Storage and mobilization of extracellular matrix proteins during sea urchin development

Spectrin labeling during oogenesis in zebrafish (Danio rerio)

Progression through mitosis and meiosis during early zebrafish ovarian development is accompanied by highly regulated series of transformations in the architecture of oocytes. These cytoskeletal-dependent membrane events may be assumed to be brought about by deployment of proteins. While the cytoskeleton and its associated proteins play a pivotal role in each of these developmental transitions, it remains unclear how specific cytoskeletal proteins participate in regulating diverse processes of oocyte development in zebrafish. Results from this study show that a pool of spectrin accumulates during oogenesis and parallels an increase in volume of oocytes at pre-vitellogenic stages of development. Spectrin labeling is restricted to the surface of oogonia, the cortex of post-pachytene oocytes and later accumulates on the cytoplasm of pre-vitellogenic and vitellogenic oocytes. Results here suggest a correlation between spectrin labeling, increased cytoplasm volume of oocytes, an increase in the number of nucleoli and accumulation of cytoplasmic organelles. Overall, these results suggest that synthesis and storage of spectrin during pre-vitellogenic stages of oogenesis primes the egg with a pre-established pool of membrane-cytoskeletal precursors for use during embryogenesis, and that the presence of spectrin at the oocyte sub-cortex is essential for maintaining oocyte structure.

Sea urchin eggs in the acid reign

Sea urchin eggs have been an indispensable model system for studying egg activation and ionic signalling for at least a century. Instrumental in the discovery of two Ca(2+)-mobilizing second messengers, cyclic ADP-ribose and NAADP, the sea urchin has revolutionized cell biology for all phyla. This review attempts to summarize what we currently know about egg acidic vesicles in the context of Ca(2+) signalling. The dynamics of Ca(2+) storage, Ca(2+) mobilization, proton fluxes and two-pore channels will be discussed.

Multi-scale mechanics from molecules to morphogenesis

Dynamic mechanical processes shape the embryo and organs during development. Little is understood about the basic physics of these processes, what forces are generated, or how tissues resist or guide those forces during morphogenesis. This review offers an outline of some of the basic principles of biomechanics, provides working examples of biomechanical analyses of developing embryos, and reviews the role of structural proteins in establishing and maintaining the mechanical properties of embryonic tissues. Drawing on examples we highlight the importance of investigating mechanics at multiple scales from milliseconds to hours and from individual molecules to whole embryos. Lastly, we pose a series of questions that will need to be addressed if we are to understand the larger integration of molecular and physical mechanical processes during morphogenesis and organogenesis.

Echinonectin is a Del-1-like molecule with regulated expression in sea urchin embryos

Echinonectin (EN) is a dimeric galactosyl-binding protein found in sea urchin eggs and embryos. It had been postulated in earlier studies that EN is secreted into the hyaline layer, a stratified matrix deposited on the apical surface of cells, and serves as an attachment substrate for cells of the blastoderm. However, the dynamics of EN expression have rendered past observations difficult to interpret on this point and others. Radioiodination experiments in this study indicate that the bulk of EN is, at any one time, maintained in its vesicular compartment beneath the plasma membrane, but that a portion of the protein is secreted onto the cell surface during early development. The primary structure of EN was determined. The protein consists of a series of coagulation factor 5/8 repeats and discoidin-like lectin domains, and bears similarity to the secreted proteins DEL-1 and lactadherin from angiogenic endothelial cells. In situ hybridization analysis indicates that EN mRNA levels are regulated to coincide with periods of reduced motility in embryonic cells, supporting the postulate that the protein is involved in cell anchoring.

Proteolytic processing of a sea urchin, ECM-localized protein into lower mol mass species possessing collagen-cleavage activity

The hyaline layer is an apically located extraembryonic matrix, which blankets the sea urchin embryo. Using gelatin substrate gel zymography, we have identified a number of gelatin-cleaving activities within the hyaline layer and defined a precursor-product processing pathway which leads to the appearance of 40- and 38-kDa activities coincident with the loss of a 50-kDa species. Proteolytic processing of the precursor required the presence of both CaCl2 and NaCl at concentrations similar to those found in sea water. The cleavage activities utilized both sea urchin and rat tail tendon gelatins as substrates but demonstrated a species-specific cleavage activity towards sea urchin collagen. The gelatin-cleaving activities were refractory to inhibition by 1,10-phenanthroline but were inhibited by benzamidine. This latter result defines the serine protease nature of the cleavage activities. Both the 40- and 38-kDa activities were found to comigrate with gelatin-cleaving activities present in the sea urchin embryo.

'Nectosome': a novel cytoplasmic vesicle containing nectin in the egg of the sea urchin, Temnopleurus hardwickii

Localization of an extracellular matrix protein, Th-nectin, in the eggs and embryos of the sea urchin Temnopleurus hardwickii was examined by both immunofluorescence and immunoelectron microscopy. The protein is associated with a tubular structure packaged in rod-shaped vesicles that were designated as 'nectosomes'. In unfertilized eggs, nectosomes are distributed uniformly throughout the cytoplasm, but after fertilization, they gradually translocate to the cortical zone where they are arranged perpendicular to the plasma membrane. The migration of the nectosomes was strongly inhibited by cytochalasin B, which suggested that microfilaments play an important role in this process. Immunocytochemical and immunoblotting analyses both ascertained that nectin is secreted into the hyaline layer. Some nectosomes remain in the apical cytoplasm of dermal cells until the gastrula stage. Ultrastructural examination revealed that the accumulation of nectosomes in the oocyte cytoplasm begins quite early in oogenesis, concomitant with the accumulation of cortical vesicles.

Zymogen activation and characterization of a major gelatin-cleavage activity localized to the sea urchin extraembryonic matrix

The hyaline layer (HL) is an apically located extracellular matrix (ECM) which surrounds the sea urchin embryo from the time of fertilization until metamorphosis occurs. While gelatin-cleavage activities were absent from freshly prepared hyaline layers, a dynamic pattern of activities developed in layers incubated at 15 or 37 degrees C in Millipore-filtered sea water (MFSW). Cleavage activities at 90, 55, 41, and 32 kDa were evident following incubation at either temperature. The activation pathway leading to the appearance of these species was examined to determine the minimum salt conditions required for processing and to establish precursor-product relationships. In both qualitative and quantitative assays, the purified 55 kDa gelatinase activity was inhibited by 1,10-phenanthroline (a zinc-specific chelator) and ethylenebis (oxyethylenenitrilo) tetraacetic acid (EGTA). Calcium reconstituted the activity of the EGTA-inhibited enzyme with an apparent dissociation constant (calcium) of 1.2 mM. Developmental substrate gel analysis was performed using various stage embryos. The 55 and 32 kDa species comigrated with gelatin-cleavage activities present in sea urchin embryos. Collectively, the results reported here document a zymogen activation pathway which generates a 55 kDa, gelatin-cleaving activity within the extraembryonic HL. This species displayed characteristics of the matrix metalloproteinase class of ECM modifying enzymes.

An increase in surface area is not required for cell division in early sea urchin development

Cell division requires an increase in surface area to volume ratio. During early development, surface area can increase, volume can decrease, or surface topography can be optimized to allow for division. While exocytosis is thought to be essential for division [Mol. Biol. Cell 10 (1999), 2735; Proc. Natl. Acad. Sci. USA 99 (2002), 3633], exocytosis doesn't always yield an increase in surface area [Proc. Natl. Acad. Sci. USA 79 (1982), 6712]. We used multiphoton laser scanning microscopy, fluorescence spectroscopy, and electron microscopy to monitor membrane trafficking, surface area, volume, and surface topography during early sea urchin development. Despite extensive membrane trafficking monitored by FM 1-43 fluorescence, we find that the net surface area of the embryo does not change prior to the eight-cell stage. During this period, embryo volume decreases by 15%, and microvilli disappear from interior facing membrane segments. Thus, the first three cell divisions utilize residual membrane liberated by decreasing cytoplasmic volume, and reducing microvilli density on interior facing membranes. Only after the eight-cell stage was a net increase in FM 1-43 fluorescence from the embryo surface detected. Our data suggest that compensatory endocytosis is downregulated after this developmental stage to yield an increase in surface area for cell division.

Biochemical analysis of hyalin gelation: an essential step in the assembly of the sea urchin extraembryonic matrix, the hyaline layer

We have examined the effects of calcium and magnesium on both the structural characteristics and the self-association reaction of hyalin, a major protein component of the sea urchin extraembryonic matrix, the hyaline layer. In the absence of calcium, the circular dichroic spectrum revealed a protein possessing a high beta sheet content. The presence of increasing concentrations of calcium resulted in an increase in beta sheet content and a coincidental decrease in alpha helix. This effect occurred with an apparent dissociation constant (calcium) of 1.5mM. The calcium-induced structural change was potentiated by magnesium. Similar concentrations of calcium protected hyalin from digestion by trypsin and this effect was potentiated by magnesium. The thermal denaturation profile of hyalin was modulated by calcium. At a concentration of 3mM, calcium protected hyalin from thermal denaturation, an effect partially mimicked, but not potentiated, by magnesium. Calcium was also found to modulate both the intensity and the wavelength of maximal, endogenous tryptophan fluorescence. The effect of calcium on hyalin tertiary structure had a concentration dependence decidedly different from those reported above with an apparent dissociation constant of 0.18 mM. Collectively, these results delineate two distinct roles for calcium in modulating hyalin structure and allow us to define the pathway leading to hyalin-gel formation.

Localization and functional role of a 41 kDa collagenase/gelatinase activity expressed in the sea urchin embryo

The egg storage compartment of the sea urchin embryo was investigated for a protein destined for export to the extracellular matrices. Using an antiserum prepared against a 41 kDa collagenase/gelatinase localized to the extraembryonic matrices (the hyaline layer and basal lamina), the egg storage compartment was mapped for this antigen. Indirect immunofluorescence analysis revealed the 41 kDa collagenase/gelatinase in the cortical granules as well as a second compartment which was dispersed throughout the egg cytoplasm. High resolution immunogold labeling defined this cytoplasmic compartment as the yolk granule organelle. Gelatin substrate gel zymography revealed the presence of a 41 kDa gelatin cleavage activity in purified yolk granules. These results suggest a role for yolk granules in regulated protein export and challenge the traditional view of this organelle as a benign storage compartment for nutrients. In additional experiments, embryos grown in the presence of the 41 kDa cleavage activity or the anti-41 kDa antiserum had severely delayed gut formation and spicule elongation. These results demonstrate a requirement for defined levels of the 41 kDa activity in the extracellular matrices of the developing embryo.

Ultrastructural aspects of the development of the hyaline layer and extracellular matrix lining the gastrointestinal tract in embryos and larvae of the starfish Pisaster ochraceus preserved by freeze substitution

Embryos and larvae of the starfish Pisaster ochraceus are surrounded by a complex extracellular matrix (ECM) layer called the hyaline layer (HL). A similar but less well-organized ECM layer lines some regions of the larval gut. Examination of material preserved by freeze substitution shows that the HL consists of a coarse outer meshwork, a boundary layer, a supporting layer, which is divided into three sublayers, H1, H2, and H3, and an intervillus layer. The development of the HL has been studied in material preserved by freeze substitution. Development begins at fertilization when exocytosis of the cortical granules releases ECM into the perivitelline space and elevates the fertilization membrane. Shortly after, plaques of dense material with attached fibers are present on the outer surface of the egg plasmalemma. Following this, these plaques and fibers are associated with the tips of short microvilli, suggesting that they may induce microvillus formation. Next, the tips of some of the microvilli are joined by short regions of the H1 sublayer. Some of these H1 regions have short segments of boundary layer material associated with their outer surfaces while others are naked. Just prior to hatching, the H1 and boundary layers completely surround the embryo, separating the developing coarse meshwork and intervillus layers. Short segments of the H2 and H3 sublayers are also present. Posthatching, the microvilli and all HL layers increase in thickness and density, particularly the H2, boundary, and coarse outer meshwork layers. The results suggest a sequential organization of HL components from ECM that is secreted into the perivitelline space.

Plasma membrane resident 'fusion complexes' mediate reconstituted exocytosis

Calcium-triggered exocytosis is thought to be mediated by membrane-associated protein complexes. In sea urchin eggs, high concentrations of calcium activate multiple 'fusion complexes' per cortical vesicle-plasma membrane docking site. Some of these fusion complexes are known to reside in the vesicle membrane. It is not known if fusion complexes also reside in the plasma membrane, or if plasma membrane-resident fusion complexes require cognate partners in the vesicle membrane. Using reconstitution, we show that N-ethylmaleimide treatment of either vesicles or plasma membrane fragments prior to reconstitution does not completely inhibit exocytosis. Treatment of both components did result in complete inhibition. Upon reconstitution, cortical vesicles and the early endosomes formed by compensatory endocytosis both contributed, on average, two fusion complexes per reconstituted docking site. The plasma membrane contributed, on average, two fusion complexes per docking site when assembled with cortical vesicles, but only one complex when reconstituted with endosomes. We conclude that there are at least two types of plasma membrane-resident fusion complexes that participate in reconstituted cortical vesicle-plasma membrane fusion. The activity of one of these fusion complexes is target-specific for cortical vesicles, while the second type also supports fusion with endosomes.

Characterization of matrix metalloprotease activities induced in the sea urchin extraembryonic matrix, the hyaline layer

Hyaline layers, freshly prepared from one-hour-old embryos, were devoid of gelatin-cleavage activity. However, upon storage at 4°C, gelatin-cleavage activities appeared; three species of apparent mol mass 94[Formula: see text]117-, 90-, and 45-kDa were seen. All three species required zinc for activity. Using gel-exclusion chromatography we separated the 94[Formula: see text]117-, and 90-kDa species from the 45-kDa activity. The two higher mol mass species were inhibited by ethylenebis (oxyethylenenitrilo) tetraacetic acid and the lost activity was restored by calcium. Reconstitution of activity occurred with an apparent dissociation constant (calcium) of 5 µM. The presence of millimolar concentrations of magnesium had a minimal inhibitory effect on activity. The thermal denaturation profile of the higher mol mass gelatin-cleavage activity was significantly different in the presence and absence of calcium. Stabilization of these activities against thermal denaturation at 60°C occurred with an apparent dissociation constant (calcium) of 0.6 mM. Magnesium had no significant effect on the thermal denaturation profile. Collectively, these results suggest at least two different modes of interaction between calcium and the higher mol mass gelatinases. These conclusions are discussed in the context of the high calcium and magnesium concentrations present in the sea water environment of the sea urchin embryo.Key words: sea urchin, embryo, matrix metalloprotease, calcium.

Characterization of sea urchin unconventional myosins and analysis of their patterns of expression during early embryogenesis

Early sea urchin development requires a dynamic reorganization of both the actin cytoskeleton and cytoskeletal interactions with cellular membranes. These events may involve the activities of multiple members of the superfamily of myosin motor proteins. Using RT-PCR with degenerate myosin primers, we identified 11 myosin mRNAs expressed in unfertilized eggs and coelomocytes of the sea urchin Strongylocentrotus purpuratus. Seven of these sea urchin myosins belonged to myosin classes Igamma, II, V, VI, VII, IX, and amoeboid-type I, and the remaining four may be from novel classes. Sea urchin myosins-V, -VI, -VII, and amoeboid-type-I were either completely or partially cloned and their molecular structures characterized. Sea urchin myosins-V, -VI, -VII, and amoeboid-type-I shared a high degree of sequence identity with their respective family members from vertebrates and they retained their class-specific structure and domain organization. Analysis of expression of myosin-V, -VI, -VII, and amoeboid-type-I mRNAs during development revealed that each myosin mRNA displayed a distinct temporal pattern of expression, suggesting that myosins might be involved in specific events of early embryogenesis. Interestingly, the onset of gastrulation appeared to be a pivotal point in modulation of myosin mRNA expression. The presence of multiple myosin mRNAs in eggs and embryos provides insight into the potential involvement of multiple specific motor proteins in the actin-dependent events of embryo development.

Lectin histochemistry of the hyaline layer around the larvae of Patiriella species (Asteroidea) with different developmental modes

Larvae of sea stars are surrounded by an extracellular matrix called the hyaline layer. The lectin-binding properties of this matrix were investigated in an ultrastructural study of Patiriella species having different modes of development. The planktonic bipinnaria and brachiolaria of P. regularis and the planktonic brachiolaria of P. calcar demonstrated the same labeling of the hyaline layer for three lectins: Con A, SBA, and WGA. In both species the outer coarse meshwork stained for all three lectins, whereas the intervillous layer displayed patchy labeling. In the benthic brachiolaria of P. exigua, the outer coarse meshwork displayed heavy labeling for all three lectins. The heavy labeling of the outer coarse meshwork of P. exigua compared with that of the other species suggests an increased number of lectin binding sites in the hyaline layer of this species. The similar ultrastructure and histochemistry of the hyaline layer of P. regularis and P. calcar may reflect similar requirements of their extracellular cover in their planktonic environment. Lectin labeling shows that hypertrophy of the hyaline layer of P. exigua, in particular the outer coarse meshwork, involves elaboration of the carbohydrate composition of the matrix. Modifications seen in the ultrastructure and histochemistry of the hyaline layer of P. exigua appear to be associated with the evolution of benthic development.

Apextrin, a novel extracellular protein associated with larval ectoderm evolution in Heliocidaris erythrogramma

During the evolution of direct development in the sea urchin Heliocidaris erythrogramma major modifications occurred, which allowed the precocious formation of adult-specific structures and led to a novel larval body that surrounds these structures. The HeET-1 gene was isolated in a differential screen for transcripts enriched in the early embryos of H. erythrogramma relative to those of its indirect-developing congener, H. tuberculata. HeET-1 was unique among the three genes found in that no homologous transcript was detected in H. tuberculata total embryonic RNA blots. To verify this apparently extreme differential expression of the HeET-1 genes in Heliocidaris, we isolated the HeET-1 homologue from H. tuberculata genomic DNA and used it to probe blots of poly(A)+ RNA prepared from H. tuberculata embryos. It is expressed in H. tuberculata embryos at levels undetectable by this technique. The predicted amino acid sequence of HeET-1 suggested that it encodes a novel secreted protein. To assess the function of HeET-1, we raised polyclonal antisera to the HeET-1-encoded protein. We find that it is present in eggs in a type of secretory vesicle and that this maternal pool is gradually secreted after fertilization. As cells acquire apical-basal polarity in the blastula the protein becomes localized to the apical extracellular matrix, leading us to name the protein apextrin. The apical extracellular localization of apextrin is maintained in the columnar cells of the larval ectoderm until their internalization at metamorphosis. Ingressing mesenchyme cells rapidly endocytose apextrin upon leaving the vegetal plate. Comparison with fibropellin III, an apical lamina component, suggests that apextrin is an extracellular protein that is in tighter association with the plasma membrane than is the hyalin layer or apical lamina. We propose that apextrin is involved in apical cell adhesion and that its high level of expression may represent an adaptive cooption necessary for strengthening the large H. erythrogramma embryo.

Measurements of mechanical properties of the blastula wall reveal which hypothesized mechanisms of primary invagination are physically plausible in the sea urchin Strongylocentrotus purpuratus

Computer simulations showed that the elastic modulus of the cell layer relative to the elastic modulus of the extracellular layers predicted the effectiveness of different force-generating mechanisms for sea urchin primary invagination [L. A. Davidson, M. A. R. Koehl, R. Keller, and G. F. Oster (1995) Development 121, 2005-2018]. Here, we measured the composite elastic modulus of the cellular and extracellular matrix layers in the blastula wall of Strongylocentrotus purpuratus embryos at the mesenchyme blastula stage. Combined, these two layers exhibit a viscoelastic response with an initial stiffness ranging from 600 to 2300 Pa. To identify the cellular structures responsible for this stiffness we disrupted these structures and correlated the resulting lesions to changes in the elastic modulus. We treated embryos with cytochalasin D to disrupt the actin-based cytoskeleton, nocodazole to disrupt the microtubule-based cytoskeleton, and a gentle glycine extraction to disrupt the apical extracellular matrix (ECM). Embryos treated less than 60 min in cytochalasin D showed no change in their time-dependent elastic modulus even though F-actin was severely disrupted. Similarly, nocodazole had no effect on the elastic modulus even as the microtubules were severely disrupted. However, glycine extraction resulted in a 40 to 50% decrease in the elastic modulus along with a dramatic reduction in the hyalin protein at the apical ECM, thus implicating the apical ECM as a major mechanical component of the blastula wall. This finding bears on the mechanical plausibility of several models for primary invagination.

Calcium-protein interactions in the extracellular environment: calcium binding, activation, and immunolocalization of a collagenase/gelatinase activity expressed in the sea urchin embryo

We have purified and characterized a collagenase/gelatinase activity expressed during sea urchin embryonic development. The native molecular mass was determined to be 160 kDa, while gelatin substrate gel zymography revealed an active species of 41 kDa, suggesting that the native enzyme is a tetramer of active subunits. Incubation in the presence of EGTA resulted in nearly complete loss of activity and this effect could be reversed by calcium. Calcium-induced reactivation appeared to be cooperative and occurred with an apparent kd value of 3.7 mM. Two modes of calcium binding to the 41-kDa subunit were detected; up to 80 moles of calcium bound with a kd value of 0.5 mM, while an additional 120 moles bound with a kd value of 5 mM. Amino acid analysis revealed a carboxy plus carboxyamide content of 24.3 mol/100 mol, indicating the availability of substantial numbers of weak Ca2+-binding sites. Calcium binding did not result in either secondary or quaternary structural changes in the collagenase/gelatinase, suggesting that Ca2+ may facilitate activation through directly mediating the binding of substrate to the enzyme. The collagenase/gelatinase activity was detected in blastocoelic fluid and in the hyalin fraction dissociated from 1-h-old embryos. Immunolocalization studies revealed two storage compartments in the egg; cortical granules and small granules/vesicles dispersed throughout the cytoplasm. After fertilization, the antigen was detected in both the apical and basal extracellular matrices, the hyaline layer, and basal lamina, respectively.

Reconstitution of calcium-triggered membrane fusion using "reserve" granules

Calcium-gated secretion of proteins involves the transfer of "reserve" granules, exocytotic vesicles that are cytoplasmic and, hence, plasma membrane-naive, from the cell interior to the surface membrane where they dock prior to fusion. Docking and subsequent priming steps are thought to require cytoplasmic factors. These steps are believed to induce fusion competence. We have tested this hypothesis by isolating reserve granules from sea urchin eggs and determining under which conditions these granules will fuse. We find that isolated reserve granules, lacking soluble cofactors, support calcium-dependent membrane fusion in vitro. Preincubation with adenosine 5'-3-O-(thio)triphosphate and guanosine 5'-3-O-(thio)triphosphate did not prevent fusion. Thus, isolated reserve granules have all the necessary components required for calcium-gated fusion prior to docking.

Regulated exocytosis and sequential construction of the extracellular matrix surrounding the sea urchin zygote

After fertilization most eggs become surrounded by a complex extracellular matrix. This study examines those matrix assembly processes that are triggered by fertilization of the sea urchin egg. The study uses antibodies that identify five different storage compartments in the egg. These compartments release their protein contents in a highly regulated fashion to assemble and modify the extraembryonic layers. The exocytosis sequence begins with a fertilization wave that progresses from the site of sperm entry and elevates the fertilization envelope above a water-filled perivitelline space. The immediate surface of the zygote then becomes covered by a newly secreted hyaline layer. Prior to fertilization some of the antigens are localized to cortical granules. Others are found in "basal laminar vesicles" that are released in a wave beginning at about 30 sec, or roughly at the same time as cortical granule exocytosis. The remaining antigens are exocytosed with a rather precise timing, but with a delay of several to tens of minutes relative to the first wave of exocytosis. "Apical vesicles," so named because antigens from this class are preferentially exocytosed toward the apical cell surface of polarized cells, include antigens that are exocytosed beginning at about 5 min postfertilization. The fourth compartment, named "echinonectin vesicles" release echinonectin, a protein that is deposited to the inner side of the hyaline layer. Surface staining of echinonectin is first detected about 10-15 min following sperm contact. Finally, maternal cadherin, which is stored in yet a fifth distinct compartment, is not detected on the surface until at least 30 min following fertilization. The data are also consistent with the notion that the tightly regulated timing of exocytosis contributes to the ordered assembly of the hyaline layer and elevation of the fertilization envelope. Finally, two of the vesicle classes continue to exocytose after the cells become polarized. In polarized cells apical and basal laminar antigens are trafficked toward opposite sides of the same cell after passing through the same trans-Golgi network-like compartment.

Ultrastructure and synthesis of the extracellular matrix of Pisaster ochraceus embryos preserved by freeze substitution

When asteroid embryos cryoprotected with propylene glycol are rapidly frozen in liquid propane and freeze substituted with ethanol, preservation of the cells and extracellular matrix (ECM) is excellent. The basal lamina, although thicker and less well defined than in conventionally fixed embryos, demonstrates a region of decreased density just below the cells that corresponds to the lamina lucida and a lamina densa. The former region is often occupied by fibrous material. In addition, as was previously described in conventionally fixed issues, the basal lamina of the ectoderm is generally thicker and more substantial than that of the endoderm, reinforcing an earlier suggestion that the structure of the basal is different in different regions of the embryo. The ECM of the blastocoel consists of thin "twig-like" elements that form a loose meshwork evenly distributed throughout the blastocoel. Bundles of 20 nm fibers, located within the meshwork, are oriented parallel to the base of the cells of the stomodeum. In the long axis of the embryo, similar fibers are present in the dorsal aspect of the animal between the stomach and the ectoderm and radiate out from the esophagus crossing the region between it and the ectoderm. Immunocytochemical work with three different monoclonal antibodies shows that glycoprotein molecules, synthesized in the Golgi apparatus, are also secreted here and form part of the matrix structure. The results suggest that the blastocoel is filled with a gel-like material reinforced with bundles of 20-nm fibers. The manner in which the observed arrangement could contribute to the development and maintainence of the shape of the embryo is discussed.

Dynamics of filamentous actin organization in the sea urchin egg cortex during early cleavage divisions: implications for the mechanism of cytokinesis

We have used confocal laser scanning microscopy in conjunction with BODIPY-phallacidin staining of filamentous actin to investigate changes in the quantity and organization of cortical actin during the first two cell cycles following fertilization in eggs of the sea urchin Strongylocentrotus purpuratus. Quantification of fluorescent phallacidin staining reveals that the amount of filamentous actin (F-actin) in the cortex undergoes cyclical increases and decreases during early cleavage divisions, peaking near the beginning of the cell cycle and decreasing to a minimum at cytokinesis. Changes in the content of cortical F-actin are accompanied by the growth and disappearance of rootlet-like bundles of actin filaments which extend from the bases of microvilli that cover the surface of the egg. Actin rootlets reach their maximum degree of development by 20 min postfertilization, and then gradually decrease in number and length over the next 40 min. Small actin rootlets persist until cleavage, disappear during cytokinesis, and reform following division. The formation of actin rootlets requires cytoplasmic alkalization and is inhibited by cytochalasin D. Cytochalasin D washout experiments demonstrate that assembly of the cortical actin cytoskeleton can be blocked until 5 min before the onset of cleavage and still allow normal cytokinesis. These results illustrate the dynamic nature of cortical actin organization during early development and demonstrate that cytokinesis occurs at the point of minimum cortical F-actin content. They further demonstrate that cytokinesis can occur in embryos in which the normal developmental sequence of changes in cortical actin organization has been blocked by treatment with cytochalasin D, suggesting that these changes do not function in the establishment of the contractile apparatus for cytokinesis, but rather serve other developmental functions. Cell Motil. Cytoskeleton 36:30-42, 1997.

Purification and metal ion requirements of a candidate matrix metalloproteinase: a 41 kDa gelatinase activity in the sea urchin embryo

Using substrate gel zymography, the sea urchin embryo was found to express a dynamic pattern of gelatinase activities with a 41 kDa species persisting throughout the course of embryonic development. We have purified to near homogeneity the 41 kDa gelatinase in the sea urchin egg. In both qualitative and quantitative assays, the 41 kDa gelatinase activity was inhibited by ethylenediaminetetracetic acid but not the serine protease inhibitor, phenylmethylsulfonylfluoride, or the chelating agent, 1,10-phenanthroline. Activity could be restored to the inactivated gelatinase by each of several divalent cations: Ca(2+) > Mn(2+) > Mg(2+) > Cu(2+). Cadmium and Zn(2+) were largely ineffective at reconstituting the inactivated enzyme. In metal ion binding assays, the relative apparent affinities of the metal ions for binding to the gelatinase were determined to be Zn(2+) > or = Cd(2+) > or = Ca(2+) > Mn(2+) > Mg(2+) > Cu(2+). While the gelatinase is clearly a metalloproteinase, metal ion binding per se is not sufficient for activity. The 41 kDa gelatinase exhibited selective substrate utilization, being most active with gelatin, substantially less active with casein, and inactive towards bovine haemoglobin and bovine serum albumin as substrates. The substrate specificity and metal ion requirements suggest that this species is a member of the matrix metalloproteinase class of extracellular matrix remodelling enzymes.

Direct membrane retrieval into large vesicles after exocytosis in sea urchin eggs

At fertilization in sea urchin eggs, elevated cytosolic Ca2+ leads to the exocytosis of 15,000-18,000 1.3-microns-diam cortical secretory granules to form the fertilization envelope. Cortical granule exocytosis more than doubles the surface area of the egg. It is thought that much of the added membrane is retrieved by subsequent endocytosis. We have investigated how this is achieved by activating eggs in the presence of aqueous- and lipid-phase fluorescent dyes. We find rapid endocytosis of membrane into 1.5-microns-diam vesicles starting immediately after cortical granule exocytosis and persisting over the following 15 min. The magnitude of this membrane retrieval can compensate for the changes in the plasma membrane of the egg caused by exocytosis. This membrane retrieval is not stimulated by PMA treatment which activates the endocytosis of clathrin-coated vesicles. When eggs are treated with short wave-length ultraviolet light, cortical granule exocytosis still occurs, but granule cores fail to disperse. After egg activation, large vesicles containing semi-intact cortical granule protein cores are observed. These data together with experiments using sequential pulses of fluid-phase markers support the hypothesis that the bulk of membrane retrieval immediately after cortical granule exocytosis is achieved through direct retrieval into large endocytotic structures.

Identification of a component of the sea urchin hyaline layer, HLC-175, which undergoes proteolytic processing during development

To define the role(s) played by the sea urchin extraembryonic matrix, the hyaline layer, we have previously purified and characterized a number of the protein components of this structure. We are currently studying the timing and significance of the proteolytic processing of these species. The localization of HLC-175 in the egg and 1-hr-old embryo was determined by indirect immunofluorescence analysis. The relationship between HLC-175 and the 109- and 81 kDa species was determined by a combination of native gel electrophoresis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under non-reducing conditions and protein gel blot analyses using the anti-175, -109 and 81 kDa antisera. Using gel exclusion chromatography we have fractionated a mixture of proteins extracted from the surface of 1-hr-old sea urchin embryos. A set of fractions eluting from the column contained three species of apparent molecular masses 175-, 109- and 81 K. These species comigrated on analysis by either non-reducing SDS-PAGE or native gel electrophoresis. Inclusion of the reducing agent, dithiothreitol, in the solubilizing solutions abolished comigration of these polypeptides. When polyclonal antisera were prepared against each of these antigens cross-reactivity between the 175- and 109 kDa species and between the 175- and 81 kDa species was detected. Developmental protein gel blot analyses revealed a precursor-product relationship between the 175- and the 109- and 81 kDa polypeptides. Indirect immunofluorescence analysis confirmed the localization of HLC-175 to the hyaline layer. The results reported here clearly identify HLC-175 as a component of the hyaline layer.(ABSTRACT TRUNCATED AT 250 WORDS)

A role for regulated secretion of apical extracellular matrix during epithelial invagination in the sea urchin

Epithelial invagination, a basic morphogenetic process reiterated throughout embryonic development, generates tubular structures such as the neural tube, or pit-like structures such as the optic cup. The ‘purse-string’ hypothesis, which proposes that circumferential bands of actin microfilaments at the apical end of epithelial cells constrict to yield a curved epithelial sheet, has been widely invoked to explain invaginations during embryogenesis. We have reevaluated this hypothesis in two species of sea urchin by examining both natural invagination of the vegetal plate at the beginning of gastrulation and invagination induced precociously by Ca2+ ionophore. Neither type of invagination is prevented by cytochalasin D. In one species, treatment with A23187 three hours before the initiation of invagination resulted in the deposition of apical extracellular matrix at the vegetal plate, rather than invagination. This apical matrix contains chondroitin sulfate, as does the lumen of the archenteron in normal gastrulae. When the expansion of this secreted matrix was resisted by an agarose gel, the vegetal plate buckled inward, creating an archenteron that appeared 3–4 hours prematurely. Pretreatment with monensin, which blocks secretion, inhibits both Ca2+ ionophore-stimulated folding and natural invagination, demonstrating that secretion is probably required for this morphogenetic event. These results indicate that alternatives to the purse-string hypothesis must be considered, and that the directed deposition of extracellular matrix may be a key Ca(2+)-regulated event in some embryonic invaginations. A bending bilayer model for matrix-driven epithelial invagination is proposed in which the deposition of hygroscopic material into a complex, stratified extra-cellular matrix results in the folding of an epithelial sheet in a manner analagous to thermal bending in a bimetallic strip.

Deployment of extracellular matrix proteins in sea urchin embryogenesis

The apical extracellular matrix of the sea urchin embryo, known as the hyaline layer (HL), is a multi-laminate organelle composed of at least 10 polypeptides. Although integrated into one ECM, HL proteins exhibit individual temporal and spatial dynamics throughout development. These molecules are stockpiled in the oocyte during vitellogenesis in at least four distinct vesicle populations. They are released onto the cell surface at fertilization in a specific order, and interact differentially with embryonic cells as development proceeds. Many experiments have suggested that the HL is vital for embryogenesis, but relatively little is known about the functions and interactions of its constituent molecules. The purpose of the present review has been to gather information on the basic characteristics of the known HL proteins together with data on their expression in the embryo, and where possible, their biological activities. Compiled, these observations may provide some insight into the workings of a uniquely embryonic organelle.

A new extracellular matrix protein of the sea urchin embryo with properties of a substrate adhesion molecule

A new embryonic extracellular matrix protein has been purified from eggs of the sea urchin Paracentrotus lividus. The molecule is a 210 kD dimer consisting of two 105 kD subunits that are held together by S-S bridges. In the unfertilized egg, the protein is found within granules uniformly distributed throughout the cytoplasm. After the egg is fertilized, the antigen is polarized to the apical surface of ectodermal and endodermal cells during all of the developmental stages examined, until the pluteus larva is formed. The protein promotes the adhesion of blastula cells to the substrate and is antigenically distinct from echinonectin, a well characterized substrate adhesion molecule. This report adds a new candidate to the list of known extracellular matrix molecules for the regulation of differentiation and morphogenesis in the sea urchin embryo.

Protein-protein interactions and structural entities within the sea urchin extraembryonic matrix, the hyaline layer

We have investigated the effects of a variety of experimental conditions on the structural integrity of the sea urchin extraembryonic matrix, the hyaline layer. Removal of Ca2+ resulted in the quantitative release of hyalin from isolated layers. Protein gel blot analyses indicated that, in the absence of Ca2+, hyalin was also quantitatively released from the layers surrounding 1-h-old embryos. However, no other polypeptides of the hyaline layer were released in significant amounts. The layers remaining after removal of hyalin were refractory to digestion with proteinase K and dissociated only in the presence of chaotropes. These results provide insights into the structural organization within the hyaline layer as well as the role of the embryonic cell surface in maintaining the structural integrity of this extraembryonic matrix.

Fibropellins, products of an EGF repeat-containing gene, form a unique extracellular matrix structure that surrounds the sea urchin embryo

The sea urchin SpEGF 1 gene belongs to a growing family of developmentally important genes which encode proteins that contain repeated epidermal growth factor-like motifs. To characterize the embryonic expression of the protein products of this gene from Strongylocentrotus purpuratus, we generated polyclonal antisera from SpEGF I fusion proteins. These antibodies recognize two glycoproteins of 145 and 185 kDa, which we have named fibropellins. These proteins are present in unfertilized oocytes and throughout early development. The fibropellins are stored in cytoplasmic vesicles in the oocyte and are released soon after fertilization in a distinct secretory event following the exocytosis of cortical granule contents. Following secretion the proteins are localized in the basal surface of the hyaline layer. At the blastula stage the fibropellins become organized into distinct fibers which form a mesh-like network over the surface of the embryo. During subsequent development to the pluteus larva stage this network increases in overall morphological complexity and becomes regionally distinct. The molecular weights of the fibropellins and their pattern of embryonic localization indicate that these proteins form a component of the hyaline layer previously described as the apical lamina.

Assembly of the sea urchin extraembryonic hyaline layer; Ca2+ and Mg2+ act independently and at different sites on the pathway leading to hyalin-gel formation

We have studied the interactions of Ca2+ with the sea urchin extraembryonic coat protein hyalin. As reported previously, Ca2+ alone was ineffective in inducing hyalin-gel (large aggregate) formation. This reaction required the additional presence of Mg2+ and NaCl. However, the results of tryptic digestion and nondenaturing agarose gel electrophoresis experiments demonstrated that Ca2+ could induce hyalin self-association into small aggregates in the absence of Mg2+ and NaCl. Magnesium did not modulate the interactions of Ca2+ with hyalin. In addition, Mg2+ had minimal effects on the conformation of hyalin. These results have been incorporated into a model delineating the pathway leading to hyalin-gel formation.

Subcellular localization of sea urchin egg spectrin: evidence for assembly of the membrane-skeleton on unique classes of vesicles in eggs and embryos

A recent study from our laboratory on the sea urchin egg suggested that spectrin was not solely restricted to the plasma membrane, but instead had a more widespread distribution on the surface of a variety of membranous inclusions. (E. M. Bonder et al., 1989, Dev. Biol. 134, 327-341). In this report we extend our initial findings and provide experimental and ultrastructural evidence for the presence of spectrin on three distinct classes of cytoplasmic vesicles. Immunoblot analysis of membrane fractions prepared from egg homogenates establishes that spectrin coisolates with vesicle-enriched fractions, while indirect immunofluorescence microscopy on cryosections of centrifugally stratified eggs demonstrates that spectrin specifically associates with cortical granules, acidic vesicles, and yolk platelets in vivo. Immunogold ultrastructural localization of spectrin on cortices isolated from eggs and early embryos details the striking distribution of spectrin on the cytoplasmic surface of the plasma membrane and the membranes of cortical granules, acidic vesicles, and yolk platelets, while quantitative studies show that relatively equivalent amounts of spectrin are present on the different membrane surfaces both before and after fertilization. These data, in combination with the localization of numerous spectrin crosslinks between actin filaments in surface microvilli, suggest that spectrin plays a pivotal role in structuring the cortical membrane-cytoskeletal complex of the egg and the embryo.

Sea urchin spectrin in oogenesis and embryogenesis: a multifunctional integrator of membrane-cytoskeletal interactions

Using indirect immunofluorescence microscopy on semithin cryosections of maturing ovarian tissue, eggs, and developing embryos, we have mapped the cellular distribution and dynamic redistribution of spectrin in oogenesis and early embryogenesis. During oogenesis, spectrin is initially found in the cortex of oogonia and previtellogenic oocytes, and later accumulates in the cytoplasm of vitellogenic oocytes on the surfaces of cortical granules, pigment granules/acidic vesicles, and yolk platelets. Following egg activation, spectrin undergoes a rapid redistribution coincident with three major developmental events including: (1) restructuring of the cell surface, (2) translocation of pigment granules/acidic vesicles to the cortex during the first cell cycle, and (3) amplification of the embryo's surface during the rapid cleavage phase of early embryogenesis. The synthesis and storage of spectrin during oogenesis appears to prime the egg with a preestablished pool of membrane-cytoskeletal precursor for use during embryogenesis. Results from this study support the hypothesis that spectrin may function as a key integrator and modulator of multiple membrane-cytoskeletal functions during embryonic growth and cellular differentiation.

Differentiation of a calsequestrin-containing endoplasmic reticulum during sea urchin oogenesis

We have used light and electron microscopic immunolocalization to study the distribution of a sea urchin calsequestrin-like protein (SCS) during sea urchin oogenesis. SCS was localized exclusively in the lumen of the endoplasmic reticulum (ER) and in the nuclear envelope of oocytes of all maturation stages. Immunoelectron microscopy also revealed that SCS is not present in golgi complexes of oocytes. Double label immunofluorescent staining of frozen sections of ovary with the SCS antiserum and an antibody to the cortical granule protein hyalin indicated a dramatic morphogenesis of the SCS-containing ER (SCS-ER) coincident with oocyte maturation. This differentiation included an apparent increase in the amount and complexity of the cytoplasmic SCS-ER network, the transient appearance of stacks of SCS-ER cisternae in synthetically active vitellogenic oocytes, and the restructuring of the SCS-ER in the cortex. Immunofluorescence of isolated oocyte cortices showed a plasma membrane-associated SCS-ER which was much less dense and regular than that found surrounding the cortical granules in the mature unfertilized egg cortex. Cytoplasmic and cortical microtubule arrays are present in oocytes and may provide the basis for the SCS-ER distributional dynamics. The results of this study underscore the dynamic nature of ER and how it's organization reflects cellular functions. We suggest that the establishment during oogenesis of the dense SCS-ER tubuloreticulum provides the egg with the calcium sequestration and release apparatus that regulates calcium fluxes during egg activation and early development.

Collagen type IV of Drosophila is stockpiled in the growing oocyte and differentially located during early stages of embryogenesis

We have developed and characterized a battery of specific polyclonal antibodies directed against specific portions of the alpha-chain of collagen type IV synthesized in Drosophila by the gene DCg1. Here, we describe the use of these antibodies together with in situ hybridization experiments in an attempt to study the expression and localization of collagen type IV during Drosophila oogenesis and early embryogenesis. The results clearly demonstrate that DCg1 is maternally expressed by follicle cells and that the collagen type IV chain produced is stockpiled in the growing oocyte. During the gastrulation stages, this component of Drosophila basement membranes concentrated on cells involved in the gradual invaginations leading to morphogenetic furrows. The presence of collagen type IV, which is an RGD-bearing molecule, during early stages of Drosophila development is discussed in comparison with the crucial, active role its vertebrate counterpart is supposed to play in morphogenetic processes.

In vitro biological activities of echinonectin

Echinonectin (EN) is a 230-kDa extracellular matrix glycoprotein found in the hyaline layer of sea urchin embryos. Dissociated embryonic cells attached strongly to EN-coated microtiter wells in a centrifugal-based in vitro adhesion assay, suggesting that EN is one of the hyaline layer proteins to which cells adhere in vivo (Alliegro et al., 1988). The present study examines the molecular properties of that adhesion using monoclonal antibodies as probes to block cell attachment, and also demonstrates that EN possesses lectin activity. EN binds tenaciously to agarose-based chromatography resins, such as Sepharose. The sugar-binding activity is associated with the polypeptide component of EN, and not with the carbohydrate moiety. Binding is inhibited with galactose and fucoidan, but not with glucose or locust bean gum. Although functional sites both for polysaccharide binding and for cell attachment are present on each subunit of the EN molecule, the sites appear to be functionally distinct because galactose and fucoidan are completely without effect on cell attachment in vitro. Proteolytic digestion of EN yields a highly limited set of immunoreactive peptides. Digestion with trypsin yields a 20-kDa fragment, chymotrypsin, a doublet at 20 kDa, and 20- and 23-kDa fragments with thermolysin. McAb's directed against these peptides block cell adhesion in vitro, suggesting that they possess the cell attachment domain of EN. This is supported by the observations that trypsin-digested EN is an effective substrate in adhesion assays and that adhesion to the tryptic fragments is also blocked by McAb's to the 20-kDa domain.

Ontogeny and characterization of mesenchyme antigens of the sea urchin embryo

A monoclonal antibody, Sp12, binds to cortical granules, the hyaline layer, and skeletogenic, chromogenic, and blastocoelar mesenchyme of sea urchin eggs and embryos. Adult urchins also express Sp12 antigens in the dermal layer of the test and spines. Antigen is expressed on the surface of primary mesenchyme cells after they have entered the blastocoel, and by two secondary mesenchyme derivatives--the blastocoelar cells after they have been released from the tip of the archenteron, and the pigment cells in prism stage embryos. Immunogold localizations show antigen on the surfaces of mesenchyme, within membrane bounded vesicles, and associated with the Golgi apparatus. Western blots of antigens immunoprecipitated from seven developmental stages reveal twelve antigens ranging in Mr from 35 k to 240 k. Most of these antigens appear, disappear or change Mr over the first five days of development. Characterizations of this complex array of antigens show that the epitope recognized by Sp12 is eliminated by proteolytic enzymes and endoglycosidase F, while immunoreactivity is only reduced by periodate oxidation. As well, calcium magnesium free seawater extracts a subset of antigens different from that retained by crude membrane preparations. It is proposed that the mesenchyme of sea urchin embryos produces a family of developmentally regulated cell surface and extracellular matrix glycoproteins which all exhibit a carbohydrate epitope recognized by Sp12.