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Karl Willert

Assistant Professor, Cellular And Molecular Medicine, UC San Diego

The main focus of Dr. Karl Willert's lab is to understand how the extracellular environment regulates cell fate choices. His lab utilizes human pluripotent stem cells both embryonic and induced pluripotent stem cells (hESC and iPSC) to explore early developmental fate choices with particular emphasis on the Wnt signaling pathway.

The first Wnt gene was identified by virtue of its over-expression in the mouse mammary gland, thereby contributing to the progression of mammary tumors. Since then, Wnt proteins and their signaling cascades have been studied in a vast number of developmental processes and human cancers. The human genome contains at least 19 distinct Wnt genes whose protein products interact with several cell surface receptors and stimulate a number of signal transduction pathways. A longterm goal of Dr. Willert's research is to understand how these highly conserved signaling molecules elicit such a breadth of biological outcomes.

In 2003, Dr. Willert successfully purified the first Wnt protein to homogeneity, and demonstrated that one particular Wnt protein, Wnt3a, elicits potent effects on self renewal of blood forming stem cells. Since these discoveries, many labs have interrogated the role of Wnt signaling in a vast variety of tissues, including skin, intestine, liver and brain. The isolation of biologically active and pure Wnt protein enabled the analysis of how Wnt proteins interact with the complex extracellular environment. Importantly, Dr. Willert found that mature Wnt proteins are modified by a covalently attached lipid, a modification that regulates Wnt secretion and distribution in the extra-cellular space.

Presently, Dr. Willert has three main research projects: (i) exploring how Wnt signaling regulates pluripotent stem cell behavior, (ii) applying a cellular microarray technology to interrogate the role of the extracellular microenvironment on stem cell pluripotency and differentiation, and (iii) studying the Wnt signal transduction pathway.

  1. Wnt signaling in pluripotent stem cells. Several studies have shown that Wnt signaling is critical to the regulation of embryonic stem cell proliferation and differentiation. In addition, recent studies have also implicated Wnt signaling in reprogramming, or inducing a pluripotent stem (iPS) cell state from differentiated cells, raising the possibility that Wnt proteins can promote de-differentiation. By using Wnt proteins with distinct signaling activities and modulating Wnt receptor expression in pluripotent stem cells and during stages of reprogramming, the Willert lab is establishing methods to manipulate the fate of embryonic stem cells.
  2. Arrayed Cellular Microenvironment Technology. Cells are exposed to a vast number of factors that control their behavior. Current approaches are extremely limited in their ability to systematically interrogate the effect of these factors on cell fate. To address this shortcoming, the Willert lab together with Dr. Shu Chien in Bioengineering has developed a novel cellular microarray technology, which allows for the screening of the effect of thousands of combinations of biological molecules, including extracellular matrix proteins, growth factors and glycans, on any cellular process of interest. The Willert lab has applied this technology to define and optimize a cellular matrix, composed of either biological or synthetic molecules, that supports undifferentiated growth of human embryonic stem cells. In addition, they have used this technology platform to interrogate the effect of the extracellular environment on hepatic stellate cell (HSC) activation. This study demonstrated that Wnt signaling regulates the state of HSC activation.
  3. Wnt signal transduction. Several signaling pathways have been proposed to transduce Wnt signals from the cell surface to the interior of the cell. The canonical Wnt signaling pathway, which involves transcriptional regulation of target genes by the key signal mediator β-catenin, is the most extensively pathway. In contrast, non-canonical Wnt signaling is poorly understood and likely involves signaling cross-talk with various other signaling pathways. Using Wnt proteins that trigger either canonical or non-canonical Wnt signaling, the Willert lab is examining how Wnt proteins and their signaling cascades control cell fate, in particular the choice of embryonic stem cells to proliferate in an undifferentiated state or to differentiate. Shared by the various Wnt signaling pathways is the signaling transducer Dishevelled (Dsh/Dvl), which encodes a cytosolic scaffolding protein. How Dvl proteins mediate the various Wnt signaling inputs remains mysterious. Dr. Willert's lab is examining Dvl proteins using various biochemical strategies in order to shed light on Dvl's mode of action.