Vascular tissue lies close to zones of neurogenesis. This prompted the authors to ask if the vascular tissue influences the fate of neural progenitors. To test this, first author Qin Shen and colleagues grew neural stem cells with endothelial feeder cells, placing the latter in a reservoir suspended just above mouse embryonic day-10 stem cells isolated from the cerebral cortex. In this type of experiment the cells are kept apart by a membrane that allows protein-sized molecules to diffuse freely from one compartment to the other.

The results of the experiment suggest that endothelial cells have two effects on progenitors, preventing their differentiation while promoting their self-renewal, and enhancing their capacity to make neurons.

First, the pump priming. When Shen put bovine pulmonary artery endothelial cells or mouse brain endothelial cells in the reservoir, the colony size of the progenitors grew over fourfold. Under the influence of control feeder cells, such as cortical cells, colony size averaged around 40 cells per clone, but with the endothelial cells this grew to around 180 cells per clone. Differentiation was also suppressed, as judged by the numbers of cells expressing LeX, Nestin, and β-tubulin. The latter is a marker of neural differentiation, while LeX and Nestin are typically thought to be markers of progenitor cells. With endothelial feeders, progenitors expressing LeX and Nestin were triple and quadruple, respectively, that seen when control feeder cells were used, while expression of β-tubulin was dramatically reduced.

Next, Shen removed the feeders and monitored changes in the clonally expanded cells. Within four days, about 30 percent of the cells were expressing β-tubulin, as opposed to only 10 percent of cells similarly treated with control feeders. What’s more, almost all of the endothelial-treated, β-tubulin-positive cells also expressed MAP-2, a dendritic marker.

To determine what types of neurons the endothelial cells may evoke, Shen and colleagues expanded the progenitors in co-culture with the bovine pulmonary artery cells. The neurons formed expressed Tbr1, a protein that is primarily found in projection neurons. In contrast, when Shen carried out the same experiment with embryonic day-15.5 stem cells, or adult subventricular zone progenitors, very few Tbr1-positive cells were formed. The finding suggests that endothelial cells may play a temporal role in deciding stem cell fate because during development, projection neurons are usually formed before glia and interneurons. “Endothelial factors are permissive, not instructive, for this fate,” write the authors.

At the molecular level, it is unclear what those factors are. The authors suspect the Notch pathway, because when they co-cultured stem cells with endothelial cells in the presence of a γ-sectretase inhibitor, the stem cells behaved as if they were grown without the endothelial cells. Moreover, the authors found that in the presence of endothelium, the Notch effector Hes1 (mammalian homologue of the Drosophila protein hairy and enhancer-of-split) was upregulated in stem cells. Hes5, however, was not.—Tom Fagan.

Reference:
Shen Q, Goderie S, Jin L, Karanth N, Sun Y, Abramova N, Vincent P, Pumiglia K, Temple S. Endothelial cells stimulate self-renewal and expand neurogenesis of neural stem cells. Science. 2004 Apr 1 [Epub ahead of print]. Abstract