Cell Fate Choices in the Nervous System 
and Elsewhere 
Spyridon Artavanis-Tsakonas, Ph.D. — Investigator 
Dr. Artavanis-Tsakonas is also Professor of Cell Biology and Biology at Yale University. He received 
his M.A. degree in chemistry from the Federal Institute of Technology (Eidgenoessische Technische 
Hochschule) in Zurich and his Ph.D. degree in molecular biology from the University of Cambridge, 
England, for work done at the MRC Laboratory of Molecular Biology. His postdoctoral work was done 
at the Biozentrum of the University of Basel with Walter Gehring and at Stanford University 
with David Hogness. 
A fundamental issue in the biology of multi- 
cellular organisms is how a cell acquires its 
specific developmental fate. The molecular rules 
by which neighboring cells choose developmen- 
tally distinct paths are unknown. We are particu- 
larly interested in how these rules apply to the 
nervous system and thus have been exploring the 
molecular biology of early neurogenesis in the 
fruit fly. 
The central nervous system in Drosophila de- 
rives from a set of precursor cells (neuroblasts) 
that segregate from the epidermal precursors 
(dermoblasts) in the very early ectoderm. Embry- 
ological, genetic, and molecular studies have 
demonstrated that neuroblast segregation de- 
pends on interactions between neighboring cells. 
Several genes capable of interfering with this pro- 
cess have been identified, and the Notch locus is 
central. As we have shown, Notch codes for a 
transmembrane protein with homology to the 
mammalian epidermal growth factor (EGF), im- 
plying involvement in cell-surface events. 
For several years we have been studying Notch 
and other genetic elements in the cell inter- 
actions underlying neuroblast differentiation. 
Through these studies it became clear that the 
mechanisms governing the segregation of the neu- 
roblasts from the epidermal precursors in the 
neural ectoderm are used not only in neurogene- 
sis but also in a broad spectrum of tissues and cell 
fate choices. 
In an attempt to examine the complexity of the 
genetic circuitry in which Notch is integrated 
and to identify genes whose products may inter- 
act directly with the Notch protein, we have been 
using genetic screens designed to identify sup- 
pressors or enhancers of specific Notch muta- 
tions. Through this analysis, five genes, which we 
operationally refer to as the '"Notch group," have 
been identified, and extensive interactions 
among them documented. Besides Notch, the set 
includes four members that code for cell-surface, 
cytoplasmic, and nuclear elements. Delta and 
Serrate both code for transmembrane proteins 
with EGF homologous extracellular domains; En- 
hancer of split and mastermind code for nuclear 
proteins; and deltex appears to be cytoplasmic. 
Among the various genetic interactions within 
the Notch group, those occurring between Notch 
and Delta most clearly suggest that they interact 
on the molecular level. To investigate this possi- 
bility, we examined the effects of Notch and 
Delta expression in Drosophila S2 cells. We 
found that Notch- and De/to-expressing cells 
formed mixed aggregates and that this occurs via 
their extracellular EGF homologous domains. In- 
terestingly, A'^o^c^-expressing cells exhibit vesic- 
ular structures containing both Delta and Notch 
proteins, implying that Delta is internalized via 
Notch as a receptor. This relationship is currently 
being analyzed by electron microscopy. 
Deletion mutagenesis of the extracellular do- 
main of Notch determined that only two of the 36 
EGF repeats are sufficient and necessary to medi- 
ate interactions with Delta. Thus the extracellu- 
lar domain of Notch seems surprisingly modular 
and could potentially serve to bind a number of 
other proteins. Some of these may bind to subsets 
of the EGF repeats or to parts of the extracellular 
domain of Notch, while others may compete for 
the same binding sites. Indeed, Serrate, which 
displays homologies to the Delta protein, inter- 
acts with Notch in a Delta-like fashion and, since 
it binds to the same EGF repeats, may compete 
with Delta for binding to Notch. 
On the basis of these results, it was proposed 
that Notch may function as a multifunctional re- 
ceptor containing a series of ligand-binding sites, 
each of which may interact with more than one 
specific ligand. Such a model provides a plausi- 
ble explanation for the pleiotropic action of 
Notch and suggests a general function for the 
gene throughout development. 
If Notch acts as a receptor, how are Notch- 
mediated extracellular signals transmitted to the 
nucleus? This question is central to our current 
work and is being addressed at several levels. 
Cloning and sequencing of deltex (which dis- 
plays allele-specific interactions with Notch, 
Delta, and mastermind) revealed the primary 
structure of its product, a 700-amino acid pro- 
tein with no known homologues. Preliminary 
results indicate that the deltex protein is cyto- 
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