Almost all cell types exhibit some form of polarity; cell polarity refers to the spatial differences in shape, structure and function within a cell, which enables the cell to carry out specialized roles as part of a tissue. Epithelial cells exhibit two types of cellular polarity: apicobasal polarity, which defines the axis between the apical and basal membranes, as well as planar polarity, which refers to the coordinated alignment of cell polarity across the tissue plane. The collective alignment of cell polarity across the tissue plane is a phenomenon known as planar cell polarity (PCP). Exemplified by the uniform orientation of bristles covering the insect epidermis or of the hairs covering the mammalian body surface, PCP patterns can align over thousands, even billions of cells. This phenomenon is controlled by the so-called PCP pathway, which integrates global directional cues to produce locally polarized cell behaviors. There has been a recent surge in interest in PCP after discoveries that various processes such as vertebrate gastrulation, mammalian ear patterning and hearing, and neural tube closure all require a conserved set of PCP genes.
The role of microtubules in the establishment of planar polarity has been controversial. Dr. Cecilia Moens and her laboratory in the Basic Sciences Division utilize the zebrafish as an experimental model to study the development of the vertebrate brain. In particular, the zebrafish floorplate is an ideal model to study complex interactions between PCP proteins, the microtubule cytoskeleton and maintenance of the polarized cellular architecture because the in vivo dynamics of PCP and microtubule fluorescent fusion proteins after the establishment of cellular PCP can be visualized in zebrafish embryos (they are small/microscopic and translucent). Dr. Moens explains the impact of their work published in a recent issue of Developmental Biology: “There is a lot of debate about how planar polarity is established in epithelia and once established, how it is maintained. A lot of models have been proposed, and most agree that polarized microtubules are essential for establishing planar polarity in the first place, but that microtubules are dispensable after that. Our study shows that microtubules are essential for maintaining planar polarity after all.”
Using the zebrafish, the authors showed the progressive establishment and maintenance of planar cell polarity in the zebrafish floorplate. To do this, they detected the posterior localization of the primary cilia, which are cell surface, microtubule-based organelles that dynamically extend from cells to receive and process molecular and mechanical signaling cues, and the asymmetric localization of PCP proteins Vangl2 and Fzd3a. GFP-Vangl2 protein localized asymmetrically to anterior membranes, whereas Fzd3a-GFP was detected primarily within dynamic vesicles that traffic along microtubules toward the base of the primary cilium. Interestingly, contrary to previous studies, the authors report that microtubules were required to maintain polarity in the floorplate. Using nocodazole, a synthetic tubulin-binding agent that disrupts microtubule polymerization, they showed that nocodazole-induced loss of microtubule polymerization disrupted basal body positioning and GFP-Vangl2 localization, and also reduced Fzd3a-GFP movements within the cytosol.
This study presented some obstacles. “We had to figure out how to disrupt microtubules and then recover them, while live imaging markers of planar polarity. It turned out that the treatments that we thought should disrupt the microtubules only partially disrupted them, so the results were inconclusive. Fortunately we had a visiting scientist in the lab at the time who had just reviewed a paper describing a more effective way to disrupt microtubules in zebrafish, so we were able to jump on that method” said Dr. Moens.
As for what comes next for the Moens laboratory, Dr. Moens revealed that they are moving on from planar polarity itself to the function of planar polarity on the polarized behaviors of neurons in the central nervous system: how they migrate and where they project their axons. “Many neurons in the brain and spinal cord project their axons across the floorplate and then right after crossing the axons turn left or right (anterior or posterior) and this depends on the planar polarity pathway. We would like to investigate this process using live imaging in the zebrafish: where are the components of the pathway needed for directional axon turning: In the floorplate? In the neuron? in both? And if we can resolve it optically we’d like to get at how the planar polarity pathway influences the growth cone to make it turn one way or the other.”
Mathewson AW, Berman DG, Moens CB. 2019. Microtubules are required for the maintenance of planar cell polarity in monociliated floorplate cells. Dev Biol, Apr 25. [Epub ahead of print]
This work was supported by the National Institutes of Health.