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#retinoic acid

14 public questions tagged with this topic.

Eye Lens Induction -ll

What is the main function of retinoic acid in eye development?

Retinoic acid signaling synthesized by retinal pigment epithelium via Raldh1-3 enzymes plays pivotal role in retinal differentiation patterning. Retinoic acid gradient along dorsal-ventral axis regulates expression of Tbx5, Vax2, and Cyp26 to specify dorsal-ventral retinal identity, promote photoreceptor differentiation, and stimulate neurite outgrowth. RA also induces Pax2 in optic stalk and supports vascular development. It does not primarily induce lens which depends on BMP-FGF, nor optic vesicle formation which precedes RA, nor cornea formation dependent on surface ectoderm, but fine-tunes retinal progenitor competence and patterning.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Retinoic acid role in retinal dorsal-ventral patterning and differentiation.

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Limb Development -lll

What is the main role of RA in limb development?

Two opposing gradients establish proximal-distal axis. Retinoic acid synthesized in flank creates proximal environment, inducing Meis1 Meis2 and proximal Hox programs governing stylopod specification. Distal FGF8 from AER induces Cyp26b1, an RA-degrading enzyme, clearing RA distally and permitting distal Hox12-13 activation and Meis repression. Excess RA proximalizes distal structures, RA inhibition distalizes. This two-signal model explains proximal dominance when AER removed early. RA does not directly determine hindlimb identity, prevent autopod apoptosis, or primarily regulate FGF transcription; its core function is imposing proximal character versus distal FGF.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: Retinoic acid proximalization two-signal model.

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Regeneration -lll

Which mechanism explains tail-to-limb conversion in RA-treated tadpoles?

Tadpole tail normally regenerates tail. When tadpoles treated with retinoic acid after tail amputation, blastema converts to limb program producing hindlimb instead of tail. RA increases positional value along proximal-distal axis activating Meis, proximal Hox genes and posteriorizes inducing limb-specific transcription factors, altering RA to FGF ratio interpretation. Tail blastema expressing different Hox code becomes competent to adopt more proximal and posterior limb identity under RA influence, representing homeotic transformation via positional respecification rather than mutation or cell death. Molecularly RA upregulates Shh, Fgf10 associated with limb initiation program replacing tail gene network.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: RA-induced tail-to-limb homeotic transformation in tadpoles.

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Regeneration -lll

In newt limb regeneration, retinoic acid typically:

In newt forelimb regeneration retinoic acid typically proximalizes positional identity reprogramming distal blastema toward proximal fate. Applied topically or via implanted beads, RA activates RAR-dependent transcription of Meis1/2 and proximal Hox9 genes, increases Prod1 expression level characteristic of proximal cells, suppresses distal HoxA13. Resulting regenerate duplicates proximal structures, converting wrist blastema into complete limb. Endogenous gradient with high RA proximally shapes normal pattern. Dose-dependent transformations demonstrated by Maden confirm proximalizing action; thus RA functions as respecifying morphogen resetting positional memory rather than eliminating identity or having no effect.

Ref: NCBI Bookshelf, Limb Regeneration: RA proximalization, Meis, Prod1 regulation in newt limb regeneration.

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Regeneration -lll

High levels of retinoic acid will convert a distal limb blastema to:

High levels of retinoic acid convert normally distal blastema fate to proximal by transcriptional reprogramming. RA via RAR-RXR binds enhancers of Meis1, Meis2, proximal Hox genes and upregulates glycosylated Prod1 isoform characteristic of proximal cells, while downregulating distal markers HoxA13. Consequently distal amputation plane that would regenerate only hand now regenerates complete arm including stylopod and zeugopod, producing proximalized duplicated limb. Dose-response experiments demonstrate progression: moderate RA duplicates forearm, high duplicates entire arm. This proximalizing effect underlies complete limb regeneration from distal blastema after exogenous retinoid exposure.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: High RA converting distal blastema to proximal structures.

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Regeneration -lll

Morphogens such as retinoic acid primarily act by:

Morphogens such as retinoic acid act not simply as mitogens but as graded informational signals establishing concentration thresholds that differentially regulate gene expression programs. RA diffusing from proximal source forms gradient shaped by binding proteins CRABP, synthesis enzymes Raldh2, and degradative CYP26, activating retinoic acid response elements at distinct thresholds. High RA induces proximal Meis genes, low RA permits distal HoxA13, creating French flag pattern of transcriptional domains. This spatial transcriptional landscape confers positional identity, allowing blastema cells to know location and regenerate appropriate structures consistent with gradient interpretation model.

Ref: Alberts, Molecular Biology of the Cell, Chapter 22: Morphogen gradients and transcriptional control by retinoic acid.

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Regeneration -lll

Retinoic acid influences limb regeneration by:

Retinoic acid influences amphibian limb regeneration by resetting proximal-distal positional value toward proximal identity. Binding RAR-RXR nuclear receptors activates transcription of Meis1, Meis2, HoxA9, and surface protein Prod1 characteristic of proximal cells, while repressing distal genes like HoxA13. Distal blastema exposed to RA behaves as proximal, generating full limb duplication when grafted. Endogenous gradient formed by Raldh2 synthesis proximally and Cyp26b1 degradation distally patterns normal limb. Hence exogenous RA proximalizes positional identity, reprogramming blastema cells to adopt more proximal fate during regeneration and development processes.

Ref: Nature Reviews, Developmental Mechanisms: RA as proximalizing morphogen in limb regeneration and Meis activation.

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Regeneration -lll

Axolotl limb regeneration treated with retinoic acid will:

Axolotl limb amputation at distal level normally regenerates only distal parts. Retinoic acid treatment proximalizes blastema positional memory activating proximal markers Meis1/2, HoxA9 and Prod1, converting distal blastema to proximal identity. Consequently distal wrist blastema treated with RA regenerates complete limb from stylopod through digits, representing serial duplication beyond amputation plane. Experiments by Maden and colleagues showed dose-dependent proximalization: low doses duplicate forearm, high doses duplicate entire arm. This property demonstrates RA as reprogramming agent altering positional value and Hox gene expression during epimorphic regeneration.

Ref: Alberts, Molecular Biology of the Cell, Chapter 22: RA proximalization and complete limb regeneration in axolotl.

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Tetrapod

What effect does retinoic acid have on limb regeneration?

Retinoic acid acts as morphogen proximalizing regenerating blastema through nuclear RAR-RXR mediated transcription. Exogenous RA applied to distal blastema upregulates proximal transcription factors Meis1, Meis2, HoxA9, Prod1 surface gradient marker, shifting positional value toward proximal base. Consequently wrist-level blastema normally restricted to hand regeneration regenerates entire limb including humerus, radius. Dose-dependent response shows increasing RA yields more proximal duplications. This illustrates RA sets proximal positional identity antagonizing distal FGF signals and illuminates how chromatin remodeling underlies positional reprogramming during regeneration and limb development.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: Retinoic acid proximalization and Hox activation in regeneration.

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Teratogens

Retinoic acid specifies:

Retinoic acid concentration gradient along embryo axis established by posterior Raldh2 synthesis and anterior Cyp26 degradation acts as morphogen instructing anterior-posterior identity via direct regulation of Hox clusters. Retinoic acid receptors heterodimerize with retinoid X receptor binding retinoic acid response elements upstream of Hox genes determining rhombomere boundaries in hindbrain and vertebral identities. Deficiency anteriorizes, excess posteriorizes development. While RA also influences limb patterning, its primary embryonic role is specifying anterior-posterior axis positioning of tissues through Hox gene collinear activation establishing body plan organization and segmental fate.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Retinoic acid specifies AP axis via Hox.

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Teratogens

High RA exposure activates enzymes causing:

Retinoic acid levels tightly regulated by synthesis via retinaldehyde dehydrogenases and degradation via cytochrome P450 family Cyp26 enzymes. Exogenous high RA dose activates retinoic acid response elements in Cyp26A1 promoter, strongly inducing RA catabolic enzymes aimed at restoring homeostasis. Enhanced enzyme activity persists beyond clearance of exogenous RA, accelerating endogenous RA turnover leading to precipitous drop below normal required for ongoing organogenesis. This induced catabolism creates long-lasting RA deficiency phase more teratogenic than initial excess, explaining paradox where similar phenotypes arise from both RA surplus and deficiency due to disrupted morphogen balance during critical windows.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: High RA induces enzymes causing RA deficiency.

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Teratogens

Which herbicide upregulates endogenous retinoic acid activity?

Glyphosate herbicide, active ingredient in Roundup broad spectrum formulations, has been implicated in developmental disruption through retinoid pathway rather than classical hormonal mimicry. Experimental studies in Xenopus embryos demonstrated glyphosate exposure increased endogenous retinoic acid activity by upregulating retinol oxidizing dehydrogenases or suppressing Cyp26 catabolic enzymes, enhancing RA signaling. Elevated RA causes microcephaly, cyclopia and neural tube defects reminiscent of exogenous RA administration, indicating indirect upregulation of endogenous morphogen. This mechanism highlights herbicide-induced teratogenic potential via perturbation of retinoic acid homeostasis leading to anterior-posterior patterning defects in vertebrate embryos.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: Glyphosate upregulates endogenous retinoic acid.

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