Spatial distribution of two maternal messengers in Paracentrotus lividus during oogenesis and embryogenesis

Maternal control of early patterning in sea urchin embryos

Sea urchin development has been studied extensively for more than a century and considered regulative since the first experimental evidence. Further investigations have repeatedly supported this standpoint by revealing the presence of inductive mechanisms that alter cell fate decisions at early cleavage stages and flexibility of development in response to environmental conditions. Some features indicate that sea urchin development is not completely regulative, but actually includes determinative events. In 16-cell embryos, mesomeres and macromeres represent multipotency, while the cell fate of most vegetal micromeres is restricted. It is known that the mature sea urchin eggs are polarized by the asymmetrical distribution of some maternal mRNAs and proteins. Spatially-distributed maternal factors are necessary for the orientation of the primary animal-vegetal axis, which is established by both maternal and zygotic mechanisms later in development. The secondary dorsal-ventral axis is conditionally specified later in development. Dorsal-ventral polarity is very liable during the early cleavages, though more recent data argue that its direction may be oriented by maternal asymmetry. In this review, we focus on the role of maternal factors in initial embryonic patterning during the first cleavages of sea urchin embryos before activation of the embryonic genome.

Novel morphological traits in the early developmental stages of Temnopleurus toreumaticus

We have re-observed in detail the development of the sea urchin species Temnopleurus toreumaticus, which is considered to be a typical indirect-developing species with a feeding larval stage. In this re-observation, we discovered two new morphological traits in the early embryonic stages of T. toreumaticus. The first trait is that, immediately after fertilization, the egg enters a stage in which wrinkles form on its surface as a result of actin polymerization. The second new trait is that the blastulae form wrinkles; in sea urchins, this has previously been known only in direct-developing species that have a nonfeeding larval stage and form wrinkles during the blastula stage, before hatching. These phenomena indicate that after fertilization, the egg of T. toreumaticus undergoes a surface transformation that is unprecedented in echinoderms, and that an indirect-developing sea urchin can form a wrinkled blastula.

Strongylocentrotus drobachiensis oocytes maintain a microtubule organizing center throughout oogenesis: Implications for the establishment of egg polarity in sea urchins

Although it has been known for over a century that sea urchin eggs are polarized cells, very little is known about the mechanism responsible for establishing and maintaining polarity. Our previous studies of microtubule organization during sea urchin oogenesis described a cortical microtubule-organizing center (MTOC) present during germinal vesicle (GV) migration in large oocytes. This MTOC was localized within the future animal pole of the mature egg. In this study we have used electron microscopy and immunocytochemistry to characterize the structure of this MTOC and have established that this organelle appears prior to GV migration. We show that the cortical MTOC contains all the components of a centrosome, including a pair of centrioles. Although a centrosome proper was not found in small oocytes, the centriole pair in these cells was always found in association with a striated rootlet, a structural remnant of the flagellar apparatus present in precursor germinal cells (PGCs). The centrioles/striated rootlet complex was asymmetrically localized to the side of the oocyte closest to the gonadal wall. These data are consistent with the previously proposed hypothesis that in echinoderms the polarity of the PGCs in the germinal epithelium influences the final polarity of the mature egg.

Maternal Paracentrotus lividus RNAs are differentially localized during the first cell division

In Paracentrotus lividus eggs, there are RNAs localized at the animal and vegetal poles. During the first cell division, some of these RNAs are associated with the mitotic spindle, whereas others are free in the cytoplasm. Among the RNAs bound to mitotic apparatus (MA), we have found the mitochondrial 16S rRNA. By immunohistochemistry we have also detected hsp60, a mitochondrial membrane protein, localized around the MA, suggesting that the entire mitochondria are associated with it. Western blotting of proteins prepared by cellular fractionation after detergent treatment of P. lividus eggs revealed that both hsp60 and cytochrome c are not associated with cytoskeletal elements. All the above data have been confirmed by immunoblot analyses of preparations of microtubules and MA in which the presence of hsp60 and cytochrome c were detected only in the MA fraction. Moreover, mitochondrial succinate dehydrogenase activity was determined in MA and cytoplasm fractions during the first cell division, and the localization and vitality of the organelles were also confirmed by in vivo staining with Mito red. A possible role for mitochondria in the asymmetric distribution of RNAs and in cell division is discussed.

Paracentrotus lividus eggs contain different RNAs at the animal and vegetal poles

Paracentrotus lividus eggs were divided by centrifugation into nucleated and anucleated halves. Fertilization and development of the two halves permitted us to establish that nucleated and anucleated fragments correspond, respectively, to the animal and vegetal parts. RNA was extracted from both egg halves and submitted to differential display. Northern blot analysis confirmed their maternal origin and showed that each transcript has a different expression pattern during development. By Northern blot and in situ hybridization experiments we ascertained that Bep2 and PlAn1 are localized in the animal part, whereas 16S rRNA, Plveg1, and L27 in the vegetal part, and that Plun1 is uniformly distributed. Moreover, by treating P. lividus eggs with detergent, in presence or not of drugs such as colchicine and cytochalasin B, we demonstrated the involvement of the cytoskeleton only in localization of Bep2, PlAn1, and Plun1, suggesting that different mechanisms are utilized for animal and vegetal distribution.

Molecular patterning along the sea urchin animal-vegetal axis

The molecular regulatory mechanisms underlying primary axis formation during sea urchin development have recently been identified. Two opposing maternally inherited systems, one animalizing and one vegetalizing, set up the animal-vegetal (A-V) axis. The vegetal system relies in part on the Wnt-beta-catenin-Tcf/Lef signaling pathway and the animal system is based on a cohort of animalizing transcription factors that includes members of the Ets and Sox classes. The two systems autonomously define three zones of cell-type specification along the A-V axis. The vegetalmost zone gives rise to the skeletogenic mesenchyme lineage; the animalmost zone gives rise to ectoderm; and the zone in which the two systems overlap generates endoderm, secondary mesenchyme, and ectoderm. Patterning along the A-V also depends on cellular interactions involving Wnt, Notch, and BMP signaling. We discuss how these systems impact the formation of the second axis, the oral-aboral axis; how they connect to later developmental events; and how they lead to cell-type-specific gene expression via cis-regulatory networks associated with transcriptional control regions. We also discuss how these systems may confer on the embryo its spectacular regulatory capacity to replace missing parts.

Identification of an antigen related to the sea urchin RNA-binding protein LP54 in mammalian central nervous system

LP54 is an RNA-binding protein involved in localization of maternal messengers in sea urchin egg and embryos. Using a polyclonal antibody directed against Paracentrotus lividus LP54 we detected a 66-kDa cross-reacting antigen in undifferentiated and differentiated SH-SY5Y human neuroblastoma cells. After treatment of undifferentiated cells with detergent, the 66-kDa antigen was found to be enriched in the cytoskeletal fraction. By Western blot the expression of this antigen was also analyzed in regions of the CNS and in tissues of the adult rat and its exclusive presence in the hippocampus and thalamus was revealed. The immunoreactivity with P. lividus antibody against LP54 in hippocampal lysate was also confirmed throughout anti-LP54 immunoaffinity column and competition experiments. The results indicates that a related protein to the sea urchin LP54 is evolutionary conserved in mammalian CNS.

Bep4 Protein Is Involved in Patterning along the Animal–Vegetal Axis in the Paracentrotus lividus Embryo

In sea urchin embryos, the initial animal-vegetal (AV) axis is specified during oogenesis but the mechanism is largely unknown. By using chemical reagents such as lithium, it is possible to shift the principal embryonic territories toward a vegetal fate. We have investigated the possibility of obtaining the same morphological effect as with lithium by utilizing Fabs against the maternal Bep4 protein that is localized in the animal part of Paracentrotus lividus egg and embryos. Incubation of fertilized eggs with Fabs against Bep4 protein causes exogastrulation at 48 h of development of P. lividus embryos, similar to embryos treated with lithium. This vegetalizing effect was ascertained by utilizing territorial markers such as EctoV, EndoI, and Ig8. The effect of Fabs against Bep4 on gene expression was observed by monitoring spatial expression of the hatching enzyme gene. A decreased expression domain compared to its normal spatial distribution was detected and this effect was again comparable to those obtained with lithium treatment. Association of Bep4 with a cadherin was demonstrated by immunoprecipitation and immunostaining experiments, and an involvement in cell signaling is discussed. In addition, treatment of embryos with anti-Bep4 Fabs causes an enhancement in the level and an expansion in the pattern of nuclear beta-catenin. Moreover, this treatment also provokes a decrease of beta-catenin in adherens junctions. Together, these data indicate that anti-Bep4 Fabs provoke a shift of the animal-vegetal boundary toward the animal pole and suggest an active role of Bep4 protein in patterning along the AV axis.

Localization and association to cytoskeleton of COLL1alpha mRNA in Paracentrotus lividus egg requires cis- and trans-acting factors

COLL1alpha mRNA is asymmetrically distributed in the Paracentrotus lividus egg. Here we examine the involvement of the cytoskeleton in the localization process of collagen mRNA. The use of drugs such as colchicine and cytochalasin B reveals a perturbation of localization collagen mRNA. Moreover, the presence of specific cis-and trans-acting factors involved in cytoskeleton binding and the localization process was investigated. By Northwestern experiment we found that the 3'UTR of COLL1alpha mRNA is also able to bind two proteins of 54 and 40 kDa in a cellular fraction containing the cytoskeleton. Finally, we found that the protein of 54 kDa is LP54, a protein that binds the 3'UTRs of P. lividus maternal bep messengers and is necessary for their localization.

Isolation of a trans-acting factor involved in localization of Paracentrotus lividus maternal mRNAs

Localization of Paracentrotus lividus bep maternal mRNAs at the animal pole occurs by association with the cytoskeleton and involves a 54-kDa protein, called LP54, that is able to bind to the 3' untranslated regions (UTRs) of bep mRNAs. We describe here the isolation and purification of this protein. Antibodies raised against purified LP54 allowed us to establish its localization in P. lividus eggs and embryos. This localization coincides with the mRNAs with which it is associated, that is, the animal pole in the egg, and, after fertilization, the regions derived from this part of the egg, and finally the oral ectoderm of the pluteus. Association with the cytoskeleton was shown by the copurification of LP54 in a microtubule preparation. Involvement in bep mRNA localization was demonstrated by microinjection of anti-LP54 antibodies in P. lividus eggs, which caused alteration of spatial distribution of bep3 mRNA.

Asymmetrical localization and segregation of Paracentrotus lividus Bep4 maternal protein

Asymmetric divisions that produce two distinct cells play a fundamental role in generating different cell types during development. Here we investigate the role of the cortex region and mitotic apparatus in asymmetrical localization and segregation of Bep4 protein in Paracentrotus lividus egg. By centrifugation of eggs with or without drugs we established an involvement of the cortex region in localization of Bep4 protein, confirmed by immunohistochemistry of isolated cortex. Association with the mitotic apparatus during cell division permits selective partitioning of Bep4 protein into the daughter cells. Direct association with spindle was also demonstrated both by Western blot and immunohistochemistry after isolation of the mitotic apparatus.

Association of the sea urchin EGF-related peptide, EGIP-D, with fasciclin I-related ECM proteins from the sea urchin Anthocidaris crassispina

Exogastrula-inducing peptides (EGIP) of the sea urchin Anthocidaris crassispina are endogenous peptides related to epidermal growth factor (EGF), which induce exogastrulation in the embryo. Recently, a protein(s) from sea urchin embryos that binds to one of the EGIP, EGIP-D (EGIP-D-binding protein, EBP) was purified. The isolation and characterization of the cDNA clones for two EBP proteins (EBP-alpha and EBP-beta) is reported. The two EBP proteins were highly similar in structure to each other; both possessed putative cell-binding sites and two repeated sequences characteristically seen in the insect neuronal cell adhesion protein, fasciclin I. The EBP showed similarity with other sea urchin proteins HLC-32, Bep1, and Bep4. It has been confirmed that bacterially expressed EBP proteins associate with EGIP-D as does native EBP, suggesting the interaction between EGF-related proteins and fasciclin I-related proteins. An EBP transcript of 1.4 kb was strongly expressed in immature ovaries but not in immature testes. A somewhat lower level of the transcript existed in unfertilized eggs and the amount gradually declined to an almost undetectable level by the pluteus stage. The EBP proteins were present throughout embryonic development at nearly constant levels. Although most of the proteins were distributed rather evenly in the cytoplasm, a small portion was detected on the apical surface of blastomeres and ectodermal cells, showing that EBP are components of the hyaline layer.

How to grow a gut: ontogeny of the endoderm in the sea urchin embryo

Gastrulation is the process of early development that reorganizes cells into the three fundamental tissue types of ectoderm, mesoderm, and endoderm. It is a coordinated series of morphogenetic and molecular changes that exemplify many developmental phenomena. In this review, we explore one of the classic developmental systems, the sea urchin embryo, where investigators from different backgrounds have converged on a common interest to study the origin, morphogenesis, and developmental regulation of the endoderm. The sea urchin embryo is remarkably plastic in its developmental potential, and the endoderm is especially instructive for its morphological and molecular responsiveness to inductive cell interactions. We start by examining and integrating the several models for the morphogenetic mechanisms of invagination and tissue elongation, the basic processes of endoderm morphogenesis in this embryo. We next critique the proposed mechanisms of inductive gene regulation in the endoderm that exemplifies a concept of modular transcriptional regulation. Finally, we end with an examination of the current molecular models to explain cell fate determination of the endoderm. Recent progress at the molecular level should soon allow us to explain the seminal experimental observations made in this embryo over a hundred years ago.

Folding and binding activity of the 3'UTRs of Paracentrotus lividus bep messengers

Bep mRNAs are localized at the animal pole of P. lividus eggs. In the present communication the secondary structures of the 3'UTRs of the bep1, bep3 and bep4 mRNAs are reported. The minimal lengths of these regions required to bind the 54-kDa protein, previously shown to be involved in localization and anchoring of these RNAs, is estimated. Microinjection of the bep3 3'UTR into egg shows that this RNA fragment is also able to become localized to one of the egg poles, as happens for the entire bep3 RNA.

Involvement of the cytoskeleton in localization of Paracentrotus lividus maternal BEP mRNAs and proteins

The maternal bep1 and bep4 mRNAs and their protein products are localized at the animal pole of Paracentrotus lividus eggs. We have examined the role of the cytoskeleton in localization both of bep RNAs and BEP proteins in unfertilized and fertilized eggs and in determining the polarity of P. lividus eggs. The use of drugs such as colchicine and cytochalasin B, which depolymerize microtubules and microfilaments respectively, revealed a perturbation of localization of bep1 and bep4 mRNAs. In contrast, the microfilament inhibitor cytochalasin B had no effect on localization of BEP1 and BEP4 antigens localization, which appears to be due only to microtubules. Moreover, the presence of bep mRNAs and BEP proteins in a microtubule preparation has been demonstrated. Maintenance of the asymmetric distribution of BEP proteins during cellular division of eggs and embryos, by association with mitotic spindle, is also shown.

Protein tyrosine kinase activity following fertilization is required to complete gastrulation, but not for initial differentiation of endoderm and mesoderm in the sea urchin embryo

The egg activation process functions to implement developmental programs that act much later in embryogenesis. One example of this is the fact that application of protein tyrosine kinase inhibitors to the fertilized sea urchin egg for a 15-min period results in a defect in the gastrulation process occurring over 24 h later (Kinsey, W. H., Dev. Biol. 172, 704-707, 1995). In the present study, we show that the window of sensitivity is not due to differential uptake of inhibitor, and establish that the inhibitor inhibits tyrosine kinase activity at the time of application. We also demonstrate that inhibition of protein tyrosine kinase activity in the zygote causes a specific defect in the morphogenetic movements associated with gastrulation without interfering with the initial specification and differentiation of endoderm and mesoderm. Differentiation events occurring concurrent with or subsequent to gastrulation were also suppressed in embryos derived from treated zygotes. These findings indicate that fertilization initiates a signaling cascade involving protein tyrosine kinase activity that is required specifically for events at gastrulation. This signaling event is required to complete the developmental program of both endoderm and mesoderm, but is different from those events necessary for initial specification of endodermal and mesodermal cell fate.

Purification of EGIP-D-binding protein from the embryos of the sea urchin Anthocidaris crassispina

Exogastrula-inducing peptides (EGIPs) are intrinsic factors that are present in eggs and embryos of the sea urchin Anthocidaris crassispina. They induce exogastrulation when added exogenously to the embryos. In the present study, we isolated an EGIP-D-binding protein (EBP) from a homogenate of mesenchyme blastulae. EBP had an apparent molecular weight of 33,000. The N-terminal amino acid sequence of EBP had a sequence homology to HLC-32 and bep4 identified in other sea urchin embryos. In addition to its ability of binding to EGIP-D, EBP also inhibited exogastrulation induced by EGIP-D. These results suggest that EBP plays an essential role in EGIP-D-induced exogastrulation.

Tyrosine kinase signaling at fertilization

The unfertilized egg is a highly differentiated cell that retains unlimited developmental potential. The execution of that potential requires signal transduction pathways that release the egg from its quiescent metabolic state, direct the union of the maternal and paternal genome, and initiate a developmental program that will guide embryogenesis. The egg is equipped with an array of cytosolic as well as cell surface receptor protein tyrosine kinases as part of a preassembled signal transduction mechanism. These protein tyrosine kinases have been found to act at several points during this egg activation process, beginning as early as the initial sperm-egg interaction. While many of these kinase functions are common to all cells, several functions unique to fertilization demonstrate the versatility of this class of protein kinases.

Spatial distribution of collagen type I mRNA in Paracentrotus lividus eggs and embryos

We have identified the presence of type I collagen (COLL1alpha) mRNA in Paracentrotus lividus unfertilized egg, indicating a maternal origin of this mRNA. By in situ whole mount hybridization the spatial distribution of COLL1alpha mRNA in egg and embryo at different developmental stages was established. Moreover, the presence of COLL1alpha gene in Paracentrotus lividus genome was analyzed by Southern blot experiments. The localization pattern indicates that the maternal mRNA is placed in the fertilized egg in a fixed position, relative to the embryonic axes. Furthermore, the embryonic expression is spatially restricted during development, suggesting involvement in sea urchin embryo cell specification events. The presence of two bands in Southern blot hybridization may indicate that two genes specific for COLL1alpha are present in the sea urchin genome.

A 54-kDa protein specifically associates the 3' untranslated region of three maternal mRNAs with the cytoskeleton of the animal part of the Paracertrotus lividus egg

Bep mRNAs, i.e., maternal messengers coding for cell surface proteins, are localized in the animal part of Paracertrotus lividus egg and embryos. Here we have examined the involvement of the cytoskeleton in asymmetric distribution of bep3 mRNA. Moreover, in order to understand whether and how cis- and trans-acting factors are necessary for bep3 mRNA localization, we have looked for in vitro-specific interactions between egg proteins and bep3 mRNA. By northwestern assay we have identified a 54-kDa protein that binds to the 3'UTR of bep3 mRNA. This 54-kDa protein also permits association of 3'UTR of bep3 with cytoskeleton elements, indicating its involvement in the localization process. Binding of 54-kDa protein to 3'UTR of bep1 and bep4 has also been demonstrated, suggesting that a binding motif is shared with these other two mRNAs of the same gene family. Northwestern analyses carried out utilizing proteins extracted from different developmental stages indicate that the 54-kDa protein is the only protein able to bind to the 3'UTR of bep3.

Centrifugation does not alter spatial distribution of 'BEP4' mRNA in paracentrotus lividus EGG

Paracentrotus lividus unfertilized eggs were centrifuged in a sucrose gradient, so to split each into two parts: a nucleated light fragment and an anucleated heavy fragment. Northern blot analyses utilizing a bep4 probe as animal marker and H2A histone gene and 12S-mit RNA as controls indicate that the eggs are elongated along the animal-vegetal axis during centrifugation and thereafter split into an animal and a vegetal half. Treatment of the eggs with colchicine before centrifugation abolishes the animal localization of bep4 mRNA.

Cloning, expression, and localization of a new member of a Paracentrotus lividus cell surface multigene family

We have isolated and characterized a cDNA clone corresponding to a new member of bep (butanol, extracted, proteins) Paracentrotus lividus multigene family coding for cell surface proteins. The cDNA, called bep3, encodes a 370 amino acid protein and shares the same structural organization in the coding region with other members of the same gene family already characterized. Expression of this clone studied by Northern blot and by whole mount hybridization shows that the bep3 messenger is transcribed during oogenesis and utilized till the gastrula stage, whereas at the prism stage, unlike other members of the same gene family, new synthesis of messenger occurs. By whole mount hybridization spatial distribution of bep3 messenger in egg and embryos is established. This messenger appears located in the animal half of the unfertilized egg and moves to the cortical zone after fertilization; it is not present in the structures derived by the vegetal part of the embryo, such as the micromeres of the 16-cell stage, the primary mesenchyme cells of the blastula, and the primary intestine of the gastrula. At the prism stage instead, hybridization of bep3 messenger is restricted to the part of the embryo that will give origin to the oral region as successively confirmed by hybridization at the pluteus stage. The result of whole mount hybridization was confirmed by Northern blot hybridization of separated meso-macromere and micromere RNAs. A Southern blot experiment demonstrates that bep3 is codified by a single copy gene. Conservation of the bep multigene family in several Mediterranean and Japanese sea urchin species has also been analyzed.

Homeobox-containing gene transiently expressed in a spatially restricted pattern in the early sea urchin embryo

In the sea urchin embryo, the lineage founder cells whose polyclonal progenies will give rise to five different territories are segregated at the sixth division. To investigate the mechanisms by which the fates of embryonic cells are first established, we looked for temporal and spatial expression of homeobox genes in the very early cleavage embryos. We report evidence that PlHbox12, a paired homeobox-containing gene, is expressed in the embryo from the 4-cell stage. The abundance of the transcripts reaches its maximum when the embryo has been divided into the five polyclonal territories--namely at the 64-cell stage--and it abruptly declines at later stages of development. Blastomere dissociation experiments indicate that maximal expression of PlHbox12 is dependent on intercellular interactions, thus suggesting that signal transduction mechanisms are responsible for its transcriptional activation in the early cleavage embryo. Spatial expression of PlHbox12 was determined by whole-mount in situ hybridization. PlHbox12 transcripts in embryos at the fourth, fifth, and sixth divisions seem to be restricted to the conditionally specified ectodermal lineages. These results suggest a possible role of the PlHbox12 gene in the early events of cell specification of the presumptive ectodermal territories.