Romand Raymond, Dollé Pascal, Hashino Eri
Institut Clinique de la Souris and Institut de Génétique et de Biologie Moléculaire et cellulaire, B.P. 10142, 67404 Illkirch Cedex, France.
J Neurobiol. 2006 Jun;66(7):687-704. doi: 10.1002/neu.20244.
The inner ear originates from an embryonic ectodermal placode and rapidly develops into a three-dimensional structure (the otocyst) through complex molecular and cellular interactions. Many genes and their products are involved in inner ear induction, organogenesis, and cell differentiation. Retinoic acid (RA) is an endogenous signaling molecule that may play a role during different phases of inner ear development, as shown from pathological observations. To gain insight into the function of RA during inner ear development, we have investigated the spatio-temporal expression patterns of major components of RA signaling pathway, including cellular retinoic acid binding proteins (CRABPs), cellular retinoid binding proteins (CRBPs), retinaldehyde dehydrogenases (RALDHs), catabolic enzymes (CYP26s), and nuclear receptors (RARs). Although the CrbpI, CrabpI, and -II genes are specifically expressed in the inner ear throughout development, loss-of-function studies have revealed that these proteins are dispensable for inner development and function. Several Raldh and Cyp26 gene transcripts are expressed at embryological day (E) 9.0-9.5 in the otocyst and show mainly complementary distributions in the otic epithelium and mesenchyme during following stages. From Western blot, RT-PCR, and in situ hybridization analysis, there is a low expression of Raldhs in the early otocyst at E9, while Cyp26s are strongly expressed. During the following days, there is an up-regulation of Raldhs and a down-regulation for Cyp26s. Specific RA receptor (Rar and Rxr) genes are expressed in the otocyst and during further development of the inner ear. At the otocyst stage, most of the components of the retinoid pathway are present, suggesting that the embryonic inner ear might act as an autocrine system, which is able to synthesize and metabolize RA necessary for its development. We propose a model in which two RA-dependent pathways may control inner ear ontogenesis: one indirect with RA from somitic mesoderm acting to regulate gene expression within the hindbrain neuroepithelium, and another with RA acting directly on the otocyst. Current evidence suggests that RA may regulate several genes involved in mesenchyme-epithelial interactions, thereby controlling inner ear morphogenesis. Our investigations suggest that RA signaling is a critical component not only of embryonic development, but also of postnatal maintenance of the inner ear.
内耳起源于胚胎外胚层基板，并通过复杂的分子和细胞相互作用迅速发育成三维结构（耳泡）。许多基因及其产物参与内耳诱导、器官发生和细胞分化。视黄酸（RA）是一种内源性信号分子，病理观察表明其可能在内耳发育的不同阶段发挥作用。为深入了解RA在内耳发育过程中的功能，我们研究了RA信号通路主要成分的时空表达模式，包括细胞视黄酸结合蛋白（CRABP）、细胞类视黄醇结合蛋白（CRBP）、视黄醛脱氢酶（RALDH）、分解代谢酶（CYP26）和核受体（RAR）。尽管CrbpI、CrabpI和 -II基因在整个发育过程中在内耳中特异性表达，但功能丧失研究表明这些蛋白对于内耳发育和功能并非必需。几种Raldh和Cyp26基因转录本在胚胎第9.0 - 9.5天在耳泡中表达，并且在随后阶段主要在耳上皮和间充质中呈现互补分布。从蛋白质印迹、逆转录 - 聚合酶链反应和原位杂交分析来看，在胚胎第9天早期耳泡中Raldh表达较低，而Cyp26表达强烈。在接下来的几天里，Raldh表达上调，Cyp26表达下调。特异性RA受体（Rar和Rxr）基因在耳泡以及内耳进一步发育过程中表达。在耳泡阶段，类视黄醇途径的大多数成分都存在，这表明胚胎内耳可能作为一个自分泌系统，能够合成和代谢其发育所需的RA。我们提出一个模型，其中两条RA依赖途径可能控制内耳个体发生：一条是间接途径，来自体节中胚层的RA作用于调节后脑神经上皮内的基因表达；另一条是直接途径，RA直接作用于耳泡。目前的证据表明RA可能调节几个参与间充质 - 上皮相互作用的基因，从而控制内耳形态发生。我们的研究表明RA信号不仅是胚胎发育的关键组成部分，也是内耳出生后维持的关键组成部分。
