RESUMO
Buffering variability in morphogen distribution is essential for reproducible patterning. A theoretically proposed class of mechanisms, termed "distal pinning," achieves robustness by combining local sensing of morphogen levels with global modulation of gradient spread. Here, we demonstrate a critical role for morphogen sensing by a gene enhancer, which ultimately determines the final global distribution of the morphogen and enables reproducible patterning. Specifically, we show that, while the pattern of Toll activation in the early Drosophila embryo is robust to gene dosage of its locally produced regulator, WntD, it is sensitive to a single-nucleotide change in the wntD enhancer. Thus, enhancer properties of locally produced WntD directly impinge on the global morphogen profile.
Assuntos
Proteínas de Drosophila/metabolismo , Drosophila/embriologia , Drosophila/genética , Drosophila/metabolismo , Elementos Facilitadores Genéticos/genética , Peptídeos e Proteínas de Sinalização Intracelular/metabolismo , Animais , Sítios de Ligação , Padronização Corporal , Proteínas de Drosophila/genética , Desenvolvimento Embrionário/genética , Gástrula/fisiologia , Dosagem de Genes , Regulação da Expressão Gênica no Desenvolvimento , Proteínas HMGB/metabolismo , Peptídeos e Proteínas de Sinalização Intracelular/genética , Morfogênese/genética , Morfogênese/fisiologia , Proteínas Repressoras/metabolismo , Alinhamento de Sequência , Transdução de Sinais/genética , Transdução de Sinais/fisiologia , Receptores Toll-Like/genética , Receptores Toll-Like/metabolismoRESUMO
Establishment of morphogen gradients in the early Drosophila embryo is challenged by a diffusible sextracellular milieu, and by rapid nuclear divisions that occur at the same time. To understand how a sharp gradient is formed within this dynamic environment, we followed the generation of graded nuclear Dorsal protein, the hallmark of pattern formation along the dorso-ventral axis, in live embryos. The dynamics indicate that a sharp extracellular gradient is formed through diffusion-based shuttling of the Spaetzle (Spz) morphogen that progresses through several nuclear divisions. Perturbed shuttling in wntD mutant embryos results in a flat activation peak and aberrant gastrulation. Re-entry of Dorsal into the nuclei at the final division cycle plays an instructive role, as the residence time of Dorsal in each nucleus is translated to the amount of zygotic transcript that will be produced, thereby guiding graded accumulation of specific zygotic transcripts that drive patterned gastrulation. We conclude that diffusion-based ligand shuttling, coupled with dynamic readout, establishes a refined pattern within the diffusible environment of early embryos.
Assuntos
Proteínas de Drosophila/fisiologia , Drosophila/embriologia , Gástrula/metabolismo , Gastrulação , Regulação da Expressão Gênica no Desenvolvimento , Morfogênese , Animais , Padronização Corporal , Núcleo Celular/fisiologia , Proteínas de Drosophila/genética , Embrião não Mamífero/fisiologia , Feminino , Peptídeos e Proteínas de Sinalização Intracelular/genética , Masculino , Mutação , Proteínas Nucleares/fisiologia , Fosfoproteínas/fisiologia , Transdução de Sinais , Fatores de Transcrição/fisiologiaRESUMO
Patterning by morphogen gradients relies on the capacity to generate reproducible distribution profiles. Morphogen spread depends on kinetic parameters, including diffusion and degradation rates, which vary between embryos, raising the question of how variability is controlled. We examined this in the context of Toll-dependent dorsoventral (DV) patterning of the Drosophila embryo. We find that low embryo-to-embryo variability in DV patterning relies on wntD, a Toll-target gene expressed initially at the posterior pole. WntD protein is secreted and disperses in the extracellular milieu, associates with its receptor Frizzled4, and inhibits the Toll pathway by blocking the Toll extracellular domain. Mathematical modeling predicts that WntD accumulates until the Toll gradient narrows to its desired spread, and we support this feedback experimentally. This circuit exemplifies a broadly applicable induction-contraction mechanism, which reduces patterning variability through a restricted morphogen-dependent expression of a secreted diffusible inhibitor.
