The Role of Arrestins in Development - Progress in Molecular Biology and Translational Science - page 224

cells using a chimeric protein consisting of the N-terminal portion of the
b

AR1) and the C terminal domain of Frizzled1
demonstrated that Frizzled proteins signal through G i/o proteins. 47-49 Sim-
ilar to classical GPCRs, they also associate with

1-adrenergic receptor ( b

arrs, 50 which can already be b

observed during embryogenesis.

4.2.1 b arr2 and canonical Wnt signaling
In the absence ofWnt ligands, -catenin is caught in amultimeric destruction

complex. This complex consists of the scaffolding protein axin, APC, and gly-
cogen synthase kinase 3 (GSK3), 51,52 which phosphorylates

b

-catenin and
thereby primes it for proteasomal degradation. 53 However, upon binding
of Wnt to Frizzled and its coreceptor proteins LRP5/6 54 , the cytoplasmic
protein disheveled (Dsh) is recruited to the cell membrane. 55 This in turn
facilitates the disassembly of the destruction complex. b -Catenin is then
stabilized and can translocate to the nucleus. 56-58

b

Several groups have

arr2 can associate with Dsh proteins. 14,50,59 Thus, the notion

reported that b

that arr2 also functions in Wnt/beta-catenin signaling is not surprising.

A classical readout for canonical Wnt signaling is the axis duplication
assay in Xenopus embryos. 60 Injection of Wnt8a mRNA into two ventral
blastomeres at the four-cell stage induces a second organizer and conse-
quently two body axes. Simultaneous knockdown of

b

b

arr2, however, is suf-
ficient to block secondary axis formation. 14 It also inhibits axis duplication
induced by a Dsh construct lacking the DEP domain, which has been shown
to activate -catenin signaling. 61 In addition, Wnt target genes are less tran-

scribed when embryos are depleted for

b

b arr2. Complementary experiments

in arr2 KO mouse embryonic fibroblast (MEFs) further helped to build a

hierarchy of Wnt signaling and

b

arr2. 16 When KO MEFs are stimulated

b

withWnt3a, -catenin accumulation is inhibited and transcriptional activity

as measured using the TOPFlash assay 62

b

arr2 must act
upstream of the destruction complex. At the membrane, Wnt signal trans-
duction is initiated by the phosphorylation of LRP6 at PPPSP motifs by
GSK3 and GRKs 5 and 6. 63,64 Wnt3a-treated

is reduced. Thus, b

b

arr2 KO MEFs displayed
similar levels of PPPSP phosphorylation as wild-type cells, implying b arr2

acts downstream of LRP6. Thus, arr2 is a positive modulator of canonical

Wnt signaling during development ( Fig. 9.2 , left panel).

b

4.2.2 b arr2 and noncanonical Wnt signaling
Interestingly,

-catenin sig-
naling. It is also involved in a noncanonical Wnt pathway ( Fig. 9.2 , right

b arr2 actions are not limited to canonical Wnt/ b