A calsequestrin-like protein in the endoplasmic reticulum of the sea urchin: localization and dynamics in the egg and first cell cycle embryo

Calsequestrin, a key protein in striated muscle health and disease

Calsequestrin (CASQ) is the most abundant Ca²⁺ binding protein localized in the sarcoplasmic reticulum (SR) of skeletal and cardiac muscle. The genome of vertebrates contains two genes, CASQ1 and CASQ2. CASQ1 and CASQ2 have a high level of homology, but show specific patterns of expression. Fast-twitch skeletal muscle fibers express only CASQ1, both CASQ1 and CASQ2 are present in slow-twitch skeletal muscle fibers, while CASQ2 is the only protein present in cardiomyocytes. Depending on the intraluminal SR Ca²⁺ levels, CASQ monomers assemble to form large polymers, which increase their Ca²⁺ binding ability. CASQ interacts with triadin and junctin, two additional SR proteins which contribute to localize CASQ to the junctional region of the SR (j-SR) and also modulate CASQ ability to polymerize into large macromolecular complexes. In addition to its ability to bind Ca²⁺ in the SR, CASQ appears also to be able to contribute to regulation of Ca²⁺ homeostasis in muscle cells. Both CASQ1 and CASQ2 are able to either activate and inhibit the ryanodine receptors (RyRs) calcium release channels, likely through their interactions with junctin and triadin. Additional evidence indicates that CASQ1 contributes to regulate the mechanism of store operated calcium entry in skeletal muscle via a direct interaction with the Stromal Interaction Molecule 1 (STIM1). Mutations in CASQ2 and CASQ1 have been identified, respectively, in patients with catecholamine-induced polymorphic ventricular tachycardia and in patients with some forms of myopathy. This review will highlight recent developments in understanding CASQ1 and CASQ2 in health and diseases.

High resolution imaging of the cortex isolated from sea urchin eggs and embryos

The cellular cortex-consisting of the plasma membrane and the adjacent outer few microns of the cytoplasm-is a critically important, dynamic and complex region in the sea urchin egg and embryo. Some 40 years ago it was discovered that isolated cortices could be obtained from eggs adhered to glass coverslips and since that time this preparation has been used in a wide range of studies, including seminal research on fertilization, exocytosis, the cytoskeleton, and cytokinesis. In this chapter, we discuss methods for isolating cortices from eggs and embryos, including those undergoing cell division. We also provide protocols for analyzing cortical architecture and dynamics using specific localization methods combined with super-resolution Structured Illumination and Stimulated Emission Depletion light microscopy and platinum replica transmission electron microscopy.

Bidirectional Ca²⁺ signaling occurs between the endoplasmic reticulum and acidic organelles

After acidic organelles induce signaling to activate ER calcium ion release, local microdomains of high calcium at ER–acidic organelle junctions feed back to activate further acidic organelle calcium release.

The endoplasmic reticulum (ER) and acidic organelles (endo-lysosomes) act as separate Ca²⁺ stores that release Ca²⁺ in response to the second messengers IP₃ and cADPR (ER) or NAADP (acidic organelles). Typically, trigger Ca²⁺ released from acidic organelles by NAADP subsequently recruits IP₃ or ryanodine receptors on the ER, an anterograde signal important for amplification and Ca²⁺ oscillations/waves. We therefore investigated whether the ER can signal back to acidic organelles, using organelle pH as a reporter of NAADP action. We show that Ca²⁺ released from the ER can activate the NAADP pathway in two ways: first, by stimulating Ca²⁺-dependent NAADP synthesis; second, by activating NAADP-regulated channels. Moreover, the differential effects of EGTA and BAPTA (slow and fast Ca²⁺ chelators, respectively) suggest that the acidic organelles are preferentially activated by local microdomains of high Ca²⁺ at junctions between the ER and acidic organelles. Bidirectional organelle communication may have wider implications for endo-lysosomal function as well as the generation of Ca²⁺ oscillations and waves.

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.

How and why does the endoplasmic reticulum move?

The ER (endoplasmic reticulum) is a fascinating organelle that is highly dynamic, undergoing constant movement and reorganization. It has many key roles, including protein synthesis, folding and trafficking, calcium homoeostasis and lipid synthesis. It can expand in size when needed, and the balance between tubular and lamellar regions can be altered. The distribution and organization of the ER depends on both motile and static interactions with microtubules and the actin cytoskeleton. In the present paper, we review how the ER moves, and consider why this movement may be important for ER and cellular function.

Calcium signalling in early embryos

The onset of development in most species studied is triggered by one of the largest and longest calcium transients known to us. It is the most studied and best understood aspect of the calcium signals that accompany and control development. Its properties and mechanisms demonstrate what embryos are capable of and thus how the less-understood calcium signals later in development may be generated. The downstream targets of the fertilization calcium signal have also been identified, providing some pointers to the probable targets of calcium signals further on in the process of development. In one species or another, the fertilization calcium signal involves all the known calcium-releasing second messengers and many of the known calcium-signalling mechanisms. These calcium signals also usually take the form of a propagating calcium wave or waves. Fertilization causes the cell cycle to resume, and therefore fertilization signals are cell-cycle signals. In some early embryonic cell cycles, calcium signals also control the progress through each cell cycle, controlling mitosis. Studies of these early embryonic calcium-signalling mechanisms provide a background to the calcium-signalling events discussed in the articles in this issue.

A single luminally continuous sarcoplasmic reticulum with apparently separate Ca2+ stores in smooth muscle

Whether or not the sarcoplasmic reticulum (SR) is a continuous, interconnected network surrounding a single lumen or comprises multiple, separate Ca2+ pools was investigated in voltage-clamped single smooth muscle cells using local photolysis of caged compounds and Ca2+ imaging. The entire SR could be depleted or refilled from one small site via either inositol 1,4,5-trisphosphate receptors (IP3R) or ryanodine receptors (RyR) suggesting the SR is luminally continuous and that Ca2+ may diffuse freely throughout. Notwithstanding, regulation of the opening of RyR and IP3R, by the [Ca2+] within the SR, may create several apparent SR elements with various receptor arrangements. IP3R and RyR may appear to exist entirely on a single store, and there may seem to be additional SR elements that express either only RyR or only IP3R. The various SR receptor arrangements and apparently separate Ca2+ storage elements exist in a single luminally continuous SR entity.

Extracellular heat shock protein 70 has novel functional effects on sea urchin eggs and coelomocytes

Numerous reports document that the 70 kDa heat shock proteins are not only intracellular proteins but are also present in blood and other extracellular compartments. How they affect cell function from the extracellular space remains unclear. Using two well-characterized cell types from the sea urchin, we show that extracellular mixtures of the constitutive and inducible forms of the 70 kDa heat shock proteins (Hsc70 and Hsp70, respectively) have dramatic effects on initiation of cell division in fertilized eggs and on the clotting reaction of hypotonically stressed coelomocytes. In suspensions of fertilized eggs to which Hsc70 or a 2:3 mixture of Hsc and Hsp70 was added, progression to the first mitotic division was accelerated. Evidence is provided that the extracellular Hsc70 passes into the egg cells in an unconventional manner, being distributed through the cytoplasm, and that it may alter the intracellular signaling cascade initiated by sperm penetration. In coelomocytes that were stimulated by hypotonic shock to mimic injury, the spreading reaction of the clotting response was significantly inhibited when either Hsp70 or Hsc70 was in the medium. These results suggest that the presence of Hsc and/or Hsp70 in the extracellular fluid may promote mitosis of dividing cells and suppress the reactivity of immune system cells.

Cellular magnesium acquisition: an anomaly in embryonic cation homeostasis

The intracellular dominance of magnesium ion makes clinical assessment difficult despite the critical role of Mg(++) in many key functions of cells and enzymes. There is general consensus that serum Mg(++) levels are not representative of the growing number of conditions for which magnesium is known to be important. There is no consensus method or sample source for testing for clinical purposes. High intracellular Mg(++) in vertebrate embryos results in part from interactions of cations which influence cell membrane transport systems. These are functionally competent from the earliest stages, at least transiently held over from the unfertilized ovum. Kinetic studies with radiotracer cations, osmolar variations, media lacking one or more of the four biological cations, Na(+), Mg(++), K(+), and Ca(++), and metabolic poison 0.05 mEq/L NaF, demonstrated that: (1) all four cations influence the behavior of the others, and (2) energy is required for uptake and efflux on different time scales, some against gradient. Na(+) uptake is energy dependent against an efflux gradient. The rate of K(+) loss is equal with or without fluoride, suggesting a lack of an energy requirement at these stages. Ca(++) efflux took twice as long in the presence of fluoride, likely due in part to intracellular binding. Mg(++) is anomalous in that early teleost vertebrate embryos have an intracellular content exceeding the surrounding sea water, an isolated unaffected yolk compartment, and a clear requirement for energy for both uptake and efflux. The physiological, pathological, and therapeutic roles of magnesium are poorly understood. This will change: (1) when (28)Mg is once again generally available at a reasonable cost for both basic research and clinical assessment, and (2) when serum or plasma levels are determined simultaneously with intracellular values, preferably as part of complete four cation profiles. Atomic absorption spectrophotometry, energy-dispersive x-ray analysis, and inductively coupled plasma emission spectroscopy on sublingual mucosal and peripheral blood samples are potential methods of value for coordinated assessments.

Calcium microdomains and cell cycle control

The cell division cycle comprises successive rounds of genome replication and segregation that are never error-free. A complex signalling network chaperones cell cycle events to ensure that cell cycle progression does not occur until any errors detected are put right. The signalling network consists of cell cycle control proteins that are phosphorylated and dephosphorylated, synthesized and degraded interactively to generate a set of sensors and molecular switches that are thrown at appropriate times to permit or trigger cell cycle progression. In early embryos, discrete calcium signals have been shown to be a key component of the molecular switch mechanism. In somatic cells in contrast, the participation of calcium signals in cell cycle control is far from clear. Recent experiments in syncytial Drosophila embryos have shown that localised calcium signals in the nucleus and mitotic spindle can be detected. It appears that the nucleus comprises a calcium signalling microdomain bounded by endoplasmic reticulum that isolates the nucleus and spindle. These findings offer a possible explanation for the apparent absence of calcium signals in somatic cells during mitosis.

Calcium at fertilization and in early development

Fertilization calcium waves are introduced, and the evidence from which we can infer general mechanisms of these waves is presented. The two main classes of hypotheses put forward to explain the generation of the fertilization calcium wave are set out, and it is concluded that initiation of the fertilization calcium wave can be most generally explained in invertebrates by a mechanism in which an activating substance enters the egg from the sperm on sperm-egg fusion, activating the egg by stimulating phospholipase C activation through a src family kinase pathway and in mammals by the diffusion of a sperm-specific phospholipase C from sperm to egg on sperm-egg fusion. The fertilization calcium wave is then set into the context of cell cycle control, and the mechanism of repetitive calcium spiking in mammalian eggs is investigated. Evidence that calcium signals control cell division in early embryos is reviewed, and it is concluded that calcium signals are essential at all three stages of cell division in early embryos. Evidence that phosphoinositide signaling pathways control the resumption of meiosis during oocyte maturation is considered. It is concluded on balance that the evidence points to a need for phosphoinositide/calcium signaling during resumption of meiosis. Changes to the calcium signaling machinery occur during meiosis to enable the production of a calcium wave in the mature oocyte when it is fertilized; evidence that the shape and structure of the endoplasmic reticulum alters dynamically during maturation and after fertilization is reviewed, and the link between ER dynamics and the cytoskeleton is discussed. There is evidence that calcium signaling plays a key part in the development of patterning in early embryos. Morphogenesis in ascidian, frog, and zebrafish embryos is briefly described to provide the developmental context in which calcium signals act. Intracellular calcium waves that may play a role in axis formation in ascidian are discussed. Evidence that the Wingless/calcium signaling pathway is a strong ventralizing signal in Xenopus, mediated by phosphoinositide signaling, is adumbrated. The central role that calcium channels play in morphogenetic movements during gastrulation and in ectodermal and mesodermal gene expression during late gastrulation is demonstrated. Experiments in zebrafish provide a strong indication that calcium signals are essential for pattern formation and organogenesis.

Dynamics and inheritance of the endoplasmic reticulum

The endoplasmic reticulum (ER) consists of a polygonal array of interconnected tubules and sheets that spreads throughout the eukaryotic cell and is contiguous with the nuclear envelope. This elaborate structure is created and maintained by a constant remodeling process that involves the formation of new tubules, their cytoskeletal transport and homotypic fusion. Since the ER is a large, single-copy organelle, it must be actively segregated into daughter cells during cell division. Recent analysis in budding yeast indicates that ER inheritance involves the polarized transport of cytoplasmic ER tubules into newly formed buds along actin cables by a type V myosin. The tubules then become anchored to a site at the bud tip and this requires the Sec3p subunit of the exocyst complex. The ER is then propagated along the cortex of the bud to yield a cortical ER structure similar to that of the mother cell. In animal cells, the ER moves predominantly along microtubules, whereas actin fibers serve a complementary role. It is not yet clear to what extent the other components controlling ER distribution in yeast might be conserved in animal cells.

Dynamics of the endoplasmic reticulum during early development of Drosophila melanogaster

In this study, we analyze for the first time endoplasmic reticulum (ER) dynamics and organization during oogenesis and embryonic divisions of Drosophila melanogaster using a Protein Disulfide Isomerase (PDI) GFP chimera protein. An accumulation of ER material into the oocyte takes place during the early steps of oogenesis. The compact organization of ER structures undergoes a transition to an expanded reticular network at fertilization. At the syncytial stage, this network connects to the nuclear envelope as each nucleus divides. Time-lapse confocal microscopy on PDI transgenic embryos allowed us to characterize a rapid redistribution of the ER during the mitotic phases. The ER network is massively recruited to the spindle poles in prophase. During metaphase most of the ER remains concentrated at the spindle poles and shortly thereafter forms several layers of membranes along the ruptured nuclear envelope. Later, during telophase an accumulation of ER material occurs at the spindle equator. We also analyzed the subcellular organization of the ER network at the ultrastructural level, allowing us to corroborate the results from confocal microscopy studies. This dynamic redistribution of ER suggests an unexpected regulatory function for this organelle during mitosis.

Ciliary protein turnover continues in the presence of inhibitors of golgi function: evidence for membrane protein pools and unconventional intracellular membrane dynamics

The intimate association of the Golgi apparatus with cilia suggests a functional alliance. To explore the relationship between the synthesis and processing of membrane constituents and the turnover or regeneration of cilia, parallel cultures of gastrula-stage sea urchin embryos were pulse-chase labeled with (3)H-leucine in the presence of monensin, brefeldin A, or colchicine. Steady-state labeled cilia were isolated, and the embryos were allowed to regenerate cilia, which were then isolated after the equivalent of two normal regeneration times. Regeneration was absent in colchicine, minimal in monensin, and inhibited about 40% by brefeldin A. Both monensin and brefeldin A effectively inhibited the post-translational processing of prominent phosphatidylinositoylated and palmitoylated membrane proteins and the axoneme-associated transmembrane Spec3 protein, yet most other membrane plus matrix and 9+2 axonemal proteins were labeled to levels indistinguishable from untreated controls. However, total protein analysis of the membrane plus matrix fractions showed a substantial increase in glycoproteins and the calsequestrin-like protein ECaSt/PDI after treatment at steady-state with all three inhibitors and after regeneration in brefeldin A. Other constituents of this compartment, such as membrane-associated tubulin, calmodulin, and a 53-kDa calcium-binding protein, were unchanged. Therefore, inhibition of Golgi function via three different mechanisms left 9+2 protein turnover undiminished but resulted in an accumulation, in the cilium, of already-processed membrane pool constituents and a normally ER-resident protein. A disproportionate elevation of HSP70 suggests that a novel stress response may be involved in inhibiting ciliary regeneration or promoting glycoprotein augmentation.

OOC-3, a novel putative transmembrane protein required for establishment of cortical domains and spindle orientation in the P(1) blastomere of C. elegans embryos

Asymmetric cell divisions require the establishment of an axis of polarity, which is subsequently communicated to downstream events. During the asymmetric cell division of the P(1) blastomere in C. elegans, establishment of polarity depends on the establishment of anterior and posterior cortical domains, defined by the localization of the PAR proteins, followed by the orientation of the mitotic spindle along the previously established axis of polarity. To identify genes required for these events, we have screened a collection of maternal-effect lethal mutations on chromosome II of C. elegans. We have identified a mutation in one gene, ooc-3, with mis-oriented division axes at the two-cell stage. Here we describe the phenotypic and molecular characterization of ooc-3. ooc-3 is required for the correct localization of PAR-2 and PAR-3 cortical domains after the first cell division. OOC-3 is a novel putative transmembrane protein, which localizes to a reticular membrane compartment, probably the endoplasmic reticulum, that spans the whole cytoplasm and is enriched on the nuclear envelope and cell-cell boundaries. Our results show that ooc-3 is required to form the cortical domains essential for polarity after cell division.

Spatial and temporal dynamics of the secretory pathway during differentiation of the Plasmodium yoelii schizont

A specialized complex of apical organelles facilitates Plasmodium merozoite invasion into the erythrocyte. Even though the apical organelles are crucial to the invasion process, relatively little is known about how they function or their biosynthesis during asexual replication. MAEBL is an erythrocyte binding protein located in the rhoptries and on the surface of mature merozoites and is expressed at the beginning of schizogony before the first nuclear division. Therefore, we have characterized MAEBL as a marker for the biosynthetic pathway of the rhoptry apical organelle during the final phase of intraerythrocytic development and as a marker for the nascent rhoptry vesicle in the immature schizont. An extensive proliferation of the endoplasmic reticulum occurred at the onset of schizogony and was seen as a complex but transient tubule array near the parasite surface. Both the rhoptry protein MAEBL and surface protein MSP-1 appeared to be present in this tubular reticular network together with endoplasmic reticulum markers. MAEBL then transits through Golgi bodies positioned near the parasite plasma membrane, directly adjacent to the network. Rhoptry organelle precursors are seen at the three to four nuclei stage of schizont development, remaining near the plasma membrane throughout schizogony. These studies constitute the first direct evidence that proteins of the rhoptry organelles transit through compartments of the 'classical' secretory pathway.

Microtubules and mitotic cycle phase modulate spatiotemporal distributions of F-actin and myosin II in Drosophila syncytial blastoderm embryos

We studied cyclic reorganizations of filamentous actin, myosin II and microtubules in syncytial Drosophila blastoderms using drug treatments, time-lapse movies and laser scanning confocal microscopy of fixed stained embryos (including multiprobe three-dimensional reconstructions). Our observations imply interactions between microtubules and the actomyosin cytoskeleton. They provide evidence that filamentous actin and cytoplasmic myosin II are transported along microtubules towards microtubule plus ends, with actin and myosin exhibiting different affinities for the cell's cortex. Our studies further reveal that cell cycle phase modulates the amounts of both polymerized actin and myosin II associated with the cortex. We analogize pseudocleavage furrow formation in the Drosophila blastoderm with how the mitotic apparatus positions the cleavage furrow for standard cytokinesis, and relate our findings to polar relaxation/global contraction mechanisms for furrow formation.

Dynamics of the endoplasmic reticulum and golgi apparatus during early sea urchin development

The endoplasmic reticulum (ER) and Golgi were labeled by green fluorescent protein chimeras and observed by time-lapse confocal microscopy during the rapid cell cycles of sea urchin embryos. The ER undergoes a cyclical microtubule-dependent accumulation at the mitotic poles and by photobleaching experiments remains continuous through the cell cycle. Finger-like indentations of the nuclear envelope near the mitotic poles appear 2-3 min before the permeability barrier of the nuclear envelope begins to change. This permeability change in turn is approximately 30 s before nuclear envelope breakdown. During interphase, there are many scattered, disconnected Golgi stacks throughout the cytoplasm, which appear as 1- to 2-microm fluorescent spots. The number of Golgi spots begins to decline soon after nuclear envelope breakdown, reaches a minimum soon after cytokinesis, and then rapidly increases. At higher magnification, smaller spots are seen, along with increased fluorescence in the ER. Quantitative measurements, along with nocodazole and photobleaching experiments, are consistent with a redistribution of some of the Golgi to the ER during mitosis. The scattered Golgi coalesce into a single large aggregate during the interphase after the ninth embryonic cleavage; this is likely to be preparatory for secretion of the hatching enzyme during the following cleavage cycle.

The role of calcium on the activity of ERcalcistorin/protein-disulfide isomerase and the significance of the C-terminal and its calcium binding. A comparison with mammalian protein-disulfide isomerase

ERcalcistorin/protein-disulfide isomerase (ECaSt/PDI) shows a 55% identity with mammalian protein-disulfide isomerase (PDI) (Lucero, H. A., Lebeche, D., and Kaminer, B. (1994) J. Biol. Chem. 269, 23112-23119) is a high capacity low affinity Ca2+-binding protein and behaves as a Ca2+ storage protein in the ER of a living cell (Lucero, H. A., Lebeche, D., and Kaminer, B. (1998) J. Biol. Chem. 273, 9857-9863). Here we show that recombinant ECaSt/PDI bound 26 mol of Ca2+/mol and a C-terminal truncated mutant bound 14 mol of Ca2+/mol, both with a Kd of 2.8 mM in 50 mM KCl and 5.2 mM in 150 mM KCl. The percentage reduction in Ca2+ binding in the mutant corresponded with the percentage reduction of deleted pairs of acidic residues, postulated low affinity Ca2+-binding sites. 5 mM Ca2+ moderately increased the PDI activity of both ECaSt/PDI and the C-terminal truncated mutant on reduced RNase and insulin. Surprisingly, ECaSt/PDI in the absence of Ca2+ prevented the spontaneous reactivation of reduced bovine pancreatic trypsin inhibitor. In the presence of 1-5 mM Ca2+ (or 10 microM polylysine) ECaSt/PDI augmented the bovine pancreatic trypsin inhibitor reactivation rate. In contrast, the C-terminal truncated ECaSt/PDI augmented rBPTI reactivation in the absence of Ca2+ and 1-5 mM Ca2+ further accelerated the reactivation rate, responses similar to those obtained with mammalian PDI.

Reorganization of filamentous actin and myosin-II in zebrafish eggs correlates temporally and spatially with cortical granule exocytosis

The zebrafish egg provides a useful experimental system to study events of fertilization, including exocytosis. We show by differential interference contrast videomicroscopy that cortical granules are: (1) released nonsynchronously over the egg surface and (2) mobilized to the plasma membrane in two phases, depending upon vesicle size and location. Turbidometric assay measurements of the timing and extent of exocytosis revealed a steady release of small granules during the first 30 seconds of egg activation. This was followed by an explosive discharge of large granules, beginning at 30 seconds and continuing for 1–2 minutes. Stages of single granule exocytosis and subsequent remodeling of the egg surface were imaged by either real-time or time-lapse videomicroscopy as well as scanning electron microscopy. Cortical granule translocation and fusion with the plasma membrane were followed by the concurrent expansion of a fusion pore and release of granule contents. A dramatic rearrangement of the egg surface followed exocytosis. Cortical crypts (sites of evacuated granules) displayed a purse-string-like contraction, resulting in their gradual flattening and disappearance from the egg surface. We tested the hypothesis that subplasmalemmal filamentous (F-) actin acts as a physical barrier to secretion and is locally disassembled prior to granule release. Experimental results showed a reduction of rhodamine-phalloidin and antimyosin staining at putative sites of secretion, acceleration of the timing and extent of granule release in eggs pretreated with cytochalasin D, and dose-dependent inhibition of exocytosis in permeabilized eggs preincubated with phalloidin. An increase in assembled actin was detected by fluorometric assay during the period of exocytosis. Localization studies showed that F-actin and myosin-II codistributed with an inward-moving, membrane-delimited zone of cytoplasm that circumscribed cortical crypts during their transformation. Furthermore, cortical crypts displayed a distinct delay in transformation when incubated continuously with cytochalasin D following egg activation. We propose that closure of cortical crypts is driven by a contractile ring whose forces depend upon dynamic actin filaments and perhaps actomyosin interactions.

Calcium and endoplasmic reticulum dynamics during oocyte maturation and fertilization in the marine worm Cerebratulus lacteus

To monitor calcium and endoplasmic reticulum (ER) dynamics during oocyte maturation and fertilization, oocytes of the marine worm Cerebratulus lacteus were injected with the calcium-sensitive indicator calcium green dextran and/or the ER-specific probe "DiI." Based on time-lapse confocal imaging of such specimens, prophase-arrested immature oocytes failed to develop normally after insemination and typically produced non-wave-like calcium transients that were lower in amplitude and less persistent than the wave-like oscillations observed during fertilizations of mature oocytes. Accordingly, the ER of DiI-loaded immature oocytes lacked an obvious substructure, whereas ER clusters, or "microdomains," began to form in maturing specimens at about the time that these oocytes became competent to undergo normal fertilization-induced calcium dynamics and cleavage. The ER microdomains of mature oocytes typically reached widths of 1-8 micrometer and disappeared approximately 1 h after fertilization, which in turn coincided with the termination of the calcium oscillations. Collectively, these findings indicate: (i) changes in ER structure are temporally correlated with the onset and cessation of the calcium oscillations required for subsequent cleavage, and (ii) such ER reorganizations may play an important role in early development by enabling mature oocytes to generate a normal calcium response.

Analysis of rab10 localization in sea urchin embryonic cells by three-dimensional reconstruction

Rabs are a subfamily of ras-like GTPases required for membrane traffic in eukaryotic cells. In this report we describe the analysis of a rab10 GTPase expressed during sea urchin development. Protein distance measurements suggest that rab10 is less evolutionarily conserved than rabs 1, 2, and 3, particularly in the hypervariable C-terminus responsible for membrane targeting. Immunoblots and immunofluorescent stainings show that rab10 protein (rab10p) is expressed during all stages of sea urchin early development and in all embryonic cell types. Iterative deconvolutions of immunofluorescently stained embryos reveal that rab10p is localized to an extensive tubular network. Rab10p is not exclusively localized to the endoplasmic reticulum, as identified by anti-calsequestrin immunofluorescence. Double-labeling experiments with anti-rab10 antisera and wheat germ agglutinin, a trans-Golgi and trans-Golgi network (TGN) marker, demonstrate that rab10p is not localized to the trans-Golgi/TGN. Three-dimensional reconstructions of immunofluorescently labeled sea urchin embryonic cells show that tubules with greater concentrations of rab10p are closely apposed to trans-Golgi/TGN in a cis orientation-suggesting localization of rab10p to the cis-Golgi network. In mammalian cell lines, Rab10 has been localized to the trans-Golgi/trans-Golgi network (Y.-T. Chen et al., 1993, Proc. Natl. Acad. Sci. USA 90, 6508-6512). The localization of rab10 may not have been evolutionarily conserved between echinoderms and mammals because of the high rate of change in the hypervariable domain.

Cell cycle regulation of organelle transport

Microtubule- and actin-based motors play a wide range of vital roles in the organisation and function of cells during both interphase and mitosis, all of which are likely to be under strict control. Here, we describe how one of these roles--the movement of membranes--is regulated through the cell cycle. Organelle movement in many species is greatly reduced in mitosis as compared to interphase, and this change occurs concomitantly with an inhibition of most membrane traffic functions. Data from in vitro studies is shedding light on how microtubule motor regulation may be achieved.

ERcalcistorin/protein-disulfide isomerase acts as a calcium storage protein in the endoplasmic reticulum of a living cell. Comparison with calreticulin and calsequestrin

ERcalcistorin/protein-disulfide isomerase (ECaSt/PDI), a high capacity low affinity Ca2+-binding protein in the endoplasmic reticulum of sea urchin eggs (Lebeche, D., and Kaminer, B. (1992) Biochem. J. 287, 741-747), shares 55% sequence identity with mammalian PDI and has PDI activity (Lucero, H., Lebeche, D., and Kaminer, B. (1994) J. Biol. Chem. 269, 23112-23119). We report on ECaSt/PDI functioning as a Ca2+ storage protein in the endoplasmic reticulum (ER) of a living cell and compare it with calsequestrin and calreticulin, high capacity low affinity Ca2+-binding proteins in the sarcoplasmic reticulum and ER, respectively. Stably transfected Chinese hamster ovary cell clones expressed these proteins, which were localized in the ER of the cell. Microsomes from cells expressing ECaSt/PDI, calreticulin, and calsequestrin accumulated 17.2 +/- 0.27, 20.0 +/- 0.82, and 38.0 +/- 0.28 nmol of Ca2+/mg of protein, respectively; control microsomes accumulated from 2.6 +/- 0.17 to 2.9 +/- 0.14 nmol of Ca2+/mg of protein. The initial rate of Ca2+ uptake was similar in microsomes from transfected and control cells. Microsomes containing an ECaSt/PDI mutant in which 45% of the acidic residue pairs in the C terminus were truncated had a reduced Ca2+ storage capacity. This supports our previous hypothesis that the degree of low affinity Ca2+ binding is dependent on the number of pairs of carboxyl groups in the molecule. The maximal Ca2+ accumulation by microsomes containing the expressed ECaSt/PDI, C-terminally truncated ECaSt/PDI, calreticulin, or calsequestrin correlates approximately with the Ca2+ binding capacity of the respective proteins.

Microtubule-organizing centers and nucleating sites in land plants

Microtubule-organizing centers (MTOCs) are morphologically diverse cellular sites involved in the nucleation and organization of microtubules (MTs). These structures are synonymous with the centrosome in mammalian cells. In most land plant cells, however, no such structures are observed and some have argued that plant cells may not have MTOCs. This review summarizes a number of experimental approaches toward the elucidation of those subcellular sites involved in microtubule nucleation and organization. In lower land plants, structurally well-defined MTOCs are present, such as the blepharoplast, multilayered structure, and polar organizer. In higher plants, much of the nucleation and organization of MTs occurs on the nuclear envelope or other endomembranes, such as the plasmalemma and smooth (tubular) endoplasmic reticulum. In some instances, one endomembrane may serve as a site of nucleation whereas others serve as the site of organization. Structural and motor microtubule-associated proteins also appear to be involved in MT nucleation and organization. Immunochemical evidence indicates that at least several of the proteins found in mammalian centrosomes, gamma-tubulin, centrin, pericentrin, and polypeptides recognized by the monoclonal antibodies MPM-2, 6C6, and C9 also recognize putative lower land plant MTOCs, indicating shared mechanisms of nucleation/organization in plants and animals. The most recent data from tubulin incorporation in vivo, mutants with altered MT organization, and molecular studies indicate the potential of these research tools in investigation of MTOCs in plants.

Large plasma membrane disruptions are rapidly resealed by Ca2+-dependent vesicle-vesicle fusion events

A microneedle puncture of the fibroblast or sea urchin egg surface rapidly evokes a localized exocytotic reaction that may be required for the rapid resealing that follows this breach in plasma membrane integrity (Steinhardt, R.A,. G. Bi, and J.M. Alderton. 1994. Science (Wash. DC). 263:390-393). How this exocytotic reaction facilitates the resealing process is unknown. We found that starfish oocytes and sea urchin eggs rapidly reseal much larger disruptions than those produced with a microneedle. When an approximately 40 by 10 microm surface patch was torn off, entry of fluorescein stachyose (FS; 1, 000 mol wt) or fluorescein dextran (FDx; 10,000 mol wt) from extracellular sea water (SW) was not detected by confocal microscopy. Moreover, only a brief (approximately 5-10 s) rise in cytosolic Ca2+ was detected at the wound site. Several lines of evidence indicate that intracellular membranes are the primary source of the membrane recruited for this massive resealing event. When we injected FS-containing SW deep into the cells, a vesicle formed immediately, entrapping within its confines most of the FS. DiI staining and EM confirmed that the barrier delimiting injected SW was a membrane bilayer. The threshold for vesicle formation was approximately 3 mM Ca2+ (SW is approximately 10 mM Ca2+). The capacity of intracellular membranes for sealing off SW was further demonstrated by extruding egg cytoplasm from a micropipet into SW. A boundary immediately formed around such cytoplasm, entrapping FDx or FS dissolved in it. This entrapment did not occur in Ca2+ -free SW (CFSW). When egg cytoplasm stratified by centrifugation was exposed to SW, only the yolk platelet-rich domain formed a membrane, suggesting that the yolk platelet is a critical element in this response and that the ER is not required. We propose that plasma membrane disruption evokes Ca2+ regulated vesicle-vesicle (including endocytic compartments but possibly excluding ER) fusion reactions. The function in resealing of this cytoplasmic fusion reaction is to form a replacement bilayer patch. This patch is added to the discontinuous surface bilayer by exocytotic fusion events.

Calcium-induced restructuring of nuclear envelope and endoplasmic reticulum calcium stores

The spatial organization of endoplasmic reticulum (ER) and nuclear envelope (NE) calcium stores is important for the regulation of localized calcium signals and sustained calcium gradients. Here, we have used a lumenal GFP fusion protein and shown that, in resting cells, large molecules can rapidly diffuse across the cell within the lumenal storage space defined by the ER and NE membranes. Increases in cytosolic calcium concentration reversibly fragmented ER tubules and prevented lumenal diffusion. However, the integrity of the NE was maintained, and a significant fraction of NE lumenal protein accumulated in an NE-associated vesicle. These dynamic properties of ER-NE calcium stores provide insights into the spatiotemporal control of calcium signaling.

Function of spindle microtubules in directing cortical movement and actin filament organization in dividing cultured cells

The mitotic spindle has long been recognized to play an essential role in determining the position of the cleavage furrow during cell division, however little is known about the mechanisms involved in this process. One attractive hypothesis is that signals from the spindle may function to induce reorganization of cortical structures and transport of actin filaments to the equator during cytokinesis. While an important idea, few experiments have directly tested this model. In the present study, we have used a variety of experimental approaches to identify microtubule-dependent effects on key cortical events during normal cell cleavage, including cortical flow, reorientation of actin filaments, and formation of the contractile apparatus. Single-particle tracking experiments showed that the microtubule disrupting drug nocodazole induces an inhibition of the movements of cell surface receptors following anaphase onset, while the microtubule stabilizing drug taxol causes profound changes in the overall pattern of receptor movements. These effects were accompanied by a related set of changes in the organization of the actin cytoskeleton. In nocodazole-treated cells, the three-dimensional organization of cortical actin filaments appeared less ordered than in controls. Measurements with fluorescence-detected linear dichroism indicated a decrease in the alignment of filaments along the spindle axis. In contrast, actin filaments in taxol-treated cells showed an increased alignment along the equator on both the ventral and dorsal cortical surfaces, mirroring the redistribution pattern of surface receptors. Together, these experiments show that spindle microtubules are involved in directing bipolar flow of surface receptors and reorganization of actin filaments during cell division, thus acting as a stimulus for positioning cortical cytoskeletal components and organizing the contractile apparatus of dividing tissue culture cells.

Visualization of exocytosis during sea urchin egg fertilization using confocal microscopy

A Ca2+ wave at fertilization triggers cortical granule exocytosis in sea urchin eggs. New methods for visualizing exocytosis of individual cortical granules were developed using fluorescent probes and confocal microscopy. Electron microscopy previously provided evidence that cortical granule exocytosis results in the formation of long-lived depressions in the cell surface. Fluorescent dextran or ovalbumin in the sea water seemed to label these depressions and appeared by confocal microscopy as disks. FM 1–43, a water-soluble fluorescent dye which labels membranes in contact with the sea water, seemed to label the membrane of these depressions and appeared as rings. In double-labeling experiments, the disk and ring labeling by the two types of fluorescent dyes were coincident to within 0.5 second. The fluorescent labeling is coincident with the disappearance of cortical granules by transmitted light microscopy, demonstrating that the labeling corresponds to cortical granule exocytosis. Fluorescent labeling was simultaneous with an expansion of the space occupied by the cortical granule, and labeling by the fluorescent dextran was found to take 0.1-0.2 second. These results are consistent with, and reinforce the previous electron microscopic evidence for, long-lived depressions formed by exocytosis; in addition, the new methods provide new ways to investigate cortical granule exocytosis in living eggs. The fluorescence labeling methods were used with the Ca2+ indicator Ca Green-dextran to test if Ca2+ and cortical granule exocytosis are closely related spatially and temporally. In any given region of the cortex, Ca2+ increased relatively slowly.(ABSTRACT TRUNCATED AT 250 WORDS)

Organization of Wolbachia pipientis in the Drosophila fertilized egg and embryo revealed by an anti-Wolbachia monoclonal antibody

Cytoplasmic incompatibility (CI) in Drosophila is related to the presence of Wolbachia, an intracellular microorganism found in many species of insects. In order to study the intracellular localization of Wolbachia in eggs and embryos, we have purified the bacteria from fly embryos and subsequently generated a monoclonal antibody (Mab Wol-1) specific for Wolbachia. Indirect immunofluorescence staining using Wol-1 reveals that during mitosis, Wolbachia are localized near spindle poles and centrosomes. Double label immunofluorescence experiments using anti-tubulin and anti-Wolbachia antibodies show that Wolbachia co-localize with centrosomal microtubules throughout the cell cycle. Direct interactions between the bacteria and centrosome-organized microtubules are implied from seven observations: (1) throughout the mitotic cycle, the position and movement of Wolbachia precisely mimic the behavior of the centrosome and apparently associated with centrosome-organized microtubules; (2) Wolbachia segregate equally to each spindle pole during mitosis; (3) Wolbachia do not associate with spindle microtubules during mitosis; (4) Wolbachia located in the egg cortex localize to the domains of cytoplasm organized by microtubules during blastoderm formation; (5) polar body nuclei that lack centrosomes but contain associated microtubules do not contain Wolbachia; (6) Wolbachia no longer associated with yolk nuclei, following differentiation and loss of centrosomes; (7) during pole cell formation, Wolbachia co-localize with the centrosome on the apical side of the nucleus as pole cells form. Quantitative data indicates that no Wolbachia growth occurs during the preblastoderm period even though rapid nuclear, and subsequent cellular, proliferation takes place during this same period. This indicates that Wolbachia are under strict growth regulation by the host suggesting that host factors play a role in regulating growth of Wolbachia in the egg. Further cellular and molecular studies of the extensive, global interactions between host and symbiont observed in this egg should provide important new insights into the evolution of host/symbiosis and the cell biology of cytoplasmic incompatibility.

Direct visualization of a vast cortical calcium compartment in Paramecium by secondary ion mass spectrometry (SIMS) microscopy: possible involvement in exocytosis

The plasma membrane of ciliates is underlaid by a vast continuous array of membrane vesicles known as cortical alveoli. Previous work had shown that a purified fraction of these vesicles actively pumps calcium, suggesting that alveoli may constitute a calcium-storage compartment. Here we provide direct confirmation of this hypothesis using in situ visualization of total cell calcium on sections of cryofixed and cryosubstituted cells analyzed by SIMS (secondary ion mass spectrometry) microscopy a method never previously applied to protists. A narrow, continuous, Ca-emitting zone located all along the cell periphery was observed on sections including the cortex. In contrast, Na and K were evenly distributed throughout the cell. Various controls confirmed that emission was from the alveoli, in particular, the emitting zone was still seen in mutants totally lacking trichocysts, the large exocytotic organelles docked at the cell surface, indicating that they make no major direct contribution to the emission. Calcium concentration within alveoli was quantified for the first time in SIMS microscopy using an external reference and was found to be in the range of 3 to 5 mM, a value similar to that for sarcoplasmic reticulum. After massive induction of trichocyst discharge, this concentration was found to decrease by about 50%, suggesting that the alveoli are the main source of the calcium involved in exocytosis.

Molecular structure and function of CD4 on murine egg plasma membrane

In the present study, the expression of the CD4 molecule on murine egg plasma membrane was confirmed by the indirect immunofluorescence (IIF) method. The full-length CD4 cDNA from murine eggs was synthesised by the reverse transcriptase-polymerase chain reaction (RT-PCR) method and its authenticity verified by Southern blot hybridisation using an end-labelled internal oligonucleotide. The results of DNA sequencing showed that the nucleotide sequence of the cDNA of CD4 from murine egg mRNA was identical to that of immune T cells. To demonstrate the direct interaction of CD4 from murine egg with murine sperm cells bearing MHC (major histocompatibility complex) class II molecule, we employed a baculovirus expression system to generate CD4 on the surface of Spodoptera frugiperda (Sf9) cells. Expression of CD4 on Sf9 cells infected with Autographa californica nuclear polyhedrosis virus (AcNPV)-CD4 was demonstrated by IIF and immunoblotting. The CD4-expressing Sf9 cells adhered to MHC class II-bearing sperm cells since the adhesion was specifically blocked by anti-CD4 monoclonal antibody (mAb) or anti-monomorphic region of MHC class II mAb. Taking our previous and present experimental results together, they strongly suggest that intercellular membrane adhesion between two gametes at the fusion step in fertilisation is mediated by the MHC class II molecule located on the posterior region of the sperm head and the CD4 molecule on egg plasma membrane.

Cytoplasm calcium-binding proteins of germ cells and embryos of the sea urchin

Synchronous, demonstrative, easily reproducible fertilization with the following embryonic development makes the process in the sea urchin extremely attractive for studying many biological enigmas. In particular, germ and embryonic cells of the sea urchin present a wide opportunity for investigating different associated phenomena launched by an increase in concentration of Ca2+ in cells ([Ca2+]i). Ca2+ ions participate in the activation of diverse processes of respiration and sperm motility (Shapiro et al., 1990; Brokaw, 1991), chemotaxis of spermatozoa to components of the egg jelly (Ward et al., 1985), acrosomal reaction (Trimmer et al., 1986; Shapiro et al., 1990), cortical reaction, formation of the fertilization membrane (Sasaki, 1984; Sardet and Chang, 1987), cellular division in the embryo (Poenie et al., 1985; Silver, 1986; Whitaker and Patel, 1990), their adhesion (McClay and Matranga, 1986), differentiation and formation of spicules (Mitsunaga et al., 1988) and metamorphosis (Carpenter et al., 1984). The present review combines information on the function of calcium-binding proteins and their targets, calmodulin regulation of NAD-kinase, exocytosis of cortical granules, Ca(2+)- and calmodulin-dependent protein phosphatase, Ca(2+)-dependent protein phosphorylation, regulation of ion-exchanger in the germ and embryonic cells as well as Ca(2+)- and calmodulin control of sperm motility in sea urchins.

Structural changes in the endoplasmic reticulum of starfish oocytes during meiotic maturation and fertilization

The endoplasmic reticulum (ER) of live starfish oocytes was observed during meiotic maturation and fertilization. The ER was visualized by injection into the cytoplasm of an oil drop saturated with the fluorescent lipophilic dye DiI; DiI spread throughout the oocyte endoplasmic reticulum and the pattern was imaged by confocal microscopy. The ER in the immature (germinal vesicle stage) oocyte was composed of interconnected membrane sheets. In response to 1-methyladenine, the sheets of ER appeared to become associated with the yolk platelets, forming spherical shells. A few of these spherical shells could sometimes be seen in immature oocytes, but their number was much greater in the egg at the first meiotic spindle stage. At about the time that the first polar body formed, the spherical shells disappeared, and the ER returned to a form like that of the immature oocyte. The spherical shells did not reappear during the second meiotic cycle. During maturation, the ER also began to move; the movement was apparent by the time of germinal vesicle breakdown and continued throughout both meiotic cycles and in eggs with second polar bodies. When eggs at the first meiotic spindle stage were fertilized, the form of the ER changed. Within 1 min after sperm addition to the observation chamber, the circular cross sections of the spherical shells of the unfertilized egg ER were no longer distinct. At this point, the form of the ER could not be discerned with the resolution of the light microscope; however, the rate of spreading of DiI from an injected oil drop decreased, providing strong evidence that the ER had become fragmented. The ER remained in this form for several minutes and then gradually, the appearance of the ER and the rate of DiI spreading returned to be like those of the unfertilized egg. Injection of inositol trisphosphate caused a similar change in the ER structure. These results indicate that the ER is a dynamic structure, the form of which changes during oocyte maturation and fertilization.

Cleavage furrow: timing of emergence of contractile ring actin filaments and establishment of the contractile ring by filament bundling in sea urchin eggs

Cleavage furrow formation at the first cell division of sea urchin and sand dollar eggs was investigated in detail by fluorescence staining of actin filaments with rhodamine-phalloidin of either whole eggs or isolated egg cortices. Cortical actin filaments were clustered at anaphase and then the clusters became fibrillar at the end of anaphase. The timing when the contractile ring actin filaments appear was precisely determined in the course of mitosis: accumulation of the contractile ring actin filaments at the equatorial cell cortex is first noticed at the beginning of telophase (shortly before furrow formation), when the chromosomal vesicles are fusing with each other. The accumulated actin filaments were not well organized at the early stage but were organized into parallel bundles as the furrowing progressed. The bundles were finally fused into a tightly packed filament belt. Wheat germ agglutinin (WGA)-binding sites were distributed on the surface of the egg in a manner similar to the actin filaments after anaphase. The WGA-binding sites became accumulated in the contractile ring together with the contractile ring actin filaments, indicating an intimate relationship between these sites and actin filament-anchoring sites on the plasma membrane. Myosin also appeared in the contractile ring together with the actin filaments. The ‘cleavage stimulus’, a signal hypothesized by Rappaport (reviewed by R. Rappaport (1986) Int. Rev. Cytol. 105, 245–281) was suggested to induce aggregation or bundling of the actin filaments in the cortical layer.

The distribution of cytoplasmic bacteria in the early Drosophila embryo is mediated by astral microtubules

Maternally inherited cytoplasmic bacteria have occasionally been observed in embryos and adults of different strains of several Drosophila species. While there is a considerable body of data on the relationship between bacteria and embryo viability, little is known about the behavior of these bacteria during the early development of Drosophila. In eggs laid by infected Drosophila melanogaster females we showed that cytoplasmic bacteria were initially concentrated in a thin cortical layer and scattered in the yolk region. During the following syncytial blastoderm mitoses the bacteria mainly accumulated towards the poles of the mitotic spindles, suggesting that astral microtubules play a role in localizing bacteria. This is supported by the observation that treatment of the infected embryos with the microtubule-disrupting drug colchicine led to the partial dissociation of the bacteria from the spindle poles, whereas cytochalasin treatment left almost all the bacterial clusters intact. Moreover, bacteria were not found near the polar bodies and yolk nuclei, which were without astral microtubules. In mitosis-defective embryos, with centrosomes dissociated from the nuclei, the bacteria were concentrated in association with the isolated astral microtubules, and in cold-treated embryos, in which microtubules regrew from isolated centrosomes after recovering, the bacteria clustered around the newly formed asters. These observations, also supported by electron microscope analysis, indicate a close relationship between cytoplasmic bacteria and astral microtubules, and suggest that the latter were able to build discrete cytoplasmic domains ensuring the proper distribution of cytoplasmic components during the blastoderm mitoses, despite the lack of cell membranes.

Roles of kinesin and kinesin-like proteins in sea urchin embryonic cell division: evaluation using antibody microinjection

Previous studies suggest that kinesin heavy chain (KHC) is associated with ER-derived membranes that accumulate in the mitotic apparatus in cells of early sea urchin embryos (Wright, B. D., J. H. Henson, K. P. Wedaman, P. J. Willy, J. N. Morand, and J. M. Scholey. 1991. J. Cell Biol. 113:817-833). Here, we report that the microinjection of KHC-specific antibodies into these cells has no effect on mitosis or ER membrane organization, even though one such antibody, SUK4, blocks kinesin-driven motility in vitro and in mammalian cells. Microinjected SUK4 was localized to early mitotic figures, suggesting that it is able to access kinesin in spindles. In contrast to KHC-specific antibodies, two antibodies that react with kinesin-like proteins (KLPs), namely CHO1 and HD, disrupted mitosis and prevented subsequent cell division. CHO1 is thought to exert this effect by blocking the activity of a 110-kD KLP. The relevant target of HD, which was raised against the KHC motor domain, is unknown; HD may disrupt mitosis by interfering with an essential spindle KLP but not with KHC itself, as preabsorption of HD with KHC did not alter its ability to block mitosis. These data indicate that some KLPs have essential mitotic functions in early sea urchin embryos but KHC itself does not.

Evidence for calcium-binding proteins and calcium-dependent regulatory proteins in sensory cells of the organ of Corti

Calcium is thought to play a major signaling role in outer hair cells to control metabolism, cytoskeletal integrity, cell shape and cell excitability. For this to happen, in resting cells the concentration of free calcium ions must be maintained at low levels so that focal increases can trigger specific events. In this paper, the localization of calcium, calcium-binding and calcium-dependent regulatory proteins in sensory cells from the guinea pig inner ear was demonstrated using immunocytochemical and histochemical techniques. We found the calcium buffer and/or calcium sensor proteins calmodulin, calbindin and calsequestrin predominantly in sensory cells and that when present, these proteins can be enriched in the outer hair cells. Calmodulin is found in the stereocilia, in the cuticular plate and in the cytoplasm and calbindin is found only in the cuticular plate and cytoplasm of both the inner and outer hair cells. The staining for these proteins in the outer hair cells is homogeneous, with no apparent compartmentalization along the lateral wall. Calsequestrin, thought to store and release calcium from membrane bound intracellular storage sites is found only in the cytoplasm of outer hair cells. There, it has a more punctuate staining pattern than does calmodulin or calbindin suggesting that it may be present in calciosomes rather than soluble in the cytoplasm. We did not detect caldesmon and S-100. Using the potassium pyroantimonate technique, we found precipitates containing calcium ions distributed throughout the cytoplasm of outer hair cells, with no evidence that the subsurface cisterns along the lateral wall act as calcium storage sites. Thus, calcium in resting cells is found in the cytoplasm along with calbindin and calmodulin and appears to have a punctate distribution consistent with a co-localization with calsequestrin. The implications of this distribution with respect to the slow shortening and elongation seen in outer hair cells are discussed.

Calsequestrinlike calcium-binding protein is expressed in calcium-accumulating cells of Pistia stratiotes

To contend with high calcium (Ca) levels in the environment, many plant species contain crystal idioblasts, specialized cells which accumulate large amounts of Ca as oxalate crystals. The biochemical processes involved in the accumulation of Ca in crystal idioblasts are unknown, as these cells constitute only a minor proportion of the total plant tissue. To address how crystal idioblasts buffer cytosolic Ca during crystal formation, we purified these cells from water lettuce and assessed their biochemistry. We show here that crystal idioblast cells contain three Ca-binding proteins not detectable in mesophyll cells. One of the Ca-binding proteins shares antigenicity with rabbit calsequestrin, a high-capacity low-affinity Ca-binding protein, and is encoded by related nucleotide sequences. Immunocytochemical localization studies further demonstrate that a calsequestrinlike protein is present primarily in crystal idioblasts and is preferentially localized in the endoplasmic reticulum, an organelle enriched in Ca as evidenced by vital staining. We thus conclude that crystal idioblasts possess a buffering system involving calsequestrinlike proteins, a process that likely plays an essential role in the bulk control of Ca in plant cells.

Non-plasmalemmal localisation of the major ganglioside in sea urchin eggs

M5 ganglioside (NeuGc alpha 2-6Glc beta 1-1'Cer) is the predominant glycosphingolipid in sea urchin eggs. Distribution of M5 ganglioside was studied in unfertilised and fertilised eggs of the sea urchin Hemicentrotus pulcherrimus by indirect immunofluorescence microscopy. In the cortices of unfertilised eggs, anti-M5 antibody strongly stained the submembranous, polygonal and tubular network of endoplasmic reticulum that was revealed by a membrane-staining dye, DiIC18(3). In addition to the cortical network of endoplasmic reticulum, at least two morphologically distinct vesicles were positive to the antibody. In the cortices isolated from fertilised eggs 30 min after insemination, the antibody stained only a similar network of endoplasmic reticulum, presumably the one reconstructed 5-10 min after fertilisation. During mitosis the endoplasmic reticulum is known to aggregate within the asters of the mitotic apparatus. Indeed, the antibody stained the asters and (more strongly) the vesicular components attaching to the periphery of the mitotic apparatus.

Polarity and reorganization of the endoplasmic reticulum during fertilization and ooplasmic segregation in the ascidian egg

During the first cell cycle of the ascidian egg, two phases of ooplasmic segregation create distinct cytoplasmic domains that are crucial for later development. We recently defined a domain enriched in ER in the vegetal region of Phallusia mammillata eggs. To explore the possible physiological and developmental function of this ER domain, we here investigate its organization and fate by labeling the ER network in vivo with DiIC16(3), and observing its distribution before and after fertilization in the living egg. In unfertilized eggs, the ER-rich vegetal cortex is overlaid by the ER-poor but mitochondria-rich subcortical myoplasm. Fertilization results in striking rearrangements of the ER network. First, ER accumulates at the vegetal-contraction pole as a thick layer between the plasma membrane and the myoplasm. This accompanies the relocation of the myoplasm toward that region during the first phase of ooplasmic segregation. In other parts of the cytoplasm, ER becomes progressively redistributed into ER-rich and ER-poor microdomains. As the sperm aster grows, ER accumulates in its centrosomal area and along its astral rays. During the second phase of ooplasmic segregation, which takes place once meiosis is completed, the concentrated ER domain at the vegetal-contraction pole moves with the sperm aster and the bulk of the myoplasm toward the future posterior side of the embryo. These results show that after fertilization, ER first accumulates in the vegetal area from which repetitive calcium waves are known to originate (Speksnijder, J. E. 1992. Dev. Biol. 153:259-271). This ER domain subsequently colocalizes with the myoplasm to the presumptive primary muscle cell region.

A novel vesicle-associated protein (VAP-1) in sea urchin eggs containing multiple RNA-binding consensus sequences

We have identified a novel high molecular weight, vesicle-associated protein (VAP-1) in the eggs of the sea urchin Strongylocentrotus purpuratus. Biochemical fractionation and immunofluorescence analysis of unfertilized eggs indicate that VAP-1 is a peripheral membrane protein associated with microsomal membrane fractions. Sequence analysis of partial VAP-1 cDNA clones reveals that the protein contains at least four RNA-binding consensus sequences. The RNA-binding sequences are separated by several glycine rich domains and this organization, RNA-binding domains separated by glycine rich sequences, is common to several RNA-binding proteins including the heterogeneous ribonuclear protein A1 and nucleolin. The characteristics of VAP-1 suggest that the protein may function as a multidomain RNA-binding protein. The possibility that VAP-1 may play a role in nuclear RNA processing is also discussed.