Dr. Aldrich is also Associate Professor of Molec- 
ular and Cellular Physiology at the Stanford Uni- 
versity School of Medicine. 
Articles 
Foster, CD., Chung, S., Zagotta, W.N., Aldrich, 
R.W., and Levitan, I.B. 1992. A peptide derived 
from the Shaker-^ channel produces short and 
long blocks of reconstituted Ca^^-dependent 
channels. Neuron 9:229-236. 
Germeraad, S., O'Dowd, D., and Aldrich, R.W. 
1992. Functional assay of a putative Drosophila 
sodium channel gene in homozygous deficiency 
neurons, f Neurogenet 8:1-16. 
Hoshi, T., Zagotta, W.N., and Aldrich, R.W. 1991. 
Two types of inactivation in Shaker channels: 
effects of alterations in the carboxy-terminal re- 
gion. Neuron 7:547-556. 
McEachern, A.E., Shelton, E.R., Bhakta, S., Ober- 
nolte, R., Bach, C, Zuppan, P., Fujisaki, J., Al- 
drich, R.W., and Jarnagin, K. 1991. Expression 
cloning of a rat B2 bradykinin receptor. Proc Natl 
Acad Sci USA 88:7724-7728. 
CELLULAR AND MOLECULAR MECHANISMS OF NEURAL CREST DEVELOPMENT 
David J. Anderson, Ph.D., Assistant Investigator 
Regulation and Function of MASH Genes 
To begin to identify nuclear regulatory factors 
that control the commitment of neural crest cells to 
various sublineages. Dr. Anderson and his col- 
leagues have cloned vertebrate homologues of Dro- 
sophila genes involved in neural development. Two 
such genes are MASH (mammalian achaete-scute 
homologous) 1 and 2, homologues of Drosophila 
achaete-scute. These genes encode members of the 
basic helix-loop-helix (bHLH) family of transcrip- 
tion factors, which includes the myogenic determi- 
nation gene MyoD. MASHl and MASH2 are >80% 
identical to their Drosophila counterparts within 
the bHLH domain, but diverge outside this region. 
Analysis of the pattern of MASHl expression using 
a monoclonal antibody has revealed that this gene, 
like its Drosophila counterpart, is expressed specifi- 
cally and transiently in spatially restricted subsets of 
precursor cells within the developing nervous sys- 
tem. Together with similar data obtained by others 
for mammalian and Drosophila MyoD {nautilus, 
nau), these results suggest that genes encoding 
bHLH proteins may constitute an evolutionarily 
conserved family of cell-type determination genes, 
of which MASHl is the first neural-specific member 
to be identified in vertebrates. 
The deduced amino acid sequence of the MASH 
proteins predicts that they should function as tran- 
scription factors. To provide experimental evidence 
in support of this prediction, recombinant MASH 
proteins were expressed in different systems and 
their DNA-binding and transcriptional regulatory ac- 
tivities assessed. Both MASHl and MASH 2 were 
found to bind to an E box, the consensus binding site 
for all bHLH proteins, but only as hetero-oligomers 
with the ubiquitously expressed bHLH protein E2A. 
Surprisingly, MASHl and MASH 2 also activated 
transcription of a muscle-specific gene, muscle cre- 
atine kinase (MCK), which contains an E box and is 
thought to be a target of MyoD. Unlike MyoD, how- 
ever, the MASH genes were unable to activate the 
full myogenic program in transfected C3H-10T'/2 
cells, even if the MyoD basic region was substituted 
for the MASHl basic region. Although MCK is un- 
likely to be a physiological target of the MASH 
genes, these data indicate that MASHl and MASH 2 
are indeed transcription factors and identify a bio- 
chemical assay system in which structure-function 
relationships in these proteins can be explored. 
Studies are now under way to identify authentic tar- 
gets of MASH proteins using a variety of experimen- 
tal strategies. 
To identify a culture system in which the regula- 
tion and function of MASH genes can be studied. Dr. 
Anderson's laboratory examined their expression in 
mouse PI 9 embryonal carcinoma cells. In such 
cells, MASHl mRNA and protein are initially unde- 
tectable, but are expressed upon induction of neuro- 
nal differentiation by retinoic acid. Furthermore, 
MASH 1 protein expression precedes and then over- 
laps expression of neuronal markers such as NCAM 
(neural cell adhesion molecule). However, MASHl 
is not detected in differentiated PI 9 neurons, sug- 
gesting that (as is the case in vivo), its expression 
388 
