Environmental Factor, June 2010, National Institute of Environmental Health Sciences
Fellows Host Wnt Signaling Expert
By Sophie Bolick
Roel Nusse, Ph.D., a Stanford University professor of Developmental Biology and Howard Hughes Medical Institute (HHMI) investigator, recently visited NIEHS as a guest of the Laboratory of Molecular Carcinogenesis (LMC) postdoctoral fellows. Speaking to a packed room on May 13, Nusse addressed "Wnt Signaling During Stem Cell Renewal and Tissue Repair," focusing on several aspects of his extensive research on the protein's influence on health and metabolism.
Hosted by Gilberto Dos Santos, a visiting fellow with the Transcriptional Responses to the Environment Group, Nusse was the most recent distinguished scientist invited to NIEHS by the fellows, presenting his research as part of the LMC seminar series.
Early in development, cells in a tissue respond to a given signal. As a tissue grows and becomes more complex, cells respond to the same signal in a different manner. Nusse likened this response to what is occurring with stem cells and explained, "Control over what stem cells are doing occurs locally. When you remove the niche, or signal, stem cells differentiate and lose their self-renewal state."
Similar mechanisms are involved in tissue homeostasis following injury and adverse environmental exposure. Stem cells sense the loss of cells and become activated after damage, filling in the remaining tissue.
Nusse has spent many years studying the Wnt signaling pathway, which is involved in these processes. "When you follow Wnt signaling in vivo, there is a lot of fantastic stuff going on," he said. Using a LacZ reporter mouse, he has studied active Wnt in stem cells in intestinal crypts, mammary glands, and lung, determining that stem cells in vivo are under control of Wnt signaling.
Wnt sufficient for stem cell expansion in culture
Nusse (http://www.hhmi.org/research/investigators/nusse_bio.html) said he wanted to answer an important question about stem cell expansion. "Now that we have purified Wnt protein, can we manipulate stem cells in culture using pure Wnt protein?" Addition of purified Wnt to embryonic stem (ES) cells in culture is sufficient for ES cell maintenance and expansion. Combining Wnt and the growth factor leukemia inhibitory factor (LIF) allows for several passages of the ES cells. When implanted in a mouse blastocyst, he explained, "The cultured ES cells are pluripotent and are able to repopulate the germ line of the mouse and generate whole animals."
The mammary gland contains a population of self-renewing mammary stem cells (MSC), which can also be expanded in culture. In the presence of Wnt, there is a dramatic increase in the number of colonies that grow. Implantation of cultured MSCs in mice produces mammary glands capable of producing milk in lactating animals, Nusse said, "So these are bona fide stem cells." The ability of single colonies to reconstitute into single organs is completely dependent on the Wnt program. "These studies may have value for organ regeneration," Nusse explained.
Wnt signaling is important in injury response
Nusse's group also investigates the role of Wnt proteins in injury and repair of lung cells in mice. "The lung is a great tissue for studying injury and subsequent repair." When mice are treated with naphthalene for 24 hours, Clara cells in the bronchioles are ablated. However, Clara cells regenerate and, seven days post injury, various Wnt family members are expressed. Normal ciliated cells adjacent to the ablated cells rapidly respond to the injury.
(Sophie Bolick, Ph.D., is a postdoctoral fellow with the Molecular and Genetic Epidemiology Group in the Laboratory of Molecular Carcinogenesis.)
Active Wnt Proteins Are Modified by Lipids
Nusse has worked on Wnt biology since he discovered the Wnt1 gene while working as a postdoctoral fellow in the lab of Harold Varmus, M.D., at the University of California, San Francisco. His career began in Amsterdam, the Netherlands, and has continued in both the United States and the Netherlands. Nusse is currently chair of the Department of Developmental Biology at Stanford University.
Nusse and his research group were the first to develop the first pure and active Wnt, Wnt3A. Purification of Wnts has led to the discovery that they are modified by the lipids palmitate and palmitoleate. Lipid modification makes Wnts hydrophobic and difficult to purify for functional studies. Since successfully establishing the purification protocol, Nusse's lab has purified additional members of the Wnt protein family. With purified Wnt, functional studies could be done.
Lipid modification plays an important role in Wnt signaling. Using the fly as a model system, Nusse's group has identified Secreted Wingless (Wg) Interacting Molecule (SWIM), which controls the effective range of Wnt signaling. Wg is the Drosophila homologue of Wnt, with both short- and long-range signaling functions. Mass spectrometry analysis demonstrates binding of SWIM to Wg by palmitate. Nusse points out "that the complex of SWIM and Wg is soluble in the absence of a detergent." This finding, in combination with SWIM RNAi experiments showing that long-range target Distalles is perturbed, indicates long-range activity of Wg requires a soluble complex.