Background

The biological system that my group is concentrating on is the so-called planar cell polarity pathway or in short PCP. PCP characterizes a type of epithelial polarity that is perpendicular to the well-known apico-basal polarity in epithelial tissues. While apico-basolateral polarity enables organs/tissues to perform vectorial functions, including transport of fluid or directed secretion of specialized components, PCP organizes the development and function of epithelia in the planar direction. This type of polarity is, however, not restricted to epithelial tissues, but also found in mesenchymal cell types throughout animal development. How individual cells hundreds of cell diameters apart acquire the same orientation within an organ (or the plane of an epithelial field) or how mesenchymal cells generate a uniform polarization leading to ordered multicellular intercalation are fascinating biological problems. In addition, recent discoveries suggest that PCP plays a critical role in many diseases, and we are focusing on the linking of genetic syndromes associated with ciliary functions (such as polycystic kidney disease) to PCP.

In each tissue PCP generation can be subdivided into three steps: (1) definition of the source of a polarizing signal; (2) reception and interpretation of the signal in single cells or groups of cells; and (3) organization of the cells in response to the signal.

Although virtually nothing is known about the first step, a model of the Fz/PCP pathway and its regulation by some of the other PCP genes has emerged. This novel signaling pathway has been identified mainly through a genetic dissection in Drosophila. Based on the initial observation that the direction of PCP signaling depends on a Frizzled (Fz) gradient, the model suggests that Fz levels are sensed by Strabismus (Stbm or Vang). This leads to the formation of a complex of Stbm and Prickle at the cell surface. Together, they act negatively on Dishevelled (Dsh) and Frizzled (Fz) possibly by interfering with the recruitment of Dsh to the membrane. The mutually exclusive interactions lead to an asymmetric distribution of the PCP core factors at the plasma membrane (see Figure 1).

Using a genome-wide RNAi screen we could recently uncover a surprising connection between local pH and charge conditions and the subcellular localization of PCP core components. We have since then focused on the role of electrochemical cues in the assembly of PCP signaling complexes at the plasma membrane. Bioelectrical signals are mediated by electrical properties of cells and are generated by specific ion channels and pumps within cell membranes. The segregation of charges achieved by ion fluxes through such transporter proteins gives rise to transmembrane voltage potentials. In addition, the polar distribution of ion channels and pumps in some cell types gives rise to electrochemical potentials on the tissue level. The resulting electrical fields carry information to the source cell as well as to its neighbors. Thus, all cells generate and receive bioelectrical signals. Despite much fascinating data on the role of endogenous bioelectric signals controlling, e.g. cell and embryonic polarity and cell division, the field as a whole is largely unknown to modern cell and developmental biologists. This is particularly surprising as these findings have already lead to biomedical applications, for example in wound and fracture healing. In order to understand the biological processes that generate endogenous electrical fields and the cellular factors that respond to them, more knowledge is clearly needed.

Projects

We recently found that a PCP core protein, Dishevelled (Dsh), requires the activity of a sodium-proton exchanger (Nhe2) at the plasma membrane in order to assume its subcellular distribution (Simons et al, Nature Cell Biology, 2009). This distribution is facilitated by the binding of Dsh to negatively charged lipids in an electrostatic manner. In addition, Dsh binds directly to its activating receptor, the G-protein coupled receptor (GPCR)-like Frizzled. Apart from Nhe2, we have also found other ion transporters such as the proton-pumping V-ATPase to be involved in in-vivo PCP signaling in Drosophila suggesting that ion fluxes, particularly those that involve protons, play a fundamental role in setting up PCP.

We now aim to understand this relationship between ion transport and PCP at the systems level. For this, we want to study cell behavior in an electrical field (EF). It has been known for quite some time that several in-vitro cell lines display oriented cell migration and cell division in EFs. Based on our previous findings, we hypothesize that PCP core proteins are responsive to electrical signals. We therefore want to study the role of PCP core proteins in EF-induced cell migration using RNAi and GFP-based live imaging techniques. We are also using Drosophila eye and wing development as in-vivo experimental models. Our systems biology approach includes a mathematical description of cell behaviors in an electrical field as well as multi-parametric analysis of the temporal and spatial distribution of PCP proteins during EF-induced polarization.

Figure 1:

(A) Dsh is polarized to the distal side of the developing wing epidermal cell (taken from Axelrod, G&D 2001)

(B) Schematic of PCP asymmetry in the wing cell (Dsh is light green, Fz is dark green)

(C) Analogy to the polarized migratory cell. Dsh-GFP (green) localizes to the trailing edge of wound edge fibroblasts.