#Wnt signaling

Stem Cell -ll

Which signaling pathway plays a crucial role in maintaining stem cell pluripotency?

Pluripotency maintenance depends on integrated extrinsic pathways modulating core transcription factor circuitry of Oct4 and Nanog. Wnt beta-catenin signaling inhibits GSK3 beta, stabilizing beta-catenin which interacts with Oct4 and derepresses Nanog promoter, while also supporting metabolic reprogramming toward glycolysis. In mouse embryonic stem cells, Wnt together with LIF sustains ground state pluripotency in 2i culture medium containing CHIR99021 and PD0325901 MEK inhibitor. JAK-STAT downstream of LIF, Hedgehog, and Notch primarily regulate lineage-specific somatic stem cells, but Wnt ac

Ref: Clevers et al., Science 2014; Gilbert, Chapter 6: Wnt signaling maintains pluripotency and stemness.

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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 of the following defines the role of R-spondin in Wnt signaling?

R-spondins 1-4 are cysteine-rich secreted potentiators containing furin and thrombospondin domains amplifying Wnt responses dramatically. They simultaneously bind leucine-rich repeat GPCRs LGR4, LGR5, LGR6 marking stem cells and transmembrane E3 ubiquitin ligases ZNRF3, RNF43 which normally ubiquitinate Frizzled for lysosomal degradation reducing surface receptor availability. R-spondin-LGR complex internalizes and inhibits ZNRF3/RNF43, stabilizing Frizzled receptors increasing membrane availability and amplifying Wnt/beta-catenin signaling several hundred-fold. This sustains intestinal crypt,

Ref: de Lau et al., Genes & Development 2014: R-spondin binds LGR and ZNRF3 to stabilize Frizzled and amplify Wnt.

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Paracrine Factor

Which molecule is responsible for negatively regulating Wnt signaling by removing lipid modifications?

Wnt activity modulated by extracellular modifiers controlling lipidation state and receptor availability. Notum encodes secreted carboxylesterase pectin acetylesterase-like deacylase related to lipases that specifically cleaves palmitoleate moiety added by Porcupine O-acyltransferase in ER on Wnt Ser209, essential for Frizzled binding hydrophobic groove. Removal of lipid blocks receptor engagement, dampening signal, shaping gradient. Axin and APC are intracellular negative regulators scaffolding destruction complex; Dishevelled is positive adaptor. Notum secreted from Wnt target tissue itself

Ref: Kakugawa et al., Nature 2015: Notum removes palmitoleate from Wnt inhibiting receptor binding and signaling.

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Paracrine Factor

Which co-receptor is required for efficient Wnt signaling through the canonical pathway?

Efficient canonical Wnt signaling requires cooperation between Frizzled seven-transmembrane receptor and single-pass co-receptor LRP5/6 forming signalosome. Frizzled alone low affinity and cannot stabilize beta-catenin efficiently, while LRP5/6 low-density lipoprotein receptor family member binds Wnt bridging Frizzled extracellular domains and intracellular Axin. Wnt induces LRP5/6 phosphorylation at PPPSP motifs by GSK3 and CK1gamma creating Axin docking sites, inactivating destruction complex and allowing beta-catenin accumulation. Genetic loss abolishes axis duplication and causes osteoporo

Ref: He et al., Development 2004: LRP5/6 co-receptor required for canonical Wnt signaling with Frizzled and Axin recruitment.

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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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Limb Development -ll

Which experiment demonstrated the ability of Wnt signaling to initiate limb formation?

Capability of Wnt signaling to initiate limbs was demonstrated by gain-of-function manipulation of canonical pathway. Overexpression of Wnt3a or stabilized beta-catenin or constitutive Tcf in lateral plate mesoderm of chick embryos induced ectopic expression of FGF10 and its downstream cascade, formation of additional apical ridge and ectopic limbs from flank with DV polarity. This proved Wnt signals operate upstream of FGF10 to establish limb fields and permit FGF10-FGF8 loop initiation, interacting with Tbx5. Knockout of FGF8 truncates limbs, AER removal time-course reveals PD specification,

Ref: Kawakami et al., Development, Gilbert Chapter 20: Wnt overexpression initiates limb formation.

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Limb Development -lll

Which molecule establishes the dorsal-ventral axis of the limb?

Dorsal-ventral polarity originates from ectoderm-mesenchyme signaling interactions conserved across vertebrates and mammals. Dorsal ectoderm secretes Wnt7a, which via canonical beta-catenin pathway induces LIM-homeodomain factor Lmx1b in underlying dorsal mesenchyme, conferring dorsal character to skin, nails, sesamoids and extensor musculature. Ventral ectoderm expresses Engrailed-1, which represses Wnt7a, confining it dorsally and permitting ventral BMP-driven fate with thick volar pads. Loss of Wnt7a ventralizes dorsal limb causing biventral double-ventral phenotype. TBX5 specifies forelimb

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: Wnt7a establishment of dorsal-ventral limb axis.

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Regeneration -l

What happens when Wnt signaling is blocked in hydra?

Head regeneration in Hydra absolutely requires functional canonical Wnt signaling to maintain and rebuild hypostome organizer. Chemical inhibition of Wnt secretion using porcupine inhibitors Wnt-C59 or IWP-2, blocking Wnt ligand palmitoylation, eliminates endogenous Wnt3 gradient at oral pole. Without Wnt3, beta-catenin target genes including Cnox-3, Brachyury1, and tentacle markers fail to reactivate after decapitation, hypostome does not form, and remaining column adopts foot fate expressing pedibin. Tentacle morphogenesis also fails because hypostome organizer provides prerequisite inductiv

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: Wnt blockade prevents head formation in Hydra.

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Regeneration -l

Which factor controls the balance between Wnt and Notum in planarian regeneration?

Planarian regeneration requires rapid re-establishment of opposing Wnt and Notum domains at wounds. Early injury signals activate MAPK Erk within minutes via calcium and ROS, leading to phosphorylation of Elk transcription factors that bind notum promoter inducing its expression specifically at anterior-facing wounds. Erk activity simultaneously represses wnt1 transcription anteriorly. Consequently balance between Wnt-driven tail and Notum-driven head is orchestrated by wound-induced Erk gradient. Pax6 specifies eye differentiation, FGF10 mediates vertebrate limb outgrowth, Hedgehog upstream m

Ref: Cell Reports, Erk regulates Notum-Wnt balance controlling anterior-posterior regeneration in planaria.

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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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