Cytoplasmic distributions of translatable messenger RNA species and the regulation of patterns of protein synthesis during sea urchin embryogenesis

Stored mRNA in early embryos of a fern Marsilea vestita: a paternal and maternal origin

In early embryos of the fern Marsilea vestita, we have shown that transcription did not begin until the 8-cell stage, even though protein synthesis was taking place. As in animal species, the pre-existence of stored maternal mRNA has been suspected in embryonic cells. To investigate the presence of such long-lived mRNA we have used in situ hybridization method using [3H]polyuridylic acid, [3H]poly(U), as a probe. Temporal and spatial changes in the distribution of poly(A)+ RNA were followed throughout all the different phases of embryogenesis. Poly(A)+ RNA was present in the cytoplasm of both unfertilized and fertilized eggs and early embryos, while the nucleus exhibited no, or a moderate, [3H]-poly(U) binding activity. New poly(A)+ RNA was detected in the nucleus only at the 16-cell stage, when cytological differentiation of the root apical cell became morphologically detectable. After in situ hydridization with the labeled probes, the numerous silver grains detected autoradiographically over the mature sperm, indicate the presence of poly(A)+ RNA molecules associated with the male genome. We discuss the possible role of poly(A)+ RNA of maternal origin in supporting early translation prior to synthesis of new mRNA. Additional studies are needed to elucidate the role of stored paternal mRNA.

Functions of maternal mRNA in early development

In this review, the types of mRNAs found in oocytes and eggs of several animal species, particularly Drosophila, marine invertebrates, frogs, and mice, are described. The roles that proteins derived from these mRNAs play in early development are discussed, and connections between maternally inherited information and embryonic pattern are sought. Comparisons between genetically identified maternally expressed genes in Drosophila and maternal mRNAs biochemically characterized in other species are made when possible. Regulation of the meiotic and early embryonic cell cycles is reviewed, and translational control of maternal mRNA following maturation and/or fertilization is discussed with regard to specific mRNAs.

Gastrulation in the sea urchin is accompanied by the accumulation of an endoderm-specific mRNA

Spatial diversification of the endoderm during gastrulation in the sea urchin Lytechinus variegatus was examined with an endoderm-specific cDNA clone. This cDNA clone, LvN1.2, was identified by a differential cDNA screen between the ectoderm and endoderm/mesoderm fractions from prism stage embryos. The LvN 1.2-kb mRNA was first detectable by Northern blots at the mesenchyme blastula stage just prior to gastrulation and then accumulated approximately 15-fold from gastrulation to the pluteus stage. In situ hybridization analysis demonstrated that the mRNA accumulated specifically in endoderm and was restricted to the hindgut-midgut regions. This restricted localization was apparent during gastrulation and predicted the morphological distinction between foregut and midgut eventually seen at prism and pluteus stages. Sequence analysis showed that the 189-amino acid open reading frame represented a novel protein. In vitro translation of synthetically produced LvN1.2 mRNA and Western blot analysis with antibodies to the protein sequence yielded the same 25-kDa polypeptide on SDS-PAGE. The LvN1.2 protein resided within discrete granules of the hindgut and midgut cells. These particles were concentrated to the luminal aspect of the cells, suggesting the LvN1.2 protein participates in the digestive function of this region of the gut.

Expression of carbonic anhydrase II gene in early brain cells as revealed by in situ hybridization and immunohistochemistry

A mouse carbonic anhydrase (CA II) complementary(c) DNA probe was used for in situ hybridization on mouse brain cultured cells in order to follow CA II gene expression during brain development. An improved method was established using biotinated probes that resulted in a high sensitivity and an absence of background; this method could be combined with immunohistochemistry. Hypothalamic cells of embryonic day (ED) 12-14 mice were cultured for various periods. Chronologic appearance of CA II messenger(m)RNA and protein was studied. The CA II gene transcripts are detectable as early as ED 12-13, although the protein they encode is not detectable until ED 17-18. Gene expression is restricted to 0.1% of the total population. Northern blot analysis confirmed the presence of CA II transcripts in embryonic hypothalamus. At postnatal stage, the majority of glial cells express both the CA II mRNA and the protein. Our results favour the early appearance of a glial lineage in a precise area of the developing CNS. The precocity of CA II gene transcription makes in situ hybridization an invaluable approach in defining the onset of nerve cell lineages during embryonic development.

Multiple levels of regulation of tubulin gene expression during sea urchin embryogenesis

The expression of the tubulin genes during embryogenesis of the sea urchin Lytechinus pictus has been analyzed. Single strand tracer excess titrations of alpha- and beta-tubulin mRNA and RNA gel blot hybridizations indicate that tubulin mRNA remains at a constant 1.3 X 10(5) transcripts per embryo during cleavage stages, increases during ciliogenesis shortly before hatching (12 hr PF), declines until midgastrula (30-35 hr PF), and then gradually increases 3-fold to about 6 X 10(5) per pluteus larva (72 hr PF). Tubulin synthesis changes in concert with its mRNA, except that during cleavage the relative rate of tubulin synthesis increases without a corresponding increase in tubulin mRNA abundance. The relative rates of tubulin gene transcription were assayed by a run-on assay in isolated nuclei. The synthesis of alpha- or beta-tubulin RNA results in little supplementation of maternal tubulin RNA during cleavage stages, but the rate increases at least 18-fold during ciliogenesis and then gradually decreases thereafter. The accumulation of tubulin mRNA after gastrulation can be accounted for by an ontogenetic increase in tubulin RNA stability, assayed by actinomycin D chase and RNA gel blot hybridization. The rates of synthesis, stabilities, and abundances of alpha- and beta-tubulin mRNAs were similar, suggesting coordinate regulation. These observations indicate the importance of translational regulation during cleavage, transcriptional regulation during ciliogenesis, and regulation of mRNA stability by the level of unpolymerized tubulin during later development.

Coordinate accumulation of five transcripts in the primary mesenchyme during skeletogenesis in the sea urchin embryo

The sea urchin larval skeleton is produced by the primary mesenchyme (PM), a group of 32 cells descended from the four micromeres of the 16-cell embryo. The development of this lineage proceeds normally in isolated cultures of micromeres. A complementary DNA (cDNA) library was generated from cytoplasmic polyadenylated RNA isolated from differentiated micromere cultures of Strongylocentrotus purpuratus. Five clones were selected on the basis of their enrichment in differentiated PM cell RNA as compared to the polyribosomal RNAs of other embryonic cell types and other developmental stages. Each cloned cDNA hybridized to a distinct RNA that was abundant in the polyribosomes of differentiated PM cells, but absent from larval ectoderm and from 16-cell embryos. These RNAs were encoded by single or low copy genes. In situ hybridization analysis of the most abundant of these RNAs (SpLM 18) demonstrated that it was specifically limited to the skeletogenic PM of intact embryos. During the development of the PM, all five RNAs exhibited the same schedule of accumulation, appearing de novo, or increasing abruptly just before PM ingression, and remaining at relatively high levels thereafter. This pattern of RNA accumulation closely paralleled the pattern of synthesis of PM-specific proteins in general (Harkey and Whiteley, 1983) and of the SpLM 18-encoded protein specifically (Leaf et al., 1987). These results indicate that at least five distinct genes in the sea urchin, each of which encodes a PM-enriched or PM-specific mRNA, are expressed with tight coordination during development of the larval skeleton. They also demonstrate that expression of these genes in the PM is regulated primarily at the level of RNA abundance rather than RNA utilization.

Post-transcriptional restriction of gene expression in sea urchin interspecies hybrid embryos

The synthesis of many paternal species-specific proteins is reduced in all stages of sea urchin interspecies hybrid embryos, due to the reduced amounts of some paternal mRNA species in hybrid embryos compared with the embryos of the paternal species (Tufaro and Brandhorst 1982). Possible explanations for this restriction were tested. Cloned cDNAs were selected that were specific for paternal RNA sequences having reduced amounts (to 2-20% of normal) in hybrid embryos derived from a cross of Stronglyocentrotus purpuratus eggs with Lytechinus pictus sperm. Several of these RNA species are barely detectable in the eggs, but they accumulate extensively (5- to 40-fold) during L. pictus embryogenesis. Thus, the restricted expression of these paternal genes in hybrid embryos is not the result of the persistence of stable maternal mRNA species stored in eggs and not replaced by zygotic transcription. The accumulation of some of these L. pictus transcripts is also reduced in the reciprocal cross (L. pictus eggs X S. purpuratus sperm); therefore, the full expression of these L. pictus genes in hybrid embryos is not dependent on species-specific maternal factors stored in the egg. The transcriptional activity of one such gene was estimated using a run-on assay in isolated nuclei; it is as actively transcribed in hybrid as it is in homospecific embryos, but in hybrid embryos the cytoplasmic transcript accumulates to only 2-15% of the normal level. Sequence analysis indicates that this gene encodes a metallothionein. Mechanisms are discussed that might account for the post-transcriptional restriction of expression of some genes in hybrid embryos.

Translational activation of maternal mRNA encoding the heat-shock protein hsp90 during sea urchin embryogenesis

Changes in protein synthesis induced by heat shock of Strongylocentrotus purpuratus gastrulae were analyzed bt two-dimensional electrophoresis. Hyperthermia induces the synthesis of polypeptides having molecular masses of 90, 70, 50, 40, and 38 kDa. One of these, hsp90, appears as a pair of polypeptides which comigrates with proteins synthesized at normal temperature in eggs and embryos; these comigrating spots produce indistinguishable patterns upon electrophoretic analysis of partial V8 protease digests, indicating that hsp90 is synthesized throughout embryogenesis. The relative rate of incorporation of methionine into hsp90 is low in eggs and zygotes, but increases abruptly in morulae, constituting a rare and striking change in protein synthesis during early development. Cell-free translation analyses indicate that most of the mRNA encoding hsp90 resides in the pool of free ribonucleoprotein particles in eggs and early embryos, but shifts to polysomes by the 64-cell stage while remaining constant in mass. Thus the increase in synthesis of hsp90 appears to be via the selective activation of translation of a stored maternal mRNA. The shift of hsp90 mRNA to polysomes is accompanied by polyadenylation. Heat shock of eggs or zygotes did not result in translational activation of hsp90 mRNA. The sea urchin hsp90 doublet of spots comigrates with hsp90 induced by heat shock of chicken embryo fibroblasts, a conserved protein abundant in many cells of a variety of species.