Département Adaptation du Vivant, UMR CNRS, Evolution des Régulations Endocriniennes, Muséum National d'Histoire Naturelle, Paris, France.
Zoologisches Institut, Zell-und Entwicklungsbiologie, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany.
PLoS One. 2018 Apr 11;13(4):e0195374. doi: 10.1371/journal.pone.0195374. eCollection 2018.
Thyroid hormone (TH) orchestrates amphibian metamorphosis. Thus, this developmental phase is often used to study TH-dependent responses in specific tissues. However, TH signaling appears early in development raising the question of the control of TH availability in specific cell types prior to metamorphosis. TH availability is under strict temporal and tissue-specific control by deiodinases. We examined the expression of the TH-inactivating enzyme, deiodinase type 3 (D3), during early retinal development. To this end we created a Xenopus laevis transgenic line expressing GFP from the Xenopus dio3 promoter region (pdio3) and followed pdio3-GFP expression in pre-metamorphic tadpoles. To validate retinal GFP expression in the transgenic line as a function of dio3 promoter activity, we used in situ hybridization to compare endogenous dio3 expression to reporter-driven GFP activity. Retinal expression of dio3 increased during pre-metamorphosis through stages NF41, 45 and 48. Both sets of results show dio3 to have cell-specific, dynamic expression in the pre-metamorphic retina. At stage NF48, dio3 expression co-localised with markers for photoreceptors, rods, Opsin-S cones and bipolar neurons. In contrast, in post-metamorphic juveniles dio3 expression was reduced and spatially confined to certain photoreceptors and amacrine cells. We compared dio3 expression at stages NF41 and NF48 with TH-dependent transcriptional responses using another transgenic reporter line: THbZIP-GFP and by analyzing the expression of T3-regulated genes in distinct TH availability contexts. At stage NF48, the majority of retinal cells expressing dio3 were negative for T3 signaling. Notably, most ganglion cells were virtually both dio3-free and T3-responsive. The results show that dio3 can reduce TH availability at the cellular scale. Further, a reduction in dio3 expression can trigger fine-tuned T3 action in cell-type specific maturation at the right time, as exemplified here in photoreceptor survival in the pre-metamorphic retina.
甲状腺激素（TH）调控着两栖动物的变态发育。因此，这个发育阶段通常被用来研究特定组织中 TH 依赖性反应。然而，TH 信号似乎在发育早期就出现了，这就提出了在变态发育之前，特定细胞类型中 TH 可用性的控制问题。TH 的可用性受到脱碘酶的严格时间和组织特异性控制。我们研究了 TH 失活酶，即脱碘酶 3（D3），在早期视网膜发育过程中的表达。为此，我们创建了一个表达 GFP 的非洲爪蟾转基因系，其 GFP 由非洲爪蟾 dio3 启动子区域（pdio3）启动（pdio3-GFP），并在预变态蝌蚪中跟踪 pdio3-GFP 的表达。为了验证该转基因系中视网膜 GFP 表达作为 dio3 启动子活性的功能，我们使用原位杂交来比较内源性 dio3 表达与报告基因驱动的 GFP 活性。在预变态期，视网膜中 dio3 的表达在 NF41、45 和 48 期通过 stage 增加。这两组结果都表明，dio3 在预变态视网膜中具有细胞特异性、动态表达。在 stage NF48，dio3 的表达与光感受器、棒状细胞、Opsin-S 视锥细胞和双极神经元的标志物共定位。相比之下，在变态后的幼体中，dio3 的表达减少，并局限于特定的光感受器和无长突细胞。我们在 NF41 和 NF48 阶段比较了 dio3 表达与另一个转基因报告基因线：THbZIP-GFP 的 TH 依赖性转录反应，以及在不同 TH 可用性情况下分析 T3 调节基因的表达。在 stage NF48，表达 dio3 的大多数视网膜细胞对 T3 信号呈阴性。值得注意的是，大多数神经节细胞实际上都是既无 dio3 又无 T3 反应性的。结果表明，dio3 可以在细胞水平上降低 TH 的可用性。此外，dio3 表达的减少可以在适当的时间触发特定细胞类型成熟的精细 T3 作用，如这里在预变态视网膜中光感受器的存活所例示的那样。
