Biomedical Engineering Reference
In-Depth Information
Dvl2, and Dvl3 was observed prior to the posterior localization of the basal
body (
Antic et al., 2010; Hashimoto et al., 2010
). Asymmetric localization of
the basal body results in tilted growth of the motile primary cilium, thereby
generating leftward directional fluid flow, which in turn gives rise to the
left/right asymmetry of the body axis (
Hashimoto et al., 2010; Song
et al., 2010
). Epidermal cells forming hair follicles on the mammalian
skin also demonstrate planar-polarized features: Celsr1, Vangl2, and Fz6
were found to localize asymmetrically. Through a mechanism not yet
understood, the action of these asymmetrically localized proteins results
in asymmetry within the developing follicle, producing a planar-polarized
tilt to hair growth (
Devenport & Fuchs, 2008
). In the auditory and
vestibular epithelia in the inner ear of developing chicken and mouse
embryos, asymmetric PCP protein crescents were observed both in the
mechanosensing hair cells and the surrounding supporting cells, which is
thought to be responsible for polarized positioning and orientation of the
kinocilium and stereocilia required for correct mechanotransduction
(
Davies, Formstone, Mason, & Lewis, 2005; Deans et al., 2007;
Montcouquiol et al., 2006; Wang, Guo, & Nathans, 2006
).
Similar protein asymmetry was also evident in several epithelial types with
multiple, planar-polarized, motile cilia. Ependymal cells on the ventricular
lining of the brain, for example, demonstrate asymmetric Vangl2 localization,
which is dependent on the Fmi orthologues Celsr2 and Celsr3 (
Guirao et al.,
2010; Tissir et al., 2010
). Another well-characterized example is the
multiciliated tracheal epithelial cells, where almost all known PCP proteins
show asymmetric localization to opposite apical proximal and distal
crescents, closely mimicking what we have learned from developing fly
wing epithelium (Eszter Vladar et al., unpublished). It should be noted,
however, that numerous other developmental events in vertebrates have
been described to be under control of the PCP genes, or a set thereof, but
for which robust asymmetric localization has not been observed (
Vladar
et al., 2009
). These are typically nonepithelial, and how mechanistically
similar they are to the epithelial PCP described here remains to be determined.
4. THE WAYS ANDMEANS TO PLANAR POLARIZE A CELL:
MECHANISMS OF ACHIEVING ASYMMETRY
The process of segregating the proximal and distal PCP proteins to
opposite regions of the adherens junction creates distinct domains at the cell
cortex. Achieving this
segregation requires an energy investment
to
