We use genetic and biochemical approaches to study three areas of developmental biology. These are:

Planar cell polarity (PCP) in epithelial cells:

The establishment of cellular polarity is a crucial step in the development of epithelial tissues. Polarization along the apical/basal axis is a universal feature of epithelial cells and is important for specialized epithelial functions such as vectorial transport. In addition to apical/basal polarity, the epithelial cells of many tissues are also polarized along an axis that is orthogonal to the apical/basal axis. This form of epithelial polarity is known as planar cell polarity or tissue polarity. Examples include the sensory cells of the ear and the hairs on the wing cells of the fly. Many studies have suggested that the orientation of planar polarization is directed by a gradient of signaling by the Frizzled transmembrane receptor protein across the tissue. Since Frizzled class proteins are known to be able to act as receptors for Wnt class ligands, this has led to the suggestion that long-range gradients of Wnts provide the positional information directing the direction of PCP. However, no such Wnts have been found despite a great deal of effort. In our recent work on the fly eye, we have uncovered a novel mechanism for directing PCP in a Wnt-independent manner. This mechanism involves the graded expression across the tissue of a cadherin-class transmembrane protein (Dachsous) that regulates Frizzled signaling through the action of another cadherin superfamily member (called Fat). Interestingly, Fat has been previously implicated as an important regulator of epithelial cell growth. We are currently investigating the detailed biochemical mechanisms underlying this novel signaling system and its role in cellular polarity and growth.

The control of cell shape, motility and the actin cytoskeleton by Src family protein tyrosine kinases:

Dynamic regulation of the actin cytoskeleton is a key feature in coordinated cell movement and cell shape changes. Much evidence suggests that protein tyrosine kinases of the SRC family (SFKs) play a key role in regulating actin cytoseletal rearrangements. We study the role of the SFKs in this process by analyzing the role of the SRC64 protein in regulating the actin cytoskeleton and cell movements during Drosophila oogenesis. We have shown that SRC64 regulates the growth dynamics of actin-based structures called ring canals as well as the movement of follicle cells during oogenesis. We are now seeking to identify the precise biochemical pathways by which SRC64 has these effects. Among our discoveries is another tyrosine kinase, TEC29, that is a target for SRC64 regulation. We anticipate that these studies will shed light on the role of SFKs in many actin-based processes in other tissues including cell migration, metastasis in the case of tumor cells, and the formation of axonal growth cones.

The control of cell fate specification by receptor tyrosine kinases:

Another goal of our research is to understand how undifferentiated cells choose specific differentiation pathways. Our approach is to study a particular example of a single cell that chooses between two alternate fates. The cell that we study is the R7 photoreceptor present in each of the ommatidial clusters (facets) in the Drosophila eye. The committment of this cell to neuronal differentiation is dependent on a signal received by the undifferentiated R7 precursor cell from a neighboring photoreceptor. The receptor for this signal is a transmembrane receptor tyrosine kinase (RTK) called Sevenless (SEV). SEV is activated by binding to a protein called BOSS that is present on the surface of the neighboring R8 cell. If either BOSS or SEV function is lacking, then the R7 precursor cell fails to differentiate as a photoreceptor and instead becomes a non-neuronal cell. Our goal is to understand which signal transduction cascades are regulated within the R7 cell after SEV is activated and how this regulation leads to neuronal differentiation. Our approach has been to conduct genetic screens to identify genes that encode proteins essential for SEV signaling. We then study these proteins in order to understand how they function in the signal transduction cascade. Our current interests are focused on three proteins: the protein tyrosine phosphatase CSW, its activator DOS and the novel ankryin repeat and KH-domain containing protein MASK.