Adhesive recognition and retinotectal specificity

An assay has been developed to test the hypothesis that neurons from a limited area of the retina will adhere preferentially to that part of the optic tectum near their normal synaptic termini. The method measures the adherence of (32)P-labeled cell bodies from either the dorsal or ventral half of the neural retina of chick embryos to dorsal and ventral tectal halves. When a labeled, single-cell suspension is prepared from dorsal half-retina, more cells bind to the ventral half of the tectum. When the labeled cells are from ventral half-retina, more bind to dorsal half-tecta. This preferential adhesion mimics the retinotectal projection found in vivo. Dorsal retinal cells show this preference shortly after dissociation with crude trypsin, and maintain it for at least 9 hr. Ventral retinal cells, however, require incubation in nutrient medium after trypsinization in order to display this selectivity. Comparable results are obtained when the cell suspension is prepared from pigmented retina. The data support an interpretation of neuronal specificity dependent on cell-surface interactions and demonstrate a clear correlation between selective adhesion and biological function.

Intercellular contact and cell-surface galactosyl transferase activity (cell culture-mouse-radioautography-contact inhibition-cis-and trans-galactosylation)

Evidence is presented suggesting the presence of galactosyl transferases and galactosyl acceptors on the outer surfaces of intact Balb/c 3T3 cells. In addition, the data indicate that these transferases may only be capable of transferring galactose from uridine diphosphate galactose to galactosyl acceptors on adjacent cells after intercellular contact is made (trans-glycosylation). Intact Balb/c 3T12 cells, by contrast, show no requirement for intercellular contact in order to carry out this reaction suggesting that these cells, which do not exhibit contact inhibition of growth, may be able to transfer galactose to acceptors situated on the same cell as the enzyme (cis-glycosylation). Electrophoretic and radioautographic assays were used to detect surface transferase activities in these two cell lines. Results of experiments on cells from sparse and dense cultures, and under conditions where intercellular contact was regulated, are consistent with the above hypothesis.

Evidence for cell-surface glycosyltransferases. Their potential role in cellular recognition

Intact chicken embryo neural retina cells have been shown to catalyze the transfer of galactose-(14)C from uridine diphosphate galactose (UDP-galactose) to endogenous acceptors of high molecular weight as well as to exogenous acceptors. Four lines of evidence indicate that the galactosyltransferases catalyzing these reactions are at least partly located on the outside surface of the plasma membrane: (a) there is no evidence for appreciable uptake of sugar-nucleotides by vertebrate cells nor did unlabeled galactose, galactose 1-phosphate, or UDP-glucose interfere with the radioactivity incorporated during the reaction; (b) the cells remained essentially intact during the course of the reaction; (c) there was insufficient galactosyltransferase activity in the cell supernatants to account for the incorporation of galactose-(14)C into cell pellets; and (d) the intact cells could transfer galactose to acceptors of 10(6) daltons, and the product of this reaction was in the extracellular fluid. Appropriate galactosyl acceptors interfered with the adhesive specificity of neural retina cells; other compounds, which were not acceptors, had no effect. These results suggested that the transferase-acceptor complex may play a role in cellular recognition.

An assay for intercellular adhesive specificity

A modification of an assay for intercellular adhesive specificity is described. The method involves the collection of radioactively labeled cells by aggregates of the same (isotypic aggregates) or different (heterotypic aggregates) types of tissue and determination of the number of cells collected by liquid scintillation counting. The use of (32)P to label the tissues permitted a much more rapid estimation of cell collection than was obtained previously. With the use of chick embryo neural retina, liver, forebrain, pectoral muscle, and heart ventricle tissue, it was shown that isotypic was always greater than heterotypic collection. Labeled neural retina cell collection by neural retina aggregates was studied as a function of time, cell suspension density, aggregate diameter, temperature, and aggregate number. Neural retina aggregates were treated with certain enzymes in an attempt to determine whether specific changes on the surface of the aggregates would interfere with labeled neural retina cell collection. Of the various proteases and glycosidases tested, only beta-galactosidase rendered the surface more nonspecific.