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

High resolution imaging of the cortex isolated from sea urchin eggs and embryos

The cellular cortex-consisting of the plasma membrane and the adjacent outer few microns of the cytoplasm-is a critically important, dynamic and complex region in the sea urchin egg and embryo. Some 40 years ago it was discovered that isolated cortices could be obtained from eggs adhered to glass coverslips and since that time this preparation has been used in a wide range of studies, including seminal research on fertilization, exocytosis, the cytoskeleton, and cytokinesis. In this chapter, we discuss methods for isolating cortices from eggs and embryos, including those undergoing cell division. We also provide protocols for analyzing cortical architecture and dynamics using specific localization methods combined with super-resolution Structured Illumination and Stimulated Emission Depletion light microscopy and platinum replica transmission electron microscopy.

Isolation of Cytokinetic Actomyosin Rings from Saccharomyces cerevisiae and Schizosaccharomyces pombe

Cytokinesis is the final stage of cell division, through which cellular constituents of mother cells are partitioned into two daughter cells resulting in the increase in cell number. In animal and fungal cells cytokinesis is mediated by an actomyosin contractile ring, which is attached to the overlying cell membrane. Contraction of this ring after chromosome segregation physically severs the mother cell into two daughters. Here we describe methods for the isolation and partial purification of the actomyosin ring from the fission yeast Schizosaccharomyces pombe and the budding yeast Saccharomyces cerevisiae, which can serve as in vitro systems to facilitate biochemical and ultrastructural analysis of cytokinesis in these genetically tractable model systems.

Integrins on eggs: focal adhesion kinase is activated at fertilization, forms a complex with integrins, and is necessary for cortex formation and cell cycle initiation

We investigate the proposal that integrins and focal adhesion kinase (FAK) form a complex that has structural and signaling functions in eggs. FAK protein is present in eggs and is phosphorylated at fertilization. pY(397)FAK localizes to the membrane 30 min after fertilization, which correlates with the expression of βC integrins and egg cortex development. The βC integrin and pY(397)FAK coimmunoprecipitate from egg cortex lysates. PF573 228 and Y11, inhibitors of FAK, interfere with pronuclear fusion and reduce the abundance of pY(397)FAK and cortical actin without affecting microvillar actin. Cyclin E normally accumulates in the nucleus 15 min after fertilization, then returns to background levels. PF573 228- or Y11-treated eggs accumulate cyclin E in the nucleus; however, levels remain high. In addition, PF573 228 interferes with the accumulation of pERK1/2 in the nucleus and in eggs initiating mitosis. Injection of eggs with a fusion protein consisting of the focal adhesion-targeting domain of FAK fused to green fluorescent protein interferes with cortex formation and produces abnormal nuclei. These data indicate that an integrin-FAK adhesion complex forms at the egg surface that functions in formation of actin arrays in the egg cortex and provides signaling inputs for cell cycle initiation.

Cell cortex composition and homeostasis resolved by integrating proteomics and quantitative imaging

The cellular actin cortex is the cytoskeletal structure primarily responsible for the control of animal cell shape and as such plays a central role in cell division, migration, and tissue morphogenesis. Due to the lack of experimental systems where the cortex can be investigated independently from other organelles, little is known about its composition, assembly, and homeostasis. Here, we describe novel tools to resolve the composition and regulation of the cortex. We report and validate a protocol for cortex purification based on the separation of cellular blebs. Mass spectrometry analysis of purified cortices provides a first extensive list of cortical components. To assess the function of identified proteins, we design an automated imaging assay for precise quantification of cortical actomyosin assembly dynamics. We show subtle changes in cortex assembly dynamics upon depletion of the identified cortical component profilin. Our widely applicable integrated method paves the way for systems-level investigations of the actomyosin cortex and its regulation during morphogenesis.

Isolation and partial purification of the Saccharomyces cerevisiae cytokinetic apparatus

Cytokinesis is the process by which a cell physically divides in two at the conclusion of a cell cycle. In animal and fungal cells, this process is mediated by a conserved set of proteins including actin, type II myosin, IQGAP proteins, F-BAR proteins, and the septins. To facilitate biochemical and ultrastructural analysis of cytokinesis, we have isolated and partially purified the Saccharomyces cerevisiae cytokinetic apparatus. The isolated apparatus contains all components of the actomyosin ring for which we tested-actin, myosin heavy and light chain, and IQGAP-as well as septins and the cytokinetic F-BAR protein, Hof1p. We also present evidence indicating that the actomyosin rings associated with isolated cytokinetic apparati may be contractile in vitro, and show preliminary electron microscopic imaging of the cytokinetic apparatus. This first successful isolation of the cytokinetic apparatus from a genetically tractable organism promises to make possible a deeper understanding of cytokinesis.

Cell polarity emerges at first cleavage in sea urchin embryos

In protostomes, cell polarity is present after fertilization whereas most deuterostome embryos show minimal polarity during the early cleavages. We now show establishment of cell polarity as early as the first cleavage division in sea urchin embryos. We find, using the apical markers G(M1), integrins, and the aPKC-PAR6 complex, that cells are polarized upon insertion of distinct basolateral membrane at the first division. This early apical-basolateral polarity, similar to that found in much larger cleaving amphibian zygotes, reflects precocious functional epithelial cell polarity. Isolated cleavage blastomeres exhibit polarized actin-dependent fluid phase endocytosis only on the G(M1), integrin, microvillus-containing apical surface. A role for a functional PAR complex in cleavage plane determination was shown with experiments interfering with aPKC activity, which results in several spindle defects and compromised blastula development. These studies suggest that cell and embryonic polarity is established at the first cleavage, mediated in part by the Par complex of proteins, and is achieved by directed insertion of basolateral membrane in the cleavage furrow.

Interaction between EB1 and p150glued is required for anaphase astral microtubule elongation and stimulation of cytokinesis

In animal cells, microtubules (MTs) of the mitotic apparatus (MA) communicate with the cell cortex to stimulate cytokinesis; however, the molecular nature of this stimulus remains elusive . A signal for cytokinesis likely involves the MT plus end binding family of proteins, which includes EB1, p150glued, APC, LIS1, and CLIP-170. These proteins modulate MT dynamics and facilitate interactions between growing MTs and their intracellular targets, including kinetochores, organelles, and the cell cortex . The dynein-dynactin complex mediates many of these microtubule capture events . We report that EB1 and p150glued interactions are required for stimulation of cytokinesis in dividing sea urchin eggs. Injected antibodies against EB1 or p150glued suppressed furrow ingression but did not prevent elongation of anaphase astral MTs toward the cortex, suggesting that EB1 and dynactin are both required for communication between the MA and the cortex. Targeted disruption of the interaction between EB1 and p150glued suppressed anaphase astral MT elongation and resulted in a delay of cytokinesis that could not be overcome by manipulation of the asters toward the cortex. We conclude that EB1 and dynactin participate in stimulation of the cleavage furrow, and their interaction promotes elongation of astral MTs at anaphase onset.

Calcium-responsive contractility during fertilization in sea urchin eggs

Fertilization triggers a reorganization of oocyte cytoskeleton, and in sea urchins, there is a dramatic increase in cortical F-actin. However, the role that myosin II plays during fertilization remains largely unexplored. Myosin II is localized to the cortical cytoskeleton both before and after fertilization and to examine myosin II contractility in living cells, Lytechinus pictus eggs were observed by time-lapse microscopy. Upon sperm binding, a cell surface deflection traversed the egg that was followed by and dependent on the calcium wave. The calcium-dependence of surface contractility could be reproduced in unfertilized eggs, where mobilization of intracellular calcium in unfertilized eggs under compression resulted in a marked contractile response. Lastly, inhibition of myosin II delayed absorption of the fertilization cone, suggesting that myosin II not only responds to the same signals that activate eggs but also participates in the remodeling of the cortical actomyosin cytoskeleton during the first zygotic cell cycle.

Analysis of microtubule movement on isolated Xenopus egg cortices provides evidence that the cortical rotation involves dynein as well as Kinesin Related Proteins and is regulated by local microtubule polymerisation

In amphibians, the cortical rotation, a translocation of the egg cortex relative to the cytoplasm, specifies the dorsoventral axis. The cortical rotation involves an array of subcortical microtubules whose alignment is mediated by Kinesin-related proteins (KRPs), and stops as M-phase promoting factor (MPF) activation propagates across the egg. To dissect the role of different motor proteins in the cortical rotation and to analyse their regulation, we have developed an open cell assay system involving reactivation of microtubule movement on isolated cortices. Microtubule movements were dependent on ATP and consisted mainly of wriggling and flailing without net displacement, consistent with a tethering of microtubules to the cortex. Reactivated movements were inhibited by anti-KRP and anti-dynein antibodies perfused together but not separately, the KRP antibody alone becoming fixed to the cortex. Neither antibody could inhibit movement in the presence of MPF, indicating that arrest of the cortical rotation is not due to MPF-dependent inhibition of motor molecules. In contrast, D(2)O treatment of live eggs to protect microtubules from progressive depolymerisation prolonged the cortical rotation. We conclude that the cortical rotation probably involves cytoplasmic dynein as well as cortical KRPs and terminates as a result of local MPF-dependent microtubule depolymerisation.

Micromechanics and ultrastructure of actin filament networks crosslinked by human fascin: a comparison with alpha-actinin

Fascin is an actin crosslinking protein that organizes actin filaments into tightly packed bundles believed to mediate the formation of cellular protrusions and to provide mechanical support to stress fibers. Using quantitative rheological methods, we studied the evolution of the mechanical behavior of filamentous actin (F-actin) networks assembled in the presence of human fascin. The mechanical properties of F-actin/fascin networks were directly compared with those formed by alpha-actinin, a prototypical actin filament crosslinking/bundling protein. Gelation of F-actin networks in the presence of fascin (fascin to actin molar ratio >1:50) exhibits a non-monotonic behavior characterized by a burst of elasticity followed by a slow decline over time. Moreover, the rate of gelation shows a non-monotonic dependence on fascin concentration. In contrast, alpha-actinin increased the F-actin network elasticity and the rate of gelation monotonically. Time-resolved multiple-angle light scattering and confocal and electron microscopies suggest that this unique behavior is due to competition between fascin-mediated crosslinking and side-branching of actin filaments and bundles, on the one hand, and delayed actin assembly and enhanced network micro-heterogeneity, on the other hand. The behavior of F-actin/fascin solutions under oscillatory shear of different frequencies, which mimics the cell's response to forces applied at different rates, supports a key role for fascin-mediated F-actin side-branching. F-actin side-branching promotes the formation of interconnected networks, which completely inhibits the motion of actin filaments and bundles. Our results therefore show that despite sharing seemingly similar F-actin crosslinking/bundling activity, alpha-actinin and fascin display completely different mechanical behavior. When viewed in the context of recent microrheological measurements in living cells, these results provide the basis for understanding the synergy between multiple crosslinking proteins, and in particular the complementary mechanical roles of fascin and alpha-actinin in vivo.

Spectrin: the ghost in the machine

It has long been speculated that spectrin, the actin crosslinking and molecular scaffold protein, is involved in the development of apicobasal polarity in epithelia. While spectrins can undoubtedly influence the protein content of specific membrane domains, recent genetic evidence indicates that this activity is not necessary for the establishment or maintenance of this axis. Instead, these studies point to critical roles in tissue stability and morphogenesis. A possible role in cellular contractility is highlighted in this review.

Apical spectrin is essential for epithelial morphogenesis but not apicobasal polarity in Drosophila

Changes in cell shape and position drive morphogenesis in epithelia and depend on the polarized nature of its constituent cells. The spectrin-based membrane skeleton is thought to be a key player in the establishment and/or maintenance of cell shape and polarity. We report that apical beta(Heavy)-spectrin (beta(H)), a terminal web protein that is also associated with the zonula adherens, is essential for normal epithelial morphogenesis of the Drosophila follicle cell epithelium during oogenesis. Elimination of beta(H) by the karst mutation prevents apical constriction of the follicle cells during mid-oogenesis, and is accompanied by a gross breakup of the zonula adherens. We also report that the integrity of the migratory border cell cluster, a group of anterior follicle cells that delaminates from the follicle epithelium, is disrupted. Elimination of beta(H) prevents the stable recruitment of alpha-spectrin to the apical domain, but does not result in a loss of apicobasal polarity, as would be predicted from current models describing the role of spectrin in the establishment of cell polarity. These results demonstrate a direct role for apical (alphabeta(H))(2)-spectrin in epithelial morphogenesis driven by apical contraction, and suggest that apical and basolateral spectrin do not play identical roles in the generation of apicobasal polarity.

Parameters that specify the timing of cytokinesis

One model for the timing of cytokinesis is based on findings that p34(cdc2) can phosphorylate myosin regulatory light chain (LC20) on inhibitory sites (serines 1 and 2) in vitro (Satterwhite, L.L., M.H. Lohka, K.L. Wilson, T.Y. Scherson, L.J. Cisek, J.L. Corden, and T.D. Pollard. 1992. J. Cell Biol. 118:595-605), and this inhibition is proposed to delay cytokinesis until p34(cdc2) activity falls at anaphase. We have characterized previously several kinase activities associated with the isolated cortical cytoskeleton of dividing sea urchin embryos (Walker, G.R., C.B. Shuster, and D.R. Burgess. 1997. J. Cell Sci. 110:1373-1386). Among these kinases and substrates is p34(cdc2) and LC20. In comparison with whole cell activity, cortical H1 kinase activity is delayed, with maximum levels in cortices prepared from late anaphase/telophase embryos. To determine whether cortical-associated p34(cdc2) influences cortical myosin II activity during cytokinesis, we labeled eggs in vivo with [(32)P]orthophosphate, prepared cortices, and mapped LC20 phosphorylation through the first cell division. We found no evidence of serine 1,2 phosphorylation at any time during mitosis on LC20 from cortically associated myosin. Instead, we observed a sharp rise in serine 19 phosphorylation during anaphase and telophase, consistent with an activating phosphorylation by myosin light chain kinase. However, serine 1,2 phosphorylation was detected on light chains from detergent-soluble myosin II. Furthermore, cells arrested in mitosis by microinjection of nondegradable cyclin B could be induced to form cleavage furrows if the spindle poles were physically placed in close proximity to the cortex. These results suggest that factors independent of myosin II inactivation, such as the delivery of the cleavage stimulus to the cortex, determine the timing of cytokinesis.

Microtubule-entrained kinase activities associated with the cortical cytoskeleton during cytokinesis

Research over the past few years has demonstrated the central role of protein phosphorylation in regulating mitosis and the cell cycle. However, little is known about how the mechanisms regulating the entry into mitosis contribute to the positional and temporal regulation of the actomyosin-based contractile ring formed during cytokinesis. Recent studies implicate p34cdc2 as a negative regulator of myosin II activity, suggesting a link between the mitotic cycle and cytokinesis. In an effort to study the relationship between protein phosphorylation and cytokinesis, we examined the in vivo and in vitro phosphorylation of actin-associated cortical cytoskeletal (CSK) proteins in an isolated model of the sea urchin egg cortex. Examination of cortices derived from eggs or zygotes labeled with 32P-orthophosphate reveals a number of cortex-associated phosphorylated proteins, including polypeptides of 20, 43 and 66 kDa. These three major phosphoproteins are also detected when isolated cortices are incubated with [32P]ATP in vitro, suggesting that the kinases that phosphorylate these substrates are also specifically associated with the cortex. The kinase activities in vivo and in vitro are stimulated by fertilization and display cell cycle-dependent activities. Gel autophosphorylation assays, kinase assays and immunoblot analysis reveal the presence of p34cdc2 as well as members of the mitogen-activated protein kinase family, whose activities in the CSK peak at cell division. Nocodazole, which inhibits microtubule formation and thus blocks cytokinesis, significantly delays the time of peak cortical protein phosphorylation as well as the peak in whole-cell histone H1 kinase activity. These results suggest that a key element regulating cortical contraction during cytokinesis is the timing of protein kinase activities associated with the cortical cytoskeleton that is in turn regulated by the mitotic apparatus.

Fascins, a family of actin bundling proteins

Fascin is an actin-bundling protein that was first isolated from cytoplasmic extracts of sea urchin eggs [Kane, 1975: J. Cell Biol. 66:305-315] and was the first bundling protein to be characterized in vitro. Subsequent work has shown that fascin bundles actin filaments in fertilized egg microvilli and filopodia of phagocytic coelomocytes [Otto et al., 1980: Cell Motil. 1:31-40; Otto and Bryan, 1981: Cell Motil. 1:179-192]. Fifteen years later, the molecular cloning of sea urchin fascin [Bryan et al., 1993: Proc. Natl. Acad. Sci. U.S.A. 90:9115-9119] has led to the identification and characterization of homologous proteins in Drosophila [Cant et al., 1994: J. Cell Biol. 125:369-380], Xenopus [Holthuis et al., 1994: Biochim. Biophys. Acta. 1219:184-188], rodents [Edwards et al,. 1995: J. Biol. Chem. 270:10764-10770], and humans [Duh et al., 1994: DNA Cell Biol. 13:821-827; Mosialos et al., 1994: J. Virol. 68:7320-7328] that bundle actin filaments into structures which stabilize cellular processes ranging from mechanosensory bristles to the filopodia of nerve growth cones. Fascin has emerged from relative obscurity as an exotic invertebrate egg protein to being recognized as a widely expressed protein found in a broad spectrum of tissues and organisms. The purpose of this review is to relate the early studies done on the sea urchin and HeLa cell fascins to the recent molecular biology that defines a family of bundling proteins, and discuss the current state of knowledge regarding fascin structure and function.