For Immediate Release: Thursday, July 17, 2003
Contact:
Bob Kuska, (301) 594-7560
Within weeks of fertilization, one of the great mysteries of life occurs: The
heart, lungs, kidneys, and other organs begin to appear in the fetus. Many start
out as a tube-shaped sheet of cells, which then bud and branch anew hundreds to
millions of times before reaching their final three-dimensional shape. Although
scientists have identified many proteins that are present as organs develop,
they know little about how these molecules interact to catalyze the process,
information that is vital in learning one day to efficiently engineer
replacement organs.
A team of NIDCR scientists recently made a critical discovery that helps to
explain in part how the process works. Reporting in the June 19, 2003 issue of
Nature, the scientists found that cells in certain areas of the bud
secrete the protein fibronectin, which helps to create a deep indentation, or
cleft, in the bud. These clefts serve to subdivide the single bud into several
smaller buds, freeing them to branch in different directions and form
increasingly more intricate, preprogrammed three dimensional patterns.
Intriguingly, the scientists found they could double the rate of branching
when they added fibronectin to organ cultures depleted of the protein. This
finding suggests a specific biological mechanism that bioengineers can exploit
in future studies to spur the natural growth of many developing organs.
Kenneth Yamada, M.D., Ph.D., senior author on the paper, noted that their
discovery occurred first in animal studies of the developing submandibular
salivary gland, located on the underside of the jaw. Yamada said his group
extended their work to the developing lung and kidney, where they found the
protein also was essential for clefting. “This is one more example of the value
of studying the craniofacial region,” said Yamada. “Oral tissues are relatively
easy to access, and so many of the biological principles that we discover there
are applicable in other parts of the body.”
According to Yamada, the Nature paper grew out of preliminary work
in which the group identified several genes that were expressed at higher levels
in the developing cleft, an indication that their protein products might be
important in forming the cleft.
Among the names on the list was fibronectin, a single gene that encodes a
variety of adhesive structural proteins generally involved in holding cells in
position and guiding their migrations. “We’ve studied fibronectin in the past,”
said Yamada, who, in fact, published a number of important early papers on the
gene and its proteins. “So, we just jumped on it.”
First, the group compared expression levels of the fibronectin gene in the
cleft with those elsewhere in the bud. This confirmed their previous finding,
reporting that fibronectin was expressed 16-fold higher in the cleft. They also
discovered thereafter that, in the area immediately adjacent to where the
fibronectin protein localized, cell to cell adhesion was decreased, which one
might expect if a cleft was forming.
“At this point, we hypothesized that, after the transient, local expression
of the protein, cells in the general area stop adhering to each other and begin
attaching to a fibronectin matrix, which serves as the structural framework of
the developing cleft,” said Takayoshi Sakai, D.D.S., lead author on the
study.
Putting their hypothesis to the test, Sakai et al. successfully demonstrated
in follow-up organ culture studies that the development of the submandibular
salivary gland is severely inhibited when fibronectin is not present. They also
showed that, without fibronectin, the budding and branching process is either
nonexistent or substantially decreased.
“In other organs, especially the kidney, it’s been shown previously that the
budding and branching seems to differ in the relative contribution of bud
outgrowth compared with cleft formation,” said Melinda Larsen, Ph.D., an author
on the study. “Nevertheless, in our studies with lung and kidney, we also found
a marked increase in fibronectin within developing cleft regions. Similar to our
work in the salivary gland, we found that blocking fibronectin inhibited
branching.”
In blocking the protein from the organ cultures, however, the group found
they could add purified fibronectin and progressively enhance the rate of
branching. Previous reports in the biomedical literature indicate that two other
molecules - TIMP-1 and epidermal growth factor - also can stimulate branching,
but Sakai et al. found the effects of fibronectin to be “substantially greater.”
In fact, the scientists said they succeeded in doubling the rate of branching.
Yamada said the Nature paper should be of interest to laboratories
involved in tissue engineering, especially those engaged in studies to develop
an artificial salivary gland. “In theory, you could have a bioengineered
polymeric structure serve as the framework for the artificial gland,” he said.
“But you would still have to use biological catalysts to trigger adequate
three-dimensional structure. You would need to insert the tissue and induce it
to fill up the framework with the appropriate amount of branching to generate
enough surface area to produce adequate amounts of saliva. Our paper suggests a
molecular mechanism that will be important in doing that.”