Development of the Drosophila Peripheral Nervous System 
in the opposite orientation of cpo transcription 
cause much less severe defects than insertions in 
the same orientation. These observations provide 
a unique opportunity to study the interactions be- 
tween enhancers and promoters in vivo. The 
study of cpo is also supported by the Muscular 
Dystrophy Association. 
The nrm gene is expressed early in the develop- 
ment of the PNS and muscles. It maps at cytologi- 
cal band 80A and was also identified in an en- 
hancer detector screen. To our knowledge only a 
single insertion has been identified in nrm, and 
the jS-galactosidase expression pattern in em- 
bryos of this strain is virtually identical to that of 
cpo during early development. Flies homozygous 
for this particular insertion are homozygous via- 
ble. A mutagenesis experiment allowed us to re- 
cover 10 recessive lethal mutations that fail to 
complement each other. Although many subtle 
defects have been observed in different mutant 
strains, no consistent morphological defects have 
been observed. 
The cloned nrm gene encodes a single 4.6-kb 
transcript that is expressed transiently in the sen- 
sory mother cells and developing neurons and 
support cells of the PNS (5-10 h). Later in embry- 
onic development (14-17 h) the transcript is 
also observed in most embryonic muscles. The 
earliest expression of nrm in muscles coincides 
with the formation of the neuromuscular junc- 
tion. Sequencing data show that the nrm gene 
encodes a novel protein that contains a signal 
peptide and eight immunoglobulin domains. The 
protein is possibly anchored in the membrane of 
the cells by a secondary modification of the pro- 
tein. On the basis of these molecular data and the 
expression pattern of nrm, we propose that nrm 
is a novel neural cell adhesion molecule that may 
play an important role in growth cone guidance 
in the PNS and may affect the formation of the 
neuromuscular junction. These hypotheses are 
presently being tested. 
To study the role of glial cells in the develop- 
ment of the PNS, we are developing a method that 
allows temporally induced arrest of most func- 
tions in specific cells. These arrests are induced 
by temperature shifts of flies that express temper- 
ature-sensitive (ts) forms of diphtheria toxin 
(DT-A'*) under the control of cell- or tissue- 
specific regulatory sequences. DT-A'* genes were 
isolated in a mutagenesis screen using the yeast 
Saccharomyces cerevisiae and subsequently 
tested in PNS neurons, namely the R1-R6 photo- 
receptor cells of transgenic fruit flies. Four DT-A'* 
have been partially characterized in yeast cells, 
and three have been tested in Drosophila. These 
toxins show similar temperature dependence, 
suggesting that they may be useful in a wide range 
of species. We are presently trying to ablate glial 
cells in the PNS and CNS, in an attempt to exam- 
ine the role of glia in the living organism. 
Expression pattern of the couch potato protein in an almost fully developed Drosophila 
embryo. This protein is expressed in all cells of the peripheral nervous system and in a 
subset of cells of the central nervous system. Anterior, left, dorsal, up. 
Research and photograph by Sandra Kooyer and Diana D 'Evelyn in the laboratory of 
Hugo Bellen. 
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