Cellular and subcellular localization of LETS protein in the nervous system

Association of thy-1 cell surface differentiation antigen with certain connective tissues in vivo

The cell surface differentiation antigen, Thy-1, has been demonstrated by immunofluorescence to be associated with some collagen-based connective tissue. In the rat, the basement membrane of kidney collecting tubules of certain blood vessels, reticulin of the lamina propria, loose connective tissue in the dermis, and collagen fibres within certain nerve cell tumours, bear the antigen. Other, apparently similar, connective tissue (especially that of muscle and liver) lacked Thy-1. In the mouse, only the connective tissue of the skin was found to bear the antigen. The possible origin of this extracellular Thy-1, and its implications for the function of the molecule, are discussed.

Cell-type-specific markers for distinguishing and studying neurons and the major classes of glial cells in culture

We have used 4 cell-type-specific markers to identify individual glial and neuronal cells in dissociated cell cultures of neonatal rat sciatic nerve, dorsal root ganglia (DRG), optic nerve, cerebellum, corpus callosum, cerebral cortex and leptomeninges. Schwann cells were identified with antibodies against rat neural antigen-1 (Ran-1), neurons with tetanus toxin, astrocytes with antibody against the glial fibrillary acidic protein (GFAP) and oligodendrocytes with antibody against galactocerebroside. All of these ligands react with cell surface molecules except for anti-GFAP antibody which binds to intracellular glial filaments. Using two-fluorochrome immunofluorescence we have studied the distribution of various glycoproteins and glycolipids on these 4 major neural cell types in short-term cultures. We have found that (1) although Ran-1 is expressed on glial and neuronal tumours, it was not found on normal astrocytes, oligodendrocytes or neurons; (2) Thy-1 was present on fibroblasts and some neurons but not on the majority of leptomeningeal cells or on oligodendrocytes or astrocytes in short-term cultures (however, it was expressed on some astrocytes in longer term cultures); (3) the 'large external transformation sensitive' (LETS) protein could be detected on fibroblasts and leptomeningeal cells but not on neurons or glial cells; (4) GM1 was present on all neurons, most oligodendrocytes and approx. 50% of other cell types; sulfatide and GM3 were only detectable on oligodendrocytes, while globoside was only found on some neurons. In addition, we were able to identify putative microglial cells by the presence of cell surface receptors for IgG and by their phagocytic activity; they did not express and of the cell-type-specific defining markers.

Fibronectin associated with the glial component of embryonic brain cell cultures

In the basic approach to investigations of neuronal--glial interactions during both normal brain development and its pathogenesis, embryonic brain cell populations were fractionated into purified neuronal and glial components. Using separation procedures based on differential adhesion and cytotoxicity, the isolated neuronal and glial phenotypes could be identified by distinct morphological and biochemical characteristics, including the visualization of glial fibrillary acid protein (GFA) within glial cells in immunohistochemical assays with monospecific anti-GFA serum. When unfractionated cerebrum cells dissociated from 10-day chick or 14-day mouse embryos were plated as monolayers and cultured for 1--14 days, monospecific antiserum against fibronectin (LETS glycoprotein) was found to react with many, but not all, of the cells as revealed by indirect immunofluorescence microscopy. The isolated neuronal and glial components of these populations were used to determine whether the appearance of membrane-associated fibronectin was characteristic of one cell type or the other, or both, and if neuronal--glial cell interaction was required for its expression. It was found that the surfaces of glial cells, completely isolated from neurons, showed an intense fluorescent reaction to the anti-fibronectin serum. In contrast, the purified neuronal cultures showed no fluorescence with either the anti-GFA or anti-fibronectin sera. These results demonstrate fibronectin as a cell surface protein associated primarily with glial cells and independent of neuronal--glial cell interaction for its expression. Furthermore, the results indicate that the fibronectin observed on glial cell surfaces in these cultures is produced endogenously and is not due to the preferential binding of fibronectin present in the culture medium. The role of fibronectin as an adhesive molecule in neuronal--glial interactions is discussed.

Cell surface fibronectin and oncogenic transformation

Fibronectin is a large glycoprotein at the cell surface of many different cell types; a related protein is present in plasma. Fibronectin is a dimer of 230,000-dalton subunits and also occurs in larger aggregates; it forms fibrillar networks at the cell surface, between cells and substrata and between adjacent cells, and it is not a typical membrane protein. Cell surface fibronectin is reduced in amount or absent on transformed cells and in many cases its loss correlates with acquisition of tumorigenicity and, in particular, metastatic ability. Exceptions to the correlations with transformation and tumorigenicity exist. Loss of fibronectin and the resulting reduced adhesion appear to be involved in pleiotropic alterations in cell behavior and may be responsible for several aspects of the transformed phenotype in vitro. Fibronectin interacts with other macromolecules (collagen/gelatin, fibrin/fibrinogen, proteoglycans) and is apparently connected to microfilaments inside the cell.

Possible role of fibronectin in malignancy

Frozen sections of tumors induced by injecting virally transformed cells into animals were stained for fibronectin by immunofluorescence. Many tumor cell lines do not express fibronectin in tumors in situ even though some of them express fibronectin in culture. Cell shape and hormones appear to influence the expression of fibronectin in culture; however, it is unclear how fibronectin expression is regulated in vivo.