Intracellular Cell-Autonomous Association of Notch and Its Ligands: A Novel Mechanism of Notch Signal Modification

Abstract

Notch (N) and its ligands, Delta (Dl) and Serrate (Ser), are membrane-spanning proteins with EGF repeats. They play an essential role in mediating proliferation and segregated differentiation of stem cells. One of the prominent features of N signal system is that its ligands are anchored to the plasma membrane, which allows the ligand/receptor association only between the neighboring cells. Various lines of evidences have verified this intercellular signal transmission, but there also have been implications that expression of Dl or Ser interferes cell-autonomously with the ability of the cell to receive N signal, implying that N and its ligands may interact in the same cell. Here, we demonstrate that N, Dl, and Ser cell-autonomously form homomeric or heteromeric complexes. The cell-autonomous heteromeric complexes are not present on the cell surface, implying that the association occurs in the endoreticulum or Golgi apparatus. Expression of Dl or Ser cell-autonomously reduces the N-mediated HES-5 promoter activity, indicating that the cell-autonomous association alters the N signal receptivity. Intracellular deletion of Dl shows elevated activity of this dominant-negative effect. In vivo overexpression study suggests that the cell-autonomous function of Dl and Ser is independent of the ligand specificity and may be modulated by Fringe (Fg), which inhibits the formation of the cell-autonomous Dl/N or Ser/N complex.

Upregulation of ASCL1 and inhibition of Notch signaling pathway characterize progressive astrocytoma

“…Similarly, MASH1 mutants fail to express Delta1 transcripts (Casarosa et al, 1999). High levels of Notch ligand Delta1 is capable of inhibiting Notch signaling by forming intracellular cell-autonomus Notch : Delta1 associations (Sakamoto et al, 2002). Thus, our data clearly demonstrate the downregulation of Notch signaling during progressive astrocytoma development.…”

“…We did not find any significant change in the expression levels of Lunatic, Radical and Manic fringe in most samples, suggesting that fringe molecules may not have any role in inhibiting notch ligands during progressive astrocytoma development (data not shown). It is reported that Notch ligands Delta1 and Jagged 1 sequester Notch proteins in the endoreticulum or Golgi apparatus of neuronal precursors as intracellular heteromeric complexes and thus reduce the effective dose of Notch signaling (Sakamoto et al, 2002). Thus, the overexpression of Notch ligands is believed to inhibit Notch signaling by forming intracellular cell-autonomus ligand : receptor associations rather than activate the Notch pathway.…”

When cell biology meets development: endocytic regulation of signaling pathways

“…Interestingly, in the absence of shibire, N ECD and N ICD can be internalized together with Delta in signaling cells of the wing disc, suggesting that a dynamin-independent internalization mechanism may also exist. Furthermore, it has been suggested that, at least in vitro, Notch and Delta can cell autonomously form homomeric and heteromeric complexes that repress Notch signaling, adding an additional dimension of complexity to signal regulation (Sakamoto et al 2002). Although more work is needed to clarify this process further, it appears that, in the signaling cell, N ECDDelta complex internalization is required for the activation of Notch signaling.…”

Lunatic fringe causes expansion and increased neurogenesis of trunk neural tube and neural crest populations

“…In Drosophila, fringe encodes a secreted glycosyl-transferase that plays an important role in boundary-specific signaling during pattern formation (Johnston et al, 1997;Barrantes et al, 1999). It localizes the signaling activity of Notch by Delta to presumptive boundary regions such as the Drosophila wing margin while blocking Notch response to Serrate (Panin and Irvine, 1998;Ju et al, 2000;Moloney et al, 2000), and is known to influence Notch signaling in a cell autonomous manner by forming complexes with Notch before secretion (Ju et al, 2000;Sakamoto et al, 2002). In this way, fringe modulates the ligand preference of Notch for Delta.…”