Search

Retinoid receptors, which are members of the nuclear hormone receptor superfamily, act as ligand-dependent transcription factors. They mediate the effects of retinoic acid primarily as heterodimers of retinoic acid receptors (RARs) and retinoid X receptors (RXRs). To analyse their function, xRXR beta synthetic mRNA was injected into Xenopus embryos in combination with normal and mutated xRAR alpha transcripts. Two informative phenotypes are reported here. Firstly, over-expression of xRXR beta with xRAR alpha results in the formation of ectopic primary neurons. Secondly, blocking retinoid signalling with a mutated xRAR alpha results in a lack of primary neurons. These two phenotypes, from contra-acting manipulations, indicate a role for retinoid signalling during neurogenesis.

In the liver and the yolk sack of 6–12 week postfertilization human embryos, we have detected RNA transcripts from the following apolipoprotein genes: AI, AII, B, CII, CIII and E. The mRNA from the apolipoprotein CIII gene was relatively more abundant in the total RNA from the yolk sack than in that from the liver. The gut and adrenals contained transcripts of all these apolipoprotein genes apart from apolipoprotein AIL The kidneys and heart contained some apolipoprotein transcripts. In conjunction with previous studies, these results suggest that in the human embryo apolipoprotein genes are transcribed in a much larger range of organs than is the case in the adult. Many of these organs lack endoderm tissues.

We have investigated the interactions between mesoderm and ectoderm that result in the formation of a regionally differentiated nervous system in Xenopus embryos. We have used genes expressed at different positions along the neural tube as regional markers of neural induction in both whole, and in experimentally manipulated embryos. By comparing transcription from the anterior marker, XIF3, with that from the posterior marker, XIHbox6, and the general neural marker XIF6, we have shown that the normal induction process requires interactions between ectoderm and mesoderm that persist through gastrulation into the late neurula stages. We have found that competence of the ectoderm to respond to induction is lost at the same early neurula stage for all three marker genes. Using rhodamine dextran-labelled mesoderm, we have established that the duration of contact between ectoderm and mesoderm required for gene activation in conjugates is the same for each of the markers. We have, however, identified regions of the mesoderm that can induce different combinations of neural marker gene expression. The anterior mesoderm induces expression of the anterior marker, XIF3, and the later migrating posterior mesoderm induces the ectoderm overlying it to express the posterior marker XIHbox6. It has been proposed that neural inducing signals reach the ectoderm by two different routes: from mesoderm lying directly beneath the ectoderm or along the plane of the ectoderm. We have assessed the contribution of each route in respect of our three neural markers and find that a signal passing directly from mesoderm to ectoderm fully accounts for neural gene expression. We were unable to detect an inducing signal that passes along the plane of the ectoderm.

A full-length cDNA clone for the Xenopus intermediate filament gene XIF3 has been isolated. It is very similar in sequence to the rat intermediate filament cDNA clone 73 that is thought to encode the neuronal intermediate filament protein ‘peripherin’. By analysing dissected embryos, we show that XIF3 is expressed predominantly in anterior and dorsal structures and most strongly in the brain of the tailbud (stage 26) embryo. In situ hybridization shows XIF3 transcripts to be localized in neural tissue and especially in regions that most probably correspond to the motor neurones of the neural tube and to some cranial nerve ganglia. New XIF3 transcripts are first found at the start of gastrulation at a low level throughout the ectoderm and are not localized to the presumptive neurectoderm. Expression subsequently increases by about 10-fold in neural tissue, and requires an interaction of the mesoderm with overlying ectoderm. Because new transcripts are found predominantly in neural tissue of the head, this response can be used as a marker of anterior neural induction.

A hamster vimentin cDNA probe has been used to isolate and characterize three Xenopus laevis intermediate filament genes, named XIF1, XIF3 and XIF6. Of these, XIF6 shows 89 % homology at the amino acid level to a portion of porcine neurofilament-M. XIF6 is transcribed solely in nervous tissue of embryos, commencing at the late neural tube stage. Expression is totally dependent on an interaction between mesoderm and ectoderm during gastrulation and can be used as a marker of neural induction. XIF1 shows 94 % homology and XIF3 83 % homology to hamster vimentin at the amino acid level over a region of the protein. Although XIF1 and XIF3 show more homology to vimentin than to any other intermediate filament gene, they have distinct temporal and spatial patterns of expression. XIF1 expression most resembles that of vimentin in higher vertebrates, being expressed in embryonic myotome and nerve cord, whilst XIF3 is unusual in that its expression is restricted predominantly to the head in tailbud embryos.

Immunocytochemical studies using a monoclonal antiporcine vimentin antibody reveal a well-organized pattern of staining in Xenopus laevis oocytes, eggs and early embryos. The positions of Xenopus vimentin and desmin in two-dimensional (2D) polyacrylamide gels were first established by immunoblotting of muscle Triton extracts with anti-intermediate filament antibodies (anti-EFA), which cross-react with all intermediate filament proteins (IFPs). The anti-porcine vimentin reacts with vimentin and desmin in muscle 2D immunoblots, but only reacts with one polypeptide in oocyte blots in the position predicted for vimentin (Mr55×103, pl 5·6). Using an anti-sense probe derived from a Xenopus vimentin genomic clone in RNase protection assays, we show that expression of vimentin begins in previtel-logenic oocytes. The level of expression remains constant throughout oogenesis and in unfertilized eggs. These data suggest that vimentin is expressed in oocytes and eggs. Most interestingly, the immunocytochemical results also show that vimentin is present in the germ plasm of oocytes, eggs and early embryos. It is therefore possible that vimentin has an important role in the formation or behaviour of early germ line cells.