G2 cell cycle arrest induced by glycopeptides isolated from the bovine cerebral cortex

Anatomical gradients in proliferation and differentiation of embryonic rat CNS accessed by buoyant density fractionation: alpha 3, beta 3 and gamma 2 GABAA receptor subunit co-expression by post-mitotic neocortical neurons correlates directly with cell buoyancy

Development of the CNS occurs as a complex cascade of pre-programmed events involving distinct phases of cell proliferation and differentiation. Here we show these phases correlate with cells of specific buoyant densities which can be readily accessed by density gradient fractionation. Sprague-Dawley dams were pulse-labelled with bromodeoxyuridine (BrdU) and selected regions of embryonic (E) CNS tissues at E11-22 dissociated with papain into single-cell suspensions. Proliferative cell populations were assessed by anti-BrdU and propidium iodide staining using flow cytometry. Cell differentiation was evaluated using molecular and immunocytochemical probes against mRNAs and antigens differentiating the neuroepithelial, neuronal and glial cell lineages. The results show the emergence of distinctive spatiotemporal changes in BrdU+ populations throughout the CNS during embryonic development, which were followed by corresponding changes in the cellular distributions of antigens distinguishing specific cell types. Fractionation of neocortical cells using discontinuous Percoll gradients revealed that an increasing number of cells increase their buoyancy during corticogenesis. Immunocytochemical and molecular characterization showed that the proliferative and progenitor cell populations are for the most part associated with lower buoyancy or higher specific buoyant densities (> 1.056 g/ml) whereas the post-mitotic, differentiated neurons generally separated into fractions of higher buoyancy or lower specific buoyant densities (< 1.043 g/ml). Immunostaining with antibodies against several GABAA receptor subunits (alpha 3, beta 3, gamma 2) revealed that the highest percent (70-90%) of immunopositive cells could be identified in the most buoyant, differentiating neurons found in the cortical plate/subplate regions, with the lowest percent of the immunopositive cells found in the least buoyant, proliferative and progenitor cell populations originating from the ventricular/subventricular zones. Taken together, these results indicate that buoyant density is a distinguishing characteristic of embryonic CNS cells transforming from primarily proliferative to mainly differentiating, and that fractionation of these cells according to their buoyant densities provides rapid access to the properties of specific cell lineages during the prenatal period of CNS development.

Negative regulators of cell proliferation

Cell proliferation is governed by the influence of both mitogens and inhibitors. Although cell contact has long been thought to play a fundamental role in cell cycling regulation, and negative regulators have long been suspected to exist, their isolation and purification has been complicated by a variety of technical difficulties. Nevertheless, over recent years an ever-expanding list of putative negative regulators have emerged. In many cases, their biological inhibitory activities are consistent with density-dependent growth inhibition. Most likely their interactions with mitogenic agents, at an intracellular level, are responsible for either mitotic arrest or continued cell cycling. A review of naturally occurring cell growth inhibitors is presented with an emphasis on those factors shown to be residents of the cell surface membrane. Particular attention is focused on a cell surface sialoglycopeptide, isolated from intact bovine cerebral cortex cells, which has been shown to inhibit the proliferation of an unusually wide range of target cells. The glycopeptide arrest cells obtained from diverse species, both fibroblasts and epithelial cells, and a broad variety of transformed cells. Signal transduction events and a limited spectrum of cells that are refractory to the sialoglycopeptide have provided insight into the molecular events mediated by this cell surface inhibitor.

The identification of a naturally occurring cell surface growth inhibitor related to a previously described bovine sialoglycopeptide

A 66-kDa sialoglycoprotein has been identified as the parental membrane molecule of an earlier described sialoglycopeptide (SGP), an 18-kDa molecule released by protease treatment of intact bovine cerebral cortex cells that was shown to be a potent inhibitor of cellular proliferation. The 66-kDa parental sialoglycoprotein (p-SGP) was purified approximately 2,400-fold, to apparent homogeneity, from bovine cerebral cortex cell membranes by its release during incubation with 3 M NaCl, preparative isoelectric focusing and lectin affinity chromatography. Although a membrane-associated molecule, the p-SGP appeared to be tightly bound to the cell membrane, since it was not released during incubations in the absence of 3 M NaCl. Incubation of the membrane preparations with 3 M urea proved to be too harsh, and the antigenicity required to follow the purification of the p-SGP was abolished. Analyses by SDS-PAGE, under reducing and nonreducing conditions, suggested that the p-SGP membrane component was a single polypeptide without subunit structure. The p-SGP was shown to be structurally related to the SGP fragment by immunoblots with IgG raised to the SGP inhibitor, and functionally related to the SGP by its ability to inhibit Swiss 3T3 proliferation at concentrations strikingly similar to that previous measured with the SGP fragment.

Inhibition of DNA synthesis and cell division by a cell surface sialoglycopeptide

We have isolated and purified a cell surface sialoglycopeptide (SGP) from bovine cerebral cortex cells that previously was shown to be a potent inhibitor of cellular protein synthesis. The following studies were carried out to characterize the potential ability of the SGP to inhibit DNA synthesis and to arrest cell division. Treatment of exponentially proliferating Swiss 3T3 cells with the SGP inhibitor resulted in a marked inhibition of thymidine incorporation within 24 h. When the SGP was removed from inhibited cultures, a sharp rise in 3H-thymidine incorporation followed within 3-4 h that peaked well above that measured in exponentially growing cultures, suggesting that the inhibitory action of the SGP was reversible and that a significant proportion of the arrested cells was synchronized in the mitotic cycle. In addition to DNA synthesis, the inhibitory action of the SGP was monitored by direct measurement of cell number. Consistent with the thymidine incorporation data, the SGP completely inhibited 3T3 cell division 20 h after its addition to exponentially growing cultures. Upon reversal there was a delay of 15 h before cell division resumed, when the arrested cells quickly doubled. Most, if not all, of the growth-arrested cells appeared to have been synchronized by the SGP. The SGP inhibited DNA synthesis in a surprisingly wide variety of target cells, and the relative degree of their sensitivity to the inhibitor was remarkably similar. Cells sensitive to the SGP ranged from vertebrate to invertebrate cells, fibroblast and epitheliallike cells, primary cells and established cell cultures, as well as a wide range of transformed cell lines.

Accumulation of cells with 4N DNA content at nonpermissive temperature in rat embryo diploid cells transformed by tsA mutant of simian virus 40

Primary rat embryo cells were transformed by a tsA mutant (tsA640) of simian virus 40 (SV40). Proliferation of all four independent diploid transformants was suppressed at a nonpermissive temperature (40.3 degrees C), being accompanied by a marked increase in the fraction of cells with a 4N DNA content (a 4N peak in the flow cytofluorogram). However, in this case, the fraction of cells with a 2N DNA content (a 2N peak in the flow cytofluorogram) was preserved. Both effects (suppression of proliferation and increase in the 4N peak) diminished when transformed cells were superinfected with wild-type SV40. The increased 4N peak was preserved, albeit not completely, for at least 24 hours, when cells were further incubated in the presence of hydroxyurea at the nonpermissive temperature. On the other hand, the preserved 2N peak all but disappeared within 24 hours, when cells were further incubated in the presence of colcemid at the nonpermissive temperature. These results suggest that the thermolabile large T antigen of SV40 directly or indirectly induces an accumulation of cells with a 4N DNA content, at the nonpermissive temperature, by prolonging the G2 (and/or late S) period.

Purification and characterization of a bovine cerebral cortex cell surface sialoglycopeptide that inhibits cell proliferation and metabolism

A sialoglycopeptide from bovine cerebral cortex cells was purified to apparent homogeneity by a procedure that included chloroform/methanol extraction, diethylaminoethyl ion exchange chromatography, wheat germ agglutinin affinity chromatography, size-exclusion HPLC, and hydrophobic interaction chromatography. The cell surface inhibitor had a molecular weight of approximately 18,000, no subunit composition was detectable on reduction and polyacrylamide gel electrophoresis analysis, and the glycopeptide apparently contained sialic acid, as illustrated by its ability to bind to Limulus polyhemus lectin. Deglycosylation of the molecule, however, did not reduce its protein synthesis inhibitory activity. As little as 20 ng of the sialoglycopeptide was capable of inhibiting protein synthesis in a wide variety of fibroblast cell lines but not in transformed cells. Mice immunized with the sialoglycopeptide produced antibodies that, when bound to protein A-agarose gel, removed the inhibitory activity from solution. The antibodies were used to identify a single isoelectric focused band and to establish the pI of 3.0 for the molecule.

Purification of a cell growth inhibitor from bovine cerebral cortex cells

A glycopeptide, isolated from bovine cerebral cortex cells and added in only nanogram levels to cells in culture, has been shown to inhibit both cell protein synthesis and cell division. When purified by gel filtration and Ulex europaeus lectin affinity chromatography, the radioiodinated preparation was subjected to high resolution isoelectric focusing and shown to contain three species of macromolecules. The glycopeptide focusing at pH 8.1 comprised over 75% of the radioiodinated material and possessed inhibitory activity against both cell protein synthesis and cell division. A second species that focused at pH 8.3 was also found to be inhibitory to cell metabolism and may have represented a variant of the major glycopeptide.

Astroglial proliferation and phenotype are modulated by neuronal plasma membrane

The rate of proliferation of rat astroglia cultured in a serum-free medium, estimated by tritiated thymidine radioautography, was diminished by more than 50% by addition of rat central nervous system axolemmal fragments to the culture medium. Addition of the axolemmal fragments also induced a phenotypic alteration of the cultured astroglia, from cells of irregular shape containing a fine meshwork of intracytoplasmic glial fibrils to star-shaped cells with thicker, cable-like glial fibrils.

A monoclonal antibody to a unique cell surface growth regulatory glycopeptide

A glycopeptide, isolated from bovine cerebral cortex cells and added to cells in only nanogram/ml levels, has been shown to inhibit both cell protein synthesis and cell division. A monoclonal antibody was used to show that the inhibitory component originated from the cell surface. Incubation of the M1 IgG monoclonal antibody with partially purified bovine glycopeptide preparations and Staphyloccocus protein A removed the inhibitor from solution. Intact mouse cerebral cortex cells were found to have a similar epitope on their surfaces. In contrast, normal rat kidney cells (NRK) did not react with the monoclonal antibody. An analysis of mouse cerebral cortex membrane preparations, incubated with the monoclonal antibody, confirmed that the primary source of the antigenic determinant was the plasma membrane.