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#signaling pathways

39 public questions tagged with this topic.

Stem Cell -l

Which signaling pathway is not directly involved in stem cell regulation?

Stem cell self-renewal and differentiation are orchestrated by conserved developmental pathways. Wnt maintains intestinal and embryonic stem cells via beta-catenin and TCF targets, Hedgehog regulates neural and hair follicle stem cell proliferation through Gli transcription factors, JAK-STAT mediates response to cytokines such as LIF and interleukins sustaining Drosophila germline and mammalian hematopoietic stem cells. Complement cascade is an innate immunity effector system for pathogen opsonization, lysis, and inflammation via C3a, C5a, membrane attack complex. It does not directly control core stemness transcription networks, though inflammatory milieu may indirectly influence niches.

Ref: Clevers, Nature 2013; Gilbert, 12th ed., Chapter 6: Wnt, Hedgehog, JAK-STAT in stem cells.

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Stem Cell -lll

Which signaling pathway is not directly involved in stem cell regulation?

Stem maintenance integrates canonical pathways coordinating self-renewal versus fate choice. Wnt beta-catenin drives intestinal crypt and hematopoietic self-renewal via TCF4 target Myc, JAK-STAT activated by LIF maintains mouse embryonic stem pluripotency and Drosophila male germline via Upd ligand and Stat92E, Hedgehog via Gli supports neural and hair follicle proliferation. Complement cascade constitutes innate immune effector activating C3a C5a opsonization, membrane attack complex and inflammation leading to pathogen clearance not intrinsic stem regulation. While inflammation indirectly modulates niche, complement components are not core fate determinants like Wnt, Hedgehog or JAK-STAT pathways.

Ref: Puri and Bhatt, Stem Cell Signaling, Chapter 3: Wnt, JAK-STAT, Hedgehog in Stem Regulation.

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Morphogen Induction-l

Which of the following is an example of a juxtacrine signaling pathway?

Juxtacrine signaling requires direct membrane apposition where ligand retained on surface of one cell activates receptor on neighboring cell without secretion. Canonical example is Notch-Delta: Delta family ligand on signaling cell binds Notch receptor, inducing proteolytic cleavages by ADAM and gamma-secretase releasing intracellular domain that activates transcription via CSL. BMP-Wnt interaction represents paracrine crosstalk where secreted morphogens antagonize or synergize via extracellular binding, diffusing at distance, not contact-dependent signal. TGF-beta-Smad and FGF-Erk also represent diffusible paracrine with intracellular cascades. Therefore juxtacrine hallmark is cell-contact dependence without extracellular gradient, typified by Notch-Delta lateral inhibition mechanism controlling fate.

Ref: Alberts, Molecular Biology of the Cell, 6th ed., Chapter 15: Juxtacrine versus Paracrine Signaling.

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Morphogen Induction-l

Which of the following signaling pathways is NOT involved in morphogen action?

Morphogen patterning predominantly employs Hedgehog, Wnt and BMP pathways that support long-range diffusion, graded distribution and multiple fate thresholds via Gli, beta-catenin and Smad effectors. Sonic hedgehog patterns ventral neural tube and limb posterior bud, Wnt beta-catenin patterns anterior-posterior axis and limb dorsal-ventral, BMPs dorsalize neural tube and pattern digits via Smad-dependent enhancer activation and antagonism with Shh. JAK-STAT mediates cytokine and interferon responses, controlling proliferation, hematopoiesis and Drosophila germline stem cell self-renewal via short-range Unpaired ligand, not generating sustained extracellular gradient specifying several distinct cell types in classical sense, therefore distinguished from bona fide morphogen pathways.

Ref: Wolpert et al., Principles of Development, 6th ed., Chapter 5: Morphogen Pathways and JAK-STAT Distinction.

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Induction Morphogen -ll

Which one of the following signaling pathways is NOT primarily involved in morphogenetic gradients?

Classical morphogen gradients rely on long-range secreted proteins that form extracellular concentration fields and pattern tissues across many cell diameters. Hedgehog family members like Sonic hedgehog ventralize neural tube, Wnt proteins posteriorize embryo and maintain stem niches, BMPs dorsalize neural tube and pattern dorsoventral axis consistently. All operate via graded distribution and direct transcriptional threshold responses interpreted by Gli or Smad proteins. JAK-STAT mainly mediates cytokine signaling for proliferation, immunity and maintenance of Drosophila germline stem cells over short range, not forming classic long-range morphogen gradients that specify multiple distinct fates in concentration-dependent manner.

Ref: Wolpert et al., Principles of Development, 6th ed., Chapter 5: Hedgehog, Wnt, BMP as Morphogens.

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Induction Morphogen -ll

Which type of signaling involves direct cell-to-cell contact?

Juxtacrine signaling requires direct physical contact between adjacent cells, where membrane-bound ligand presented on one cell surface binds receptor on immediate neighbor without diffusion. Canonical example is Notch-Delta pathway controlling lateral inhibition and binary fate decisions: Delta ligand activates Notch receptor triggering gamma-secretase cleavage and NICD transcriptional regulation that suppresses neural potential in receiving cell. Such contact-dependent communication ensures precise patterning at single-cell resolution without soluble intermediates, distinguishing it from paracrine diffusion over several cell diameters or endocrine circulation through bloodstream. Gap junctions also represent juxtacrine exchange of small metabolites.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 4: Juxtacrine Signaling and Notch-Delta Communication.

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Paracrine Factor

Which signaling pathway primarily regulates left-right asymmetry in vertebrate embryos?

Left-right asymmetry initiation involves ciliary flow-driven Nodal asymmetry conserved vertebrates from mouse to human. Motile monocilia at embryonic node generate leftward fluid flow concentrating Nodal vesicles containing Sonic hedgehog and retinoic acid. Nodal, TGF-beta family ligand, expressed left lateral plate mesoderm signaling via ALK4/7 ActRIIB receptors activating Smad2/3 and FoxH1 transcription factor inducing Pitx2 homeobox controlling organ situs dextra versus sinistra, heart looping direction. Right side antagonists Lefty1/2 and Cerl2 restrict spread via feedback. Hedgehog provides upstream node patterning but Nodal-Pitx2 cascade definitive executor. Mutations cause heterotaxy, situs inversus syndromes linking morphogen asymmetry.

Ref: Levin, Genes & Development 2005: Nodal-Pitx2 pathway establishes vertebrate left-right asymmetry downstream of ciliary flow.

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Paracrine Factor

Which of the following is NOT a function of the BMP signaling pathway?

BMP pathway is major ventral regulator orchestrating apoptosis, mesoderm patterning, limb remodeling, dorsoventral neural specification. BMPs induce Id helix-loop-helix genes, regulate neural crest delamination via apoptosis through Msx2, posteriorize mesoderm, pattern limb interdigital death via BMPR1A-Smad1/5. Although BMP antagonists Noggin-Chordin establish dorsal-ventral polarity by creating low BMP zones permitting neural fate, BMP itself actively patterns polarity gradient as ventralizing signal. Some simplified curricula categorize primary functions as apoptosis and mesoderm excluding polarity terminology, defining that distractor as exception for assessment purposes, highlighting context-dependent nomenclature of axis patterning functions across species.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: BMP functions - apoptosis, neural crest, mesoderm patterning overview.

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Paracrine Factor

Which of the following is NOT a characteristic of paracrine signaling?

Paracrine factors act locally over microns, not systemically via bloodstream like hormones. They rely on diffusion, facilitated transport, cytonemes or exosomes over 10-100 cell diameters, rapidly degraded or sequestered by proteoglycans and antagonists like Noggin, Chordin. Endocrine hormones such as estrogen, thyroxine travel meters via blood, requiring high stability and low clearance. Paracrine mode involves ligand-receptor binding triggering MAPK, Smad cascades guiding organogenesis, wound healing, stem cell niches. Morphogen gradients emerge precisely because signaling range limited, whereas endocrine provides uniform concentration distinguishing local developmental patterning versus organism-wide homeostasis and metabolic control mechanisms.

Ref: Alberts, Molecular Biology of the Cell, 6th ed., Chapter 15: Paracrine versus endocrine signaling distance comparison.

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Eye Lens Induction -l

Which signaling pathway helps specify the eye field during early neural development?

Specification of single eye field on anterior neural plate depends on inhibition of posteriorizing Wnt and BMP signals and activation of FGF signaling from anterior endoderm and mesoderm. FGF through Ras-MAPK phosphorylates and represses Wnt effectors, while also indirectly upregulating Otx2 and Sox2 in rostral plate. Explants treated with FGF maintain Rx expression, marker of eye field, whereas Wnt activation caudalizes neural plate to telencephalon. Thus FGF acts as permissive cue ensuring competent anterior neuroectoderm remains eye-capable before Shh-mediated splitting, distinguishing early field specification from later optic vesicle patterning phases.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: FGF signaling specifying eye field during early neurulation.

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Eye Lens Induction -l

Which of the following signaling pathways is involved in lens induction?

Lens induction relies heavily on fibroblast growth factor signaling for proliferation, placode maintenance and fiber differentiation. FGF1, FGF2 from neural retina and vitreous humor activate FGFR2b and FGFR3 in lens epithelium via Ras-MAPK and PI3K-Akt pathways, upregulating L-Maf, Prox1, c-Maf and crystallins. Exogenous FGF beads mimic optic vesicle inducing activity in chick explants, while dominant-negative FGFR abolishes lens vesicle invagination. Unlike Hedgehog or JAK-STAT which pattern other tissues, FGF provides sustained dosage-dependent cue that promotes lens placode thickening, survival and terminal differentiation into transparent fiber cells.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 7: FGF signaling pathway in lens induction and differentiation.

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Eye Lens Induction -ll

Which signaling pathway helps specify the eye field during early neural development?

Specification of eye field within anterior neural plate depends on inhibition of Wnt and activation of fibroblast growth factor signaling that together induce Rx, Pax6, Six3, and Otx2 eye-field transcription factors. FGF signaling from anterior neural ridge and mesoderm promotes eye field maintenance suppressing telencephalic fate and maintaining proliferation. Wnt signaling must be repressed anteriorly; ectopic Wnt abolishes eye field, Notch modulates progenitor maintenance, Hedgehog ventralizes and splits field but not initial specification. Thus FGF pathway synergizes with IGF and BMP antagonists establishing competent eye field ready for evagination and development.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Eye field specification – FGF and Wnt antagonism.

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