Biomedical Engineering Reference
In-Depth Information
were carried out in late embryogenesis, starting from embryonic day 18
when the kinocilium has already migrated from the center to the lateral
edge of the cell, raising the question whether the asymmetric partition of
PCP proteins is the initial event that sets up the polarity and organizes
the epithelium, or simply a molecular readout of polarity signals induced
by as yet unidentified cues.
Celsr1
Crsh
and
Celsr1
Scy
homozygotes as well as compound heterozygotes
Celsr1
Crsh/Scy
exhibit craniorachischisis, a severe neural tube defect due to a
failure to initiate neural tube closure in the cervical region (
Curtin et al.,
2003
). A role for
Celsr1
in neural tube closure was recently confirmed by iden-
tification of six
Celsr1
mutations in human fetuses with craniorachischisis. In
in vitro
assays, those mutations all impair trafficking of Celsr1 protein, reducing
its membrane localization (
Robinson et al., 2011
).
As
Celsr1
Crsh
and
Celsr1
Scy
homozygous mutants are embryonic lethal, a
conditional allele (
Celsr1
f
) was generated to allow tissue-specific inactivation
upon expression of Cre recombinase (
Ravni, Qu, Goffinet, & Tissir, 2009
),
a null allele
Celsr1
ko
was obtained by crosses with germline-expressing Cre
mice, and Western blot with an antibody against the N-terminal region of
Celsr1 confirmed absence of Celsr1 protein in embryonic mouse brain ex-
tracts. Unlike heterozygous
Celsr1
Crsh
and
Celsr1
Scy
, heterozygous
Celsr1
ko/þ
mice have no perceptible phenotype. In contrast, homozygous
Celsr1
ko/ko
mutants display abnormal behavior such as circling and hyperactivity. About
20% of them die at embryonic stages with various degrees of neural tube
closure defects, and many have a looping tail. Some have striking skin hair
patterning defects with whorls and crests instead of regular caudally and dis-
tally pointing hairs on the body and limbs (
Fig. 7.2
)(
Ravni et al., 2009
).
When
Celsr1
is inactivated in crosses with
Emx1-Cre
mice, inactivation of
the gene in the apical ectodermal ridge induces a whorl in distal hindlimb,
showing that the action of Celsr1 is ectodermal cell autonomous. The mech-
anism of Celsr1 action in the hair bulb has been scrutinized by the Fuchs lab
(
Devenport & Fuchs, 2008
). They found that, prior to hair growth, Celsr1
becomes asymmetrically localized along the anterior/posterior (A/P) axis in
basal epidermal cells in hair follicles and in interfollicular epithelium. This
asymmetric localization is essential for A/P orientation of skin hair. Hair fol-
licles fail to adopt the A/P orientation in skin explants isolated from E13.5
embryos, before the polarization of Celsr1. In contrast, when explants are
isolated from E14.5 embryos, when Celsr1 was fully polarized, epidermis
keeps the Celsr1 polarization established
in vivo
, and the A/P polarity. Con-
sistent with this, hair follicles are misaligned in E18.5 embryos with
