Autoradiographic study of the production of secretory material by the subcommissural organ of frogs (Rana temporaria) after injection of several radioactive precursors, with special reference to the glycosilation and turnover rate of the secretory material

The subcommissural organ and the Reissner fiber: old friends revisited

The subcommissural organ (SCO) is an ancient and conserved brain gland secreting into cerebrospinal fluid (CSF) glycoproteins that form the Reissner fiber (RF). The present investigation was designed to further investigate the dynamic of the biosynthetic process of RF glycoproteins prior and after their release into the CSF, to identify the RF proteome and N-glycome and to clarify the mechanism of assembly of RF glycoproteins. Various methodological approaches were used: biosynthetic labelling injecting ³⁵S-cysteine and ³H-galactose into the CSF, injection of antibodies against galectin-1 into the cerebrospinal fluid, light and electron microscopical methods; isolated bovine RF was used for proteome analyses by mass spectrometry and glycome analysis by xCGE-LIF. The biosynthetic labelling study further supported that a small pool of SCO-spondin molecules rapidly enter the secretory pathways after its synthesis, while most of the SCO-spondin molecules are stored in the rough endoplasmic reticulum for hours or days before entering the secretory pathway and being released to assemble into RF. The proteomic analysis of RF revealed clusterin and galectin-1 as partners of SCO-spondin; the in vivo use of anti-galectin-1 showed that this lectin is essential for the assembly of RF. Galectin-1 is not secreted by the SCO but evidence was obtained that it would be secreted by multiciliated ependymal cells lying close to the SCO. Further, a surprising variety and complexity of glycan structures were identified in the RF N-glycome that further expands the potential functions of RF to a level not previously envisaged. A model of the macromolecular organization of Reissner fiber is proposed.

Biosynthesis and molecular biology of the secretory proteins of the subcommissural organ

Ependymal cells are specialized in the synthesis and release of different factors into the cerebrospinal fluid (CSF). The subcommissural organ (SCO) is one of the most active areas of the ventricular walls secreting into the CSF. This gland is localized in the roof of the third ventricle covering the posterior commissure. Glycoproteins synthesized in SCO cells are released into the ventricular CSF where they aggregate, in a highly ordered fashion, forming an elongated supramacromolecular structure known as the Reissner's fiber (RF). RF grows caudally and extends along the brain aqueduct, the fourth ventricle, and the whole length of the central canal of the spinal cord. The SCO cells synthesize glycoproteins of high molecular weight. A precursor form of 540 kDa is synthesized in bovine and chick SCO cells, and a transcript of 10--14 kb is expressed selectively in the bovine SCO cells. The processing of this molecule generates at least one protein of about 450 kDa (RF-Gly-I), which, after being released, is involved in the formation of RF. Additionally, biochemical data indicate that bovine SCO cells synthesize a second precursor compound of 320 kDa, which is also detected in rat, rabbit, and dog. We postulate that RF is formed by two different complexes, one of which has a very high molecular mass (700 kDa or more) and is made up of at least six polypeptides, with the polypeptide of 450 kDa being its main component. The molecules that form RF in different species have different primary structures but they express common epitopes associated to the existence of cysteine bridges, which are probably crucial for polymerization of RF. Molecular procedures involving the use of anti-RF antibodies have led to the isolation of cDNA clones encoding two proteins known as RF-GLY-I and SCO-spondin. In the last 3 years, five partial cDNA sequences encoding SCO-spondin-like proteins have been obtained (Y08560, Y08561, AJ132107, AJ132106, AJ133488). These clones along with RF-GLY-I and SCO-spondin were computer-assembled generating a cDNA consensus sequence of 14.4 kb. Analyses of the long consensus sequence revealed an extended open reading frame (ORF-1) spanning from base 1,634 to 14,400 that encodes for a putative protein of 4,256 amino acids (approximately 450 kDa). The Mr of the predicted protein is consistent with the observed Mr of the largest protein recognized with anti-RF antibodies in SCO and RF extracts. However, the absence of consensus sequences typically present near the 5J'-end of the translation initiation site suggests the existence of a second open reading frame (ORF-2) extending from base 1 to base 14,400 in frame with the ORF-1 and probably encoding for the largest protein precursor (540 kDa). An antibody raised against a peptide sequence, deduced from the open reading frame encoded by a SCO cDNA, reacted specifically with the bovine and rat SCO-RF complex, thus indicating that the protein encoded by the cloned cDNA is part of RF. Immunoblots of bovine SCO extracts using the anti-peptide serum revealed bands of 540 kDa and 450 kDa, but it did not react with the proteins of 320 and 190 kDa. These data support the existence of two precursors for the bovine RF-glycoproteins (540 and 320 kDa) with the 450-kDa protein being a processed form of the 540-kDa precursor. We postulate that the cloned cDNAs encode for a protein that corresponds to the 540-kDa precursor and that at least part of this sequence is present in the processed form of 450 kDa that is secreted to form the RF.

The subcommissural organ

The subcommissural organ (SCO) is a phylogenetically ancient and conserved structure. During ontogeny, it is one of the first brain structures to differentiate. In many species, including the human, it reaches its full development during embryonic life. The SCO is a glandular structure formed by ependymal and hypendymal cells highly specialized in the secretion of proteins. It is located at the entrance of the aqueduct of Sylvius. The ependymal cells secrete into the ventricle core-glycosylated proteins of high molecular mass. The bulk of this secretion is formed by glycoproteins that would derive from two different precursors of 540 and 320 kDa and that, upon release into the ventricle aggregate, form a threadlike structure known as Reissner's fiber (RF). By addition of newly released glycoproteins to its proximal end, RF grows caudally and extends along the aqueduct, fourth ventricle, and the whole length of the central canal of the spinal cord. RF material continuously arrives at the dilated caudal end of the central canal, known as the terminal ventricle or ampulla. When reaching the ampulla, the RF material undergoes chemical modifications, disaggregates, and then escapes through openings in the dorsal wall of the ampulla to finally reach local blood vessels. The SCO also appears to secrete a cerebrospinal fluid (CSF)-soluble material that is different from the RF material that circulates in the ventricular and subarachnoidal CSF. Cell processes of the ependymal and hypendymal cells, containing a secretory material, terminate at the subarachnoidal space and on the very special blood capillaries supplying the SCO. The SCO is sequestered within a double-barrier system, a blood-brain barrier, and a CSF-SCO barrier. The function of the SCO is unknown. Some evidence suggests that the SCO may participate in different processes such as the clearance of certain compounds from the CSF, the circulation of CSF, and morphogenetic mechanisms.

Seasonal variation in the secretory activity of the subcommissural organ (SCO) of reptiles

Seasonal variations in the secretory activity of the subcommissural organ (SCO) of snakes and turtles was studied by immunocytochemistry, lectins, and electron microscopy. In animals sacrificed in summer, immunoreactive material, mostly devoid of sialic acid, occupied the whole cytoplasm. Cells showed many distended cisternae of rough endoplasmic reticulum and secretory granules. In animals sacrificed in winter, patches of immunoreactive sialic acid-rich material occupied the apical cytoplasm. Cells lacked distended cisternae and the secretory granules formed clusters. These results suggest a decreased synthesis and release of secretory material in the SCO of lethargic reptiles.

Analysis of the secretory glycoproteins of the subcommissural organ of the dogfish (Scyliorhinus canicula)

The subcomissural organ (SCO) is an ancient and conserved brain gland secreting glycoproteins into the cerebrospinal fluid which condense to form Reissner's fiber (RF). The SCO of an elasmobranch species, the dogfish Scyliorhinus canicula, was investigated applying morphological and biochemical methods. The SCO of 34 dogfishes were processed for the following techniques: (1) conventional transmission electron microscopy; (2) light and electron microscopy lectin histochemistry (Concanavalin A, Con A; wheat germ agglutinin, WGA; Limax flavus agglutinin, LFA); (3) light and electron microscopy immunocytochemistry using antisera raised against the glycoproteins of the bovine RF (anti-bovine RF), and the secretory material of the dogfish SCO (anti-dogfish SCO). The former reacts with the SCO of virtually all vertebrate species [19] (conserved epitopes); the latter reacts only with the SCO of elasmobranchs [Cell Tissue Res., 276 (1994) 515-522] (class-specific epitopes). At the light microscopic level both antisera immunoreacted selectively with the SCO and RF; no other structure of the central nervous system was reactive. Within the SCO the binding sites for WGA (affinity = glucosamine, sialic acid) and LFA (affinity = sialic acid) displayed the same density and intracellular distribution. At the ultrastructural level two types of granules were distinguished. Type I granules (200-400 nm) were numerous, reacted with both antisera, bound WGA but not Con A. Type II granules (0.8-1.8 microns) reacted with the anti-bovine RF serum but not with the anti-dogfish SCO serum, bound Con A and WGA. The content of dilated cisternae of the rough endoplasmic reticulum reacted with both antisera and bound Con A; it did not bind WGA. The SCOs of 4500 dogfishes were extracted in ammonium bicarbonate. This extract was used for SDS-PAGE and blotting. Blots were processed for immunolabeling using anti-bovine RF and anti-dogfish SCO sera, and for lectin binding (Con A, WGA and LFA). The anti-bovine RF revealed four compounds with apparent molecular weights of 750, 380, 145 and 35 kDa. The two former also reacted with the anti-dogfish SCO serum and bound Con A. Only the 380 kDa compound bound WGA and LFA. The findings indicate that both the conserved and the class-specific epitopes are part of the same compounds (780, 380 kDa), which would be stored in type I granules. The lectin binding properties of these compounds point to the 780 kDa compound as a precursor form and the 380 kDa polypeptide as a processed form.

Incorporation of D3H glucosamine to the adult and developing cochlear tectorial membrane of normal and hypothyroid rats

The uptake of D-3H-glucosamine by the developing cochlea of normal and hypothyroid rats was examined using light microscopic radioautography. During postnatal development, normal and hypothyroid rat cochleas exhibited a layer of radiolabelling in the tectorial membrane (TM). This layer first appeared in the TM region which covers the spiral limbus and the Kölliker's organ (KO), then progressively reached the apical part of the TM covering the organ of Corti. Radiolabelling was significantly greater in hypothyroid than in normal cochleas. These findings suggests that the enormous size reached by the TM in the congenital hypothyroidism could be related to an increase of epithelial secretion, at least for carbohydrates. It also suggests that TM, in normal and hypothyroid cochleas, could be formed during development by the addition of successive layers. Older layers could be displaced upwards by the new ones. Cochleas of normal young adult rats, treated with D-3H-glucosamine, showed a very scarce and diffuse radiolabelling. Cochleas of hypothyroid young adult rats exhibited a thickened and distorted TM, which incorporated a significant amount of carbohydrates. These results suggest that TM secretion is highly reduced in young adult normal animals, while in young adult hypothyroid ones it is still active. During cochlear maturation, thyroxine seems to be necessary, not only for the synthesis of normal glycoproteins (as suggested by previous reports), but also for the control of glycoprotein secretion.

Developmental expression of glial markers in ependymocytes of the rat subcommissural organ: role of the environment

The rat subcommissural organ (SCO), principally composed of modified ependymocytes (a type of glial cell), is a suitable model for the in vivo study of glial differentiation. An immunohistochemical study of the ontogenesis of rat SCO-ependymocytes from embryonic day 13 to postnatal day 10 shows that these cells express transitory glial fibrillary acidic protein (GFAP) from embryonic day 19 until postnatal day 3. However, S100 protein (S100) is never expressed in the SCO-cells, contrasting with the ventricle-lining cells of the third ventricle, which contain S100 as early as embryonic day 17. Environmental factors could be responsible for the repression of GFAP and S100 in adult rats, because GFAP and S100 are observed in ependymocytes of SCO 3 months after being grafted from newborn rat into the fourth ventricle of an adult rat. Neuronal factors might be involved in the control of the expression of S100, since after the destruction of serotonin innervation by neurotoxin at birth, S100 can be observed in some SCO-ependymocytes of adult rats. On the other hand, GFAP expression is apparently not affected by serotonin denervation, suggesting the existence of several factors involved in the differentiation of SCO-cells.

Incorporation of D-[3H]-glucosamine and L-[3H]-fucose into the developing rat cochlea

The uptake of two tritiated carbohydrates, D-[3H]-glucosamine and L-[3H]-fucose, to the developing rat cochlea was examined using light and electron microscopic radioautography. Both carbohydrates, administered to in vitro developing rat cochleas, shared a similar ultrastructural labeling pattern on the microvilli and apical cell region and on the tectorial membrane (TM) fibrils. On embryonic day 18, the radiolabeling appeared on the apical surface of the undifferentiated epithelium that will develop into both spiral limbus and Kölliker's organ (KO), while on postnatal day (PD) 1, it was only located on the apical surface of the KO. When D-[3H]-glucosamine was administered in vivo to newborn rats, the radiolabeling was observed in the TM covering the KO at PD 3. Lastly, D-[3H]-glucosamine administered in vivo to PD 7 rats, appeared at PD 9 in the TM region lying just above the organ of Corti. The present findings support the previously suggested leading role of the spiral limbus and KO in the secretion of the TM during cochlear development. The secretion of carbohydrates, and probably of other matrix components, starts on the spiral limbus and KO region and progressively extends to the organ of Corti.

A comparative immunocytochemical and immunochemical analysis of glycoproteins synthesized in the bovine subcommissural organ

To extend our previous immunochemical investigations in the chick embryo (Karoumi et al., 1990 b), we raised antibodies in the rabbit against crude extracts of the subcommissural organ (SCO) of the bovine. The antiserum labeled A99 was absorbed by crude brain extracts and its specificity was tested by different techniques. Comparison of crude SCO and cerebral hemispheres supernatants after immunoblotting allow to identify specific 98, 60, 52, 42, 38, and 32 kDa polypeptides in the SCO profile. Immunoaffinity chromatography on A99 immunoadsorbent of crude SCO, cerebral hemispheres (CH) and classical ependyma (CE) supernatants was followed by electrophoretical analysis and electrotransfer. Concanavalin A (Con A) and wheat germ agglutinin (WGA) labeling procedures demonstrated the presence of numerous glycopeptides specific of crude SCO supernatants and having an apparent molecular weight ranging from 240 to 50 kDa. In the CH-eluted fraction, 50 and 52 kDa glycopeptides were revealed by ConA and WGA, whereas in the CE-immunopurified fraction no band was visualized. The similarity of the chick embryo and bovine electrophoretic pattern corresponding to the SCO eluted fractions speaks in favour of a high degree of conservation of the SCO secretory material and an evolutionary stability of the antigens recognized by A99IgG.

Comparative marker analysis of the ependymocytes of the subcommissural organ in four different mammalian species

The subcommissural organ (SCO), classified as one of the circumventricular organs, is composed mainly of modified ependymal cells, attributable to a glial lineage. Nevertheless, in the rat, these cells do not possess glial markers such as glial fibrillary acidic protein (GFAP), protein S100, or the enzyme glutamine synthetase (GS). They receive a synaptic 5-HT input and show pharmacological properties for uptake of GABA resembling the uptake mechanism of neurons. In this study, we examine the phenotype of several mammalian SCO (cat, mouse, rabbit) and compare them with the corresponding features of the rat SCO. In all these species, the SCO ependymocytes possess vimentin as an intermediate filament, but never express GFAP or neurofilament proteins. They do not contain GS as do glial cells involved in GABA metabolism, and when they contain protein S100 (rabbit, mouse), its rate is low in comparison to classical glial or ependymal cells. Thus, these ependymocytes display characteristics that differentiate them from other types of glial cells (astrocytes, epithelial ependymocytes and tanycytes). Striking interspecies differences in the capacity of SCO-ependymocytes for uptake of GABA might be related to their innervation and suggest a species-dependent plasticity in their function.