Three-dimensional spatio-angular fluorescence microscopy with a polarized dual-view inverted selective-plane illumination microscope (pol-diSPIM)

Polarized fluorescence microscopy is a valuable tool for measuring molecular orientations, but techniques for recovering three-dimensional orientations and positions of fluorescent ensembles are limited. We report a polarized dual-view light-sheet system for determining the three-dimensional orientations and diffraction-limited positions of ensembles of fluorescent dipoles that label biological structures, and we share a set of visualization, histogram, and profiling tools for interpreting these positions and orientations. We model our samples, their excitation, and their detection using coarse-grained representations we call orientation distribution functions (ODFs). We apply ODFs to create physics-informed models of image formation with spatio-angular point-spread and transfer functions. We use theory and experiment to conclude that light-sheet tilting is a necessary part of our design for recovering all three-dimensional orientations. We use our system to extend known two-dimensional results to three dimensions in FM1-43-labelled giant unilamellar vesicles, fast-scarlet-labelled cellulose in xylem cells, and phalloidin-labelled actin in U2OS cells. Additionally, we observe phalloidin-labelled actin in mouse fibroblasts grown on grids of labelled nanowires and identify correlations between local actin alignment and global cell-scale orientation, indicating cellular coordination across length scales.

MARK2 regulates directed cell migration through modulation of myosin II contractility and focal adhesion organization

Cancer cell migration during metastasis is mediated by a highly polarized cytoskeleton. MARK2 and its invertebrate homolog Par1B are kinases that regulate the microtubule cytoskeleton to mediate polarization of neurons in mammals and embryos in invertebrates. However, the role of MARK2 in cancer cell migration is unclear. Using osteosarcoma cells, we found that in addition to its known localizations on microtubules and the plasma membrane, MARK2 also associates with the actomyosin cytoskeleton and focal adhesions. Cells depleted of MARK proteins demonstrated that MARK2 promotes phosphorylation of both myosin II and the myosin phosphatase targeting subunit MYPT1 to synergistically drive myosin II contractility and stress fiber formation in cells. Studies with isolated proteins showed that MARK2 directly phosphorylates myosin II regulatory light chain, while its effects on MYPT1 phosphorylation are indirect. Using a mutant lacking the membrane-binding domain, we found that membrane association is required for focal adhesion targeting of MARK2, where it specifically enhances cell protrusion by promoting FAK phosphorylation and formation of focal adhesions oriented in the direction of migration to mediate directionally persistent cell motility. Together, our results define MARK2 as a master regulator of the actomyosin and microtubule cytoskeletal systems and focal adhesions to mediate directional cancer cell migration.

Efficient Front-Rear Coupling in Neutrophil Chemotaxis by Dynamic Myosin II Localization

Efficient chemotaxis requires rapid coordination between different parts of the cell in response to changing directional cues. Here, we investigate the mechanism of front-rear coordination in chemotactic neutrophils. We find that changes in the protrusion rate at the cell front are instantaneously coupled to changes in retraction at the cell rear, while myosin II accumulation at the rear exhibits a reproducible 9-15-s lag. In turning cells, myosin II exhibits dynamic side-to-side relocalization at the cell rear in response to turning of the leading edge and facilitates efficient turning by rapidly re-orienting the rear. These manifestations of front-rear coupling can be explained by a simple quantitative model incorporating reversible actin-myosin interactions with a rearward-flowing actin network. Finally, the system can be tuned by the degree of myosin regulatory light chain (MRLC) phosphorylation, which appears to be set in an optimal range to balance persistence of movement and turning ability.

Filopodia and focal adhesions: An integrated system driving branching morphogenesis in neuronal pathfinding and angiogenesis

Single cell branching during development in vertebrates is typified by neuronal branching to form neurites and vascular branches formed by sprouting angiogenesis. Neurons and endothelial tip cells possess subcellular protrusions that share many common features from the morphological to the molecular level. Both systems utilize filopodia as their cellular protrusion organelles and depend on specific integrin-mediated adhesions to the local extracellular matrix for guidance in their pathfinding. We discuss the similar molecular machineries involved in these two types of cell branch formation and use their analogy to propose a new mechanism for angiogenic filopodia function, namely as adhesion assembly sites. In support of this model we provide primary data of angiogenesis in zebrafish in vivo showing that the actin assembly factor VASP participates in both filopodia formation and adhesion assembly at the base of the filopodia, enabling forward progress of the tip cell. The use of filopodia and their associated adhesions provide a common mechanism for neuronal and endothelial pathfinding during development in response to extracellular matrix cues.

Local pulsatile contractions are an intrinsic property of the myosin 2A motor in the cortical cytoskeleton of adherent cells

Pulsatile dynamics of myosin 2A occurs in single cells, is unique to myosin 2A and not 2B, and is a result of the kinetics of the myosin-2A motor, whereas the myosin-2B motor is insufficient to induce this dynamic behavior. This pulsatile contraction is an inherent property of myosin-2A/F-actin networks in adherent cells.

The role of nonmuscle myosin 2 (NM2) pulsatile dynamics in generating contractile forces required for developmental morphogenesis has been characterized, but whether these pulsatile contractions are an intrinsic property of all actomyosin networks is not known. Here we used live-cell fluorescence imaging to show that transient, local assembly of NM2A “pulses” occurs in the cortical cytoskeleton of single adherent cells of mesenchymal, epithelial, and sarcoma origin, independent of developmental signaling cues and cell–cell or cell–ECM interactions. We show that pulses in the cortical cytoskeleton require Rho-associated kinase– or myosin light chain kinase (MLCK) activity, increases in cytosolic calcium, and NM2 ATPase activity. Surprisingly, we find that cortical cytoskeleton pulses specifically require the head domain of NM2A, as they do not occur with either NM2B or a 2B-head-2A-tail chimera. Our results thus suggest that pulsatile contractions in the cortical cytoskeleton are an intrinsic property of the NM2A motor that may mediate its role in homeostatic maintenance of tension in the cortical cytoskeleton of adherent cells.

FMN2 Makes Perinuclear Actin to Protect Nuclei during Confined Migration and Promote Metastasis

Cell migration in confined 3D tissue microenvironments is critical for both normal physiological functions and dissemination of tumor cells. We discovered a cytoskeletal structure that prevents damage to the nucleus during migration in confined microenvironments. The formin-family actin filament nucleator FMN2 associates with and generates a perinuclear actin/focal adhesion (FA) system that is distinct from previously characterized actin/FA structures. This system controls nuclear shape and positioning in cells migrating on 2D surfaces. In confined 3D microenvironments, FMN2 promotes cell survival by limiting nuclear envelope damage and DNA double-strand breaks. We found that FMN2 is upregulated in human melanomas and showed that disruption of FMN2 in mouse melanoma cells inhibits their extravasation and metastasis to the lung. Our results indicate a critical role for FMN2 in generating a perinuclear actin/FA system that protects the nucleus and DNA from damage to promote cell survival during confined migration and thus promote cancer metastasis.

The FAK-Arp2/3 interaction promotes leading edge advance and haptosensing by coupling nascent adhesions to lamellipodia actin

Focal adhesion kinase (FAK) is an important regulator of focal adhesion dynamics during cell migration. Distinct functions of FAK—kinase activation and Arp2/3 binding—enable cells to mechanosense or haptotax during spreading and migration.

Cell migration is initiated in response to biochemical or physical cues in the environment that promote actin-mediated lamellipodial protrusion followed by the formation of nascent integrin adhesions (NAs) within the protrusion to drive leading edge advance. Although FAK is known to be required for cell migration through effects on focal adhesions, its role in NA formation and lamellipodial dynamics is unclear. Live-cell microscopy of FAK^−/− cells with expression of phosphorylation deficient or a FERM-domain mutant deficient in Arp2/3 binding revealed a requirement for FAK in promoting the dense formation, transient stabilization, and timely turnover of NA within lamellipodia to couple actin-driven protrusion to adhesion and advance of the leading edge. Phosphorylation on Y397 of FAK promotes dense NA formation but is dispensable for transient NA stabilization and leading edge advance. In contrast, transient NA stabilization and advance of the cell edge requires FAK–Arp2/3 interaction, which promotes Arp2/3 localization to NA and reduces FAK activity. Haptosensing of extracellular matrix (ECM) concentration during migration requires the interaction between FAK and Arp2/3, whereas FAK phosphorylation modulates mechanosensing of ECM stiffness during spreading. Taken together, our results show that mechanistically separable functions of FAK in NA are required for cells to distinguish distinct properties of their environment during migration.

YAP Nuclear Localization in the Absence of Cell-Cell Contact Is Mediated by a Filamentous Actin-dependent, Myosin II- and Phospho-YAP-independent Pathway during Extracellular Matrix Mechanosensing

Cell-cell contact inhibition and the mechanical environment of cells have both been shown to regulate YAP nuclear localization to modulate cell proliferation. Changes in cellular contractility by genetic, pharmacological, and matrix stiffness perturbations regulate YAP nuclear localization. However, because contractility and F-actin organization are interconnected cytoskeletal properties, it remains unclear which of these distinctly regulates YAP localization. Here we show that in the absence of cell-cell contact, actomyosin contractility suppresses YAP phosphorylation at Ser(112), however, neither loss of contractility nor increase in YAP phosphorylation is sufficient for its nuclear exclusion. We find that actin cytoskeletal integrity is essential for YAP nuclear localization, and can override phosphoregulation or contractility-mediated regulation of YAP nuclear localization. This actin-mediated regulation is conserved during mechanotransduction, as substrate compliance increased YAP phosphorylation and reduced cytoskeletal integrity leading to nuclear exclusion of both YAP and Ser(P)(112)-YAP. These data provide evidence for two actin-mediated pathways for YAP regulation; one in which actomyosin contractility regulates YAP phosphorylation, and a second that involves cytoskeletal integrity-mediated regulation of YAP nuclear localization independent of contractility. We suggest that in non-contact inhibited cells, this latter mechanism may be important in low stiffness regimes, such as may be encountered in physiological environments.

Thematic Minireview Series: The State of the Cytoskeleton in 2015

The study of cytoskeletal polymers has been an active area of research for more than 70 years. However, despite decades of pioneering work by some of the brightest scientists in biochemistry, cell biology, and physiology, many central questions regarding the polymers themselves are only now starting to be answered. For example, although it has long been appreciated that the actin cytoskeleton provides contractility and couples biochemical responses with mechanical stresses in cells, only recently have we begun to understand how the actin polymer itself responds to mechanical loads. Likewise, although it has long been appreciated that the microtubule cytoskeleton can be post-translationally modified, only recently have the enzymes responsible for these modifications been characterized, so that we can now begin to understand how these modifications alter the polymerization and regulation of microtubule structures. Even the septins in eukaryotes and the cytoskeletal polymers of prokaryotes have yielded new insights due to recent advances in microscopy techniques. In this thematic series of minireviews, these topics are covered by some of the very same scientists who generated these recent insights, thereby providing us with an overview of the State of the Cytoskeleton in 2015.

Move your microvilli

Polarized epithelial cells create tightly packed arrays of microvilli in their apical membrane, but the fate of these microvilli is relatively unknown when epithelial cell polarity is lost during wound healing. In this issue, Klingner et al. (2014. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201402037) show that, when epithelial cells become subconfluent, actomyosin contractions locally within the apical cortex cause their microvilli to become motile over the dorsal/apical surface. Their unexpected observations may have implications for epithelial responses in wound healing and disease.

Incoherent structured illumination improves optical sectioning and contrast in multiphoton super-resolution microscopy

Three-dimensional super-resolution imaging in thick, semi-transparent biological specimens is hindered by light scattering, which increases background and degrades both contrast and optical sectioning. We describe a simple method that mitigates these issues, improving image quality in our recently developed two-photon instant structured illumination microscope without requiring any hardware modifications to the instrument. By exciting the specimen with three laterally-structured, phase-shifted illumination patterns and post-processing the resulting images, we digitally remove both scattered and out-of-focus emissions that would otherwise contaminate our raw data. We demonstrate the improved performance of our approach in biological samples, including pollen grains, primary mouse aortic endothelial cells cultured in a three-dimensional collagen matrix and live tumor-like cell spheroids.

Myosin II controls cellular branching morphogenesis and migration in three dimensions by minimizing cell-surface curvature

In many cases cell function is intimately linked to cell shape control. We utilized endothelial cell branching morphogenesis as a model to understand the role of myosin-II in shape control of invasive cells migrating in 3D collagen gels. We applied principles of differential geometry and mathematical morphology to 3D image sets to parameterize cell branch structure and local cell surface curvature. We find that Rho/ROCK-stimulated myosin-II contractility minimizes cell-scale branching by recognizing and minimizing local cell surface curvature. Utilizing micro-fabrication to constrain cell shape identifies a positive feedback mechanism in which low curvature stabilizes myosin-II cortical association, where it acts to maintain minimal curvature. The feedback between myosin-II regulation by and control of curvature drives cycles of localized cortical myosin-II assembly and disassembly. These cycles in turn mediate alternating phases of directionally biased branch initiation and retraction to guide 3D cell migration.

Rac1-dependent phosphorylation and focal adhesion recruitment of myosin IIA regulates migration and mechanosensing

BACKGROUND

Cell migration requires coordinated formation of focal adhesions (FAs) and assembly and contraction of the actin cytoskeleton. Nonmuscle myosin II (MII) is a critical mediator of contractility and FA dynamics in cell migration. Signaling downstream of the small GTPase Rac1 also regulates FA and actin dynamics, but its role in regulation of MII during migration is less clear.

RESULTS

We found that Rac1 promotes association of MIIA with FA. Live-cell imaging showed that, whereas most MIIA at the leading edge assembled into dorsal contractile arcs, a substantial subset assembled in or was captured within maturing FA, and this behavior was promoted by active Rac1. Protein kinase C (PKC) activation was necessary and sufficient for integrin- and Rac1-dependent phosphorylation of MIIA heavy chain (HC) on serine1916 (S1916) and recruitment to FA. S1916 phosphorylation of MIIA HC and localization in FA was enhanced during cell spreading and ECM stiffness mechanosensing, suggesting upregulation of this pathway during physiological Rac1 activation. Phosphomimic and nonphosphorylatable MIIA HC mutants demonstrated that S1916 phosphorylation was necessary and sufficient for the capture and assembly of MIIA minifilaments in FA. S1916 phosphorylation was also sufficient to promote the rapid assembly of FAs to enhance cell migration and for the modulation of traction force, spreading, and migration by ECM stiffness.

CONCLUSIONS

Our study reveals for the first time that Rac1 and integrin activation regulates MIIA HC phosphorylation through a PKC-dependent mechanism that promotes MIIA association with FAs and acts as a critical modulator of cell migration and mechanosensing.

A contractile and counterbalancing adhesion system controls the 3D shape of crawling cells

How adherent and contractile systems coordinate to promote cell shape changes is unclear. Here, we define a counterbalanced adhesion/contraction model for cell shape control. Live-cell microscopy data showed a crucial role for a contractile meshwork at the top of the cell, which is composed of actin arcs and myosin IIA filaments. The contractile actin meshwork is organized like muscle sarcomeres, with repeating myosin II filaments separated by the actin bundling protein α-actinin, and is mechanically coupled to noncontractile dorsal actin fibers that run from top to bottom in the cell. When the meshwork contracts, it pulls the dorsal fibers away from the substrate. This pulling force is counterbalanced by the dorsal fibers' attachment to focal adhesions, causing the fibers to bend downward and flattening the cell. This model is likely to be relevant for understanding how cells configure themselves to complex surfaces, protrude into tight spaces, and generate three-dimensional forces on the growth substrate under both healthy and diseased conditions.

Spinning disk confocal imaging of neutrophil migration in zebrafish

Live-cell imaging techniques have been substantially improved due to advances in confocal microscopy instrumentation coupled with ultrasensitive detectors. The spinning disk confocal system is capable of generating images of fluorescent live samples with broad dynamic range and high temporal and spatial resolution. The ability to acquire fluorescent images of living cells in vivo on a millisecond timescale allows the dissection of biological processes that have not previously been visualized in a physiologically relevant context. In vivo imaging of rapidly moving cells such as neutrophils can be technically challenging. In this chapter, we describe the practical aspects of imaging neutrophils in zebrafish embryos using spinning disk confocal microscopy. Similar setups can also be applied to image other motile cell types and signaling processes in translucent animals or tissues.

Spatially isotropic four-dimensional imaging with dual-view plane illumination microscopy

Optimal four-dimensional imaging requires high spatial resolution in all dimensions, high speed and minimal photobleaching and damage. We developed a dual-view, plane illumination microscope with improved spatiotemporal resolution by switching illumination and detection between two perpendicular objectives in an alternating duty cycle. Computationally fusing the resulting volumetric views provides an isotropic resolution of 330 nm. As the sample is stationary and only two views are required, we achieve an imaging speed of 200 images/s (i.e., 0.5 s for a 50-plane volume). Unlike spinning-disk confocal or Bessel beam methods, which illuminate the sample outside the focal plane, we maintain high spatiotemporal resolution over hundreds of volumes with negligible photobleaching. To illustrate the ability of our method to study biological systems that require high-speed volumetric visualization and/or low photobleaching, we describe microtubule tracking in live cells, nuclear imaging over 14 h during nematode embryogenesis and imaging of neural wiring during Caenorhabditis elegans brain development over 5 h.

Instant super-resolution imaging in live cells and embryos via analog image processing

Existing super-resolution fluorescence microscopes compromise acquisition speed to provide subdiffractive sample information. We report an analog implementation of structured illumination microscopy that enables 3D super-resolution imaging with 145 nm lateral and 350 nm axial resolution, at acquisition speeds up to 100 Hz. By performing image processing operations optically instead of digitally, we removed the need to capture, store, and combine multiple camera exposures, increasing data acquisition rates 10–100x over other super-resolution microscopes and acquiring and displaying super-resolution images in real-time. Low excitation intensities allow imaging over hundreds of 2D sections, and combined physical and computational sectioning allow similar depth penetration to confocal microscopy. We demonstrate the capability of our system by imaging fine, rapidly moving structures including motor-driven organelles in human lung fibroblasts and the cytoskeleton of flowing blood cells within developing zebrafish embryos.

Stiffness-controlled three-dimensional extracellular matrices for high-resolution imaging of cell behavior

Regulation of cell functions by the physical properties of the extracellular matrix (ECM) has emerged as a crucial contributor to development and disease. Two specific physical properties of the ECM, stiffness and dimensionality, each influence cell signaling and function. As these ECM physical properties are linked to other properties that also regulate cell behavior, e.g., integrin ligand density, parsing the specific contributions of ECM stiffness and dimensionality has proven difficult. Here we detail a simple protocol, which can be completed in 1-2 d, for combining three-dimensional (3D) ECM engagement with controlled underlying ECM stiffness. In these 'sandwich gels', cells are sandwiched between a 3D fibrillar ECM and an ECM-coupled polyacrylamide gel of defined compliance, allowing the study of the specific effects of ECM compliance on cell function in physiologically relevant 3D ECMs. This type of system enables high-resolution time-lapse imaging and is suitable for a wide range of cell types and molecular perturbations.

Microscopy in 3D: a biologist's toolbox

The power of fluorescence microscopy to study cellular structures and macromolecular complexes spans a wide range of size scales, from studies of cell behavior and function in physiological 3D environments to understanding the molecular architecture of organelles. At each length scale, the challenge in 3D imaging is to extract the most spatial and temporal resolution possible while limiting photodamage/bleaching to living cells. Several advances in 3D fluorescence microscopy now offer higher resolution, improved speed, and reduced photobleaching relative to traditional point-scanning microscopy methods. We discuss a few specific microscopy modalities that we believe will be particularly advantageous in imaging cells and subcellular structures in physiologically relevant 3D environments.

Distinct ECM mechanosensing pathways regulate microtubule dynamics to control endothelial cell branching morphogenesis

The compliance and dimensionality of the ECM regulate distinct changes in microtubule growth speed and growth persistence.

During angiogenesis, cytoskeletal dynamics that mediate endothelial cell branching morphogenesis during vascular guidance are thought to be regulated by physical attributes of the extracellular matrix (ECM) in a process termed mechanosensing. Here, we tested the involvement of microtubules in linking mechanosensing to endothelial cell branching morphogenesis. We used a recently developed microtubule plus end–tracking program to show that specific parameters of microtubule assembly dynamics, growth speed and growth persistence, are globally and regionally modified by, and contribute to, ECM mechanosensing. We demonstrated that engagement of compliant two-dimensional or three-dimensional ECMs induces local differences in microtubule growth speed that require myosin II contractility. Finally, we found that microtubule growth persistence is modulated by myosin II–mediated compliance mechanosensing when cells are cultured on two-dimensional ECMs, whereas three-dimensional ECM engagement makes microtubule growth persistence insensitive to changes in ECM compliance. Thus, compliance and dimensionality ECM mechanosensing pathways independently regulate specific and distinct microtubule dynamics parameters in endothelial cells to guide branching morphogenesis in physically complex ECMs.

Tropomodulins are negative regulators of neurite outgrowth

Regulation of the actin cytoskeleton is critical for neurite formation. Tropomodulins (Tmods) regulate polymerization at actin filament pointed ends. Previous experiments using a mouse model deficient for the neuron specific isoform Tmod2 suggested a role for Tmods in neuronal function by impacting processes underlying learning and memory. However, the role of Tmods in neuronal function on the cellular level remains unknown. Immunofluorescence localization of the neuronal isoforms Tmod1 and Tmod2 in cultured rat primary hippocampal neurons revealed that Tmod1 is enriched along the proximal part of F-actin bundles in lamellipodia of spreading cells and in growth cones of extending neurites, while Tmod2 appears largely cytoplasmic. Functional analysis of these Tmod isoforms in a mouse neuroblastoma N2a cell line showed that knockdown of Tmod2 resulted in a significant increase in the number of neurite-forming cells and in neurite length. While N2a cells compensated for Tmod2 knockdown by increasing Tmod1 levels, over-expression of exogenous Tmod1 had no effect on neurite outgrowth. Moreover, knockdown of Tmod1 increased the number of neurites formed per cell, without effect on the number of neurite-forming cells or neurite length. Taken together, these results indicate that Tmod1 and Tmod2 have mechanistically distinct inhibitory roles in neurite formation, likely mediated via different effects on F-actin dynamics and via differential localizations during early neuritogenesis.

Tropomodulin1 is required for membrane skeleton organization and hexagonal geometry of fiber cells in the mouse lens

Hexagonal packing geometry is a hallmark of close-packed epithelial cells in metazoans. Here, we used fiber cells of the vertebrate eye lens as a model system to determine how the membrane skeleton controls hexagonal packing of post-mitotic cells. The membrane skeleton consists of spectrin tetramers linked to actin filaments (F-actin), which are capped by tropomodulin1 (Tmod1) and stabilized by tropomyosin (TM). In mouse lenses lacking Tmod1, initial fiber cell morphogenesis is normal, but fiber cell hexagonal shapes and packing geometry are not maintained as fiber cells mature. Absence of Tmod1 leads to decreased gammaTM levels, loss of F-actin from membranes, and disrupted distribution of beta2-spectrin along fiber cell membranes. Regular interlocking membrane protrusions on fiber cells are replaced by irregularly spaced and misshapen protrusions. We conclude that Tmod1 and gammaTM regulation of F-actin stability on fiber cell membranes is critical for the long-range connectivity of the spectrin-actin network, which functions to maintain regular fiber cell hexagonal morphology and packing geometry.

Local cortical tension by myosin II guides 3D endothelial cell branching

A key feature of angiogenesis is directional control of endothelial cell (EC) morphogenesis and movement [1]. During angiogenic sprouting, endothelial "tip cells" directionally branch from existing vessels in response to biochemical cues such as VEGF or hypoxia and migrate and invade the surrounding extracellular matrix (ECM) in a process that requires ECM remodeling by matrix metalloproteases (MMPs) [2-4]. Tip EC branching is mediated by directional protrusion of subcellular pseudopodial branches [5, 6]. Here, we seek to understand how EC pseudopodial branching is locally regulated to directionally guide angiogenesis. We develop an in vitro 3D EC model system in which migrating ECs display branched pseudopodia morphodynamics similar to those in living zebrafish. Using this system, we find that ECM stiffness and ROCK-mediated myosin II activity inhibit EC pseudopodial branch initiation. Myosin II is dynamically localized to the EC cortex and is partially released under conditions that promote branching. Local depletion of cortical myosin II precedes branch initiation, and initiation can be induced by local inhibition of myosin II activity. Thus, local downregulation of myosin II cortical contraction allows pseudopodium initiation to mediate EC branching and hence guide directional migration and angiogenesis.

Tmod3 regulates polarized epithelial cell morphology

Although the role of the actin cytoskeleton in morphogenesis of polarized epithelial sheets is generally accepted as centrally important, the regulation of actin dynamics in this process remains unclear. Here, we show that the pointed-end capping protein Tmod3 contributes to epithelial cell shape within confluent monolayers of polarized epithelial cells. Tmod3 localizes to lateral cell membranes in polarized epithelia of several cell types. Reduction of Tmod3 levels by shRNA leads to a loss of F-actin and tropomyosins from lateral cell membranes, and a decrease in epithelial cell height, without effects on localisation of tight junction or adherens junction proteins, or any apparent changes in cell-cell adhesion. Instead, distribution of alphaII-spectrin on lateral membranes is disrupted upon reduction of Tmod3 levels, suggesting that loss of Tmod3 function leads to destabilization and disassembly of tropomyosin-coated actin filaments followed by disorganization of the spectrin-based membrane skeleton on lateral membranes. These data demonstrate for the first time a role for pointed-end capping in morphology regulation of polarized epithelial cells through stabilization of F-actin on lateral membranes. We propose that Tmod3-capped tropomyosin-actin filaments provide crucial links in the spectrin membrane skeleton of polarized epithelial cells, enabling the membrane skeleton to maintain cell shape.

Tropomodulin 3 binds to actin monomers

Regulation of the actin cytoskeleton by filament capping proteins is critical to myriad dynamic cellular functions. The ability of these proteins to bind both filaments as well as monomers is often central to their cellular functions. The ubiquitous pointed end capping protein Tmod3 (tropomodulin 3) acts as a negative regulator of cell migration, yet mechanisms behind its cellular functions are not understood. Analysis of Tmod3 effects on kinetics of actin polymerization and steady state monomer levels revealed that Tmod3, unlike previously characterized tropomodulins, sequesters actin monomers with an affinity similar to its affinity for capping pointed ends. Furthermore, Tmod3 is found bound to actin in high speed supernatant cytosolic extracts, suggesting that Tmod3 can bind to monomers in the context of other cytosolic monomer binding proteins. The Tmod3-actin complex can be efficiently cross-linked with 1-ethyl-3-(dimethylaminopropyl)carbodiimide/N-hydroxylsulfosuccinimide in a 1:1 complex. Subsequent tryptic digestion and liquid chromatography/tandem mass spectrometry revealed two binding interfaces on actin, one distinct from other actin monomer binding proteins, and two potential binding sites in Tmod3, which are independent of the previously characterized leucine-rich repeat structure involved in pointed end capping. These data suggest that the Tmod3 isoform may regulate actin dynamics differently in cells than the previously described tropomodulin isoforms.

Nebulin: does it measure up as a ruler?

A recent study has shown that the giant protein nebulin maintains the lengths of actin filaments in striated muscle cells. Although on the surface, nebulin looks like a molecular ruler, it may be playing a more complex role in regulating dynamics at the pointed end of actin filaments in striated muscle.

Cell migration without a lamellipodium: translation of actin dynamics into cell movement mediated by tropomyosin

The actin cytoskeleton is locally regulated for functional specializations for cell motility. Using quantitative fluorescent speckle microscopy (qFSM) of migrating epithelial cells, we previously defined two distinct F-actin networks based on their F-actin-binding proteins and distinct patterns of F-actin turnover and movement. The lamellipodium consists of a treadmilling F-actin array with rapid polymerization-dependent retrograde flow and contains high concentrations of Arp2/3 and ADF/cofilin, whereas the lamella exhibits spatially random punctae of F-actin assembly and disassembly with slow myosin-mediated retrograde flow and contains myosin II and tropomyosin (TM). In this paper, we microinjected skeletal muscle alphaTM into epithelial cells, and using qFSM, electron microscopy, and immunolocalization show that this inhibits functional lamellipodium formation. Cells with inhibited lamellipodia exhibit persistent leading edge protrusion and rapid cell migration. Inhibition of endogenous long TM isoforms alters protrusion persistence. Thus, cells can migrate with inhibited lamellipodia, and we suggest that TM is a major regulator of F-actin functional specialization in migrating cells.

Aberrant myofibril assembly in tropomodulin1 null mice leads to aborted heart development and embryonic lethality

Tropomodulin1 (Tmod1) caps thin filament pointed ends in striated muscle, where it controls filament lengths by regulating actin dynamics. Here, we investigated myofibril assembly and heart development in a Tmod1 knockout mouse. In the absence of Tmod1, embryonic development appeared normal up to embryonic day (E) 8.5. By E9.5, heart defects were evident, including aborted development of the myocardium and inability to pump, leading to embryonic lethality by E10.5. Confocal microscopy of hearts of E8–8.5 Tmod1 null embryos revealed structures resembling nascent myofibrils with continuous F-actin staining and periodic dots of α-actinin, indicating that I-Z-I complexes assembled in the absence of Tmod1. Myomesin, a thick filament component, was also assembled normally along these structures, indicating that thick filament assembly is independent of Tmod1. However, myofibrils did not become striated, and gaps in F-actin staining (H zones) were never observed. We conclude that Tmod1 is required for regulation of actin filament lengths and myofibril maturation; this is critical for heart morphogenesis during embryonic development.

Tropomodulins: life at the slow end

Dynamic exchange of actin monomers at filament ends is crucial for the functional architecture of many cytoskeletal-dependent processes. Recent evidence indicates that tropomodulins (Tmods), a conserved family of actin-capping proteins that bind to the pointed (slow-growing) end of actin filaments, regulate a variety of actin structures, including dynamic actin networks found in some motile cells. Actin structures that are more stable, such as sarcomeric thin filaments, require capping by Tmods to specify filament lengths and to provide filament stability. Here, we discuss the functional differences between the capping of pointed and barbed ends within the context of these actin-filament systems, and how Tmods uniquely contribute to their regulation and organization.

Pointed-end capping by tropomodulin3 negatively regulates endothelial cell motility

Actin filament pointed-end dynamics are thought to play a critical role in cell motility, yet regulation of this process remains poorly understood. We describe here a previously uncharacterized tropomodulin (Tmod) isoform, Tmod3, which is widely expressed in human tissues and is present in human microvascular endothelial cells (HMEC-1). Tmod3 is present in sufficient quantity to cap pointed ends of actin filaments, localizes to actin filament structures in HMEC-1 cells, and appears enriched in leading edge ruffles and lamellipodia. Transient overexpression of GFP-Tmod3 leads to a depolarized cell morphology and decreased cell motility. A fivefold increase in Tmod3 results in an equivalent decrease in free pointed ends in the cells. Unexpectedly, a decrease in the relative amounts of F-actin, free barbed ends, and actin-related protein 2/3 (Arp2/3) complex in lamellipodia are also observed. Conversely, decreased expression of Tmod3 by RNA interference leads to faster average cell migration, along with increases in free pointed and barbed ends in lamellipodial actin filaments. These data collectively demonstrate that capping of actin filament pointed ends by Tmod3 inhibits cell migration and reveal a novel control mechanism for regulation of actin filaments in lamellipodia.

The lens membrane skeleton contains structures preferentially enriched in spectrin-actin or tropomodulin-actin complexes

The spectrin-based membrane skeleton plays an important role in determining the distributions and densities of receptors, ion channels, and pumps, thus influencing cell shape and deformability, cell polarity, and adhesion. In the paradigmatic human erythrocyte, short tropomodulin-capped actin filaments are cross-linked by spectrin into a hexagonal network, yet the extent to which this type of actin filament organization is utilized in the membrane skeletons of nonerythroid cells is not known. Here, we show that associations of tropomodulin and spectrin with actin in bovine lens fiber cells are distinct from that of the erythrocyte and imply a very different molecular organization. Mechanical disruption of the lens fiber cell membrane skeleton releases tropomodulin and actin-containing oligomeric complexes that can be isolated by gel filtration column chromatography, sucrose gradient centrifugation and immunoadsorption. These tropomodulin-actin complexes do not contain spectrin. Instead, spectrin is associated with actin in different complexes that do not contain tropomodulin. Immunofluorescence staining of isolated fiber cells further demonstrates that tropomodulin does not precisely colocalize with spectrin along the lateral membranes of lens fiber cells. Taken together, our data suggest that tropomodulin-capped actin filaments and spectrin-cross-linked actin filaments are assembled in distinct structures in the lens fiber cell membrane skeleton, indicating that it is organized quite differently from that of the erythrocyte membrane skeleton.

While E1A can facilitate epithelial cell transformation by several dominant oncogenes, the C-terminus seems only to regulate rac and cdc42 function, but in both epithelial and fibroblastic cells

Epithelial and fibroblast cells were differentially susceptible to transformation by oncogenic src, ras, mos, raf, rac, and cdc42 and the influence of adenovirus E1A. In contrast to NIH 3T3 cells, which are easily transformed by all the oncogenes tested, epithelial cells were more resistant to transformation by the same oncogenes. Transformation efficiency of both primary and immortal epithelial cells by E1B, V12ras, v-src, v-raf, and v-mos was increased by cotransfection of E1A 12S, which enables these cells to overcome the M1/M2 mortality blocks, which are not present in NIH 3T3 cells. NIH 3T3 cell transformation by these oncogenes was not altered by E1A. Although V12cdc42 or V12rac1 alone could produce foci on NIH 3T3 cells, morphological conversion was observed only in the presence of a hypertransforming E1A mutant and not WT E1A. Epithelial cells were not transformed by V12cdc42 or V12rac1, even in the presence of WT or mutant E1A, but could be transformed by coexpression of mos/raf and rac/cdc42, and the resultant phenotype was affected by the E1A C-terminus. Hypertransformation, which has previously been reported with ras and E1A C-terminal mutants, turns out to be due to a synergy with rac/cdc42, but not ERK/MAPK or PI3K ras effectors. Like V12rac, expression of the E1A hypertransforming mutant resulted in the upregulation of vinculin and VASP, concomitant with the altered organization of the actin cytoskeleton in these cells. The results show that in addition to requiring abrogation of M1/M2 mortality blocks, primary epithelial cells require activation of the ERK MAPK cascade and rearrangement of the actin CSK to achieve transformation. In addition, the E1A C-terminus regulates rac/cdc42 function in both epithelial and fibroblast cells to affect the extent of transformation progression.

Stabilization and remodeling of the membrane skeleton during lens fiber cell differentiation and maturation

Actin filaments are integral components of the plasma membrane-associated cytoskeleton (membrane skeleton) and are believed to play important roles in the determination of cell polarity, shape, and membrane mechanical properties, however the roles of actin regulatory proteins in controlling the assembly, stability, and organization of actin filaments in the membrane skeleton are not well understood. Tropomodulin is a tropomyosin and actin-binding protein that stabilizes tropomyosin-actin filaments by capping their pointed ends and is associated with the spectrin-actin membrane skeleton in erythrocytes, skeletal muscle cells, and lens fiber cells, a specialized epithelial cell type. In this study, we have investigated the role of tropomodulin and other membrane skeleton components in lens fiber cell differentiation and maturation. Our results demonstrate that tropomodulin is expressed concomitantly with lens fiber cell differentiation and assembles onto the plasma membrane only after fiber cells have begun to elongate and form apical-apical contacts with the undifferentiated epithelium. In contrast, other membrane skeleton components, spectrin, actin, and tropomyosin, are constitutively expressed and assembled on the plasma membranes of both undifferentiated and differentiated fiber cells. Tropomodulin, but not other membrane skeleton components, is also enriched at a novel structure at the apical and basal ends of newly elongated fiber cells at the fiber cell-epithelium and fiber cell-capsule interface, respectively. Once assembled, tropomodulin and its binding partners, tropomyosin and actin, remain membrane-associated and are not proteolyzed during fiber cell maturation and aging, despite proteolysis of alpha-spectrin and other cytoskeletal filament systems such as microtubules and intermediate filaments. We propose that actin filament stabilization by tropomodulin, coupled with partial proteolysis of other cytoskeletal components, represents a programmed remodeling of the lens membrane skeleton that may be essential to maintain plasma membrane integrity and transparency of the extremely elongated, long-lived cells of the lens. The unique localization of tropomodulin at fiber cell tips further suggests a new role for tropomodulin at cell-cell and cell-substratum contacts; this may be important for cell migration and/or adhesion during differentiation and morphogenesis.

Tropomodulin and tropomyosin mediate lens cell actin cytoskeleton reorganization in vitro

PURPOSE

To determine the role of the actin cytoskeleton regulatory proteins tropomyosin and tropomodulin (Tmod) in the reorganization of the actin cytoskeleton during lens epithelial cell differentiation.

METHODS

Primary cultures of chick lens epithelial cells were allowed to differentiate in vitro to form lentoid bodies. Localization of F-actin, Tmod, and tropomyosin were determined by immunofluorescent staining followed by confocal microscopy. Tropomyosin and Tmod isoform expression was determined by immunoprecipitation and western blot analysis.

RESULTS

In undifferentiated epithelial cells F-actin was organized in polygonal arrays of stress fibers and was also associated with the adherens belt. In contrast, F-actin in differentiated cells was predominantly associated with membranes in a reticular or fibrillar pattern and was organized in curvilinear fibrils in the cytoplasm. Tmod was not detected in the undifferentiated epithelial cells but was expressed upon cell differentiation and assembled into F-actin and non-F-actin structures. Tmod isoforms expressed in the lens cell cultures were identical with those expressed in the embryonic chick lens fiber cells. Tropomyosin was associated with the polygonal arrays of stress fibers in the undifferentiated epithelial cells and was recruited to cortical F-actin at the cell periphery during differentiation. This occurred coincident with a shift in tropomyosin isoform expression.

CONCLUSIONS

Expression and sequential assembly of low-molecular-weight tropomyosin and Tmod into the cortical actin cytoskeleton of differentiated lens cells may help to reorganize the actin cytoskeleton during morphogenetic differentiation. Moreover, lens epithelial cell differentiation may include the generation of novel Tmod-containing, non-F-actin cytoskeletal structures.

Identification of a novel mechanism of regulation of the adherens junction by E1A, Rac1, and cortical actin filaments that contributes to tumor progression

Transformation progression toward more malignant behavior often results from a loss of epithelial cell behavior, especially cell-cell adhesion. E1A cooperates with ras to transform primary epithelial cells such that they maintain epithelial cell differentiation, including the proper localization of adherens junctions (AJs). Second exon mutants of E1A 12S cooperate with ras to produce a more aggressively transformed phenotype, termed hypertransformation, that includes the loss of adhesion. Such hypertransformation can also be achieved by the addition of activated Rac1 to cells expressing wild-type E1A and ras, suggesting that actin reorganization may be important for the hypertransformed phenotype. Primary epithelial cells expressing hypertransforming mutants of E1A or V12Rac1 exhibit the loss of cortical actin filaments. In these cells, AJ complexes do not incorporate alpha-catenin, fail to associate with the cytoskeleton, and fail to localize to the plasma membrane, resulting in the destabilization of the AJ components and a loss of function. Loss of these epithelial cell characteristics predisposes these cells to a more malignant phenotype due to the loss of cell-cell adhesion. Taken together, these results suggest a novel mechanism of regulation of AJ function in tumor progression that involves the correct targeting of the AJ components, and this is affected by the status of cortical actin, which can be differentially affected by E1A or Rac1.

The C terminus of E1A regulates tumor progression and epithelial cell differentiation

The E1A gene of adenovirus has been considered both a dominant oncogene and a tumor suppressor. It has been reported to induce epithelial cell but to prevent myoblast differentiation. E1A enables oncogenes that are unable to transform primary cells on their own to do so, yet suppresses tumor progression toward invasion and metastasis. To try to reconcile the seemingly, conflicting E1A phenotypes, we examined the expression of epithelial cell specific and characterizing proteins in immortalized or tumorigenically transformed primary epithelial cells expressing wild-type E1A or a C-terminal mutant that has lost tumor suppressive abilities. All the cell types continued to express cytokeratin. Epithelial cell morphology, social behavior, and growth characteristics were retained by cells expressing wild-type E1A, even in the presence of an activated ras oncogene. Mutant E1A-expressing cells were less well differentiated even in the absence of ras. They were specifically defective in cell-cell junctional complexes, such as tight and adherens junctions and desmosomes. There was also a preference for those actin structures prominent in fibroblasts: stress fibers and filopodia, while in the wild-type E1A expressing cells, cortical actin and circumferential actin filaments were dominant. Thus the E1A-mutant-expressing cells were already predisposed to a more advanced tumor stage even when they were only immortalized and not transformed. The results suggest the possibility that the C terminus of E1A may be involved in regulating epithelial mesenchymal transitions, which have previously been linked to tumor progression.

Expression of the pRb-binding regions of E1A enables efficient transformation of primary epithelial cells by v-src

Primary cultures of rat embryo fibroblasts have been shown to be resistant to transformation by dominant oncogenes such as v-src. We sought to determine if similar resistance is displayed by primary epithelial cells, and, if so, whether an immortalizing oncogene such as E1A could enhance transformation of primary epithelial cells by v-src. Transformation of primary rat epithelial cells by v-src was synergistically enhanced when E1A expression plasmids were cotransfected with a v-src expression plasmid. Foci were more numerous and observed earlier (9 to 14 days) with E1A plus v-src than with v-src alone (18 to 28 days). This cotransformation ability was abrogated by deletions in CR1 or CR2 of E1A, which encode the binding regions for the pRb family and are responsible for E1A-mediated cell cycle activation. Mutations in the p300 binding site or the second exon, which abolish immortalization, did not affect v-src cooperation, in contrast to ras and adenovirus E1B. While kinase activation was required for growth in soft agar, differential activation of Src kinase did not correlate with transformation efficiency. Cell morphology and actin structures were not dramatically impacted by E1A expression; thus, hypertransformation, as previously described for ras cotransformation, was not observed with v-src and second-exon mutants of E1A. However, growth rates for cells expressing both E1A and v-Src were higher than those for cells expressing only v-Src. These results suggest that functions involved in cell cycle activation encoded by E1A first exon can enhance v-src transformation of primary epithelial cells.

Rac1 and extracellularly regulated kinase activation are sufficient for E1A-dependent cooperative transformation of primary epithelial cells, but progression can only be modulated by E1A or Rac1

Ras transformation of fibroblast cell lines requires activation of multiple distinct signal pathways that act synergistically. E1A-ras cotransformation of primary epithelial cells is enhanced by distinct mutations in the second exon of E1A, resulting in "hypertransformation" and metastasis. The molecular and cellular differences in the in vitro properties of such transformed cells are characterized here. Hypertransformed cells grew faster and to higher saturation densities; had smaller, more refractile cell morphologies with pronounced actin microspikes; and were less adhesive when compared with wild-type (WT) E1A+ras-expressing cells. No significant differences were observed in extracellularly regulated kinase activity levels between the hypertransformed and WT transformed cells. Activated raf and Rac1 together were sufficient for transformation of primary epithelial cells with E1A, whereas neither alone was competent to cooperate with E1A. In the presence of activated ras and WT E1A, activated Rac1 expression effected all of the hypertransformation properties. Dominant-negative Rac1 expression was suppressive of the hypertransformation phenotype, including cell morphology, actin cytoskeletal structures, decreased growth rates, and increased adhesion. Thus, hypertransformation is not the result of extracellularly regulated kinase differences but can be effected by perturbations in Rac1 signals, as well as E1A 12S COOH-terminal mutants.

L-Arogenate Is a Chemoattractant Which Can Be Utilized as the Sole Source of Carbon and Nitrogen by Pseudomonas aeruginosa

L-Arogenate is a commonplace amino acid in nature in consideration of its role as a ubiquitous precursor of L-phenylalanine and/or L-tyrosine. However, the questions of whether it serves as a chemoattractant molecule and whether it can serve as a substrate for catabolism have never been studied. We found that Pseudomonas aeruginosa recognizes L-arogenate as a chemoattractant molecule which can be utilized as a source of both carbon and nitrogen. Mutants lacking expression of either cyclohexadienyl dehydratase or phenylalanine hydroxylase exhibited highly reduced growth rates when utilizing L-arogenate as a nitrogen source. Utilization of L-arogenate as a source of either carbon or nitrogen was dependent upon (sigma)(sup54), as revealed by the use of an rpoN null mutant. The evidence suggests that catabolism of L-arogenate proceeds via alternative pathways which converge at 4-hydroxyphenylpyruvate. In one pathway, prephenate formed in the periplasm by deamination of L-arogenate is converted to 4-hydroxyphenylpyruvate by cyclohexadienyl dehydrogenase. The second route depends upon the sequential action of periplasmic cyclohexadienyl dehydratase, phenylalanine hydroxylase, and aromatic aminotransferase.

Induction of a complex between rasGAP and a novel 110 kD protein is required for immortalization of primary epithelial cells by the E1A 12S oncoprotein of adenovirus

Although established cell lines can be transformed with oncogenic ras, primary epithelial cells cannot, but require the coexpression of an immortalizing oncogene, such as the E1A region of adenovirus. We have previously shown that immortalization of primary epithelial cells by E1A 12S requires the expression of five regions encoded by both the first and second exons of the gene. However, only three of these regions, located in the first exon, are required for cotransformation of primary cells with an activated ras oncogene. Thus, the expression of oncogenic ras is able to abrogate the need for the E1A function(s) encoded by the second exon that are required for immortalization. This suggested the possibility that the functions encoded by the second exon of E1A may involve or interact with the normal ras signal transduction pathway. The results described herein demonstrate that immortalization-competent 12S gene products induce the expression of a novel 110 kD protein, p110, that forms a stable complex with rasGAP. Failure to induce the p110-rasGAP complex results in the concomitant loss of ability of 12S to immortalize primary epithelial cells. The appearance of this complex parallels the expression of the 12S protein and is sensitive to the levels of E1A 12S. p110 induction is independent of the ability of 12S to activate the cell cycle and of the presence of adenovirus E1B and is not observed in the presence of the large T antigen of SV40. Thus, it is not a general response to proliferation or tumorigenic transformation, but rather seems to be specific to the immortalization function(s) of E1A 12S.

Neuropsychological and neuroanatomical correlates of confabulation

In the present exploratory investigation we report nine confabulatory patients of comparable age, education, and general level of intelligence in the acute epoch of recovery after rupture and clipping of ACoA aneurysms. Five of the nine cases had "spontaneous" confabulation, severe anterograde amnesia, markedly poor attentional and executive functions, and denial of illness. These patients all had multiple lesions that involved basal forebrain, ventral frontal lobe, and striatum. The other four patients manifested only "momentary" or "provoked" confabulations. These patients also had severe anterograde amnesia but showed relatively mild deficits in executive functions. These patients had lesions restricted to the basal forebrain except for one who had additional orbital frontal damage. Analysis of these two groups of confabulatory patients suggests that there is a common profile of deficits and anatomic foundation associated with confabulation; "spontaneous" confabulation appears to require extensive, simultaneous disruption of medial basal forebrain and frontal cognitive systems resulting in profound executive and memory deficits, whereas more limited lesions to the basal forebrain or orbital frontal cortex will result in "transient" or "provoked" confabulatory responses and a more restricted profile of cognitive deficits.

Neisseria gonorrhoeae possesses two nicotinamide adenine dinucleotide-independent lactate dehydrogenases

An important metabolic capability of Neisseria gonorrhoeae is the utilization of host-derived lactate. Two isoenzymes of the membrane-associated, pyridine dinucleotide-independent type of lactate dehydrogenase (iLDH) participate in lactate assimilation, but exhibit distinctive properties. Isoenzyme iLDH-I utilized lactate exclusively as substrate, exhibiting a preference for the D-isomer. In contrast, isoenzyme iLDH-II exhibited broad substrate specificity (lactate, phenyllactate, and 4-hydroxyphenyllactate), but was stereospecific for the L-isomers. These results explain the difficulty in isolating mutants unable to utilize lactate.

A monofunctional prephenate dehydrogenase created by cleavage of the 5' 109 bp of the tyrA gene from Erwinia herbicola

A cohesive phylogenetic cluster that is limited to enteric bacteria and a few closely related genera possesses a bifunctional protein that is known as the T-protein and is encoded by tyrA. The T-protein carries catalytic domains for chorismate mutase and for cyclohexadienyl dehydrogenase. Cyclohexadienyl dehydrogenase can utilize prephenate or L-arogenate as alternative substrates. A portion of the tyr A gene cloned from Erwinia herbicola was deleted in vitro with exonuclease III and fused in-frame with a 5' portion of lacZ to yield a new gene, denoted tyrA*, in which 37 N-terminal amino acids of the T-protein are replaced by 18 amino acids encoded by the polycloning site/5' portion of the lacZ alpha-peptide of pUC19. The TyrA* protein retained dehydrogenase activity but lacked mutase activity, thus demonstrating the separability of the two catalytic domains. While the Km of the TyrA* dehydrogenase for NAD+ remained unaltered, the Km for prephenate was fourfold greater and the Vmax was almost twofold greater than observed for the parental T-protein dehydrogenase. Activity with L-arogenate, normally a relatively poor substrate, was reduced to a negligible level. The prephenate dehydrogenase activity encoded by tyrA* was hypersensitive to feedback inhibition by L-tyrosine (a competitive inhibitor with respect to prephenate), partly because the affinity for prephenate was reduced and partly because the Ki value for L-tyrosine was decreased from 66 microM to 14 microM. Thus, excision of a portion of the chorismate mutase domain is shown to result in multiple extra-domain effects upon the cyclohexadienyl dehydrogenase domain of the bifunctional protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Lesion localization in apractic agraphia

Apractic agraphia is an impairment in writing in which the actual orthographic production of letters and words is abnormal despite normal sensorimotor function, visual feedback, and word and letter knowledge. We report one case and review the limited clinicoanatomical literature. Analysis of available cases supports the hypothesis that apractic agraphia is one of several related clinical disorders that are due to the loss of spatially and kinesthetically modulated movements. It is produced by lesions in the superior parietal lobule, usually in the hemisphere dominant for language.

Cyclohexadienyl dehydratase from Pseudomonas aeruginosa. Molecular cloning of the gene and characterization of the gene product

The gene encoding cyclohexadienyl dehydratase (denoted pheC) was cloned from Pseudomonas aeruginosa by functional complementation of a pheA auxotroph of Escherichia coli. The gene was highly expressed in E. coli due to the use of the high-copy number vector pUC18. The P. aeruginosa cyclohexadienyl dehydratase expressed in E. coli was purified to electrophoretic homogeneity. The latter enzyme exhibited identical physical and biochemical properties as those obtained for cyclohexadienyl dehydratase purified from P. aeruginosa. The activity ratios of prephenate dehydratase to arogenate dehydratase remained constant (about 3.3-fold) throughout purification, thus demonstrating a single protein having broad substrate specificity. The cyclohexadienyl dehydratase exhibited Km values of 0.42 mM for prephenate and 0.22 mM for L-arogenate, respectively. The pheC gene was 807 base pairs in length, encoding a protein with a calculated molecular mass of 30,480 daltons. This compares with a molecular mass value of 29.5 kDa determined for the purified enzyme by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Since the native molecular mass determined by gel filtration was 72 kDa, the enzyme probably is a homodimer. Comparison of the deduced amino acid sequence of pheC from P. aeruginosa with those of the prephenate dehydratases of Corynebacterium glutamicum, Bacillus subtilis, E. coli, and Pseudomonas stutzeri by standard pairwise alignments did not establish obvious homology. However, a more detailed analysis revealed a conserved motif (containing a threonine residue known to be essential for catalysis) that was shared by all of the dehydratase proteins.

Cloning, sequencing, and expression of the P-protein gene (pheA) of Pseudomonas stutzeri in Escherichia coli: implications for evolutionary relationships in phenylalanine biosynthesis

The pheA gene encoding the bifunctional P-protein (chorismate mutase:prephenate dehydratase) was cloned from Pseudomonas stutzeri and sequenced. This is the first gene of phenylalanine biosynthesis to be cloned and sequenced from Pseudomonas. The pheA gene was expressed in Escherichia coli, allowing complementation of an E. coli pheA auxotroph. The enzymic and physical properties of the P-protein from a recombinant E. coli auxotroph expressing the pheA gene were identical to those of the native enzyme from P. stutzeri. The nucleotide sequence of the P. stutzeri pheA gene was 1095 base pairs in length, predicting a 365-residue protein product with an Mr of 40,844. Codon usage in the P. stutzeri pheA gene was similar to that of Pseudomonas aeruginosa but unusual in that cytosine and guanine were used at nearly equal frequencies in the third codon position. The deduced P-protein product showed sequence homology with peptide sequences of the E. coli P-protein, the N-terminal portion of the E. coli T-protein (chorismate mutase:prephenate dehydrogenase), and the monofunctional prephenate dehydratases of Bacillus subtilis and Corynebacterium glutamicum. A narrow range of values (26-35%) for amino acid matches revealed by pairwise alignments of monofunctional and bifunctional proteins possessing activity for prephenate dehydratase suggests that extensive divergence has occurred between even the nearest phylogenetic lineages.

Reversed lateralization of cognitive functions in right handers. Exceptions to classical aphasiology

Most current and past research on the cerebral organization of cognitive functions has presupposed certain specialized hemisphere operations. At least for right handers, language and praxis are to be organized in the left hemisphere, while affective prosody, configurational spatial capacity, and global attention are lateralized in the right hemisphere. Deviations from these presuppositions, as in crossed aphasics and perhaps left handers, are generally considered to be 'exceptions' and either to disprove the rules or to be irrelevant to the rule. We report 4 very 'exceptional' cases, right handers with almost entirely reversed lateralization of functions. Analysis of the intrahemispheric relationships between functions suggests that there may be a specific neurobiology to the interrelationships between and among cognitive functions, handedness, and the intrahemisphere localization of the function.

Remnants of an ancient pathway to L-phenylalanine and L-tyrosine in enteric bacteria: evolutionary implications and biotechnological impact

The pathway construction for biosynthesis of aromatic amino acids in Escherichia coli is atypical of the phylogenetic subdivision of gram-negative bacteria to which it belongs (R. A. Jensen, Mol. Biol. Evol. 2:92-108, 1985). Related organisms possess second pathways to phenylalanine and tyrosine which depend upon the expression of a monofunctional chorismate mutase (CM-F) and cyclohexadienyl dehydratase (CDT). Some enteric bacteria, unlike E. coli, possess either CM-F or CDT. These essentially cryptic remnants of an ancestral pathway can be a latent source of biochemical potential under certain conditions. As one example of advantageous biochemical potential, the presence of CM-F in Salmonella typhimurium increases the capacity for prephenate accumulation in a tyrA auxotroph. We report the finding that a significant fraction of the latter prephenate is transaminated to L-arogenate. The tyrA19 mutant is now the organism of choice for isolation of L-arogenate, uncomplicated by the presence of other cyclohexadienyl products coaccumulated by a Neurospora crassa mutant that had previously served as the prime biological source of L-arogenate. Prephenate aminotransferase activity was not conferred by a discrete enzyme, but rather was found to be synonymous with the combined activities of aspartate aminotransferase (aspC), aromatic aminotransferase (tyrB), and branched-chain aminotransferase (ilvE). This conclusion was confirmed by results obtained with combinations of aspC-, tyrB-, and ilvE-deficient mutations in E. coli. An example of disadvantageous biochemical potential is the presence of a cryptic CDT in Klebsiella pneumoniae, where a mutant carrying multiple enzyme blocks is the standard organism used for accumulation and isolation of chorismate.(ABSTRACT TRUNCATED AT 250 WORDS)