(339d) The Role of a-Catenin in Cadherin-Mediated Mechanical Signaling in Human Embryonic Stem Cells

Controlling
stem cell renewal and differentiation is crucial to tissue engineering and
regenerative medicine as a whole. Substrate stiffness and cell-cell junctional
forces have been shown to be critical factors that influence proliferation,
survival, and differentiation in human mesenchymal stem cells and other cell
types. However, how mechanical cues regulate proliferation and self-renewal in
human embryonic stem cells (hESCs) and other pluripotent cell types is poorly
understood. Work from our and other laboratories supports an emerging picture
in which the protein αE-catenin functions as a central mechanosensor at
cell-cell junctions. Genetic and developmental evidence likewise suggest that
αE-catenin acts as a potent regulator of the Hippo pathway, a central
regulator of cell proliferation, by preventing nuclear translocation of the
transcriptional coactivator yorkie-associated protein (YAP).

Using
hESCs as a model system, we sought to determine the molecular mechanisms that
connect force-dependent changes in αE-catenin conformation to downstream
signal transduction that controls cell fate in pluripotent stem cells. To do
so, we made use of an R551A
αE-catenin mutant which mimics the force-activated conformation of
αE-catenin present at cell-cell junctions. We found that in epithelial
cells, R551A αE-catenin reduces cell proliferation, sequesters YAP at the
junctions, and disrupts apical-basal polarity by recruiting the tight junction
protein ZO-1 to the lateral cell-cell junctions. In contrast, there are no
observable differences in protein localization or proliferation rates when R551A
αE-catenin was expressed in hESCs. Instead, R551A αE-catenin
preferentially localized to the apical junctions, indicative of its recruitment
to either tight junctions or the polarity complex (Figure 1). Despite
this difference, hESCs cultured on engineered polyacrylamide gels of varying
stiffness still exhibit differential Hippo pathway activation and marked
differences in cytoskeletal structures. These results suggests that, while
hESCs are critically sensitive to mechanical inputs, they do so via a mechanism
distinct from the one previously described in epithelial stem cells¹.
Ongoing work examines the role of αE-catenin and its conformation in
controlling germ layer organization during hESC differentiation, and aims to
discover the identity of the proteins responsible for mechanosensing
specifically in pluripotent stem cells.

¹
Schlegelmilch,
K. et al. (2011) Cell. 144(5):782-95

Figure 1. A Immunofluorescent
staining of R2/7 epithelial cells expressing wild-type and R551A αE-catenin.
B hESCs expressing wild-type and R551A αE-catenin are stained with
YAP, actin, vinculin and ZO-1. Below each image is a y-z projection of a
confocal stack. C hESCs cultured on polyacrylamide gels with varying
stiffness. Cells are stained with YAP, pluripotency marker Nanog and nuclear
dye Hoechst.