An investigation of microtubule organization and functions in living Drosophila embryos by injection of a fluorescently labeled antibody against tyrosinated alpha-tubulin

Array tomography of in vivo labeled synaptic receptors

Array tomography (AT) allows one to localize sub-cellular components within the structural context of cells in 3D through the imaging of serial sections. Using this technique, the z-resolution can be improved physically by cutting ultra-thin sections. Nevertheless, conventional immunofluorescence staining of those sections is time consuming and requires relatively large amounts of costly antibody solutions. Moreover, epitopes are only readily accessible at the section's surface, leaving the volume of the serial sections unlabeled. Localization of receptors at neuronal synapses in 3D in their native cellular ultrastructural context is important for understanding signaling processes. Here, we present in vivo labeling of receptors via fluorophore-coupled tags in combination with super-resolution AT. We present two workflows where we label receptors at the plasma membrane: first, in vivo labeling via microinjection with a setup consisting of readily available components and self-manufactured microscope table equipment and second, live receptor labeling by using a cell-permeable tag. To take advantage of a near-to-native preservation of tissues for subsequent scanning electron microscopy (SEM), we also apply high-pressure freezing and freeze substitution. The advantages and disadvantages of our workflows are discussed.

Fluorescent Single-Walled Carbon Nanotubes for Protein Detection

Nanosensors have a central role in recent approaches to molecular recognition in applications like imaging, drug delivery systems, and phototherapy. Fluorescent nanoparticles are particularly attractive for such tasks owing to their emission signal that can serve as optical reporter for location or environmental properties. Single-walled carbon nanotubes (SWCNTs) fluoresce in the near-infrared part of the spectrum, where biological samples are relatively transparent, and they do not photobleach or blink. These unique optical properties and their biocompatibility make SWCNTs attractive for a variety of biomedical applications. Here, we review recent advancements in protein recognition using SWCNTs functionalized with either natural recognition moieties or synthetic heteropolymers. We emphasize the benefits of the versatile applicability of the SWCNT sensors in different systems ranging from single-molecule level to in-vivo sensing in whole animal models. Finally, we discuss challenges, opportunities, and future perspectives.

Dynamics of cortical domains in early Drosophila development

Underlying the plasma membrane of eukaryotic cells is an actin cortex that includes actin filaments and associated proteins. A special feature of all polarized and epithelial cells are cortical domains, each of which is characterized by specific sets of proteins. Typically, an epithelial cell contains apical, subapical, lateral and basal domains. The domain-specific protein sets contain evolutionarily conserved proteins, as well as cell-type-specific factors. Among the conserved proteins are, the Par proteins, Crumbs complex and the lateral proteins Scribbled and Discs large 1. Organization of the plasma membrane into cortical domains is dynamic and depends on cell type, differentiation and developmental stage. The dynamics of cortical organization is strikingly visible in early Drosophila embryos, which increase the number of distinct cortical domains from one, during the pre-blastoderm stage, to two in syncytial blastoderm embryos, before finally acquiring the four domains that are typical for epithelial cells during cellularization. In this Review, we will describe the dynamics of cortical organization in early Drosophila embryos and discuss the processes and mechanisms underlying cortical remodeling.

Quantitative microscopy uncovers ploidy changes during mitosis in live Drosophila embryos and their effect on nuclear size

Time-lapse microscopy is a powerful tool to investigate cellular and developmental dynamics. In Drosophila melanogaster, it can be used to study division cycles in embryogenesis. To obtain quantitative information from 3D time-lapse data and track proliferating nuclei from the syncytial stage until gastrulation, we developed an image analysis pipeline consisting of nuclear segmentation, tracking, annotation and quantification. Image analysis of maternal-haploid (mh) embryos revealed that a fraction of haploid syncytial nuclei fused to give rise to nuclei of higher ploidy (2n, 3n, 4n). Moreover, nuclear densities in mh embryos at the mid-blastula transition varied over threefold. By tracking synchronized nuclei of different karyotypes side-by-side, we show that DNA content determines nuclear growth rate and size in early interphase, while the nuclear to cytoplasmic ratio constrains nuclear growth during late interphase. mh encodes the Drosophila ortholog of human Spartan, a protein involved in DNA damage tolerance. To explore the link between mh and chromosome instability, we fluorescently tagged Mh protein to study its subcellular localization. We show Mh-mKO2 localizes to nuclear speckles that increase in numbers as nuclei expand in interphase. In summary, quantitative microscopy can provide new insights into well-studied genes and biological processes.

Summary: A new 3D time-lapse microscopy image analysis pipeline consisting of nuclear segmentation, tracking, annotation and quantification revealed karyotype changes in Drosophila embryos.

Delivery of antibodies to the cytosol: debunking the myths

The use of antibodies to target their antigens in living cells is a powerful analytical tool for cell biology research. Not only can molecules be localized and visualized in living cells, but interference with cellular processes by antibodies may allow functional analysis down to the level of individual post-translational modifications and splice variants, which is not possible with genetic or RNA-based methods. To utilize the vast resource of available antibodies, an efficient system to deliver them into the cytosol from the outside is needed. Numerous strategies have been proposed, but the most robust and widely applicable procedure still remains to be identified, since a quantitative ranking of the efficiencies has not yet been done. To achieve this, we developed a novel efficiency evaluation method for antibody delivery based on a fusion protein consisting of a human IgG 1 Fc and the recombination enzyme Cre (Fc-Cre). Applied to suitable GFP reporter cells, it allows the important distinction between proteins trapped in endosomes and those delivered to the cytosol. Further, it ensures viability of positive cells and is unsusceptible to fixation artifacts and misinterpretation of cellular localization in microscopy and flow cytometry. Very low cytoplasmic delivery efficiencies were found for various profection reagents and membrane penetrating peptides, leaving electroporation as the only practically useful delivery method for antibodies. This was further verified by the successful application of this method to bind antibodies to cytosolic components in living cells.

Three-dimensional segmentation of nuclei and mitotic chromosomes for the study of cell divisions in live Drosophila embryos

Drosophila embryogenesis is an established model to investigate mechanisms and genes related to cell divisions in an intact multicellular organism. Progression through the cell cycle phases can be monitored in vivo using fluorescently labeled fusion proteins and time-lapse microscopy. To measure cellular properties in microscopic images, accurate and fast image segmentation methods are a critical prerequisite. To quantify static and dynamic features of interphase nuclei and mitotic chromosomes, we developed a three-dimensional (3D) segmentation method based on multiple level sets. We tested our method on 3D time-series images of live embryos expressing histone-2Av-green fluorescence protein. Our method is robust to low signal-to-noise ratios inherent to high-speed imaging, fluorescent signals in the cytoplasm, and dynamic changes of shape and texture. Comparisons with manual ground-truth segmentations showed that our method achieves more than 90% accuracy on the object as well as voxel levels and performs consistently throughout all cell cycle phases and developmental stages from syncytial blastoderm to postblastoderm mitotic domains.

Targeting antibodies to the cytoplasm

A growing number of research consortia are now focused on generating antibodies and recombinant antibody fragments that target the human proteome. A particularly valuable application for these binding molecules would be their use inside a living cell, e.g., for imaging or functional intervention. Animal-derived antibodies must be brought into the cell through the membrane, whereas the availability of the antibody genes from phage display systems allows intracellular expression. Here, the various technologies to target intracellular proteins with antibodies are reviewed.

Quantitative microinjection of Drosophila embryos: general strategy

INTRODUCTIONMicroinjection of Drosophila embryos is a common technique used by a wide range of investigators, but some applications require a refined strategy for handling embryos. This article outlines the general procedures for microinjection and quantification of aqueous solutions during high-resolution observation of early development in the fly embryo. It also describes the design of suitable support slides for the manipulation of Drosophila embryos under upright and inverted microscopes.

Characterization of sub-nuclear changes in Caenorhabditis elegans embryos exposed to brief, intermediate and long-term anoxia to analyze anoxia-induced cell cycle arrest

Background

The soil nematode C. elegans survives oxygen-deprived conditions (anoxia; <.001 kPa O₂) by entering into a state of suspended animation in which cell cycle progression reversibly arrests. The majority of blastomeres of embryos exposed to anoxia arrest at interphase, prophase and metaphase. The spindle checkpoint proteins SAN-1 and MDF-2 are required for embryos to survive 24 hours of anoxia. To further investigate the mechanism of cell-cycle arrest we examined and compared sub-nuclear changes such as chromatin localization pattern, post-translational modification of histone H3, spindle microtubules, and localization of the spindle checkpoint protein SAN-1 with respect to various anoxia exposure time points. To ensure analysis of embryos exposed to anoxia and not post-anoxic recovery we fixed all embryos in an anoxia glove box chamber.

Results

Embryos exposed to brief periods to anoxia (30 minutes) contain prophase blastomeres with chromosomes in close proximity to the nuclear membrane, condensation of interphase chromatin and metaphase blastomeres with reduced spindle microtubules density. Embryos exposed to longer periods of anoxia (1–3 days) display several characteristics including interphase chromatin that is further condensed and in close proximity to the nuclear membrane, reduction in spindle structure perimeter and reduced localization of SAN-1 at the kinetochore. Additionally, we show that the spindle checkpoint protein SAN-1 is required for brief periods of anoxia-induced cell cycle arrest, thus demonstrating that this gene product is vital for early anoxia responses. In this report we suggest that the events that occur as an immediate response to brief periods of anoxia directs cell cycle arrest.

Conclusion

From our results we conclude that the sub-nuclear characteristics of embryos exposed to anoxia depends upon exposure time as assayed using brief (30 minutes), intermediate (6 or 12 hours) or long-term (24 or 72 hours) exposures. Analyzing these changes will lead to an understanding of the mechanisms required for initiation and maintenance of cell cycle arrest in respect to anoxia exposure time as well as order the events that occur to bring about anoxia-induced cell cycle arrest.

Reassessing the role and dynamics of nonmuscle myosin II during furrow formation in early Drosophila embryos

The early Drosophila embryo undergoes two distinct membrane invagination events believed to be mechanistically related to cytokinesis: metaphase furrow formation and cellularization. Both involve actin cytoskeleton rearrangements, and both have myosin II at or near the forming furrow. Actin and myosin are thought to provide the force driving membrane invagination; however, membrane addition is also important. We have examined the role of myosin during these events in living embryos, with a fully functional myosin regulatory light-chain-GFP chimera. We find that furrow invagination during metaphase and cellularization occurs even when myosin activity has been experimentally perturbed. In contrast, the basal closure of the cellularization furrows and the first cytokinesis after cellularization are highly dependent on myosin. Strikingly, when ingression of the cellularization furrow is experimentally inhibited by colchicine treatment, basal closure still occurs at the appropriate time, suggesting that it is regulated independently of earlier cellularization events. We have also identified a previously unrecognized reservoir of particulate myosin that is recruited basally into the invaginating furrow in a microfilament-independent and microtubule-dependent manner. We suggest that cellularization can be divided into two distinct processes: furrow ingression, driven by microtubule mediated vesicle delivery, and basal closure, which is mediated by actin/myosin based constriction.

Evidence for a role of the (alpha)-tubulin C terminus in the regulation of cyclin B synthesis in developing oocytes

Microinjected mAb YL1/2, an (alpha)-tubulin antibody specific for the tyrosinated form of the protein, blocks the cell cycle in developing oocytes. Here, we have investigated the mechanism involved in the mAb effect. Both developing starfish and Xenopus oocytes were injected with two different (alpha)-tubulin C terminus antibodies. The injected antibodies blocked cell entry into mitosis through specific inhibition of cyclin B synthesis. The antibody effect was independent of the presence or absence of polymerized microtubules and was mimicked by injected synthetic peptides corresponding to the tyrosinated (alpha)-tubulin C terminus, whereas peptides lacking the terminal tyrosine were ineffective. These results indicate that tyrosinated (alpha)-tubulin, or another protein sharing the same C-terminal epitope, is involved in specific regulation of cyclin B synthesis in developing oocytes.

Interdependence of filamentous actin and microtubules for asymmetric cell division

Asymmetric cell divisions are crucial to the generation of cell fate diversity. They contribute to unequal distribution of cellular factors to the daughter cells. Asymmetric divisions are characterized by a 90 degrees rotation of the mitotic spindle. There is increasing evidence that a tight cooperation between cortical, filamentous actin and astral microtubules is indispensable for successful spindle rotation. Over the past years, the dynactin complex has emerged as a key candidate to mediate actin/microtubule interaction at the cortex. This review discusses our current understanding of how spindle rotation is accomplished by the interplay of filamentous actin and microtubules in a variety of experimental systems.

Ubiquitous expression of a Drosophila adenomatous polyposis coli homolog and its localization in cortical actin caps

We report the expression pattern of a new adenomatous polyposis coli (APC) homolog called E-APC during Drosophila development. E-APC protein is expressed in all embryonic and larval cells we have examined. In the early blastoderm embryo, we see a striking concentration of E-APC in the cortical actin caps. Microtubules are closely associated with these caps. Since human APC has been reported to bind to microtubules, we investigated whether the cortical E-APC co-localizes with tubulin. However, this was not the case, implying that the putative tubulin-binding property of human APC is not well conserved.

Reaction-diffusion microtubule concentration patterns occur during biological morphogenesis

Reaction-diffusion processes can lead to a macroscopic concentration pattern from an initially homogeneous solution, and thus provide a physical-chemical mechanism for biological pattern formation and morphogenesis. The central prediction of reaction-diffusion theory is that the patterns contain periodic concentration variations in some of the reactives. Microtubules assembled in vitro spontaneously self-organise and form stationary striped macroscopic structures. In agreement with reaction-diffusion theory. Here we show, in agreement with reaction-diffusion theory, that these preparations contain substantial microtubule concentration variations. Similar striped microtubule patterns arise during Drosophila embryogenesis. A characteristic of these patterns is their dependence on sample dimensions. In Drosophila eggs shortened by ligation, we found that the microtubule pattern varied with egg fragment length in the same way as the in vitro microtubule pattern varied with sample length, and as expected from theory. This is evidence that reaction-diffusion structures occur during Drosophila morphogenesis.

Centrosomes and microtubule organisation during Drosophila development

Are the microtubule-organising centers of the different cell types of a metazoan interchangeable? If not, what are the differences between them? Do they play any role in the differentiation processes to which these cells are subjected? Nearly one hundred years of centrosome research has established the essential role of this organelle as the main microtubule-organising center of animal cells. But only now are we starting to unveil the answers to the challenging questions which are raised when the centrosome is studied within the context of a pluricellular organism. In this review we present some of the many examples which illustrate how centrosomes and microtubule organisation changes through development in Drosophila and discuss some of its implications.

Centriole and centrosome dynamics during the embryonic cell cycles that follow the formation of the cellular blastoderm in Drosophila

We have used immunofluorescence and electron microscopy to examine centrosome dynamics during the first postblastodermic mitoses in the Drosophila embryo. The centrosomal material, as recognized by antibodies against CP190 and gamma-tubulin, does not show the typical shape changes observed in syncytial embryos, but remains compact throughout mitosis. Centrioles, however, behave as during the syncytial mitoses, with each daughter cell inheriting two separated centrioles at the end of telophase. During interphase in epithelial cells that have a distinct G1 phase, two isolated centrioles are found, suggesting that the separation of sister centrioles is tightly coupled to a mitotic oscillator in both the "abbreviated" and the "complete" embryonic division cycles. The centrioles of the Drosophila embryo sharply differed from the sperm basal body, having a cartwheel structure with nine microtubular doublets and a central tubule. This "immature" centriolar morphology was shown to persist throughout embryonic development, clearly demonstrating that these centrioles are able to replicate despite their apparently neotenic structure.

Microinjection of anti-alpha-tubulin antibody (DM1A) inhibits progesterone-induced meiotic maturation and deranges the microtubule array in follicle-enclosed oocytes of the frog, Rana pipiens

Microinjection of anti-alpha-tubulin (Dm1A) inhibited progesterone-induced meiotic maturation in large follicle-enclosed oocytes of the frog, Rana pipiens. DM1A (46 nl; 10 mg/ml) injection significantly increased the ED50 value for progesterone as determined by germinal vesicle dissolution (GVD) bioassay. By contrast, low doses of microinjected DM1A (46 nl; 2.5 mg/ml), anti-actin (clone KJ43A), anti-cytokeratin (C-11), anti-intermediate filament antibody (IFA), generic IgG (46 nl; 20 mg/ml) or sodium azide (46 nl; 1 mg/ml), an antibody preservative, were without inhibitory effect in this bioassay. Microinjected, affinity-purified DM1A (46 nl; 7.5 mg/ml) was also inhibitory, but preabsorption with pure tubulin prior to injection significantly reduced the inhibitory effect. DM1A injection had no effect on centrifugation-induced germinal vesicle migration (GVM). Previous work indicated that drugs (e.g. demecolcine and nocodazole), which destabilise microtubules, enhance both centrifugation-induced GVM and progesterone-induced GVD in Rana oocytes. Taking these results together, it is suggested that DM1A injection may have differential effects on microtubules in this cell. Thus, while the majority of microtubules were apparently depolymerised by DM1A (46 nl; 10 mg/ml) injection, a small subpopulation appeared to be stabilised as bundles. Confocal immunofluorescence microscopy of follicle-enclosed oocytes after DM1A injection revealed a major loss of microtubules throughout the cell; however, apparent sparse bundles of microtubules arranged in an approximately 600 microns shell were associated with the injectate region 24 h post-injection. By contrast, control follicle-enclosed oocytes topically labelled with DM1A post-fixation had extensive microtubule arrays similar to those previously reported in Xenopus oocytes. Intracellular recording after DM1A injection and progesterone treatment yielded an intermediate membrane potential (Vm = -31.8 mV) compared with control (immature) DM1A-injected cells (Vm = -44.7 mV) or potassium balanced salt solution (KBS)-injected cells matured with progesterone (Vm = -13.9 mV). These results suggest that DM1A injection does not completely inhibit electrophysiological changes initiated by progesterone. Working hypotheses are proposed that suggest a role for microtubules in the action of progesterone which normally lifts the prophase I block in the Rana follicle-enclosed oocyte.

Live analysis of free centrosomes in normal and aphidicolin-treated Drosophila embryos

In a number of embryonic systems, centrosomes that have lost their association with the nuclear envelope and spindle maintain their ability to duplicate and induce astral microtubules. To identify additional activities of free centrosomes, we monitored astral microtubule dynamics by injecting living syncytial Drosophila embryos with fluorescently labeled tubulin. Our recordings follow multiple rounds of free centrosome duplication and separation during the cortical division. The rate and distance of free sister centrosome separation corresponds well with the initial phase of associated centrosome separation. However, the later phase of separation observed for centrosomes associated with a spindle (anaphase B) does not occur. Free centrosome separation regularly occurs on a plane parallel to the plasma membrane. While previous work demonstrated that centrosomes influence cytoskeletal dynamics, this observation suggests that the cortical cytoskeleton regulates the orientation of centrosome separation. Although free centrosomes do not form spindles, they display relatively normal cell cycle-dependent modulations of their astral microtubules. In addition, free centrosome duplication, separation, and modulation of microtubule dynamics often occur in synchrony with neighboring associated centrosomes. These observations suggest that free centrosomes respond normally to local nuclear division signals. Disruption of the cortical nuclear divisions with aphidicolin supports this conclusion; large numbers of abnormal nuclei recede into the interior while their centrosomes remain on the cortex. Following individual free centrosomes through multiple focal planes for 45 min after the injection of aphidicolin reveals that they do not undergo normal modulation of their astral dynamics nor do they undergo multiple rounds of duplication and separation. We conclude that in the absence of normally dividing cortical nuclei many centrosome activities are disrupted and centrosome duplication is extensively delayed. This indicates the presence of a feedback mechanism that creates a dependency relationship between the cortical nuclear cycles and the centrosome cycles.

The 95F unconventional myosin is required for proper organization of the Drosophila syncytial blastoderm

The 95F myosin, a class VI unconventional myosin, associates with particles in the cytoplasm of the Drosophila syncytial blastoderm and is required for the ATP- and F-actin-dependent translocation of these particles. The particles undergo a cell cycle-dependent redistribution from domains that surround each nucleus in interphase to transient membrane invaginations that provide a barrier between adjacent spindles during mitosis. When 95F myosin function is inhibited by antibody injection, profound defects in syncytial blastoderm organization occur. This disorganization is seen as aberrant nuclear morphology and position and is suggestive of failures in cytoskeletal function. Nuclear defects correlate with gross defects in the actin cytoskeleton, including indistinct actin caps and furrows, missing actin structures, abnormal spacing of caps, and abnormally spaced furrows. Three-dimensional examination of embryos injected with anti-95F myosin antibody reveals that actin furrows do not invaginate as deeply into the embryo as do normal furrows. These furrows do not separate adjacent mitoses, since microtubules cross over them. These inappropriate microtubule interactions lead to aberrant nuclear divisions and to the nuclear defects observed. We propose that 95F myosin function is required to generate normal actin-based transient membrane furrows. The motor activity of 95F myosin itself and/or components within the particles transported to the furrows by 95F myosin may be required for normal furrows to form.

Drosophila cytoplasmic dynein, a microtubule motor that is asymmetrically localized in the oocyte

The unidirectional movements of the microtubule-associated motors, dyneins, and kinesins, provide an important mechanism for the positioning of cellular organelles and molecules. An intriguing possibility is that this mechanism may underlie the directed transport and asymmetric positioning of morphogens that influence the development of multicellular embryos. In this report, we characterize the Drosophila gene, Dhc64C, that encodes a cytoplasmic dynein heavy chain polypeptide. The primary structure of the Drosophila cytoplasmic dynein heavy chain polypeptide has been determined by the isolation and sequence analysis of overlapping cDNA clones. Drosophila cytoplasmic dynein is highly similar in sequence and structure to cytoplasmic dynein isoforms reported for other organisms. The Dhc64C dynein transcript is differentially expressed during development with the highest levels being detected in the ovaries of adult females. Within the developing egg chambers of the ovary, the dynein gene is predominantly transcribed in the nurse cell complex. In contrast, the encoded dynein motor protein displays a striking accumulation in the single cell that will develop as the oocyte. The temporal and spatial pattern of dynein accumulation in the oocyte is remarkably similar to that of several maternal effect gene products that are essential for oocyte differentiation and axis specification. This distribution and its disruption by specific maternal effect mutations lends support to recent models suggesting that microtubule motors participate in the transport of these morphogens from the nurse cell cytoplasm to the oocyte.

The Drosophila maternal-effect mutation grapes causes a metaphase arrest at nuclear cycle 13

grapes (grp) is a second chromosome (36A-B) maternal-effect lethal mutation in Drosophila melanogaster. We demonstrate that the syncytial nuclear divisions of grp-derived embryos are normal through metaphase of nuclear cycle 12. However, as the embryos progress into telophase of cycle 12, the microtubule structures rapidly deteriorate and midbodies never form. Immediately following the failure of midbody formation, sister telophase products collide and form large tetraploid nuclei. These observations suggest that the function of the midbody in the syncytial embryo is to maintain separation of sister nuclei during telophase of the cortical divisions. After an abbreviated nuclear cycle 13 interphase, these polyploid nuclei progress through prophase and arrest in metaphase. The spindles associated with the arrested nuclei are stable for hours even though the microtubules are rapidly turning over. The nuclear cycle 13 anaphase separation of sister chromatids never occurs and the chromosomes, still encompassed by spindles, assume a telophase conformation. Eventually neighboring arrested spindles begin to associate and form large clusters of spindles and nuclei. To determine whether this arrest was the result of a disruption in normal developmental events that occur at this time, both grp-derived and wild-type embryos were exposed to X-irradiation. Syncytial wild-type embryos exhibit a high division error rate, but not a nuclear-cycle arrest after exposure to low doses of X-irradiation. In contrast, grp-derived embryos exhibit a metaphase arrest in response to equivalent doses of X-irradiation. This arrest can be induced even in the early syncytial divisions prior to nuclear migration. These results suggest that the nuclear cycle 13 metaphase arrest of unexposed grp-derived embryos is independent of the division-cycle transitions that also occur at this stage. Instead, it may be the result of a previously unidentified feedback mechanism.

A cytoplasmic dynein motor in Drosophila: identification and localization during embryogenesis

We have characterized a cytoplasmic dynein motor isoform that is present in extracts of Drosophila embryos. A prominent high molecular weight (HMW) polypeptide (&gt; 400 kDa) is enriched in microtubules prepared from nucleotide-depleted embryonic extracts. Based on its ATP-sensitive microtubule binding activity, 20 S sedimentation coefficient, sensitivity to UV-vanadate and nucleotide specificity, the HMW polypeptide resembles cytoplasmic dyneins prepared from other organisms. The Drosophila cytoplasmic dynein acts as a minus-end motor that promotes microtubule translocation in vitro. A polyclonal antibody raised against the dynein heavy chain polypeptide was used to localize the dynein antigen in whole-mount preparations of embryos by immunofluorescence microscopy. These studies show that the dynein motor is associated with microtubules throughout embryogenesis, including mitotic spindle microtubules and microtubules of the embryonic nervous system.

Isolation of cytoskeletal proteins from Drosophila

In this summary of methods, we have attempted to present a series of protocols relevant to the study of proteins that associate with each other inside cells. These techniques have found their main application, in our hands, to cytoskeletal structures, but the utility of these techniques is limited to this application. As we learn more about the physiology and organization of eukaryotic cells, information from a variety of studies and systems indicates that many processes occur in a highly spatially organized manner in cells, using multiprotein complexes. The techniques described here will find application to the study of how organization and associations between proteins contribute to many of cellular processes.

Different reactivity with monoclonal anti-tubulin antibodies between native and fixed mitotic microtubules in sea urchin eggs

The effect on fixation on the reactivities of mitotic microtubules with monoclonal anti-tubulin antibodies was investigated by the indirect immunofluorescence procedure. All of the seven antibodies used intensely stained mitotic microtubules in sea urchin eggs lysed and fixed with methanol at -20 degrees C, whereas only two of them stained the stabilized microtubules in the lysed eggs before the fixation. The other five did not stain the mitotic microtubules even after microtubule components other than tubulin were removed by treating the lysed eggs with 0.4 M KCl solution containing taxol. These results exclude the possibility that the fixation affects proteins, which interact with microtubules including microtubule-associated proteins (MAPs) and interfere with the binding of monoclonal antibodies with tubulin, and strongly suggest that the fixation directly affects the three-dimensional conformation of tubulin. Furthermore, microinjection of these antibodies indicated the results as follows [combining the results reported previously; Oka et al., 1990: Cell Struct. Funct. 15: 373-378]: The antibodies which stained mitotic microtubules stabilized in the lysed eggs induced disassembly of native mitotic microtubules in the living eggs, but those which did not stain the stabilized microtubules did not disassemble the native microtubules. From these results, it is suggested that the monoclonal antibodies which stain microtubules in the eggs lysed but not fixed are useful for microinjection experiments.

Characterization and biological activities of anti-Brugia pahangi tubulin monoclonal antibodies

Three monoclonal antibodies (mAb) specific to beta-tubulin were used to investigate the heterogeneity of tubulins from nematodes and mammals. Western blot analysis of one-dimensional SDS-PAGE showed that anti-Brugia pahangi tubulin mAb 1B6 and P3D react with epitope(s) specific to nematode beta-tubulin and recognize tubulin from adults and microfilariae of B. pahangi, adult B. malayi and Dirofilaria immitis, eggs of Haemonchus contortus and adult Ascaris suum. However, the same mAb did not recognize tubulin from trophozoites of Giardia lamblia, pig brain or 3T3 mouse fibroblast cells. In two-dimensional SDS-PAGE, mAb 1B6 recognized one isoform of beta-tubulin and mAb P3D recognized two beta-tubulin isoforms. Limited proteolysis showed that mAb 1B6 reacted with the amino-terminal fragments of beta-tubulin. In contrast, mAb P3D recognized the carboxy-terminal fragments of beta-tubulin. In ELISA, mAb P3D reacted with an 18 amino acid peptide corresponding to residues 430-448 of B. pahangi beta-tubulin. These observations confirm that the epitope of mAb P3D is located on the extreme carboxy-terminal region. Immunogold labelling of adult B. pahangi sections with mAb P3D revealed the presence of beta-tubulin isoforms in the cuticle, hypodermal layer and somatic muscle blocks of B. pahangi. Under in vitro conditions, mAb P3D caused 80% reduction in worm viability, during exposure over 48 h.

Mutations in the Drosophila melanogaster gene three rows permit aspects of mitosis to continue in the absence of chromatid segregation

We have cloned the three rows (thr) gene, by a combination of chromosome microdissection and P element tagging. We describe phenotypes of embryos homozygous for mutations at the thr locus. Maternal mRNA and protein appear to be sufficient to allow 14 rounds of mitosis in embryos homozygous for thr mutations. However, a small percentage of cells in syncytial blastoderm stage thr embryos sink into the interior of the embryo as if they have failed to divide properly. Following cellularisation all cells complete mitosis 14 normally. All cells become delayed at mitosis 15 with their chromosomes remaining aligned on the spindle in a metaphase-like configuration, even though both cyclins A and B have both been degraded. As cyclin B degradation occurs at the metaphase-anaphase transition, subsequent to the microtubule integrity checkpoint, the delay induced by mutations at the thr locus defines a later point in mitotic progression. Chromosomes in the cells of thr embryos do not undertake anaphase separation, but remain at the metaphase plate. Subsequently they decondense. A subset of nuclei go on to replicate their DNA but there is no further mitotic division.

In vivo observation of the puff-specific protein no-on transient A (NONA) in nuclei of Drosophila embryos

The spatial distribution of no-on transient A (NONA), a protein associated with specific puffs on polytene chromosomes, was followed in nuclei of living Drosophila embryos by microinjection of fluorescently labeled monoclonal antibody to NONA. The injected antibodies remained active until the larval stage, revealing the distribution of the NONA protein throughout embryogenesis. Most injected animals completed embryonic development and hatched as normal larvae. NONA was restricted to the cytoplasm until the end of cycle 11. We document an active uptake of the NONA-antibody complex into early interphase nuclei from nuclear cycle 14 onwards, following each mitosis. Significant differences in the distribution of the protein between fixed and living embryos were apparent, particularly at high resolution. The NONA protein was localized in the nuclei of living embryos at discrete sites, most of which lay at the periphery and some of which were tightly clustered. The constellation of sites changed with time; in some nuclei these changes were fast whereas in other nuclei the pattern was quite stable. These data suggest that specific protein complexes associated with active interphase chromatin, and possibly chromatin in general, are mobile in the living organism.

Mutations affecting the cytoskeletal organization of syncytial Drosophila embryos

Cytoplasmic organization, nuclear migration, and nuclear division in the early syncytial Drosophila embryo are all modulated by the cytoskeleton. In an attempt to identify genes involved in cytoskeletal functions, we have examined a collection of maternal-effect lethal mutations induced by single P-element transposition for those that cause defects in nuclear movement, organization, or morphology during the syncytial embryonic divisions. We describe three mutations, grapes, scrambled, and nuclear-fallout, which define three previously uncharacterized genes. Females homozygous for these mutations produce embryos that exhibit extensive mitotic division errors only after the nuclei migrate to the surface. Analysis of the microfilament and microtubule organization in embryos derived from these newly identified mutations reveal disruptions in the cortical cytoskeleton. Each of the three mutations disrupts the actin-based pseudocleavage furrows and the cellularization furrows in a distinct fashion. In addition to identifying new genes involved in cytoskeletal organization, these mutations provide insights into cytoskeletal function during early Drosophila embryogenesis.

Consequences of topoisomerase II inhibition in early embryogenesis of Drosophila revealed by in vivo confocal laser scanning microscopy

The regulation of DNA topology by topoisomerase II from Drosophila melanogaster has been studied extensively by biochemical methods but little is known about its roles in vivo. We have performed experiments on the inhibition of topoisomerase II in living Drosophila blastoderm embryos. We show that the enzymatic activity can be specifically disrupted by microinjection of antitopoisomerase II antibodies as well as the epipodophyllotoxin VM26, a known inhibitor of topoisomerase II in vitro. By labeling the chromatin of live embryos with tetramethylrhodamine-coupled histones, the effects of inhibition on nuclear morphology and behaviour was followed in vivo using confocal laser scanning microscopy. Both the antibodies and the drug prevented or hindered the segregation of chromatin daughter sets at the anaphase stage of mitosis. In addition, high concentrations of inhibitor interfered with the condensation of chromatin and its proper arrangement into the metaphase plate. The observed effects yielded non-functional nuclei, which were drawn into the inner yolk mass of the embryo. Concurrently, undamaged nuclei surrounding the affected region underwent compensatory division, leading to the restoration of the nuclear population, and thereby demonstrating the regulative capacity of Drosophila blastoderm embryos.

F-actin domains in the syncytial blastoderm of the dipteran Ceratitis capitata

Laser scanning confocal microscopy on rhodamine-phalloidin-treated syncytial embryos of the dipteran Ceratitis capitata allowed us to recognize four different kinds of actin filament distribution in close spatial proximity. One domain is represented by microfilaments localized in the plasma membrane within the microprojections and membrane infoldings. At a slightly lower focal level, rhodamine-phalloidin labelling is concentrated in small irregular aggregates, which are localized around the dividing nuclei. Our results indicate that the organization of the actin aggregates follows that of the microtubules of the mitotic apparatus and suggest that the dynamic reorganization of these structures during mitosis may be microtubule-dependent. A three-dimensional network of thin actin filaments fills the whole periplasm and links the spindles together. A fourth actin domain is localized at the poles of the spindles in correspondence with the centrosomal region. The complex network of cortical filament bundles described in the present study may represent the ultrastructural basis of the tension leading to segregation of daughter nuclei at late telophase and to their lateral migration along the embryo surface.

Maternal effect mutations of the sponge locus affect actin cytoskeletal rearrangements in Drosophila melanogaster embryos

In the syncytial blastoderm stage of Drosophila embryogenesis, dome-shaped actin "caps" are observed above the interphase nuclei. During mitosis, this actin rearranges to participate in the formation of pseudocleavage furrows, transient membranous invaginations between dividing nuclei. Embryos laid by homozygous sponge mothers lack these characteristic actin structures, but retain other actin associated structures and processes. Our results indicate that the sponge product is specifically required for the formation of actin caps and metaphase furrows. The specificity of the sponge phenotype permits dissection of both the process of actin cap formation and the functions of actin caps and metaphase furrows. Our data demonstrate that the distribution of actin binding protein 13D2 is unaffected in sponge embryos and suggest that 13D2 is upstream of actin in cortical cap assembly. Although actin caps and metaphase furrows have been implicated in maintaining the fidelity of nuclear division and the positions of nuclei within the cortex, our observations indicate that these structures are dispensible during the early syncytial blastoderm cell cycles. A later requirement for actin metaphase furrows in preventing the nucleation of mitotic spindles between inappropriate centrosomes is observed. Furthermore, the formation of actin caps and metaphase furrows is not a prerequisite for the formation of the hexagonal array of actin instrumental in the conversion of the syncytial embryo into a cellular blastoderm.

Surface cap modifications in cold-treated Drosophila melanogaster embryos

When early Drosophila embryos were allowed to develop at 0 degree C, several abnormalities in the surface cap organization were observed. Scanning electron microscopy showed that exposure to cold mainly lead to the deformation of the cortical caps and to their partial fusion with adjacent caps. The process of cellularization was presumably affected and large uncellularized areas were observed. Rhodamine-phalloidin staining showed that cap deformation was closely related to the altered microfilament distribution, which was presumably responsible for the failure of large syncytial areas to cellularize. During the process of cellularization, F-actin localization did not depend on the microtubules forming the baskets around the elongating nuclei, but was related to the subpopulation of microtubules radiating from the centrosomes toward the plasma membrane. Only these microtubules seemed to be affected by cold treatment.

Involvement of microtubules and microfilaments in centrosome dynamics during the syncytial mitoses of the early Drosophila embryo

To examine the role of microfilaments and microtubules in centrosome dynamics we exposed Drosophila embryos to culture medium containing cytochalasin B and to low temperature. The results show that the splitting of the centrosomal material does not occur when the embryos are treated with cytochalasin before centrosome duplication at late telophase. The fragmentation of the centrosomal material, caused by cold exposure, is also prevented by cytochalasin incubation. These results indicate that both microtubules and microfilaments may be involved in determining centrosome shape during the syncytial mitoses which lead to the formation of the blastoderm in early Drosophila embryos.

From egg to pole cells: ultrastructural aspects of early cleavage and germ cell determination in insects

Insect eggs are giant and very complex cells covered by an extremely resistant shell. Both the egg cell and surrounding eggshell express anteroposterior and ventrodorsal polarity. The molecular and cytoplasmic organization of both axes originates during oogenesis and leads to the production of an ooplasmic system which consists of euplasm and deutoplasm (yolk) and contains a nucleus as well as extranuclear determinants of maternal origin. Both are part of the store of information for early embryogenesis. In addition, the deutoplasm serves as raw material and early nutrient supply for building the embryo. The insect egg cell, which is arrested in the first maturation division when released from the ovary during oviposition, will be activated by different stimuli among different species to complete meiosis and start embryogenesis. The zygote nucleus undergoes a number of synchronous mitotic divisions leading to cleavage energids which initially form a syncytial blastoderm and subsequently the cellular blastoderm. In many insects, prior to blastoderm formation, polar granules (or oosome material) are incorporated in a single cell or a small number of cells which bud off at the posterior pole. These so called pole cells give rise to the primordial germ cells. Therefore, polar granules or the oosome material mark the germ line, and while structural counterparts of determinants of body pattern formation have so far not been found, the polar granules or oosome serve as an autonomous ooplasmic determinant for the pole or germ cells. Anteroposterior body polarity can arise independent of the germ plasm.

Role of the cytoskeleton during early development

Oocytes, eggs, and embryos from a diverse array of species have evolved cytoskeletal specializations which allow them to meet the needs of early embryogenesis. While each species studied possesses one or more specializations which are unique, several cytoskeletal features are widely conserved across different animal phyla. These features include highly-developed cortical cytoskeletal domains associated with developmental information, microtubule-mediated pronuclear transport, and rapid intracellular signal-regulated control of cytoskeletal organization.

Microfilament distribution in cold-treated Drosophila embryos

Cold treatment of Drosophila embryos is observed to result in general alteration of microfilament distribution leading to deformation of the surface caps and to perturbation of the process of cleavage furrow extension. After exposure to low temperature the cortical actin caps underwent several morphological changes, despite the arrested nuclear cycle. These observations are discussed in relation to centrosome behavior during the cell cycle.

The effects of microinjection of rhodamine-phalloidin on mitosis and cytokinesis in early stage Drosophila embryos

Rhodamine-phalloidin was microinjected into early stage Drosophila embryos, which were then allowed to develop for various times, fixed, and examined by fluorescence microscopy. A gradient of effects was seen. Close to the site of injection an area of diffuse bright fluorescence was found which included lumps and long strands of fluorescent material. Around this region particular cytoplasmic domains showed a denser F-actin distribution. These domains included the nuclear islands of the preblastoderm, the cortical caps of the syncytial blastoderm, and the contractile ring network which forms during cellularization of the blastoderm. It is proposed that these domains are regions of preferential actin polymerization under the appropriate cellular conditions and that the injected phalloidin causes incorporation of additional polymer into existing structures. Further away the pattern of phalloidin staining corresponded to that found with fixed material. In contrast to the domains of apparent additional F-actin polymerization a reduction of actin incorporated into small aggregates was found, both in syncytial blastoderm stages and during cellularization. This occurred in regions where additional actin had been incorporated into adjacent actin-rich structures. A storage role for the aggregates, which are depleted when F-actin is polymerized, is proposed. Both mitosis and cytokinesis were found to be slowed but the inhibition was only transient. However, most embryos died without differentiating. Rarely, differentiated tissues formed and the musculature was strongly stained by rh-phalloidin. When embryos were injected immediately prior to the start of cellularization cytokinesis was inhibited only locally and continued normally elsewhere. This finding argues against the hypothesis that contraction of an actomyosin network over the whole surface is the only force involved in the cellularization of the blastoderm and that local factors, e.g., plasmalemma extension, must be involved.

Abnormal behavior of the yolk centrosomes during early embryogenesis of Drosophila melanogaster

After the 10th nuclear cycle the yolk centrosomes follow an irregular pathway. Unlike the somatic centrosomes, which move to the opposite poles of the nuclei to form the bipolar spindles, the yolk centrosomes remain as pairs at one pole of the yolk nuclei or shift feebly and nucleate irregular spindles, most of which have only one main pole. The yolk centrosomes are no longer observed near the yolk nuclei, but progressively move away into the surrounding cytoplasm. Despite the irregular behavior of the centrosomes and although the yolk nuclei cease to divide, the yolk centrosome duplication cycle continues. The early development of Drosophila thus provides an excellent natural system for the study of the uncoupling of the nuclear and centrosomal cycles.

Spatial distribution of posttranslationally modified tubulins in polarized cells of developing Artemia

In many differentiated cells, posttranslationally modified tubulins exhibit restricted subcellular distribution, leading to the proposal that they are required for the production and maintenance of polarity. To study this possibility, we used immunological approaches to examine tubulin isoforms in developing Artemia larvae and to determine their location in several types of cells within the organism. The amount of tubulin in relation to total protein remained relatively constant during early larval development while detyrosinated tubulin increased, an event correlated with the differentiation of larval gut muscle cells. Except for epidermal cells of the developing thorax, each type of cell within the Artemia larvae exhibited characteristic staining patterns which were very similar for each antitubulin antibody. Within epidermal cells, microtubules containing acetylated tubulin appeared patchy or punctate in their distribution, an image not seen with the other antibodies. In most polarized cells, staining for tubulin and actin colocalized in discrete areas, demonstrating enrichment of both proteins within the same cellular compartment and suggesting functional interactions. Mitotic figures were stained with qualitatively equal intensity by all of the antitubulin antibodies, but asters were not observed. Midbodies were intensely stained with phalloidin as well as the antibodies to tubulin. It was clear that microtubules exhibited a preferential localization in cells of Artemia but in no case was a tubulin isoform found exclusively in one area of a cell. The results support the contention that microtubules influence the organization of polarized cell structure and function but they do not permit the conclusion that this capability is dependent on the localization of posttranslationally modified tubulins to restricted subcellular positions.

Quartet: a Drosophila developmental mutation affecting chromosome separation in mitosis

The Drosophila mutation, quartet, affects development at points in the life cycle that require intense mitotic activity. Examination of embryos affected by the maternal effect of quartet has revealed defects that can be attributed to incomplete chromosome separation at mitosis. These defects include uneven spacing of nuclei, strands of DNA creating bridges between nuclei, and abnormal amounts of DNA per nucleus. Nuclei in quartet-affected embryos also have a greater-than-normal number of centrosomes. Immunofluorescent examination of the spindles in quartet-affected embryos has revealed tripolar spindles and adjacent spindles that share a common spindle pole. Finally, chromosome separation distance was measured in anaphase and telophase spindles in quartet-affected embryos and found to be blocked in anaphase. Examination of mitotic figures in quartet larvae revealed a reduced mitotic index and an elevated frequency of abnormal mitotic figures. quartet could encode a function necessary for the disengagement of chromosomes in mitosis, for kinetochore function or for function of a spindle motor. Mutations in quartet prevent the post-translational modification of three abundant proteins. These proteins may be involved in chromosome separation in mitosis.

Alterations in the establishment and maintenance of epithelial cell polarity as a basis for disease processes

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Distribution of microtubules containing post-translationally modified alpha-tubulin during Drosophila embryogenesis

The distribution of microtubules (MTs) enriched in detyrosinated alpha-tubulin (Glu-tubulin) was studied in Drosophila embryos by immunofluorescence microscopy by using a monoclonal antibody (ID5) which was raised against a 14-residue synthetic peptide spanning the carboxyterminal sequence of Glu-tubulin (Wehland and Weber: J. Cell Sci. 88:185-203, 1987). While all MT arrays contained tyrosinated alpha-tubulin (Tyr-tubulin), MTs rich in Glu-tubulin were not found during early stages of development even by using an image intensification camera. Elevated levels of microtubular Glu-tubulin were first detected after CNS condensation in neurone processes. In addition, sperm tails, which remained remarkably stable inside the embryo until late stages of development, were decorated by ID5. This was in marked contrast to the distribution of microtubule arrays containing acetylated alpha-tubulin, which could already be detected during the cellular blastoderm stage. Additional experiments with taxol suggested that the absence of MTs rich in Glu-tubulin during early stages of development was not due to the rapid turnover rate of MTs, which would be too fast for alpha-tubulin to be detyrosinated. The possible significance of the differential detyrosination and acetylation of microtubules during development is discussed.

Identification of microtubule-associated proteins in the centrosome, spindle, and kinetochore of the early Drosophila embryo

We have developed affinity chromatography methods for the isolation of microtubule-associated proteins (MAPs) from soluble cytoplasmic extracts and have used them to analyze the cytoskeleton of the early Drosophila embryo. More than 50 Drosophila embryo proteins bind to microtubule affinity columns. To begin to characterize these proteins, we have generated individual mouse polyclonal antibodies that specifically recognize 24 of them. As judged by immunofluorescence, some of the antigens localize to the mitotic spindle in the early Drosophila embryo, while others are present in centrosomes, kinetochores, subsets of microtubules, or a combination of these structures. Since 20 of the 24 antibodies stain microtubule structures, it is likely that most of the proteins that bind to our columns are associated with microtubules in vivo. Very few MAPS seem to be identically localized in the cell, indicating that the microtubule cytoskeleton is remarkably complex.

Genetic analysis of the claret locus of Drosophila melanogaster

The claret (ca) locus of Drosophila melanogaster comprises two separately mutable domains, one responsible for eye color and one responsible for proper disjunction of chromosomes in meiosis and early cleavage divisions. Previously isolated alleles are of three types: (1) alleles of the claret (ca) type that affect eye color only, (2) alleles of the claret-nondisjunctional (cand) type that affect eye color and chromosome behavior, and (3) a meiotic mutation, non-claret disjunctional (ncd), that affects chromosome behavior only. In order to investigate the genetic structure of the claret locus, we have isolated 19 radiation-induced alleles of claret on the basis of the eye color phenotype. Two of these 19 new alleles are of the cand type, while 17 are of the ca type, demonstrating that the two domains do not often act as a single target for mutagenesis. This suggests that the two separately mutable functions are likely to be encoded by separate or overlapping genes rather than by a single gene. One of the new alleles of the cand type is a chromosome rearrangement with a breakpoint at the position of the claret locus. If this breakpoint is the cause of the mutant phenotype and there are no other mutations associated with the rearrangement, the two functions must be encoded by overlapping genes.

Abnormal centrosomes in cold-treated Drosophila embryos

In this study we examine the effect on the centrosomes of cold treatment of early Drosophila embryos. Prolonged cold treatment during the mitotic divisions which lead to the formation of the blastoderm causes arrest at metaphase of the nuclear divisions. When examined with immunofluorescence microscopy the mitotic spindles show marked pole splitting with the formation of supernumerary and irregularly sized centers, all able to nucleate microtubules. In embryos recovered for longer periods the additional organizing centers become ring-shaped and lose their nucleating properties. Cold treatment of embryos during the cellularization of the blastoderm results in marked fragmentation of the centrosomes, but nucleating capacity is preserved. Sometimes the centrioles come away from the pericentriolar material and their structure is seen to be modified.

Centrosomes, and not nuclei, initiate pole cell formation in Drosophila embryos

An injection of aphidicolin into early Drosophila embryos inhibits DNA synthesis and nuclear division, while centrosome replication and many other aspects of the mitotic cycle continue. If aphidicolin is injected at nuclear cycle 7-8, the normal migration of nuclei to the embryo cortex is completely inhibited. In most of these embryos, however, centrosomes continue to migrate in a coordinated manner to the cortex, where they reorganize tubulin, actin, and the overlying plasma membrane. Remarkably, the centrosomes that migrate to the posterior pole of such embryos initiate pole cell formation in the absence of nuclei. These observations demonstrate that centrosomes alone are able to direct a major reorganization of the cortical cytoskeleton when they arrive at the surface of the embryo. They also suggest that the coordinated movement of nuclei to the embryo cortex is mediated by forces acting on the centrosome rather than on the nucleus itself.

Drosophila spectrin: the membrane skeleton during embryogenesis

The distribution of alpha-spectrin in Drosophila embryos was determined by immunofluorescence using affinity-purified polyclonal or monoclonal antibodies. During early development, spectrin is concentrated near the inner surface of the plasma membrane, in cytoplasmic islands around the syncytial nuclei, and, at lower concentrations, throughout the remainder of the cytoplasm of preblastoderm embryos. As embryogenesis proceeds, the distribution of spectrin shifts with the migrating nuclei toward the embryo surface so that, by nuclear cycle 9, a larger proportion of the spectrin is concentrated near the plasma membrane. During nuclear cycles 9 and 10, as the nuclei reach the cell surface, the plasma membrane-associated spectrin becomes concentrated into caps above the somatic nuclei. Concurrent with the mitotic events of the syncytial blastoderm period, the spectrin caps elongate at interphase and prophase, and divide as metaphase and anaphase progress. During cellularization, the regions of spectrin concentration appear to shift: spectrin increases near the growing furrow canal and concomitantly increases at the embryo surface. In the final phase of furrow growth, the shift in spectrin concentration is reversed: spectrin decreases near the furrow canal and concomitantly increases at the embryo surface. In gastrulae, spectrin accumulates near the embryo surface, especially at the forming amnioproctodeal invagination and cephalic furrow. During the germband elongation stage, the total amount of spectrin in the embryo increases significantly and becomes uniformly distributed at the plasma membrane of almost all cell types. The highest levels of spectrin are in the respiratory tract cells; the lowest levels are in parts of the forming gut. The spatial and temporal changes in spectrin localization suggest that this protein plays a role in stabilizing rather than initiating changes in structural organization in the embryo.