Differential expression of two cadherins in Xenopus laevis

Using a cadherin fraction from Xenopus tissue culture cells as an immunogen, two monoclonal antibodies were obtained that allowed the characterization of two distinct cadherins in the Xenopus embryo. The two cadherins differ in molecular weight, in their time of appearance during development and in their spatial pattern of expression. One of the antigens was identified as E-cadherin. It appears in the embryonic ectoderm during gastrulation when epidermal differentiation commences and it disappears from the neural plate area upon neural induction. The second antigen could not be allocated to any of the known cadherin subtypes and was termed U-cadherin. It is present in the egg and becomes deposited in newly formed inner cell membranes during cleavage, the outer apical membranes of the embryo remaining devoid of the cadherin throughout development. U-cadherin is found on membranes of all cells up to the late neurula stages. A conspicuous polarized expression of the antigen on the membranes of individual inner cells suggests its participation in the segregation of cell layers and organ anlagen. These findings are discussed in the context of current hypotheses on the role of cadherins in establishing the spatial structure of the embryo.     

Differential expression of two neural cell-specific beta-tubulin mRNAs during rat brain development.

We recently demonstrated that beta-tubulin mRNA expression is regulated during rat brain development. This is manifested by a dramatic decrease in both 1.8- and 2.9-kilobase (kb) mRNAs when extensive neurite elongation is occurring. Coincident with these decreases is the increased production of a 2.5-kb mRNA. (J.F. Bond and S.R. Farmer, Mol. Cell. Biol. 3:1333-1342, 1983). In the present study, we have isolated and characterized three different cDNAs corresponding to beta-tubulin mRNAs (R beta T.1, R beta T.2, and R beta T.3). Hybridization of 3' untranslated region subclones of R beta T.1 and R beta T.2 cDNAs to RNA from a variety of rat tissues and cells revealed that these two cDNAs are neural cell specific. R beta T.1 corresponds to an abundant 1.8-kb mRNA expressed only at early stages of rat brain development. R beta T.2 corresponds to the 2.5-kb mRNA expressed at later stages. These data strongly suggest that there is differential expression of the beta-tubulin multigene family during rat brain development.     

Regulated gene expression of hyaluronan synthases during Xenopus laevis development

Here reported is the developmental gene expression pattern of the three known vertebrate hyaluronan synthases (XHas1, XHas2 and XHas3) and a comparative analysis of their mRNAs spatio-temporal distribution during Xenopus laevis development. We found that while XHas2 shows a steady-state expression from gastrula to late tailbud stage, XHas1 is mainly present in the early phases of development while XHas3 is predominantly transcribed in tailbud embryos. XHas1, XHas2 and XHas3 show distinct tissue expression patterns. In particular, XHas1 is localized in ectodermal derivatives and in cranial neural crest cells, whereas XHas2 is mainly found in mesoderm-derived structures and in trunk neural crest cells. Moreover, the expression pattern of XHas2 overlaps that of MyoD in cells committed to a muscle fate. Unlike the other hyaluronan synthases, XHas3 mRNA distribution is very restricted. In particular, XHas3 is expressed in the otic vesicles and closely follows the inner ear development. In conclusion, XHas1, XHas2 and XHas3 mRNAs have distinct and never overlapping spatial expression domains, which would suggest that these three enzymes may play different roles during embryogenesis.     

Expression of muscle LIM protein during early development in Xenopus laevis

We have isolated the Xenopus homologue of Muscle LIM protein (MLP, CRP3) and examined its expression during early embryonic development. MLP is only expressed in the differentiated heart during early development and is expressed in a subset of other striated muscles during later stages. There is no MLP expression during primary myogenesis in the somites, although it is found in adult skeletal muscle.     

Differential gene expression in the anterior neural plate during gastrulation of Xenopus laevis

We have isolated three cDNA clones that are preferentially expressed in the cement gland of early Xenopus laevis embryos. These clones were used to study processes involved in the induction of this secretory organ. Results obtained show that the induction of this gland coincides with the process of neural induction. Genes specific for the cement gland are expressed very early in the anterior neural plate of stage-12 embryos. This suggests that the anteroposterior polarity of the neural plate is already established during gastrulation. At later stages of development, two of the three genes have secondary sites of expression. The expression of these genes can be induced in isolated animal caps by incubation in 10 mM-NH4Cl, a treatment that is known to induce cement glands.     

Differential stem cell contributions to thymocyte succession during development of Xenopus laevis.

The contribution of two embryonic stem cell compartments to the developing thymus in the amphibian Xenopus was examined throughout the larval, postmetamorphic, and adult periods. Hematopoietic chimeras were produced by transplanting either the ventral blood islands (VBI) or the dorsal stem cell compartment (DSC) from diploid donors onto triploid hosts. The DNA content of isolated nuclei harvested from the thymus and circulating E populations was analyzed using propidium iodide staining and flow cytometry. The DNA content of mitotic figures derived from PHA reactive splenocytes was analyzed using the Feulgen reaction and microdensitometry. These data suggested that both the VBI and DSC contribute to the thymocyte populations from the earliest developmental stages examined. Moreover, the contribution of both stem cell compartments was cyclic. However, the periods of these cycles were different. Both VBI- and DSC-derived cells entered the thymus 4 days postfertilization. VBI-derived thymocytes were at a minimum at 28 days postfertilization, reached a maximum at 35 days postfertilization and a second minimum at 42 days postfertilization. However, DSC-derived cells reached a maximum at 28 days, a minimum at 35 days, and a second maximum at 42 days. The PHA-reactive splenocyte population followed a similar temporal pattern. In contrast, the VBI-derived E population was at a maximum during early development and steadily declined throughout the larval period. DSC-derived E were undetectable during early development but steadily increased throughout the larval period. Both VBI- and DSC-derived hematopoietic cells persisted after metamorphosis and contributed to all populations examined in adult frogs. Because of temporal differences in the VBI and DSC contributions to the developing thymus, these data suggest heterogeneity within the thymocyte population associated with the embryonic origin of the colonizing stem cells.     

Differential expression of creatine kinase isozymes during development of Xenopus laevis: An unusual heterodimeric isozyme appears at metamorphosis

The creatine kinase (CK) repertoire of Xenopus laevis, which is more complex than that of most other vertebrates, involves at least four genomic loci, all showing developmental and tissue-specific expression. The differential expression of this multilocus CK isozyme system was investigated by immunohistology. Specific monoclonal antibodies (mAb) against the three cytoplasmic CK isozymes of Xenopus laevis were isolated and characterized. Two of these mAbs, anti-CK-IV (DM16) and anti-CK-III (JRM4), were specific for CK-IV and CK-III subunits respectively, as well as for the corresponding homodimeric isozymes, CK-IV/IV and CK-III/III. Anti-CK-II (MRX7) mAb recognizes CK-II subunits and CK-II/III heterodimers; the homodimeric CK-II/III does not occur. Immunohistological localization on larval and adult tissue sections reveals that CK-IV epitopes, beside a generalized tissue distribution, are especially concentrated in the cytoplasm of some particular cells such as the photoreceptors in the outer segment of the retina, certain nerve cells of the spinal cord and spinal ganglia, and in larval hepatocytes. The CK-III III isozyme is specifically expressed in skeletal muscle, its appearance and accumulation occurring in parallel with myoblast differentiation. The CK-II antigen is detected first at the time of metamorphosis is skeletal muscles, as well as in the heart, eyes and brain. In striated musculature the expression of CK-II subunits during metamorphosis results in almost complete replacement of CK-III/III homodimers by CK-II/III heterodimers, as indicated by the progressive masking of CK-III epitope and the corresponding appearance of CK-II antigen. In the adult eyes, CK-II antigens localize at the same particular site of photoreceptors as do CK-IV antigens. Since that antigen represents a heterodimeric CK-II/III isozyme, this implies the activation of both CK-II and CK-III genes, none of which is expressed in larval retina.     

The distribution of E-cadherin during Xenopus laevis development

A vast amount of experimental evidence suggests that cell surface molecules involved in cell-to-cell and/or cell-to-substrate interactions participate in the control of basic events in morphogenesis. E-cadherin is a cell adhesion molecule directly implicated in the control of Ca2(+)-dependent interactions between epithelial cells. We report here the patterns of expression of E-cadherin in developmental stages of Xenopus laevis ranging from early embryo to adult using immunofluorescence microscopy. Although its distribution shares some similarities with those of L-CAM in the chicken and E-cadherin/Uvomorulin in the mouse, the distribution of E-cadherin in Xenopus presents several peculiar and unique features. In early stages of Xenopus development, E-cadherin is not expressed. The molecule is first detectable in the ectoderm of late gastrulas (stage 13-13.5 NF). At this time both the external and the sensory layer of the nonneural ectoderm accumulate high levels of E-cadherin while the ectoderm overlying the neural plate and regions of the involuting marginal zone (IMZ) not yet internalized by the movements of gastrulation are E-cadherin-negative. Unlike most other species, endodermal cells express no or very low levels of E-cadherin up to stage 20 NF. Endodermal cells become strongly E-cadherin-positive only when a well-differentiated epithelium forms in the gut. No mesodermal structures are stained during early development. In the placodes, in contrast to other species, E-cadherin disappears very rapidly after placode thickening. During further embryonic development E-cadherin is present in the skin, the gut epithelium, the pancreas, many monostratified epithelia and most glands. Hepatocytes are stained weakly while most other tissues, including the pronephros, are negative. In the mesonephros, the Wolffian duct and some tubules are positive. During metamorphosis a profound restructuring of the body plan takes place under the control of thyroid hormones, which involves the degeneration and subsequent regeneration of several tissues such as the skin and the gut. All newly formed epithelia express high levels of E-cadherin. Surprisingly, degenerating epithelia of both skin and intestine maintain high levels of the protein even after starting to become disorganized and to degenerate. In the adult, staining is strong in the skin, the glands, the lungs, the gut epithelium and the pancreas, weak in the liver and absent from most other tissues. Our results show that the expression of E-cadherin in Xenopus is strongly correlated with the appearance of differentiated epithelia.