Catherine Dulac

Molecular characterization of the Schwann cell myelin protein, SMP: structural similarities within the immunoglobulin superfamily

The Schwann cell myelin protein (SMP), previously defined in quail and chick by a monoclonal antibody, is in vivo exclusively expressed by myelinating and nonmyelinating Schwann cells and oligodendrocytes. The isolation of the complete nucleotide sequence of SMP is reported here. The predicted polypeptide chain reveals that SMP is a transmembrane molecule of the immunoglobulin superfamily showing sequence similarities with several surface glycoproteins expressed in the nervous and immune systems. In spite of a 43.5% overall sequence identity between rat myelin-associated glycoprotein (MAG) and quail SMP, SMP does not seem to be the avian homolog of MAG, since their expression, regulation, and functions are significantly different. Unusual sequence arrangements shared by SMP, MAG, and two lymphoid antigens suggest the existence of a particular subgroup in the immunoglobulin superfamily.

A surface protein expressed by avian myelinating and nonmyelinating Schwann cells but not by satellite or enteric glial cells

Searching for specific markers of neural crest-derived cell lineages, we immunized mice with glycoproteins purified from adult quail peripheral myelin. We obtained a monoclonal antibody that reacts with myelin and peripheral glial cells. This antibody, to Schwann cell myelin protein (SMP), is specific for the membranes of all Schwann cells, irrespective of whether they are associated with myelinated nerves. SMP persists on Schwann cells in long-term cultures in vitro, but is absent from satellite cells of peripheral ganglia, both in vivo and in vitro. The antigen (a protein doublet of Mr 75,000-80,000) is present in, but not restricted to, the myelin lamellae, since it is distributed along the whole myelinating Schwann cell membrane. In the CNS, SMP appears as a single band of Mr 80,000. SMP is first detectable by immunofluorescence at E6 in the quail, which is at least 6 days earlier than the first appearance of already described markers related to myelination.

New Insights into the Development of Neural Crest Derivatives

Publisher Summary This chapter focuses on new insights into the development of neural crest (NC) derivatives. NC is a highly pluripotent structure, the constitutive cells of which while exhibiting a striking migratory behavior spread over an embryo and settle in certain selected areas where they differentiate into a large variety of cell types. By means of the quail or chick-exchange system of definite regions of the neural primordium, prior to the onset of crest-cell migration, a fate map of the NC was constructed. The basic experimental paradigm was to exchange defined territories in an isotopic and isochronic manner between the embryos of two species. The cephalic NC gives rise to a large array of peripheral ganglia. In addition to melanocytes and endocrine cells, the cephalic crest also yields the mesectoderm from which a large majority of the head skeleton originates. The common precursors for glia and neurons are abundant in the NC. The in vitro clonal technique revealed that the cells with the potentiality to give rise to neurons, glia, melanocytes, and ectomesenchyme exist in the migratory cephalic NC, thus showing that the neural, melanocytic, and mesenchymal lineages are not completely segregated at the time of crest-cell dispersion.

Negative Regulation of Schwann Cell Myelin Protein Gene Expression by the Dorsal Root Ganglionic Microenvironment

In vivo, the surface glycoprotein Schwann cell myelin protein (SMP) is expressed in the quail peripheral nervous system exclusively by Schwann cells. It is not detectable at any developmental stage either in enteric glia or in ganglionic satellite cells. We demonstrate here that the satellite glial cells of the dorsal root ganglia start to express SMP on their surface when they are dissociated into single cells and cultivated in vitro. Activation of SMP synthesis is a rapid event observed in mass cultures of dorsal root ganglia dissociated cells as soon as 4 h after the onset of the culture. Confocal microscope analysis revealed that satellite cells may acquire the Schwann cell marker when still in close contact with the neuronal soma. Clonal cultures of satellite cells from E8 dorsal root ganglia demonstrated that the progeny of these SMP‐negative cells steadily express SMP. This, together with similar results previously obtained with enteric glia, suggests that the SMP‐positive phenotype is a constitutive trait of the peripheral glial cell lineage which is inhibited in satellite cells in vivo by the microenvironment prevailing in the peripheral nervous system ganglia.

A Monoclonal Antibody Defining a Carbohydrate Epitope Restricted to Glial Cells

To follow cell lineage segregation of glial cell precursors in the central and peripheral nervous systems (CNS and PNS), we made monoclonal antibodies against purified avian myelin glycoproteins and made detailed studies of antibodies reacting with cellular antigens expressed early in development. We describe here the cellular and molecular characterization of the 4B3 epitope, which is first expressed at day 2.5 in quail embryos and appears strictly specific to glial cell membranes in the PNS and CNS. We demonstrate that, not only oligodendrocytes and astrocytes but also myelinating and non‐myelinating Schwann cells, satellite cells of sensory and autonomic ganglia and enteric glial cells are 4B3‐positive. This epitope was identified as an N‐linked carbohydrate determinant shared by several unrelated molecules, one of them already described as the SMP glycoprotein.

5 – Cell progenitors in the neural crest

This chapter focuses on cell progenitors in the neural crest. In vivo and in vitro studies described in this chapter reveal that, from the beginning of its migration, the neural crest is a globally pluripotent, heterogeneous population of cells that contains progenitors endowed with different individual proliferation and developmental potentials. Highly pluripotent stem-like cells, that in the rat have been demonstrated to have self-renewal capacity, coexist with more restricted precursors and with cells already committed to specific lineages. The proportion of oligopotent and committed cells increases as development progresses, by a mechanism that seems to be at least in part stochastic. However, several experimental data point to the importance, at all stages of development of the neural crest, and of local environmental signals in guiding the process of crest cell differentiation. The growth factor GGF2 has been suggested to exercise an instructive action on the glial cell lineage determination. Moreover, it is shown that the stability of the non-Schwann cell glial phenotype, as defined by the absence of SMP expression, depends upon continued environmental control.

New Molecular Insights on the Development of the Peripheral Nervous System

In the study of cell differentiation, a necessary step is to determine the onset of specific gene activities, in the various cell types emerging during embryogenesis. Production of monoclonal antibodies (Mab) able to distinguish specific antigenic determinants carried by differentiated cells, constitutes a mean to fulfill such a goal.