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

Actin is not an essential component in the mechanism of calcium-triggered vesicle fusion

Actin has been suggested as an essential component in the membrane fusion stage of exocytosis. In some model systems disruption of the actin filament network associated with exocytotic membranes results in a decrease in secretion. Here we analyze the fast Ca2+-triggered membrane fusion steps of regulated exocytosis using a stage-specific preparation of native secretory vesicles (SV) to directly test whether actin plays an essential role in this mechanism. Although present on secretory vesicles, selective pharmacological inhibition of actin did not affect the Ca2+-sensitivity, extent, or kinetics of membrane fusion, nor did the addition of exogenous actin or an anti-actin antibody. There was also no discernable affect on inter-vesicle contact (docking). Overall, the results do not support a direct role for actin in the fast, Ca2+-triggered steps of regulated membrane fusion. It would appear that actin acts elsewhere within the exocytotic cycle.

The heterotrimeric motor protein kinesin-II localizes to the midpiece and flagellum of sea urchin and sand dollar sperm

We have utilized immunoblotting and light microscopic immunofluorescent staining methods to examine the expression and localization of sea urchin kinesin-II, a heterotrimeric plus end-directed microtubule motor protein (previously referred to as KRP(85/95)), in sea urchin and sand dollar sperm. We demonstrate the presence of the 85 K and 115 K subunits of kinesin-II in sperm and localize these proteins to the sperm flagella and midpiece. The kinesin-II localization pattern is punctate and discontinuous, and in the flagella it is quite distinct from the continuous labeling present in sperm labeled with anti-flagellar dynein. The kinesin-II staining is largely insensitive to prefixation detergent extraction, suggesting that it is not associated with membranous elements in the sperm. In the midpiece the kinesin-II staining is similar to the pattern present in sperm labeled with an anti-centrosomal antibody. To our knowledge, this is the first localization of kinesin-like proteins in mature sperm and corroborates the recent identification and localization of kinesin-like proteins in the flagella and basal body of the unicellular green alga Chlamydomonas. We hypothesize that kinesin-II in the sperm may play functional roles in intraflagellar transport and/or the formation of flagella during spermatogenesis.

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.

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.

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.

Relationship between spatial pattern of basal bodies and membrane skeleton (epiplasm) during the cell cycle of Tetrahymena: cdaA mutant and anti-membrane skeleton immunostaining

Microtubular basal bodies and epiplasm (membrane skeleton) are the main components of the cortical skeleton of Tetrahymena. The aim of this report was to study functional interactions of basal bodies and epiplasm during the cell cycle. The cortex of Tetrahymena cells was stained with anti-epiplasm antibody. This staining produced a bright epiplasmic layer with a dark pattern of unstained microtubular structures. The fluorescence of the anti-epiplasm antibody disappeared at sites of newly formed microtubular structures, so the new basal body domains and epiplasmic layer could be followed throughout the cell cycle. Different patterns of deployment of new basal bodies were observed in early and advanced dividers. In advanced dividers the fluorescence of the epiplasmic layer diminished locally within the forming fission line where the polymerization of new basal bodies largely extincted. In wild type Tetrahymena, the completion of the micronuclear metaphase/anaphase transition was associated with a transition from the pattern of new basal body deployment and epiplasm staining of the early divider to the pattern of the advanced dividers. The signal for the fission line formation in Tetrahymena (absent in cdaA1 Tetrahymena mutationally arrested in cytokinesis) brings about 1) transition of patterns of deployment of basal bodies and epiplasmic layer on both sides of the fission line; and 2) coordination of cortical divisional morphogenesis with the micronuclear mitotic cycle.

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)

Cytoskeleton--plasma membrane interactions

Proteins at the boundary between the cytoskeleton and the plasma membrane control cell shape, delimit specialized membrane domains, and stabilize attachments to other cells and to the substrate. These proteins also regulate cell locomotion and cytoplasmic responses to growth factors and other external stimuli. This diversity of cellular functions is matched by the large number of biochemical mechanisms that mediate the connections between membrane proteins and the underlying cytoskeleton, the so-called membrane skeleton. General organizational themes are beginning to emerge from examination of this biochemical diversity.

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.

Molecular links between the cytoskeleton and membranes

This review covers recent advances in non-erythroid spectrin re-distributions during development, structural motifs recently discovered in ankyrin, band 4.2, band 4.1, ezrin, talin, and myosin I, and our present understanding of actin-membrane interactions at focal adhesions and in liver, platelet, and Dictyostelium discoideum plasma membranes.

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

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