Stanley Hoffman

Cytotactin and its proteoglycan ligand mark structural and functional boundaries in somatosensory cortex of the early postnatal mouse

The expression of the extracellular matrix molecules cytotactin, which is synthesized by glia, and cytotactin-binding (CTB) proteoglycan, which is synthesized by neurons, was examined in the developing brain of the mouse, specifically in the cortical barrel field, using highly specific polyclonal antibodies to the purified molecules. Both molecules appeared early in the development of the cortex but were excluded from the centers of the developing barrels at the time of entry and arborization of thalamocortical axons. Of the two major forms of cytotactin (220 and 200 kDa), the larger form predominated during development of the mouse brain and also predominated in mixed neuron-glia cultures but not in pure glial cultures. Both cytotactin and CTB proteoglycan were recognized by various lectins that have been shown in other studies to demarcate the barrel field: both molecules were recognized by lentil lectin and concanavalin A and CTB proteoglycan was also recognized by peanut and wheat germ agglutinins. The HNK-1 carbohydrate antigen, present on cytotactin, CTB proteoglycan, and other adhesion molecules, was also found in the barrel walls and diminished in the barrel hollows. Cytotactin and CTB proteoglycan were preferentially expressed in barrel walls through P12. After this time, their expression became uniform even though the histological pattern of barrel walls and hollows was maintained. The fusion of a row of barrels which results from peripheral damage to a row of whiskers was accompanied by the loss of patterned expression of both molecules following electrocauterization of a row of whisker follicles at P1.5. We conclude that activity from the periphery is important not only to development of anatomical pattern but also of the molecular pattern and that the expression of both glial and neuronal proteins can respond to such activity. The results are consistent with previous studies showing that incoming thalamocortical axons play a primary role in barrel field formation. They also suggest that both the migration of cortical neurons on glia and the refinement of the mapping between the peripheral whisker field and its cortical representation may depend upon the distribution of substrate adhesion molecules.

Expression of adhesion molecules and the establishment of boundaries during embryonic and neural development

Evidence is accumulating that molecules involved in cell-cell and cell-substratum interactions are important in the establishment and maintenance of borders between cell groups during development. In this report, we review evidence supporting this conclusion, particularly in regard to the role of adhesion molecules in the formation of cell collectives and in the modulation of cell and neurite movements.

Expression of adhesion molecules during the formation and differentiation of the avian endocardial cushion tissue

The expression of cytotactin, the cytotactin-binding (CTB) proteoglycan, and the neural cell adhesion molecule, N-CAM, was examined during the development of the avian endocardial cushion tissue (ECT). N-CAM was present in the cardiac mesoderm from its earliest time of development. At the time when endothelial cells converted to mesenchyme and began to migrate, they ceased their expression of N-CAM. Cytotactin and CTB proteoglycan were present in the cardiac jelly (into which the ECT cells migrate) in patterns that were correlated with cell migration. At early times of migration (stage 18), the region of the cardiac jelly near the endocardium contained cytotactin in the vicinity of the migrating cells. During later migration (stage 22), cytotactin remained associated with the leading zone of cell migration, but its expression began to decrease in areas where cells had accumulated. After ECT cell migration had ceased, cytotactin expression decreased, remaining high only in the peripheral portion of the aorticopulmonary septum and absent from its ridges. CTB proteoglycan was expressed during early migration at high levels in and adjacent to the myocardium. By stage 22, its distribution had become more uniform throughout the ECT regions and in the myocardium. The combined results of this study suggest that cytotactin, CTB proteoglycan, and N-CAM each play a distinct, critical role in pattern formation in the early heart.

Expression and function of cell adhesion molecules during the early development of the heart.

The functioning of the heart is dependent upon the proper development and organization of its complex structures: the muscle, the conduction system, and the fibrous skeleton that forms the chambers and valves of the heart. These structures arise through a series of dynamic remodelings of tissues during development,l-’ utilizing the primary processes of cell adhesion, cell migration, cell proliferation, programmed cell death, and differentiation: Failures in these processes, particularly during the septation of the heart, can result in anomalies some of which are seen in human patients.’ To determine at the molecular level how cell adhesion affects other primary processes, we have focused on a cell-cell adhesion molecule, N-CAM, and two cellsubstrate adhesion molecules (SAMs), cytotactin and cytotactin-binding (CTB) proteoglycan. The data suggest that all three of these molecules play important roles in heart development. N-CAM mediates cell-cell adhesion in the myocardium, and at least some N-CAM molecules in the heart contain a novel insert not found in brain or gizzard N-CAM. The distributions of cytotactin and CTB proteoglycan and their roles in cell-substrate adhesion and cell migration suggest that these molecules are involved in the formation of the endocardia1 cushion tissue, the developmental precursor of most of the septation in the heart.

The Mechanism of Binding of Neural Cell Adhesion Molecules

The experimental results reviewed in this paper strongly suggest that the molecular mechanism of N-CAM-mediated cell adhesion involves the direct interaction of N-CAM molecules on one cell with N-CAM molecules on a second cell. The rate of this aggregation has a high-order dependence on the local N-CAM concentration, and is inversely related to the sialic acid content of the N-CAM molecules involved. In accordance with their relative sialic acid concentrations, the relative rates of aggregation mediated by E and A forms of N-CAM are A-A greater than A-E greater than E-E. Further removal of sialic acid from N-CAM below the level found in the A form gives little further enhancement of aggregation. These results provide one basis upon which to interpret the modulation hypothesis (Edelman, 1983) for control of N-CAM function, i.e. the adhesive strength of N-CAM bonds in an in vitro system can be altered in a graded manner over a wide range by variations in the local surface density of N-CAM or by chemical modification of N-CAM (differential sialylation). It is important to stress that these results do not preclude the possibility of other forms of modulation of N-CAM function or the function of other molecules in cell-cell interactions. It will be much more difficult to assess the role of N-CAM and the modulation of its function on pattern formation in vivo. It is pertinent to mention, however, that recent experiments on transformed neural cells (Greenberg et al., 1984) show loss of N-CAM following transformation with accompanying loss of aggregation and increased motility of the transformed cells. Aside from the possible implications for metastasis (transformation has for the first time been shown to affect a defined CAM and alter cellular sociology), these findings are consonant with the notion that alteration of surface N-CAM affects expression of other cellular processes. Clearly additional experiments are required to define the mechanisms by which this occurs. In addition to mapping the prevalence and form of N-CAM during embryonic development, it will be necessary to pruturb both its functions and the modulation of its function using reagents that will discriminate among various forms of the molecule in the embryo itself. These problems will require elegant solutions but must be solved if completely satisfactory answers to questions about the role of N-CAM in vivo are to be obtained.

Molecular features of cell-cell adhesion molecules

Publisher Summary Two cell adhesion molecules (CAMs), the neural cell adhesion molecule (N-CAM) and the neuron–glia cell adhesion molecule (Ng-CAM), have been shown to play crucial roles in intercellular interactions. The N-CAM and the Ng-CAM are critical for the formation and maintenance of neuronal contacts. In addition, the liver cell adhesion molecule (L-CAM) is expressed with N-CAM throughout development in a coordinate fashion, suggesting a general role for N-CAM and L-CAM in morphogenesis and histogenesis. N-CAM, Ng-CAM, and L-CAM are large cell surface glycoproteins that are subject to a variety of post-translational modifications, which may regulate their expression and activities. L-CAM appears as a single glycoprotein and, unlike N-CAM and Ng-CAM, both its structure and activity are dependent on calcium ions. A variety of data indicates that the NILE glycoprotein is the mammalian equivalent of Ng-CAM. N-CAM is distinguished by large amounts of polysialic acid on at least one of its four Asn-linked oligosaccharides and includes two polypeptides that are translated from two large messenger RNAs derived from a single gene.

Cytotactin and cytotactin-binding proteoglycan : an interactive pair of extracellular matrix proteins

Morphogenesis results from milieu-dependent cues to cells in collectives that drive subpopulations of cells along distinct developmental pathways by differentially affecting primary cellular processes such as cell adhesion, migration, proliferation, death, and differentiation. At the cellular level, these signals are believed to be transduced by an array consisting of the nucleus, the cytoskeleton, and specific cell surface and surface-associated proteins involved in cell adhesion and communication? These proteins include the integral membrane proteins that mediate cell-cell adhesion ( CAMS), the integral membrane proteins that form specialized junctions between cells, and the extracellular matrix proteins that mediate cell-substrate adhesion (SAMs) and their receptors. In this article, we shall present data suggesting that the extracellular matrix proteins, cytotactin and cytotactin-binding (CTB) proteoglycan, play major roles in the development and regeneration of the nervous system. In addition, these proteins are present at a variety of nonneural sites. These proteins differ from other described extracellular matrix proteins in their structure, their distributions in vivo, and their effects on cell behavior.