Biomedical Engineering Reference
In-Depth Information
which influence fundamental cell processes including morphogenesis, polar-
ity, movement, and cell division (
Jaffe & Hall, 2005
). The specific roles of
these downstream PCP targets have not yet been extensively characterized
in the mammalian inner ear, but a few studies into the role of Rac have
begun to yield some insights.
Rac1 was previously shown to be a downstream effector of PCP signaling
during convergence extension movements in mammalian cells (
Habas,
Dawid, & He, 2003
). Based on these results, inner ear-specific
Rac1
condi-
tional mutants were generated (
Grimsley-Myers, Sipe, Geleoc, & Lu,
2009
). Resulting cochleae were severely shortened with disrupted cellular
patterning and misoriented stereociliary bundles in all hair cell rows. Initial
establishment of PCP appeared normal, as indicated by the asymmetric
migration of the kinocilium. Yet subsequent asymmetricmembrane localiza-
tion of the core PCP protein Fz3 was disrupted, implicatingRac1 in themain-
tenance but not initial establishment of PCP in the mammalian cochlea
(
Grimsley-Myers et al., 2009
). Rac1was also shown to be crucial for continued
stereociliary bundle and accompanying kinociliummorphogenesis. Functional
redundancy among the GTPases has been suggested, as not all hair cells in
Rac1
mutants are disrupted. Homozygous
Rac3
knockouts showed no morpholog-
ical cochlear defects, yet double homozygous conditional knockout mutants
for
Rac3
and
Rac1
displayed more severe cochlea defects than
Rac1
mutants
alone (
Grimsley-Myers et al., 2009
). Along with
Rac1
and
Rac3
,
Cdc42
RNA expression has also been detected in the developing cochlea
(
Grimsley-Myers et al., 2009
), although a
Cdc42
cochlea mutant has yet to
be reported.
The p21-activated kinases (PAKs) are downstream effectors of Rac and
are known to regulate cytoskeletal dynamics (
Bokoch, 2003
). Immunohis-
tochemistry using an antibody against activated phospho-PAK in develop-
ing cochlea tissue showed asymmetric lateral membrane localization in inner
and outer hair cells in a developmental gradient concomitant with the wave
of hair cell differentiation (
Grimsley-Myers et al., 2009
). In
Vangl2
Lp
mu-
tants, pPAK was present but mislocalized in a pattern that strongly correlated
with hair bundle misorientation. Additionally,
in vitro
cultures treated with
IPA-3, a specific chemical inhibitor of PAK, displayed bundle defects that
were similar to those observed in
Rac1
mutants (
Grimsley-Myers et al.,
2009
). Combined, these data suggest that PAK acts downstream of Rac1
to regulate bundle orientation and morphogenesis.
Finally, c-Jun N-terminal kinases (JNKs) have also been shown to act as
cytoplasmic mediators of PCP signaling. In particular, JNKs have been
