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by Tim Bonfield
Long before an embryo develops the bones of its face, the progenitor cells of the facial skeleton must decide where to migrate, when to divide and when to differentiate. How do they know when to take these steps?
“Early on, you have neural crest cells — the progenitors of the facial bones — sandwiched between the developing brain, the neuroectoderm, and the developing skin, the surface ectoderm. Proper development depends on receiving molecular signals from both the neuroectoderm and surface ectoderm, but what happens if the neural crest cells cannot receive or transmit these signals?”
Asking this question is researcher Samantha Brugmann, PhD, an expert in craniofacial developmental at Cincinnati Children’s who works in both the Divisions of Plastic Surgery and Developmental Biology.
Many cells, including neural crest cells, use the primary cilia to receive molecular signals. These tiny, finger-like protrusions play a large role in prenatal development by helping cells sense where they are and identify what surrounds them.
When primary cilia fail to do their job, the resulting disorder is called a ciliopathy. These disorders typically result in defects in the growing brain, kidney, skeleton and the developing face.
“It turns out there are multiple genes involved in the proper formation and function of the cilia,” Brugmann says. “Different genetic mutations can result in different outcomes.”
In facial development, ciliopathies are frequently characterized by cleft lip, cleft palate, hypertelorism (widely set eyes), micrognathia (a very small lower jaw) and other rare conditions.
Brugmann’s team, which includes Ching Feng Chang, PhD, and graduate student Betsy Schock, studies ciliopathies in both chicken and mouse model systems. In the chicken model they have identified a novel avian gene, called C2CD3, that leads to cilia-related malformations. A paper identifying the gene and characterizing the affected molecular pathway is currently in revision for publication in the journal Development.
The findings are significant because Brugmann’s team has classified a long utilized, but poorly understood avian mutant, talpid2, as a ciliopathy. This mutant exhibits features similar to human ciliopathies, such as cleft lip, cleft palate and micrognathia, which makes it especially useful for research.
In January 2014, Brugmann was awarded a five-year, $1.25 million grant from the National Institute of Dental and Craniofacial Research to expand her study to a mouse model. She plans to explore the roles played by a specific ciliary protein, Kif3a, and how that protein functions in interpreting the Sonic Hedgehog (Shh) signaling pathway.
To conduct the study, the team is producing a line of “conditional knock-out” mice, bred to lack the gene that makes the Kif3a protein — but only in the neural crest cells. Mice that lack the gene entirely do not survive long enough to reach birth.
“These mice have a similar phenotype to what we see in the chicken — cleft palate, increased cartilage development, a wide midface and micrognathia with aglossia — the lack of a tongue,” Brugmann says. “We believe the wide midface may be the result of too much Shh signaling, while the tongue and jaw defects may reflect too little Shh signaling.”
When people are born with these sorts of defects, the only treatment has been surgery — often involving multiple invasive procedures. As Brugmann’s work helps explain why these defects occur, it might lead to a preventive treatment. But that goal remains years away.
“A genetic therapy may be possible, but it would need to be performed before birth and that would be a potential ethical challenge,” Brugmann says. “Still, understanding the flow of that molecular pathway could reveal other starting points for possible therapeutic intervention.”
For example, scientists already know that cholesterol interacts with the Sonic Hedgehog pathway, which is one reason why pregnant women are advised to avoid certain cholesterol-lowering drugs. Could a cholesterol-based therapy prevent facial malformations? It’s too early to tell. And it may turn out that other ways to manage the Hedgehog pathway would be more effective.
“First we have to learn the nuts and bolts of this process,” Brugmann says.
Cilia are microscopic, hair-like structures or organelles that extend from the surface of nearly all mammalian cells. There are two types of cilia - motile and non-motile, also known as primary cilia. For years, scientists believed primary cilia were useless vestiges of evolution. More recently, researchers have discovered that primary cilia play crucial roles during embryonic development. They act like tiny antennae that help cells sense their surroundings and pinpoint their location. This helps guide the developing cells to their ultimate fate. When primary cilia do not form properly or malfunction, it can disrupt the formation of bones, the brain, the liver and other organs. In recent years, a number of seemingly unrelated genetic birth defects have been re-classified as ciliopathies.
Samantha Brugmann, PhD
Samantha Brugmann, PhD, studies how defects in primary cilia can lead to facial deformities, using an embryonic avian model.
Dr. Samantha Brugmann reviews some of the malformations produced in embryonic mice.
Avian cells extending primary cilia
In this scanning electron microscope image, colored projections show avian cells extending primary cilia. Normally, 60 to 70 percent of cells are extending cilia at any given moment. However, when a gene mutation affects ciliogenesis, only 19 percent of cells extend cilia. (Images provided by a collaborator at Miami University.)
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