The Role of Arrestins in Development - Progress in Molecular Biology and Translational Science - page 226

b

phenotype, implying that arr2 levels must be very delicately regulated or

the pathway is deranged. Closer analysis revealed that

arr2 is localized to
cell membranes of DMZ cells while they undergo CE movements.
In DMZ explants,

b

arr2 was shown to activate RhoA and JNK via interac-
tion with Dsh. This requires

b

arr2 and clathrin-coated pit-mediated inter-
nalization of Frizzled proteins and Dsh. 68 Interestingly, Wnt11 stimulation
of Frizzled7 in the presence of Dsh and

b

arr2 is not sufficient to induce inter-
nalization. However, if Frizzled forms a ternary complex with Wnt11 and
the transmembrane protein Ryk, internalization and signaling are restored. 68
When

b

arr2 is then depleted, Dsh stays at the cell membrane. In addition,
knockdown of Ryk causes CE defects that can be rescued by overexpression
of

b

b

arr2, while CE phenotypes induced by ectopic expression of Ryk can be
reversed by depletion of

arr2. This underscores that certain components of
the Wnt/JNK pathway must be fine-tuned, including

b

arr2.
The Wnt/JNK pathway leads to cytoskeletal rearrangement, and thus
cell movement, by activating RhoA, Rac, and Cdc42. Further experiments
in MEFs revealed that

b

arr2-mediated noncanonical Wnt signal transduc-
tion is also mediated by Rac, but not by Cdc42. Consequently, the

b

arr2
extension phenotype could be rescued by constitutively active RhoA or
Rac1, but not by active Cdc42. Neither was it possible to reverse the effect
by overexpression of canonical Wnt target genes,

b

indicating that the

observed effects of arr2 on CE movements are not due to its role in canon-

ical Wnt signaling, which can counteract the Wnt/JNK pathway.

b

4.3. b arrs in Hh signaling
The Hh signaling pathway is one of the key regulators of embryonic devel-
opment, crucial for many aspects of organogenesis and growth control.
Hh signaling is transduced by the atypical GPCR Smo (reviewed in
Ref. 69 ). Binding of Hh ligand to its transmembrane receptor Patched 1
(Ptch1) relieves Ptch1 inhibition of Smo and allows Smo to signal to a com-
plex of proteins that includes the transcription factors Ci (in Drosophila ) and
Gli (in vertebrates). 70 Data frommammalian cell culture, mice, and zebrafish
have indicated that Smo trafficking into the primary cilium by intraflagellar
transport (IFT) particles is an essential step in Hh downstream signaling
(reviewed in Refs. 71,72 ). Smo translocation results in the accumulation
of transcriptionally active Gli at the tip of the cilium, from which is then
transported out of the cilium to promote Hh target gene expression 71
( Fig. 9.3 ).