#Hedgehog signaling

Paracrine Factor

What is the function of the primary cilium in Hedgehog signaling?

Vertebrate Hedgehog signaling strictly requires primary cilium, solitary microtubule-based organelle protruding from most cells containing acetylated tubulin axoneme and basal body. Patched localizes near cilium base, upon Hedgehog binding exits cilium, Smoothened accumulates at ciliary membrane and transports Gli-Sufu complexes via intraflagellar transport proteins IFT88, IFT172, Kif3A, Kif7 to tip where phosphorylation by PKA, GSK3 changes convert Gli repressor to activator. Ciliary mutants lacking IFT display Hedgehog loss phenotype resembling Shh knockout, cyclopia, polydactyly despite normal receptors. Primary cilium acts as signaling hub compartmentalizing components facilitating proteolytic processing and activation, not degrading Patched.

Ref: Goetz & Anderson, Nature Reviews Genetics 2010: Primary cilium required for Hedgehog transduction and Gli processing.

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Paracrine Factor

Which of the following proteins is NOT involved in the Hedgehog signaling pathway?

Hedgehog pathway core components include secreted cholesterol-modified ligand Hedgehog, twelve-pass receptor Patched with sterol sensing domain, seven-pass transducer Smoothened homologous to Frizzled GPCR, transcription factor Gli/Ci with five zinc fingers, suppressor Sufu sequestering Gli, and Kif7 kinesin. Upon activation, Smoothened accumulates in primary cilium membrane blocking proteolytic cleavage of Gli into repressor GliR, generating activator GliA driving targets Patched, Hhip. LRP5/6 is single-pass Wnt co-receptor binding Wnt with Frizzled, recruiting Axin, stabilizing beta-catenin for TCF activation. LRP not involved in Hedgehog binding or transduction, distinguishing pathway specificity for patterning and growth.

Ref: Varjosalo & Taipale, Genes & Development 2008: Hedgehog pathway components - Patched, Smoothened, Gli; LRP excluded.

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Paracrine Factor

Which of the following is the primary transcription factor activated in the Hedgehog signaling pathway?

Gli family members Gli1, Gli2, Gli3 are zinc finger transcription factors homologous to Drosophila Cubitus interruptus mediating Hedgehog transcriptional response. In resting state, Suppressor of Fused Sufu sequesters Gli in cytoplasm, promoting sequential phosphorylation by PKA, GSK3, CK1 creating beta-TrCP degron generating repressor Gli3R that blocks targets like Patched1. Upon Hedgehog activation, Smoothened inhibits Sufu and PKA processing, allowing full-length Gli activators Gli1, Gli2A to enter nucleus, inducing Patched1, Gli1 itself, cyclins, Myc, Bcl2. TCF is Wnt effector, STAT cytokine effector, beta-catenin coactivator not Hedgehog specific, making Gli definitive readout.

Ref: Briscoe & Thérond, Nature Reviews Mol Cell Biol 2013: Gli/Ci transcription factors mediate Hedgehog target gene activation.

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Paracrine Factor

Which of the following Hedgehog proteins is primarily responsible for neural tube patterning?

Sonic hedgehog patterning vertebrate central nervous system is classic morphogen gradient regulated by cholesterol and palmitate modifications restricting diffusion. Secreted from notochord and floor plate, cholesterol-modified Shh diffuses dorsally forming ventral-high dorsal-low gradient, binding Patched on neuroepithelial progenitors reducing Gli repressor formation via Smoothened derepression and PKA inhibition. High concentration inhibits Gli3R, activating Nkx2.2, Olig2 for V3 and motor neurons, intermediate concentrations activate Dbx, Pax6 for interneurons. Indian hedgehog regulates bone growth plate chondrocytes, Desert supports spermatogenesis, but only Sonic establishes ventral neural tube identities demonstrated by Shh knockout dorsalization and holoprosencephaly phenotype.

Ref: Jessell, Nature Reviews Neuroscience 2000: Sonic hedgehog as ventral neural tube morphogen patterning motor neurons.

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Cell Signaling Dev Bio & Other Pathways

Activated Hedgehog signaling results in

Gene activation, is consistent with established principles of cell signaling, receptor pharmacology and cellular regulation. Experimental measurements of binding parameters, genetic loss-of-function studies and pharmacological interventions all converge on the same interpretation. Related options address neighboring concepts but do not satisfy the precise criterion stated in the question.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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Cell Signaling Dev Bio & Other Pathways

In vertebrates, Hedgehog signaling requires

Primary cilium, is consistent with established principles of cell signaling, receptor pharmacology and cellular regulation. Experimental measurements of binding parameters, genetic loss-of-function studies and pharmacological interventions all converge on the same interpretation. Related options address neighboring concepts but do not satisfy the precise criterion stated in the question.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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Cell Signaling Dev Bio & Other Pathways

In absence of Hedgehog, Ci is processed into

Repressor, is consistent with established principles of cell signaling, receptor pharmacology and cellular regulation. Experimental measurements of binding parameters, genetic loss-of-function studies and pharmacological interventions all converge on the same interpretation. Related options address neighboring concepts but do not satisfy the precise criterion stated in the question.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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