containing molecules were absent; instead the precursor glucuronyl carbohydrate (GC)-containing molecules accumulated. Other relevant glycolipids and proteins were not affected. The GC molecules in the mutant were localized at the same anatomical sites in the nervous system as the SGC molecules in the wild type. In vitro binding studies showed that, similar to sulfoglucuronyl glycolipids, glucuronyl glycolipids interacted with SBP-1, but with a lower binding capacity. In vitro studies with explant cultures of cerebellum indicated that neurite outgrowth and cell migration were not significantly affected in the mutant, possibly owing to interaction of SBP-1 with GC molecules. The results suggested that in vivo SBP-1-GC interaction was sufficient to allow normal neurite outgrowth and cell migration in the mutant, giving rise to a wild-type phenotype. However, the role of other compensatory molecules involved in these processes cannot be completely ruled out.

Biosynthesis of HNK-1 glycans on O-linked oligosaccharides attached to the neural cell adhesion molecule (NCAM): the requirement for core 2 beta 1,6-N-acetylglucosaminyltransferase and the muscle-specific domain in NCAM

The HNK-1 glycan, sulfo-->3GlcAbeta1-->3Galbeta1-->4GlcNAcbeta1-->R, is highly expressed in neuronal cells and apparently plays critical roles in neuronal cell migration and axonal extension. The HNK-1 glycan synthesis is initiated by the addition of beta1,3-linked GlcA to N-acetyllactosamine followed by sulfation of the C-3 position of GlcA. The cDNAs encoding beta1,3-glucuronyltransferase (GlcAT-P) and HNK-1 sulfotransferase (HNK-1ST) have been recently cloned. Among various adhesion molecules, the neural cell adhesion molecule (NCAM) was shown to contain HNK-1 glycan on N-glycans. In the present study, we first demonstrated that NCAM also bears HNK-1 glycan attached to O-glycans when NCAM contains the O-glycan attachment scaffold, muscle-specific domain, and is synthesized in the presence of core 2 beta1,6-N-acetylglucosaminyltransferase, GlcAT-P, and HNK-1ST. Structural analysis of the HNK-1 glycan revealed that the HNK-1 glycan is attached on core 2 branched O-glycans, sulfo-->3GlcAbeta1-->3Galbeta1-->4GlcNAcbeta1-->6(Galbeta1-->3)GalNAc. Using synthetic oligosaccharides as acceptors, we found that GlcAT-P and HNK-1ST almost equally act on oligosaccharides, mimicking N- and O-glycans. By contrast, HNK-1 glycan was much more efficiently added to N-glycans than O-glycans when NCAM was used as an acceptor. These results are consistent with our results showing that HNK-1 glycan is minimally attached to O-glycans of NCAM in fetal brain, heart, and the myoblast cell line, C2C12. These results combined together indicate that HNK-1 glycan can be synthesized on core 2 branched O-glycans but that the HNK-1 glycan is preferentially added on N-glycans over O-glycans of NCAM, probably because N-glycans are extended further than O-glycans attached to NCAM containing the muscle-specific domain.

The specificity of monoclonal antibody A2B5 to c-series gangliosides

To examine the specificity of monoclonal antibody A2B5, four A2B5-reactive gangliosides (designated as G-1, G-2, G-3 and G-4) were purified from bonito fish brain. Ganglioside-1, -2, and -3 migrated above GD1b, below GQ1b, and far below GQ1b on thin-layer chromatography. Ganglioside-4 had the slowest chromatographic mobility and migrated below G-3. The structures of these gangliosides were characterized by overlay analysis with glycolipid-specific ligands, product analysis after sialidase or mild acid treatment, and electrospray ionization-mass spectrometry (ESI-MS). Accordingly, G-1, G-2 and G-3 were identified to be GT3, GQ1c and GP1c, respectively. The ganglioside G-4 was shown to have the following structure: NeuAc-NeuAc-NeuAc-Galbeta1-3Gal NAcbeta1-4(NeuAc-NeuAc-NeuAcalpha2-3)Galbeta1-4Glcbeta1-1'Cer. The antibody A2B5 reacted with these c-series gangliosides, but not with GD3 and other gangliosides and neutral glycosphingolipids. The antigenic epitope for A2B5 was assumed to include the trisialosyl residue connected to the inner galactose of the hemato- or ganglio-type oligosaccharide structure of gangliosides. Phylogenetic analysis of brain gangliosides using the A2B5 preparation demonstrated that c-series gangliosides are enriched in lower animals, especially bony fish of different species. The monoclonal antibody A2B5 would be a useful tool for examining the distribution and function of c-series gangliosides.

The HNK-1 carbohydrate epitope in the eye: basic science and functional implications

The HNK-1 carbohydrate epitope is part of many cell membrane and extracellular matrix molecules. It has been implicated in cell to cell and cell to extracellular matrix adhesion, and antibodies to the HNK-1 epitope are emerging as a versatile tool in eye research. They have been used to identify a novel cell type in the human eye, the subepithelial matrix cells that reside in the inner connective tissue layer (ICTL) of the ciliary body. Although these cells resemble fibroblasts in ultrastructure, they form a distinct cell population that differs in its antigenic profile from fibroblasts of other tissues. These cells are associated with the elastic fiber system of the ICTL. Other structures in the human eye that harbor the HNK-1 epitope in a nonrandom pattern are the ciliary and iris epithelia, the zonular lamella, the lens capsule, the retina, glial cells of the optic and ciliary nerves, and scleral fibroblasts. The HNK-1 epitope in the eye appears early during embryonic development and is phylogenetically conserved, but many interspecies differences exist in its distribution. The role of the HNK-1 epitope may be to structurally stabilize the ciliary body and the retina, and to participate in zonular attachments. The HNK-1 epitope has been linked with many common eye diseases. The subepithelial matrix cells seem to be susceptible to undergo irreversible damage as a result of glaucoma, thermal injury, and tissue compression. This epitope has proved to be useful in identifying intraocular deposits of exfoliation syndrome. It can explain the adhesiveness of exfoliation material. Intraocular exfoliation material differs in HNK-1 immunoreactivity from the extraocular fibrillopathy of exfoliation syndrome and its presence in fellow eyes also argues against the concept of unilateral exfoliation syndrome. The HNK-1 epitope is found in the extracellular matrix of secondary cataract and anterior subcapsular cataract, and it may contribute to their pathogenesis. Finally, the HNK-1 epitope can be used to trace neuroepithelial derivatives of the optic vesicle in developmental anomalies and in tumors of the eye. Eventual identification of molecules that bear the HNK-1 epitope in the eye will likely shed light on many aspects of ocular physiology and pathobiology

Molecular cloning and expression of a second glucuronyltransferase involved in the biosynthesis of the HNK-1 carbohydrate epitope

A cDNA encoding a novel glucuronyltransferase was cloned from a rat brain cDNA library. The cDNA sequence contained an open reading frame encoding 324 amino acids, with type II transmembrane topology. The amino acid sequence revealed 49% homology to rat GlcAT-P, a glucuronyltransferase involved in the biosynthesis of the HNK-1 carbohydrate epitope of glycoproteins, [Terayama et al. (1997) Proc. Natl. Acad. Sci. USA 94, 6093-6098] and the highest sequence homology was found in the catalytic region. Northern blot analysis indicated that this newly cloned glucuronyltransferase is expressed in the nervous system, consistent with the selective localization of the HNK-1 carbohydrate epitope in the nervous system. Transfection of this cDNA into COS-1 cells induced the expression of the HNK-1 carbohydrate epitope on cell surfaces, and induced the morphological changes in these cells. These results indicated that this newly cloned cDNA is a second glucuronyltransferase involved in the biosynthesis of the HNK-1 carbohydrate epitope.

Spatio-temporal diversity in the microenvironments for neural cell adhesion molecule, neural cell adhesion molecule-polysialic acid, and L1-cell adhesion molecule expression by sensory neurons and their targets during cochleo-vestibular innervation

Sixteen phases in the microenvironments were defined for the structural development and innervation of the cochleo-vestibular ganglion and its targets. In each phase the cell adhesion molecules, neural cell adhesion molecule, neural cell adhesion molecule-polysialic acid, and L1-cell adhesion molecule, were expressed differentially by cochleo-vestibular ganglion cells, their precursors, and the target cells on which they synapse. Detected by immunocytochemistry in staged chicken embryos, in the otocyst, neural cell adhesion molecule, but not L1-cell adhesion molecule, was localized to the ganglion and hair cell precursors. Ganglionic precursors, migrating from the otocyst, only weakly expressed neural cell adhesion molecule. Epithelial hair cell precursors, remaining in the otocyst, expressed neural cell adhesion molecule, but not L1-cell adhesion molecule. Post-migratory ganglion cell processes expressed both molecules in all stages. The cell adhesion molecules were most heavily expressed by axons penetrating the otic epithelium and accumulated in large amounts in the basal lamina. In the basilar papilla (cochlea), cell adhesion molecule expression followed the innervation gradient. Neural cell adhesion molecule and L1 were heavily concentrated on axonal endings peripherally and centrally. In the rhombencephalon, primitive epithelial cells expressed neural cell adhesion molecule, but not L1-cell adhesion molecule, except in the floorplate. The neuroblasts and their axons expressed L1-cell adhesion molecule, but not neural cell adhesion molecule, when they began to migrate and form the dorsal commissure. There was a stage-dependent, differential distribution of the cell adhesion molecules in the floorplate. Commissural axons expressed both cell adhesion molecules, but their polysialic acid disappeared within the floorplate at later stages. In conclusion, the cell adhesion molecules are expressed by the same cells at different times and places during their development. They are positioned to play different roles in migration, target penetration, and synapse formation by sensory neurons. A multiphasic model provides a morphological basis for experimental analyses of the molecules critical for the changing roles of the microenvironment in neuronal specification.

Expression cloning of a human sulfotransferase that directs the synthesis of the HNK-1 glycan on the neural cell adhesion molecule and glycolipids

The HNK-1 carbohydrate is expressed on various adhesion molecules in the nervous system and is suggested to play a role in cell-cell and cell-substratum interactions. Here we describe the isolation and functional expression of a cDNA encoding a human sulfotransferase that synthesizes the HNK-1 carbohydrate epitope. A mutant Chinese hamster ovary cell line, Lec2, which stably expresses human neural cell adhesion molecule (N-CAM) (Lec2-NCAM), was first established. Lec2-NCAM was co-transfected with a human fetal brain cDNA library, a cDNA encoding the rat glucuronyltransferase that forms a precursor of the HNK-1 carbohydrate, and a vector encoding the polyoma large T antigen. The transfected Lec2-NCAM cells expressing the HNK-1 glycan were enriched by fluorescence-activated cell sorting. Sibling selection of recovered plasmids resulted in a cDNA encoding a sulfotransferase, HNK-1ST, that directs the expression of the HNK-1 carbohydrate epitope on the cell surface. The deduced amino acid sequence indicates that the enzyme is a type II membrane protein. Sequence analysis revealed that there is a short amino acid sequence in the presumed catalytic domain, which is highly homologous to the corresponding sequence in other Golgi-associated sulfotransferases so far cloned. The amount of HNK-1ST transcript is high in fetal brain compared with fetal lung, kidney, and liver. Expression of HNK-1ST resulted in the formation of the HNK-1 epitope on N-CAM and a soluble chimeric form of HNK-1ST was shown to add a sulfate group to a precursor, GlcAbeta1-->3Galbeta1-->4GlcNAcbeta1-->R, forming sulfo-->3GlcAbeta1-->3Galbeta1-->4GlcNAcbeta1-->R. The results combined together indicate that the cloned HNK-1ST directs the synthesis of the HNK-1 carbohydrate epitope on both glycoproteins and glycolipids in the nervous tissues.

Cloning and functional expression of a novel glucuronyltransferase involved in the biosynthesis of the carbohydrate epitope HNK-1

The HNK-1 carbohydrate epitope is characteristically expressed on a series of cell adhesion molecules and also on some glycolipids in the nervous system over a wide range of species from insect to mammal. The HNK-1 epitope is involved in cell-cell and/or cell-substrate interaction and recognition during the development of the nervous system. In this study, we isolated a novel glucuronyltransferase from rat brain, which is a key enzyme of the biosynthesis of the HNK-1 epitope on glycoproteins. Based on the partial amino acid sequences, we isolated cDNA encoding the glucuronyltransferase. The primary structure deduced from the cDNA sequence predicted a type II transmembrane protein with 347 amino acids and had no detectable similarity with any other proteins of known functions, including glucuronyltransferases of the liver and olfactory epithelium. Expression of a soluble recombinant form of the enzyme in COS-1 cells produced an active glucuronyltransferase. The selective expression of the glucuronyltransferase gene in the nervous system was consistent with the almost exclusive localization of the HNK-1 epitope in the nervous system. Transfection of the glucuronyltransferase cDNA into COS-1 cells induced not only expression of the HNK-1 epitope on the cell surface but also marked morphological changes of the cells, suggesting that the HNK-1 epitope associates with the cell-substratum interaction.

Retinoic acid-induced neural tube defects with multiple canals in the chick: immunohistochemistry with monoclonal antibodies

When retinoic acid was injected into chicken yolks before incubation, various types of neural tube defect (NTD) were induced in 38-46% of the embryos after 48-96 h of incubation. The cranial NTD consisted of a delay in closing of the neural plate in 48-h embryos and some remained as disorganized, hyperplastic masses in older embryos. In spinal NTD of 48-h embryos the posterior neuropore remained widely open. In older embryos with a closed posterior neuropore, the neural tube appeared dissociated or disorganized locally at the trunk level. The tissue consisted of a dorsally-situated, neural-plate-like structure and a ventrally-located cell mass containing multiple canals. Although the location was different, this arrangement was similar to the overlap zone which appears between primary and secondary neurulation in normal development. Immunohistochemistry was performed using monoclonal antibodies which selectively stained various components of chick tissue. Considering the similarity in neural tube formation between chick and human, this experimental NTD may provide clues to understanding the etiology of human myelomeningocele.

Immunohistological study on brains of Alzheimer's disease using antibodies to fetal antigens, C-series gangliosides and microtubule-associated protein 5

An immunohistological study of Alzheimer's brains was performed using antibodies to C-series gangliosides and microtubule-associated protein 5 (MAP5), and their staining patterns were compared with those of antibodies to tau and beta-amyloid precursor protein. Antibodies to C-series gangliosides and MAP5, both of which are known to preferentially expressed in the fetal brains, immunostained dystrophic neurites of senile plaques, neurofibrillary tangles and neuropil threads abundant in 3rd and 5th layers in the cerebral cortex, all of which are considered to be pathological hallmarks of Alzheimer's disease. The immunostaining patterns of these structures by antibodies to C-series gangliosides and MAP5 were similar to those by the antibody to tau. These three antibodies also immunostained some neurons in Alzheimer's brain, although their staining patterns were slightly different from one another; i.e., both diffuse and granular patterns were seen by the antibody to tau, but only granular pattern by the antibodies to C-series gangliosides and MAP5. These neurons immunostained by these three types of antibodies appeared to be the precursors of the classical neurofibrillary tangles, as positively stained neurons were not seen in the brains of non-demented cases. The presence of fetal antigens such as the C-series gangliosides and MAP5 in Alzheimer's brain may suggest that regeneration or sprouting of neurons is ongoing in association with the re-induction of gene expression characteristic for the brain in the early stage of development.

Ganglion formation from the otic placode and the otic crest in the chick embryo: mitosis, migration, and the basal lamina

We have studied the morphogenesis of the cochleo-vestibular (CVG) and distal cranial ganglia in the early chick embryo (White Leghorn embryos). Light microscopy and immunocytochemical staining for fibronectin and laminin were used to trace the cellular contributions to these ganglia from the otic placode and otocyst. Serial semi-thin plastic sections (3-5 microns) stained with toluidine blue at each Hamburger-Hamilton stage (St.) from 10 to 21 were used. We were able to trace individual groups of cells derived from these epithelial structures into the anlagen of the CVG and the distal parts of cranial n. ganglia VII, IX, and X. For immunostaining, antisera were used to visualize the basal lamina in 15-microns cryostat sections from St. 14 to 21 embryos. Described here for the first time is the otic crest, a ridge of epithelium surrounding the placode. Cells migrate from the otic crest (St. 11 to 14) during the period when the otocyst is forming. These cells become continuous spatially with those derived from the epibranchial placodes and the presumptive ganglia of cranial nerves VII, IX, and X. Furthermore, rostral otic crest cells merge with neural crest cells, which appose the myelencephalon, and they join with the newly formed neuroblasts of the CVG, which migrate from the ventral epithelium of the otocyst at St. 14 to 21. This region of the epithelium forms the bulk of the CVG; it also has many more mitotic figures than the rest of the otocyst. Cells in the rostralmost CVG (vestibular part) are the first to complete their migration and send axons into both the medulla and incipient crista ampullaris. Immuno-staining for fibronectin and laminin shows that these two basal-lamina-associated glycoproteins appear in a continuous layer beneath the otic epithelium just prior to CVG migration. Thus there is no evidence that the migration is launched by a prior decomposition of the basal lamina. The cells migrating from the epithelium bridge the basal lamina with their leading processes while the trailing processes are withdrawing from the epithelium. These trailing processes must withdraw after the neuroblast migrates, since most of the neuroblasts undergo mitosis in subsequent stages. The migrating cells appear to push out of the epithelium by displacing immunostained fragments of the basal lamina ahead of their leading processes. This suggests that the exodus of cells is accompanied by forces within the epithelium itself. Whether this is generated by the migratory neuroblasts themselves or by other sources remains to be determined.

Shaping, invagination, and closure of the chick embryo otic vesicle: scanning electron microscopic and quantitative study

Scanning electron microscopy, light microscopy, and morphometric analysis were used to study the morphological changes of the otic placode and vesicle before and during invagination and closure processes. Our results reveal that the otic placode undergoes shaping between stages HH9 and HH12; during this period the rostrocaudal axis is shortened, while the lateromedial axis of the placode lengthens. The presence of long cytokinesis bridges during this period suggests that cellular displacements after mitosis may participate in the shaping of the otic placode. The shaping process appears to facilitate the approach of the otic placode to the neural tube. From stage HH12 on, the otic anlage gradually becomes a U-shaped structure with its medial portion in intimate apposition to the rhombencephalic neural tube. The coincidence in time between the beginning of intimate otic anlage-rhombencephalon contact and active invagination suggests that these two processes are related. Changes occurring at the edges of the otic vesicle until their disappearance in stage HH17 suggest that, in addition to a process of invagination, the edges of the otic anlage become bent. During closure, cells at the edges of the otic vesicle differ in apical morphology according to their topographical location: The cells between the rostral and lateral edges have elongated apices, in contrast with the polygonal shape of the cell apices in other places of the edges. In the opposite side (between the caudal and medial edges) cell death is observed. Closure of the otic vesicle conceptualized as a zipper-like model is discussed. We propose that early development of the otic anlage takes place in four stages: 1) shaping (stages HH9-11); 2) triggering of the invagination (stage HH12); 3) early invagination and lateral bending (stages HH13-15); and 4) late invagination and closure (stages HH16-17).

Differences in lipid composition between isolated growth cones from the forebrain and those from the brainstem in the fetal rat

The lipid composition of nerve growth cone membranes isolated from rat fetal forebrain or brainstem by the sucrose density gradient method was analyzed biochemically and immunochemically. In the forebrain, growth cone membrane (GCM) contained lower levels of gangliosides than those from other heavier fractions, but it was not the case in the fetal brainstem at the same developmental stage. The distinctive features in the ganglioside composition of GCM are the predominance of GD3 and the presence of c-series gangliosides that are due to fetal expression in mammals. A unique acidic glycolipid, sulfoglucuronylparagloboside (SGPG), which is not present in adult brains, was first detected in both forebrain and brainstem GCM. Including such minor species, the ganglioside composition in forebrain or brainstem GCM was almost identical to other membrane fractions from the forebrain or brainstem. The compositional ratios of the major lipid classes in membranes, cholesterol and phospholipids, seemed to be common to forebrain GCM and brainstem GCM, as indicated by the identical values of phospholipid-to-protein (PL/Pr), cholesterol-to-protein (Ch/Pr), and cholesterol-to-phospholipid (Ch/PL) ratios for both. This study has revealed that GCM isolated from forebrain which is supposed to be at an earlier stage of neuronal differentiation than brainstem has less amounts of total gangliosides, high proportion of GD3 to GD1a and enriched c-series gangliosides as compared to brainstem GCM.

Developmentally expressed O-acetyl ganglioside GT3 in fetal rat cerebral cortex

Monoclonal antibody M6704, established against the chick neural tube, was shown to recognize a trisialosyl residue, NeuAc alpha 2-8NeuAc alpha 2-8NeuAc alpha 2-3-R of C-series gangliosides. Using this antibody, the developmental changes of C-series gangliosides in fetal rat cerebral cortex have been examined. Two dimensional thin layer chromatography (TLC) enzyme-immunostaining analysis revealed that alkali treatment resulted in a great increase in GT3 that amounted to more than 85% of the total GT3 detected. The alkalilabile form was easily degraded to form GT3 by the action of the receptor-destroying enzyme of influenza C virus, sialate O-acetylesterase, indicating that the antigen was most probably 9-O-acetyl-NeuAc containing GT3. The ganglioside was highly enriched at the 14th gestation day, gradually decreased, and was not detected in adult rat cerebral cortex.

A new membrane antigen revealed by monoclonal antibodies is associated with motoneuron axonal pathways

Chick embryonic motoneurons selectively grow out from the spinal cord as the first step of their selective axonal growth. In order to detect the molecules responsible for motoneuron outgrowth from the cord, we produced and immunohistochemically screened many monoclonal antibodies (MAbs) against cord and somite. We found that two of them, called M7412 and M7902, selectively bound to the cell surface of the anterior half of the sclerotome, where motoneurons selectively extend their axons. Immunohistochemistry and immunoblot results were identical for these antibodies and the antigen was called M7412 antigen. Although neural crest cells also migrate into the anterior half of the sclerotome, the expression of M7412 antigen by sclerotome cells was independent of the neural crest, because neural crest removal did not affect the appearance of the antigen. Furthermore, MAb M7412 bound to the mesenchymal cells along presumptive major nerve trunks in the limb and to the structures surrounding myotubes in muscles during the formation of intramuscular nerve branches. These results suggest that M7412 antigen might be a substrate for general, but not specific, growth of motoneuron axons. If this is the case, we must also infer that some molecule inhibitory for motoneuron growth is localized in the posterior half of sclerotome, because at upper cervical levels the M7412 antigen was also expressed intensely in the posterior sclerotome, whereas motoneurons still grew only into the anterior half. The M7412 antigen was transiently expressed in such various tissues as somite; muscles; blood vessels; spinal cord cells, especially motoneurons innervating the limb; and dorsal root and other peripheral ganglion cells. The M7412 antigenic molecule was extractable with NP40 from a membrane fraction of whole chick embryos and its molecular weight was estimated to be 70 kDa from immunoblot analysis. Thus, our monoclonal antibodies have revealed a new membrane-associated molecule which is likely to play a role in cell-cell interactions during development of motoneurons.