Translational control of activin in Xenopus laevis embryos

Activin is a potent mesoderm inducing factor present in embryos of Xenopus laevis. Recent evidence has implicated activin in the inhibition of neural development in addition to the well-established induction of mesoderm in ectodermal explants. These diverse effects are critically dependent on the concentration of activin yet little is known about the mechanisms regulating the level of activin in the embryo. We report that the 3′ untranslated region (3′ UTR) of activin βB mRNA inhibits the translation of activin in embryos. Microinjection of activin mRNA from which the 3′ UTR has been deleted is 8–10-fold more potent in inducing mesoderm than mRNA containing the 3′ UTR. Truncation of the 3′ UTR also leads to a marked enhancement of activin protein levels in embryos but has no effect when the truncated mRNA is translated in vitro. The 3′ UTR also confers translational inhibition on a heterologous mRNA. These data show that a maternal factor(s) present in X. laevis regulates the translation of injected activin βB mRNA. This factor(s) could be responsible for regulating the levels of endogenous activin βB protein during mesoderm induction and the specification of ectodermal derivatives such as neural and epidermal tissues. © 1995 Wiley-Liss, Inc.     

Relationships between Presence of the Eye Cup and Maintenance of Lens-Forming Capacity in Larval Xenopus laevis

The lentectomized eye of larval Xenopus laevis can regenerate a lens by a process of lens-transdifferentiation of the cornea and pericorneal epidermis. These tissues can form the lens only when they become in direct communication with the environment of the vitreous chamber (neural retina) indicating that the eye cup plays a fundamental role in this process.
In this work the role of the eye cup in the maintainance of the lens-forming capacity of the cornea and pericorneal epidermis was studied by allowing these tissues to cover the enucleated orbit for different periods, and then implanting them into the vitreous chamber of the contralateral eye. Under these experimental conditions the maintainance of the lens-forming capacity of the cornea and pericorneal epidermis showed no significant correlation with the time from enucleation to implantation.     

Retinoid dynamics in chicken eye during pre-and postnatal development

Changes in the steady state level of retinols, retinaldehydes and retinyl esters in the trans and 11-cis forms and trans retinoic acid were measured in whole chicken eye during development from day 6in ovo to day 3 post-hatch. These retinoids, quantified by different HPLC systems, were detected in this time sequence: trans-retinol and trans-retinyl esters in the first weekin ovo, 11-cis-retinol in the second week. The highest level of 11-cis-retinaldehyde and 11-cis-retinyl esters was reached at the end of developmentin ovo; however, their levels increased further after hatching. The retinoic acid level decreased at the end of the first week, rising again at the end of the second week.The enzyme activities involved in the metabolism of these retinoids-acyl-CoA: retinol acyltransferase, trans-retinol dehydrogenase, 11-cis-retinol dehydrogenase, trans-retinyl ester hydrolase and trans: 11-cis-retinol isomerase were also estimated and they were detectable already in the first week of developmentin ovo.At day 6 of the biosynthesis of retinoic acid by the retinaldehyde dehydrogenase activity from retina cytosol was also shown.     

Role of fibroblast growth factors as inducing agents in early embryonic development

To assess the potential role of a molecule in development we need to know three things: 1) what are the biological activities of the molecule, 2) what is its expression pattern, and 3) what are the consequences of removing it from the embryo? In the case of the FGF family in Xenopus embryos we have quite a lot of information about all three questions. Most members of the family can induce mesoderm from isolated animal caps, thus mimicking the natural “ventral vegetal” inducing signal operative in the blastula. This activity can be exerted on isolated, disaggregated cells and does not involve a change in division rate. When overexpressed from injected mRNA, the activity of FGFs depends largely on whether or not they possess a signal sequence, showing the importance of secretion in the inductive process. In addition to the mesoderm-inducing activity, there are effects of overexpression on whole embryos which lead to a suppression of anterior structures. Three types of FGF have so far been cloned from Xenopus: direct homologs of each of the mammalian types FGF-2 and FGF-3, and eFGF (“embryonic FGF”), which is equidistant in sequence from mammalian FGF-4 and FGF-6. Attempts to find homologs of mammalian FGF-5 and FGF-7 in Xenopus have proved unsuccessful. All three types of Xenopus FGF are expressed in early development. FGF-2 and eFGF are present in the oocyte and fertilized egg, and are thus both available at the time of mesoderm induction. FGF-3 and eFGF are both expressed from the embryonic genome during gastrulation and concentrated in the forming mesoderm. FGF-2 is expressed from the embryonic genome during neurulation in the brain, and a little later in the branchial arch mesenchyme and in the forming myotomes. These expression patterns suggest that there are several functions for the FGFs. The most successful strategy for inhibition of the FGF system has been the use of a dominant negative receptor construct introduced by Kirschner and colleagues. Overexpression of this construct can abolish the FGF responsiveness of animal caps. In whole embryos, the absence of FGF signaling causes a reduction, although not a total ablation, of mesoderm formation. There is also a severe effect on axis formation in which formation of the posterior parts is reduced consequent on an inhibition of invagination and elongation of the dorsal mesoderm. Thus, the present evidence suggests that the FGF system contributes to, although is not solely responsible for, mesoderm induction in vivo. It is also necessary for normal gastrulation movements, particularly in the dorsal mesoderm, and is likely to have several later functions, particularly in development of the central nervous system and the myotomes. © 1994 Wiley-Liss, Inc.     

Vertical versus planar induction in amphibian early development

In the Urodeles, the archenteron roof invaginates as a single continuous sheet of cells, vertically inducing the neural anlage in the overlying ectoderm during invagination. The induction comprises first the activation process, leading, to forebrain differentiation tendencies, and then the superimposed transformation process, which changes presumptive forebrain development into that of hindbrain and spinal cord acting with a caudally increasing intensity. The activating action, being maximal anteriorly, decreases caudally to nearly zero. In the double-layered Xenopus embryo, the internal mesodermal marginal zone shows much more independent and earlier regional segregation and involution than the external marginal zone in the Urodeles; its prechordal mesoderm already initiating vertical neural induction in overlying ectoderm at stages 10 to 10⁺ before any visible archenteron invagination. In Xenopus incomplete exogastrulae the prechordal mesoderm involutes normally prior to evagination of the endoderm and mesodem. Artificially produced Xenopus total exogastrulae, made at stage 9 before mesoderm involution, behave just like axolotl total exogastrulae, showing no neural differentiation. The notion of planar neural induction in Xenopus can only be applied in exogastrulae and Keller explants for the transforming action, which is maximal in the caudal archenteron roof. In normal Xenopus development, the formation of the entire nervous system is essentially due to vertical induction by the successively involuting prechordal and notochordal mesoderm. The different behavior of Xenopus embryos in comparison with Urodele embryos can essentially be explained by the double-layered character of the animal moiety of the Xenopus embryo.     

非洲爪蟾眼的形态发生研究

详细观察和描述了非洲爪蟾Xenopus laevis眼的发生和发育变化过程,并分别对各发育时期视网膜的厚度进行了定量分析.非洲爪蟾眼的发牛开始于眼原基的形成,进而形成视泡;晶状体的发生是在视杯外壁增厚的同时诱导覆盖其上的胚胎外胚层内层增厚,形成预定晶状体板;在视网膜和晶状体共同诱导下,预定角膜上皮变为透明的角膜.在视杯出现之前,预定RPE的厚度由厚变薄,NR层不断地增厚直至结构功能完善.     

小鼠植入前胚胎致密化与囊胚形成的分子调控

哺乳动物胚胎植入前的发育中致密化和囊胚形成分别标志着第一次、第二次细胞分化（即细胞命运决定）的起始,是胚胎正常发育的必要条件。因此对影响致密化和囊胚形成的蛋白及调节因子的研究尤为重要。本文探讨了与致密化相关的细胞黏附蛋白、连接蛋白、细胞骨架等分子和囊胚形成相关的紧密连接蛋白、钠钾三磷酸腺苷激酶等分子的一系列调控,以及致密化和囊胚形成在细胞命运决定中的重要作用。     

Patterning of the avian intermediate mesoderm by lateral plate and axial tissues

Amniote kidney tissue is derived from the intermediate mesoderm (IM), a strip of mesoderm that lies between the somites and the lateral plate. While much has been learned concerning the later events which regulate the differentiation of IM into tubules and other types of kidney tissue, much less is known concerning the earlier events which regulate formation of the IM itself. In the current study, the chick pronephros was used as a model system to identify tissues that play a role in patterning the IM and the critical time periods during which such patterning events take place. Explant studies revealed that the prospective pronephric IM is already specified to express kidney genes by stage 6, shortly after its gastrulation through the primitive streak, and earlier than previously reported. Transplant and explant experiments revealed that the lateral plate contains an activity that can repress IM formation in tissues that are already specified to express IM genes. In contrast, Hensen's node can promote formation of IM in the lateral plate. Paraxial tissues (presomitic mesoderm plus neural plate and notochord) were found to influence the morphogenesis of the nephric duct, but did not induce IM tissue to an appreciable extent. Combining lateral plate and paraxial tissue in vivo or in vitro led to induction of IM genes in the paraxial mesoderm but not in the lateral plate mesoderm. Based on these results and those of others, we propose a two-step model for the patterning of the IM. While tissue is still in the primitive streak, the prospective IM is relatively uncommitted. By stage 6, shortly after cells leave the primitive streak, a field of cells is generate which is specified to give rise to IM (Step 1). Subsequently, competing signals from the lateral plate and axial tissues modulate the number of cells that commit to an IM fate (Step 2).     

Asters play only a dispensable role in the induction of the cleavage furrow in the blastomeres of early Xenopus embryos

Kuroda et al. (2001) of our laboratory have previously revealed that exposure of early Xenopus embryos to 150 mm urethane results in complete suppression of formation of the asters and the cleavage furrow, as well as significant reduction of the size of the spindle in the blastomeres, allowing only 1 or 2 cycles of mitosis but not cytokinesis. In the course of closer examination of the effect of urethane on the cleavage of blastomeres of early Xenopus embryos, we unexpectedly discovered that exposure of early Xenopus embryos to 75 mm urethane did not prevent cell division at all, though asters were not detected in the blastomeres. Instead, they contained a spindle that appeared rather normal. They also formed the diastema, a thin yolk-free structure, which is considered to play an essential role in the induction of the cleavage furrow. Essentially the same results were obtained in the exposure of embryos to vinblastine, a well-known microtubule inhibitor: exposure of embryos to 20 micro g/mL vinblastine resulted in complete suppression of cleavage of the blastomeres, where formation of both the spindle and asters were perfectly suppressed. By contrast, exposure of embryos to 5 microg/mL vinblastine did not prevent cleavage in the blastomeres though asters were not detected, whereas the rather normal spindle was formed. Thus, there was a close correlation between the formation of the normal spindle, not asters, and that of the cell division furrow and the diastema in the blastomeres of early Xenopus embryos. We suggest that while the spindle plays an essential role, asters are likely to play only a dispensable role in the induction of the cleavage furrow in even very large cells like the blastomeres of early Xenopus embryos.     

Aggregation of crystallins induced by pulsed laser UV light (308 nm)

Here we compile and analyze the data on photoaggregation of a model protein carboanhydrase and the main eye lens proteins α-, β-, γ-crystallins under the action of pulsed UV irradiation from a Xe-Cl laser (308 nm) with broad variation of pulse energy density and repetition rate. The aggregation efficacy proves to be a nonlinear function of these parameters and protein concentration. A theoretical model is proposed that qualitatively explains the experimental data. It is shown that N-arm-truncated βA3-crystallin is more prone to UV-induced aggregation than the full-sized protein; such defects caused by mutation or aging may aggravate the development of lenticular opacity. Analyzed is the effect of some low-molecular compounds on the aggregation of β-crystallin and its mixture with α-crystallin. A combination of short peptides prepared on this basis markedly impedes crystallin aggregation and retards the development of UV-induced cataract in rats.