10/25/05 -- UCSF scientists have illuminated a key step in a signaling pathway
that helps orchestrate embryonic development. The finding, they say, could lead to insights into the development of stem cells,
as well as birth defects and cancers, and thus fuel therapeutic strategies.
The 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.
Now, 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.
The 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.
The 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.
Scientists 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.
Finally, 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."
Co-authors 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.
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
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
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.