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

Eph and Ephrin function in dispersal and epithelial insertion of pigmented immunocytes in sea urchin embryos

The mechanisms that underlie directional cell migration are incompletely understood. Eph receptors usually guide migrations of cells by exclusion from regions expressing Ephrin. In sea urchin embryos, pigmented immunocytes are specified in vegetal epithelium, transition to mesenchyme, migrate, and re-enter ectoderm, distributing in dorsal ectoderm and ciliary band, but not ventral ectoderm. Immunocytes express Sp-Eph and Sp-Efn is expressed throughout dorsal and ciliary band ectoderm. Interfering with expression or function of Sp-Eph results in rounded immunocytes entering ectoderm but not adopting a dendritic form. Expressing Sp-Efn throughout embryos permits immunocyte insertion in ventral ectoderm. In mosaic embryos, immunocytes insert preferentially in ectoderm expressing Sp-Efn. We conclude that Sp-Eph signaling is necessary and sufficient for epithelial insertion. As well, we propose that immunocytes disperse when Sp-Eph enhances adhesion, causing haptotactic movement to regions of higher ligand abundance. This is a distinctive example of Eph/Ephrin signaling acting positively to pattern migrating cells.

DOI:http://dx.doi.org/10.7554/eLife.16000.001

During animal development, numerous cells move around the embryo to form and shape the growing tissues. As these cells move, they are guided to their destination by molecular cues. The embryo’s tissues produce these cues and the cues can either repel or attract migrating cells. Ephrins are a large and well-studied family of proteins that serve as guidance cues and are found on the surface of certain types of cells. Some migrating cells have receptors for Ephrin and are repelled from tissues that contain Ephrin proteins. In these cases, the repulsive interaction between Ephrins and cells with receptors ensures that migrating cells avoid certain locations and reach the correct final destination.

The sea urchin is an important model organism for studying how animals develop and in particular how genes control animal development. This is in part because these animals can be easily manipulated in the laboratory and are more closely related to animals with backbones than many other model organisms. Sea urchins also have a relatively simple set of genes; many of which are similar to the human form of the gene. In sea urchin embryos, pigmented cells called immunocytes are known to migrate from one region of the embryo to another where they form part of its immune system. However it was not clear what guides this migration.

Sea urchins produce one type of Ephrin protein and its associated receptor, and now Krupke et al. show that immunocytes carry the receptor for Ephrin and migrate to embryonic tissues that produce high levels of this Ephrin. This finding suggested that the Ephrin is actually attracting the immunocytes to their final destination rather than repelling them. Further experiments supported this idea and revealed that immunocytes that lack the Ephrin receptor fail to enter the right tissue. Similarly, altering the pattern of Ephrin in the embryo’s tissues altered immunocyte migration in a predictable way.

These findings of Krupke et al. suggest that Ephrin and its receptor have changed their biological functions during evolution of animals. This raises a number of questions for future research including whether the molecular mechanisms used by Ephrin and its receptor to attract immunocytes in sea urchins is the same as that used to repel cells in other species.

DOI:http://dx.doi.org/10.7554/eLife.16000.002

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.

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.

Fertilization and nicotinic acid adenine dinucleotide phosphate induce pH changes in acidic Ca(2+) stores in sea urchin eggs

The second messenger nicotinic acid adenine dinucleotide phosphate (NAADP) releases Ca(2+) from the acidic Ca(2+) stores of many organisms, including those of the sea urchin egg. We investigated whether the pH within the lumen of these acidic organelles changes in response to stimuli. Fertilization activates the egg by Ca(2+) release dependent upon NAADP, and accordingly, we report that fertilization also alters organellar pH in a spatio-temporally complex manner. Upon sperm fusion, vesicles deep in the egg center slowly acidify, whereas cortical vesicles undergo a rapid alkalinization. The cortical vesicle alkalinization is independent of exocytosis and cytosolic pH but coincides with the NAADP-dependent fertilization Ca(2+) wave. Microinjection of NAADP mimicked the fertilization cortical response, suggesting that it occurred within NAADP-sensitive acidic Ca(2+) stores. Our data show that NAADP and physiological stimuli alter the pH within intracellular organelles and suggest that NAADP signals through pH as well as Ca(2+).

Integrins on eggs: the betaC subunit is essential for formation of the cortical actin cytoskeleton in sea urchin eggs

Eggs of several metazoans have been demonstrated to express integrins; however, their function is unclear. Previous studies have shown that the betaC integrin subunit is expressed on unfertilized sea urchin eggs and proteolytically removed at fertilization. Here we report that the betaC subunit is reexpressed on the egg surface immediately after fertilization. Using morpholino antisense oligonucleotides to block translation, we show that without betaC expression, eggs undergo cleavage resulting in loosely adherent cells that fail to develop beyond a blastula. Without betaC containing integrins, the cortical actin network of the egg does not form, yet contractile rings appear. Coinjection of RNA encoding the betaC or chicken beta1 subunit, but lacking the morpholino target sequence, rescues the cortical actin network and normal embryos result. Coinjection of RNA encoding the betaC subunit lacking the cytoplasmic domain fails to rescue. These studies demonstrate that the cortical actin cytoskeleton is anchored by betaC integrins and contractile ring actin is not. We suggest that one important function of egg integrins is to organize the actin cortex.

The spectrin skeleton of newly-invaginated plasma membrane

As a cell's shape and volume change. its surface area must re-adjust. How is the plasma membrane's spectrin skeleton implicated? For erythrocytes, cells of fixed surface area, spectrin responses to mechanical disturbances have been studied, but for more typical cells with changeable surface areas, they have not. In rapidly shrinking cells, surface membrane at an adherent substratum invaginates, forming transient vacuole-like dilations (VLDs). We exploited this readily inducible surface area perturbation to pose a simple question: is newly invaginated plasma membrane naked or is it supported by a spectrin skeleton? The spectrin skeleton was examined immunocytochemically in L6 cells (rat skeletal muscle) before and after VLD formation, using fixation in cold methanol and 4112, an antibody against beta-fodrin and beta-spectrin. 4112 was visualized by confocal fluorescence microscopy, while paired phase contrast images independently located the VLDs. To generate VLDs, cells were hypotonically swelled then reshrunk in isotonic medium. Swollen L6 cells maintained their plasma membrane (sarcolemma) spectrin skeleton. Within minutes of subsequent shrinkage, VLDs of 1-2 microm diameter invaginated at the substratum surface of myotubes. Both sarcolemma and VLDs were lined by a relatively uniform spectrin skeleton. Z-series suggested that some of the spectrin skeleton-lined sarcolemma became internalized as vacuoles.

Calcium transients accompany ooplasmic segregation in zebrafish embryos

Through the injection of f-aequorin (a calcium-specific luminescent reporter), and the use of an imaging photon detector, transient localized elevations of free cytosolic calcium in the forming blastodisc (BD) and animal hemisphere cortex were visualized that correlated with ooplasmic segregation. The introduction of an appropriate concentration of the weak (KD = 1.5 micromol/L) calcium buffer 5,5'-dibromo-BAPTA results in the dissipation of these calcium domains, and inhibits cytoplasmic streaming and the subsequent formation of a BD at the animal pole. These inhibitory actions are dependent on the final cytosolic concentration of buffer within the egg: > or = 1.3 mmol/L blocks ooplasmic streaming; < 1.3 mmol/L eggs segregate normally. Injection of 5,5'-dimethyl-BAPTA (KD = 0.15 micromol/L) to a final concentration of 1.5 mmol/L as a control has no effect on ooplasmic streaming. These results suggest that localized domains of elevated free cytosolic calcium are essential for ooplasmic segregation in zebrafish. Furthermore, a hypothetical model is presented linking these calcium transients to the contraction of a cortically located actin microfilament network as a possible mechanism providing the driving force for segregation.

Germline cyst formation in Drosophila

In a wide variety of organisms, gametes develop within clusters of interconnected germline cells called cysts. Four major principles guide the construction of most cysts: synchronous division, a maximally branched pattern of interconnection between cells, specific changes in cyst geometry, and cyst polarization. The fusome is a germline-specific organelle that is associated with cyst formation in many insects and is likely to play an essential role in these processes. This review examines the cellular and molecular processes that underlie fusome formation and cyst initiation, construction, and polarization in Drosophila melanogaster. The studies described here highlight the importance of cyst formation to the subsequent development of functional gametes.

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.

Cortical granules of the sea urchin translocate early in oocyte maturation

Cortical granules are secretory vesicles poised at the cortex of an egg that, upon stimulation by sperm contact at fertilization, secrete their contents. These contents modify the extracellular environment and block additional sperm from reaching the egg. The role of cortical granules in blocking polyspermy is conserved throughout much of phylogeny. In the sea urchin, cortical granules accumulate throughout the cytoplasm during oogenesis, but in mature eggs the cortical granules are attached to the plasma membrane, having translocated to the cortex at some earlier time. To study the process of cortical granule translocation to the cell surface we have devised a procedure for maturation of sea urchin oocytes in vitro. Using this procedure, we examined the rate of oocyte maturation by observing the movement and breakdown of the germinal vesicle, the formation of polar bodies and the formation of the egg pronucleus. We find that oocyte maturation takes approximately 9 hours in the species used here (Lytechinus variegatus), from the earliest indication of maturation (germinal vesicle movement) to formation of a distinct pronucleus. We then observed the translocation of cortical granules in these cells by immunolocalization using a monoclonal antibody to hyalin, a protein packaged specifically in cortical granules. We found that the translocation of cortical granules in in vitro-matured oocytes begins with the movement of the germinal vesicle to the oocyte cell surface, and is 50% complete 1 hour after germinal vesicle breakdown. In the in vitro-matured egg, 99% of the cortical granules are at the cortex, indistinguishable from translocation in oocytes that mature in vivo. We have also found that eggs that mature in vitro are functionally identical to eggs that mature in vivo by four criteria. (1) The matured cells undergo a selective turnover of mRNA encoding cortical granule contents. (2) The newly formed pronucleus begins transcription of histone messages. (3) Cortical granules that translocate in vitro are capable of exocytosis upon activation by the calcium ionophore, A23187. (4) The mature egg is fertilizable and undergoes normal cleavage and development. In vitro oocyte maturation enables us to examine the mechanism of cortical granule translocation and other processes that had previously only been observed in static sections of fixed ovaries.

The structure, function, and assembly of actin filament bundles

The cellular organization, function, and molecular composition of selected biological systems with prominent actin filament bundles are reviewed. An overall picture of the great variety of functions served by actin bundles emerges from this overview. A unifying theme is that the actin cross-linking proteins are conserved throughout the eukaryotic kingdom and yet assembled in a variety of combinations to produce actin bundles of differing functions. Mechanisms of actin bundle formation in vitro are considered illustrating the variety of physical and chemical driving forces in this exceedingly complex process. Our limited knowledge regarding the formation of actin filament bundles in vivo is contrasted with the elegant biophysical studies performed in vitro but nonetheless reveals that interactions with membranes, nucleation sites, and other organizational components must contribute to formation of actin bundles in vivo.

alpha-spectrin is required for germline cell division and differentiation in the Drosophila ovary

During Drosophila oogenesis, developing germline cysts are spanned by a large cytoplasmic structure called a fusome, containing alpha-spectrin and the adducin-like product of the hu-li tai shao (hts) gene. We found that fusomes contain two additional membrane skeletal proteins: beta-spectrin and ankyrin. hts was shown previously to be required for cyst formation and oocyte differentiation; the role of the fusome itself, however, and the organization and function of its other components, remains unclear. Using the FRT/FLP recombinase system to generate clones of alpha-spectrin-deficient cells in the ovary, we have shown that alpha-spectrin is also required for cyst formation and oocyte differentiation, but that its role in each process is distinct from that of Hts protein. Furthermore, alpha-spectrin is required for these processes in germline cells, but not in the follicle cells that surround each cyst. We have also found that the organization of membrane skeletal proteins is more dependent on alpha-spectrin in the fusome than at the plasma membrane in other cells. Our results suggest that the fusome and its associated membrane skeleton play a central role in regulating the divisions and differentiation of cyst cells.

Characterisation and role of integrins during gametic interaction and egg activation

It has recently been proposed that some of the processes induced by fertilisation in mammals may be mediated by integrins. By performing immunofluorescence labelling and Western blots with antibodies directed against some of the alpha and beta subunits of integrins, we show here the presence of some of these proteins in human and hamster oocytes. Among them, alpha 2 and alpha 5 were also present on in vitro preparations of sea urchin egg cortices. In addition, antibodies raised against these two proteins were the most effective at inhibiting attachment and fusion of human spermatozoa with hamster oocytes. We suggest that alpha 2 and alpha 5 integrin chains may be common mediators in adhesion-fusion mechanisms triggered by fertilisation. Using similar techniques, we show that eggs are rich in three cytoskeletal proteins known to be linked to the beta chain of integrins: talin, vinculin and alpha-actinin. Moreover, we found that talin and alpha-actinin were associated with proteins phosphorylated on tyrosine after fertilisation in sea urchin eggs. We suggest that integrins might be involved during fertilisation and trigger egg activation through cytoskeletal structures.

Stimulation of cortical actin polymerization in the sea urchin egg cortex by NH4Cl, procaine and urethane: elevation of cytoplasmic pH is not the common mechanism of action

Previous studies have demonstrated that the penetrating weak base NH4Cl and the anesthetics procaine and urethane disrupt the normal attachment of cortical granules to the cortex of the sea urchin egg. Hylander and Summers (1981: Dev. Biol. 86:1-11) hypothesized that this effect may be caused by a pH-induced polymerization of cortical actin. We have tested this hypothesis by measuring the intracellular pH of eggs of the sea urchins S. purpuratus and A. punctulata treated with NH4Cl, procaine, or urethane, and determining the effects of these agents on the organization of cortical actin. Intracellular pH was determined by the ratiometric measurement of the fluorescent dye BCECF, and filamentous actin organization was examined by confocal laser scanning microscopy of BODIPY-phallocidin stained eggs. Treatment of eggs with either NH4Cl or procaine resulted in a rapid and reversible increase in cytoplasmic pH of up to 1 pH unit and a dose-dependent increase in the intensity of fluorescent staining of the cortex, indicating an increase in the content of filamentous actin. While urethane also induced a dramatic polymerization of cortical actin, no effect on cytoplasmic pH could be detected. These results demonstrate that NH4Cl, procaine and urethane all induce an increase in the amount of filamentous actin in the sea urchin egg cortex that may participate in the detachment of cortical granules. However, these compounds do not share a common mechanism of action based on the elevation of cytoplasmic pH.

Isolation and characterization of a sea urchin zygote cortex that supports in vitro contraction and reactivation of furrowing

The isolation of the cortex of the sea urchin blastomere by detergent lysis was explored with the aim of analyzing components important in the structure and function of the cortical cytoskeleton, and their relationship to such phenomena as contraction. Buffered EGTA medium supplemented with isotonic glycerol and with magnesium, at a level close to the reported internal cellular concentration, yields stable cytoskeletal cortices that retain their spherical shape. Cortices prepared this way contain actin, myosin, fascin and spectrin, components normally associated with the cortical cytoskeleton in a similar distribution to that in intact zygotes. They retain the organized cortical filamentous structure, including the actin-fascin bundles that form cores of microvilli. ATP and NaCl caused changes in cortical shape, described as either contraction or expansion, respectively. Spectrin, but not myosin, was partially extracted by NaCl, resulting in expansion of the cortex that suggests a role for spectrin in maintenance of cortical structure. ATP (but not ADP nor ATP gamma S), which caused the partial removal of myosin and spectrin, led to the contraction of the cortex, consistent with a role for myosin in cortical tension. In cortices isolated from dividing eggs, the zygotes retained their cleavage furrows and ATP induced continuation of furrow progression. This preparation appears to be a useful in vitro model for cytokinesis.

The fodrin-ankyrin cytoskeleton of choroid plexus preferentially colocalizes with apical Na+K(+)-ATPase rather than with basolateral anion exchanger AE2

A unique feature of the choroid plexus as a single-layer epithelium is its localization of Na+K(+)-ATPase at its apical (lumenal) surface. In contrast, a band 3 (AE1)-related anion exchanger protein has been localized to the basolateral surface of the choroid plexus. Both Na+K(+)-ATPase and AE1 in other tissues have been shown to bind via ankyrin to the spectrin-actin-based membrane cytoskeleton. Since linkage of integral membrane proteins to the membrane cytoskeleton is important for their restriction to specialized domains of the cell surface, we investigated the polarity of the choroid plexus membrane cytoskeleton. We developed isoform-specific antibodies to confirm the identity of choroid plexus band 3-related polypeptide as AE2. We demonstrated that ankyrin, fodrin/spectrin, actin, myosin, and alpha-actinin are predominantly apical in choroid plexus and preferentially colocalize with apical Na+K(+)-ATPase rather than with basolateral anion exchanger AE2. Colchicine administration did not alter the polarity of apical cytoskeletal and transport proteins or basolateral AE2 in choroid plexus, suggesting that biosynthetic targeting of these proteins is not microtubule dependent. In choroid plexus papilloma, Na+K(+)-ATPase and AE2 were decreased in amount and failed to preserve their polarized distributions.

The 270 kDa splice variant of erythrocyte beta-spectrin (beta I sigma 2) segregates in vivo and in vitro to specific domains of cerebellar neurons

Spectrin isoforms arise from four distinct genes, three of which generate multiple alternative transcripts. With no biochemical restrictions on the assembly of alpha beta heterodimers, more than 25 distinct heterodimeric spectrin species may exist. Whether (and why) this subtle but substantial diversity is realized in any single cell is unknown. To address this question, sequence-specific antibodies to alternatively spliced regions of alpha- and beta-spectrin have been prepared. Reported here is the localization in rat cerebellar neurons at light and electron microscopic levels of an antibody against a unique sequence (beta I sigma 2-A = PGQHKDGQKSTGDERPT) from the 270 kDa transcript of the red cell beta-spectrin gene (spectrin beta I sigma 2). In this version, the 3′ sequence of erythroid beta-spectrin (beta I sigma 1) is replaced with an alternative sequence that shares substantial homology with the 3′ sequence of non-erythroid beta-spectrin (beta II sigma 1). The antibody to beta I sigma 2-A stains a single protein band at 270 kDa, determined by western blotting, in both rat cerebellum and in cultured cerebellar granule cells, and does not react with beta II sigma 1 spectrin (beta-fodrin). This antibody stains the dendritic spines of Purkinje cells in the molecular layer, and is concentrated at postsynaptic densities (PSDs) adjacent to synapsin I (which is confined to the presynaptic membrane). The soma of Purkinje cells do not stain. In the granular layer, cytoplasmic organelles and the postsynaptic densities of granular cells stain strongly. Astrocytes are also stained. In all cells, plasma membrane staining is confined to postsynaptic densities (PSD). The beta I sigma 2 isoform co-immunoprecipitates with non-erythroid alpha-spectrin (alpha II sigma), even though the distribution of alpha II sigma within neurons only partially overlaps that of beta I sigma 2. No hybrid beta I sigma 2 and beta II sigma 1 (beta-fodrin) spectrin complexes appear to exist. Spectrin beta I sigma 2 is also polarized in cultured rat cerebellar granule cells, where it is abundant in cell bodies but not neurites. The overall distribution of beta I sigma 2 is as a subset of the distribution of spectrins 240/235E previously detected with a generally reactive erythrocyte alpha beta-spectrin antibody. These findings establish the highly precise segregation of a beta-spectrin isoform to distinct cytoplasmic and membrane surface domains, indicate that it is complexed (partially) with non-erythroid alpha-spectrin, and demonstrate that cytoskeletal targeting mechanisms are preserved in cultured granular cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Spectrin redistributes to the cytosol and is phosphorylated during mitosis in cultured cells

Dramatic changes in morphology and extensive reorganization of membrane-associated actin filaments take place during mitosis in cultured cells, including rounding up; appearance of numerous actin filament-containing microvilli and filopodia on the cell surface; and disassembly of intercellular and cell-substratum adhesions. We have examined the distribution and solubility of the membrane-associated actin-binding protein, spectrin, during interphase and mitosis in cultured CHO and HeLa cells. Immunofluorescence staining of substrate-attached, well-spread interphase CHO cells reveals that spectrin is predominantly associated with both the dorsal and ventral plasma membranes and is also concentrated at the lateral margins of cells at regions of cell-cell contacts. In mitotic cells, staining for spectrin is predominantly in the cytoplasm with only faint staining at the plasma membrane on the cell body, and no discernible staining on the membranes of the microvilli and filopodia (retraction fibers) which protrude from the cell body. Biochemical analysis of spectrin solubility in Triton X-100 extracts indicates that only 10-15% of the spectrin is soluble in interphase CHO or HeLa cells growing attached to tissue culture plastic. In contrast, 60% of the spectrin is soluble in mitotic CHO and HeLa cells isolated by mechanical "shake-off" from nocodazole-arrested synchronized cultures, which represents a four- to sixfold increase in the proportion of soluble spectrin. This increase in soluble spectrin may be partly due to cell rounding and detachment during mitosis, since the amount of soluble spectrin in CHO or HeLa interphase cells detached from the culture dish by trypsin-EDTA or by growth in spinner culture is 30-38%. Furthermore, mitotic cells isolated from synchronized spinner cultures of HeLa S3 cells have only 2.5 times as much soluble spectrin (60%) as do synchronous interphase cells from these spinner cultures (25%). The beta subunit of spectrin is phosphorylated exclusively on serine residues both in interphase and mitosis. Comparison of steady-state phosphorylation levels of spectrin in mitotic and interphase cells demonstrates that solubilization of spectrin in mitosis is correlated with a modest increase in the level of phosphorylation of the spectrin beta subunit in CHO and HeLa cells (a 40% and 70% increase, respectively). Two-dimensional phosphopeptide mapping of CHO cell spectrin indicates that this is due to mitosis-specific phosphorylation of beta-spectrin at several new sites. This is independent of cell rounding and dissociation from other cells and the substratum, since no changes in spectrin phosphorylation take place when cells are detached from culture dishes with trypsin-EDTA.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Fertilization alters the orientation of pigment granule saltations in Arbacia eggs

Unfertilized eggs of the sea urchin Arbacia punctulata contain pigment granules distributed throughout their cytoplasm. During the first 15 minutes after fertilization, these vesicles move out to the cortex where they become firmly anchored. We have used time-lapse video differential interference microscopy to analyze the motility of these organelles in unfertilized and fertilized Arbacia eggs. Pigment granules exhibit saltatory movement in both unfertilized and fertilized eggs. Quantitation of vesicle saltations before and after fertilization demonstrates that while there is no significant difference in the speed or path-length of vesicle movement, there is a dramatic change in the orientation of these saltations. Saltations in the unfertilized egg are very non-radial and are as likely to be directed toward the cortex as away. In contrast, saltations in the fertilized egg are more radially oriented and more likely to be cortically directed. This transition must reflect underlying changes in the cellular structures necessary for pigment granule saltations. The change in the orientation of pigment granule saltations following fertilization requires both a transient increase in the cytoplasmic concentration of Ca2+ and an elevation of cytoplasmic pH. Similarly, the ability of pigment granules to adhere to the cortex requires both the transient elevation of cytoplasmic Ca2+ and the alkalinization of the cytoplasm. As the reorganization of cortical actin at fertilization is regulated by these ionic fluxes, and both movement and adhesion are sensitive to cytochalasins, we hypothesize that the alterations in directed motility and adhesion reflect underlying changes in the actin cytoskeleton.

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.