Histologische, histochemische und experimentelle Studien am Subkommissuralorgan von Anuren (Mit Hinweisen auf den Epiphysenkomplex)

The dispersed cell culture as model for functional studies of the subcommissural organ: preparation and characterization of the culture system

The subcommissural organ (SCO) is an enigmatic secretory gland of the brain, which is believed to be derived from ependymal (glial) precursor cells. We here developed a dispersed cell culture system of the bovine SCO as an approach to functional analyses of this brain gland. Tissue of the bovine SCO obtained from the slaughterhouse was papain dissociated either directly after dissection or after preparation of SCO explants. The latter had been maintained for 4-6 weeks in organ culture. The dispersed cells were cultured for up to 14 days and continuously tested for their secretory state by immunostaining of their secretory product. With respect to the morphology of the SCO cells (shape, processes, nucleus), no difference was found between the culture of freshly dissociated SCOs and that of dissociated SCO explants. In all cases, the dissociation caused a dedifferentiation; typical elongated cells were formed increasingly after 1 day of culture. Thereafter, only the cellular size increased, whereas the shape and the viability of the cells remained unchanged. Proliferating SCO cells were never observed. The culture obtained from fresh SCO tissue contained more glia cells and fibrocytes than the culture prepared from SCO explants. The proliferation of glia cells and fibrocytes was suppressed by blocking the mitotic activity with cytosine-beta-D-arabino furanoside (CAF). The cytophysiological features of the cultured dispersed cells of both origins did not differ as demonstrated by classical histology, by immunocytochemistry for the secretory products of the SCO, by the characteristics of calcium influx into the cytoplasm ([Ca2+]i) and cyclic adenosine monophosphate (cAMP) after stimulation with adenosine-5-triphosphate, substance P or serotonin, and by the activation of the transcription factor cAMP-responsive element-binding protein. Because of the maintenance of their viability, their capacity to release the secretory product into the culture medium, their receptive capacity, and their signal transduction pathways, we conclude that the dispersed cell culture system, especially that obtained from SCO explants, represents an appropriate and useful model for functional studies of the mammalian SCO.

Spatial distribution of Reissner's fiber glycoproteins in the filum terminale of the rat and rabbit

The subcommissural organ secretes into the third ventricle glycoproteins that condense to form the Reissner's fiber (RF). At the distal end of the central canal of the spinal cord, the RF-glycoproteins accumulate in the form of an irregular mass known as massa caudalis. Antibodies against RF-glycoproteins and a set of lectins were used at the light and electron microscopic level to investigate the spatial distribution of the massa caudalis material in the rat and rabbit filum terminale. In the sacral region of the rat, the central canal presents gaps between the ependymal cells through which RF-glycoproteins spread out. The bulk of massa caudalis material, however, escapes through openings in the dorsal wall of the terminal ventricle. In the rabbit, the massa caudalis is formed within the ependymal canal, at the level of the second coccygeal vertebra, it accumulates within preterminal and terminal dilatations of the central canal, and it escapes out through gaps in the dorsal ependymal wall of the terminal ventricle. The existence of wide intercellular spaces and a large orifice (neuroporous) in the dorsal ependymal wall of the terminal ventricle, and the passage of RF-material through them, appear to be conserved evolutionary features. After leaving the terminal ventricle of the rat and rabbit, RF-glycoproteins establish a close spatial association with the numerous blood vessels irrigating the filum terminale, suggesting that in these species the blood vessels are the site of destination of the RF-glycoproteins escaping from the central canal, thus resembling the situation found in lower vertebrates. When passing from the RF stage to the massa caudalis stage, the rabbit RF-glycoproteins lose their sialic acid residues, exposing galactose as the terminal residue. Since this sialic acid-galactose modification of RF-glycoproteins had also been described in lamprey larvae, it may be regarded as a conserved evolutionary feature associated with the formation of the massa caudalis.

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.

Neural input and neural control of the subcommissural organ

The neural control of the subcommissural organ (SCO) has been partially characterized. The best known input is an important serotonergic innervation in the SCO of several mammals. In the rat, this innervation comes from raphe nuclei and appears to exert an inhibitory effect on the SCO activity. A GABAergic innervation has also been shown in the SCO of the rat and frog Rana perezi. In the rat, GABA and the enzyme glutamate decarboxylase are involved in the SCO innervation. GABA is taken up by some secretory ependymocytes and nerve terminals, coexisting with serotonin in a population of synaptic terminals. Dopamine, noradrenaline, and different neuropeptides such as LH-RH, vasopressin, vasotocin, oxytocin, mesotocin, substance P, alpha-neoendorphin, and galanin are also involved in SCO innervation. In the bovine SCO, an important number of fibers containing tyrosine hydroxylase are present, indicating that in this species dopamine and/or noradrenaline-containing fibers are an important neural input. In Rana perezi, a GABAergic innervation of pineal origin could explain the influence of light on the SCO secretory activity in frogs. A general conclusion is that the SCO cells receive neural inputs from different neurotransmitter systems. In addition, the possibility that neurotransmitters and neuropeptides present in the cerebrospinal fluid may also affect the SCO activity, is discussed.

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.

Identification and partial characterization of the secretory glycoproteins of the bovine subcommissural organ-Reissner's fiber complex. Evidence for the existence of two precursor forms

The subcommissural organ (SCO) is a brain gland whose secretory material is released into the cerebrospinal fluid where it condenses into a thread-like structure known as Reissner's fiber (RF). This fiber extends along the aqueduct, fourth ventricle and central canal of the spinal cord. The present investigation was designed to identify and partially characterize the secretory products of the bovine SCO in their intracellular location and after they have been released and packed into RF form. 5,000 SCOs were dissected out under a microscope, whereas RF of 30,000 cows were collected by perfusing the central canal of the spinal cord with artificial cerebrospinal fluid. Extracts of SCO and RF were used for (i) raising polyclonal antibodies; (ii) immunoblotting; (iii) lectin binding on electrotransfers: concanavalin A (affinity = mannose, glucose) and Limax flavus agglutinin (affinity = sialic acid); (iv) immunoaffinity chromatography; (v) preparative SDS-PAGE and raising of polyclonal antibodies against each of the secretory glycoproteins identified in the immunoblots. All antibodies and the two lectins were also applied to tissue sections of the SCO and RF of several species. The immunocytochemical study of the bovine SCO using an anti-RF serum showed that the secretory material present in the rough endoplasmic reticulum (RER), secretory granules and in RF is strongly immunoreactive. Con A binding sites were only found in the endoplasmic reticulum, whereas Limax flavus agglutinin revealed secretory granules and RF, only. In the blots the immunostaining was used to identify secretory polypeptides. The glycosylated nature of the latter was established by their affinity for Con A and/or Limax flavus agglutinin. Furthermore, this latter lectin allowed us to distinguish whether the intracellular source of a secretory glycoprotein is from a pre-Golgi (RER) or a post-Golgi (secretory granules) compartment. Four glycoproteins were identified in the SCO with apparent molecular weights of 540, 450, 320 and 190 kDa. The three former were also purified by immunoaffinity chromatography. The 540 and 320 kDa forms are present in the SCO but missing in RF, have affinity for Con A, but not for LFA. It is suggested that these two compounds correspond to two precursor forms. The 450 and 190 kDa glycoproteins are present in both, the SCO and RF, and have affinity for Con A and Limax flavus agglutinin. These most likely correspond to processed forms. The presence of more than one precursor was further substantiated by immunocytochemical findings using antisera against the 540, 450 and 320 kDa forms.(ABSTRACT TRUNCATED AT 400 WORDS)

Secretory glycoproteins of the rat subcommissural organ are N-linked complex-type glycoproteins. Demonstration by combined use of lectins and specific glycosidases, and by the administration of Tunicamycin

Two experimental protocols were used to investigate the secretory glycoproteins of the subcommissural organ (SCO). Protocol I: Lectins, specific exoglycosidases and immunocytochemistry were sequentially applied to the same section or to adjacent semithin sections of the rat SCO fixed in Bouin's fluid and embedded in methacrylate. Lectins used: concanavalin A (con A), wheat germ agglutinin, Limulus polyphemus agglutinin, Ricinus communis agglutinin and Arachis hypogeae agglutinin. Glycosidases used: neuroaminidase, beta-galactosidase, alpha-mannosidase, alpha-glucosidase and beta-N-acetyl-glucosaminidase. For immunocytochemistry an antiserum against bovine Reissner's fiber (AFRU) was used. Lectins and glycosidases were used in sequences that allowed the cleaved sugar residue to be identified as well as that appearing exposed as a terminal residue. This approach led to the following conclusions: (1) the terminal sugar chain of the secreted glycoproteins has the sequence sialic acid-galactose-glucosamine-; (2) the con A-binding material present in the rough endoplasmic reticulum corresponds to mannose; (3) the apical secretory granules and Reissner's fibers displayed a strong con A affinity after removing sialic acid, thus indicating the presence of internal mannosyl residues in the secreted material; (4) after removing most of the sugar moieties the secretory material continued to be strongly immunoreactive with AFRU. Protocol II: Rats were injected into the lateral ventricle with Tunica-mycin and killed 12, 24, 50 and 60 h after the injection. The SCO of rats from the last two groups showed a complete absence of con A binding sites. The results from the two experiments confirm that the secretory glycoproteins of the rat SCO are N-linked complex-type glycoproteins with the conformation previously suggested (Rodríguez et al. 1986).

Ontogenetical study of the chick embryo subcommissural organ by lectin-histofluorescence and electronmicroscopy

The secretory activity of the subcommissural organ (SCO) was studied during embryogenesis of the chick (Gallus gallus) using two lectins labelled with fluorescein isothiocyanate, concanavalin A (Con A) and wheat-germ agglutinin (WGA). While WGA labels the apical or ventricular border of the organ, Con A labels both, the apical and vascular poles of the cells. Glycoproteinaceous secretory products, visualized by Con A appear early, at 5 days, in the ependymal epithelium and expand progressively in a rostrocaudal direction. A correlation is established between histofluorescence and the ultrastructural aspects of the ependymocytes. This throws light on the role of the endoplasmic reticulum in the synthesis, storage and transport of the material produced by the SCO, and points to the existence of two poles of exchange between the secretory cells and the extracellular medium, i.e., the ventricular and the vascular one. WGA reactivity at the apical border is linked up with the formation of Reissner's fibre and shows that the secretory product of the SCO cells undergoes at least partly modifications during its intracytoplasmic transport preceding apical discharge.

A comparative histochemical and immunocytochemical study on the secretory material in the subcommissural organ of Rana temporaria L

The secretory material within the cells of the subcommissural organ (SCO) of Rana temporaria has been studied histochemically by application of a classical staining method and immunocytochemically by use of an antiserum (gift of Prof. Sterba, Leipzig) raised against bovine liquor fibre (LF) material, the LF being the secretory product of the SCO. The immunocytochemical method appears to be more specific and more sensitive than the histochemical staining method. The LF can also be visualized immunocytochemically; the fibre reacting equally positively from the SCO until the spinal cord's end. A significant positive linear correlation exists both between the amounts of histochemically and of immunocytochemically stained material and between their concentrations. The suitability of the immunocytochemical staining method for quantification of the secretory material within the cells of the SCO is discussed.

Histochemical and immunohistochemical properties of the CRF-granules and other "Gomori-positive" substances of the rat

In adrenalectomized rats, histochemical and immunohistochemical properties of the following secretion products have been investigated: 1. CRF-granules in the outer layer of the median eminence; 2. neurosecretory material (NSM) in the supraoptico-hypophysial system of the hypothalamus; 3. secretory granules in the TSH-cells of the anterior lobe of the hypophysis; 4. secretory granules in the ependymal cells of the subcommissural organ (SCO); 5. beta-cell-granules in the islets of Langerhans in the pancreas. All these substances are characterized by their stainability with the so-called "Gomori method". The experiments have included studies into: a) the extractability of the substances by various solvents; b) the digestability of the substances by pepsin or trypsin; c) their histochemically detectable content of disulfide groups, arginine and periodic acid-Schiff (PAS) reactive carbohydrates; d) their reaction with porcine-neurophysin-II-antibodies. All substances exhibited a positive reaction for disulfide groups. Based on their solubility properties, their resistance to pepsin or trypsin, their respective content of PAS-reactive carbohydrates and their failure to react with anti-neurophysin serum the "Gomori-positive" granules in TSH-, SCO- and pancreatic beta-cells can be distinguished from one another and from CRF- and neurosecretory granules. In contrast, CRF-granules and NSM showed identical properties. Taking into consideration data from the biochemical and histochemical literature, the present findings suggest that CRF-granules and NSM consist of closely related biochemical substances.

Enzymatic organization of the subcommissural organ

In the subcommissural organ (SCO) of the guinea pig, rat, golden hamster, and mouse the activity and distribution of enzymes related to the energy-supplying metabolism and of some marker enzymes of different cell organelles have been investigated by means of mostly modified histochemical methods. The results were compared with findings in the ciliated ependyma of the ventricular wall and with those in the ependyma of the choroid plexus of the third ventricle. In the ependymal part of the SCO only a moderate activity of hexokinase is observed in its specialized columnar cells whereas a high activity is present both in the ciliated ependyma and the choroid plexus. - The staining pattern of glucose-6-phosphatase is similar to that of hexokinase but this enzyme is found is the SCO only. - Likewise hexokinase, glycogen granules and enzymes related to glycogen metabolism (phosphoglucomutase, uridine-diphosphoglucose pyrophosphorylase, glycogen synthetase and phosphorylase) are regularly found most numerous and active in the nuclear and supra-nuclear area of the ependymal part. These enzymes are less active in both the other ependymal regions. - Uridine-diphosphoglucose dehydrogenase could not be demonstrated in the SCO. The NADP-linked enzymes of the pentose phosphate shunt, glucose-6-phosphate and 6-phosphogluconate dehydrogenase, show a moderate activity which decreases also from the nuclear towards the apical area of the ependymal cells of the SCO. Enzymes of the glycolytic pathway, such as glucosephosphate isomerase, fructose-6-phosphate kinase, fructose-I,6-diphosphate aldolase, glyceraldehyde-3-phosphate and lactate dehydrogenase, are highly active in the SCO and are located mainly in the supranuclear area, too. Fructose-1,6-diphosphatase could not be demonstrated thus indicating that in the SCO the pathway is most probably only glycolytic but not gluconeogenetic. Compared to the ependyma of the ventricular wall and of the choroid plexus, in the SCO the M type subunits of lactate dehydrogenase predominate. Glycolytic enzymes are also very active in the choroid plexus but less in the ciliated ependyma. Compared to the ciliated ependyma and especially to the ependyma of the choroid plexus, the activities of enzymes which are only present in mitochondria (NAD-linked isocitrate dehydrogenase, succinate dehydrogenase, NAD-linked malate dehydrogenase after preextraction, cytochrome oxidase, 3-hydroxybutyrate and glycerolphosphate and glutamate dehydrogenase) are relatively low. Mitochondria are accumulated near the superior pole of the nuclei as well as in the most apical part of the ependymal cells. - The staining pattern of NADP-linked isocitrate and malate dehydrogenase as well as of NADH dehydrogenase suggests that these enzymes are localized both in and out of mitochondria. The extramitochondrial activity of the first two enzymes might be localized in the cytosol. The extramitochondrial activity of NADH dehydrogenase might be localized in the endoplasmic reticulum...