Fat2 polarizes the WAVE complex in trans to align cell protrusions for collective migration

For a group of cells to migrate together, each cell must couple the polarity of its migratory machinery with that of the other cells in the cohort. Although collective cell migrations are common in animal development, little is known about how protrusions are coherently polarized among groups of migrating epithelial cells. We address this problem in the collective migration of the follicular epithelial cells in Drosophila melanogaster. In this epithelium, the cadherin Fat2 localizes to the trailing edge of each cell and promotes the formation of F-actin-rich protrusions at the leading edge of the cell behind. We show that Fat2 performs this function by acting in trans to concentrate the activity of the WASP family verprolin homolog regulatory complex (WAVE complex) at one long-lived region along each cell's leading edge. Without Fat2, the WAVE complex distribution expands around the cell perimeter and fluctuates over time, and protrusive activity is reduced and unpolarized. We further show that Fat2's influence is very local, with sub-micron-scale puncta of Fat2 enriching the WAVE complex in corresponding puncta just across the leading-trailing cell-cell interface. These findings demonstrate that a trans interaction between Fat2 and the WAVE complex creates stable regions of protrusive activity in each cell and aligns the cells' protrusions across the epithelium for directionally persistent collective migration.

Modulating RhoA effectors induces transitions to oscillatory and more wavelike RhoA dynamics in C. elegans zygotes

Pulsatile RhoA dynamics underlie a wide range of cell and tissue behaviors. The circuits that produce these dynamics in different cells share common architectures based on fast positive and delayed negative feedback through F-actin, but they can produce very different spatiotemporal patterns of RhoA activity. However, the underlying causes of this variation remain poorly understood. Here we asked how this variation could arise through modulation of actin network dynamics downstream of active RhoA in early C. elegans embryos. We find that perturbing two RhoA effectors - formin and anillin - induce transitions from non-recurrent focal pulses to either large noisy oscillatory pulses (formin depletion) or noisy oscillatory waves (anillin depletion). In both cases these transitions could be explained by changes in local F-actin levels and depletion dynamics, leading to changes in spatial and temporal patterns of RhoA inhibition. However, the underlying mechanisms for F-actin depletion are distinct, with different dependencies on myosin II activity. Thus, modulating actomyosin network dynamics could shape the spatiotemporal dynamics of RhoA activity for different physiological or morphogenetic functions. [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text].

Filament-guided filament assembly provides structural memory of filament alignment during cytokinesis

During cytokinesis, animal cells rapidly remodel the equatorial cortex to build an aligned array of actin filaments called the contractile ring. Local reorientation of filaments by active equatorial compression is thought to underlie the emergence of filament alignment during ring assembly. Here, combining single molecule analysis and modeling in one-cell C. elegans embryos, we show that filaments turnover is far too fast for reorientation of individual filaments by equatorial compression to explain the observed alignment, even if favorably oriented filaments are selectively stabilized. By tracking single formin/CYK-1::GFP particles to monitor local filament assembly, we identify a mechanism that we call filament-guided filament assembly (FGFA), in which existing filaments serve as templates to orient the growth of new filaments. FGFA sharply increases the effective lifetime of filament orientation, providing structural memory that allows cells to build highly aligned filament arrays in response to equatorial compression, despite rapid turnover of individual filaments.

Roadmap for the multiscale coupling of biochemical and mechanical signals during development

The way in which interactions between mechanics and biochemistry lead to the emergence of complex cell and tissue organization is an old question that has recently attracted renewed interest from biologists, physicists, mathematicians and computer scientists. Rapid advances in optical physics, microscopy and computational image analysis have greatly enhanced our ability to observe and quantify spatiotemporal patterns of signalling, force generation, deformation, and flow in living cells and tissues. Powerful new tools for genetic, biophysical and optogenetic manipulation are allowing us to perturb the underlying machinery that generates these patterns in increasingly sophisticated ways. Rapid advances in theory and computing have made it possible to construct predictive models that describe how cell and tissue organization and dynamics emerge from the local coupling of biochemistry and mechanics. Together, these advances have opened up a wealth of new opportunities to explore how mechanochemical patterning shapes organismal development. In this roadmap, we present a series of forward-looking case studies on mechanochemical patterning in development, written by scientists working at the interface between the physical and biological sciences, and covering a wide range of spatial and temporal scales, organisms, and modes of development. Together, these contributions highlight the many ways in which the dynamic coupling of mechanics and biochemistry shapes biological dynamics: from mechanoenzymes that sense force to tune their activity and motor output, to collectives of cells in tissues that flow and redistribute biochemical signals during development.

Actin bundle architecture and mechanics regulate myosin II force generation

The actin cytoskeleton is a soft, structural material that underlies biological processes such as cell division, motility, and cargo transport. The cross-linked actin filaments self-organize into a myriad of architectures, from disordered meshworks to ordered bundles, which are hypothesized to control the actomyosin force generation that regulates cell migration, shape, and adhesion. Here, we use fluorescence microscopy and simulations to investigate how actin bundle architectures with varying polarity, spacing, and rigidity impact myosin II dynamics and force generation. Microscopy reveals that mixed-polarity bundles formed by rigid cross-linkers support slow, bidirectional myosin II filament motion, punctuated by periods of stalled motion. Simulations reveal that these locations of stalled myosin motion correspond to sustained, high forces in regions of balanced actin filament polarity. By contrast, mixed-polarity bundles formed by compliant, large cross-linkers support fast, bidirectional motion with no traps. Simulations indicate that trap duration is directly related to force magnitude and that the observed increased velocity corresponds to lower forces resulting from both the increased bundle compliance and filament spacing. Our results indicate that the microstructures of actin assemblies regulate the dynamics and magnitude of myosin II forces, highlighting the importance of architecture and mechanics in regulating forces in biological materials.

Apical Relaxation during Mitotic Rounding Promotes Tension-Oriented Cell Division

Global tissue tension anisotropy has been shown to trigger stereotypical cell division orientation by elongating mitotic cells along the main tension axis. Yet, how tissue tension elongates mitotic cells despite those cells undergoing mitotic rounding (MR) by globally upregulating cortical actomyosin tension remains unclear. We addressed this question by taking advantage of ascidian embryos, consisting of a small number of interphasic and mitotic blastomeres and displaying an invariant division pattern. We found that blastomeres undergo MR by locally relaxing cortical tension at their apex, thereby allowing extrinsic pulling forces from neighboring interphasic blastomeres to polarize their shape and thus division orientation. Consistently, interfering with extrinsic forces by reducing the contractility of interphasic blastomeres or disrupting the establishment of asynchronous mitotic domains leads to aberrant mitotic cell division orientations. Thus, apical relaxation during MR constitutes a key mechanism by which tissue tension anisotropy controls stereotypical cell division orientation.

The Dynamics of P Granule Liquid Droplets Are Regulated by the Caenorhabditis elegans Germline RNA Helicase GLH-1 via Its ATP Hydrolysis Cycle

P granules are phase-separated liquid droplets that play important roles in the maintenance of germ cell fate in Caenorhabditis elegans Both the localization and formation of P granules are highly dynamic, but mechanisms that regulate such processes remain poorly understood. Here, we show evidence that the VASA-like germline RNA helicase GLH-1 couples distinct steps of its ATPase hydrolysis cycle to control the formation and disassembly of P granules. In addition, we found that the phenylalanine-glycine-glycine repeats in GLH-1 promote its localization at the perinucleus. Proteomic analyses of the GLH-1 complex with a GLH-1 mutation that interferes with P granule disassembly revealed transient interactions of GLH-1 with several Argonautes and RNA-binding proteins. Finally, we found that defects in recruiting the P granule component PRG-1 to perinuclear foci in the adult germline correlate with the fertility defects observed in various GLH-1 mutants. Together, our results highlight the versatile roles of an RNA helicase in controlling the formation of liquid droplets in space and time.

Differential Expression of a Classic Cadherin Directs Tissue-Level Contractile Asymmetry during Neural Tube Closure

Embryos control force generation at tissue boundaries, but how they do so remains poorly understood. Here we show how tissue-specific expression of the type II cadherin, Cadherin2, patterns actomyosin contractility along tissue boundaries to control zippering and neural tube closure in the basal chordate, Ciona robusta. Cadherin2 is differentially expressed and homotypically enriched in neural cells along the neural/epidermal (Ne/Epi) boundary, where RhoA and myosin are activated during zipper progression. Homotypically enriched Cadherin2 sequesters the Rho GTPase-activating protein, Gap21/23, to homotypic junctions. Gap21/23 in turn redirects RhoA/myosin activity to heterotypic Ne/Epi junctions. By activating myosin II along Ne/Epi junctions ahead of the zipper and inhibiting myosin II along newly formed Ne/Ne junctions behind the zipper, Cadherin2 promotes tissue-level contractile asymmetry to drive zipper progression. We propose that dynamic coupling of junction exchange to local changes in contractility may control fusion and separation of epithelia in many other contexts.

RhoA Mediates Epithelial Cell Shape Changes via Mechanosensitive Endocytosis

Epithelial remodeling involves ratcheting behavior whereby periodic contractility produces transient changes in cell-cell contact lengths, which stabilize to produce lasting morphogenetic changes. Pulsatile RhoA activity is thought to underlie morphogenetic ratchets, but how RhoA governs transient changes in junction length, and how these changes are rectified to produce irreversible deformation, remains poorly understood. Here, we use optogenetics to characterize responses to pulsatile RhoA in model epithelium. Short RhoA pulses drive reversible junction contractions, while longer pulses produce irreversible junction length changes that saturate with prolonged pulse durations. Using an enhanced vertex model, we show this is explained by two effects: thresholded tension remodeling and continuous strain relaxation. Our model predicts that structuring RhoA into multiple pulses overcomes the saturation of contractility and confirms this experimentally. Junction remodeling also requires formin-mediated E-cadherin clustering and dynamin-dependent endocytosis. Thus, irreversible junction deformations are regulated by RhoA-mediated contractility, membrane trafficking, and adhesion receptor remodeling.

Genetic induction and mechanochemical propagation of a morphogenetic wave

Tissue morphogenesis arises from coordinated changes in cell shape driven by actomyosin contractions. Patterns of gene expression regionalize cell behaviours by controlling actomyosin contractility. Here we report two modes of control over Rho1 and myosin II (MyoII) activation in the Drosophila endoderm. First, Rho1-MyoII are induced in a spatially restricted primordium via localized transcription of the G-protein-coupled receptor ligand Fog. Second, a tissue-scale wave of Rho1-MyoII activation and cell invagination progresses anteriorly away from the primordium. The wave does not require sustained gene transcription, and is not governed by regulated Fog delivery. Instead, MyoII inhibition blocks Rho1 activation and propagation, revealing a mechanical feedback driven by MyoII. We find that MyoII activation and invagination in each row of cells drives adhesion to the vitelline membrane mediated by integrins, apical spreading, MyoII activation and invagination in the next row. Endoderm morphogenesis thus emerges from local transcriptional initiation and a mechanically driven cycle of cell deformation.

Dynamic interplay of cell fate, polarity and force generation in ascidian embryos

A fundamental challenge in developmental biology is to understand how forces produced by individual cells are patterned in space and time and then integrated to produce stereotyped changes in tissue-level or embryo-level morphology. Ascidians offer a unique opportunity to address this challenge by studying how small groups of cells collectively execute complex, but highly stereotyped morphogenetic movements. Here we highlight recent progress and open questions in the study of ascidian morphogenesis, emphasizing the dynamic interplay of cell fate determination, cellular force generation and tissue-level mechanics.

The PAR proteins: from molecular circuits to dynamic self-stabilizing cell polarity

PAR proteins constitute a highly conserved network of scaffolding proteins, adaptors and enzymes that form and stabilize cortical asymmetries in response to diverse inputs. They function throughout development and across the metazoa to regulate cell polarity. In recent years, traditional approaches to identifying and characterizing molecular players and interactions in the PAR network have begun to merge with biophysical, theoretical and computational efforts to understand the network as a pattern-forming biochemical circuit. Here, we summarize recent progress in the field, focusing on recent studies that have characterized the core molecular circuitry, circuit design and spatiotemporal dynamics. We also consider some of the ways in which the PAR network has evolved to polarize cells in different contexts and in response to different cues and functional constraints.

Protein Clustering Shapes Polarity Protein Gradients

In this issue of Developmental Cell, Dickinson et al. (2017) and Rodriguez et al. (2017), along with Wang et al. (2017) in Nature Cell Biology, show how PAR protein oligomerization can dynamically couple protein diffusion and transport by cortical flow to control kinase activity gradients and polarity in the C. elegans zygote.

Isoforms Confer Characteristic Force Generation and Mechanosensation by Myosin II Filaments

Myosin II isoforms with varying mechanochemistry and filament size interact with filamentous actin (F-actin) arrays to generate contractile forces in muscle and nonmuscle cells. How myosin II force production is shaped by isoform-specific motor properties and environmental stiffness remains poorly understood. Here, we used computer simulations to analyze force production by an ensemble of myosin motors against an elastically tethered actin filament. We found that force output depends on two timescales: the duration of F-actin attachment, which varies sharply with the ensemble size, motor duty ratio, and external load; and the time to build force, which scales with the ensemble stall force, gliding speed, and environmental stiffness. Although force-dependent kinetics were not required to sense changes in stiffness, the myosin catch bond produced positive feedback between the attachment time and force to trigger switch-like transitions from transient attachments, generating small forces, to high-force-generating runs. Using parameters representative of skeletal muscle myosin, nonmuscle myosin IIB, and nonmuscle myosin IIA revealed three distinct regimes of behavior, respectively: 1) large assemblies of fast, low-duty ratio motors rapidly build stable forces over a large range of environmental stiffness; 2) ensembles of slow, high-duty ratio motors serve as high-affinity cross-links with force buildup times that exceed physiological timescales; and 3) small assemblies of low-duty ratio motors operating at intermediate speeds are poised to respond sharply to changes in mechanical context-at low force or stiffness, they serve as low-affinity cross-links, but they can transition to force production via the positive-feedback mechanism described above. Together, these results reveal how myosin isoform properties may be tuned to produce force and respond to mechanical cues in their environment.

Clustering of low-valence particles: structure and kinetics

We compute the structure and kinetics of two systems of low-valence particles with three or six freely oriented bonds in two dimensions. The structure of clusters formed by trivalent particles is complex with loops and holes, while hexavalent particles self-organize into regular and compact structures. We identify the elementary structures which compose the clusters of trivalent particles. At initial stages of clustering, the clusters of trivalent particles grow with a power-law time dependence. Yet at longer times fusion and fission of clusters equilibrates and clusters form a heterogeneous phase with polydispersed sizes. These results emphasize the role of valence in the kinetics and stability of finite-size clusters.

Ray transfer matrix for a spiral phase plate

We present a ray transfer matrix for a spiral phase plate. Using this matrix we determine the stability of an optical resonator made of two spiral phase plates and trace stable ray orbits in the resonator. Our results should be relevant to laser physics, optical micromanipulation, quantum information, and optomechanics.

Bond flexibility and low valence promote finite clusters of self-aggregating particles

Systems of complex particles such as proteins or colloidal particles have a widely observed tendency to form nonconnected nanometer-size clusters at steady state, but the underlying mechanisms remain poorly understood. We report here a numerical study on the self-aggregation of low-valence particles with flexible bonds (i.e., free bond orientations) in two dimensions and predict the formation of a stable cluster phase for average valences ranging from 2 to 3.6. For the intermediate case of trivalent particles, we show that a cluster phase is present over a wide range of concentrations and interaction energies. The clusters are polydisperse in size, have a fractal dimension of 1.5, and tend to fully saturate their bonds at high interaction energies. The number of unformed bonds scales linearly with the number of particles in a cluster, which implies the absence of phase transition in the explored region of interaction energies and concentrations. We discuss possible implications of our model for membrane protein clustering.

Force generation, transmission, and integration during cell and tissue morphogenesis

Cell shape changes underlie a large set of biological processes ranging from cell division to cell motility. Stereotyped patterns of cell shape changes also determine tissue remodeling events such as extension or invagination. In vitro and cell culture systems have been essential to understanding the fundamental physical principles of subcellular mechanics. These are now complemented by studies in developing organisms that emphasize how cell and tissue morphogenesis emerge from the interplay between force-generating machines, such as actomyosin networks, and adhesive clusters that transmit tensile forces at the cell cortex and stabilize cell-cell and cell-substrate interfaces. Both force production and transmission are self-organizing phenomena whose adaptive features are essential during tissue morphogenesis. A new era is opening that emphasizes the similarities of and allows comparisons between distant dynamic biological phenomena because they rely on core machineries that control universal features of cytomechanics.

Quantitative variation in autocrine signaling and pathway crosstalk in the Caenorhabditis vulval network

BACKGROUND

Biological networks experience quantitative change in response to environmental and evolutionary variation. Computational modeling allows exploration of network parameter space corresponding to such variations. The intercellular signaling network underlying Caenorhabditis vulval development specifies three fates in a row of six precursor cells, yielding a quasi-invariant 3°3°2°1°2°3° cell fate pattern. Two seemingly conflicting verbal models of vulval precursor cell fate specification have been proposed: sequential induction by the EGF-MAP kinase and Notch pathways, or morphogen-based induction by the former.

RESULTS

To study the mechanistic and evolutionary system properties of this network, we combine experimental studies with computational modeling, using a model that keeps the network architecture constant but varies parameters. We first show that the Delta autocrine loop can play an essential role in 2° fate specification. With this autocrine loop, the same network topology can be quantitatively tuned to use in the six-cell-row morphogen-based or sequential patterning mechanisms, which may act singly, cooperatively, or redundantly. Moreover, different quantitative tunings of this same network can explain vulval patterning observed experimentally in C. elegans, C. briggsae, C. remanei, and C. brenneri. We experimentally validate model predictions, such as interspecific differences in isolated vulval precursor cell behavior and in spatial regulation of Notch activity.

CONCLUSIONS

Our study illustrates how quantitative variation in the same network comprises developmental patterning modes that were previously considered qualitatively distinct and also accounts for evolution among closely related species.

Cellular symmetry breaking during Caenorhabditis elegans development

The nematode worm Caenorhabditis elegans has produced a wellspring of insights into mechanisms that govern cellular symmetry breaking during animal development. Here we focus on two highly conserved systems that underlie many of the key symmetry-breaking events that occur during embryonic and larval development in the worm. One involves the interplay between Par proteins, Rho GTPases, and the actomyosin cytoskeleton and mediates asymmetric cell divisions that establish the germline. The other uses elements of the Wnt signaling pathway and a highly reiterative mechanism that distinguishes anterior from posterior daughter cell fates. Much of what we know about these systems comes from intensive study of a few key events-Par/Rho/actomyosin-mediated polarization of the zygote in response to a sperm-derived cue and the Wnt-mediated induction of endoderm at the four-cell stage. However, a growing body of work is revealing how C. elegans exploits elements/variants of these systems to accomplish a diversity of symmetry-breaking tasks throughout embryonic and larval development.

Sequential activation of apical and basolateral contractility drives ascidian endoderm invagination

BACKGROUND

Epithelial invagination is a fundamental morphogenetic behavior that transforms a flat cell sheet into a pit or groove. Previous studies of invagination have focused on the role of actomyosin-dependent apical contraction; other mechanisms remain largely unexplored.

RESULTS

We combined experimental and computational approaches to identify a two-step mechanism for endoderm invagination during ascidian gastrulation. During Step 1, which immediately precedes invagination, endoderm cells constrict their apices because of Rho/Rho-kinase-dependent apical enrichment of 1P-myosin. Our data suggest that endoderm invagination itself occurs during Step 2, without further apical shrinkage, via a novel mechanism we call collared rounding: Rho/Rho-kinase-independent basolateral enrichment of 1P-myosin drives apico-basal shortening, whereas Rho/Rho-kinase-dependent enrichment of 1P and 2P myosin in circumapical collars is required to prevent apical expansion and for deep invagination. Simulations show that boundary-specific tension values consistent with these distributions of active myosin can explain the cell shape changes observed during invagination both in normal embryos and in embryos treated with pharmacological inhibitors of either Rho-kinase or Myosin II ATPase. Indeed, we find that the balance of strong circumapical and basolateral tension is the only mechanism based on differential cortical tension that can explain ascidian endoderm invagination. Finally, simulations suggest that mesectoderm cells resist endoderm shape changes during both steps, and we confirm this prediction experimentally.

CONCLUSIONS

Our findings suggest that early ascidian gastrulation is driven by the coordinated apposition of circumapical and lateral endoderm contraction, working against a resisting mesectoderm. We propose that similar mechanisms may operate during other invaginations.

Prognosis and comorbidities in stage III and IV endometrial cancer

e16553

Background: While endometrial cancer may be associated with many comorbid conditions, none have been characterized as changing overall prognosis. The aim of this study was to identify medical conditions or laboratory values, that may serve as prognostic factors in stage III and IV endometrial cancer. Methods: A retrospective chart review identified 112 women with stage III or IV endometrial cancer between years 1993–1998. Information about medical comorbidities and presenting lab values were collected using electronic medical records. Progression free survival (PFS) and overall survival (OS) were analyzed using the Kaplan-Meier survival method and the log rank test. Results: The average age was 64.9 yrs. 79 women (70.5%) had stage III disease and 33 women (29.5%) had stage IV disease. For those with a baseline creatinine <1.2 (n = 91), the PFS and OS were not significantly different from those with a baseline creatinine ≥1.2 (n = 17; p = 0.554 and p = 0.487, respectively). There was a non-significant trend toward worse PFS for the 41 patients with hypertension (HTN) compared to the 62 without (48.0 and 61.2 months, p = 0.191). The overall survival was significantly worse for those with HTN (38.7 months vs. 56.0 months p = 0.046). For those with known coronary artery disease, no significant difference in PFS or OS was found (p = 0.792 and p = 0.312 respectively). Those with diabetes (n = 15) did not have a significantly different PFS compared to those who did not (n = 88; p = 0.728). The OS was worse at 20.1 months for those with diabetes compared to 54.3 months for those without (p = 0.001). Baseline albumin had a significant effect on both PFS and OS. Those with an albumin <3.5 (n = 54) had a PFS of 46.2 months compared to 94.0 months for those with an albumin ≥3.5 (n = 23; p = 0.007), and the OS for those with low albumin was 44.8 months compared to 83.4 months for those with the higher albumin (p = 0.005). Conclusions: These results suggest that past medical history and some baseline laboratory values may be important in considering prognosis. In particular, patients with a history of HTN or diabetes have a worse overall survival. Those with a baseline albumin of <3.5 have a worse PFS and OS.

No significant financial relationships to disclose.

Pushing the frontiers of development

A joint meeting of the Japanese and French societies for Developmental Biology, entitled `Frontiers in Developmental Biology', was recently held in Giens, France. The organizers, Patrick Lemaire and Shinichi Aizawa, showcased some of the rapid progress in the field that has been made possible through the use of modern large-scale network analyses, and of an increasingly sophisticated array of tools and ideas from microscopy, mathematics and computer science.

FGF3 in the floor plate directs notochord convergent extension in the Ciona tadpole

Convergent extension (CE) is the narrowing and lengthening of an embryonic field along a defined axis. It underlies a variety of complex morphogenetic movements, such as mesoderm elongation and neural tube closure in vertebrate embryos. Convergent extension relies on the same intracellular molecular machinery that directs planar cell polarity (PCP) in epithelial tissues, including non-canonical Wnt signaling components. However, it is not known what signals coordinate CE movements across cell fields. In the simple chordate Ciona intestinalis, the notochord plate consists of just 40 cells, which undergo mediolateral convergence (intercalation) to form a single cell row. Here we present evidence that a localized source of FGF3 in the developing nerve cord directs notochord intercalation through non-MAPK signaling. A dominant-negative form of the Ciona FGF receptor suppresses the formation of polarized actin-rich protrusions in notochord cells, resulting in defective notochord intercalation. Inhibition of Ciona FGF3 activity results in similar defects, even though it is expressed in an adjacent tissue: the floor plate of the nerve cord. In Xenopus mesoderm explants, inhibiting FGF signaling perturbs CE and disrupts membrane localization of Dishevelled (Dsh), a key regulator of PCP and CE. We propose that FGF signaling coordinates CE movements by regulating PCP pathway components such as Dsh.

Processing bodies and germ granules are distinct RNA granules that interact in C. elegans embryos

In somatic cells, untranslated mRNAs accumulate in cytoplasmic foci called processing bodies or P-bodies. P-bodies contain complexes that inhibit translation and stimulate mRNA deadenylation, decapping, and decay. Recently, certain P-body proteins have been found in germ granules, RNA granules specific to germ cells. We have investigated a possible connection between P-bodies and germ granules in Caenorhabditis elegans. We identify PATR-1, the C. elegans homolog of the yeast decapping activator Pat1p, as a unique marker for P-bodies in C. elegans embryos. We find that P-bodies are inherited maternally as core granules that mature differently in somatic and germline blastomeres. In somatic blastomeres, P-bodies recruit the decapping activators LSM-1 and LSM-3. This recruitment requires the LET-711/Not1 subunit of the CCR4-NOT deadenylase and correlates spatially and temporally with the onset of maternal mRNA degradation. In germline blastomeres, P-bodies are maintained as core granules lacking LSM-1 and LSM-3. P-bodies interact with germ granules, but maintain distinct dynamics and components. The maternal mRNA nos-2 is maintained in germ granules, but not in P-bodies. We conclude that P-bodies are distinct from germ granules, and represent a second class of RNA granules that behaves differently in somatic and germline cells.

Asymmetric Cell Division: A CAB Driver for Spindle Movements

To divide asymmetrically, a cell must position the mitotic spindle relative to localized cell fate determinants. Recent work in the early ascidian embryo reveals the function of a single factor that coordinates this act to control cleavage pattern and cell fate determination.

Cellular morphogenesis in ascidians: how to shape a simple tadpole

Ascidians are invertebrate chordates that form tadpole larvae with a surprisingly small number of cells. Recently, the emergence of powerful molecular tools to study cell fate determination in ascidians has been complemented by studies, often at cellular resolution, of morphogenetic processes. These studies point to a complex interplay among mechanisms that control cell fate and polarity and those that govern cell shape change and morphogenesis. The relative simplicity and stereotypy of ascidian development suggests that it will be possible to understand, and possibly to mathematically model, this dynamic coupling between cell fate and shape change.

Conditional dominant mutations in the Caenorhabditis elegans gene act-2 identify cytoplasmic and muscle roles for a redundant actin isoform

Animal genomes each encode multiple highly conserved actin isoforms that polymerize to form the microfilament cytoskeleton. Previous studies of vertebrates and invertebrates have shown that many actin isoforms are restricted to either nonmuscle (cytoplasmic) functions, or to myofibril force generation in muscle cells. We have identified two temperature-sensitive and semidominant embryonic-lethal Caenorhabditis elegans mutants, each with a single mis-sense mutation in act-2, one of five C. elegans genes that encode actin isoforms. These mutations alter conserved and adjacent amino acids predicted to form part of the ATP binding pocket of actin. At the restrictive temperature, both mutations resulted in aberrant distributions of cortical microfilaments associated with abnormal and striking membrane ingressions and protrusions. In contrast to the defects caused by these dominant mis-sense mutations, an act-2 deletion did not result in early embryonic cell division defects, suggesting that additional and redundant actin isoforms are involved. Accordingly, we found that two additional actin isoforms, act-1 and act-3, were required redundantly with act-2 for cytoplasmic function in early embryonic cells. The act-1 and -3 genes also have been implicated previously in muscle function. We found that an ACT-2::GFP reporter was expressed cytoplasmically in embryonic cells and also was incorporated into contractile filaments in adult muscle cells. Furthermore, one of the dominant act-2 mutations resulted in uncoordinated adult movement. We conclude that redundant C. elegans actin isoforms function in both muscle and nonmuscle contractile processes.

Cortical flows powered by asymmetrical contraction transport PAR proteins to establish and maintain anterior-posterior polarity in the early C. elegans embryo

The C. elegans PAR proteins PAR-3, PAR-6, and PKC-3 are asymmetrically localized and have essential roles in cell polarity. We show that the one-cell C. elegans embryo contains a dynamic and contractile actomyosin network that appears to be destabilized near the point of sperm entry. This asymmetry initiates a flow of cortical nonmuscle myosin (NMY-2) and F-actin toward the opposite, future anterior, pole. PAR-3, PAR-6, and PKC-3, as well as non-PAR proteins that associate with the cytoskeleton, appear to be transported to the anterior by this cortical flow. In turn, PAR-3, PAR-6, and PKC-3 modulate cortical actomyosin dynamics and promote cortical flow. PAR-2, which localizes to the posterior cortex, inhibits NMY-2 from accumulating at the posterior cortex during flow, thus maintaining asymmetry by preventing inappropriate, posterior-directed flows. Similar actomyosin flows accompany the establishment of PAR asymmetries that form after the one-cell stage, suggesting that actomyosin-mediated cortical flows have a general role in PAR asymmetry.

Robustness, flexibility, and the role of lateral inhibition in the neurogenic network

BACKGROUND

Many gene networks used by developing organisms have been conserved over long periods of evolutionary time. Why is that? We showed previously that a model of the segment polarity network in Drosophila is robust to parameter variation and is likely to act as a semiautonomous patterning module. Is this true of other networks as well?

RESULTS

We present a model of the core neurogenic network in Drosophila. Our model exhibits at least three related pattern-resolving behaviors that the real neurogenic network accomplishes during embryogenesis in Drosophila. Furthermore, we find that it exhibits these behaviors across a wide range of parameter values, with most of its parameters able to vary more than an order of magnitude while it still successfully forms our test patterns. With a single set of parameters, different initial conditions (prepatterns) can select between different behaviors in the network's repertoire. We introduce two new measures for quantifying network robustness that mimic recombination and allelic divergence and use these to reveal the shape of the domain in the parameter space in which the model functions. We show that lateral inhibition yields robustness to changes in prepatterns and suggest a reconciliation of two divergent sets of experimental results. Finally, we show that, for this model, robustness confers functional flexibility.

CONCLUSIONS

The neurogenic network is robust to changes in parameter values, which gives it the flexibility to make new patterns. Our model also offers a possible resolution of a debate on the role of lateral inhibition in cell fate specification.

Inter-individual differences in repair of DNA base oxidation, measured in vitro with the comet assay

There is a need for a reliable, robust and sensitive assay for DNA repair, suitable for use with human lymphocyte samples in molecular epidemiological investigations. The comet assay (single cell alkaline gel electrophoresis) has been modified to measure the ability of a simple subcellular extract of lymphocytes to carry out the initial step of repair, i.e. incision, on a DNA substrate carrying specific lesions--namely, oxidized bases introduced by visible light in the presence of photosensitizer. The cell extract is free of non-specific nuclease activity, incising DNA only if the DNA has been treated with photosensitizer and light. The activity varies between individuals, but consistency is seen between samples from each individual taken on occasions several months apart. The lack of activity of extract from Ogg1(-) mouse cells (deficient in the glycosylase that excises 8-oxoguanine) in this assay confirms that the activity measured is predominantly excision repair of oxidized bases. This new DNA repair assay is simple, rapid and requires only small quantities of lymphocyte extract (obtainable from 10 ml blood).

Resolution of skin lesions and long-term survival in a dog with superficial necrolytic dermatitis and liver cirrhosis

A nine-year-old, neutered female Shetland sheepdog was presented with crusted, ulcerative skin lesions affecting the footpads, commissures of the lips and the lateral canthi of the eyes. Histopathological examination of skin biopsies revealed changes consistent with superficial necrolytic dermatitis and biochemical analysis demonstrated elevated liver enzymes. Abdominal radiography revealed a small liver which, on ultrasonography, appeared diffusely mottled and showed changes suggestive of periportal fibrosis. On exploratory laparotomy, the pancreas appeared normal, but the liver was small and had multiple nodules throughout the parenchyma. This appearance was confirmed as cirrhosis on histopathological examination. The dog was placed on a hepatic support diet and treated with colchicine, essential fatty acid supplementation and raw egg yolks. After four weeks, the skin lesions had resolved and the dog remained free of clinical signs over a 22-month follow-up period.

Role of risk assessment in reducing homicides by people with mental illness

BACKGROUND

Improved risk assessment has been stressed as the way to reduce homicides by people with mental illness. The feasibility of predicting rare events needs examining.

AIMS

To examine the findings of public inquiries into homicides by people with mental illness to see if they support the claim that better risk assessment would have averted the tragedy.

METHOD

Analysis was made of the findings of the public inquiries between 1988 and 1997 in relation to the predictability and preventability of the homicides.

RESULTS

Of the homicides considered by the inquiry panels, 27.5% were judged to have been predictable, 65% preventable and 60% of the patients had a long-term history containing violence or substantial risk factors for violence.

CONCLUSIONS

Improved risk assessment has only a limited role in reducing homicides. More deaths could be prevented by improved mental health care irrespective of the risk of violence. If services become biased towards those assessed as high risk, then ethical concerns arise about the care of both violent and non-violent patients.

Modularity in animal development and evolution: elements of a conceptual framework for EvoDevo

For at least a century biologists have been talking, mostly in a black-box sense, about developmental mechanisms. Only recently have biologists succeeded broadly in fishing out the contents of these black boxes. Unfortunately the view from inside the black box is almost as obscure as that from without, and developmental biologists increasingly confront the need to synthesize known facts about developmental phenomena into mechanistic descriptions of complex systems. To evolutionary biologists, the emerging understanding of developmental mechanisms is an opportunity to understand the origins of variation not just in the selective milieu but also in the variability of the developmental process, the substrate for morphological change. Ultimately, evolutionary developmental biology (EvoDevo) expects to articulate how the diversity of organic form results from adaptive variation in development. This ambition demands a shift in the way biologists describe causality, and the central problem of EvoDevo is to understand how the architecture of development confers evolvability. We argue in this essay that it makes little sense to think of this question in terms of individual gene function or isolated morphometrics, but rather in terms of higher-order modules such as gene networks and homologous characters. We outline the conceptual challenges raised by this shift in perspective, then present a selection of case studies we believe to be paradigmatic for how biologists think about modularity in development and evolution. J. Exp. Zool. (Mol. Dev. Evol.) 285:307-325, 1999.

Common errors of reasoning in child protection work

OBJECTIVE

Repeated public inquiries into child abuse tragedies in Britain demonstrate the level of public concern about the services designed to protect children. These inquiries identify faults in professionals' practice but the similarities in their findings indicate that they are having insufficient impact on improving practice. This study is based on the hypothesis that the recurrent errors may be explicable as examples of the typical errors of human reasoning identified by psychological research.

METHODS

The sample comprised all child abuse inquiry reports published in Britain between 1973 and 1994 (45 in total). Using a content analysis and a framework derived from psychological research on reasoning, a study was made of the reasoning of the professionals involved and the findings of the inquiries.

RESULTS

It was found that professionals based assessments of risk on a narrow range of evidence. It was biased towards the information readily available to them, overlooking significant data known to other professionals. The range was also biased towards the more memorable data, that is, towards evidence that was vivid, concrete, arousing emotion and either the first or last information received. The evidence was also often faulty, due, in the main, to biased or dishonest reporting or errors in communication. A critical attitude to evidence was found to correlate with whether or not the new information supported the existing view of the family. A major problem was that professionals were slow to revise their judgements despite a mounting body of evidence against them.

CONCLUSIONS

Errors in professional reasoning in child protection work are not random but predictable on the basis of research on how people intuitively simplify reasoning processes in making complex judgements. These errors can be reduced if people are aware of them and strive consciously to avoid them. Aids to reasoning need to be developed that recognize the central role of intuitive reasoning but offer methods for checking intuitive judgements more rigorously and systematically.

Depolarization and cAMP elevation rapidly recruit TrkB to the plasma membrane of CNS neurons

Here, we describe a novel mechanism for the rapid regulation of surface levels of the neurotrophin receptor TrkB. Unlike nodose ganglion neurons, both retinal ganglion cells (RGCs) and spinal motor neurons (SMNs) in culture display only low levels of surface TrkB, though high levels are present intracellularly. Within minutes of depolarization or cAMP elevation, surface TrkB levels increase by nearly 4-fold, and this increase is not blocked by cycloheximide. These findings suggest that activity and cAMP elevation rapidly recruit TrkB to the plasma membrane by translocation from intracellular stores. We propose that a fundamental difference between peripheral nervous system (PNS) and central nervous system (CNS) neurons is the activity dependence of CNS neurons for responsiveness to their peptide trophic factors and that differences in membrane compartmentalization of the receptors underlie this difference.

Changing the response of professionals to child abuse

Britain has developed a child protection system that relies on good interprofessional communication. However, some doctors are questioning the benefits to the child of triggering a child abuse referral. The system has become disproportionately skewed towards investigation and risk assessment, leaving few resources for meeting the needs of children, or helping parents provide better care. The Department of Health (DoH) is proposing a policy to redress the balance; creating a more cooperative and less adversarial relationship with parents, and paying more attention to assessing family needs and long-term family functioning. This paper examines the history of the current system and argues that, while the proposed changes are desirable, it needs to be acknowledged that they may reduce the accuracy of risk assessments. Professionals, therefore, need the backing of the general public to implement such a fundamental shift in emphasis.

Vein graft stenosis and the heparin responsiveness of human vascular smooth muscle cells

BACKGROUND

Vascular smooth muscle cell (VMSC) proliferation is an essential component of myointimal hyperplasia, which is implicated in the failure of 30% to 50% of vascular interventions, such as coronary angioplasty and peripheral vein grafting. We have shown that cells derived from stenotic lesions in infrainguinal vein grafts were significantly more resistant than controls to growth inhibition by heparin.

METHODS AND RESULTS

In a prospective study, we correlated antiproliferative responses to heparin in vitro with graft patency after 1 year. Sixty-two patients with infrainguinal vein grafts were entered into a graft surveillance program for > or = 1 year. At operation, saphenous vein segments were explanted for VSMC culture. Cell proliferation in response to fetal calf serum was later determined in the presence and absence of heparin. In 35 cell cultures, including 13 from the above-mentioned patients, [3H]heparin binding was also estimated. VSMCs from patients with patent grafts were significantly more sensitive to growth inhibition by heparin than cells from patients with stenoses (median, 54% versus 20.9%, P<0.001), and [3H]heparin binding was strongly correlated with inhibition of proliferation (r=0.81).

CONCLUSIONS

Responsiveness to heparin in cultured VSMCs is a strong predictor of outcome for infrainguinal vein grafts, and reduced sensitivity to heparin is correlated with decreased heparin binding. Relative resistance to the antiproliferative action of heparin may be a marker for aberrant regulation of VSMC growth.

Inhibition of human vascular smooth muscle cell proliferation by lovastatin: the role of isoprenoid intermediates of cholesterol synthesis

Restenosis remains the largest single obstacle to the long-term success of invasive vascular interventions. Lovastatin, an HMG-CoA reductase inhibitor, has been shown to reduce myointimal hyperplasia in animal models of restenosis and in one clinical coronary restenosis trial. We have assessed the effect of lovastatin on the growth of cultured human vascular smooth muscle cells derived from saphenous vein and vascular graft stenoses. Lovastatin (2 microM) inhibited proliferation over 14 days in saphenous vein (and graft stenoses) derived vascular smooth muscle cells by 42% and 32% respectively: this was not significantly different. Lovastatin (10 microM) reduced [methyl 3H]-thymidine uptake by 51% in saphenous vein-derived cells. These concentrations were significantly higher than those achieved in plasma during therapeutic dosage. Lovastatin-induced inhibition of vascular smooth muscle cell proliferation and [methyl 3H]-thymidine uptake was completely reversed by adding mevalonate (100 microM) but cholesterol (10-40 micrograms ml-1) had no effect. Isopentenyl adenine (25-50 microM) did not affect the inhibition of [methyl 3H]-thymidine uptake by lovastatin (10 microM), but farnesol (20 microM), another isoprenoid precursor of cholesterol synthesis, reversed the antiproliferative effect.

Consistent responses of the human vascular smooth muscle cell in culture: implications for restenosis

PURPOSE

The mechanisms whereby restenoses occur at discrete sites within the vasculature remain uncertain. We have recently reported that vascular smooth muscle cells (VSMC) derived from patients with graft stenoses are resistant to growth inhibition by heparin. In this study, we have examined whether VSMC proliferation rates and responses to inhibition by heparin vary according to the individual or the anatomic site of origin.

METHODS

Long saphenous veins from seven patients were divided into proximal, middle, and distal portions, and VSMC were cultured separately from each. VSMC proliferation in response to 15% fetal calf serum +/- 100 micrograms/ml heparin was measured by counting triplicate samples at 0, 3, 7, 10, and 14 days. This experiment was repeated from the second to the sixth passage (n = 6) and for artery and vein pairs derived from four additional patients.

RESULTS

Differences between vein segment cultures of individual veins were found not to differ significantly from experimental error for either proliferation or heparin inhibition and were not altered by repeated passage (ANOVA). There were, however, significant differences in sensitivity to heparin inhibition between patients (p = 0.02) (ANOVA). There were no significant differences between paired samples of artery and vein for either proliferation or heparin inhibition (Mann-Whitney test).

CONCLUSIONS

VSMC growth characteristics reflect the individual patient and are maintained in cell culture.

Growth inhibition of human vascular smooth muscle cells by fenofibrate: a possible therapy for restenosis

OBJECTIVE

The aim was to assess the growth inhibitory effect of fibrates on human vascular smooth muscle cells. Restenosis is the most important factor limiting the long term success of invasive vascular interventions and there is as yet no effective preventive treatment. Platelet derived growth factor (PDGF) is considered to be an important growth promoting agent for vascular smooth muscle cells (VSMC) and fenofibric acid (a hypolipidaemic drug) has been reported to be a PDGF antagonist.

METHODS

The effect of the fibrate drugs fenofibrate, clofibrate, bezafibrate, and gemfibrozil were examined on the proliferation of cultured human vascular smooth muscle cells derived from saphenous vein (n = 20) and graft stenoses (n = 7).

RESULTS

Fenofibrate (100 microM) produced potent inhibition (48%) of VSMC proliferation at a concentration equivalent to that of its circulating metabolite fenofibric acid, but none of the other drugs produced any significant effect on growth. VSMC derived from graft stenoses were equally sensitive to inhibition as saphenous vein derived controls, in contrast to our previous work which reported that graft stenosis derived VSMC were resistant to growth inhibition by the physiological inhibitor heparin. The antiproliferative effect of fenofibrate was independent of inhibition of cellular cholesterol synthesis or toxicity. Fenofibrate inhibited VSMC growth induced by 15% fetal calf serum, PDGF, and basic fibroblast growth factor to a similar degree, indicating that it is not a specific PDGF antagonist.

CONCLUSIONS

Fenofibrate is not a specific PDGF antagonist. Fenofibric acid, one of the principal metabolites of fenofibrate, did not produce any inhibition of growth, suggesting that oral administration of fenofibrate would not be efficacious. Fenofibrate is the first potent inhibitor to be described for VSMC derived from human myo-intimal hyperplastic lesions.

Effect of calcium channel blockers on the growth of human vascular smooth muscle cells derived from saphenous vein and vascular graft stenoses

Vascular restenosis after invasive interventions is an important clinical problem for which no preventive pharmacologic therapy exists. Calcium channel blockers have been shown to inhibit myointimal hyperplasia in animal models of restenosis and in some small and flawed clinical coronary restenosis trials. We examined the inhibitory effect of amlodipine, verapamil, and diltiazem on the growth of cultured human vascular smooth muscle cells (VSMC) derived from saphenous vein (n = 20) and graft stenoses (n = 7), in 14-day proliferation assays and [methyl 3H]thymidine uptake studies. Amlodipine and verapamil produced significant inhibition (30%) of VSMC proliferation and DNA synthesis at 10 microM but not at 500 nM-1 microM. To our knowledge, this is the first study to examine the antiproliferative effect of calcium channel blockers in VSMC derived from human graft stenoses. Growth inhibition of VSMC from graft stenoses was not significantly different from that of control saphenous vein-derived cells. We conclude, therefore, that calcium channel blockers inhibit human VSMC proliferation in vitro, regardless of whether the cells were grown from graft stenoses or saphenous vein. However, the concentrations at which these calcium channel blockers elicit antiproliferative effects may not be attainable during therapeutic dosing in humans.