Changes in chemical composition of chick embryos treated with a beta-xyloside and a lathyrogen

The Basement Membrane Proteoglycans Perlecan and Agrin: Something Old, Something New

Several members of the proteoglycan family are integral components of basement membranes; other proteoglycan family members interact with or bind to molecular residents of the basement membrane. Proteoglycans are polyfunctional molecules, for they derive their inherent bioactivity from the amino acid motifs embedded in the core protein structure as well as the glycosaminoglycan (GAG) chains that are covalently attached to the core protein. The presence of the covalently attached GAG chains significantly expands the "partnering" potential of proteoglycans, permitting them to interact with a broad spectrum of targets, including growth factors, cytokines, chemokines, and morphogens. Thus proteoglycans in the basement membrane are poised to exert diverse effects on the cells intimately associated with basement membranes.

Disruption of proteoglycans in neural tube fluid by beta-D-xyloside alters brain enlargement in chick embryos

Following neurulation, the anterior end of the neural tube undergoes a dramatic increase in size due mainly to the enlarging of the brain cavity. This cavity is filled with so-called neural tube fluid (NTF), whose positive pressure has been shown to play a key role in brain morphogenesis. This fluid contains a water-soluble matrix, rich in chondroitin sulfate (CS), which has been proposed as an osmotic regulator of NTF pressure genesis. The purpose of the present study is to observe the influence of CS on NTF osmolality and its relation to NTF hydrostatic pressure and brain expansion. NTF was obtained by means of microaspiration from the mesencephalic cavity of chick embryos. The osmolality of NTF between H.H. stages 20 and 29 was measured on the basis of its cryoscopic point. CS synthesis was disrupted by using beta-D-xyloside and the induced variations in brain volume were measured by means of morphometry. We also measured the variations in NTF osmolality, hydrostatic pressure, and the concentration of CS and sodium induced by means of beta-D-xyloside. Our data reveal that, at the earliest stages of development analyzed, variations in NTF osmolality show a characteristic pattern that coincides with the developmental changes in the previously described fluid pressure. Chick embryos treated with beta-D-xyloside, a chemical that disrupts CS synthesis, displayed a notable increase in brain volume but no other apparent developmental alterations. Morphometric analysis revealed that this increase was due to hyperenlargement of the brain cavity. Beta-D-xyloside brings about specific changes in the biochemical composition of NTF, which entails a large increase in CS concentration, mainly in the form of free chains, and in that of sodium. As a result, the fluid's osmolality and brain intraluminal pressure increased, which could account for the increase in size of the brain anlage. These data support the hypothesis that the intraluminal pressure involved in embryonic brain enlargement is directly dependent on NTF osmolality, and that the concentrations of CS and its associated microions could play a key role in the regulation of this process.

Questioning the reliability of p-nitrophenyl-beta-D-xyloside as probe to study the metabolic effects of abrogated proteoglycan synthesis in cultured cells

p-Nitrophenyl-beta-D-xylopyranoside (PNP-Xyl) and similar aglycone derivatives of xylosides are proposed selective inhibitors of proteoglycan synthesis which are used frequently to analyse the metabolic and cellular effects of abrogated proteoglycan formation and, hence, tentatively, the functions of these complex molecules. Using rat liver fat storing cell (FSC) cultures as a model, the possibility was tested that p-nitrophenol (PNP), which might be generated by the enzymatic hydrolysis of PNP-Xyl, could mediate some of those effects ascribed previously to PNP-Xyl induced inhibition of proteoglycan synthesis. PNP-Xyl and PNP inhibited dose-dependently the proliferation of FSC reaching 50% inhibition at about 1.9 and 0.6 mM, respectively. The inhibition of proliferation was not accompanied by signs of toxic cell damage and was fully reversible after withdrawal of the drugs. After an initial 4-fold stimulation of the formation of [35S]sulfate-labeled medium glycosaminoglycans (GAG) by PNP-Xyl at 0.1 mM, higher concentrations of this compound (about 0.5 mM) but also PNP decreased progressively the synthesis of sulfated medium GAG. A proliferation inhibiting concentration of PNP (0.75 mM) induced disorganization and reduced the expression of desmin- and smooth muscle iso-alpha-actin containing cytoskeletal filaments. These effects were similar to related effects reported previously for PNP-Xyl. Incubation of FSC with 5 mM PNP-Xyl resulted in a time-dependent increase of PNP in medium and cells; intracellular concentrations of PNP were reached sufficient to inhibit the mitotic activity of FSC. In lysates of FSC 0.65 nmol PNP/hr/micrograms DNA or 1 x 10(5) cells were generated from PNP-Xyl (5 mM) added as substrate. Exemplified with PNP-Xyl-treated FSC cultures, the results suggest for other cell and organ systems also that PNP, which is enzymatically cleaved from PNP-Xyl, might mediate at least some of the major effects attributed previously to the inhibition of proteoglycan synthesis. The aglycone may interfere with the effects of PNP-Xyl on proteoglycan metabolism and, therefore, could complicate in an unpredictable manner the interpretation of metabolic inhibitory studies using these compounds.

beta-D-xylosides and their analogues as artificial initiators of glycosaminoglycan chain synthesis. Aglycone-related variation in their effectiveness in vitro and in ovo

A series of aryl and alkyl O-beta-D-xylosides and their analogues with S, NH or CH2 in the glycosidic linkage were prepared and examined for their ability to act as artificial chain initiators of chondroitin (dermatan) sulphate synthesis in embryonic chick cartilage, foetal rat skin and 6-week-old-rat aorta under conditions where normal protein-core synthesis was inhibited by cycloheximide. For all these tissues in culture, phenyl O-beta-D-xyloside and phenyl beta-D-thioxyloside were clearly more effective than the corresponding N-xyloside and homo-C-xyloside. Introduction of a carboxy group to the para position of their aglycone yielded derivatives with far lower initiator activity. In a concentration range lower than 0.1 mM, the effectiveness of alkyl beta-D-thioxyloside was greatly influenced by the carbon number (n) of the alkyl group and was at a maximum at n = 7 or 8 for the cartilage, at n = 5 for the skin and at n = 4 for the aorta. In the beta-xyloside-treated cartilages, the average length of newly formed chondroitin sulphate chains reflected the chain-initiator activity of added xyloside, i.e. the higher the initiator activity, the shorter the average chain length. In the skin and aorta, none of the drugs could relieve the inhibition of heparan sulphate synthesis caused by cycloheximide. Fertilized hens' eggs were each injected on day 9 with 9.2 mumol of beta-xyloside and the skeletal systems of embryos were examined a week later. The embryos treated with beta-xylosides of relatively high initiator activity showed a 30-40% decrease in the overall growth rate of skeletons, whereas those treated with beta-xylosides of low initiator activity showed little or no decrease in the growth rate. The results are consistent with the notion that the observed change in skeletal morphology results mainly, if not completely, from beta-xyloside-induced synthesis of core-protein-free chondroitin sulphate, and further suggest that a procedure employing a series of beta-xyloside homologues with various initiator activities will furnish an easily applied criterion on which to test the specificity of xyloside action on biological processes.

Development of the biochemical and morphological changes induced by administration of a beta-xyloside to chick embryos

4-Methylumbelliferyl beta-D-xyloside was administered to 9-day-old chick embryos, and the morphological and chemical changes in the embryo were followed daily. Increases in wet weight, Na and Cl content, and visible edema were detectable at 10 days and fully apparent at 11 days. Dry weight increased to the same extent in control and treated embryos for four days, but then diverged. The degree of sulfation of chondroitin sulfate was slightly less in treated than control embryos at 10 days, and reached a steady low value at 11 days. Analysis of glycosaminoglycans in skin, muscle, and aorta showed an increase in chondroitin and its sulfates in the two former tissues but not the latter. In muscle and aorta, the degree of sulfation of chondroitin sulfate was markedly reduced; but in skin the results suggested a more complex picture in which the normal metabolism of glycosaminoglycans was altered. A possible physiological role is suggested for chondroitin sulfate in embryonic soft tissues.

Fluid and glycosaminoglycan excretion by chick embryos treated with a beta-D-xyloside

Wet weights and glycosaminoglycan content were determined for embryo, amnion, and allantois of control chick embryos and embryos injected with 4-methylumbelliferyl beta-D-xyloside at nine days of age. There was an immediate increase in total uronic acid content, but not in uronic acid concentration, in the embryo. No difference could be detected either in fluid volume, nor in content or type of glycosaminoglycan, in the amnion of the two groups. The fluid content of the allantois fo control eggs increased steadily between nine and 14 days, but in treated embryos the fluid content of the allantois remained low for at least a week. Less than 2 mg of uronic acid was present in allantoic fluid of control 16-day-old embryos, while treated embryos had accumulated more than 8 mg. More than 95% of the latter uronic acid was accounted for as chondroitin sulfate linked to methylumbelliferone and with a degree of sulfation of 50-60%. Thus beta-xyloside-treated embryos excrete large amounts of chondroitin sulfate and very little fluid.