Spectrin and calmodulin in spreading mouse blastomeres

Solution structure of the calponin homology (CH) domain from the smoothelin-like 1 protein: a unique apocalmodulin-binding mode and the possible role of the C-terminal type-2 CH-domain in smooth muscle relaxation

The SMTNL1 protein contains a single type-2 calponin homology (CH) domain at its C terminus that shares sequence identity with the smoothelin family of smooth muscle-specific proteins. In contrast to the smoothelins, SMTNL1 does not associate with F-actin in vitro, and its specific role in smooth muscle remains unclear. In addition, the biological function of the C-terminal CH-domains found in the smoothelin proteins is also poorly understood. In this work, we have therefore determined the solution structure of the CH-domain of mouse SMTNL1 (SMTNL1-CH; residues 346-459). The secondary structure and the overall fold for the C-terminal type-2 CH-domain is very similar to that of other CH-domains. However, two clusters of basic residues form a unique surface structure that is characteristic of SMTNL1-CH. Moreover, the protein has an extended C-terminal alpha-helix, which contains a calmodulin (CaM)-binding IQ-motif, that is also a distinct feature of the smoothelins. We have characterized the binding of apo-CaM to SMTNL1-CH through its IQ-motif by isothermal titration calorimetry and NMR chemical shift perturbation studies. In addition, we have used the HADDOCK protein-protein docking approach to construct a model for the complex of apo-CaM and SMTNL1-CH. The model revealed a close interaction of SMTNL1-CH with the two Ca(2+) binding loop regions of the C-terminal domain of apo-CaM; this mode of apo-CaM binding is distinct from previously reported interactions of apo-CaM with IQ-motifs. Finally, we comment on the putative role of the CH-domain in the biological function of SMTNL1.

Non-equivalence of embryonic and somatic cell nuclei affecting spindle composition in clones

Cloning by nuclear transfer remains inefficient but is more efficient when nuclei from embryonic cells or embryonic stem cells (ECNT) are employed as compared with somatic cells (SCNT). The factors determining efficiency have not been elucidated. We find that somatic and embryonic nuclei differ in their ability to organize meiotic and mitotic spindles of normal molecular composition. Calmodulin, a component of meiotic and mitotic spindle chromosome complexes (SCCs), displays sharply reduced association with the SCC forming after SCNT but not ECNT. This defect persists in mitotic spindles at least through the second mitosis, despite abundant calmodulin expression in the cell, and correlates with slow chromosome congression. We propose that somatic cell nuclei lack factors needed to direct normal SCC formation in oocytes and early embryos. These results reveal a striking control of SCC formation by the transplanted nucleus and provide the first identified molecular correlate of donor stage-dependent restriction in nuclear potency.

Ca2+ and calmodulin regulate the binding of filamin A to actin filaments

Filamin A (FLNa) cross-links actin filaments (F-actin) into three-dimensional gels in cells, attaches F-actin to membrane proteins, and is a scaffold that collects numerous and diverse proteins. We report that Ca(2+)-calmodulin binds the actin-binding domain (ABD) of FLNa and dissociates FLNa from F-actin, thereby dissolving FLNa.F-actin gels. The FLNa ABD has two calponin homology domains (CH1 and CH2) separated by a linker. Recombinant CH1 but neither FLNa nor its ABD binds Ca(2+)-calmodulin in the absence of F-actin. Extending recombinant CH1 to include the negatively charged region linker domain makes it, like full-length FLNa, unable to bind Ca(2+)-calmodulin. Ca(2+)-calmodulin does, however, dissociate the FLNa ABD from F-actin provided that the CH2 domain is present. These findings identify the first evidence for direct regulation of FLNa, implicating a mechanism whereby Ca(2+)-calmodulin selectively targets the FLNa.F-actin complex.

? and ? spectrin distribution during the differentiation of pyriform cells in follicles of lizardPodarcis sicula

Using alpha and beta spectrin mammalian antibodies on Western blotting, we demonstrated that lizard ovarian follicles contain two isoforms of alpha spectrin, Mr 94 and 134 kDa, and a 230 kDa beta spectrin, and that their pattern modifies in relation to pyriform cell differentiation. In fact, a positive immunoreaction is firstly evident within follicular epithelium of previtellogenic follicles when small cells differentiate into pyriform cells via intermediate cells. Later on, immunostain is present in pyriform cells and in the oocyte cortex that previously appears unstained. It is noteworthy that immunostain is also present on small cells located in contact with the oocyte membrane, but not on those located under the basal lamina and among pyriform cells, not engaged in pyriform cell differentiation. During the subsequent stages of previtellogenic phase, spectrin immunostain over the follicular epithelium and in the oocyte cortex does not change. By contrast, in vitellogenic follicles, when the follicular epithelium is constituted only by small cells, immunostain is evident at the level of the oocyte cortex and the cytoplasm of regressing pyriform cells. The present data strongly suggest that the alpha and beta spectrin pattern put in evidence during the different phases of lizard oocyte growth is related to the differentiation of small into pyriform cells, where such protein may guarantee a relationship between surface glycoproteins (Andreuccetti et al., 2001: Anat Rec 263:1-9), and the cytoskeleton distribution (Maurizii et al., 2000: Raf Mol Reprod Dev 57:159-166). Furthermore, the distribution of spectrin mRNA, similar to that observed for the protein, demonstrates that spectrin, once synthesized within pyriform cells, is transferred through intercellular bridges in the oocyte cortex, thus confirming that pyriform cells are nurse that significantly are involved in the oocyte growth. Finally, the present data demonstrate that alpha spectrin of lizard ovarian follicles has Mr quite different from those so far reported and may constitute a new group of isoforms. This important result will be the focus of future experiments. Mol. Reprod. Dev. 67: 101-107, 2004.

Tropomyosin in preimplantation mouse development: identification, expression, and organization during cell division and polarization

Tropomyosin is an actin-binding cytoskeletal protein which has been extensively characterized in a variety of cell types and tissues, with the exception of very early developmental stages during which cellular polarization first occurs. We have identified five polypeptides in mouse preimplantation conceptuses which show many of the characteristics of tropomyosin. They form the major portion of the heat-stable cytoskeletal protein fraction of blastomeres and have the characteristic isoelectric and SDS-PAGE migration characteristics on 1-D and 2-D gels. All five polypeptides were synthesized in late 2- and 4-cell, and all 8-cell stages, with three of the five polypeptides showing lower synthetic levels in fertilized eggs and early 2-cell conceptuses. These heat-stable proteins showed specific differences from proteins isolated from mouse 3T3 fibroblasts by the same method, namely higher Mr isoforms were not represented, also some of the isoforms can be labeled by incorporation of [14C]proline. The cellular distribution of tropomyosin in early stage conceptuses was examined using monoclonal and affinity-purified polyclonal antibodies. Tropomyosin becomes associated both with the blastomere cortex postfertilization and with the cleavage furrow during cytokinesis. The interphase cortical association is uniform until the 8-cell stage, when tropomyosin becomes associated with the developing apical pole and is excluded from the basolateral cortex. This polar localization is inherited along with the pole at the 8- to 16-cell division, but experiments in which cell division is artificially prolonged show that tropomyosin localization does not represent a permanent marking of the pole. We conclude that the early mouse conceptus contains a unique and specific set of tropomyosins which respond to polarizing signals.

Regulation of preimplantation development of mouse embryos: effects of inhibition of myosin light-chain kinase, a Ca2+/calmodulin-dependent enzyme

We have examined the effects of ML-9 and wortmannin, which are, respectively, specific reversible and irreversible inhibitors of myosin light-chain kinase, a Ca2+/calmodulin-dependent enzyme, on preimplantation development of the mouse in an attempt to establish a regulatory role for this enzyme in preimplantation development. When late two-cell stage embryos were treated continuously with ML-9 or wortmannin at a concentration of 0, 1, 5, 10, or 15 microM, compaction and formation of the blastocyst were inhibited in a dose-dependent manner. Stage-specific treatment with ML-9 at 25 microM induced stage-specific responses of embryos after the eight-cell stage during the processes of compaction and cavitation. These morphological responses included aborted compaction, decompaction of compacted embryos, and the inability of embryos to form a cavity. These morphological effects were reversible, but, since cell proliferation was inhibited, the "recovered" embryos were small. Counting of cells on day 4 of culture, in both continuously treated and stage-specifically treated embryos, showed that the effect of ML-9 on cell proliferation was also dose-dependent. Wortmannin also had stage-specific effects at 15 microM, but these effects were irreversible and were more deleterious than those of ML-9. With neither inhibitor was there any apparent effect at the two-cell or the four-cell stage, although wortmannin inhibited cell division when applied stage-specifically at the four-cell stage. These results indicate that myosin light-chain kinase may be an important enzyme in the first steps of differentiation and in the maintenance of the differentiated state during preimplantation development of the mouse.

Drosophila development requires spectrin network formation

The head-end associations of spectrin give rise to tetramers and make it possible for the molecule to form networks. We analyzed the head-end associations of Drosophila spectrin in vitro and in vivo. Immunoprecipitation assays using protein fragments synthesized in vitro from recombinant DNA showed that interchain binding at the head end was mediated by segment 0-1 of alpha-spectrin and segment 18 of beta-spectrin. Point mutations equivalent to erythroid spectrin mutations that are responsible for human hemolytic anemias diminished Drosophila spectrin head-end interchain binding in vitro. To test the in vivo consequence of deficient head-end interchain binding, we introduced constructs expressing head-end interchain binding mutant alpha-spectrin into the Drosophila genome and tested for rescue of an alpha-spectrin null mutation. An alpha-spectrin minigene lacking the codons for head-end interchain binding failed to rescue the lethality of the null mutant, whereas a minigene with a point mutation in these codons overcame the lethality of the null mutant in a temperature-dependent manner. The rescued flies were viable and fertile at 25 degrees C, but they became sterile because of defects in oogenesis when shifted to 29 degrees C. At 29 degrees C, egg chamber tissue disruption and cell shape changes were evident, even though the mutant spectrin remained stably associated with cell membranes. Our results show that spectrin's capacity to form a network is a crucial aspect of its function in nonerythroid cells.

Cytoskeletal elements in mammalian spermiogenesis and spermatozoa

Identification of the cytoskeletal elements and their role in the formation as well as the maintenance of head membrane compartmentalization is a much debated issue in mammalian spermatozoa. Data which have emerged during the last ten years are summarized. Those which have converged in a common opinion, such as the distribution of actin in mammalian spermiogenesis, are distinguished from those which have to be confirmed, such as the role of actin related proteins and actin in mature spermatozoa.

Actin-binding and microtubule-associated proteins in the organ of Corti

Actin-binding and microtubule-associated proteins regulate microfilament and microtubule number, length, organization and location in cells. In freeze-dried preparations of the guinea pig cochlea, both actin and tubulin are found in the sensory and supporting cells of the organ of Corti. Fodrin (brain spectrin) co-localized with actin in the cuticular plates of both inner and outer hair cells and along the lateral wall of the outer hair cells. Alpha-actinin co-localized with actin in the cuticular plates of the hair cells and in the head and foot plates of the supporting cells. It was also found in the junctional regions between hair cells and supporting cells. Profilin co-localized with actin in the cuticular plates of the sensory hair cells. Myosin was detected only in the cuticular plates of the outer hair cells and in the supporting cells in the region facing endolymph. Gelsolin was found in the region of the nerve fibers. Tubulin is found in microtubules in all cells of the organ of Corti. In supporting cells, microtubules are bundled together with actin microfilaments and tropomyosin, as well as being present as individual microtubules arranged in networks. An intensely stained network of microtubules is found in both outer and inner sensory hair cells. The microtubules in the outer hair cells appear to course throughout the entire length of the cells, and based on their staining with antibodies to the tyrosinated form of tubulin they appear to be more dynamic structures than the microtubules in the supporting cells. The microtubule-associated protein MAP-2 is present only in outer hair cells within the organ of Corti and co-localizes with tubulin in these cells. No other MAPs (1,3,4,5) are present. Tau is found in the nerve fibers below both inner and outer hair cells and in the osseous spiral lamina. It is clear that the actin-binding and microtubule-associated proteins present in the cochlea co-localize with actin and tubulin and that they modulate microfilament and microtubule structure and function in a manner similar to that seen in other cell types. The location of some of these proteins in outer hair cells suggests a role for microfilaments and microtubules in outer hair cell motility.

Organization of the membrane skeleton in spreading mouse blastomeres. I. Morphological analysis

Differentiation in the mouse embryo begins at the 8-cell stage when the blastomeres spread against each other in a process called compaction. The spreading behavior of blastomeres on lectin-coated coverslips mimics that of blastomeres in the embryo, and we have utilized this model system to obtain an en face view of the membrane skeleton in the spreading blastomeres. Embryos were cultured on the coverslips for periods ranging from 20 sec to 6 hr, and the cells were disrupted to expose the cytoplasmic face of the adherent membranes and their associated filaments. The "membrane lawn" preparations were fixed, critical point dried, rotary shadowed, and the replicas examined by transmission electron microscopy. Using this technique we found that the plasmalemma of rounded blastomeres is associated with a lacy 3-dimensional filamentous meshwork that is transformed into a thin mat of densely woven filaments when the cells flatten. The overall organization of the membrane skeleton is similar in flattening 2- and 8-cell embryos, but there are significant differences in the time required for spreading to take place, in the means whereby the membrane skeletons are reorganized, and in the extent of maximal flattening. The significance of these observations for the compaction process is discussed.

Cytoskeleton of the mouse egg and embryo: reorganization of planar elements

Examination of detergent-extracted mouse eggs and embryos reveals the existence of two cytoskeletal networks. One network is the typical thin filament network observed in somatic cells while the other is composed of large planar elements. These latter cytoskeletal structures, with individual widths of 60.0 +/- 6.8 nm, alter their spatial organization in a developmental stage-specific manner. The planar elements are composed of filaments with a diameter of 10 nm aligned side-by-side with these filaments exhibiting a linear periodicity of 20.0 +/- 1.6 nm. A biochemical fraction containing components of the planar elements has been prepared from different stages of development and disappearance of prominent polypeptides from this fraction correlates with the altered spatial organization of the planar elements. Ultrastructure and biochemistry of cytoskeletal planar elements in eggs and embryos of the mouse are comparable with cytoskeletal sheets of Syrian hamster eggs and embryos, suggesting these cytoskeletal components may have a functional role in mammalian embryogenesis. Because such structures have not been identified in eggs or embryos of species other than mammals, their function may be unique to mammalian embryogenesis.

Identification of spectrin and calmodulin in rabbit spermiogenesis and spermatozoa

Actin was localized in the four subacrosomal bulges (Camatini et al., Eur. J. Cell Biol. 45:276-281, 1987), which characterize the sperm head of rabbit spermatozoa (Phillips, J. Ultrastruct. Res. 38:591-604, 1972), and in the postacrosomal region (Welch and O'Rand, Dev. Biol. 109:411-417, 1985; Camatini et al., Eur. J. Cell Biol. 45:276-281, 1987). Specific antibodies and indirect immunogold labelling on testis Lowicryl K4M sections and spermatozoa cryosections were used to study the distribution of calmodulin and a spectrin-like protein. This protein was also present close to the shaping membranes of the head. The results presented suggest a membrane-cytoskeletal role of actin, spectrin, and calmodulin.

The cortical cytoskeleton and its role in sperm penetration of the mammalian egg

In this study isolated cortical regions of both penetrated and nonpenetrated Syrian hamster eggs were examined in whole mounts and platinum replicas of detergent-extracted cortical patches. Two types of cytoskeletal organization were observed in the egg cortex: Loose networks (LN regions) with integrated localized dense networks (LDN regions). Decoration with heavy meromyosin and labeling with antiactin/protein G gold both indicate that the cortical cytoskeleton consists mainly of a LN of actin microfilaments and several types of nonactin filaments, whereas LDN regions dispersed within the LN were comprised of nonactin filaments. Cortical patches and replicas of eggs incubated with sperm for 10-15 min provide evidence that cortical microfilaments may be intimately associated with penetrating spermatozoa. The results of this investigation provide the first high resolution view of the cortical cytoskeletal domain of a mammalian egg and suggest that actin microfilaments might play a role in sperm penetration of the egg cortex.

Antispectrin antibodies stain the oocyte nucleus and the site of fertilization channels in the egg of Discoglossus pictus (Anura)

In Discoglossus pictus eggs, only the dimple contains ionic channels active at fertilization; in particular, chloride channels are found in the central portion of the dimple, which is also the site of sperm penetration. Moreover the dimple hosts an imposing cytoskeleton, consisting of a cortical network and bundles of microfilaments extending from the microvilli. Since spectrin cross links actin and is connected through ankyrin to anion transporters in the plasma membrane of erythrocytes as well as to anion channels in other cells, we studied, in D. pictus egg, the relationship between the localization of spectrin and the high polarization of ionic channels and cytoskeletal organization. By means of immunocytochemistry, we localized spectrin exclusively in the egg dimple. In an attempt to trace back the source of spectrin localization, we immunostained sections of D. pictus ovary and localized spectrin in the nuclei of previtellogenic oocytes, where actin is also present. Antispectrin staining remained until germinal vesicle breakdown. By contrast, a cortical localization was found only when the oocytes divided into two hemispheres and into the germinative area (GA), which, after germinal vesicle breakdown, gives rise to the dimple. At this stage the antispectrin signal was particularly strong in the GA. Using Rho-pialloidin, we also established that spectrin is generally present where F-actin is found. However, spectrin and F-actin do not have the same pattern of fluorescence. In conclusion, our data suggest that spectrin may play a role in oocyte and egg polarity. In eggs, it could be instrumental in anchoring to the cytoskeleton membrane proteins such as receptors and ionic channels, including chloride-permeable channels.

Maintenance of compaction and adherent-type junctions in mouse morula-stage embryos

Timed morulae of different stages of development were exposed to cytochalasin B causing depolymerisation of microfilaments and to ECCD-1 antibodies interacting with Ca2(+)-dependent adhesion molecules or cultured in the absence of calcium. All three treatments decompacted mid-morula-stage embryos within one hour. Late morulae were resistant to ECCD-1 antibody treatment and relatively resistant to calcium-free cultivation, but not to cytochalasin B treatment. Scanning electron microscopy revealed that the decompacting treatments not only loosened the interblastomere contacts but also resulted in rearrangement of the cell surface microvilli. Transmission electron microscopy showed that normal, untreated embryos had specialized membrane junctions in the most apical regions of the interblastomere contacts. Immunoelectron microscopy revealed that these apical junction areas contained vinculin, a protein typical of adherent junctions. Upon decompaction the apical junctions disappeared completely. When transferred back to the normal medium, the embryos rapidly started to recompact. Simultaneously the apical junctions and cell surface microvilli reassumed the organization characteristic of the morula stage. Late morulae that were resistant to treatment had normal apical junctional areas. During subcultivation in the normal medium, the treated morulae developed into morphologically normal blastocysts. These data indicate that adherent-type junctions and cell surface microvilli participate in the initiation and maintenance of compaction of morula-stage embryos.

Immunocytochemical studies of spectrin in hamster cardiac tissue

The spectrins are a family of cytoskeletal-membrane proteins that have a wide tissue distribution. In the present study, we employed polyclonal antibodies made against mammalian and avian erythroid spectrins as well as mammalian brain spectrin to assess their presence and distributions in the mammalian heart. Western blot analyses revealed that all three antibodies were specific for a 240,000 molecular weight alpha-spectrin subunit found in hamster erythrocyte ghost homogenates, whole hamster heart, and isolated hamster cardiac myofibril homogenates. Spectrin staining was absent from the Triton X-100-extracted supernatant fraction of myofibril preparations, suggesting that the protein is linked to the myofibril precipitate after exposure to the detergent. Frozen, unfixed, 2-microns-thick; sections of adult. Syrian golden hamster cardiac tissue exhibited strong immunofluorescent staining of intercalated discs and Z-bands using all three antibodies. In addition, the mammalian erythroid spectrin antibodies showed staining of the sarcolemma, and in cross section, revealed a delicate internal network of staining that appears to surround individual myofibrils. This may be T-tubule-associated staining. Myofibrils isolated from cardiac myocytes using Triton X-100 show positive Z-band staining using all three antibodies. Double staining with Texas Red-labeled monoclonal desmin and FITC-labeled polyclonal spectrin antibodies revealed that both stained the myofibrillar Z-line regions. These results demonstrate that spectrin is closely associated with the membranes, myofibrils, and intermediate filaments in the mammalian heart.

Functional diversity among spectrin isoforms

The purpose of this review on spectrin is to examine the functional properties of this ubiquitous family of membrane skeletal proteins. Major topics include spectrin-membrane linkages, spectrin-filament linkages, the subcellular localization of spectrins in various cell types and a discussion of major functional differences between erythroid and nonerythroid spectrins. This includes a summary of studies from our own laboratories on the functional and structural comparison of avian spectrin isoforms which are comprised of a common alpha subunit and a tissue-specific beta subunit. Consequently, the observed differences among these spectrins can be assigned to differences in the properties of the beta subunits.

Spectrin synthesis in the preimplantation mouse embryo

The preimplantation mouse embryo expresses two polypeptides, Mr 240,000 and Mr 235,000, that are immunologically cross-reactive with antibody to the alpha and beta subunits of mouse brain spectrin. We investigated the synthesis of the spectrin subunits in the Triton-soluble and Triton-insoluble fractions of fertilized eggs, two-cell embryos, compacted morulae, and blastocysts labeled with L-[35S]methionine. Synthesis of embryonic spectrin began in the Triton-soluble fraction with significant levels of alpha-spectrin synthesis first detected in the morula stage and significant levels of beta-spectrin synthesis detected in the blastocyst stage. Incorporation of newly synthesized alpha- and beta-spectrin into the cytoskeletal fraction took place in the blastocyst when equal amounts of both subunits were assembled. Previous studies have shown Triton-insoluble spectrin to be concentrated in regions of cell-cell contact in the embryo (J. S. Sobel and M. A. Alliegro, 1985, J. Cell Biol. 100, 333-336). The temporal and spatial correlation between the assembly of newly synthesized spectrin and its concentration in regions of cell apposition is consistent with the hypothesis that cell contact may influence the assembly of embryonic spectrin.

Cytoskeleton in preimplantation mouse development

This paper reviews the constituents of the cytoskeleton in the cells of the preimplantation mouse embryo and how they change as the development proceeds. The cytoskeleton can be divided into two distinct groups, that in the cytosplasm and that associated with the membrane. The first and better-known group contains microfilaments, microtubules and intermediate filaments, the second such components of the cell and nuclear membrane as spectrin-like protein and nuclear lamin. The filamentous components of the cytoplasmic cytoskeleton adhere to the nuclear and cell membrane at attachment points where specific proteins such as vinculin may mediate the interaction. Each cell of the early embryo has all of these components, but their morphological organization and molecular constitution alter as the embryo develops. These modifications are especially pronounced when the cleavage-stage embryo compacts and when the blastocysts forms and differentiates. These events represent the most critical stages of morphogenesis and cytodifferentiation in the preimplantation embryo. The cytoskeleton may thus have an important role in the control of the early mammalian development.