Notch: The Past, the Present, and the Future

Spyros Artavanis-Tsakonas*; Marc A.T. Muskavitch†    * Department of Cell Biology, Harvard Medical School, Boston, MA, USA and Department of Biology and Genetics of Development, College de France, Paris, France
† Department of Biology, Boston College, Chestnut Hill, MA, USA

Abstract

Proliferating investigations of the Notch pathway have given rise to the Notch “field,” which has grown exponentially over the past 30 years. This field, founded by investigations of embryology and genetics in Drosophila, now encompasses many metazoa, including humans. The increasingly diverse scope of the field has engendered an expanding understanding that normal Notch pathway function is central to most developmental decision-making in animals, and that pathway dysfunction is implicated in many diseases, including cancer. We provide a personal view of the foundations and rapid evolution of the Notch field; and we discuss a variety of outstanding conundrums and questions regarding Notch biology, for which answers will be found and refined during the next 30 years.

The first review to emphasize Notch, published by Ted Wright (1970), started with the inspiring sentence: If one was asked to choose the single, most important genetic variation concerned with the expression of the genome during embryogenesis in Drosophila melanogaster, the answer would have to be the Notch locus. The second Notch review arrived 18 years later, after cloning and sequencing of the locus (Artavanis-Tsakonas, 1988).

Today, a casual inspection of the Notch gene in Flybase (http://flybase.org/) reveals some instructive statistics. There are 345 classic alleles listed, 305 alleles on transgenic constructs, and 2250 references. These statistics, which include only fly-related work, are greatly expanded if we include research in all species. Notch biology can rightfully claim “field” status today, worthy of a book, such as this one.

The goal of this review is to give the reader some perspective on the history of the Notch field, which has become a very diverse field, rather than to review comprehensively particular aspects of Notch biology, and to try to define constants of the pathway, as well as some of the current pivotal questions. Many reviews apart from the current volume, covering more comprehensively particular aspects of Notch molecular biology, have appeared in recent years (Artavanis-Tsakonas et al., 1999; Baron, 2003; Bray, 2006; Fortini and Bilder, 2009; Gordon et al., 2008; Kopan and Ilagan, 2009; Louvi and Artavanis-Tsakonas, 2006). Moreover, the reader will find many details in subsequent chapters. Given our goals, we wish to apologize from the outset that we fail to refer to many original and important studies, and we warn the reader that our citations from the primary literature generally serve as examples, rather than providing comprehensive coverage of any topic.

1 The Beginnings: Embryology and Genetics

Almost a century has passed since T. H. Morgan’s group described a mutant in Drosophila that they named Notch because it generated serrations on the wing margin (Fig. 1.1). The Notch gene has thus contributed to the progress of genetics as a discipline from the very start. It also provided a fundamental link between genetics and developmental biology through the work of Donald F. Poulson.

Don Poulson, in the early 1930s, was conducting work at Caltech for his doctoral thesis under the supervision of A. H. Sturtevant and Th. Dobzhansky, studying the embryonic phenotypes associated with chromosomal deletions. The relationship between genes and embryonic development was very much in doubt at the time. The general belief, by the dominant figures in biology, who were undoubtedly the embryologists, was that the parameters followed by geneticists, i.e., phenotypes associated with mutations in genes, reflected only terminal traits—for instance, a notched wing, rather than activities that governed morphogenesis of the wing. …What is the role of the gene in development? Are there certain genes that are essential for the developmental process, or are genes only determinants of superficial characters? There are biologists to this day who believe that the latter is true, although there are few geneticists in their company… wrote Don Poulson in the introduction to his doctoral thesis.

The evidence lacking was a clear correlation of embryonic phenotypes with specific mutations. Poulson, who in order to describe the embryonic phenotypes linked to deletions of chromosomes, the bearers of genes, single-handedly described the embryology of Drosophila melanogaster with an extraordinary accuracy, examined the lethal phenotypes of chromosomal deficiencies (Demerec, 1950; Poulsons, 1936). Among them was Notch8, a small X-linked deficiency encompassing the Notch locus, which had been genetically characterized in Morgan’s laboratory (Dexter, 1914; Mohr, 1919) (Fig. 1.2). Notch behaved as a dominant, haploinsufficient, X-linked mutation; and heterozygous females had the characteristic “Notch” wings, while homozygous Notch females or hemizygous Notch males died as embryos.

Poulson’s analysis of the Notch lethal phenotype revealed a specific and reproducible phenotype. In his words, Although development in the early stages up to four hours is normal, Notch8 embryos fail to form the germ layers as evidenced by the absence of mesoderm and endoderm from the embryos at the time when the gut is normally completed. The organs and tissues, which are formed (although they may become highly abnormal) are all of ectodermal origin. There is no differentiation of ectoderm into hypoderm and the embryo is without skin. Those organs which undergo most differentiation and development are the nervous system and the hind-gut [Fig. 1.3 (Poulsons, 1936)]. One could thus argue that Poulson forged the link between the action of a genetic locus, Notch, and embryonic morphogenesis. In our view, this seminal discovery has not been given the credit it deserves. In many ways, this link between genes and development was most famously acknowledged almost a half century later, when it was granted the weighty imprimatur of the 1995 “fly Nobel Prize.” Later analyses refined and extended Poulson’s observations, demonstrating conclusively that when Notch activity is lost, cells under normal circumstances would give rise to epidermal precursors, the dermoblasts, switch fate, and become neuroblasts. These excessive neuroblasts continue their normal differentiation to produce a morphologically deranged, inviable embryo that displays hypertrophy of the nervous system at the expense of epidermal structures. It was because of this neural hypertrophy that the phenotype was later baptized with the term “neurogenic” (Lehmann et al., 1983).

Already in the 1920s, many Notch alleles had been identified by Morgan and his students, all of which yielded the typical notched-wing phenotype and bristle abnormalities in the females, as well as embryonic lethality, testifying to the pleiotropic nature of Notch activity. As Notch alleles started to accumulate, the spectrum of amorphic, hypomorphic, neomorphic, gain-of-function, recessive visible, and recessive lethal Notch alleles gave complex and often hard-to-interpret genetic complementation patterns, especially in the absence of clues as to the biochemical nature of the protein. Fanciful interpretations suggested that the Notch locus might be best represented by a spiral genetic map, while biochemical studies of Notch mutants were thought to suggest that Notch was the structural locus of several mitochondrial enzymes (Foster, 1973; Thorig et al., 1981a, b).

The most sophisticated genetic analyses of the Notch locus were undertaken during the 1950s and 1960s, when only a single laboratory devoted serious...