Involvement of Huntingtin in Development and Ciliary Beating Regulation of Larvae of the Sea Urchin, Hemicentrotus pulcherrimus

The multiple functions of the wild type Huntington's disease protein of the sea urchin Hemicentrotus pulcherrimus (Hp-Htt) have been examined using the anti-Hp-Htt antibody (Ab) raised against synthetic oligopeptides. According to immunoblotting, Hp-Htt was detected as a single band at around the 350 kDa region at the swimming blastula stage to the prism larva stage. From the 2-arm pluteus stage (2aPL), however, an additional smaller band at the 165 kDa region appeared. Immunohistochemically, Hp-Htt was detected in the nuclei and the nearby cytoplasm of the ectodermal cells from the swimming blastula stage, and the blastocoelar cells from the mid-gastrula stage. The Ab-positive signal was converged to the ciliary band-associated strand (CBAS). There, it was accompanied by several CBAS-marker proteins in the cytoplasm, such as glutamate decarboxylase. Application of Hp-Htt morpholino (Hp-Htt-MO) has resulted in shortened larval arms, accompanied by decreased 5-bromo-2-deoxyuridin (BrdU) incorporation by the ectodermal cells of the larval arms. Hp-Htt-MO also resulted in lowered ciliary beating activity, accompanied by a disordered swirling pattern formation around the body. These Hp-Htt-MO-induced deficiencies took place after the onset of CBAS system formation at the larval arms. Thus, Hp-Htt is involved in cell proliferation and the ciliary beating pattern regulation signaling system in pluteus larvae.

Initial report of γ-aminobutyric acidergic locomotion regulatory system and its 3-mercaptopropionic acid-sensitivity in metamorphic juvenile of sea urchin, Hemicentrotus pulcherrimus

The γ-aminobutyric acid (GABA) signal transmission system (GSTS) contributes to larval swimming through the regulation of ciliary beating. However, whether this system also contributes to the primary podia (PP)-generated motility of juveniles remained unclear. The present study aimed to elucidate the involvement of the GSTS in the motility of metamorphic juveniles (juveniles) (1) by immunohistochemically elucidating the location of molecular constituents of the PP, and (2) by inhibiting the activity of GΑΒΑ decarboxylase (GAD) with 3-mercaptopropionic acid (3-MPA). During metamorphosis, the echinus rudiment protrudes its PP out of the body surface in 8-arm plutei. The PP expresses immunopositive signal (-IS) of GAD, GABA, GABA_A receptor and tropomyosin, and is constituted with the GABA-IS negative distal tip and the GABA/GAD-IS gaiter region. The latter radiates distal projections to the disc that contains a GAD-IS cellular network. The juvenile body cavity houses a GABA/βIII-tubulin-IS Penta-radial ring (PrR) that extends branches into each PP and several bridges to the GAD/GABA-IS Penta-radial plate (PrP) on the oral side but does not reach to the gaiter region. 3-MPA reversibly inhibits the juvenile motility and GABA-IS expression in the PrR/PrP complex. This indicates that the complex is the major contributor to the GABAergic motility in juveniles.

Immunohistochemical and ultrastructural properties of the larval ciliary band-associated strand in the sea urchin Hemicentrotus pulcherrimus

BACKGROUND

The swimming activity of sea urchin larvae is dependent on the ciliary band (CB) on the larval surface and is regulated by several neurotransmitters, including serotonin (5HT), dopamine, and γ-aminobutyric acid (GABA). However, the CB signal transmission mechanism remains unknown. The present study investigated the structural relationship between the CB and external signal receptors by immunohistochemical and transmission electron microscopic analyses of sea urchin, Hemicentrotus pulcherrimus, larvae.

RESULTS

Glutamate decarboxylase (GAD; GABA synthetase) was detected in a strand of multiple cells along the circumoral CB in 6-arm plutei. The GAD-expressing strand was closely associated with the CB on the oral ectoderm side. The ciliary band-associated strand (CBAS) also expressed the 5HT receptor (5HThpr) and encephalopsin (ECPN) throughout the cytoplasm and comprised 1- to 2-μm diameter axon-like long stretched regions and sporadic 6- to 7-μm diameter bulbous nucleated regions (perikarya) that protruded into the oral ectoderm side. Besides the laterally polarized morphology of the CBAS cells, Epith-2, which is the epithelial lateral cell surface-specific protein of the sea urchin embryo and larva, was expressed exclusively by perikarya but not by the axon-like regions. The CBAS exposed its narrow apical surface on the larval epithelium between the CB and squamous cells and formed adherens junctions (AJs) on the apical side between them. Despite the presence of the CBAS axon-like regions, tubulins, such as α-, β-, and acetylated α-tubulins, were not detected. However, the neuroendocrine cell marker protein synaptophysin was detected in the axon-like regions and in bouton-like protrusions that contained numerous small ultrastructural vesicles.

CONCLUSIONS

The unique morphology of the CBAS in the sea urchin larva epithelium had not been reported. The CBAS expresses a remarkable number of receptors to environmental stimuli and proteins that are probably involved in signal transmission to the CB. The properties of the CBAS explain previous reports that larval swimming is triggered by environmental stimuli and suggest crosstalk among receptors and potential plural sensory functions of the CBAS.

Mechanisms of the epithelial-to-mesenchymal transition in sea urchin embryos

Sea urchin mesenchyme is composed of the large micromere-derived spiculogenetic primary mesenchyme cells (PMC), veg2-tier macromere-derived non-spiculogenetic mesenchyme cells, the small micromere-derived germ cells, and the macro- and mesomere-derived neuronal mesenchyme cells. They are formed through the epithelial-to-mesenchymal transition (EMT) and possess multipotency, except PMCs that solely differentiate larval spicules. The process of EMT is associated with modification of epithelial cell surface property that includes loss of affinity to the apical and basal extracellular matrices, inter-epithelial cell adherens junctions and epithelial cell surface-specific proteins. These cell surface structures and molecules are endocytosed during EMT and utilized as initiators of cytoplasmic signaling pathways that often initiate protein phosphorylation to activate the gene regulatory networks. Acquisition of cell motility after EMT in these mesenchyme cells is associated with the expression of proteins such as Lefty, Snail and Seawi. Structural simplicity and genomic database of this model will further promote detailed EMT research.

Unc-5/netrin-mediated axonal projection during larval serotonergic nervous system formation in the sea urchin, Hemicentrotus pulcherrimus

The molecular structure and role of two splice-isoforms of Unc-5 (Hp-Unc-5v1 and v2) in Unc-5/netrin interaction during serotonergic axonal projection were elucidated in this study. Hp-Unc-5v1 was found to be comprised of two immunoglobulin-like domains, two thrombospondin domains in the extracellular region, and ZU-5, DB, and Death domains in the cytoplasmic region, whereas Hp-Unc-5v2 lacked one thrombospondin domain, the transmembrane domain, and all cytoplasmic domains. Hp-Unc-5v1 was transcribed in unfertilized eggs, which continued until the 3-day post-fertilization (-dpf) 2-arm pluteus stage, but was suspended at the mesenchyme blastula stage (mB1), whereas Hp-Unc-5v2 was not transcribed in unfertilized eggs, but was from after fertilization to the same developmental stage of mB1 as Hp-Unc-5v1. Relative accumulation of transcripts of both splice-isoforms peaked at the prism stage and declined thereafter, and they were localized at the vegetal pole region of early gastrulae, around the blastopore in mid- to late gastrulae, at fore- and mid-gut regions and on the basal side of dorsal ectoderm in 28-hour post-fertilization prism larvae, and within axons at and after the 2-dpf pluteus stage. Hp-Unc-5v2:GFP was detected in the entire serotonergic cell body and extracellularly on the basal surface of oral ectoderm in 2-dpf plutei and exclusively within axons in 4-dpf plutei. Overexpression of Hp-Unc-5v2 resulted in decreased axonal projection in plutei. Knockdown of Hp-Unc-5v1 by morpholino antisense oligonucleotide resulted in severe deficiency of axonal projection. Interference of Unc-5/netrin interaction using an exogenous synthetic SQDFGKTW peptide from the VI domain in Hp-netrin inhibited axonal projection and larval swimming.

Mesomere-derived glutamate decarboxylase-expressing blastocoelar mesenchyme cells of sea urchin larvae

The ontogenetic origin of blastocoelar glutamate decarboxylase (GAD)-expressing cells (GADCs) in larvae of the sea urchin Hemicentrotus pulcherrimus was elucidated. Whole-mount in situ hybridisation (WISH) detected transcription of the gene that encodes GAD in H. pulcherrimus (Hp-gad) in unfertilised eggs and all blastomeres in morulae. However, at and after the swimming blastula stage, the transcript accumulation was particularly prominent in clumps of ectodermal cells throughout the embryonic surface. During the gastrula stage, the transcripts also accumulated in the endomesoderm and certain blastocoelar cells. Consistent with the increasing number of Hp-gad transcribing cells, immunoblot analysis indicated that the relative abundance of Hp-Gad increased considerably from the early gastrula stage until the prism stage. The expression pattern of GADCs determined by immunohistochemistry was identical to the pattern of Hp-gad transcript accumulation determined using WISH. In early gastrulae, GADCs formed blastocoelar cell aggregates around the blastopore with primary mesenchyme cells. The increase in the number of blastocoelar GADCs was inversely proportional to the number of ectodermal GADCs ranging from a few percent of total GADCs in early gastrulae to 80% in late prism larvae; this depended on ingression of ectodermal GADCs into the blastocoel. Some of the blastocoelar GADCs were fluorescein-positive in the larvae that developed from the 16-cell stage chimeric embryos; these comprised fluorescein-labeled mesomeres and unlabelled macromeres and micromeres. Our finding indicates that some of the blastocoelar GADCs are derived from the mesomeres and thus they are the new group of mesenchyme cells, the tertiary mesenchyme cells.

Characterization and Endocytic Internalization of Epith-2 Cell Surface Glycoprotein during the Epithelial-to-Mesenchymal Transition in Sea Urchin Embryos

The epithelial cells of the sea urchin Hemicentrotus pulcherrimus embryo express an Epith-2, uncharacterized glycoprotein, on the lateral surface. Here, we describe internalization of Epith-2 during mesenchyme formation through the epithelial-to-mesenchymal transition (EMT). Epith-2 was first expressed on the entire egg surface soon after fertilization and on the blastomeres until the 4-cell stage, but was localized to the lateral surface of epithelial cells at and after the 16-cell stage throughout the later developmental period. However, primary mesenchyme cells (PMC) and secondary mesenchyme cells (SMC) that ingress by EMT lost Epith-2 from their cell surface by endocytosis during dissociation from the epithelium, which was associated with the appearance of cytoplasmic Epith-2 dots. The cytoplasmic Epith-2 retained a similar relative molecular mass to that of the cell surface immediately after ingression through the early period of the spreading to single cells. Then, Epith-2 was completely lost from the cytoplasm. Tyrosine residues of Epith-2 were phosphorylated. The endocytic retraction of Epith-2 was inhibited by herbimycin A (HA), a protein tyrosine kinase (PTK) inhibitor, and suramin, a growth factor receptor (GFR) inhibitor, suggesting the involvement of the GFR/PTK (GP) signaling pathway. These two GP inhibitors also inhibited PMC and SMC spreading to individual cells after ingression, but the dissociation of PMC and SMC from the epithelium was not inhibited. In suramin-treated embryos, dissociated mesenchyme cells migrated partially by retaining their epithelial morphology. In HA-treated embryos, no mesenchyme cells migrated. Thus, the EMT occurs in relation to internalization of Epith-2 from presumptive PMC and SMC.

Development of the GABA-ergic signaling system and its role in larval swimming in sea urchin

The present study aimed to elucidate the development and γ-amino butyric acid (GABA)-ergic regulation of larval swimming in the sea urchin Hemicentrotus pulcherrimus by cloning glutamate decarboxylase (Hp-gad), GABAA receptor (Hp-gabrA) and GABAA receptor-associated protein (Hp-gabarap), and by performing immunohistochemistry. The regulation of larval swimming was increasingly dependent on the GABAergic system, which was active from the 2 days post-fertilization (d.p.f.) pluteus stage onwards. GABA-immunoreactive cells were detected as a subpopulation of secondary mesenchyme cells during gastrulation and eventually constituted the ciliary band and a subpopulation of blastocoelar cells during the pluteus stage. Hp-gad transcription was detected by RT-PCR during the period when Hp-Gad-positive cells were seen as a subpopulation of blastocoelar cells and on the apical side of the ciliary band from the 2 d.p.f. pluteus stage. Consistent with these observations, inhibition of GAD with 3-mercaptopropioninc acid inhibited GABA immunoreactivity and larval swimming dose dependently. Hp-gabrA amplimers were detected weakly in unfertilized eggs and 4 d.p.f. plutei but strongly from fertilized eggs to 2 d.p.f. plutei, and Hp-GabrA, together with GABA, was localized at the ciliary band in association with dopamine receptor D1 from the two-arm pluteus stage. Hp-gabarap transcription and protein expression were detected from the swimming blastula stage. Inhibition of the GABAA receptor by bicuculline inhibited larval swimming dose dependently. Inhibition of larval swimming by either 3-mercaptopropionic acid or bicuculline was more severe in older larvae (17 and 34 d.p.f. plutei) than in younger ones (1 d.p.f. prism larvae).

Development of a dopaminergic system in sea urchin embryos and larvae

The mechanisms that regulate the organized swimming movements of sea urchin blastulae are largely unknown. Using immunohistochemistry, we found that dopamine (DA) and the Hemicentrotus pulcherrimus homolog of the dopamine receptor D1 (Hp-DRD1) were strongly co-localized in 1-2 microm diameter granules (DA/DRD1 granules). Furthermore, these granules were arranged across the entire surface of blastulae as they developed locomotory cilia before hatching, and remained evident until metamorphosis. DA/DRD1 granules were associated with the basal bodies of cilia, and were densely packed in the ciliary band by the eight-arm pluteus stage. The transcription of Hp-DRD1 was detected from the unfertilized egg stage throughout the period of larval development. Treatment with S-(-)-carbidopa, an inhibitor of aromatic-l-amino acid decarboxylase, for 20-24 h (i) from soon after insemination until the 20 h post-fertilization (20 hpf) early gastrula stage and (ii) from the 24 hpf prism larva stage until the 48 hpf pluteus stage, inhibited the formation of DA granules and decreased the swimming activity of blastulae and larvae in a dose-dependent manner. Exogenous DA rescued these deprivations. The formation of DRD1 granules was not affected. However, in 48 hpf plutei, the serotonergic nervous system (5HT-NS) developed normally. Morpholino antisense oligonucleotides directed against Hp-DRD1 inhibited the formation of DRD1 granules and the swimming of larvae, but did not disturb the formation of DA granules. Thus, the formation of DRD1 granules and DA granules occurs chronologically closely but mechanically independently and the swimming of blastulae is regulated by the dopaminergic system. In plutei, the 5HT-NS closely surrounded the ciliary bands, suggesting the functional collaboration with the dopaminergic system in larvae.

The neurotoxic effects of monocrotophos on the formation of the serotonergic nervous system and swimming activity in the larvae of the sea urchin Hemicentrotus pulcherrimus

The neurotoxicity of monocrotophos (MCP) in the development of the serotonergic nervous system and swimming activity of larvae of the sea urchin, Hemicentrotus pulcherrimus, was examined. Lethal dose 50% of MCP was 43μg/ml. Overall morphology was not affected in larvae that received up to 30μg/ml of MCP soon after fertilization until the 53h post-fertilization pluteus stage. However, while 70±0.6% of larvae in 5μg/ml MCP swam actively, the proportion decreased to 30±1.7% in 30μg/ml MCP. Accordingly, immunoblotting indicated that MCP decreased the relative intensity of immunoreaction of serotonin receptor protein. Whole-mount immunohistochemistry indicated that MCP inhibited serotonergic axon growth, reduced the number of serotonergic cells at the apical ganglion, and perturbed formation of the serotonin receptor cell network. The present study demonstrated that sea urchin larva is a useful model for evaluating the working mechanism of environmental toxicants in neurogenesis and behavior.

Spatio-temporal expression of a Netrin homolog in the sea urchin Hemicentrotus pulcherrimus (HpNetrin) during serotonergic axon extension

A netrin homolog of the sea urchin, Hemicentrotus pulcherrimus (HpNetrin) was characterized in terms of its expression behavior. The predicted amino acid sequence was an ortholog of hemichordate netrin-1. Reverse transcriptase-PCR, immunoblotting, and whole mount immunohistochemistry showed that gene transcription and protein expression occurred from 15-hour post-fertilization (hpf) swimming blastula stage to, at least 53-hpf pluteus stage. HpNetrin was detected on the entire basal surface of the ectoderm in swimming and 16-hpf mesenchyme blastulae. However, by 24-hpf prism stage, immunoreaction on the oral ectoderm decreased, whereas it increased in the aboral ectoderm including near the animal plate ectoderm (area-I). By 48-hpf pluteus stage, the HpNetrin-rich area-I comprised a 40mm wide dorsal midline belt (DMB) that stretched from the dorsal posterior edge of the animal plate to the posterior end of the larval body. Serotonergic cells were first detected in the HpNetrin-moderate area between the anterior DMB and the HpNetrin-poor oral ectoderm near the animal plate in 24-hpf prism larvae. By 48-hpf pluteus stage, these cells extended axons toward the middle-ridge to form a neural plexus of the apical ganglion. At 53-hpf pluteus stage, these axons extended further away from the apical ganglion directly or through the DMB toward the HpNetrin-poor contralateral ectoderm. The protein expression and axon extension pattern were reproduced in embryonic cell-aggregates formed from artificially dissociated 20-hpf gastrulae and resembled small blastula. In Hpnetrin morpholino anti-sense oligonucleotide-injected plutei, serotonergic axon extension was severely inhibited. Thus, HpNetrin functions as a serotonergic axon guidance cue in this basal deuterostome.

Serotonin stimulates [Ca2+]i elevation in ciliary ectodermal cells of echinoplutei through a serotonin receptor cell network in the blastocoel

A full-length serotonin receptor mRNA from the 5Hthpr gene was sequenced from larvae of the sea urchin, Hemicentrotus pulcherrimus.The DNA sequence was most similar to 5HT-1A of the sea urchin Strongylocentrotus purpuratus found by The Sea Urchin Genome Project,and the protein sequence predicted the presence of seven transmembrane domains. Immunohistochemistry with anti-5HThpr antibodies indicated that the protein was expressed on blastocoelar cells that comprised the major blastocoelar network (serotonin receptor cell network). These network cells inserted their processes into the ectoderm in various regions, including the ciliary band region. Serotonin injected into the blastocoel stimulated a transient elevation of cytoplasmic Ca2+ concentration([Ca2+]i) in the ectoderm, as detected by Oregon-Green dextran, injected earlier in development. The calcium transient propagated as a wave at about 175 μm s–1, but was not detectable in the serotonin receptor-positive cell network. In larvae treated with p-chlorophenylalanine, a potent and irreversible serotonin synthesis inhibitor, serotonin application did not stimulate[Ca2+]i, the serotonin receptor cell network did not develop properly, and the swimming behavior of the larvae was abnormal. However, formation of a different nervous system comprising synaptotagmin-possessed neurites was not affected by p-chlorophenylalanine treatment. These results imply that serotonin secreted from the apical ganglion into the blastocoel stimulates the elevation of [Ca2+]i in the larval ectodermal cells through the serotonin receptor cell network.

Exclusive expression of hedgehog in small micromere descendants during early embryogenesis in the sea urchin, Hemicentrotus pulcherrimus

Hedgehog (hh) is a multifunctional extracellular protein, and known as an essential signal molecule in morphogenetic movement in animal embryos. We have cloned, sequenced, and studied dynamic localization of Hphh, a hedgehog homologue of the sea urchin, Hemicentrotus pulcherrimus. The origin of Hphh transcribing cells was also verified during early embryogenesis. The amino acid sequence of Hphh shows high homology to Lvhh, an hh homologue cloned in the sea urchin, Lytechinus variegatus. Reverse transcriptase polymerase chain reaction showed that the transcription of Hphh occurred at and after 19 h post-fertilization (19 hpf) mesenchyme blastula stage until, at least, 69 hpf 4-arm pluteus stage. Whole mount in situ hybridization showed Hphh transcription sites in a few cells at the tip of archenteron in 30 hpf gastrulae. At around 45 hpf 2-arm pluteus stage, the number of Hphh transcribed cells was 8, and unequally split to two groups, 5 cells in left coelomic sac and 3 cells in right coelomic sac. A cell lineage tracing by staining the small micromeres with 5-Bromo-2-deoxyuridine showed that Hphh was transcribed exclusively in all the small micromere descendants and comprised the coelomic sacs in 69 hpf plutei.

The 5-HT receptor cell is a new member of secondary mesenchyme cell descendants and forms a major blastocoelar network in sea urchin larvae

A gene encoding the serotonin (5-hydroxytryptamine, 5-HT) receptor (5-HT-hpr) was identified from the sea urchin, Hemicentrotus pulcherrimus. Partial amino acid sequence deduced from the cDNA showed strong similarity to Aplysia californica 5-HT2 receptor. Immunoblotting analysis of this 5-HT-hpr protein (5-HT-hpr) with an antibody raised against a deduced peptide showed two bands. Their relative molecular masses were 69 and 53 kDa, respectively. The larger band alone disappeared after N-glycopeptidase F digestion, indicating the protein was N-glycosylated. Immunolocalization analysis showed that cells expressing the 5-HT-hpr (SRC) first appeared near the tip of the archenteron in 33-h post-fertilization (33 hpf) prism larvae. Their cell number doubled in 2 h, and 5-HT-hpr protein expression increased further without cell proliferation. SRC spread ventrally on the basal surface of the oral ectoderm in 36 hpf prism larvae, and then clockwise on the ventral ectoderm to the posterior region to complete formation of the SRC network in 48 hpf early plutei. The SRC network was comprised of 7 main tracts: 4 spicule system-associated tracts and 3 spicule system-independent tracts. The network extended short fibers to the larval body surface through the ectoderm, implicating a signal transmission system that receives exogenous signal. Double-stain immunohistochemistry with antibodies to primary mesenchyme cells showed that SRC were not stained by the antibody. In embryos deprived of secondary mesenchyme cell (SMC) by microsurgery, the number of SRC decreased considerably. These two data indicate that SRC are SMC descendants, adding a new member to the SMC lineage.

Expression of tryptophan 5-hydroxylase gene during sea urchin neurogenesis and role of serotonergic nervous system in larval behavior

Tryptophan 5-hydroxylase (TPH) is the rate-limiting enzyme in the biosynthesis of serotonin. cDNA cloning of TPH was carried out, and the occurrence of spatiotemporal transcription of TPH message was examined in larvae of the sea urchin, Hemicentrotus pulcherrimus (HpTPH), with in situ hybridization by using the tyramide signal amplification (TSA) technique and Northern hybridization. Based on deduced amino acids sequence of HpTPH, phylogenetically sea urchin locates at the closest position to vertebrates among invertebrates, and HpTPH had common conserved sequences in a catalytic domain. Initiation of HpTPH transcription occurred at the late gastrula stage exclusively in serotonin cells of apical ganglion (SAG) that was composed of a cluster of HpTPH-positive cells and the negative cells in between. In situ hybridization showed that the mRNA expression pattern was similar to the immunohistochemical localization of serotonin cells reported before (Bisgrove and Burke [1986] Dev. Growth Differ. 28:557-569; Yaguchi et al. [2000] Dev. Growth Differ. 42:479-488). p-Chlorophenylalanine (CPA), an irreversible inhibitor of TPH activity, considerably decreased serotonin content in the serotonin cells, whereas the HpTPH expression pattern and timing, and the extension of neurofibers from SAG cells were apparently unaffected, suggesting CPA exclusively perturbed synthesis of serotonin but not nervous system organization. CPA-treated larvae did not swim, despite the occurrence of ciliary beating in culture chamber, suggesting that proper serotonin synthesis is necessary for normal swimming of the larvae.

Asymmetric formation and possible function of the primary pore canal in plutei of Temnopleurus hardwicki

The development and possible function of the primary pore canal (PPC) in plutei of the sea urchin Temnopleurus hardwicki was examined by immunochemistry, electron microscopy and microsurgery. Left and right PPC that extended from coelomic sacs in plutei contained a bundle of cilia with a 9 + 2 structure that was initially detected as a group of anti-acetylated tubulin antibody-binding granules in the epithelium of coelomic sacs in 28 h postfertilization (PF) prism larvae. The granules extended to be a bundle of fibers toward the larval dorsal surface, concurrent with formation of the PPC on both sides, over the next 4 h. The cilia in both PPC beat actively. However, the PPC on the right side disappeared by approximately 55 h PF, establishing left-right asymmetry by 60 h PF (the four-arm pluteus stage). The numbers of cilia in the left and right PPC in 56 h PF plutei were five and eight, respectively. Microsurgical removal of the coelomic sac from both sides or the left side only from 26 h PF prism larvae decreased body width to 64 and 91% of normal width by 50 h PF pluteus stage, respectively, whereas that of the right PPC did not. These observations suggest that PPC contribute to the maintenance of normal body width, and that there is asymmetrical activity between the left and right PPC.

Essential role of growth factor receptor-mediated signal transduction through the mitogen-activated protein kinase pathway in early embryogenesis of the echinoderm

In this study it was shown that growth factor receptors (GFR) play a crucial role in early embryogenesis of the echinoderms Hemicentrotus pulcherrimus and Clypeaster japonicus by transmitting signals to the mitogen-activated protein kinase (MAPK) pathway. The phosphorylation ratio of extracellular signal-regulated kinase 1 (ERK1) changed dynamically during early embryogenesis and showed a peak at the swimming blastula (sBl) stage. Suramin, an inhibitor of GFR, when applied during the sBl stage perturbed morphogenesis, including primary mesenchyme cell (PMC) migration, cell proliferation, archenteron elongation, spiculogenesis, pigment cell differentiation and phosphorylation of myosin light chains (MLC). Genistein, a receptor-type protein tyrosine kinase inhibitor, severely inhibited PMC migration, gastrulation and the phosphorylation of MLC. Manumycin A, a Ras inhibitor, inhibited spiculogenesis and invagination. PD98059, a MAPK/ERK kinase inhibitor, perturbed early PMC migration and pigment cell differentiation, but not spiculogenesis and gastrulation (although these two events were significantly delayed). PMC ingression was not perturbed by genistein, suramin, manumycin A or PD98059. All of the inhibitors perturbed the phosphorylation of ERK1, which was completely restored by exogenous platelet-derived growth factor (PDGF)-AB. PDGF-AB also partially restored elongation of the archenteron by restoring cell proliferation that had been perturbed by suramin.

An RGDS peptide-binding receptor, FR-1R, localizes to the basal side of the ectoderm and to primary mesenchyme cells in sand dollar embryos

Immunoblotting using polyclonal antibodies (pAb) raised against an FR-1 receptor (FR-1R), a 57 kDa Arg-Gly-Asp-Ser (RGDS)-binding protein, of the sand dollar Clypeaster japonicus showed that the pAb monospecifically bound to the protein. FR-1R was present in purified plasma membrane, suggesting that the protein is a membrane-bound protein. The molecular structure of FR-1R did not change throughout the early embryogenesis, whereas its expression changed significantly during this period. FR-1R was present in the cortex of unfertilized eggs and was then transferred to the hyaline layer soon after the fertilization. The hyaline layer retained FR-1R immunoreactivity during early embryogenesis. FR-1R appeared on the basal side of the ectoderm at the morula stage and was retained basolaterally, at least, to the early gastrula stage. In mesenchyme blastulae, FR-1R was also present on the surface of primary mesenchyme cells (PMC). FR-1R was localized on the basal side of the ectoderm in early gastrulae, exclusively at the place where PMC formed ventrolateral aggregates, and at the apical tuft ectoderm. In vitro, PMC bound to FR-1R and its binding was inhibited in the presence of a synthetic RGDS peptide or the pAb. The pAb introduced into the blastocoele perturbed PMC migration and gastrulation. FR-1R was weakly recognized by antihuman integrin beta5 subunit pAb.

Disappearance of an epithelial cell surface-specific glycoprotein (Epith-1) associated with epithelial-mesenchymal conversion in sea urchin embryogenesis

Cell surface modification during mesenchyme ingression was examined using a monoclonal antibody (mAb), anti-Epith-1 mAb, raised against a protein (Epith-1) that was confined to the lateral surface of the epithelial cells in embryo of the sea urchin, Temnopleurus hardwicki. The mAb epitope was N-glycosylated oligosaccharides of 160 kDa monomeric Epith-1 protein. The glycoprotein was negatively charged, and its isoelectric point (IP) was 4.98. The mAb, however, is not immunologically cross-reactive with other sea urchin embryos including Hemicentrotus pulcherrimus, Strongylocentrotus nudus, and Scaphechinus mirabilis. Epith-1 is present initially in the cytoplasm of unfertilized eggs. Cytoplasmic Epith-1 shifted to the cell surface to be integrated in plasma membrane during the first cleavage, and remained there during early embryogenesis by retaining the same relative molecular mass (Mr). During primary and secondary mesenchyme ingression periods, however, Epith-1 disappears from the presumptive mesenchyme cell surface that was associated with internalization of the protein. In plutei, an additional anti-Epith-1 mAb-positive protein appears at the 142 kDa region, which was not associated with any visible alteration of the histologic localization of the protein in larvae. Anti-Epith-1 mAb IgG did not inhibit the reaggregation of epithelial cells in vitro, which suggests that either the protein is not involved in cell-cell adhesion or that the mAb is not recognizing the active site of the protein.

Pamlin-induced tyrosine phosphorylation of SUp62 protein in primary mesenchyme cells during early embryogenesis in the sea urchin, Hemicentrotus pulcherrimus

Ingression of primary mesenchyme cells (PMC) is associated with the encounter of basal lamina including pamlin. It was found that sea urchin embryos have a protein that binds antihuman focal adhesion kinase (FAK) antibodies, yet it has a 62 kDa homo-dimeric structure. Thus, this protein was distinctive from known FAK, and was named SUp62. In mesenchyme blastulae, one of the subunits increased its apparent molecular mass slightly but distinctively, then restored the original molecular mass in early gastrulae. This temporal and stage-specific shifting of the molecular mass was associated with the occurrence of tyrosine phosphorylation of a subunit that did not increase the apparent molecular mass. Herbimycin A induced the hyperphosphorylation of tyrosine residues of SUp62, and inhibited the occurrence of molecular mass shifting. Immunohistochemistry showed a strong positive signal of SUp62 and phosphotyrosine in PMC. Herbimycin A also severely but reversibly inhibited PMC dissociation, migration and gastrulation. Tyrosine phosphorylation of SUp62 was induced when PMC were incubated with pamlin in vitro, and it was initiated within 10 min after onset of the incubation. It reached its peak in 1 h, and declined gradually in the next 1 h, indicating that pamlin-induced tyrosine phosphorylation of SUp62 occurs closely associated with acquiring PMC migration activity.

Initial analysis of immunochemical cell surface properties, location and formation of the serotonergic apical ganglion in sea urchin embryos

In the present study it was found that serotonergic apical ganglion (SAG)-forming cells in plutei of the sea urchin, Hemicentrotus pulcherrimus, possessed a characteristic pear shape with broad apical sides and a pointed basal side in the acron epithelium. The basal side extended axons through the space between the epithelium and the basal lamina toward the midline of the embryo that aligned parallel to the embryonic anteroposterior axis. Serotonergic apical ganglion-forming cells had epithelial cell surface-specific proteins on their entire surface. The SAG in 4-arm plutei was composed of a 4-cell trunk region that aligned at right angles to the embryonic anteroposterior axis, and forked into two branches of one to two cells at both ends. Two branches extended toward the oral and the other two toward the aboral region, respectively. Double-stained immunohistochemistry using antiserotonin antibodies and oral ectoderm-specific anti-Ecto V monoclonal antibody or aboral ectoderm-specific anti-Ars antibodies indicated that SAG was in the aboral ectoderm region. Serotonergic apical ganglion cells were first detected in late gastrulae and increased in number rapidly between 36 and 48 h after fertilization, and then slowly afterwards. A 5-bromo-2-deoxyuridine incorporation study indicated that none of the increased SAG cells were in the S phase during the aforementioned period, suggesting that SAG cells do not proliferate by cell division, but acquire the property in particular cells by transdifferentiation using a mechanism that has yet to be elucidated.

Primary mesenchyme cell-ring pattern formation in 2D-embryos of the sea urchin

Primary mesenchyme cell (PMC) migration during PMC-ring pattern formation was analyzed using computer-assisted time-lapse video microscopy in spread embryos (2D-embryo) of the sea urchin, Mespilia globulus, and a computer simulation. The PMC formed a near normal ring pattern in the 2D-embryos, which were shown to be an excellent model for the examination of cell behavior in vivo by time-lapse computer analysis. The average migration distance of the ventro-lateral PMC aggregate-forming cells (AFC) and that of the dorso-ventral PMC cable-forming cells (CFC) showed no significant difference. All PMC took a rather straightforward migration path to their destinations with little lag time after ingression. This in vivo cell behavior fitted well to a computer simulation with a non-diffusable chemotaxis factor in the cyber-cell migration field. This simulation suggests that PMC recognize their destination from a very early moment of cell migration from the vegetal plate, and implicates that a chemoattractive region is necessary for making the PMC migration pattern. The left- and right-lateral AFC and dorso and ventral CFC were each derived from an unequally divided one-quarter segment of the vegetal plate. This suggests that AFC and CFC have a distinctive ancestor in the vegetal plate, and the PMC are a heterogeneous population at least in terms of their destination in the PMC-ring pattern.

Initial analysis of the molecular image of pamlin, a sea urchin cell adhesion protein, by transmission electron microscopy

Pamlin, an important extracellular protein required early for sea urchin embryogenesis, is readily isolated from the embryos of Hemicentrotus pulcherrimus. A molecular image analysis of pamlin was conducted using immuno-electron microscopy, rotary shadowing and negative staining technique-applied electron microscopy. The electron microscopy showed that a monoclonal antibody to the pamlin alpha-subunit bound to a position 13.5 nm from one end of a purified 255 kDa pamlin molecule, which is a 132 nm long and 6.8 nm wide linear structure. The pamlin structure is composed of three subunits, a 47 nm long 52 kDa alpha-subunit that attaches to one end of a 105 nm long 180 kDa beta-subunit, and a 15.6 nm diameter globular 23 kDa gamma-subunit that binds to the middle of the beta-subunit. The alpha- and beta-subunits together form a 125-140nm linear structure. Intermolecular aggregation frequently occurred between the free end of two beta-subunits of the alphabetagamma pamlin molecule, leaving the entire alpha-subunit surface free. Occasionally associations between the ends of alpha-subunits, or between an alpha-subunit and the middle of a beta-subunit also occurred, but no aggregations of pamlin formed through the gamma-subunit. These homophilic molecular aggregations of pamlin formed a large supramolecular network. In addition, the single pamlin molecule rounded at one end under high calcium ion concentration to form a 'loop', suggesting the presence of a calcium sensitive region in the molecule.

Histological distribution of FR-1, a cyclic RGDS-peptide, binding sites during early embryogenesis, and isolation and initial characterization of FR-1 receptor in the sand dollar embryo

A fibronectin-related synthetic cyclic H-Cys-Arg-Gly-Asp-Ser-Pro-Ala-Ser-Ser-Cys-OH (RGDSPASS) peptide (FR-1) binding site in the embryo of the sand dollar Clypeaster japonicus was specified using dansyl-labeled FR-1 (Dns-FR-1) and horseradish peroxidase-labeled FR-1, and an FR-1 receptor was isolated using FR-1-affinity column chromatography. The FR-1 introduced to the blastocoel of blastulae inhibited primary mesenchyme cell (PMC) migration in mesenchyme blastulae, and complete gastrulation and spicule differentiation in gastrulae. The Dns-FR-1 bound to the entire basal side of the ectoderm in mesenchyme blastulae, and then restricted to the basal side of the ectoderm at the apical tuft region and the vegetal hemisphere in early gastrulae. The cytoplasm of the archenteron also bound to Dns-FR-1. In PMC, Dns-FR-1 bound to the nucleus and cytoplasmic reticular features. In unfertilized eggs, Dns-FR-1 bound to the entire cytoplasm, particularly to the oval-shaped granules and the nuclear envelope, but only to the cytoplasm after fertilization. Relative molecular mass (Mr) of the FR-1-binding protein was 240 kDa under non-reducing conditions and 57 kDa under reducing conditions. The FR-1 receptor protein bound anti-sea urchin integrin (Spl) betaL subunit antibodies raised against the embryos of Strongylocentrotus purpuratus. Immunohistochemistry showed that the antibody binding site was similar to the histochemical distribution of Dns-FR-1. However, Mr of the FR-1 receptor is distinctively larger than that of the Spl betaL subunit.

Pamlin, a primary mesenchyme cell adhesion protein, in the basal lamina of the sea urchin embryo

Pamlin, a primary mesenchyme cell (PMC) adhesion protein, was isolated from the blastocoel of embryos of the sea urchin Hemicentrotus pulcherrimus. PMCs isolated from mesenchyme blastulae bound exclusively to pamlin. Pamlin is a distinctive extracellular matrix (ECM) component from reported ECM molecules in sea urchin embryos in its motility on SDS-PAGE gels both with and without 2-mercaptoethanol and histological localization. A monoclonal antibody was raised against pamlin, and this protein was immunoaffinity purified. The Mr of pamlin shown by SDS-PAGE gel analysis under nonreduced conditions was 255 kDa. Under reduced conditions, pamlin was cleaved to 180-, 52-, and 23-kDa subunits, indicating the 255-kDa protein is an SS-bonded heterotrimer. PMCs bound exclusively to the 52-kDa subunit. Mannose residues occur in the larger two subunits, but not in the smallest subunit. Pamlin does not run into 4% nondenatured PAGE gels, suggesting that the native glycoprotein forms a large polymeric supramolecular configuration in vivo. Immunohistochemistry showed that pamlin is seen on the entire basal lamina in the blastocoel and hyaline layer of mesenchyme blastulae. In vitro PMC migration assays on pamlin show that an optimum amount of pamlin for PMC migration was 2.5 micrograms/ml and that a synthetic RGDS peptide inhibited PMC migration dose dependently.

A fibronectin-related synthetic peptide, Pro-Ala-Ser-Ser, inhibits fibronectin binding to the cell surface, fibronectin-promoted cell migration in vitro, and cell migration in vivo

The biological activity of the amino acid sequence consisting of the immediate carboxyl terminus side of the Arg-Gly-Asp-Ser (RGDS) amino acid sequence in the cell-binding domain of intact fibronectin (FN) molecules was examined using synthetic peptides [RGDS, Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP), Arg-Gly-Asp-Ser-Pro-Ala-Ser-Ser-Lys-Pro (RGDSPASSKP), Pro-Ala (PA), Pro-Ala-Ser (PAS), Pro-Ala-Ser-Ser (PASS), and Pro-Ala-Ser-Ser-Lys (PASSK)]. These peptides were applied to the primary mesenchyme cells (PMCs) of the sea urchin, Clypeaster japonicus. In vitro immunohistochemistry indicated that the binding of exogenous FN to the PMC surface was inhibited by the peptides RGDSPASSKP and PASS, but not by RGDS, GRGDSP, PA, or PAS. PASS and RGDS introduced into the blastocoel also inhibited PMC migration in vivo. FN-promoted PMC migration in vitro was also inhibited by PASS and RGDS. The present results indicate that the PASS peptide inhibits FN binding to the PMC surface and promotes PMC migration, suggesting that the FN molecule uses the PASS amino acid sequence to bind to the PMC surface and to promote PMC migration in the blastocoel.

Inhibition of cell migration in sea urchin embryos by beta-D-xyloside

This investigation examines the effect of exogenous xylosides on primary mesenchyme cell behavior in Strongylocentrotus purpuratus embryos. In confirmation of studies in some other species the addition of 2 mM p-nitrophenyl-beta-D-xylopyranoside blocks the migration but not the initial ingression of primary mesenchyme cells. The blastocoel matrix of treated embryos appears deficient in a 15- to 30-nm-diameter granular component that is observed extensively on the basal lamina and on filopodia of migrating primary mesenchyme cells in untreated embryos. Other blastocoel components appear unaffected by ultrastructural criteria. The incorporation of 35SO4(2-) per embryo into ethanol precipitates of isolated blastocoel matrices was reduced significantly after xyloside treatment but the distribution of 35SO4(2-) after polyacrylamide gel electrophoresis or the glycosaminoglycan composition was unaffected. Chromatography on Sepharose CL-2B demonstrates a reduction in size of sulfated components of the blastocoel. While over 60% of the 35S-labeled material from the blastocoel of normal mesenchyme blastulae is voided from a Sepharose CL-2B column run in a dissociative solvent, only 10% from xyloside treated embryos is voided. Instead, there is a large included peak with Kav of 0.33. This material is acid soluble but cetylpyridinium chloride precipitable. It apparently consists largely of free glycosaminoglycan chains. Based on analysis of chondroitinase ABC digestion products this material consists of 41% chondroitin-6-sulfate and 58% dermatan sulfate. These results are consistent with a role in cell migration for intact chondroitin sulfate/dermatan sulfate proteoglycans in the sea urchin blastocoel matrix.

Behavior of sea urchin primary mesenchyme cells in artificial extracellular matrices

The primary mesenchyme cells (PMCs) were separated from the mesenchyme blastulae of Pseudocentrotus depressus using differential adhesiveness of these cells to plastic Petri dishes. These cells were incubated in various artificial extracellular matrices (ECMs) including horse serum plasma fibronectin, mouse EHS sarcoma laminin, mouse EHS sarcoma type IV collagen, and porcine skin dermatan sulfate. The cell behavior was monitored by a time-lapse videomicrograph and analysed with a microcomputer. The ultrastructure of the artificial ECM was examined by transmission electron microscopy (TEM), while the ultrastructure of the PMCs was examined by scanning electron microscopy (SEM). The PMCs did not migrate in type IV collagen gel, laminin or dermatan sulfate matrix either with or without collagen gel, whereas PMCs in the matrix which was composed of fibronectin and collagen gel migrated considerably. However, the most active and extensive PMC migration was seen in the matrix which contained dermatan sulfate in addition to fibronectin and collagen gel. This PMC migration involved an increase not only of migration speed but also of proportion of migration-promoted cells. These results support the hypothesis that the mechanism of PMC migration involves fibronectin, collagen and sulfated proteoglycans which contain dermatan sulfate.

Role of fibronectin in primary mesenchyme cell migration in the sea urchin

We studied the effect of fibronectin (FN) on the behavior of primary mesenchyme cells isolated from sea urchin mesenchyme blastulae in vitro using a time-lapse technique. The migration of isolated primary mesenchyme cells reconstituted in seawater and horse serum is dependent on the presence or absence of exogenous FN in the culture media. The cells in FN, 4 and 40 micrograms/ml, show a high percentage of migration and migrate long distances, whereas a higher concentration of FN at 400 micrograms/ml tends to inhibit migration.

Effects of prolonged antitubulin culture on the ultrastructure of anterior limb bud cells of the chick embryo

The anterior limb bud mesenchyme cells of stage 24 chick embryos were dissociated by trypsinization followed by gentle pipetting, and placed in a tissue culture medium of F12 containing 10% fetal calf serum and antibiotics. As the cells became nearly confluent, some of them were exposed to colchicine or vinblastine sulfate for durations as long as 48 hr. The control and antitubulin-treated cells were processed for transmission electron microscopy and the ultrastructure of the cells was compared. Annulate lamellae (AL) were observed in small amounts in both control and antitubulin-treated cells. The amount of AL did not markedly differ in the control versus antitubulin-treated cells. Furthermore, few multinucleated cells were observed in antitubulin-treated cultures. These results indicate that prolonged culture of cells in antitubulins need not, in itself, lead to a condition of enhanced AL development as reported in several other studies using various cell types.

Effects of prolonged antitubulin culture on annulate lamellae in mouse alpha L929 fibroblasts

The fine structure of alpha L929 fibroblasts cultured in colchicine or vinblastine sulfate for periods as long as 48 hr was compared to control cells not exposed to antitubulins . In response to prolonged antitubulin culture, several changes in cell ultrastructure were noted: Control fibroblasts contain cytoplasmic annulate lamellae (AL), but prolonged exposure to either vinblastine sulfate or colchicine results in enhanced development of AL. Single pore complexes are present in the rough-surfaced endoplasmic reticulum (rER) in both control and antitubulin-treated cells, but stacked porous cytomembranes also occur under both conditions. Polyribosomes often are closely associated or continuous with the pore complexes. Many antitubulin-treated cells become multinucleate. Some nuclei in both control and antitubulin-treated cells contain large and multiple nucleoli. The large and multiple nucleoli are either attached directly to the inner membrane of the nuclear envelope or to infoldings of the nuclear envelope. Antitubulin-treated cells, after 48-hr exposure, appear also to contain enhanced quantities of smooth-surfaced endoplasmic reticulum (sER) and cytoplasmic filaments (and in some cells, lysosomes and rER as well) when compared to untreated cells. In both control and colchicine-treated cells, AL can exhibit continuity with either rER or sER. Further, all three membrane systems may at times be continuous, but the quantity of these membranes appears to be greater in colchicine-treated cells than in control cells. The results are discussed with respect to possible functional significance.

Occurrence of fibronectin on the primary mesenchyme cell surface during migration in the sea urchin embryo

The distribution of fibronectin in situ in the sea urchin embryo was examined by using indirect immunofluorescence with an antibody raised against human plasma fibronectin. Fibronectin was detected on the surfaces of primary mesenchyme cells in the mid-mesenchyme blastula stage, when these cells are migratory. However, it was not detected on these cells at the early mesenchyme blastula or early gastrula stages. Also, it was not detected in the blastocoel nor on the basal surface of the blastular wall. The migration of the primary mesenchyme cells is therefore correlated with a stage-dependent occurrence of cell surface-associated fibronectin.