Forces for morphogenesis investigated with laser microsurgery and quantitative modeling

Relocations of cell convergence sites and formation of pharyngula-like shapes in mechanically relaxed Xenopus embryos

Influence of the relaxation of mechanical tensions upon collective cell movements, shape formation, and expression patterns of tissue-specific genes has been studied in Xenopus laevis embryos. We show that the local relaxation of tensile stresses within the suprablastoporal area (SBA) performed at the early-midgastrula stage leads to a complete arrest of normal convergent cell intercalation towards the dorsal midline. As a result, SBA either remains nondeformed or protrudes a strip of cells migrating ventralwards along one of the lateral lips of the opened blastopore. Already, few minutes later, the tissues in the ventral lip vicinity undergo abnormal transversal contraction/longitudinal extension resulting in the abnormal cell convergence toward ventral (rather than dorsal) embryo midline. Within a day, the dorsally relaxed embryos acquire pharyngula-like shapes and often possess tail-like protrusions. Their antero-posterior and dorso-ventral polarity, as well as expression patterns of pan-neural (Sox3), muscular cardiac actin, and forebrain (Otx2) genes substantially deviate from the normal ones. We suggest that normal gastrulation is permanently controlled by mechanical stresses within the blastopore circumference. The role of tissue tensions in regulating collective cell movements and creating pharyngula-like shapes are discussed.

Mechanotransduction in development: a growing role for contractility

Mechanotransduction research has focused historically on how externally applied forces can affect cell signalling and function. A growing body of evidence suggests that contractile forces that are generated internally by the actomyosin cytoskeleton are also important in regulating cell behaviour, and suggest a broader role for mechanotransduction in biology. Although the molecular basis for these cellular forces in mechanotransduction is being pursued in cell culture, researchers are also beginning to appreciate their contribution to in vivo developmental processes. Here, we examine the role for mechanical forces and contractility in regulating cell and tissue structure and function during development.

Decrease in laser ablation threshold for epithelial tissue microsurgery in a living Drosophila embryo during dorsal closure

In this work, we use a two-photon fluorescence microscope for combined imaging and laser tissue ablation of a living Drosophila Melanogaster embryo. By using tightly focused near-infrared femtosecond pulses at MHz repetition rate and of sub-nanojoule energy we are able to produce microsurgery on the epithelial tissue within a Drosophila embryo at the final stages of its embryonic development. Ablation was performed on labelled and unlabelled embryos during and after dorsal closure. We observed that ablation of GFP-labelled tissue required lower energy deposition than unlabelled tissue ensuring that the tissue ablation is mediated by multiphoton absorption of Green Fluorescent Protein (GFP). In addition, the energy deposition to produce ablation is further decreased during dorsal closure. These results show the presence of additional tensile forces on the tissue during dorsal closure. Furthermore, an increased activity of actin near the laser wounds was observed as the tissue heals.

Kinetic and mechanical analysis of live tube morphogenesis

Ribbon is a nuclear Broad Tramtrack Bric-a-brac (BTB) -domain protein required for morphogenesis of the salivary gland and trachea. We recently showed that ribbon mutants exhibit decreased Crumbs and Rab11-coincident apical vesicles and increased apical Moesin activity and microvillar structure during tube elongation. To learn how these molecular and morphological changes affect the dynamics of tubulogenesis, we optimized an advanced two-photon microscope to enable high-resolution live imaging of the salivary gland and trachea. Live imaging revealed that ribbon mutant tissues exhibit slowed and incomplete lumenal morphogenesis, consistent with previously described apical defects. Because Moesin activity correlates with cortical stiffness, we hypothesize that ribbon mutants suffer from increased apical stiffness during morphogenesis. We develop this hypothesis through mechanical analysis, using the advantages of live imaging to construct computational elastic and analytical viscoelastic models of tube elongation, which suggest that ribbon mutant tubes exhibit three- to fivefold increased apical stiffness and twofold increased effective apical viscosity.

Nature and anisotropy of cortical forces orienting Drosophila tissue morphogenesis

The morphogenesis of developing embryos and organs relies on the ability of cells to remodel their contacts with neighbouring cells. Using quantitative modelling and laser nano-dissection, we probed the mechanics of a morphogenetic process, the elongation of Drosophila melanogaster embryos, which results from polarized cell neighbour exchanges. We show that anisotropy of cortical tension at apical cell junctions is sufficient to drive tissue elongation. We estimated its value through comparisons between in silico and in vivo data using various tissue descriptors. Nano-dissection of the actomyosin network indicates that tension is anisotropically distributed and depends on myosin II accumulation. Junction relaxation after nano-dissection also suggests that cortical elastic forces are dominant in this process. Interestingly, fluctuations in vertex position (points where three or more cells meet) facilitate neighbour exchanges. We delineate the contribution of subcellular tensile activity polarizing junction remodelling, and the permissive role of vertex fluctuations during tissue elongation.

Mechanical control of tissue morphogenesis

Mechanical forces participate in morphogenesis from the level of individual cells to whole organism patterning. This article reviews recent research that has identified specific roles for mechanical forces in important developmental events. One well defined example is that dynein-driven cilia create fluid flow that determines left-right patterning in the early mammalian embryo. Fluid flow is also important for vasculogenesis, and evidence suggests that fluid shear stress rather than fluid transport is primarily required for remodeling the early vasculature. Contraction of the actin cytoskeleton, driven by nonmuscle myosins and regulated by the Rho family GTPases, is a recurring mechanism for controlling morphogenesis throughout development, from gastrulation to cardiogenesis. Finally, novel experimental approaches suggest critical roles for the actin cytoskeleton and the mechanical environment in determining differentiation of mesenchymal stem cells. Insights into the mechanisms linking mechanical forces to cell and tissue differentiation pathways are important for understanding many congenital diseases and for developing regenerative medicine strategies.

Apoptotic force and tissue dynamics during Drosophila embryogenesis

Understanding cell morphogenesis during metazoan development requires knowledge of how cells and the extracellular matrix produce and respond to forces. We investigated how apoptosis, which remodels tissue by eliminating supernumerary cells, also contributes forces to a tissue (the amnioserosa) that promotes cell-sheet fusion (dorsal closure) in the Drosophila embryo. We showed that expression in the amnioserosa of proteins that suppress or enhance apoptosis slows or speeds dorsal closure, respectively. These changes correlate with the forces produced by the amnioserosa and the rate of seam formation between the cell sheets (zipping), key processes that contribute to closure. This apoptotic force is used by the embryo to drive cell-sheet movements during development, a role not classically attributed to apoptosis.

Actomyosin purse strings: renewable resources that make morphogenesis robust and resilient

Dorsal closure in Drosophila is a model system for cell sheet morphogenesis and wound healing. During closure two sheets of lateral epidermis move dorsally to close over the amnioserosa and form a continuous epidermis. Forces from the amnioserosa and actomyosin-rich, supracellular purse strings at the leading edges of these lateral epidermal sheets drive closure. Purse strings generate the largest force for closure and occur during development and wound healing throughout phylogeny. We use laser microsurgery to remove some or all of the purse strings from developing embryos. Free edges produced by surgery undergo characteristic responses as follows. Intact cells in the free edges, which previously had no purse string, recoil away from the incision and rapidly assemble new, secondary purse strings. Next, recoil slows, then pauses at a turning point. Following a brief delay, closure resumes and is powered to completion by the secondary purse strings. We confirm that the assembly of the secondary purse strings requires RhoA. We show that alpha-actinin alternates with nonmuscle myosin II along purse strings and requires nonmuscle myosin II for its localization. Together our data demonstrate that purse strings are renewable resources that contribute to the robust and resilient nature of closure.

Measurement techniques for cellular biomechanics in vitro

Living cells and tissues experience mechanical forces in their physiological environments that are known to affect many cellular processes. Also of importance are the mechanical properties of cells, as well as the microforces generated by cellular processes themselves in their microenvironments. The difficulty associated with studying these phenomena in vivo has led to alternatives such as using in vitro models. The need for experimental techniques for investigating cellular biomechanics and mechanobiology in vitro has fueled an evolution in the technology used in these studies. Particularly noteworthy are some of the new biomicroelectromechanical systems (Bio-MEMS) devices and techniques that have been introduced to the field. We describe some of the cellular micromechanical techniques and methods that have been developed for in vitro studies, and provide summaries of the ranges of measured values of various biomechanical quantities. We also briefly address some of our experiences in using these methods and include modifications we have introduced in order to improve them.

In vivo nanosecond laser axotomy: cavitation dynamics and vesicle transport

Nanosecond laser pulses (lambda = 355 nm) were used to cut mechanosensory neurons in Caenorhabditis elegans and motorneurons in Drosophila melanogaster larvae. A pulse energy range of 0.8-1.2 microJ and < 20 pulses in single shot mode were sufficient to generate axonal cuts. Viability post-surgery was >95% for C. elegans and 60% for Drosophila. Cavitation bubble dynamics generated due to laser-induced plasma formation were observed in vivo by time-resolved imaging in both organisms. Bubble oscillations were severely damped in vivo and cavitation dynamics were complete within 100 ns in C. elegans and 800 ns in Drosophila. We report the use of this system to study axonal transport for the first time and discuss advantages of nanosecond lasers compared to femtosecond sources for such procedures.

Emergent properties during dorsal closure in Drosophila morphogenesis

Dorsal closure is an essential stage of Drosophila development that is a model system for research in morphogenesis and biological physics. Dorsal closure involves an orchestrated interplay between gene expression and cell activities that produce shape changes, exert forces and mediate tissue dynamics. We investigate the dynamics of dorsal closure based on confocal microscopic measurements of cell shortening in living embryos. During the mid-stages of dorsal closure we find that there are fluctuations in the width of the leading edge cells but the time-averaged analysis of measurements indicate that there is essentially no net shortening of cells in the bulk of the leading edge, that contraction predominantly occurs at the canthi as part of the process for zipping together the two leading edges of epidermis and that the rate constant for zipping correlates with the rate of movement of the leading edges. We characterize emergent properties that regulate dorsal closure, i.e., a velocity governor and the coordination and synchronization of tissue dynamics.

"Developmental mechanics": cellular patterns controlled by adhesion, cortical tension and cell division

How embryos are shaped during development has inspired the work of many, embryologists, geneticists, but also mathematicians such as Turing, and physicists. Despite the inherent complexity of the problems it tackles, developmental biology has produced one of the most spectacular conceptual achievements, demonstrating that embryos are built with conserved molecules that orchestrate pattern and morphogenesis. As the logic of development now emerges, new challenges arise, such as how tissue mechanics is controlled. Quantitative approaches and computational models are essential to predict tissue organization and cell shapes. I review briefly how physical concepts have fueled this research in the past decades.

Epithelial morphogenesis in embryos: asymmetries, motors and brakes

Epithelial cells play a central role in many embryonic morphogenetic processes, during which they undergo highly coordinated cell shape changes. Here, we review some common principles that have recently emerged through genetic and cellular analyses performed mainly with invertebrate genetic models, focusing on morphogenetic processes involving epithelial sheets. All available data argue that myosin II is the main motor that induces cell shape changes during morphogenesis. We discuss the control of myosin II activity during epithelial morphogenesis, as well as the recently described involvement of microtubules in this process. Finally, we examine how forces unleashed by myosin II can be measured, how embryos use specific brakes to control molecular motors and the potential input of mechano-sensation in morphogenesis.

Enabled plays key roles in embryonic epithelial morphogenesis in Drosophila

Studies in cultured cells and in vitro have identified many actin regulators and begun to define their mechanisms of action. Among these are Enabled (Ena)/VASP proteins, anti-Capping proteins that influence fibroblast migration, growth cone motility, and keratinocyte cell adhesion in vitro. However, partially redundant family members in mammals and maternal Ena contribution in Drosophila previously prevented assessment of the roles of Ena/VASP proteins in embryonic morphogenesis in flies or mammals. We used several approaches to remove maternal and zygotic Ena function, allowing us to address this question. We found that inactivating Ena does not disrupt cell adhesion or epithelial organization, suggesting its role in these processes is cell type-specific. However, Ena plays an important role in many morphogenetic events, including germband retraction, segmental groove retraction and head involution, whereas it is dispensable for other morphogenetic movements. We focused on dorsal closure, analyzing mechanisms by which Ena acts. Ena modulates filopodial number and length, thus influencing the speed of epithelial zippering and the ability of cells to match with correct neighbors. We also explored filopodial regulation in cultured Drosophila cells and embryos. These data provide new insights into developmental and mechanistic roles of this important actin regulator.

Biophysical response to pulsed laser microbeam-induced cell lysis and molecular delivery

Cell lysis and molecular delivery in confluent monolayers of PtK(2) cells are achieved by the delivery of 6 ns, lambda = 532 nm laser pulses via a 40x, 0.8 NA microscope objective. With increasing distance from the point of laser focus we find regions of (a) immediate cell lysis; (b) necrotic cells that detach during the fluorescence assays; (c) permeabilized cells sufficient to facilitate the uptake of small (3 kDa) FITC-conjugated Dextran molecules in viable cells; and (d) unaffected, viable cells. The spatial extent of cell lysis, cell detachment, and molecular delivery increased with laser pulse energy. Hydrodynamic analysis from time-resolved imaging studies reveal that the maximum wall shear stress associated with the pulsed laser microbeam-induced cavitation bubble expansion governs the location and spatial extent of each of these regions independent of laser pulse energy. Specifically, cells exposed to maximum wall shear stresses tau(w, max) > 190 +/- 20 kPa are immediately lysed while cells exposed to tau(w, max) > 18 +/- 2 kPa are necrotic and subsequently detach. Cells exposed to tau(w, max) in the range 8-18 kPa are viable and successfully optoporated with 3 kDa Dextran molecules. Cells exposed to tau(w, max) < 8 +/- 1 kPa remain viable without molecular delivery. These findings provide the first direct correlation between pulsed laser microbeam-induced shear stresses and subsequent cellular outcome.

Zasp is required for the assembly of functional integrin adhesion sites

The integrin family of heterodimeric transmembrane receptors mediates cell–matrix adhesion. Integrins often localize in highly organized structures, such as focal adhesions in tissue culture and myotendinous junctions in muscles. Our RNA interference screen for genes that prevent integrin-dependent cell spreading identifies Z band alternatively spliced PDZ-motif protein (zasp), encoding the only known Drosophila melanogaster Alp/Enigma PDZ-LIM domain protein. Zasp localizes to integrin adhesion sites and its depletion disrupts integrin adhesion sites. In tissues, Zasp colocalizes with βPS integrin in myotendinous junctions and with α-actinin in muscle Z lines. Zasp also physically interacts with α-actinin. Fly larvae lacking Zasp do not form Z lines and fail to recruit α-actinin to the Z line. At the myotendinous junction, muscles detach in zasp mutants with the onset of contractility. Finally, Zasp interacts genetically with integrins, showing that it regulates integrin function. Our observations point to an important function for Zasp in the assembly of integrin adhesion sites both in cell culture and in tissues.

Evolutionary origin of the amnioserosa in cyclorrhaphan flies correlates with spatial and temporal expression changes of zen

Higher cyclorrhaphan flies including Drosophila develop a single extraembryonic epithelium (amnioserosa), which closes the germband dorsally. In most other insects two extraembryonic epithelia, serosa and amnion, line the inner eggshell and the ventral germband, respectively. How the two extraembryonic epithelia evolved into one is unclear. Recent studies have shown that, in the flour beetle Tribolium and in the milkweed bug Oncopeltus, the homeobox gene zerknüllt (zen) controls the fusion of the amnion with the serosa before dorsal closure. To understand the origin of the amnioserosa in evolution, we examined the expression and function of zen in the extraembryonic tissue of lower Cyclorrhapha. We show that Megaselia abdita (Phoridae) and Episyrphus balteatus (Syrphidae) develop a serosa and a dorsal amnion, suggesting that a dorsal amnion preceded the origin of the amnioserosa in evolution. Using Krüppel (Kr) and pannier (pnr) homologues of Megaselia as markers for serosal and amniotic tissue, respectively, we show that after zen RNAi all extraembryonic tissue becomes indistinguishable from amniotic cells, like in Tribolium but unlike in Drosophila, in which zen controls all aspects of extraembryonic development. Compared with Megaselia and Episyrphus, zen expression in Drosophila is extended to cells that form the amnion in lower Cyclorrhapha and is down-regulated at the developmental stage, when serosa cells in lower Cyclorrhapha begin to expand. These expression differences between species with distinct extraembryonic tissue organizations and the conserved requirement of zen for serosa development suggest that the origin of an amnioserosa-like epithelium was accompanied by expression changes of zen.

Requirements for adherens junction components in the interaction between epithelial tissues during dorsal closure in Drosophila

Dynamic interactions between epithelial sheets are a regular feature of morphogenetic processes. Dorsal closure in Drosophila relies on the coordinated movements of two epithelia, the epidermis and the amnioserosa, and provides an excellent model system for a genetic and cell biological approach. Here, we have analyzed the contribution of junctional organization of these epithelia to dorsal closure. We observe a stringent requirement for adherens junctions at the leading edge, the interface between the amnioserosa and the epidermis, for the transmission of the forces generated during the process. We also find that interactions between Armadillo and E-cadherin play an important role in maintaining the adhesion at the leading edge, revealing the particular dynamics of this interface. Our results show that regulated cell adhesion is a crucial element of the interactions that shape epithelial sheets in morphogenetic processes.

A wave of EGFR signaling determines cell alignment and intercalation in the Drosophila tracheal placode

Invagination of organ placodes converts flat epithelia into three-dimensional organs. Cell tracing in the Drosophila tracheal placode revealed that, in the 30-minute period before invagination, cells enter mitotic quiescence and form short rows that encircle the future invagination site. The cells in the rows align to form a smooth boundary (;boundary smoothing'), accompanied by a transient increase in myosin at the boundary and cell intercalation oriented in parallel with the cellular rows. Cells then undergo apical constriction and invaginate, followed by radially oriented mitosis in the placode. Prior to invagination, ERK MAP kinase is activated in an outward circular wave, with the wave front often correlating with the smoothing cell boundaries. EGFR signaling is required for myosin accumulation and cell boundary smoothing, suggesting its propagation polarizes the planar cell rearrangement in the tracheal placode, and coordinates the timing and position of intrinsic cell internalization activities.

Using Bcr-Abl to examine mechanisms by which abl kinase regulates morphogenesis in Drosophila

Signaling by the nonreceptor tyrosine kinase Abelson (Abl) plays key roles in normal development, whereas its inappropriate activation helps trigger the development of several forms of leukemia. Abl is best known for its roles in axon guidance, but Abl and its relatives also help regulate embryonic morphogenesis in epithelial tissues. Here, we explore the role of regulation of Abl kinase activity during development. We first compare the subcellular localization of Abl protein and of active Abl, by using a phosphospecific antibody, providing a catalog of places where Abl is activated. Next, we explore the consequences for morphogenesis of overexpressing wild-type Abl or expressing the activated form found in leukemia, Bcr-Abl. We find dose-dependent effects of elevating Abl activity on morphogenetic movements such as head involution and dorsal closure, on cell shape changes, on cell protrusive behavior, and on the organization of the actin cytoskeleton. Most of the effects of Abl activation parallel those caused by reduction in function of its target Enabled. Abl activation leads to changes in Enabled phosphorylation and localization, suggesting a mechanism of action. These data provide new insight into how regulated Abl activity helps direct normal development and into possible biological functions of Bcr-Abl.

Plasma and cavitation dynamics during pulsed laser microsurgery in vivo

We compare the plasma and cavitation dynamics underlying pulsed laser microsurgery in water and in fruit fly embryos (in vivo)--specifically for nanosecond pulses at 355 and 532 nm. We find two key differences. First, the plasma-formation thresholds are lower in vivo--especially at 355 nm--due to the presence of endogenous chromophores that serve as additional sources for plasma seed electrons. Second, the biological matrix constrains the growth of laser-induced cavitation bubbles. Both effects reduce the disrupted region in vivo when compared to extrapolations from measurements in water.

Dpp signalling orchestrates dorsal closure by regulating cell shape changes both in the amnioserosa and in the epidermis

During the final stages of embryogenesis, the Drosophila embryo exhibits a dorsal hole covered by a simple epithelium of large cells termed the amnioserosa (AS). Dorsal closure is the process whereby this hole is closed through the coordination of cellular activities within both the AS and the epidermis. Genetic analysis has shown that signalling through Jun N-terminal Kinase (JNK) and Decapentaplegic (Dpp), a Drosophila member of the BMP/TGF-beta family of secreted factors, controls these activities. JNK activates the expression of dpp in the dorsal-most epidermal cells, and subsequently Dpp acts as a secreted signal to control the elongation of lateral epidermis. Our analysis shows that Dpp function not only affects the epidermal cells, but also the AS. Embryos defective in Dpp signalling display defects in AS cell shape changes, specifically in the reduction of their apical surface areas, leading to defective AS contraction. Our data also demonstrate that Dpp regulates adhesion between epidermis and AS, and mediates expression of the transcription factor U-shaped in a gradient across both the AS and the epidermis. In summary, we show that Dpp plays a crucial role in coordinating the activity of the AS and its interactions with the LE cells during dorsal closure.

Cell adhesion molecule Echinoid associates with unconventional myosin VI/Jaguar motor to regulate cell morphology during dorsal closure in Drosophila

Echinoid (Ed) is a homophilic immunoglobulin domain-containing cell adhesion molecule (CAM) that localizes to adherens junctions (AJs) and cooperates with Drosophila melanogaster epithelial (DE)-cadherin to mediate cell adhesion. Here we show that Ed takes part in many processes of dorsal closure, a morphogenetic movement driven by coordinated cell shape changes and migration of epidermal cells to cover the underlying amnioserosa. Ed is differentially expressed, appearing in epidermis but not in amnioserosa cells. Ed functions independently from the JNK signaling pathway and is required to regulate cell morphology, and for assembly of actomyosin cable, filopodial protrusion and coordinated cell migration in dorsal-most epidermal cells. The effect of Ed on cell morphology requires the presence of the intracellular domain (Ed(intra)). Interestingly, Ed forms homodimers in vivo and Ed(intra) monomer directly associates with unconventional myosin VI/Jaguar (Jar) motor protein. We further show that ed genetically interacts with jar to control cell morphology. It has previously been shown that myosin VI is monomeric in vitro and that its dimeric form can associate with and travel processively along actin filaments. Thus, we propose that Ed mediates the dimerization of myosin VI/Jar in vivo which in turn regulates the reorganization and/or contraction of actin filaments to control changes in cell shape. Consistent with this, we found that ectopic ed expression in the amnioserosa induces myosin VI/Jar-dependent apical constriction of this tissue.

Three-dimensional laser microsurgery in light-sheet based microscopy (SPIM)

Advances in the life sciences rely on the ability to observe dynamic processes in live systems and in environments that mimic in-vivo situations. Therefore, new methodological developments have to provide environments that resemble physiologically and clinically relevant conditions as closely as possible. In this work, plasma-induced laser nanosurgery for three-dimensional sample manipulation and sample perturbation is combined with optically sectioning light-sheet based fluorescence microscopy (SPIM) and applied to three-dimensional biological model systems. This means: a) working with a biological system that is not confined to essentially two dimensions like cell cultures on cover glasses, b) gaining intrinsic optical sectioning capabilities by an efficient three-dimensional fluorescence imaging system, and c) using arbitrarily-shaped three-dimensional ablation-patterns by a plasma-induced laser ablation system that prevent damage to surrounding tissues. Spatial levels in our biological applications range from sub-microns during delicate ablation of single microtubules over the confined disruption of cell membranes in an MDCK-cyst to the macroscopic cutting of a millimeter-sized Zebrafish caudal fin with arbitrary three-dimensional ablation patterns. Dynamic processes like laser-induced hemocyte migration can be studied with our SPIM-microscalpel in intact, live embryos.

Traction forces exerted through N-cadherin contacts

BACKGROUND INFORMATION

Mechanical forces play an important role in the organization, growth and function of living tissues. The ability of cells to transduce mechanical signals is governed by two types of microscale structures: focal adhesions, which link cells to the extracellular matrix, and adherens junctions, which link adjacent cells through cadherins. Although many studies have examined forces induced by focal adhesions, there is little known about the role of adherens junctions in force-regulation processes. The present study focuses on the determination of force transduction through cadherins at a single cell level.

RESULTS

We characterized for the first time the distribution of forces developed by the cell through cadherin contacts. A N-cadherin (neural cadherin)-Fc chimaera, which mimicks the cell adhesion molecule N-cadherin, was immobilized on a muFSA (micro-force sensor array), comprising a dense array of vertical elastomer pillars, which were used both as a cell culture support for N-cadherin-expressing C2 myogenic cells and as detectors for force mapping. We coated the top of the pillars on which cells adhere and recruit adhesion complexes and F-actin. Individual pillar bending allowed the measurement of forces that mainly developed at the cell edge and directed toward their centre. Similar force distribution and amplitude were detected with an unrelated cell line of neuronal origin. Further comparison with forces applied by cells on pillars coated with fibronectin indicates that mechanical stresses transduced through both types of adhesions were comparable in distribution, orientation and amplitude.

CONCLUSIONS

These results present a versatile method to measure and map forces exerted by cell-cell adhesion complexes. They show that cells transduce mechanical stress through cadherin contacts which are of the same order as magnitude of those previously characterized for focal adhesions. Altogether, they emphasize the mechanotransduction role of cytoskeleton-linked adhesion receptors of the cadherin family in tissue cohesion and reshaping.

Upregulation of forces and morphogenic asymmetries in dorsal closure during Drosophila development

Tissue dynamics during dorsal closure, a stage of Drosophila development, provide a model system for cell sheet morphogenesis and wound healing. Dorsal closure is characterized by complex cell sheet movements, driven by multiple tissue specific forces, which are coordinated in space, synchronized in time, and resilient to UV-laser perturbations. The mechanisms responsible for these attributes are not fully understood. We measured spatial, kinematic, and dynamic antero-posterior asymmetries to biophysically characterize both resiliency to laser perturbations and failure of closure in mutant embryos and compared them to natural asymmetries in unperturbed, wild-type closure. We quantified and mathematically modeled two processes that are upregulated to provide resiliency--contractility of the amnioserosa and formation of a seam between advancing epidermal sheets, i.e., zipping. Both processes are spatially removed from the laser-targeted site, indicating they are not a local response to laser-induced wounding and suggesting mechanosensitive and/or chemosensitive mechanisms for upregulation. In mutant embryos, tissue junctions initially fail at the anterior end indicating inhomogeneous mechanical stresses attributable to head involution, another developmental process that occurs concomitant with the end stages of closure. Asymmetries in these mutants are reversed compared to wild-type, and inhomogeneous stresses may cause asymmetries in wild-type closure.

Measuring mechanical properties of embryos and embryonic tissues

Cell- and tissue-scale mechanics integrates numerous processes within the developing embryo to bring about both local cell movements and global tissue sculpting during morphogenesis. Rapid advances in molecular genetic approaches have outpaced quantitative approaches to study mechanics in early embryos. In this chapter, we present a device that is capable of carrying out a stress-relaxation test to measure the time-dependent elastic modulus of embryonic tissue explants a few hundreds of micrometers on a side. The device can detect forces from 25 nN to several micronewtons and allows the determination of extremely low modulii, as low as 5 Pa, found in frog embryonic tissues during gastrulation. We describe this device along with the methodology of its use and discuss the general challenges of working with embryonic tissues.

Mechanisms of laser cellular microsurgery

This chapter reviews the optics of pulsed laser microbeams and the use of basic instrumentation to provide pulsed laser microbeam capabilities within a microscope platform. Moreover, we review the principal mechanisms by which laser microbeams produce microsurgical effects in cellular targets. We discuss the principal photothermal, photomechanical, and photochemical damage mechanisms as well as their relationship to critical laser microbeam parameters, including wavelength, pulse duration, and numerical aperture. We relate this understanding of damage mechanisms to laser microbeam applications reported in the literature.

Investigating Relaxation Processes in Cells and Developing Organisms: From Cell Ablation to Cytoskeleton Nanosurgery

Dynamic microscopy of living cells and organisms alone does not reveal the high level of complexity of cellular and subcellular organization. All observable processes rely on the activity of biochemical and biophysical processes and many occur at a physiological equilibrium. Experimentally, it is not trivial to apply a perturbation that targets a specific process without perturbing the overall equilibrium of a cell. Drugs and more recently RNAi certainly have general and undesired effects on cell physiology and metabolism. In particular, they affect the entire cell. Pulsed lasers allow to severe biological tissues with a precision in the range of hundreds of nanometers and to achieve ablation on the level of a single cell or a subcellular compartment. In this chapter, we present an efficient implementation of a picosecond UV-A pulsed laser-based nanosurgery system and review the different mechanisms of ablation that can be achieved at different levels of cellular organization. We discuss the performance of the ablation process in terms of the energy deposited onto the sample and compare our implementation to others recently employed for cellular and subcellular surgery. Above the energy threshold of ionization, we demonstrate how to achieve single-cell ablation through the induction of mechanical perturbation and cavitation in living organisms. Below this threshold, we induce cytoskeleton severing inside live cells. By combining nanosurgery with fast live-imaging fluorescence microscopy, we show how the apparent equilibrium of the cytoskeleton can be perturbed regionally inside a cell.

A history of laser scissors (microbeams)

This introductory chapter reviews the history of microbeams starting with the original UV microbeam work of Tchakhotine in 1912 and covers the progress and application of microbeams through 2006. The main focus of the chapter is on laser "scissors" starting with Marcel Bessis' and colleagues work with the ruby laser microbeam in Paris in 1962. Following this introduction, a section is devoted to describing the different laser microbeam systems and then the rest of the chapter is devoted to applications in cell and developmental biology. The approach is to focus on the organelle/structure and describe how the laser microbeam has been applied to studying its structure and/or function. Since considerable work has been done on chromosomes and the mitotic spindle (Section V.A and C), these topics have been divided in distinct subsections. Other topics discussed are injection of foreign DNA through the cell membrane (optoporation/optoinjection), cell migration, the nucleolus, mitochondria, cytoplasmic filaments, and embryos fate-mapping. A final technology section is devoted to discussing the pros and cons of building/buying your own laser microbeam system and the option of using the Internet-based RoboLase system. Throughout the chapter, reference is made to other chapters in the book that go into more detail on the subjects briefly mentioned.

Endurance exercise differentially stimulates heart and axial muscle development in zebrafish (Danio rerio)

Mechanical load is an important factor in the differentiation of cells and tissues. To investigate the effects of increased mechanical load on development of muscle and bone, zebrafish were subjected to endurance swim training for 6 h/day for 10 wk starting at 14 days after fertilization. During the first 3 wk of training, trained fish showed transiently increased growth compared with untrained (control) fish. Increased expression of proliferating cell nuclear antigen suggests that this growth is realized in part through increased cell proliferation. Red and white axial muscle fiber diameter was not affected. Total cross-sectional area of red fibers, however, was increased. An improvement in aerobic muscle performance was supported by an increase in myoglobin expression. At the end of 10 wk of training, heart and axial muscle showed increased expression of the muscle growth factor myogenin and proliferating cell nuclear antigen, but there were major differences between cardiac and axial muscle. In axial muscle, expression of the "slow" types of myosin and troponin C was increased, together with expression of erythropoietin and myoglobin, which enhance oxygen transport, indicating a shift toward a slow aerobic phenotype. In contrast, the heart muscle shifts to a faster phenotype but does not become more aerobic. This suggests that endurance training differentially affects heart and axial muscle.

Nonmuscle myosin II generates forces that transmit tension and drive contraction in multiple tissues during dorsal closure

BACKGROUND

The morphogenic movements that characterize embryonic development require the precise temporal and spatial control of cell-shape changes. Drosophila dorsal closure is a well-established model for epithelial sheet morphogenesis, and mutations in more than 60 genes cause defects in closure. Closure requires that four forces, derived from distinct tissues, be precisely balanced. The proteins responsible for generating each of the forces have not been determined.

RESULTS

We document dorsal closure in living embryos to show that mutations in nonmuscle myosin II (encoded by zipper; zip/MyoII) disrupt the integrity of multiple tissues during closure. We demonstrate that MyoII localization is distinct from, but overlaps, F-actin in the supracellular purse string, whereas in the amnioserosa and lateral epidermis each has similar, cortical distributions. In zip/MyoII mutant embryos, we restore MyoII function either ubiquitously or specifically in the leading edge, amnioserosa, or lateral epidermis and find that zip/MyoII function in any one tissue can rescue closure. Using a novel, transgenic mosaic approach, we establish that contractility of the supracellular purse string in leading-edge cells requires zip/MyoII-generated forces; that zip/MyoII function is responsible for the apical contraction of amnioserosa cells; that zip/MyoII is important for zipping; and that defects in zip/MyoII contractility cause the misalignment of the lateral-epidermal sheets during seam formation.

CONCLUSIONS

We establish that zip/MyoII is responsible for generating the forces that drive cell-shape changes in each of the force-generating tissues that contribute to closure. This highly conserved contractile protein likely drives cell-sheet movements throughout phylogeny.

Specialized extraembryonic cells connect embryonic and extraembryonic epidermis in response to Dpp during dorsal closure in Drosophila

Dorsal closure in Drosophila embryogenesis involves expansion of the dorsal epidermis, followed by closure of the opposite epidermal edges. This process is driven by contractile force generated by an extraembryonic epithelium covering the yolk syncytium known as the amnioserosa. The secreted signaling molecule Dpp is expressed in the leading edge of the dorsal epidermis and is essential for dorsal closure. We found that the outermost row of amnioserosa cells (termed pAS) maintains a tight basolateral cell-cell adhesion interface with the leading edge of dorsal epidermis throughout the dorsal closure process. pAS was subject to altered cell motility in response to Dpp emanating from the dorsal epidermis, and this response was essential for dorsal closure. alphaPS3 and betaPS integrin subunits accumulated in the interface between pAS and dorsal epidermis, and were both required for dorsal closure. Looking at alphaPS3, type I Dpp receptor, and JNK mutants, we found that pAS cell motility was altered and pAS and dorsal epidermis adhesion failed under the mechanical stress of dorsal closure, suggesting that a Dpp-mediated mechanism connects the squamous pAS to the columnar dorsal epidermis to form a single coherent epithelial layer.

Transiently Reorganized Microtubules Are Essential for Zippering during Dorsal Closure in Drosophila melanogaster

There is emerging evidence that microtubules in nondividing cells can be employed to remodel the intracellular space. Here, we demonstrate an essential role for microtubules in dorsal closure, which occurs toward the end of Drosophila melanogaster embryogenesis. Dorsal closure is a morphogenetic process similar to wound healing, whereby a gap in the epithelium is closed through the coordinated action of different cell types. Surprisingly, this complex process requires microtubule function exclusively in epithelial cells and only for the last step, the zippering, which seals the gap. Preceding zippering, the epithelial microtubules reorganize to attain an unusual spatial distribution, which we describe with subcellular resolution in the intact, living organism. We provide a clearly defined example where cells of a developing organism transiently reorganize their microtubules to fulfill a specialized morphogenetic task.

Differential expression of the adhesion molecule Echinoid drives epithelial morphogenesis in Drosophila

Epithelial morphogenesis requires cell movements and cell shape changes coordinated by modulation of the actin cytoskeleton. We identify a role for Echinoid (Ed), an immunoglobulin domain-containing cell-adhesion molecule, in the generation of a contractile actomyosin cable required for epithelial morphogenesis in both the Drosophila ovarian follicular epithelium and embryo. Analysis of ed mutant follicle cell clones indicates that the juxtaposition of wild-type and ed mutant cells is sufficient to trigger actomyosin cable formation. Moreover, in wild-type ovaries and embryos, specific epithelial domains lack detectable Ed, thus creating endogenous interfaces between cells with and without Ed; these interfaces display the same contractile characteristics as the ectopic Ed expression borders generated by ed mutant clones. In the ovary, such an interface lies between the two cell types of the dorsal appendage primordia. In the embryo, Ed is absent from the amnioserosa during dorsal closure, generating an Ed expression border with the lateral epidermis that coincides with the actomyosin cable present at this interface. In both cases, ed mutant epithelia exhibit loss of this contractile structure and subsequent defects in morphogenesis. We propose that local modulation of the cytoskeleton at Ed expression borders may represent a general mechanism for promoting epithelial morphogenesis.

Pulsed laser microbeam-induced cell lysis: time-resolved imaging and analysis of hydrodynamic effects

Time-resolved imaging was used to examine the use of pulsed laser microbeam irradiation to produce cell lysis. Lysis was accomplished through the delivery of 6 ns, lambda=532 nm laser pulses via a 40x, 0.8 NA objective to a location 10 microm above confluent monolayers of PtK2 cells. The process dynamics were examined at cell surface densities of 600 and 1000 cells/mm2 and pulse energies corresponding to 0.7x, 1x, 2x, and 3x the threshold for plasma formation. The cell lysis process was imaged at times of 0.5 ns to 50 micros after laser pulse delivery and revealed the processes of plasma formation, pressure wave propagation, and cavitation bubble dynamics. Cavitation bubble expansion was the primary agent of cell lysis with the zone of lysed cells fully established within 600 ns of laser pulse delivery. The spatial extent of cell lysis increased with pulse energy but decreased with cell surface density. Hydrodynamic analysis indicated that cells subject to transient shear stresses in excess of a critical value were lysed while cells exposed to lower shear stresses remained adherent and viable. This critical shear stress is independent of laser pulse energy and varied from approximately 60-85 kPa for cell monolayers cultured at a density of 600 cells/mm2 to approximately 180-220 kPa for a surface density of 1000 cells/mm2. The implications for single cell lysis and microsurgery are discussed.

Cell distribution of stress fibres in response to the geometry of the adhesive environment

Cells display a large variety of shapes when plated in classical culture conditions despite their belonging to a common cell type. These shapes are transitory, since cells permanently disassemble and reassemble their cytoskeleton while moving. Adhesive micropatterns are commonly used to confine cell shape within a given geometry. In addition the micropattern can be designed so as to impose cells to spread upon adhesive and nonadhesive areas. Modulation of the pattern geometry allows the analysis of the mechanisms governing the determination of cell shape in response to external adhesive conditions. In this study, we show that the acquisition of cell shape follows two stages where initially the cell forms contact with the micropattern. Here, the most distal contacts made by the cell with the micropattern define the apices of the cell shape. Then secondly, the cell borders that link two apices move so as to minimise the distance between the two apices. In these cell borders, the absence of an underlying adhesive substrate is overcome by stress fibres forming between the apices, which in turn are marked by an accumulation of focal adhesions. By inhibiting myosin function, cell borders on nonadhesive zones become more concave, suggesting that the stress fibres work against the membrane tension in the cell border. Moreover, this suggested that traction forces are unevenly distributed in stationary, nonmigrating, cells. By comparing the stress fibres in cells with one, two, or three nonadherent cell borders it was reasoned that stress fibre strength is inversely proportional to number. We conclude that cells of a given area can generate the same total sum of tractional forces but that these tractional forces are differently spaced depending on the spatial distribution of its adherence contacts.

La nanochirurgie laser en biologie cellulaire

Since their first use in the early 60's, pulsed lasers have become increasingly popular for their ability to ablate biological tissue. Short laser pulses allow high precision surgery for biological and medical applications with minimal invasiveness. Performing highly targeted manipulation and ablation allows experiments impossible so far in development biology, cellular biology or even assisted reproductive technologies and laser surgery has been increasingly used over the last five years to answer key questions in Biology. Recently, picosecond UV and femtosecond IR laser pulses have been used to cleave microtubules and to severe actin stress fibers in vivo with a spatial precision in the submicrometer range to study their dynamics without affecting cell viability. We review recent findings on the underlying principles of pulsed laser nanosurgery mechanisms showing how the use of ultra short laser pulses increases precision and non-invasiveness of laser surgery. We show how the understanding of the surgical process allows one to distinguish between single cell ablation in living organisms or intracellular nanosurgery in living cells and we review recent applications to the study of forces and the quantification of cytoskeleton dynamics.

JNK signaling coordinates integrin and actin functions during Drosophila embryogenesis

Epithelial movements are key morphogenetic events in animal development. They are driven by multiple mechanisms, including signal-dependent changes in cytoskeletal organization and in cell adhesion. Such processes must be controlled precisely and coordinated to accurately sculpt the three-dimensional form of the developing organism. By observing the Drosophila epidermis during embryonic development using confocal time-lapse microscopy, we have investigated how signaling through the Jun-N-terminal kinase (JNK) pathway governs the tissue sheet movements that result in dorsal closure (DC). We find that JNK controls the polymerization of actin into a cable at the epidermal leading edge as previously suggested, as well as the joining (zipping) of the contralateral epithelial cell sheets. Here, we show that zipping is mediated by regulation of the integrins myospheroid and scab. Our data demonstrate that JNK signaling regulates a set of target genes that cooperate to facilitate epithelial movement and closure.

Effects of decreased muscle activity on developing axial musculature in nicb107 mutant zebrafish (Danio rerio)

The present paper discusses the effects of decreased muscle activity (DMA) on embryonic development in the zebrafish. Wild-type zebrafish embryos become mobile around 18 h post-fertilisation, long before the axial musculature is fully differentiated. As a model for DMA, the nic(b107) mutant was used. In nic(b107) mutant embryos, muscle fibres are mechanically intact and able to contract, but neuronal signalling is defective and the fibres are not activated, rendering the embryos immobile. Despite the immobility, distinguished slow and fast muscle fibres developed at the correct location in the axial muscles, helical muscle fibre arrangements were detected and sarcomere architecture was generated. However, in nic(b107) mutant embryos the notochord is flatter and the cross-sectional body shape more rounded, also affecting muscle fibre orientation. The stacking of sarcomeres and myofibril arrangement show a less regular pattern. Finally, expression levels of several genes were changed. Together, these changes in expression indicate that muscle growth is not impeded and energy metabolism is not changed by the decrease in muscle activity but that the composition of muscle is altered. In addition, skin stiffness is affected. In conclusion, the lack of muscle fibre activity did not prevent the basal muscle components developing but influenced further organisation and differentiation of these components.

Organizing moving groups during morphogenesis

The directed migration of cells drives the formation of many complex organ systems. Although in this morphogenetic context cells display a strong preference for migrating in organized, cohesive groups, little is known about the mechanisms that coordinate their movements. Recent studies on several model systems have begun to dissect the organization of these migrating tissues in vivo and have shown that cell guidance is mediated by a combination of chemical and mechanical cues.

ROCK-I and ROCK-II cooperatively regulate closure of eyelid and ventral body wall in mouse embryo

Rho-associated kinase (ROCK) is a serine/threonine kinase working in the Rho signaling to actin cytoskeleton. We previously reported that loss of ROCK-I results in the eyelid open at birth (EOB) and omphalocele phenotype in mice, while loss of ROCK-II results in placental dysfunction leading to intrauterine growth retardation and fetal death. Here, we report that after backcross to the C57BL/6 N genetic background, ROCK-II knockout (KO) neonates are born also with open eyelid and umbilical hernia, a phenotype similar to that of ROCK-I KO mice. ROCK-II KO embryos show impaired extension of the eyelid epithelial sheet with disorganized actin bundles in the leading edge of the sheet. These results suggest that ROCK-I and ROCK-II cooperatively regulates the assembly of actin bundles essential for closure of the eyelid and ventral body wall in mouse embryos. Consistently, ROCK-I(+/-)ROCK-II(+/-) double heterozygous mice also show the EOB and omphalocele phenotype.