Differences in abundance of individual RNAs in normal and animalized sea urchin embryos

Expression of spicule matrix protein gene SM30 in embryonic and adult mineralized tissues of sea urchin Hemicentrotus pulcherrimus

We have isolated a cDNA clone for spicule matrix protein, SM30, from sea urchin Hemicentrotus pulcherrimus and have studied the expression of this gene in comparison with that of another spicule matrix protein gene, SM50. In cultured micromeres as well as in intact embryos transcripts of SM30 were first detectable around the onset of spicule formation and rapidly increased with the growth of spicules, which accompanied accumulation of glycosylated SM30 protein(s). When micromeres were cultured in the presence of Zn2+, spicule formation and SM30 expression were suppressed, while both events resumed concurrently after the removal of Zn2+ from the culture medium. Expression of SM50, in contrast, started before the appearance of spicules and was not sensitive to Zn2+. Differences were also observed in adult tissues; SM30 mRNA was detected in spines and tube feet but not in the test, while SM50 mRNA was apparent in all of these mineralized tissues at similar levels. These results strongly suggest that the SM30 gene is regulated by a different mechanism to that of the SM50 gene and that the products of these two genes are differently involved in sea urchin biomineralization. A possible role of SM30 protein in skeleton formation is discussed.

Developmental variability of metallothionein Mtn gene expression in the species of the Drosophila melanogaster subgroup

Developmental expression of the Drosophila melanogaster metallothionein Mtn gene has been analysed. Transcripts of this gene accumulate during the vitellogenic phase of oogenesis in a ring of follicular cells at the oocyte-nurse cell margin and in the follicular cells surrounding the oocyte. There is also strong expression of the Mtn gene during the second half of embryogenesis in hemocytes, the endoderm midgut, and Malpighian tubules. A banded expression pattern is observed transiently in the midgut at stage 13. The two Mtn alleles, Mtn and Mtn, show quantitative differences in their expression patterns. Copper intoxication of flies does not induce ectopic expression of the Mtn gene, but rather leads to over-expression of the gene in the structures where it is normally transcribed. Mtn transcription is not altered in homozygous mutants of four genes (lab, wg, dpp, bap) known to be involved in midgut morphogenesis. Expression of Mtn has been also studied in six other species of the melanogaster subgroup. This analysis demonstrates that regulation of Mtn gene transcription has changed during evolution of the Drosophila lineage. For example, Mtn is expressed specifically in the Malpighian tubules of D. melanogaster, while in D. mauritiana and D. sechellia the amnioserosa is a specific location of expression. Nonetheless, expression of Mtn in the midgut is common to the seven species, suggesting a basic role for the MTN protein during embryogenesis in this organ, possibly in the release of metallic ions from vitellogenins. In contrast, two genes also expressed in the embryonic midgut, lab and dFRA, display identical patterns in all species of the melanogaster subgroup. The diversity of Mtn patterns in closely related Drosophila species exemplifies the rapid evolution of a gene regulatory system.

Polyubiquitin RNA characteristics and conditional induction in sea urchin embryos

A cDNA of the sea urchin Strongylocentrotus purpuratus was identified as encoding polyubiquitin and used to detect a single gene with transcripts containing multiple ubiquitin coding units. Polyubiquitin transcripts exist as a 3.2-kb RNA in polyribosomes and as three higher molecular weight RNAs in purified nuclei. The amount of polyubiquitin RNA is essentially constant at 10(4) -10(5) transcripts per embryo during the egg-to-blastula period and then declines during further development. Heat shock elicits a transient increase in the level of polyubiquitin RNA, while Zn(II) ions induce a sustained accumulation, that is influenced by developmental parameters: One round of Zn(II) induction elicits the accumulation of the nuclear 7.6- and 5.6-kb RNAs, as well as the 3.2-kb polysomal RNA; however, a second round of induction yields only the 5.6- and 3.2-kb RNAs, suggestive of a change in pre-mRNA size or processing. Polyubiquitin RNA is expressed equally in ectodermal and mesoendodermal tissues and is induced in both tissue fractions by treatment of pluteus larvae with Zn(II). However, in isolated and cultured tissue fractions, polyubiquitin RNA is not inducible by Zn(II), in contrast to the full inducibility of metallothionein mRNAs. Polyubiquitin RNA induction thus appears to be conditioned by the integrity of the embryo, as well as by previous exposure to inducer.

Synonymous nucleotide substitution rates of beta-tubulin and histone genes conform to high overall genomic rates in rodents but not in sea urchins

Sea urchin and rodent genomes have been posited to evolve rapidly as indicated by divergences in single copy nuclear DNA sequences. We have examined whether the synonymous substitution rates of three highly conserved genes, beta-tubulin, histone H4, and histone H3, adhere to these high genomic substitution rates by comparing sequences between two sea urchins, Strongylocentrotus purpuratus and Lytechinus pictus, and between rodents and humans. Whereas the rate of change between the 3' untranslated regions of the beta-tubulin cDNA of S. purpuratus (Sp-beta 1), sequenced in this study, and of L. pictus (Lp-beta 3) was consistent with the overall rate of change estimated from previous DNA hybridization results between these species, the synonymous substitution rates for the carboxyl domains of these beta-tubulins, as well as for the late histones H4 and H3, were significantly depressed. In contrast, synonymous nucleotide substitution rates between rodents and between rodent and human for the carboxyl domain proper of identical beta-tubulin isotypes and for histone H4 and H3.1 did not differ from the overall rate of change for the rodent genomes. Moreover, an analysis of paralogous human and mouse beta-tubulin sequences supported the conclusion that the synonymous substitution rates in the mouse were higher than those in the human. Differences in constraint on evolutionary change were not evident strictly from the conserved amino acid sequences and base compositions of these genes. Other constraining influences seemed more relevant to the departure of the synonymous substitution rates of the sea urchin beta-tubulin and histone coding regions from the average genomic rate.

Spec3: embryonic expression of a sea urchin gene whose product is involved in ectodermal ciliogenesis

We have characterized the temporal and spatial expression of Spec3 mRNA in embryos of the sea urchin, Strongylocentrotus purpuratus. This mRNA, 2.0 kb in length, is present at low levels in unfertilized eggs but accumulates rapidly during cleavage, increasing 50-fold by hatching blastula stage. Message levels then decline abruptly, remain constant during mesenchyme blastula and gastrula stages, and increase again during prism and pluteus stages. This accumulation pattern is quite similar to that of the ectodermally expressed beta-tubulin mRNAs described recently by Harlow and Nemer (1987a). In situ hybridization shows that although Spec3 message accumulates in all blastomeres at early blastula stages, it later becomes restricted to ectoderm. By late blastula stage, hybridization is strongest in the animal hemisphere. At gastrula, signals are variable over ectoderm, and by pluteus, grains are concentrated in the ciliary band, though present in other ectodermal cells as well. Deciliation and regeneration of cilia in gastrula-stage embryos results in a four- to fivefold increase in Spec3 mRNA levels, implying that the Spec3 gene product is associated with ciliogenesis. Spec3 mRNA is encoded by a single gene in the haploid genome, and characterization of the gene shows that it contains three exons that encode an open reading frame for a hydrophobic protein of 21.6 kD. The reading frame reveals that the carboxy-terminal part of the protein contains two long hydrophobic stretches, 31 and 37 residues long, separated by short hydrophilic regions of six to eight residues. The presence of these two distinct hydrophobic stretches suggests that the Spec3 protein contains two alpha-helical domains that either span the lipid bilayer or are associated with some other hydrophobic environment.

Metallothionein genes MTa and MTb expressed under distinct quantitative and tissue-specific regulation in sea urchin embryos

Sea urchin embryo metallothionein (MT) mRNAs MTa and MTb have distinct cDNA sequences and are transcripts of different genes of a multigene family. These MT mRNAs differ in size and in their 3'-untranslated sequences. They encode proteins that are unusual among MT isotypes in that the relative positions of their cysteine residues are partially out of register, suggesting potential differences in function. In pluteus larvae MTa mRNA is expressed abundantly and exclusively in the ectoderm, while MTb mRNA, which is restricted to the endomesoderm at a low endogenous level, can be induced to a high level by heavy metal ions (M2+). MT mRNA is present in the maternal reservoir of the egg and is predominantly (greater than 95%) MTa mRNA. Endogenous expression in the embryo, which is at a much higher level than in the egg, requires M2+ for gene transcription, is developmentally regulated, and is greater than 90% MTa mRNA. When induced by added M2+, however, MTa and MTb mRNAs accumulate to almost equal levels. The differences in the ratios of MTa/MTb expressed endogenously and inductively are not attributable to differences in the stabilities of these MT mRNAs, which were observed under conditions of M2+ depletion, or in their inducibilities, which were observed at moderate to high M2+ levels. We found, instead, that the MTa gene responds to M2+ at a lower threshold level than MTb, so that at very low M2+ concentrations the ratio of induced MTa/MTb mRNA is high and equivalent to the endogenous ratio. Thus, endogenous expression of the MTa gene is selectively enhanced in the ectoderm by determinants that are responsive at low M2+ threshold concentrations.

Developmental and tissue-specific regulation of beta-tubulin gene expression in the embryo of the sea urchin Strongylocentrotus purpuratus

Four beta-tubulin mRNAs in the embryo of the sea urchin Strongylocentrotus purpuratus are transcribed from at least 3 of the 9-12 beta-tubulin genes. A beta 1 tubulin mRNA of 1.8 kb, transcribed from a unique beta 1 gene, is expressed with high specificity in the pluteus ectoderm. Another 1.8-kb mRNA, beta 2, and a 2.5-kb beta 3 mRNA are moderately ectoderm specific. In contrast, a 3.0-kb beta 4 mRNA is highly specific for the endomesoderm tissue fraction. Certain similarities in developmental and tissue-specific expression suggest that these beta-tubulin genes may be related in their mode of regulation to counterparts among the genes for actin, another cytoskeletal protein. Measurements of absolute amounts revealed a distinct developmental profile for each beta-tubulin mRNA. An increase in the total amount of beta-tubulin mRNA in the early blastula was correlated with an increase in transcription rate per nucleus; whereas, later in the mesenchyme blastula stage, the beta-tubulin mRNA level decreased sharply as the rate of beta-tubulin gene transcription on a per embryo basis remained constant. Thus, during development through the blastula stages, there was a switch to a predominantly posttranscriptional regulation of beta-tubulin mRNA expression, probably through a decrease in mRNA stability.

Metallothionein genes MTa and MTb expressed under distinct quantitative and tissue-specific regulation in sea urchin embryos

Sea urchin embryo metallothionein (MT) mRNAs MTa and MTb have distinct cDNA sequences and are transcripts of different genes of a multigene family. These MT mRNAs differ in size and in their 3'-untranslated sequences. They encode proteins that are unusual among MT isotypes in that the relative positions of their cysteine residues are partially out of register, suggesting potential differences in function. In pluteus larvae MTa mRNA is expressed abundantly and exclusively in the ectoderm, while MTb mRNA, which is restricted to the endomesoderm at a low endogenous level, can be induced to a high level by heavy metal ions (M2+). MT mRNA is present in the maternal reservoir of the egg and is predominantly (greater than 95%) MTa mRNA. Endogenous expression in the embryo, which is at a much higher level than in the egg, requires M2+ for gene transcription, is developmentally regulated, and is greater than 90% MTa mRNA. When induced by added M2+, however, MTa and MTb mRNAs accumulate to almost equal levels. The differences in the ratios of MTa/MTb expressed endogenously and inductively are not attributable to differences in the stabilities of these MT mRNAs, which were observed under conditions of M2+ depletion, or in their inducibilities, which were observed at moderate to high M2+ levels. We found, instead, that the MTa gene responds to M2+ at a lower threshold level than MTb, so that at very low M2+ concentrations the ratio of induced MTa/MTb mRNA is high and equivalent to the endogenous ratio. Thus, endogenous expression of the MTa gene is selectively enhanced in the ectoderm by determinants that are responsive at low M2+ threshold concentrations.

An altered series of ectodermal gene expressions accompanying the reversible suspension of differentiation in the zinc-animalized sea urchin embryo

Early stage treatment of the sea urchin embryo with zinc ions is known to prevent its gastrulation. The treated embryo, termed "animalized" and classically regarded as a permanent blastula with possibly exaggerated ectodermal differentiation, can be viewed, instead, as being in a state of reversibly suspended differentiation. This proposition is supported by the following observations: (1) An embryo exposed to Zn2+ through its blastula stages and resuspended in fresh sea water retains the simple blastula morphology for at least 4 days; however, if the Zn2+ is also depleted by a chelator during this period, development resumes and reaches the pluteus stage. (2) A suppression of ectodermal differentiation in the zinc-animalized embryo can be inferred from the blockage of the developmental initiation of Spec 1 and CyIIIa actin mRNA accumulation, since the genes encoding them are specifically expressed in differentiated (aboral) ectoderm. (3) Chelation allows the zinc-blocked accumulation of these ectodermal mRNAs to proceed. The later the treatment with chelator, the more slowly these mRNA accumulations resume, and the longer the interval between them and the subsequent morphological differentiation. (4) The enhancement of some early ectodermal functions in the zinc-animalized embryo is indicated by the increased concentrations of mRNAs, encoded by a set of genes, Blast j1 and Spec 3, that normally display peak levels in the blastula. The association of these genes with ectoderm is based on their being specifically expressed, albeit at low levels, in the pluteus ectoderm, and their being suppressed when presumptive ectoderm is made to differentiate as endoderm in the case of the embryo treated with lithium. The program of cell division in the zinc-animalized embryo remains essentially normal. Differentiation becomes reversibly suspended, with the enhancement of certain early mRNA expressions and the reversible suppression of certain late mRNA expressions, characteristic of differentiated tissues.

Sea urchin metallothionein sequence: key to an evolutionary diversity

The metallothioneins (MTs) constitute a diverse family of proteins, which are enriched in cysteines and bind heavy metals. The amino acid sequence of sea urchin MT has been obtained from its mRNA sequence and compared with MT sequences of various sources. A largely conserved sequence of 10 amino acids, the "central segment," is located near the center of the MT molecules of Neurospora, yeast, and Drosophila and the center of putative domains in mammalian and sea urchin MTs. The sea urchin carboxyl-terminal-half MT resembles the mammalian 9-cysteine amino-terminal MT domain I, both in the presence of this central segment and in the relative placement of cysteine residues. Conversely, the sea urchin amino-terminal-half MT, containing 11 cysteines, resembles the mammalian carboxyl-terminal MT domain II in its exclusive enrichment in vicinal cysteines. The reversed order of these sea urchin and mammalian MT halves appears to be just one aspect of a diversity based on the elaboration of structures containing the central segment. Still another variation in this diversity is the duplication of the central segment, apparent in Drosophila and crab MTs.

Primary differentiation and ectoderm-specific gene expression in the animalized sea urchin embryo

Primary differentiation in sea urchin embryos, animalized by zinc, has been gauged by the formation of characteristic endodermal and mesodermal tissue derivatives and by the accumulation of the ectoderm-specific Spec 1 mRNA. Increasing the dosage of zinc diminishes the differentiation of secondary mesenchyme, primary mesenchyme, endoderm, and ectoderm, in decreasing order. Treatment is effective only during the blastula stages, involving successive periods of sensitivity for these tissues. Removal of zinc with chelator results in the resumption of differentiation to increasing degree for this series of tissues. The developmental initiation of Spec 1 gene expression, normally at the earliest blastula stage, can be delayed by zinc for at least 30 hr before being implemented by treatment of the animalized embryos with a chelator. We conclude (1) that those processes in the blastula which are required for differentiation and are suppressed by zinc are distinguishable from the determinative processes, which are not affected by the animalizing agent and occur earlier during midcleavage; (2) that animalization by zinc involves a graded failure of primary tissues to form; and (3) that animalization involves a pause in the schedule of differentiation, which can be reinstated by removal of the animalizing agent, thereby providing a survival value inherent in a flexible schedule of development.

Developmental regulation, induction, and embryonic tissue specificity of sea urchin metallothionein gene expression

Metallothionein (MT) is shown to be present in sea urchin embryos on the basis of its characteristic properties as a small protein (6-7 Da) of extraordinarily high cysteine content, whose biosynthesis is readily induced by heavy metals. Induction by Zn2+ results in the accumulation of the cysteine-rich MT protein, a 0.8 kb MT mRNA and a 2.9 kb nuclear RNA. The amount of MT mRNA is regulated intrinsically through the course of embryogenesis to the pluteus stage: A maternal MT mRNA is poly(A)-deficient and is polyadenylated after fertilization. New MT mRNA begins to accumulate between the seventh and eighth cell cleavage, reaches a maximum at the mesenchyme blastula stage, decreases during gastrulation, and rises again in the early pluteus stage. "Animalizing" embryos with Zn2+ during early embryogenesis causes a sustained accumulation of MT mRNA to levels greater than 25 times the normal amount. MT mRNA is present in high amount in the ectoderm of the pluteus, but is barely detectable in the mesoderm-endoderm tissue fraction. Treatment of either the pluteus or its isolated tissue fractions with Zn2+ results in the induction of MT mRNA accumulation in the mesoderm-endoderm but not in the already MT mRNA-enriched ectoderm. Furthermore, differences in Zn2+ induction of the MT gene in the blastula and gastrula are consistent with a developmental pattern in which MT gene expression is maintained constitutively at a high level in the ectoderm and at a low level in the mesoderm-endoderm tissues, which are, however, preferentially inducible by Zn2+.

Detection of mrnas in sea urchin embryos by in situ hybridization using asymmetric RNA probes

Asymmetric RNA probes, which contain only the mRNA coding strand, provide a large increase in hybridization efficiency in situ over that observed with either symmetric (both strands represented) RNA or DNA probes. Asymmetric RNA probes are synthesized in vitro by transcription from recombinants formed between sequences encoding sea urchin mRNAs and the transcription vector R7 delta 7. Using a probe representing early variant histone mRNA sequences we have characterized hybridization to sections of sea urchin embryos with respect to thermal stability of the hybrids formed, optimum temperature, effect of sequence divergence on hybrid thermal stability, and dependence of the hybridization signals on probe concentration and hybridization time. Estimates from the observed signals indicate that a large fraction of target RNAs is both retained in sections and hybridized with probe at saturation. Coupled with measurements of nonspecific background binding of heterologous probes, these data indicate that the method has sufficient sensitivity to detect many moderately abundant mRNAs (20-75 molecules per cell in the 1500-cell pluteus). In situ hybridizations to embryos at different developmental stages show that while histone mRNAs are uniformly distributed in cleaving embryos, different cell lineages of older embryos show large differences in accumulation of these mRNAs.