Distinct Effects of the Soluble Versus Membrane-Bound Form of the Notch Ligand Delta-4 On Human Cd34+Cd38^low Cell Expansion and Differentiation

¹ INSERM, U790, 39 rue C. Desmoulins, Villejuif, F-94805, Institut Gustave Roussy, Villejuif, F-94805, Univ Paris-Sud, Orsay, F-91405
² Novartis, Institutes for Biomedical Research, Cambridge MA 02139

^* To whom correspondence should be addressed. E-mail: elauret{at}igr.fr.

	Abstract

Though Notch ligands are considered to activate signaling through direct cell-cell contact, the existence of soluble forms has been demonstrated. However their roles remain controversial : soluble forms have been reported to mimick the biological activity of membrane-bound form, while other studies rather suggested an antagonistic activity toward their full-length counterparts. We previously observed that membrane-bound Delta4-expressing S17 stroma (mbD4/S17) reduced human CD34⁺CD38^low cell proliferation and favored self-renewal. Here we assessed the effects of a soluble form (solD4), by exposing CD34⁺CD38^low cells to S17 feeders engineered to express solD4 (solD4/S17). In contrast to mbD4/S17, i) solD4/S17 increased 10-fold cell production after 2 weeks, through enhanced cell proliferation ; ii) it did not preserve CFC and LTC-IC potential of output CD34⁺ cells. mbD4 and solD4 appeared to also differ in their signaling. Indeed, mbD4, but not solD4, strongly activated both CSL (the nuclear mediator of Notch signaling) in Hela cells overexpressing Notch1 and transcription of some classical Notch target genes in CD34⁺CD38^low cells. Furthermore, both biological effects and CSL activation elicited by mbD4 were strictly dependent upon the -secretase complex, whereas solD4 enhanced cell expansion in a partially -secretase-independent manner. Altogether these results suggest that part of solD4 activity did not rely upon canonical Notch pathway.