Cell Interactions in Neurogenesis 
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 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. He joined the Yale faculty in 1981. 
A fundamental issue in the development of 
multicellular organisms is how an individ- 
ual cell acquires a specific developmental fate. 
The molecular rules directing neighboring cells 
into developmentally discrete paths are un- 
known. With a particular interest in how these 
rules apply to the nervous system, we have been 
studying the molecular biology of early neuro- 
genesis in the fruit fly Drosophila. 
The central nervous system in Drosophila de- 
rives from a set of precursor cells, the neuro- 
blasts, which segregate from the epidermal pre- 
cursors, the dermoblasts, in the very early 
ectoderm. For the past several years, we have 
been analyzing a group of six zygotically acting 
genes that are involved in this process. Known as 
neurogenic loci, these are Delta (Dl), Enhancer 
of split [E(spl)], mastermind {mam), big brain 
{bib}, neuralized (neu), and Notch (N). Muta- 
tions in any one of these can result in the misrout- 
ing of epidermal lineage into a neuronal develop- 
mental pathway. 
The first neurogenic locus to be characterized 
in some detail was Notch. We found that it codes 
for a transmembrane protein homologous to the 
mammalian epidermal growth factor (EGF), im- 
plying an involvement in cell surface events. In- 
deed, our work and that of others — embryologi- 
cal and molecular studies of some of the other 
neurogenic loci — strengthened that notion and 
led to the hypothesis that neuroblast segregation 
depends on cell interactions, with at least some 
of the neurogenic loci coding for proteins in- 
volved in intercellular signaling. 
In studying the cellular mechanisms in which 
Notch is integrated, we have been following two 
major experimental paths. On the one hand, we 
are exploring the cell biology of Notch and the 
nature of its genetic interactions; on the other, 
we are trying to gain insights into the functional 
meaning of certain sequence motifs found in the 
Notch protein. For instance, we are characteriz- 
ing other genes sharing the EGF sequence motif 
with Notch. In addition, during the past year, we 
initiated a molecular and genetic analysis of cer- 
tain nonneurogenic genes that play a role in the 
differentiation of the ectoderm. 
We started this analysis with the gene fizzy 
(fzy) . Mutations of this gene aff^ecting the pheno- 
type have suggested that it functions to promote 
subsequent differentiation in response to the 
postulated "epidermalizing" component of the 
signal transmitted by the neurogenic gene prod- 
ucts. As such, the fzy protein has a pivotal role in 
implementing the response of presumptive der- 
moblasts to developmental cues transmitted by 
those products. We have been analyzing the ge- 
netic and phenotypic behavior of fzy mutations 
and have initiated its molecular analysis by the 
cloning of genomic sequences from the fzy 
region. 
In an attempt to examine the complexity of the 
genetic circuitry in which Notch is integrated, 
and to identify genes whose products may di- 
rectly interact with the Notch protein, we de- 
signed a genetic screen aimed at identifying sup- 
pressors of certain Notch mutations associated 
with the gene's extracellular EGF homologous 
domain. This screen has led to the identification 
of a surprisingly restricted set of interacting loci. 
These include the neurogenic genes Delta, 
which also codes for a transmembrane protein 
with EGF homology, and mastermind. In addi- 
tion, a third gene was shown capable of acting as 
a suppressor. We identified this gene to be the 
deltex locus, hitherto unlinked with neurogenic 
gene action. We have further characterized the 
phenotype of deltex mutations and have demon- 
strated allele-specific interactions between del- 
tex and Notch alleles. 
To investigate the possibility of intermolecular 
association between the products of Notch and 
Delta genes, we have examined the effects of 
their expression on aggregation in Drosophila S2 
cells. We find that A^o^c^-expressing cells, by a 
calcium-dependent process, form mixed aggre- 
gates specifically with cells that express Delta. 
Furthermore, we have determined that Notch and 
Delta proteins interact via their extracellular 
domains. 
Some of the first phenotypic interactions to be 
described between neurogenic loci were those 
involving Notch and Enhancer of split. Molecu- 
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