Seminars in Immunology 15 (2003) 99–106 Fine-tuning Notch1 activation by endocytosis and glycosylation Ute Koch, Julie S. Yuan, James A. Harper, Cynthia J. Guidos ∗ Program in Developmental Biology, Department of Immunology, University of Toronto, Hospital for Sick Children Research Institute, Rm 8104, 555 University Avenue, Toronto, Ont., Canada M5G 1X8 Abstract Recent studies have shown that disruption of Notch1 signaling in lymphocyte progenitors (LP) inhibits T cell development and promotes B cell development in the thymus. Conversely, inappropriate activation of Notch1 in LP inhibits B cell development and causes ectopic T cell development in the bone marrow. These observations imply that Notch1 activation must be spatially regulated to ensure that LP generate B cells in the bone marrow and T cells in the thymus. However, Notch ligands are expressed in both tissues. Studies in flies and worms have revealed that Notch activation is extremely sensitive to small changes in the amount of receptor or ligand expressed, and defined multiple mechanisms that limit Notch activation to discrete cells at specific times during development. Here, we describe how some of these mechanisms might regulate Notch activity in LP during the T/B lineage decision. © 2003 Elsevier Science Ltd. All rights reserved. Keywords: T/B lineage commitment; Lymphocyte progenitors; Endocytosis; Glycosyltransferases 1. Notch receptors and ligands The Notch pathway is highly conserved in mammals, and inherited or acquired mutations in Notch receptors, ligands, or regulators cause a plethora of developmental defects, complex disease syndromes, and oncogenesis in humans and mice [1]. Notch receptors and Notch ligands are transmembrane proteins that contain many cysteine-rich EGF-like repeats in their extracellular (EC) domains. Notch ligands also contain a characteristic sequence motif termed DSL near their N-termini that is required for binding Notch proteins. The biochemical basis of Notch signaling has been recently reviewed [2], so will only be described briefly here. Receptor–ligand interactions induce prote- olytic cleavage events that liberate the Notch intracellular domain (N IC ) from the plasma membrane and allow its transport to the nucleus. N IC then nucleates the assembly of a complex containing a CSL (CBF1, Su(H), LAG-1) transcription factor as well as transcriptional co-activators that induce the expression of the major effectors of the Notch pathway: Hairy/Enhancer of Split (HES) genes. Drosophila has one Notch protein and two structurally re- lated Notch ligands, Delta and Serrate. However, mammals have four Notch proteins (Notch1–4), three Delta-like (Dll) ligands (Dll-1, Dll-3, and Dll-4), two Serrate-like ligands ∗ Corresponding author. Tel.: +1-416-813-5026; fax: +1-416-813-8823. E-mail address: cynthia.guidos@sickkids.ca (C.J. Guidos). (Jagged-1 and Jagged-2), and seven HES genes. Genetic studies suggest that Delta and Serrate have both redundant and non-redundant functions in flies [3,4], but the degree to which different receptors, ligands, and HES genes have unique versus overlapping functions in mammals is not yet clear. 2. Notch signaling regulates binary cell fate decisions The Notch signaling pathway is widely used to regulate cell fate choices during development of invertebrate and ver- tebrates. Perhaps its most well-studied function is to regulate the choice between two alternative cell fates during neuro- genesis. Typically, Notch signals inhibit progenitors from adopting a neuronal (primary) fate by default. In some con- texts, this inhibition appears to be transient, and is thought to maintain the competence of progenitors to respond to later inductive signals that promote a secondary non-neuronal fate [5]. Accordingly, loss-of-function mutations in Notch pathway genes cause premature neuronal differentiation in mice [6–8]. In addition to inhibiting neuronal differentia- tion, Notch signals may also promote the self-renewal of neuronal stem cells in the mouse brain [9–11]. However, in other contexts, Notch signaling both inhibits the primary fate and instructively induces the secondary fate [12–15]. Notch signaling plays a similar role in determining how lymphocyte progenitors (LP) choose between the T or B cell 1044-5323/03/$ – see front matter © 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1044-5323(03)00006-X