UCSF research reveals how the embryo brings its heart together

Early in the life of every vertebrate embryo, be it human or hamster, there is
a moment when the heart comes together—literally. Scientists at the
University of California, San Francisco have discovered a molecule that directs
the two halves of the primordial heart to join as one.

Under the molecule’s influence, separate tubes of the would-be heart—
primordial heart buds, essentially —migrate toward each other from opposite
sides of the embryo, the researchers found. As the two halves join, the
rudimentary heart begins to beat.

The discovery is based on studies of zebrafish, tiny creatures which are
utterly transparent as embryos. Many pivotal early changes that take place in
the human embryo can be witnessed in zebrafish as if they were unfolding in a
test tube. The fish is also fairly easy to study genetically.

The research, published in the July 13 issue of the journal Nature, shows that
the normal union of the two heart tubes in the zebrafish embryo requires the
presence of S1P, a molecule already known to be involved with cell
proliferation and survival.  S1P, an abbreviation for sphingosine 1-phosphate,
is also active during wound healing, and the researchers suspect that its
capacity to draw cells together is crucial there as well.

The heart buds join to form the zebrafish heart 22 hours into the embryo’s
life, and the heart begins beating soon thereafter. In the mouse embryo, the
crucial union that will produce the heart occurs on the eighth day of
development, and in human embryos, the heart forms and begins beating at three
weeks, says Didier Stainier, PhD, UCSF associate professor of biochemistry and
biophysics and senior author of the Nature study.

“S1P is a very old molecule,” Stainier says. “It is present in
organisms as distant as yeast and humans, and it appears to have been used over
and over again throughout evolution for different roles: for cell
proliferation, for wound healing, and now as we have found, for bringing the
two parts of the primordial heart together.”

If it serves this role for the zebrafish heart, Stainier suggests, it probably
performs the same function in the human embryo.

“And as we start to understand the molecular pathways involving S1P by studying
heart formation in zebrafish, we should learn something about how it regulates
other processes such as wound healing,” he says.

In 1996, Stainier and colleagues published the identity of 58 different
mutations affecting zebrafish heart development. These they had selected from a
much
larger number of mutations produced by exposing the zebrafish genome to a
chemical agent. They zeroed in on the most interesting genes by examining the
developmental problems
their mutations caused. One mutation that prevented the primordial heart halves
to join was named miles apart by a post-doctoral researcher pining for his
lover across the sea. In the embryos affected by the miles apart mutation, the
researchers found, the two primordial heart tubes failed to converge to the
midline.

In the current research reported in Nature, Stainier’s lab isolated the miles
apart gene and went on to show that the gene codes for a receptor, or docking
site, for S1P. They determined that the receptors are concentrated in the
midline region toward which the two heart buds migrate. Presumably, when S1P
molecules dock with their miles apart receptors, they create a field that
attracts or allows the primordial heart buds to migrate to the midline. There
they find each other and fuse.

The discovery that S1P is essential for the normal union of the zebrafish heart
is surprising, Stainier says, because most molecules that guide developing
cells to their destiny—all discovered in the last five to ten years—are
proteins, the conventional cellular workhorses produced directly from genetic
instructions. But S1P is a lipid—an oily substance produced not from a
genetic code but as a product of chemical reactions triggered by enzymes.

This is the first lipid molecule shown to be crucial in vertebrate development
and greatly expands the roster of molecular candidates scientists must consider
in efforts to tease apart the puzzles of how vertebrates develop.

“Proteins can be identified from newly sequenced genes, and they can be
localized using antibodies. But lipids are the products of enzymatic pathways.
No gene codes for them, and they are much harder to study.”

Stainier has been studying zebrafish heart development for ten years. The fish
undergo most of the early life-defining changes found in human embyros, yet
their embryos are transparent and readily accessible. Because it is also
amenable to genetic study, the zebrafish has become an important model organism
for studying vertebrate development.

The blue-and-silver-striped fish, a native of the Ganges River, is less than an
inch long when mature.  At the critical time when S1P and its miles apart
receptor join forces to urge the animal’s heart to come together, the
transparent embryo is no larger than the head of a pin. Still, the researchers
can scrutinize the tiny embryos, focusing on the maladies created by different
mutations.

“This study underscores the value of studying from mutations to genes, rather
than the other way around, to understand the genetics of vertebrate
development,” Stainier says. “The vertebrate genome is full of nuggets, and
this approach of random mutagenesis in zebrafish is the best way to get at
those nuggets.” Random mutagenesis is the practice of producing mutations
throughout the genome and then studying the genes that underlie mutations of
particular interest.

The July 13 issue of Nature containing the report on heart formation includes
a perspective on the research by Wolfgang Driever, a prominent developmental
biologist in Germany.

First author on the paper with Stainier is Erik Kupperman, PhD, post-doctoral
researcher in Stainier’s lab. Co-authors on the paper and collaborators on the
research are Songzhu An, MD, PhD, UCSF assistant professor of medicine; Nick
Osborne, a graduate student and Steven Waldron, a technician, both in Stainier’
s lab. All co-authors in Stainier’s lab are in the UCSF biochemistry and
biophysics department and UCSF programs in developmental biology, genetics and
human genetics.

The research is funded by the American Heart Association, the Packard
Foundation and the UCSF Program in Human Genetics.