Mechanisms of Embryonic Induction 
in Vertebrates 
Richard L. Matts, M.D., Ph.D. — Assistant Investigator 
Dr. Maas is also Assistant Professor of Medicine at Harvard Medical School and Associate Physician at 
Brigham and Women's Hospital, Boston. He received his A.B. degree in chemistry from Dartmouth College 
and an M.D. -Ph.D. degree from Vanderbilt University School of Medicine. 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. 
THE goal of our research is to understand the 
role that the homeobox genes play in control- 
ling vertebrate organogenesis. These genes are 
defined by their expression of a 60-amino acid, 
helix-turn-helix, DNA-binding domain. Highly 
conserved in evolution, they are present in spe- 
cies as divergent as Drosophila, yeast, and hu- 
mans. Mutations in homeobox genes of fruit fly 
and mouse result in specific developmental de- 
fects. Our work thus far has focused on the char- 
acterization of two such genes that appear to play 
important roles in the formation of the mamma- 
lian kidney and eye, respectively. A long-term 
goal is to understand the target genes that these 
homeobox genes interact with, using a combina- 
tion of biochemical, embryologic, and genetic 
techniques. 
Murine Homeobox Genes Expressed in 
Mouse Embryonic Kidney 
To determine which homeobox genes are ex- 
pressed in the developing mouse kidney, we un- 
dertook a polymerase chain reaction (PGR) 
screen of reverse-transcribed, microdissected 
kidney RNA from mouse embryos at day 1 5 . This 
experiment yielded 27 different homeobox- 
containing genes, some 11 of which correspond 
to new genes. Among these novel sequences, we 
identified several with 85-98 percent sequence 
similarity to known murine Hox genes at the nu- 
cleotide level. In addition, we identified two 
other genes, closely related to each other, that 
appear to be new members of the Hox-1 and 
Hox- 3 clusters in the mouse. Mapping experi- 
ments indicate that the Hox-1 member is 
Hox- 1.8. 
The structures of several cDNA clones of Hox- 
1.8 are being determined. Thus far, five different 
alternatively processed forms have been identi- 
fied. Surprisingly, all these forms share a com- 
mon feature: due to the presence of upstream ter- 
mination codons, none would actually encode a 
translatable homeodomain. We suspect that a 
homeodomain-encoding form exists, because the 
homeodomain is preserved intact at the sequence 
level. Current efforts are aimed at securing the 5' 
end of the Hox- 1.8 gene in order to determine 
whether splice forms exist that would encode a 
functional homeodomain. 
A current working hypothesis of our laboratory 
is that many murine Hox genes may, as a general 
rule, encode both homeobox-containing and 
homeobox-less forms. Such forms may interact in 
heterodimeric combinations with one another, or 
with other Hox genes, to alfect the capacity of the 
homeobox-containing form to bind to DNA. 
The expression of the //ox- 7. S transcripts has 
been analyzed as a function of mouse embryogen- 
esis. As determined by Northern blot analysis, sig- 
nificant expression appears at day 1 0 of embryo- 
genesis, peaks at day 13, and subsides by day 15. 
Expression in adults is confined to the kidney and 
to skeletal muscle. Interestingly, the adult kidney 
appears to express a smaller transcript form of 
approximately 1.5 kb, in addition to the larger 
class of 2.8-3.9 kb observed in both embryos and 
kidney. This size range corresponds to extensive 
alternate processing, as noted above. 
We have further analyzed the expression of 
Hox- 1 .8 duTing embryogenesis by in situ hybrid- 
ization. Regionally restricted expression is ob- 
served in the submucosa of the foregut and mid- 
gut and also in somites. Of particular interest to 
the potential role of this gene in nephrogenesis is 
its expression in the condensing collecting duct 
system, in the region that comprises the develop- 
ing calyces. Three-dimensional reconstruction of 
the Hox- 1.8 expression pattern shows that ex- 
pression is localized to this part of the developing 
kidney at embryonic day 1 3 • 
Identification of a Pax Gene Involved 
in Formation of the Vertebrate Eye 
The formation of the vertebrate eye has long 
served as an attractive model system for studying 
basic features of embryonic induction. The eye 
forms as a consequence of outgrowth of the dien- 
cephalon and a subsequent interaction of this 
neuroectoderm-derived structure with the sur- 
face ectoderm, resulting in an invagination of the 
latter to form the lens vesicle. Additional neuroec- 
toderm and mesodermal ingrowth anterior to the 
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