Planar Cell Polarity

Introduction

During embryogenesis, cells in the developing organism migrate in concert and morph to form various structures. Through complex mechanisms, these cells are able to move in the same direction, maintain their relationship to tissues of other lineages and orient correctly. One striking example can be seen in the orientation of hairs on drosophila wings, which orient themselves distally (Fig. 1).

Figure 1. Hairs orient distally (right) on drosophila wings. [1]

One of the most striking examples of this pathway occurs during xenopus gastrulation. In this process, migration of cells to form the structure occurs in concert with anterior-posterior elongation of the structure.

Figure 2. Depiction of cell movements during Xenopus gastrulation. (A) Dorsal view of the direction of migration and extension during gastrula formation. (B) Convergence of the notochord (dark) and somatic (grey) cells at stage 10.5, subsequent continuation of gastrulation, resulting in the tail bud. [2]

The question arises, what signals do cells receive to coordinate these movements? The pathway by which cells receive positional identity is the Wnt planar cell polarity (PCP) pathway. While this signaling pathway was first discovered in drosophila, Xenopus are commonly used to further examine the signaling involved in this process.

Planar Cell Polarity

The process of planar cell activity during Xenopus gastrulation is  depicted.In this process, mesenchymal cells (A) are initially randomly oriented. During gastrulation, signaling stimulates cells to extend along the mediolaterial axis (B). Finally, cells are arranged in a woven fashion, causing the tissue to narrow along the mediolaterial axis, while elongating along the anterior-posterior axis (C). These movements are defined as convergence and extension, respectively. This allows the organism to grow along its axis. It is particularly interesting because of the coordination between cells necessary to achieve such organization.

Mechanisms Controlling Xenopus Gastrulation Orientation

Model System

An important tool for studying gastrulation during Xenopus development is the Keller explant (depicted in figure 3).

Figure 3. Preparation of a Keller explant (A), and specifications for analysis of outcome measures cell behavior (B) and explant elongation (C).

In this method, a piece of embryo from the early gastrulation stage embryos are isolated and maintained in culture. Using this technique, perturbations can be made and the resulting phenotypes recorded. [3]

Experiments

It has been determined that Wnt signaling pathways (canonical/b-catenin, calcium-dependent, planar cell polarity, see wnt.stanford.edu for additional information) are important in embryo development, particularly in axis formation, so the Wnt pathways were candidates for control of polarity. Wnt ligands are classified as canonical or as non-canonical, depending on the signaling pathway they activate. However, several of these ligands can activate more than one Wnt signaling cascade.

Using Keller explants, researchers demonstrated that injection of the non-canonical Wnt ligand Xwnt5a blocked convergent extension,[4] while there was no effect in the embryos injected with control protein preporlactin (Fig. 4). Injection with CamKII , a downstream component of the signaling triggered by Wnt5a, yielded a similar result as was seen in explants injected Xwnt-5A.

Figure 4.  Keller explants of Xenopus embryos injected with control (preprolactin) or experimental (Xwnt-5a, CamKIIT286D) compounds. While the control explants undergo extension along the anterior-posterior axis (indicated by P <->A), those injected with Xwnt-5A or CamKII do not.[4]

Non-canonical Wnts were identified as important molecules controlling polarity during gastrulation. Experiments using dominant negative dishevelled, a downstream molecule in the non-canonical Wnt pathway, further confirmed the importance of this pathway in planar cell polarity (Fig. 5).[5] In this experiment, control cells are aligned mediolaterally (CTL). However, cells expressing a dominant negative form of dishevelled (Xdd1) fail to align properly or to elongate.

Figure 5. While cells from control Keller explants align normally (CTL), this can be disrupted through introduction of dominant negative (Xdd1) or mutant (Xdsh) dishevelled proteins. [5]

Taken together, these studies show the importance of non-canonical Wnt signaling in planar cell polarity. Modulating Wnt ligands or directly activating/inhibiting downstream molecules affects planar cell polarity.

Strengths and Weaknesses

In the papers addressed here, there are many strong points made that clarify the role of Wnt signaling pathways in planar cell polarity. Particularly, the paper indicating the role of dishevelled in Xenopus planar cell polarity was the first linking vertebrate signaling mechanisms to those previously established in drosophila. However, due to the redundancy in Wnt pathway components that can compensate for one another, the exact contribution of each of these signaling components still remains to be determined.[6]

In addition to studies in Xenopus, knockout mouse models are employed to study the role of Wnts in axis specification, although this is complicated by the embroynic lethality of deletion of many of these genes. Therefore, complex knockout mouse models, either with deletions targeted to specific cell/tissue types or combining multiple gene deletions, are being used.[6]

Research focused on Wnt signaling emerged only 30 years ago, so there is still much to be studied. Although studies have begun to determine the role of Wnts in many processes, further study of the complex regulation and signaling cross-talk may shed new light on how these developmental processes are controlled. Furthermore, this may provide additional insight into how tissue formation is regulated during regeneration and healing in the adult organism.

Conclusions

Thus, using the experimental Xenopus model, the importance of the Wnt planar cell polarity pathway in development is established. Planar cell polarity signaling has been implicated in a number of developmental defects including neural tube defects,  polycystic kidneys, pulmonary hypertension, and metastasis during cancer progression.[7] The study of this pathway using Xenopus as an experimental model may help to elucidate these conditions, which occur both during development and in adult organisms.

Resources

Note: All references are provided as hyperlinks to PubMed.

http://www.sigmaaldrich.com/life-science/your-favorite-gene-search/pathway-overviews/wnt-signaling.html

http://www.stanford.edu/group/nusselab/cgi-bin/wnt/

http://www.genome.jp/kegg-bin/show_pathway?org_name=xla&mapno=04310&mapscale=1.0&show_description=show

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