Recent advances in the genetics of the Notch-signalling pathway

Petter Portin

he Notch-signalling pathway is one of the most profoundly studied ubiquitous cell-cell signalling routes in the animal kingdom. The Notch gene, like so many other conserved genes in animals, was first found in the 1920«s in Drosophila melanogaster where its loss-of-function mutations are recessive embryonic lethals due to hypertrophy of the neural tissue at the expense of the epidermal tissue. In the heterozygous condition these mutations cause notches at the edges of the wings [hence its name].

In a recent issue of Genes & Development (1 June 2000) two interesting reports on Notch-signalling in mammals appeared. The Notch protein is a transmembrane signalling protein with several known ligands of which the Delta protein, also first found in Drosophila melanogaster, is the best characterised. Previously two members of the Delta family, named murine DII1 and murine DII3, have been known in mammals. Now Shutter et al. (Genes & Development 14, 1313 - 1318, 2000) report the cloning and characterisation of a new member of the Delta family of Notch ligands in mice, which they named DII4. Interestingly in situ analysis revealed a highly selective expression pattern of DII4 within the vascular endothelium, which directly suggests a role for DII4 in the control of endothelial cell biology. This observation was not unexpected because the Notch-signalling pathway is known as an important regulator of neurogenesis and myogenesis both in the fruit fly and mammals. In zebrafish the gridlock gene, regulated by Notch activation, is required for development of the aorta.

A few pages on (Genes & Development 14, 1343 - 1352, 2000), the same group (Krebs et al.) reports that Notch-signalling is in actual fact essential for vascular morphogenesis in mice. The mouse, like humans, has four copies of the Notch gene in its genome (while the fruit fly has only one). Krebs et al. generated Notch4-deficient mice by gene targeting. Embryos homozygous for this mutation developed normally, and homozygous mutant adults were viable and fertile. However, the Notch4 mutant displayed genetic interactions with a targeted mutation of the Notch1 gene. Embryos homozygous for mutations of both the Notch4 and Notch1 genes often displayed a more severe phenotype than Notch1 homozygous mutant embryos. Both Notch1 mutant and Notch1 / Notch4 double mutant embryos displayed severe defects in angiogenic vascular remodelling. These results together with those of Shutter et al. reveal an essential role for the Notch-signalling pathway in regulating embryonic vascular morphogenesis and remodelling.

Two weeks later in Nature (22 June, 2000) Huppert et al. (Nature 405, 966 - 970) further elucidated the role of Notch-signalling pathway in mice. This report involves the Presenil1 gene, the homologue of which in mutant form is presumed to be the causative agent of the early-onset Alzheimer disease in man.

One hypothesis on the functioning of the Notch-signalling pathway is that ligand binding to Notch triggers Presenilin1-dependent proteolytic release of the Notch intracellular domain from the cell membrane; this domain then enters the nucleus in small amounts, where it regulates downstream target genes of the pathway. Huppert et al. found positive evidence for this hypothesis by showing that a single point mutation at the intramembranous processing site of Notch1, which changed valine at the position 1.744 to glysine, resembled the null Notch1 phenotype.

Already 29 February 2000 a fourth interesting recent report on Notch-signalling appeared in Proceedings of The National Academy of Sciences USA (Kurata et al., PNAS 97, 2117-2122) involving the role of the Notch gene in the appendage identity in Drosophila melanogaster. It is previously known that in D. melanogaster the Notch gene through its various target genes regulates wing and haltere formation and eye morphogenesis. Now Kurata et al. demonstrate that Notch-signalling regulates eyeless expression during eye induction (eyeless is a master control gene of eye development). Significantly Notch-signalling is also involved in the determination of other appendages, i.e. wings, antennae, and legs of the fruit fly through regulating the respective control genes vestigial and Distal-less.

All together the four papers, briefly reviewed here, show that in addition to being ubiquitous in the animal kingdom, the Notch-signalling pathway is also unexpectedly versatile possibly involved in fact in all cell-cell communication systems of developing animals.