#β-catenin

Paracrine Factor

What happens to β-catenin in the absence of Wnt signaling?

Without Wnt ligand, destruction complex remains constitutively active repressing signaling and preventing ectopic axes. Axin polymerizes with APC scaffolding GSK3beta and CK1alpha kinases, sequentially phosphorylating beta-catenin N-terminal Ser-Thr cluster Ser45,33,37,41. Phosphorylated beta-catenin binds beta-TrCP adaptor of SCF E3 ubiquitin ligase, polyubiquitinated, degraded in 26S proteasome, keeping cytoplasmic and nuclear pool low preventing TCF/LEF activation and target transcription. Upon Wnt binding Frizzled-Dishevelled recruits Axin to membrane via phosphorylated LRP5/6 intracellula

Ref: Clevers & Nusse, Cell 2012: Wnt absence leads to beta-catenin phosphorylation and proteasomal degradation.

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Paracrine Factor

Which enzyme phosphorylates β-catenin, marking it for degradation in the absence of Wnt signaling?

Beta-catenin degradation circuit includes scaffold proteins regulating turnover tightly and rapidly. In absence of Wnt, cytosolic destruction complex assembles Axin dimer, APC tumor suppressor, CK1alpha, GSK3beta. CK1alpha primes Ser45 phosphorylation, then Glycogen synthase kinase 3 phosphorylates Ser33, Ser37, Thr41 creating phosphodegron recognized by beta-TrCP SCF E3 ubiquitin ligase leading to polyubiquitination and proteasomal degradation in proteasome. Protein kinase A phosphorylates other sites increasing activity, JAK and MAP kinase not primary degradative kinases. This constitutive t

Ref: Stamos & Weis, Annual Review Biochemistry 2013: Destruction complex phosphorylates beta-catenin via GSK3 and CK1.

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Paracrine Factor

Which of the following Wnt signaling pathways is β-catenin dependent?

Canonical Wnt pathway depends strictly on beta-catenin as transcriptional coactivator linking membrane to nucleus. Wnt binding to Frizzled cysteine-rich domain and LRP5/6 co-receptor recruits Dishevelled scaffold polymerizing via DIX domain, disassembling destruction complex composed of Axin, APC, GSK3, CK1alpha. Stabilized beta-catenin accumulates cytosol, enters nucleus via importin, binds TCF/LEF replacing Groucho corepressor recruiting Legless to activate targets like c-Myc, Cyclin D1, Axin2. Non-canonical pathways include Planar Cell Polarity via Rho-JNK orienting hairs and convergent ext

Ref: Logan & Nusse, Annual Review Cell Dev Biol 2004: Canonical Wnt pathway stabilizes beta-catenin via Frizzled and LRP.

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Regeneration -l

What is the effect of inhibiting β-catenin in regenerating planaria?

Beta-catenin protein level translates Wnt signal strength into transcriptional fate choices. Inhibition via double-stranded RNA or small molecule tankyrase inhibition prevents nuclear accumulation and removes activation of posterior target genes. Posterior blastema deprived of Wnt output defaults to anterior genetic program expressing sfrp-1, notum, and prep forming ectopic head complete with brain and eyes. Anterior wound also forms head leading to animal with two heads, duplicated central nervous systems pointing outward. This multiple-head phenotype demonstrates binary switch where beta-cat

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: β-catenin inhibition leads to two-headed planaria via anteriorization.

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Regeneration -l

What happens when β-catenin is knocked down in planaria?

Beta-catenin acts as transcriptional co-activator dictating posterior identity where high levels specify tail. Knockdown via RNA interference abolishes Wnt target activation, converting posterior wound fate to anterior, while pre-existing tails still present may begin anterior transformation. Because anterior identity requires active Wnt inhibition via Notum, loss of beta-catenin does not prevent head formation; instead tail regeneration program is selectively disrupted leading to formation of ectopic head or failure of posterior blastema patterning. Thus only tail regeneration appears affecte

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: β-catenin RNAi and posterior defects in planaria.

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Regeneration -lll

Overexpression of β-catenin in Hydra will cause:

Hydra homeostasis and budding regulated by canonical Wnt-beta-catenin pathway concentrated at oral organizer. Overexpression of stable beta-catenin via transgenesis or inhibition of GSK3beta disrupts degradation complex, increasing nuclear beta-catenin throughout body column. This lowers threshold for organizer formation leading to ectopic activation of Wnt3, formation of multiple organizing centers generating supernumerary buds along body column with ectopic tentacles and heads. Enhanced budding rate reflects increased organizer potential, mirroring beta-catenin overexpression in vertebrate a

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: Beta-catenin overexpression and ectopic budding in Hydra.

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Sea Urchin -ll

Inhibiting nuclear accumulation of β-catenin results in:

β-catenin nuclear entry is absolutely required for endomesoderm specification in vegetal hemisphere of sea urchin embryo. Vegetal Disheveled normally blocks GSK-3β mediated degradation, allowing TCF-dependent transcription of Pmar1 and downstream endodermal genes such as FoxA, GataE and Brachyury. If nuclear accumulation experimentally prevented by overexpressing GSK-3β, dominant-negative TCF, or sequestering β-catenin in cytoplasm, vegetal gene regulatory network never initiates. Embryos fail to gastrulate, form no gut or mesenchyme, and develop as hollow ciliated balls composed entirely of a

Ref: Wikramanayake et al., PNAS 1998, β-catenin requirement; Gilbert 12th ed., Chapter 8: Animalization after β-catenin inhibition.

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Sea Urchin -ll

β-catenin primarily specifies which sea urchin cells?

Nuclear β-catenin is earliest known vegetal determinant in sea urchin embryos controlling axis specification. Vegetally localized Disheveled inhibits glycogen synthase kinase-3β within destruction complex, stabilizing β-catenin and permitting entry into vegetal nuclei, especially micromeres and macromere descendants. Complex with TCF/LEF transcription factor activates endomesodermal gene regulatory network including Pmar1, Krox, Otx and Brachyury. Without nuclear β-catenin embryos become animalized forming only ectoderm. Lithium chloride hyperactivates pathway. Thus β-catenin primarily specifi

Ref: Logan et al., Development 1999, β-catenin nuclearization specifies vegetal fate; Gilbert Chapter 8: Wnt signaling.

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Sea urchin -lll

Blocking β-catenin nuclear entry forms:

Beta-catenin nuclear entry drives vegetal gene regulatory network including Pmar1, Wnt8, blimp1b required for endomesoderm specification. Blocking nuclear entry via overexpression of cadherin cytoplasmic tail that sequesters beta-catenin, or dominant-negative TCF that occupies promoters without activation, or overactive GSK-three, eliminates endoderm and mesenchyme specification completely. Resulting embryos lack archenteron and skeleton, becoming permanent blastulae covered with long cilia called ciliated ectodermal balls, expressing only animal ectoderm markers like spec1. This animalized ph

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 10: Beta-catenin blocking forms animalized ciliated ectodermal ball.

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Sea urchin -lll

Which protein prevents β-catenin degradation?

Beta-catenin stability is controlled by destruction complex containing Axin, APC, and GSK-three kinase that phosphorylates conserved serine residues marking beta-catenin for proteasomal degradation via beta-TrCP. Wnt pathway activation recruits Dishevelled to plasma membrane via Frizzled receptor, where Dishevelled inhibits GSK-three kinase activity through protein interactions, preventing phosphorylation. Inhibition allows beta-catenin accumulation and nuclear translocation to activate TCF targets like siamois and Pmar1. In sea urchin, maternal Dishevelled localized vegetally performs this in

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 10: Dishevelled prevents beta-catenin degradation sea urchin vegetal.

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Sea urchin -lll

β-catenin accumulation in sea urchin micromeres leads primarily to formation of:

Vegetal micromeres in sea urchin inherit high levels of Dishevelled and consequently nuclear beta-catenin due to vegetal cortex localization established before fertilization. Beta-catenin activates endomesoderm gene regulatory network including Pmar1, which represses HesC repressor, allowing expression of Wnt8, blimp1b, krox, and Delta ligand. These cells become primary mesenchyme producing larval skeleton and act as organizers inducing adjacent macromeres to become endoderm forming gut and secondary mesenchyme pigment cells. Without beta-catenin accumulation, embryos become animalized ciliate

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 10: Beta-catenin in micromeres induces mesoderm endoderm.

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amphibian Development-l

β-catenin localization determines:

Localization of beta-catenin after cortical rotation creates nuclear gradient high dorsally, low ventrally that patterns secondary axis. Dorsally Dishevelled inhibits GSK-three destruction complex containing Axin and APC, stabilizing beta-catenin that accumulates in nuclei and activates organizer genes siamois, twin, goosecoid via TCF. Ventrally active GSK-three phosphorylates beta-catenin for ubiquitin-mediated proteasomal degradation, preventing dorsal fate ventrally. Resulting gradient instructs dorsal-ventral axis, not animal-vegetal axis which depends on VegT and Vg1, nor mesoderm specifi

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: Beta-catenin determines dorsal-ventral axis in amphibians.

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