A glial cell line promotes the outgrowth of neurites from embryonic Xenopus retina.

A glial cell line (XR1 cell line) derived from Xenopus retinal neuroepithelium was examined for neurite outgrowth promoting activity. A monolayer of the XR1 cells serves as an excellent substrate upon which embryonic retinal explants attach and freely elaborate neurites. The XR1 neurite outgrowth promoting activity is not secreted into the medium, but is laid down directly on the substrate where it remains active after lysing the cells by hypoosmotic shock. A polyclonal antiserum raised against membranes of the XR1 cells was effective in blocking neurite outgrowth on XR1 conditioned collagen. It is proposed that the neurite outgrowth promoting factors produced by the XR1 cells are associated with the extracellular matrix and possibly glial specific.

Xenopus fibrillin is expressed in the organizer and is the earliest component of matrix at the developing notochord-somite boundary.

We identify a Xenopus fibrillin homolog (XF), and show that its earliest developmental expression is in presumptive dorsal mesoderm at gastrulation, and that XF expression is regulated by mesoderm-inducing factors in animal cap assays. XF protein is also first detected in presumptive mesoderm, but is concentrated specifically into extracellular-matrix structures that begin to develop de novo by mid-gastrulation at both of the bilateral presumptive notochord-somite boundaries. Later in embryogenesis, XF protein is localized to the extracellular matrix at tissue boundaries, where it is found surrounding the notochord, the somites, and the neural tube, as well as under the epidermis. This pattern of protein deposition combines to give the appearance of an "embryonic skeleton," suggesting that one role for XF is to serve as a mechanical element in the embryo prior to bone deposition.

Expression of an extracellular deletion of Xotch diverts cell fate in Xenopus embryos.

Xotch is a Xenopus homolog of Notch, a receptor involved in cell fate decisions in Drosophila. Using an extracellular deletion construct, Xotch delta E, we show that Xotch has a similar role in Xenopus embryos. Broad expression causes the loss of dorsal structures and the expansion and disorganization of the brain. Single blastomere injections of Xotch delta E induce autonomous neural and mesodermal hypertrophy, even in the absence of cell division. Xotch delta E inhibits the early expression of epidermal and neural crest markers yet enhances and extends the response of animal caps to mesodermal and neural induction. Our data suggest a mechanism for the function of Notch homologs in which they delay differentiation and leave undetermined cells competent to respond to later inductive signals.

Growth cone interactions with a glial cell line from embryonic Xenopus retina.

We have isolated a nonneuronal cell line from Xenopus retinal neuroepithelium (XR1 cell line). On the basis of immunocytochemical characterization using monoclonal antibodies generated in our laboratory as well as several other glial-specific antibodies, we have established that the XR1 cells are derived from embryonic astroglia. A monolayer of XR1 cells serves as an excellent substrate upon which embryonic retinal explants attach and elaborate neurites. This neurite outgrowth promoting activity appears not to be secreted into the medium, as medium conditioned by XR1 cells is ineffective in promoting outgrowth. Cell-free substrates were prepared to examine whether outgrowth promoting activity is also associated with the XR1 extracellular matrix (ECM). Substrates derived from XR1 cells grown on collagen are still capable of promoting outgrowth following osmotic shock and chemical extraction. This activity does not appear to be associated with laminin or fibronectin. Scanning electron microscopy was used to examine growth cones of retinal axons on XR1 cells and other substrates that supported neurite outgrowth. Growth cones and neurites growing on a monolayer of XR1 cells, or on collagen conditioned by XR1 cells, closely resemble the growth cones of retinal ganglion cells in vivo. A polyclonal antiserum (NOB1) generated against XR1 cells effectively and specifically inhibits neurite outgrowth on XR1-conditioned collagen. We therefore propose that neurite outgrowth promoting factors produced by these cells are associated with the extracellular matrix and may be glial specific.