Computer simulation of glioma growth and morphology

Despite major advances in the study of glioma, the quantitative links between intra-tumor molecular/cellular properties, clinically observable properties such as morphology, and critical tumor behaviors such as growth and invasiveness remain unclear, hampering more effective coupling of tumor physical characteristics with implications for prognosis and therapy. Although molecular biology, histopathology, and radiological imaging are employed in this endeavor, studies are severely challenged by the multitude of different physical scales involved in tumor growth, i.e., from molecular nanoscale to cell microscale and finally to tissue centimeter scale. Consequently, it is often difficult to determine the underlying dynamics across dimensions. New techniques are needed to tackle these issues. Here, we address this multi-scalar problem by employing a novel predictive three-dimensional mathematical and computational model based on first-principle equations (conservation laws of physics) that describe mathematically the diffusion of cell substrates and other processes determining tumor mass growth and invasion. The model uses conserved variables to represent known determinants of glioma behavior, e.g., cell density and oxygen concentration, as well as biological functional relationships and parameters linking phenomena at different scales whose specific forms and values are hypothesized and calculated based on in vitro and in vivo experiments and from histopathology of tissue specimens from human gliomas. This model enables correlation of glioma morphology to tumor growth by quantifying interdependence of tumor mass on the microenvironment (e.g., hypoxia, tissue disruption) and on the cellular phenotypes (e.g., mitosis and apoptosis rates, cell adhesion strength). Once functional relationships between variables and associated parameter values have been informed, e.g., from histopathology or intra-operative analysis, this model can be used for disease diagnosis/prognosis, hypothesis testing, and to guide surgery and therapy. In particular, this tool identifies and quantifies the effects of vascularization and other cell-scale glioma morphological characteristics as predictors of tumor-scale growth and invasion.

A peptide zipcode sufficient for anterograde transport within amyloid precursor protein

Fast anterograde transport of membrane-bound organelles delivers molecules synthesized in the neuronal cell body outward to distant synapses. Identification of the molecular "zipcodes" on organelles that mediate attachment and activation of microtubule-based motors for this directed transport is a major area of inquiry. Here we identify a short peptide sequence (15 aa) from the cytoplasmic C terminus of amyloid precursor protein (APP-C) sufficient to mediate the anterograde transport of peptide-conjugated beads in the squid giant axon. APP-C beads travel at fast axonal transport rates (0.53 mum/s average velocity, 0.9 mum/s maximal velocity) whereas beads coupled to other peptides coinjected into the same axon remain stationary at the injection site. This transport appears physiologic, because it mimics behavior of endogenous squid organelles and of beads conjugated to C99, a polypeptide containing the full-length cytoplasmic domain of amyloid precursor protein (APP). Beads conjugated to APP lacking the APP-C domain are not transported. Coinjection of APP-C peptide reduces C99 bead motility by 75% and abolishes APP-C bead motility, suggesting that the soluble peptide competes with protein-conjugated beads for axoplasmic motor(s). The APP-C domain is conserved (13/15 aa) from squid to human, and peptides from either squid or human APP behave similarly. Thus, we have identified a conserved peptide zipcode sufficient to direct anterograde transport of exogenous cargo and suggest that one of APP's roles may be to recruit and activate axonal machinery for endogenous cargo transport.

Statistical diffusion tensor histology reveals regional dysmyelination effects in the shiverer mouse mutant

Shiverer is an important model of central nervous system dysmyelination characterized by a deletion in the gene encoding myelin basic protein with relevance to human dysmyelinating and demyelinating diseases. Perfusion fixed brains from shiverer mutant (C3Fe.SWV Mbp(shi)/Mbp(shi)n = 6) and background control (C3HeB.FeJ, n = 6) mice were compared using contrast enhanced volumetric diffusion tensor magnetic resonance microscopy with a nominal isotropic spatial resolution of 80 mum. Images were accurately coregistered using non-linear warping allowing voxel-wise statistical parametric mapping of tensor invariant differences between control and shiverer groups. Highly significant differences in the tensor trace and both the axial and radial diffusivity were observed within the major white matter tracts and in the thalamus, midbrain, brainstem and cerebellar white matter, consistent with a high density of myelinated axons within these regions. The fractional anisotropy was found to be much less sensitive than the trace and eigenvalues to dysmyelination and associated microanatomic changes.

Fast anterograde transport of herpes simplex virus: role for the amyloid precursor protein of alzheimer's disease

Anterograde transport of herpes simplex virus (HSV) from its site of synthesis in the neuronal cell body out the neuronal process to the mucosal membrane is crucial for transmission of the virus from one person to another, yet the molecular mechanism is not known. By injecting GFP-labeled HSV into the giant axon of the squid, we reconstitute fast anterograde transport of human HSV and use this as an assay to uncover the underlying molecular mechanism. HSV travels by fast axonal transport at velocities four-fold faster (0.9 microm/sec average, 1.2 microm/sec maximal) than that of mitochondria moving in the same axon (0.2 microm/sec) and ten-fold faster than negatively charged beads (0.08 microm/sec). Transport of HSV utilizes cellular transport mechanisms because it appears to be driven from inside cellular membranes as revealed by negative stain electron microscopy and by the association of TGN46, a component of the cellular secretory pathway, with GFP-labeled viral particles. Finally, we show that amyloid precursor protein (APP), a putative receptor for the microtubule motor, kinesin, is a major component of viral particles, at least as abundant as any viral encoded protein, while another putative motor receptor, JIP 1/2, is not detected. Conventional kinesin is also associated with viral particles. This work links fast anterograde transport of the common pathogen, HSV, with the neurodegenerative Alzheimer's disease. This novel connection should prompt new ideas for treatment and prevention strategies.

The role of the cytoskeleton in the life cycle of viruses and intracellular bacteria: tracks, motors, and polymerization machines

Recent advances in microbiology implicate the cytoskeleton in the life cycle of some pathogens, such as intracellular bacteria, Rickettsia and viruses. The cellular cytoskeleton provides the basis for intracellular movements such as those that transport the pathogen to and from the cell surface to the nuclear region, or those that produce cortical protrusions that project the pathogen outwards from the cell surface towards an adjacent cell. Transport in both directions within the neuron is required for pathogens such as the herpesviruses to travel to and from the nucleus and perinuclear region where replication takes place. This trafficking is likely to depend on cellular motors moving on a combination of microtubule and actin filament tracks. Recently, Bearer et al. reconstituted retrograde transport of herpes simplex virus (HSV) in the giant axon of the squid. These studies identified the tegument proteins as the viral proteins most likely to recruit retrograde motors for the transport of HSV to the neuronal nucleus. Similar microtubule-based intracellular movements are part of the biological behavior of vaccinia, a poxvirus, and of adenovirus. Pathogen-induced surface projections and motility within the cortical cytoplasm also play a role in the life cycle of intracellular pathogens. Such motility is driven by pathogen-mediated actin polymerization. Virulence depends on this actin-based motility, since virulence is reduced in Listeria ActA mutants that lack the ability to recruit Arp2/3 and polymerize actin and in vaccinia virus mutants that cannot stimulate actin polymerization. Inhibition of intracellular movements provides a potential strategy to limit pathogenicity. The host cell motors and tracks, as well as the pathogen factors that interact with them, are potential targets for novel antimicrobial therapy.

Arp2/3 complex is required for actin polymerization during platelet shape change

Platelets undergo a series of actin-dependent morphologic changes when activated by thrombin receptor activating peptide (TRAP) or when spreading on glass. Polymerization of actin results in the sequential formation of filopodia, lamellipodia, and stress fibers, but the molecular mechanisms regulating this polymerization are unknown. The Arp2/3 complex nucleates actin polymerization in vitro and could perform this function inside cells as well. To test whether Arp2/3 regulated platelet actin polymerization, we used recombinant Arp2 protein (rArp2) to generate Arp2-specific antibodies (alpha Arp2). Intact and Fab fragments of alpha Arp2 inhibited TRAP-stimulated actin-polymerizing activity in platelet extracts as measured by the pyrene assay. Inhibition was reversed by the addition of rArp2 protein. To test the effect of Arp2/3 inhibition on the formation of specific actin structures, we designed a new method to permeabilize resting platelets while preserving their ability to adhere and to form filopodia and lamellipodia on exposure to glass. Inhibition of Arp2/3 froze platelets at the rounded, early stage of activation, before the formation of filopodia and lamellipodia. By morphometric analysis, the proportion of platelets in the rounded stage rose from 2.85% in untreated to 63% after treatment with alpha Arp2. This effect was also seen with Fab fragments and was reversed by the addition of rArp2 protein. By immunofluorescence of platelets at various stages of spreading, the Arp2/3 complex was found in filopodia and lamellipodia. These results suggest that activation of the Arp2/3 complex at the cortex by TRAP stimulation initiates an explosive polymerization of actin filaments that is required for all subsequent actin-dependent events.

Association of a nonmuscle myosin II with axoplasmic organelles

Association of motor proteins with organelles is required for the motors to mediate transport. Because axoplasmic organelles move on actin filaments, they must have associated actin-based motors, most likely members of the myosin superfamily. To gain a better understanding of the roles of myosins in the axon we used the giant axon of the squid, a powerful model for studies of axonal physiology. First, a approximately 220 kDa protein was purified from squid optic lobe, using a biochemical protocol designed to isolate myosins. Peptide sequence analysis, followed by cloning and sequencing of the full-length cDNA, identified this approximately 220 kDa protein as a nonmuscle myosin II. This myosin is also present in axoplasm, as determined by two independent criteria. First, RT-PCR using sequence-specific primers detected the transcript in the stellate ganglion, which contains the cell bodies that give rise to the giant axon. Second, Western blot analysis using nonmuscle myosin II isotype-specific antibodies detected a single approximately 220 kDa band in axoplasm. Axoplasm was fractionated through a four-step sucrose gradient after 0.6 M KI treatment, which separates organelles from cytoskeletal components. Of the total nonmuscle myosin II in axoplasm, 43.2% copurified with organelles in the 15% sucrose fraction, while the remainder (56.8%) was soluble and found in the supernatant. This myosin decorates the cytoplasmic surface of 21% of the axoplasmic organelles, as demonstrated by immunogold electron-microscopy. Thus, nonmuscle myosin II is synthesized in the cell bodies of the giant axon, is present in the axon, and is associated with isolated axoplasmic organelles. Therefore, in addition to myosin V, this myosin is likely to be an axoplasmic organelle motor.

Actin dynamics in platelets

The human blood platelet circulates in the blood as a non-adherent disk. Upon receiving signals of blood vessel damage, the platelet reorganizes its actin cytoskeleton which transforms it into a spiky dynamic adherent glue. This transformation involves a temporal sequence of four morphologically distinct steps which is reproducible in vitro. The actin dynamics underlying these shape changes depend on a large number of actin-binding proteins. Maintenance of the discoid shape requires actin-binding proteins that inhibit these reorganizations, whereas transformation involves other proteins, some to disassemble old filaments and others to polymerize new ones. F-Actin-affinity chromatography identified a large set of actin-binding proteins including VASP, Arp2 and 2E4/kaptin. Recent discoveries show that VASP inhibits filament disassembly and Arp2/3 is required to polymerize new filaments. Morphological analysis of the distribution of these actin-binding proteins in spread platelets together with biochemical measurements of their interactions with actin lead to a model of interactions with actin that mediate shape change.

Motility-related proteins as markers for head and neck squamous cell cancer

HYPOTHESIS

Increased cell motility is a hallmark of cancer cells. Proteins involved in cell motility may be used as molecular markers to characterize the malignant potential of tumors.

METHODS

Molecular biology and immunohistochemistry techniques were used to investigate the expression of a selected panel of motility-related proteins (Rho A, Rac 2, Cdc42, PI3K, 2E4, and Arp2) in normal, premalignant, and squamous cell cancer cell lines of human head and neck origin. To assess the clinical potential of these proteins as molecular markers for cancer, immunohistochemistry was performed on paraffin-fixed head and neck cancer specimens (n = 15).

RESULTS

All six motility-associated proteins were overexpressed in the premalignant and squamous cell cancer cell lines relative to normal keratinocytes. Immunohistochemistry with Rho A and Rac 2 showed increased staining in areas of cancer but not in normal tissue.

CONCLUSION

Proteins involved in cell motility can be used as markers for head and neck squamous cell carcinoma. The head and neck cell lines used in this study may be used as a model to further investigate cell motility. Molecular markers of motility could have a significant impact on the diagnosis and staging of cancers originating from differentiated non-motile cells.

VASP protects actin filaments from gelsolin: an in vitro study with implications for platelet actin reorganizations

An initial step in platelet shape change is disassembly of actin filaments, which are then reorganized into new actin structures, including filopodia and lamellipodia. This disassembly is thought to be mediated primarily by gelsolin, an abundant actin filament-severing protein in platelets. Shape change is inhibited by VASP, another abundant actin-binding protein. Paradoxically, in vitro VASP enhances formation of actin filaments and bundles them, activities that would be expected to increase shape change, not inhibit it. We hypothesized that VASP might inhibit shape change by stabilizing filaments and preventing their disassembly by gelsolin. Such activity would explain VASP's known physiological role. Here, we test this hypothesis in vitro using either purified recombinant or endogenous platelet VASP by fluorescence microscopy and biochemical assays. VASP inhibited gelsolin's ability to disassemble actin filaments in a dose-dependent fashion. Inhibition was detectable at the low VASP:actin ratio found inside the platelet (1:40 VASP:actin). Gelsolin bound to VASP-actin filaments at least as well as to actin alone. VASP inhibited gelsolin-induced nucleation at higher concentrations (1:5 VASP:actin ratios). VASP's affinity for actin (K(d) approximately 0.07 microM) and its ability to promote polymerization (1:20 VASP actin ratio) were greater with Ca(++)-actin than with Mg(++)-actin (K(d) approximately 1 microM and 1:1 VASP), regardless of the presence of gelsolin. By immunofluorescence, VASP and gelsolin co-localized in the filopodia and lamellipodia of platelets spreading on glass, suggesting that these in vitro interactions could take place within the cell as well. We conclude that VASP stabilizes actin filaments to the severing effects of gelsolin but does not inhibit gelsolin from binding to the filaments. These results suggest a new concept for actin dynamics inside cells: that bundling proteins protect the actin superstructure from disassembly by severing, thereby preserving the integrity of the cytoskeleton.

Retrograde axonal transport of herpes simplex virus: evidence for a single mechanism and a role for tegument

Herpes simplex virus type I (HSV) typically enters peripheral nerve terminals and then travels back along the nerve to reach the neuronal cell body, where it replicates or enters latency. To monitor axoplasmic transport of HSV, we used the giant axon of the squid, Loligo pealei, a well known system for the study of axoplasmic transport. To deliver HSV into the axoplasm, viral particles stripped of their envelopes by detergent were injected into the giant axon, thereby bypassing the infective process. Labeling the viral tegument protein, VP16, with green fluorescent protein allowed viral particles moving inside the axon to be imaged by confocal microscopy. Viral particles moved 2.2 +/- 0.26 micrometer/sec in the retrograde direction, a rate comparable to that of the transport of endogenous organelles and of virus in mammalian neurons in culture. Electron microscopy confirmed that 96% of motile (stripped) viral particles had lost their envelope but retained tegument, and Western blot analysis revealed that these particles had retained protein from capsid but not envelope. We conclude that (i) HSV recruits the squid retrograde transport machinery; (ii) viral tegument and capsid but not envelope are sufficient for this recruitment; and (iii) the giant axon of the squid provides a unique system to dissect the viral components required for transport and to identify the cellular transport mechanisms they recruit.

2E4/Kaptin (KPTN)--a candidate gene for the hearing loss locus, DFNA4

Stereocilia of the inner ear play an integral role in the mechanotransduction of sound. Their structural support is derived from actin filaments and actin-binding proteins. We have identified a novel actin-binding protein, 2E4-kaptin (KPTN), which appears to be involved in this structural network. Using double label immunofluorescence, we now show that KPTN extends beyond the barbed ends of actin filaments at the tips of stereocilia, and using cloned human cDNA, we mapped KPTN to chromosome 19q13.4. A combination of FISH, radiation hybrid mapping and YAC screening localized KPTN between markers D19S412 and NIB1805, making this gene an excellent functional and positional candidate for DFNA4, a form of autosomal dominant non-syndromic hearing loss. We identified a second family with inherited deafness that also maps to the DFNA4 region. To screen KPTN for deafness-causing mutations, we first determined its genomic structure and then completed a mutational analysis by direct sequencing and SSCP in affected family members. Although no deafness-causing mutations were identified in the coding region, KPTN remains an excellent candidate gene for hearing loss; by synteny, its murine orthologue also remains a candidate gene for the Nijmegan waltzer (nv) mouse mutant, which has vestibular defects and a variable sensorineural hearing loss.

2E4 (kaptin): a novel actin-associated protein from human blood platelets found in lamellipodia and the tips of the stereocilia of the inner ear

Platelet activation, crucial for hemostasis, requires actin polymerization, yet the molecular mechanisms by which localized actin polymerization is mediated are not clear. Here we report the characterization of a novel actin-binding protein, 2E4, originally isolated from human blood platelets and likely to be involved in the actin rearrangements occurring during activation. 2E4 binds to filamentous (F)-actin by F-actin affinity chromatography and is eluted from F-actin affinity columns and extracted from cells with ATP. Its presence at the leading edge of platelets spread on glass and in the lamellipodia of motile fibroblasts suggests a role in actin dynamics. Using localization to obtain clues about function, we stained the sensory epithelium of the embryonic inner ear to determine whether 2E4 is at the barbed end of actin filaments during their elongation. Indeed, 2E4 was present at the tips of the elongating stereocilium. 2E4 is novel by DNA sequence and has no identifiable structural motifs. Its unusual amino acid sequence, its ATP-sensitive actin association and its location at sites of actin polymerization in cells suggest 2E4 plays a unique role in the actin rearrangements that accompany platelet activation and stereocilia formation.

Association of actin filaments with axonal microtubule tracts

Axoplasmic organelles move on actin as well as microtubules in vitro and axons contain a large amount of actin, but little is known about the organization and distribution of actin filaments within the axon. Here we undertake to define the relationship of the microtubule bundles typically found in axons to actin filaments by applying three microscopic techniques: laser-scanning confocal microscopy of immuno-labeled squid axoplasm; electronmicroscopy of conventionally prepared thin sections; and electronmicroscopy of touch preparations-a thin layer of axoplasm transferred to a specimen grid and negatively stained. Light microscopy shows that longitudinal actin filaments are abundant and usually coincide with longitudinal microtubule bundles. Electron microscopy shows that microfilaments are interwoven with the longitudinal bundles of microtubules. These bundles maintain their integrity when neurofilaments are extracted. Some, though not all microfilaments decorate with the S1 fragment of myosin, and some also act as nucleation sites for polymerization of exogenous actin, and hence are definitively identified as actin filaments. These actin filaments range in minimum length from 0.5 to 1.5 microm with some at least as long as 3.5 microm. We conclude that the microtubule-based tracks for fast organelle transport also include actin filaments. These actin filaments are sufficiently long and abundant to be ancillary or supportive of fast transport along microtubules within bundles, or to extend transport outside of the bundle. These actin filaments could also be essential for maintaining the structural integrity of the microtubule bundles.

An axoplasmic myosin with a calmodulin-like light chain

Organelles in the axoplasm from the squid giant axon move along exogenous actin filaments toward their barbed ends. An approximately 235-kDa protein, the only band recognized by a pan-myosin antibody in Western blots of isolated axoplasmic organelles, has been previously proposed to be a motor for these movements. Here, we purify this approximately 235-kDa protein (p235) from axoplasm and demonstrate that it is a myosin, because it is recognized by a pan-myosin antibody and has an actin-activated Mg-ATPase activity per mg of protein 40-fold higher than that of axoplasm. By low-angle rotary shadowing, p235 differs from myosin II and it does not form bipolar filaments in low salt. The amino acid sequence of a 17-kDa protein that copurifies with p235 shows that it is a squid optic lobe calcium-binding protein, which is more similar by amino acid sequence to calmodulin (69% identity) than to the light chains of myosin II (33% identity). A polyclonal antibody to this light chain was raised by using a synthetic peptide representing the calcium binding domain least similar to calmodulin. We then cloned this light chain by reverse transcriptase-PCR and showed that this antibody recognizes the bacterially expressed protein but not brain calmodulin. In Western blots of sucrose gradient fractions, the 17-kDa protein is found in the organelle fraction, suggesting that it is a light chain of the p235 myosin that is also associated with organelles.

Actin-based motility of isolated axoplasmic organelles

We previously showed that axoplasmic organelles from the squid giant axon move toward the barbed ends of actin filaments and that KI-washed organelles separated from soluble proteins by sucrose density fractionation retain a 235-kDa putative myosin. Here, we examine the myosin-like activities of KI-washed organelles after sucrose density fractionation to address the question whether the myosin on these organelles is functional. By electron microscopy KI-washed organelles bound to actin filaments in the absence of ATP but not in its presence. Analysis of organelle-dependent ATPase activity over time and with varying amounts of organelles revealed a basal activity of 350 (range: 315-384) nmoles Pi/mg/min and an actin-activated activity of 774 (range: 560-988) nmoles/mg/min, a higher specific activity than for the other fractions. By video microscopy washed organelles moved in only one direction on actin filaments with a net velocity of 1.11 +/- .03 microns/s and an instantaneous velocity of 1.63 +/- 0.29 microns/s. By immunogold electronmicroscopy, 7% of KI-washed organelles were decorated with an anti-myosin antibody as compared to 0.5% with non-immune serum. Thus, some axoplasmic organelles have a tightly associated myosin-like activity.

Changes in tyrosine-phosphorylated p190 and its association with p120 type I and p100 type II rasGAPs during myelomonocytic differentiation of human leukemic cells

A M(r) 190,000 protein (p190) functions as a GTPase-activating protein (GAP) for Rho and Rac family proteins, which are involved in regulating cytoskeletal actin and membrane ruffling. Tyrosine-phosphorylated p190 also complexes with rasGAP, a regulator of Ras activity, thus possibly linking Ras and Rho pathways. Leukemic cells induced to differentiate along myelomonocytic lineages have increased filamentous actin (as evidenced by phalloidin staining) and extended pseudopodia, and become irregularly shaped and flattened, suggesting altered Rho and Rac function. We, therefore, hypothesized that changes in p190 and its association with rasGAP are an integral part of these shape changes. During phorbol 13-myristate 25-acetate-induced monocytic differentiation of HL60 promyelocytic and RWLeu4 chronic myelogenous leukemic cells, the total amount of p190 decreases rapidly but returns to initial levels by 12 h. In RWLeu4, this was accompanied by commensurate changes in p190 tyrosine phosphorylation and association with p120 type I rasGAP. Association of p190 and type I rasGAP was demonstrated by immunoprecipitation with antibodies to either protein. An additional band at M(r) 100,000 (p100) was detected in immunoprecipitates after 12 h of phorbol 13-myristate 25-acetate treatment. Reverse transcription-PCR and immunoblot analyses suggest that p100 is type II rasGAP, an alternatively spliced product of p120 type I rasGAP. p100 was expressed only in response to direct protein kinase C activators, but all classes of differentiation agents increased tyrosine-phosphorylated p190. Rho and Rac are known to be involved in regulating actin polymerization. The results presented here show that the association of p190 with type I rasGAP parallels increases in actin polymerization and cell adhesion. This suggests a role for p190-rasGAP interactions in phorbol 13-myristate 25-acetate-induced cytoskeletal reorganization.

Cytoskeletal domains in the activated platelet

Platelets circulate in the blood as discoid cells which, when activated, change shape by polymerizing actin into various structures, such as filopodia and stress fibers. In order to understand this process, it is necessary to determine how many other proteins are involved. As a first step in defining the full complement of actin-binding proteins in platelets, filamentous (F)-actin affinity chromatography was used. This approach identified > 30 different proteins from ADP-activated human blood platelets which represented 4% of soluble protein. Although a number of these proteins are previously identified platelet actin-binding proteins, many others appeared to be novel. Fourteen different polyclonal antibodies were raised against these apparently novel proteins and used to sort them into nine categories based on their molecular weights and on their location in the sarcomere of striated muscle, in fibroblasts and in spreading platelets. Ninety-three percent of these proteins (13 of 14 proteins tested) were found to be associated with actin-rich structures in vivo. Four distinct actin filament structures were found to form during the initial 15 min of activation on glass: filopodia, lamellipodia, a contractile ring encircling degranulating granules, and thick bundles of filaments resembling stress fibers. Actin-binding proteins not localized in the discoid cell became highly concentrated in one or another of these actin-based structures during spreading, such that each structure contains a different complement of proteins. These results present crucial information about the complexity of the platelet cytoskeleton, demonstrating that four different actin-based structures form during the first 15 min of surface activation, and that there remain many as yet uncharacterized proteins awaiting further investigation that are differentially involved in this process.

Distribution of Xrel in the early Xenopus embryo: a cytoplasmic and nuclear gradient

In Drosophila embryos, the dorsal gene, a member of the rel family of transcription factors, is a maternal gene required for dorsal/ventral axis specification and mesoderm and neural migration and differentiation. In this report, the presence and distribution of Xrel-1 protein, a Xenopus homologue of the rel family of transcription factors, is described during early embryogenesis. Antiserum to v-rel, an avian homologue of dorsal, was used in immunoblot to detect rel proteins in Xenopus embryos. Anti-v-rel serum recognized a single band of approximately 57 kDa in Western blots of extracts of Xenopus embryos. Antiserum was also generated against bacterially expressed Xrel-1 fusion protein, a Xenopus rel homologue expressed during oogenesis. Anti-Xrel-1 antiserum recognized the same approximately 57 kDa band in Western blots as anti-v-rel. In developmental Westerns, this 57 kDa protein was present throughout early embryogenesis. Whole mounts and histologic sections of staged embryos were stained with anti-rel antiserum. Asymmetric staining of the cytoplasm of the animal cap became apparent by the eight-cell stage, with little or no staining of the vegetal hemisphere. Staining of nuclei in cells of the animal cap down to the equatorial zone became apparent between Stage 7 and 8, while vegetal nuclei never stained. Unlike Drosophila dorsal, no dorsal-ventral gradient of Xrel-1 could be detected. Anti-actin antibodies stained embryos uniformly, while preimmune serum or anti-v-rel antiserum immunoabsorbed against bacterially expressed Xrel-1 protein failed to stain. Nuclear staining diminished during gastrulation, but reappeared during neurulation. In conclusion, Xrel-1 protein is enriched in the cytoplasm of the animal pole of early Xenopus embryos, and enters the nuclei of the cells of the animal cap and presumptive mesoderm at Stage 7-1/2. The presence of this putative transcription factor in the nuclei of these cells prior to mid-blastula transition suggests that Xrel-1 may be involved in programming animal cells to respond to vegetal-inducing factors.

Evidence for myosin motors on organelles in squid axoplasm

Squid axoplasm has proved a rich source for the identification of motors involved in organelle transport. Recently, squid axoplasmic organelles have been shown to move on invisible tracks that are sensitive to cytochalasin, suggesting that these tracks are actin filaments. Here, an assay is described that permits observation of organelles moving on unipolar actin bundles. This assay is used to demonstrate that axoplasmic organelles move on actin filaments in the barbed-end direction, suggesting the presence of a myosin motor on axoplasmic organelles. Indeed, axoplasm contains actin-dependent ATPase activity, and a pan-myosin antibody recognized at least four bands in Western blots of axoplasm. An approximately 235-kDa band copurified in sucrose gradients with KI-extracted axoplasmic organelles, and the myosin antibody stained the organelle surfaces by immunogold electron microscopy. The myosin is present on the surface of at least some axoplasmic organelles and thus may be involved in their transport through the axoplasm, their movement through the cortical actin in the synapse, or some other aspect of axonal function.

Role of actin polymerization in cell locomotion: molecules and models

Actin filaments forming at the anterior margin of a migrating cell are essential for the formation of filopodia, lamellipodia, and pseudopodia, the "feet" that the cell extends before it. These structures in turn are required for cell locomotion. Yet the molecular nature of the "nucleator" that seeds the polymerization of actin at the leading edge is unknown. Recent advances, including video microscopy of actin dynamics, discovery of proteins unique to the leading edge such as ponticulin, the Mab 2E4 antigen, and ABP 120, and novel experimental models of actin polymerization such as the actin-based movements of intracellular parasites, promise to shed light on this problem in the near future.

An actin-associated protein present in the microtubule organizing center and the growth cones of PC-12 cells

The pathfinding ability of the growth cone depends upon the integrity of a dynamic actin filament network. However, although a number of actin-binding proteins have been found in growth cones, it is not known how these proteins come to be concentrated there or how they might interact to produce these important actin filaments. In this report, an actin-associated protein recognized by the monoclonal antibody 2E4 is demonstrated to be present in PC-12 cells. In undifferentiated cells, this protein is present in an apparently inactive state in a perinuclear location that corresponds to that of the microtubule organizing center and not of the Golgi apparatus. Conversely, after NGF-induced differentiation, the antigen is found enriched in the neurite and growth cone and disappears from the perinuclear position. This disappearance is directly proportional to the length of the neurite. The antigen-antibody complex binds the ends of actin filaments in vitro in an ATP-sensitive manner, and the antibody stains the outermost edge of the actin filament ruffle in the leading edge of migrating fibroblasts. Hence, it is possibly involved in the membrane-associated polymerization of actin filaments such as that observed in growth cones.

Direct observation of actin filament severing by gelsolin and binding by gCap39 and CapZ

Dynamic behavior of actin filaments in cells is the basis of many different cellular activities. Remodeling of the actin filament network involves polymerization and depolymerization of the filaments. Proteins that regulate these behaviors include proteins that sever and/or cap actin filaments. This report presents direct observation of severing of fluorescently-labeled actin filaments. Coverslips coated with gelsolin, a multi-domain, calcium-dependent capping and severing protein, bound rhodamine-phalloidin-saturated filaments along their length in the presence of EGTA. Upon addition of calcium, attached filaments bent as they broke. Actophorin, a low molecular weight, monomer sequestering, calcium-independent severing protein did not sever phalloidin-saturated filaments. Both gCap 39, a gelsolin-like, calcium-dependent capping protein that does not sever filaments, and CapZ, a heterodimeric, non-calcium-dependent capping protein, bound the filaments by one end to the coverslip. Visualization of individual filaments also revealed severing activity present in mixtures of actin-binding proteins isolated by filamentous actin affinity chromatography from early Drosophila embryos. This activity was different from either gelsolin or actophorin because it was not inhibited by phalloidin, but was calcium independent. The results of these studies provide new information about the molecular mechanisms of severing and capping by well-characterized proteins as well as definition of a novel type of severing activity.

Actin in the Drosophila embryo: is there a relationship to developmental cue localization?

Recent genetic manipulations have revealed that the cytoplasm of the early Drosophila embryo contains localized information that specifies the future embryonic axes. It is the restricted distribution or activity of particular gene products, either messenger RNA or protein, that is crucial for this specification. While some of the genes responsible for this information have been sequenced and the nature and distribution of their products examined, it is not known how this localization is established or maintained. The actin-based cytoskeleton is a likely candidate for the formation of a cytomatrix that would allow such distributions and yet no direct evidence has yet been found that implicates actin in positional cue localization. In this review I summarize what is known about actin filament behavior in Drosophila embryos and compare it to the distribution of positional cues. My purpose is to juxtapose these two bodies of information such that the relationship between them may be revealed.

Morphology of mammalian sperm membranes during differentiation, maturation, and capacitation

The mammalian spermatozoon is a highly polarized cell whose surface membrane can be divided into five functionally, structurally, and biochemically distinct domains. These domains are formed during spermatogenesis, continue to be modified during passage through the epididymis, and are further refined in the female reproductive tract. The integrity of these domains appears to be necessary for the sperm to perform its function--fusion with the egg and subsequent fertilization. The domains can be identified morphologically by their surface contours and texture, the content, distribution, and organization of intramembranous particles after freeze-fracture, and by the density of surface and cytoplasmic electron-dense coatings in thin sections. By using a variety of labels that stain carbohydrates (lectins), lipids (filipin and polymyxin B), and monoclonal antibodies to specific membrane constituents, the biochemical composition of these contiguous membrane regions has also been partly elucidated. We review here what is known about the structure, composition, and behavior of each membrane domain in the mature sperm and include some information regarding domain formation during spermatogenesis. The sperm is an excellent model system to study the creation and maintenance of cell polarity, granule exocytosis, and fertilization. Hopefully this review will provide impetus for future studies aimed more directly at addressing the relationship of its morphology to its functions.

Platelet membrane skeleton revealed by quick-freeze deep-etch

Actin polymerization is an essential component of platelet activation. Since actin appears to polymerize at its membrane-associated end, knowledge of the structural relationship of actin filaments to membrane is an important part of understanding that polymerization process. A membrane-associated actin-containing cytoskeleton has been described in human platelets biochemically and is composed, at least in part, by an association between glycoprotein Ib and the actin-binding protein originally isolated from macrophages. Many other actin-associated proteins with known sub-membranous localization in other systems have been found in platelets, including alpha-actinin, vinculin, and low levels of spectrin and the red cell protein Band 4.1. Because of the density of the platelet cytoplasm, the structure of the membrane-skeleton has not yet been visualized. We have used quick freeze-deep etch techniques to observe the sub-membranous cytoplasm and report visualization of a periodic, submembranous filament system not before seen in the platelet. This filamentous system was more easily observed in thrombin-stimulated platelets, but appeared to be present in resting, discoid cells as well. The filaments could also be readily observed when platelets are lysed after fixation, stained with tannic acid, and embedded for thin-sectioning. This membrane cytoskeleton was composed of 9 nm thick filaments lying 15 nm apart, and 15 nm from the membrane. The filaments appeared to lie in parallel and to encircle the cell. Similar filaments could be seen associated with intracytoplasmic membrane systems in activated cells.

A Simple Method for Quick-Freezing

In conventional freeze-fracture replicas produced from tissue cryoprotected with glycerol, the hydrophobic inner surfaces of membranes are revealed, but hydrophillic structures are obscured in the surrounding ice. Quick-freezing of tissue obviates the need for glycerol, which prevents the removal of this ice by etching or freeze-drying, but the major problem in freezing without glycerol cryoprotection is ice crystal formation. We describe here a simple method for quick-freezing tissue, in the absence of glycerol, on a nitrogen-cooled copper block with a hand-held specimen holder. This method freezes samples well enough to preserve molecular detail that can be revealed by subsequent etching. We show some examples of the quality of this freezing with respect to the visualization of molecular detail in isolated protein molecules such as ferritin and catalase. Furthermore, we show examples of in situ cellular structures that are revealed by this method, and we compare the structure seen in these replicas with structures preserved by quick-freezing at liquid helium temperatures.

Endothelial fenestral diaphragms: a quick-freeze, deep-etch study

The route by which water, solutes, and macromolecules traverse the endothelial cell has long been a subject of study for both physiologists and cell biologists. Recent physiologic studies describe a slit-shaped pore (5.1-5.7-nm wide) as the communicating channel, although no channel of such dimensions has been visible in electron microscopic preparations. That this channel should be found within the fenestral diaphragm has long been suggested. In this report, by the aid of a new technique in tissue processing, we are able to demonstrate a possible morphologic correlate within the fenestral diaphragm of fenestrated capillaries. Quick-freezing and deep-etching of whole tissue blocks allows the sublimation of water from the endothelial pores, thus leaving the channels through the diaphragms empty and readily replicated with a platinum-carbon shadow. The structure of the diaphragm was revealed thus to be composed of radial fibrils of 7 nm in diameter, interweaving in a central mesh, and creating by their geometric distribution, wedge-shaped channels around the periphery of the pore. The average channel had a maximum arc length of 5.46 nm. Fenestrated endothelia from various tissues, including endocrine and exocrine pancreas, adrenal cortex, and kidney peritubular capillaries, displayed the same diaphragmatic structure, whereas continuous capillaries in muscle had no such diaphragm. Photographic augmentation of electron micrographs of etched replicas displayed marked enhancement at n = 8, confirming an octagonal symmetry of the fenestral diaphragm. Finally, cationic ferritin, clearly visible as a marker after etching, heavily bound to the flowerlike structure within the fenestral pore. We conclude that the fenestral diaphragm contains the structure responsible for fenestrated capillary permeability and that the communicating channel has the shape of a wedge.

Fusion of phospholipid vesicles arrested by quick-freezing. The question of lipidic particles as intermediates in membrane fusion

We have examined the early events in Ca2+-induced fusion of large (0.2 microns diameter) unilamellar cardiolipin/phosphatidylcholine and phosphatidylserine/phosphatidylethanolamine vesicles by quick-freezing freeze-fracture electron microscopy, eliminating the necessity of using glycerol as a cryoprotectant. Freeze-fracture replicas of vesicle suspensions frozen after 1-2 s of stimulation revealed that the majority of vesicles had already undergone membrane fusion, as evidenced by dumbbell-shaped structures and large vesicles. In the absence of glycerol, lipidic particles or the hexagonal HII phase, which have been proposed to be intermediate structures in membrane fusion, were not observed at the sites of fusion. Lipidic particles were evident in less than 5% of the cardiolipin/phosphatidylcholine vesicles after long-term incubation with Ca2+, and the addition of glycerol produced more vesicles displaying the particles. We have also shown that rapid fusion occurred within seconds of Ca2+ addition by the time-course of fluorescence emission produced by the intermixing of aqueous contents of two separate vesicle populations. These studies therefore have produced no evidence that lipidic particles are necessary intermediates for membrane fusion. On the contrary, they indicate that lipidic particles are structures obtained at equilibrium long after fusion has occurred and they become particularly prevalent in the presence of glycerol.

Modifications of anionic-lipid domains preceding membrane fusion in guinea pig sperm

The relationship between anionic-lipid concentration and the functional properties of plasma-membrane domains was explored using the guinea-pig sperm membrane as a model, with polymyxin B (PXB) as a probe. Areas of plasmalemma specialized for fusion during the acrosome reaction had a higher affinity for the probe than adjacent nonfusigenic regions. In addition, capacitation--a process preceding acrosome:plasma-membrane fusion--markedly enlarged the area susceptible to PXB binding over the acrosomal cap. Protease treatment mimicked capacitation by increasing the acrosome-reaction incidence as well as PXB binding, at enzyme concentrations not affecting the surface coat nor altering filipin/sterol localization. Both proteolytic digestion and capacitation failed to augment PXB- or filipin-affinity in nonfusigenic zones, such as the post-acrosomal segment, including its particle-free maculae. Incubation of sperm in capacitating medium supplemented with 32P-labeled phosphate, followed by lipid extraction, thin-layer chromatography, and autoradiography, revealed a radioactive band comigrating with cardiolipin and phosphatidic acid. Vermiform protrusions elicited by PXB in the outer lamellae of cardiolipin-phosphatidylcholine liposomes resembled those seen in fusional regions of sperm membrane. We conclude that (a) differing concentrations of anionic lipids are found in adjacent domains of the sperm plasma membrane; (b) these domains mirror the functional regions of the membrane, with higher anionic-lipid concentrations localized over fusional zones; (c) the surface coat does not participate in the maintenance of such domains; (d) anionic-lipid synthesis may contribute to their formation; and (e) anionic-lipid concentrations increase as the membrane becomes fusionally competent, indicating that cellular modulation of lipid domains accompanies regulation of membrane function.

beta-Hydroxysterol distribution as determined by freeze-fracture cytochemistry

Filipin, a polyene antibiotic, fluoresces and forms 15-25 nm aggregates when combined with beta-hydroxysterols, rendering sterols detectable by fluorescence microscopy and by electron microscopy of thin sections and freeze-fracture replicas. We applied filipin in a glutaraldehyde fixative to tissue-cultured cells of Drosophila melanogaster larvae, in which sterol concentration can be regulated. Since the number of filipin-sterol aggregates observed in membranes was found to be proportional to the amount of sterol experimentally inserted, utilizing filipin is a valid method for quantifying, as well as for mapping, sterol distribution in biological membranes. Other antibiotics may be similarly used for localizing some species of negatively charged phospholipids. In addition to cytochemical identification of specific lipids, rapid freezing and deep etching of unfixed, non-cryoprotected cells may permit us to examine membrane lipids in different physical states: liquid-crystalline and gel. Combining these several techniques has resulted in new data concerning the disposition of lipids during the intimate juxtaposition of membranes preceding fusion. For example, in guinea-pig sperm, foci of closely apposed membranes are bereft of beta-hydroxysterols and intramembranous particles. Such regions of membrane sometimes exist in a crystalline state and may be rimmed by negatively charged phospholipids. As previously noted in other areas of cytochemistry, the in situ localization of specific substances provides information unobtainable by morphological or biochemical techniques alone.

Anionic lipid domains: correlation with functional topography in a mammalian cell membrane

Polymyxin B was used to explore distribution of anionic phospholipids in sperm plasma membranes by electron microscopy of freeze-fracture replicas. After exposure to Hepes/Tris-buffered polymyxin at 4 mM, phosphatidylcholine liposomes showed no perturbations nor did they fluoresce with dansylated incubation. When phosphatidylethanolamine was included in the liposomes, they became perturbed and fluoresced. Plasma membranes of Drosophila larval cells, containing or lacking cholesterol, were also disrupted by polymyxin. The cell membranes of guinea pig sperm were likewise disrupted but in specific functional areas. Fusional membrane domains showed protrusions; the stable membrane of the flagellum revealed diffuse bubbling. Regions of well-defined particle arrays and the postacrosomal segment maintained smooth contours. By fluorescence microscopy, we detected the same heterogeneous binding of the polymyxin dansyl derivative.