Spectrin synthesis in the preimplantation mouse embryo

Paternal exposure to cyclophosphamide alters cell-cell contacts and activation of embryonic transcription in the preimplantation rat embryo

Paternal exposure to chronic low doses of cyclophosphamide, an anticancer agent, results in aberrant embryonic development of the progeny. We hypothesized that paternal exposure to cyclophosphamide disturbs zygotic gene activity regulating proper progression through preimplantation development and that this disturbance results in improper cell-cell interactions. To test this hypothesis, we analyzed cell-cell interactions and the expression of cytoskeletal elements in preimplantation embryos sired by male rats gavaged with saline or 6 mg kg(-1) day(-1) cyclophosphamide for 5 wk. Embryos from control litters had 4-12 cells on Day 2 of gestation; cell-cell contacts were observed consistently. Embryos from litters sired by cyclophosphamide-treated males were frequently abnormal and had lower cell numbers and decreased cell-cell contacts. Steady state concentrations of the mRNAs for cell adhesion molecules (cadherins and connexin 43) and structural proteins (beta-actin, collagen, and vimentin) were low in two- and four-cell control embryos; expression increased dramatically by the eight-cell stage. In contrast, embryos sired by cyclophosphamide-treated males displayed the highest expression of most trancripts at the two-cell stage. In parallel with the mRNA profiles, E-cadherin immmunoreactivity was nearly absent in two-cell control embryos and was strong by the eight-cell stage; immunoreactivity in embryos sired by drug-treated fathers was strong at the two-cell stage but absent at later stages. Thus, drug exposure of the paternal genome led to dysregulated expression of structural elements and decreased cell interactions during preimplantation embryonic development.

Dynamic rearrangement of the spectrin membrane skeleton during the generation of epithelial polarity in Drosophila

The origin of epithelial cell polarity during development is a fundamental problem in cell biology. Central to this process is the establishment of asymmetric membrane domains that will ultimately form the apical and basolateral surfaces. The spectrin-based membrane skeleton has long been thought to participate in the generation of this asymmetry. Drosophila melanogaster contains two known (beta)-spectrin isoforms: a conventional (beta)-spectrin chain, and the novel isoform (beta)(Heavy)-spectrin. These two proteins are restricted to the basolateral and apical membrane domains, respectively. To assay for the emergence of membrane asymmetry, we have characterized the distribution of these two (beta)-spectrins during the formation of the primary epithelium in the fly embryo. Our results show that the syncytial embryo contains a maternally established apical membrane skeleton containing (beta)(Heavy)-spectrin into which the basolateral (beta)-spectrin membrane skeleton is added. We have called this process basolateral interpolation. Although basolateral membrane skeleton addition begins during cellularization, it does not become fully established until the formation of a mature zonula adherens at mid to late gastrulation. The behavior of (beta)-spectrin is consistent with a primary role in establishing and/or maintaining the basolateral domain while the behavior of (beta)(Heavy)-spectrin suggests that its primary role is associated with a specialized DE-cadherin complex associated with the furrow canals and with the maturation of the zonula adherens. Thus, the apical spectrin membrane skeleton appears to play a distinct rather than analogous role to the basolateral spectrin membrane skeleton, during the emergence of cell polarity. We find that there are several parallels between our observations and previous studies on the establishment of primary epithelial polarity in vertebrates, suggesting that basolateral interpolation of the membrane skeleton may be a common mechanism in many organisms.

Developmental studies of dystrophin and other cytoskeletal proteins in cultured muscle cells

We studied the developmental changes of localization of dystrophin and other cytoskeletal proteins, especially actin, spectrin and dystrophin related protein (DRP) using immunocytochemistry and quick-freezing and deep-etching (QF-DE) method. In developmental studies of mouse and human muscle cultures, some myoblasts had positive-reactions to spectrin, DRP, and F-actin, but not dystrophin. In aneurally cultured myotubes, dystrophin, DRP, and spectrin were localized diffusely in the cytoplasm and later in discontinuous patterns on the plasma membrane, when myotubes became mature. Spectrin and DRP had more positive reactions in immature myotubes, compared with those of dystrophin. In some areas of myotubes, dystrophin/spectrin and spectrin/actin were localized reciprocally. In innervated cultured human muscle cells, dystrophin and DRP were localized in neuro-muscular junctions, which were co-localized with clusters of acetylcholine receptors. By using the QF-DE method, dystrophin was localized just underneath the plasma membrane, and closely linked to actin-like filaments (8-10 nm in diameter), most of which were decorated with myosin subfragment 1. In actin-poor regions, spectrin was detected as well-organized filamentous structures in highly interconnected networks with various diameters. DRP was distributed irregularly with granular appearance inside the cytoplasm and also under the plasma membrane in immature mouse myotubes. Our present studies show that dystrophin, spectrin, and DRP are localized differently at the developmental stages of myotubes. These results suggest that dystrophin, spectrin, and DRP are organized independently in developing myotubes and these cytoskeletal proteins might play different functions in the preservation of plasma membrane stability in developing myotubes.

Identification of an M(r) 60,000 polypeptide unique to the meiotic spindle of the mouse oocyte

The mouse oocyte expresses an M(r) 60,000 (p60) polypeptide that is associated with the first and second meiotic spindles. Immunoreactive p60 was not detectable in the meiotic spindles of male germ cells or in mitotic spindles. P60 was identified with a polyclonal antibody whose predominant activity is directed against ankyrin. However, immunoadsorption experiments demonstrated that p60 is not an ankyrin isoform and represents a secondary activity of the polyclonal antibody. Circumstantial evidence suggest that p60 may be a microtubule-associated protein. Since the most obvious difference between the female meiotic spindle and other spindles is the long half-life of the former, we hypothesize that p60 may function in the maintenance of the long-lived female meiotic apparatus.

Cell surface localisation and stability of uvomorulin during early mouse development

We have examined immunocytochemically the subcellular distribution of the cell adhesion molecule uvomorulin in cleavage stage mouse embryos using conventional and confocal microscopy, under a range of detergent extraction and fixation regimes. Only traces of uvomorulin were detectable on the surface of unfertilised oocytes, whereas between 6 and 11 h after activation detergent-resistant surface expression was evident. This shift correlates with previously demonstrated changes in the pattern of synthesis and accumulation of uvomorulin from precursor state in unfertilised oocytes to mature protein after fertilisation. Embryos at subsequent stages up to the 8-cell stage exhibited a uniform distribution of uvomorulin on free surfaces and its concentration in regions of contact between blastomeres. At the 8-cell stage, during compaction, there was increased intercellular adhesion with concomitant accumulation of uvomorulin at intercellular contacts, whilst free surface uvomorulin was reduced and became relatively more susceptible to detergent extraction. When compact 8-cell embryos were decompacted in calcium-free medium, uvomorulin at contacts decreased while free surface and cytoplasmic staining increased. Blastomeres disaggregated from 4- and 8-cell embryos showed traces or 'footprints' of anti-uvomorulin staining in regions previously in apposition. These footprints disappeared over 45-60 min, during which time uvomorulin distribution became uniform. Possible mechanisms underlying the rearrangements which take place both at fertilisation and during compaction and experimental decompaction are discussed.

The cytoskeleton and associated proteins during cleavage, compaction and blastocyst differentiation in the pig

The organization of the cytoskeleton during early pig embryogenesis was investigated by using fluorescence and electron microscopy. The early morphogenesis of the pig embryo differed from that of the mouse, the standard model of the early mammalian development. In the pig, both compaction and polarization were gradual, and definitive polarization of cell surface microville occurred first shortly before blastocyst formation; the compaction and polarization of the mouse embryo are completed as early as at the 8 cell stage. Furthermore, the pig morula undergoes cycles of compaction and decompaction throughout its development. Distinct changes in the distribution of actin and the actin-associated proteins alpha-fodrin, vinculin and E-cadherin coincided with these events. In the pig, all these molecules were evenly distributed at all aspects of the blastomeres during early cleavage and then gradually accumulated in regions of intercellular contacts toward the blastocyst stage; microfilaments in trophectoderm cells formed a cortical meshwork associated with apical microvilli and adherent junctions (zonula adherens). In the mouse, the corresponding changes occur earlier, at the 8 cell stage. Microtubules formed a network-like cortical layer beneath the microvilli at the free outer surfaces of pig blastomeres. Cytokeratin bundles were not observed until the early blastocyst, where they characteristically associated with newly formed desmosomes. In both species a close correlation between morphologically defined developmental stages and the organization of the cytoskeleton: actin and actin-associated proteins are involved in polarization and compaction, whereas the appearance of intermediate filament bundles coincides with the building of the first epithelium, the trophectoderm; it is in the timing of events that a contrast between species is observed.

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.

Cell polarity and development of the first epithelium

In the 4 1/2 to 5 days between fertilization and implantation, the mouse conceptus must gain the abilities to implant and produce an embryo. Each of these is the sole developmental responsibility of one of two cell types forming the blastocyst, trophectoderm and inner cell mass (ICM), respectively. Trophectoderm is a polarized transporting epithelium while the ICM is an aggregate of non-epithelial pluripotent stem cells. These two cell types originate from the division of polar blastomeres when their cleavage furrows parallel their apical surfaces. Blastomeres polarize in response to asymmetric cell--cell contact, and understanding the mechanism of this induction is regarded as the key to understanding the origin of trophectoderm and ICM. Here we propose a model based on transcellular ion current loops for the induction of cell polarity during the development of the first epithelium, trophectoderm.

Mosaicism in the mouse trophectoderm

The issue of mosaicism in the mouse trophectoderm is examined by reviewing two sets of evidence: one arguing for a mosaic, the other for a non-mosaic character. Evidence for mosaicism includes documented cellular contribution from the inner cell mass to the trophectoderm, and data that reveal the gradual pace of the allocation process that separates the inner cell mass and trophectoderm lineages. Evidence suggesting a non-mosaic character for the trophectoderm is based on the polarization process undergone by exterior cells in the eight-celled embryo, the heritability of the changes brought about by this process, and the formation of gap junctions between the resulting apolar, trophectoderm progenitor cells. Since inner-cell-mass cells are developmentally labile, spatially heterogeneous and translocate to the polar trophectoderm, it is concluded that the polar trophectoderm is a mosaic tissue.

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.