Mechanisms of Embryonic Induction in Vertebrates 
Richard L. Maas, M.D., Ph.D. — Assistant Investigator 
Dr. Maas is also Assistant Professor of Medicine at Harvard Medical School. He received his A.B. degree in 
chemistry from Dartmouth College and an M.D.Ph.D. degree from Vanderbilt University School of Medi- 
cine. Following his thesis work with John Oates, he trained as a medical house officer at Brigham and 
Women's Hospital and completed a postdoctoral fellowship in Philip Leder's laboratory in the Department 
of Genetics at Harvard Medical School. 
AN emerging theme in vertebrate develop- 
ment is that temporally and spatially regu- 
lated signals between cell populations direct pat- 
tern formation. The process by which such 
signals effect differentiation of a tissue is called 
induction. A major goal in our laboratory is to 
understand what these inductive signals are and 
how their activities are orchestrated. 
Recently two major classes of genes have been 
found to have an important role in vertebrate de- 
velopment. Homeobox genes, the first of these 
classes, control the identity of individual body 
parts. They have in common a 180-base pair se- 
quence element, the homeodomain. This genetic 
element, highly conserved in evolution, has been 
shown to confer the sequence-specific binding of 
the homeoprotein to DNA, with the remainder of 
the protein functioning as a transcriptional acti- 
vator or repressor. Moreover, in the examples an- 
alyzed, the homeodomain alone can interact with 
DNA. Although the target genes for homeobox 
proteins are known in some cases, particularly in 
Drosophila, and include other homeotic genes, 
they are not well identified or understood in 
mammals. 
From work in both Xenopus and Drosophila, it 
has become clear that certain growth factors con- 
stitute a second class of gene products important 
in morphogenesis. One hypothesis is that individ- 
ual homeobox genes, acting alone or in concert, 
also regulate, perhaps directly, the function of 
individual growth-controlling peptides. It is al- 
ready known that members of a family of peptide 
growth factors, called transforming growth fac- 
tor-|8s (TGF-i8s) , can influence the expression of 
certain homeobox genes. A possibility that indi- 
vidual homeobox genes might also regulate TGF- 
i8s or other growth factors is suggested by coinci- 
dental patterns of expression in developing 
vertebrate embryos, as well as from genetic ex- 
periments in Drosophila. This raises the general 
question, What are the transcriptional targets of 
individual homeobox-containing genes during 
vertebrate embryogenesis? We have taken a 
multidisciplinary approach to this question, 
with both general and specific avenues of 
investigation. 
Homeobox Genes in the Developing 
Mouse Heart 
The embryonic development of the vertebrate 
heart consists of an interaction between two con- 
centric layers of epithelium — an outer epithelial 
layer, or myocardium, and an inner epithelial 
layer, or endocardium. The outer epithelial layer 
has been shown to elaborate a soluble factor that, 
in concert with TGF-/31, triggers the transforma- 
tion of the inner epithelium into an undifferen- 
tiated mesenchyme. This subsequently differen- 
tiates into the connective tissue of the valves of 
the adult heart. 
In the mouse embryo, TGF-^1 is expressed in 
the atrioventricular canal, the site of valve forma- 
tion, at the time of differentiation. Interestingly, 
a homeobox gene of the msh class, Hox- 7.1, ap- 
pears to share during development a common 
pattern of expression with TGF-|8 1 in the atrioven- 
tricular canal. We have cloned the TGF-/?! pro- 
moter and regulatory region and are testing its 
ability to interact with recombinant Hox- 7. 1 ho- 
meobox proteins. We also plan to determine the 
sequence of DNA responsible for binding Hox- 
7. 1, with a view to comparing the binding sites of 
msh class homeoboxes with those of the Anten- 
napedia class, which are already known. 
Using a degenerate set of primers engineered to 
the most highly conserved regions of the homeo- 
box, we have also amplified and analyzed addi- 
tional homeobox sequences from reverse- 
transcribed embryonic cardiac RNA from day 10 
of mouse embryogenesis. 
Homeobox Gene Expression in Developing 
Embryonic Mouse Kidney 
A second system studied in our laboratory is the 
developing metanephros, or kidney. Approxi- 
mately 20 different homeobox genes have been 
identified in the day- 15 kidney of the embryonic 
mouse. These genes have been divided into three 
major groups: 1) genes identical to known ho- 
meobox sequences, 2) genes that are clearly dif- 
ferent from any known sequences, and 3) genes 
that are very closely related to known sequences, 
but still different. Immediate goals for the ho- 
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