New Regulatory Mechanism Discovered For Cell Identity And Behavior In Forming Organs

Reporting their work in the Oct. 15 Genes & Development, the researchers said the mechanism coordinates cell identity and behavior in the forming organs of embryos.

"Our study helps address the current challenge of finding out where cell specificity comes from, how cells do what they do in the context of disease and development, and how these activities are regulated," said Aaron Zorn, Ph.D., a researcher in the division of Developmental Biology at Cincinnati Children's and the study's corresponding author. "This helps inform research into how we tell early stem cells what to become. If someone has diabetes, for example, how do we tell a cell to become a pancreas cell so it will produce insulin?"

The study involved embryos of Xenopus frogs, a species indigenous to Africa often used in early biomedical studies. The scientists discovered a signaling protein very common in developmental biology, Wnt11 (Wingless), has to be inhibited by the modulating protein Sfrp5 (Secreted Frizzled Related Protein), a known antagonist of Wnt. Without this restriction, Wnt signaling runs amok and the frog's foregut, liver and pancreas form improperly from a cascade of disorganized cell growth.

"We point out that Wnt has two key roles here – one controlling the cell expression pathway to tell cells what they are supposed to be, and the other controlling the pathway for cell movement, behavior and adhesion," said Dr. Zorn, also associate professor of pediatrics at the University of Cincinnati (UC) College of Medicine. "Without Sfrp5 controlling what Wnt does in both pathways, things go horribly wrong in the developing foregut and its organs."

The Wnt signaling pathway is a complex network of proteins best known for their role in stimlating cell behavior during embryo development and in cancer. They also are involved in normal physiological processes in adult animals. Parts of the Wnt pathway have been conserved between species during the long course of evolution, all the way from simple roundworms to humans.

Previous research in Xenopus has established that a low level of activity from a molecule called B-catenin – which promotes cell-to-cell adhesion and is part of the Wnt pathway – is necessary to maintain accurate foregut formation and initiate liver and pancreas development. Unknown before the study by Dr. Zorn's team was which Wnt genes are involved and how Wnt and B-catenin activity are regulated along the frog's developing anterior-posterior body axis.

During the very early phases of embryo development – when the organism is still essentially flattened layers of cells called an endoderm – Dr. Zorn's team found Wnt's stimulation of B-catenin must be restricted in the anterior region so the tissue of forming foregut organs maintain its integrity. Their experiments showed that Sfrp5 steps in at the right time and place to repress Wnt signaling, allowing the cells to form an epithelial sheet, or lining – an essential step in organ development.

In one experiment, when researchers removed the Sfrp5 protein, the resulting Sfrp5-depleted Xenopus embryos had smaller foregut cavities filled with unorganized early-stage endoderm cells, which were incapable of properly forming liver and pancreatic organs.

Dr. Zorn and colleagues said their results have possible implications in metastatic cancer. For one, Sfrp proteins are already known to be tumor suppressors that are genetically inactivated in some cancers as they progress to aggressive carcinomas. Carcinomas typically originate in epithelial cells – which form linings surrounding the surfaces and cavities of many body structures – then spread into surrounding organs and tissues.

In cancer development, the research team is suggesting a loss of Sfrp function may unleash Wnt to trigger elevated B-catenin expression, allowing its stimulation of cell-to-cell adhesion to proliferate quickly. Rapid cell proliferation and adhesion are common in cancerous and pre-cancerous conditions. It could also let Wnt send improper signals that cause a loss of structural integrity in epithelial cells, allowing cancer to spread, or metastasize.

"We talked about this mechanism in the context of cancer because the control of cell specificity, and of movement and behavior, also occurs in cancer," Dr. Zorn said. "Cells will start to proliferate out of control and then, when a cancer starts to go metastatic, they will also start to change behavior. They become motile, moving spontaneously and actively, and they become invasive."

The early stage nature of the study means it would be premature to suggest the Wnt11-Sfrp5 mechanism might become the basis of diagnostic or therapeutic strategies for patients, Dr. Zorn said. The next step is to use these results as a basis for future studies, probably involving mice, to verify the mechanism's applicability to mammalian embryo development and see how it affects disease, he said.

Participating in the study were the Cincinnati Children's Research Foundation; Department of Pediatrics, UC College of Medicine; Department of Cell Biology and Anatomy, University of Arizona Health Sciences Center and the State Key Laboratory of Phytochemistry and Plant Resources at the Kunming Institute of Botany, Kunming, China. Other researchers include lead author, Yan Li, and Scott A. Rankin, Debora Sinner, Alan P. Kenney and Paul A. Kreig.

Funding support came from the National Institutes of Health.