study, reported in Nature (Oct. 13, 2005), focuses on the Hedgehog family of signaling molecules, which play a central role
in directing development of the early embryo's growth and spatial plan, as well as its later organ and limb development. Defects
in Hedgehog signaling are a significant cause of some birth defects and cancers.
Secreted from one cell, a Hedgehog signal shoots to the surface
receptor of a second cell, and then, in a rapid-fire succession of biochemical reactions, relays a message into the cell's
nucleus. There, it issues an instruction, prompting the cell to divide, or specialize into a particular cell type, or migrate
to help form another part of the embryo, and so on. This transaction, known as signal transduction, is a ceaseless activity
of embryonic development. Scientists have long known that Hedgehog signaling requires the activity of a protein
known as Smoothened. This has been
demonstrated in animals ranging from insects to humans. They have also known
that defects in Smoothened, which only functions within Hedgehog signaling, are responsible for some cases of human cancers
- most prominently a skin cancer known as basal cell carcinoma and a childhood brain cancer known as medulloblastoma -- as
well as some birth defects. However, they have not known how Smoothened executes
its function, nor where it is located in the target cell.
through a series of studies conducted in several types of cells in culture, and in zebrafish and mouse embryos, the UCSF scientists
have answered both questions. In the process, they have revealed the critical role of a cellular component that until now
has been a mystery: an antenna-like structure attached to cells known as the primary cilium.
The primary cilium, it turns out, serves as the fulcrum in a series of acrobatic like moves between the Hedgehog signal
and the Smoothened protein. Once Hedgehog has latched on to its receptor on the target cell's surface, it prompts the cell
to move Smoothened, located in vesicles around the cell's nucleus, to the primary cilium. The positioning of Smoothened on
the cilium, in turn, prompts downstream signaling of Hedgehog signals into the nucleus, where the instructions are issued. Just how or what the primary cilium is doing to promote Smoothened's activity
is not clear, say the researchers. However, its involvement in the process is a revelation.
Scientists elsewhere reported in Nature in (Nov. 6, 2003) that removal of the primary cilium from cells led to defects in neural patterning resulting
from Hedgehog signaling. However, they didn't know why. "This study takes two
mysteries - how Smoothened functions and the role of the primary cilium - and suggests a mechanism by which they are connected,"
says the senior author of the study, Jeremy Reiter, MD, PhD, a fellow in the UCSF Program in Developmental and Stem Cell Biology,
which is part of the UCSF Institute for Stem Cell and Tissue Biology.
implications for medical research, he says, are significant. Hedgehog signals
play an important role in prompting embryonic and adult stem cells to differentiate into some of the specialized cells that
make up the body's tissues -- such as those of the brain, pancreas and skin. The new finding, says Reiter, will advance scientists efforts
to use signaling molecules to direct the differentiation of embryonic stem cells in the culture dish, with the goal of using
them to replace or replenish damaged tissues in patients. The discovery could be particularly important for neural stem cell
research, says Arnold Kriegstein, MD, PhD, director of the UCSF Institute for Stem Cell and Tissue Biology. Kriegstein, a
neural stem cell scientist, was not an author on the study. "Hedgehog signaling
plays a critical role in prompting the differentiation of neural stem cells into the various forms of neurons in the brain,"
he says. "The discovery of the importance of the cilium in Hedgehog signaling should significantly advance our understanding
of the mechanisms involved," he says.
finding should fuel research into the causes of certain birth defects (such as holoprosencephaly and limb defects) and cancers,
says Reiter. Smoothened is already known to be a proto-oncogene, a normal gene that, if mutated, is capable of causing cancers.
But its close involvement with the primary cilium suggests that the latter may also be implicated, suggesting a possible target
for therapy. More broadly, says Reiter, the primary cilium's role in Hedgehog signaling indicates it is likely to function
in other signaling pathways, as well. The scientists moved in on the role of
Smoothened and the primary cilium incrementally. First, driven by their interest in Smoothened, they set out to determine
where it was expressed in the embryo. They did so by developing highly specific antibodies to the protein and applying them
to the tissue of an eight-day mouse embryo. The study revealed that Smoothened was modestly upregulated in cells of the node,
an important early organizer tissue within the mouse embryo, and was expressed predominantly along the primary cilium of these
nodal cells. This was a significant surprise. Second, to examine whether
Smoothened's movement from vesicles around the nucleus to the cilium was regulated by Hedgehog signals, they carried out two
studies, one involving cultured epithelial and fibroblasts cells expressing Smoothened, another involving a mouse embryo.
In both cases, one set of cells was exposed to Hedgehog signals. Another set was exposed to cyclopamine, a drug that blocks
Smoothened's function. In the cells exposed to the Hedgehog signals, Smoothened moved from the vesicles of the cell body to
the cilium. In the cells exposed to cyclopamine, Smoothened was undetectable on the cilium.
have known that cyclopamine inhibits Hedgehog signaling and can prevent Hedgehog-dependent cancers from spreading. The demonstration
that the drug affected Smoothened movement to the cilium suggests how cyclopamine inhibits the Hedgehog pathway, the researchers
say, and shows that the correlation between Smoothened on the cilium and pathway activation is very tight. Third, they examined whether the Smoothened protein included an amino acid sequence that other seven-transmembrane
proteins require to move to the primary cilium and, if so, whether this sequence - a so-called "motif" - was essential to
its relocation there. The answer to both questions was yes: A study of mouse cells in which Smoothened was mutated to lack
the motif revealed that Smoothened no longer moved to the primary cilium.
to determine Smoothened's function, they tested the mutant form of Smoothened that no longer could move to the primary cilium
in epithelial cells in culture and in zebrafish embryos to see if the protein still functioned. It did not. "Thus, not only does Smoothened ciliary localization depend up on Hedgehog signaling, but Hedgehog
signaling depends on a Smoothened ciliary localization motif," says Reiter. "Whether
Smoothened functions at the cilium in all cell types remains to be determined. In addition, how Smoothened activates the Hedgehog
pathway at the cilium remains unclear," he says. "But the current finding lays the groundwork for future studies that could
ultimately have clinical benefit."
of the study were Kevin C. Corbit, Pia Aanstad, Veena Singla, Andrew R. Norman and Didier Y.R. Stainier, PhD. All are members
of the UCSF Program in Developmental and Stem Cell Biology and the UCSF Diabetes Center. Aanstad and Stainier are also members of the UCSF Department of Biochemistry
and Biophysics. All are also members of the UCSF Institute for Stem Cell and Tissue Biology.
Source: University of California - San Francisco
More Hedgehog Research at Bio.com
10/25/05 -- Curis, Inc. (NASDAQ: CRIS), a therapeutic drug development
company, today announced the publication of data showing that stimulating the Hedgehog signaling pathway was therapeutically
efficacious in preclinical models of both acute and chronic ischemic heart disease. Myocardial ischemia, the interruption
of blood flow and oxygen to heart muscle, is the leading cause of heart attacks. In the U.S., approximately 1.1
million individuals experience new or recurrent myocardial infarctions each year and, of these, about 40% eventually develop
congestive heart failure, a form of chronic heart disease. The research for these studies was performed in the laboratory
of Dr. Douglas Losordo in the Division of Cardiovascular Research at the St. Elizabeth's Medical Center of Boston, Massachusetts. Bio.com
The Hedgehog pathway
governs major aspects of heart and blood vessel development in the fetus and remains active in the adult for tissue maintenance
and repair. When blood flow to the heart is blocked experimentally, which simulates an acute heart attack, the pathway is
upregulated. In the reported animal studies, therapeutic activation of the Hedgehog pathway near the zone of ischemic damage
in the heart appeared to protect heart cells from death, preserve heart muscle function, improve blood flow, and attract bone-marrow
derived progenitor cells from the blood that help build new blood vessels. Activating the Hedgehog pathway appears to result
in the coordinated release of various factors that promote these diverse protective and regenerative effects, which potentially
could result in more robust and durable efficacy in the clinic than treatment with single factors alone.
"This study adds
to the growing body of preclinical data supporting the use of Hedgehog agonists to promote tissue repair post ischemia. Heart
disease is one of the leading killers of Americans and although early, this data is encouraging that our novel approach could
show significant promise. In keeping with our collaboration strategy, which allows for the development of a broad portfolio
of promising assets providing significant value potential, we would plan to partner this program for human studies," said
Daniel R. Passeri, President and Chief Executive Officer of Curis, Inc.