during neurogenesis is transient. Although MASHl 
expression precedes neuronal differentiation, trans- 
fection of MASHl expression constructs into PI 9 
cells failed to induce neuronal differentiation in the 
absence of retinoic acid. Nevertheless, the exoge- 
nously expressed MASH 1 protein is able to bind to 
DNA in nuclear extracts from the transfected cells. 
These data suggest that MASHl, unlike its muscle 
relative MyoD, is not sufficient for neurogenesis. 
Unexpectedly, MASH2 displayed a pattern of ex- 
pression in PI 9 cells complementary to that of 
MASHl. Low levels of MASH2 mRNA were detect- 
able in uninduced cells and declined following reti- 
noic acid treatment in parallel with the onset of neu- 
ronal differentiation. This suggests that the two 
MASH genes, despite their near sequence identity in 
the bHLH region, play very different roles in mam- 
malian development. Consistent with this idea, pre- 
liminary in situ hybridization analysis revealed that 
the highest levels of MASH2 expression are found in 
the ectoplacental cone, a primitive extraembryonic 
membrane. Furthermore, uniform low levels of 
MASH 2 mRNA are detected throughout the embryo. 
Gene mapping studies performed in collabora- 
tion with Dr. Nancy Jenkins of the National Cancer 
Institute have indicated that MASHl maps to murine 
chromosome 10 2Lnd MASH 2 to chromosome 7. This 
is to be contrasted with the Drosophila achaete- 
scute genes, which exist in a tightly linked chromo- 
somal complex. Each MASH gene is closely linked 
to an insulin-like growth factor (IGF) gene; MASHl 
to IGF-1 and MASH2 to IGF-2. This suggests that 
they may represent a duplicated chromosomal re- 
gion. Furthermore, MASH2 is located in a region of 
chromosome 7 that is known to display the phenom- 
enon of imprinting. The significance of this finding 
remains to be explored. 
To examine the biological function of the MASH 
genes in vivo, loss-of-function mutations are being 
generated by the technique of homologous recombi- 
nation in embryonic stem (ES) cells. Currently sev- 
eral ES lines containing a "knockout" mutation in 
the MASHl gene have been isolated. These lines 
have been injected into mouse blastocyst-stage em- 
bryos, generating chimeric mice. If this chimerism 
proves to extend to the germline, then homozygous 
mutant animals will result. The effects of this muta- 
tion on embryonic nervous system development 
will then be explored in detail. 
Control of Cell Fate by Glucocorticoids 
in the Sympathoadrenal Lineage 
In the sympathoadrenal lineage, two aspects of 
chromaffin cell development are dependent upon 
glucocorticoids: the inhibition of neuronal differ- 
entiation and the induction of the epinephrine- 
synthesizing enzyme phenylethanolamine A'^methyl- 
transferase (PNMT). In the developing embryo, 
these two aspects are temporally separated. This 
raises the question of how the same inducing signal 
can control two different biological responses at dif- 
ferent stages of development. 
Using cultures of purified chromaffin precursor 
cells. Dr. Anderson and his student Arie Michelsohn 
have shown that both effects of glucocorticoid ap- 
pear to be mediated by the same type of receptor. 
The temporal separation appears to be due to a cell- 
autonomous clock that changes the way the cells 
respond to the steroid, according to an intrinsic de- 
velopmental schedule. This two-step pathway ap- 
pears to provide a "checkpoint" mechanism that 
ensures that only chromaffin cells, and not sympa- 
thetic neurons, will be able to synthesize epineph- 
rine. (This project was supported by a grant from the 
National Institutes of Health.) 
Dr. Anderson is also Associate Professor of Biol- 
ogy at the California Institute of Technology and 
Adjunct Assistant Professor of Anatomy and Cell 
Biology at the University of Southern California 
School of Medicine. 
Books and Chapters of Books 
Anderson, D.J. 1992. Molecular control of neural 
development. In An Introduction to Molecular 
Neurobiology (Hall, Z.W., Ed.). Sunderland, MA: 
Sinauer Associates, pp 355-387. 
Articles 
Anderson, D.J., Carnahan, J.F., Michelsohn, A., and 
Patterson, P.H. 1991- Antibody markers identify a 
common progenitor to sympathetic neurons and 
chromaffin cells in vivo and reveal the timing of 
commitment to neuronal differentiation in the 
sympathoadrenal \me2L%e. f Neurosci 11:3507- 
3519. 
Carnahan, J. F., Anderson, D.J., and Patterson, P.H. 
1991. Evidence that enteric neurons may derive 
from the sympathoadrenal lineage. Dev Biol 
148:552-561. 
Johnson, J. E., Birren, S.J., Saito, T., and Anderson, 
D.J. 1992. DNA binding and transcriptional regu- 
latory activity of mammalian achaete-scute homol- 
ogous (MASH) proteins revealed by interaction 
with a muscle-specific enhancer. Proc Natl Acad 
Sci USA 89:3596-3600. 
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