Load Adaptation of Lamellipodial Actin Networks.

Actin filaments polymerizing against membranes power endocytosis, vesicular traffic, and cell motility. In vitro reconstitution studies suggest that the structure and the dynamics of actin networks respond to mechanical forces. We demonstrate that lamellipodial actin of migrating cells responds to mechanical load when membrane tension is modulated. In a steady state, migrating cell filaments assume the canonical dendritic geometry, defined by Arp2/3-generated 70° branch points. Increased tension triggers a dense network with a broadened range of angles, whereas decreased tension causes a shift to a sparse configuration dominated by filaments growing perpendicularly to the plasma membrane. We show that these responses emerge from the geometry of branched actin: when load per filament decreases, elongation speed increases and perpendicular filaments gradually outcompete others because they polymerize the shortest distance to the membrane, where they are protected from capping. This network-intrinsic geometrical adaptation mechanism tunes protrusive force in response to mechanical load.

Inhibitory signalling to the Arp2/3 complex steers cell migration.

Cell migration requires the generation of branched actin networks that power the protrusion of the plasma membrane in lamellipodia. The actin-related proteins 2 and 3 (Arp2/3) complex is the molecular machine that nucleates these branched actin networks. This machine is activated at the leading edge of migrating cells by Wiskott-Aldrich syndrome protein (WASP)-family verprolin-homologous protein (WAVE, also known as SCAR). The WAVE complex is itself directly activated by the small GTPase Rac, which induces lamellipodia. However, how cells regulate the directionality of migration is poorly understood. Here we identify a new protein, Arpin, that inhibits the Arp2/3 complex in vitro, and show that Rac signalling recruits and activates Arpin at the lamellipodial tip, like WAVE. Consistently, after depletion of the inhibitory Arpin, lamellipodia protrude faster and cells migrate faster. A major role of this inhibitory circuit, however, is to control directional persistence of migration. Indeed, Arpin depletion in both mammalian cells and Dictyostelium discoideum amoeba resulted in straighter trajectories, whereas Arpin microinjection in fish keratocytes, one of the most persistent systems of cell migration, induced these cells to turn. The coexistence of the Rac-Arpin-Arp2/3 inhibitory circuit with the Rac-WAVE-Arp2/3 activatory circuit can account for this conserved role of Arpin in steering cell migration.

Subdural haematoma mimics.

A subdural haematoma (SDH) is a frequently encountered pathology seen on an emergency room computed tomography (CT) head scan. An extra-axial crescentic density along the convexity of the brain or within the interhemispheric fissure is generally thought to represent a SDH; however, SDH mimics are known to occur in nature, and can be broadly classified under the subcategories of normal anatomy, artefacts, tumour, inflammation, infection, ischaemia, trauma, and iatrogenic. Understanding the typical characteristics of a SDH, knowledge of normal anatomy, close inspection of the morphology of the subdural process, changes to the adjacent structures, and rigorous attention to clinical details may reveal subtle clues that distinguish a true SDH from a mimic. This is crucial in appropriately directing clinical management. This review amalgamates most of the rare subdural processes that have been reported to mimic SDH, and discusses the imaging and clinical features that help to differentiate between them. This topic is highly valuable for radiology trainees, general radiologists, and emergency room physicians, and may serve as a refresher for the practising neuroradiologist.

Electron tomography and simulation of baculovirus actin comet tails support a tethered filament model of pathogen propulsion.

Several pathogens induce propulsive actin comet tails in cells they invade to disseminate their infection. They achieve this by recruiting factors for actin nucleation, the Arp2/3 complex, and polymerization regulators from the host cytoplasm. Owing to limited information on the structural organization of actin comets and in particular the spatial arrangement of filaments engaged in propulsion, the underlying mechanism of pathogen movement is currently speculative and controversial. Using electron tomography we have resolved the three-dimensional architecture of actin comet tails propelling baculovirus, the smallest pathogen yet known to hijack the actin motile machinery. Comet tail geometry was also mimicked in mixtures of virus capsids with purified actin and a minimal inventory of actin regulators. We demonstrate that propulsion is based on the assembly of a fishbone-like array of actin filaments organized in subsets linked by branch junctions, with an average of four filaments pushing the virus at any one time. Using an energy-minimizing function we have simulated the structure of actin comet tails as well as the tracks adopted by baculovirus in infected cells in vivo. The results from the simulations rule out gel squeezing models of propulsion and support those in which actin filaments are continuously tethered during branch nucleation and polymerization. Since Listeria monocytogenes, Shigella flexneri, and Vaccinia virus among other pathogens use the same common toolbox of components as baculovirus to move, we suggest they share the same principles of actin organization and mode of propulsion.

Spectrally coded optical nanosectioning (SpecON) with biocompatible metal-dielectric-coated substrates.

Fluorescence nanosectioning within a submicron region above an interface is desirable for many disciplines in the life sciences. A drawback, however, to most current approaches is the a priori need to physically scan a sculptured point spread function in the axial dimension, which can be undesirable for optically sensitive or highly dynamic samples. Here we demonstrate a fluorescence imaging approach that can overcome the need for scanning by exploiting the position-dependent emission spectrum of fluorophores above a simple biocompatible nanostructure. To achieve this we have designed a thin metal-dielectric-coated substrate, where the spectral modification to the total measured fluorescence can be used to estimate the axial fluorophore distribution within distances of 10-150 nm above the substrate with an accuracy of up to 5-10 nm. The modeling and feasibility of the approach are verified and successfully applied to elucidate nanoscale adhesion protein and filopodia dynamics in migrating cells. It is likely that the general principle can find broader applications in, for example, single-molecule studies, biosensing, and studying fast dynamic processes.

Rac function is crucial for cell migration but is not required for spreading and focal adhesion formation.

Cell migration is commonly accompanied by protrusion of membrane ruffles and lamellipodia. In two-dimensional migration, protrusion of these thin sheets of cytoplasm is considered relevant to both exploration of new space and initiation of nascent adhesion to the substratum. Lamellipodium formation can be potently stimulated by Rho GTPases of the Rac subfamily, but also by RhoG or Cdc42. Here we describe viable fibroblast cell lines genetically deficient for Rac1 that lack detectable levels of Rac2 and Rac3. Rac-deficient cells were devoid of apparent lamellipodia, but these structures were restored by expression of either Rac subfamily member, but not by Cdc42 or RhoG. Cells deficient in Rac showed strong reduction in wound closure and random cell migration and a notable loss of sensitivity to a chemotactic gradient. Despite these defects, Rac-deficient cells were able to spread, formed filopodia and established focal adhesions. Spreading in these cells was achieved by the extension of filopodia followed by the advancement of cytoplasmic veils between them. The number and size of focal adhesions as well as their intensity were largely unaffected by genetic removal of Rac1. However, Rac deficiency increased the mobility of different components in focal adhesions, potentially explaining how Rac - although not essential - can contribute to focal adhesion assembly. Together, our data demonstrate that Rac signaling is essential for lamellipodium protrusion and for efficient cell migration, but not for spreading or filopodium formation. Our findings also suggest that Rac GTPases are crucial to the establishment or maintenance of polarity in chemotactic migration.

Pushing with actin: from cells to pathogens.

Actin polymerization is harnessed by cells to generate lamellipodia for movement and by a subclass of pathogens to facilitate invasion of their infected hosts. Using electron tomography (ET), we have shown that lamellipodia are formed via the generation of subsets of actin filaments joined by branch junctions. Image averaging produced a 2.9 nm resolution model of branch junctions in situ and revealed a close fit to the electron density map of the actin-related protein 2/3 (Arp2/3)-actin complex in vitro. Correlated live-cell imaging and ET was also used to determine how actin networks are created and remodelled during the initiation and inhibition of protrusion in lamellipodia. Listeria, Rickettsia and viruses, such as vaccinia virus and baculovirus, exploit the actin machinery of host cells to generate propulsive actin comet tails to disseminate their infection. By applying ET, we have shown that baculovirus generates at its rear a fishbone-like array of subsets of branched actin filaments, with an average of only four filaments engaged in pushing at any one time. In both of these studies, the application of ET of negatively stained cytoskeletons for higher filament resolution and cryo-ET for preserving overall 3D morphology was crucial for obtaining a complete structure-function analysis of actin-driven propulsion.

Actin branching in the initiation and maintenance of lamellipodia.

Using correlated live-cell imaging and electron tomography we found that actin branch junctions in protruding and treadmilling lamellipodia are not concentrated at the front as previously supposed, but link actin filament subsets in which there is a continuum of distances from a junction to the filament plus ends, for up to at least 1 µm. When branch sites were observed closely spaced on the same filament their separation was commonly a multiple of the actin helical repeat of 36 nm. Image averaging of branch junctions in the tomograms yielded a model for the in vivo branch at 2.9 nm resolution, which was comparable with that derived for the in vitro actin-Arp2/3 complex. Lamellipodium initiation was monitored in an intracellular wound-healing model and was found to involve branching from the sides of actin filaments oriented parallel to the plasmalemma. Many filament plus ends, presumably capped, terminated behind the lamellipodium tip and localized on the dorsal and ventral surfaces of the actin network. These findings reveal how branching events initiate and maintain a network of actin filaments of variable length, and provide the first structural model of the branch junction in vivo. A possible role of filament capping in generating the lamellipodium leaflet is discussed and a mathematical model of protrusion is also presented.

Microsatellite DNA mutations in double-crested cormorants (Phalacrocorax auritus) associated with exposure to PAH-containing industrial air pollution.

Hamilton Harbour, Ontario, Canada is one of the most polluted sites on the Great Lakes, and is subject to substantial airborne pollution due to emissions from both heavy industry and intense vehicle traffic. Mutagenic Polycyclic aromatic hydrocarbons (PAHs) are present at very high concentrations in the air and sediment of Hamilton Harbour. We used five variable DNA microsatellites to screen for mutations in 97 families of Double-crested Cormorants (Phalacrocorax auritus) from three wild colonies, two in Hamilton Harbour and one in cleaner northeastern Lake Erie. Mutations were identified in all five microsatellites at low frequencies, with the majority of mutations found in chicks from the Hamilton Harbour site closest to industrial sources of PAH contamination. Microsatellite mutation rates were 6-fold higher at the Hamilton Harbour site closest to the industrial sources of PAH contamination than the other Hamilton Harbour site, and both were higher than the reference colony. A Phase I metabolite of the PAH benzo[a]pyrene identified by LC-MS/MS in bile and liver from Hamilton Harbour cormorant chicks suggests that these cormorants are exposed to and metabolizing PAHs, highlighting their potential to have caused the observed mutations.

Cofilin cooperates with fascin to disassemble filopodial actin filaments.

Cells use a large repertoire of proteins to remodel the actin cytoskeleton. Depending on the proteins involved, F-actin is organized in specialized protrusions such as lamellipodia or filopodia, which serve diverse functions in cell migration and sensing. Although factors responsible for directed filament assembly in filopodia have been extensively characterized, the mechanisms of filament disassembly in these structures are mostly unknown. We investigated how the actin-depolymerizing factor cofilin-1 affects the dynamics of fascincrosslinked actin filaments in vitro and in live cells. By multicolor total internal reflection fluorescence microscopy and fluorimetric assays, we found that cofilin-mediated severing is enhanced in fascin-crosslinked bundles compared with isolated filaments, and that fascin and cofilin act synergistically in filament severing. Immunolabeling experiments demonstrated for the first time that besides its known localization in lamellipodia and membrane ruffles, endogenous cofilin can also accumulate in the tips and shafts of filopodia. Live-cell imaging of fluorescently tagged proteins revealed that cofilin is specifically targeted to filopodia upon stalling of protrusion and during their retraction. Subsequent electron tomography established filopodial actin filament and/or bundle fragmentation to precisely correlate with cofilin accumulation. These results identify a new mechanism of filopodium disassembly involving both fascin and cofilin.

Lakota elders' views on traditional versus commercial/addictive tobacco use; oral history depicting a fundamental distinction.

This qualitative study is intended to elucidate Lakota elders' views on traditional tobacco and commercial/addictive tobacco use, capturing the oral history that depict the cultural protocol regarding traditional tobacco, called Cansasa. Commercial tobacco use has significantly impacted the Northern Plains Indians. National surveillance systems report that tobacco use is more prevalent among American Indian/Alaska Natives than any other population, and is notably higher than the national average. Lung cancer among Native Americans is highest in the Northern Plains and Alaska, where smoking prevalence is also the highest, and smoking is responsible for nearly 90 % of all lung cancer cases. Yet, the use of traditional tobacco is largely ignored by surveillance and seems to have a distinct, positive role. Using a community-based participatory research approach, semi-structured interviews, and qualitative analysis tools, the research team, including 2 Lakota tribe elders, Lakota speaking tribal college students, and university faculty, sought to discern tribal elders' distinctions between traditional and the addictive commercial tobacco. The team interviewed thirty Lakota elders, transcribed the interviews and field notes, and analyzed them using immersion/crystallization organizing framework. The research design engaged the Lakota tribal community in all stages, from planning to publication. Analysis revealed a clear distinction between traditional and commercial tobacco: tribal elders conveyed strong positive messages connected to traditional tobacco use (i.e., spirituality, respect, health and wellness, humility, and thoughtfulness) versus strong negative messages linked to addictive tobacco (i.e., crime, loss of control and self-esteem, lack of respect to self and others, sickness and death). These messages, along with stories in the Lakota language that were told and recorded during the interviews, can guide new ways to address addictive tobacco prevention in this community, to enhance cultural pride, and to serve as a cross-generation bridge regarding tobacco use.

The Dominant Anterior Thoracic Artery of the Spinal Cord.

BACKGROUND AND PURPOSE: Dominant radiculomedullary arteries such as the artery of lumbar enlargement and the artery of cervical enlargement are well-documented. However, variability exists as to the size, number, and location of other radiculomedullary arteries contributing supply to the anterior spinal artery. The aim of this anatomic study was to document the prevalence and characteristics of the dominant anterior thoracic artery in cadaveric specimens. MATERIALS AND METHODS: Microsurgical dissection of cadaveric human spinal cord specimens (n = 50) was conducted. The artery of lumbar enlargement was injected with colored latex until the small-caliber arterial vessels were filled. The dominant anterior thoracic artery was identified, injected, and filled with diluted industrial paint. The course, diameter, and location of the dominant anterior thoracic artery, artery of lumbar enlargement, and artery of cervical enlargement were documented. RESULTS: The artery of lumbar enlargement was identified between T3 and L2 in all 50 specimens (100%), and the artery of cervical enlargement was identified in 84% of specimens (42/50). At least 1 dominant anterior thoracic artery distinct from the artery of lumbar enlargement and the artery of cervical enlargement was identified between T1 and T11 in 47 of the 50 specimens (94%). The most frequent origin of the dominant anterior thoracic artery was at the level of T4 on the left. The average size of the dominant anterior thoracic artery was 0.446 mm (range, 0.300-0.759 mm on the left and 0.270-0.569 mm on the right). CONCLUSIONS: A dominant anterior thoracic artery is present in 94% of individuals. Variations of the arterial supply to the anterior thoracic cord are of great importance due to their implications for ischemic events as well as surgical and endovascular procedures.

Actin filament tracking in electron tomograms of negatively stained lamellipodia using the localized radon transform.

The aim of this work was to develop a protocol for automated tracking of actin filaments in electron tomograms of lamellipodia embedded in negative stain. We show that a localized version of the Radon transform for the detection of filament directions enables three-dimensional visualizations of filament network architecture, facilitating extraction of statistical information including orientation profiles. We discuss the requirements for parameter selection set by the raw image data in the context of other, similar tracking protocols.

Clustering of VASP actively drives processive, WH2 domain-mediated actin filament elongation.

Vasodilator-stimulated phosphoprotein (VASP) is a key regulator of dynamic actin structures like filopodia and lamellipodia, but its precise function in their formation is controversial. Using in vitro TIRF microscopy, we show for the first time that both human and Dictyostelium VASP are directly involved in accelerating filament elongation by delivering monomeric actin to the growing barbed end. In solution, DdVASP markedly accelerated actin filament elongation in a concentration-dependent manner but was inhibited by low concentrations of capping protein (CP). In striking contrast, VASP clustered on functionalized beads switched to processive filament elongation that became insensitive even to very high concentrations of CP. Supplemented with the in vivo analysis of VASP mutants and an EM structure of the protein, we propose a mechanism by which membrane-associated VASP oligomers use their WH2 domains to effect both the tethering of actin filaments and their processive elongation in sites of active actin assembly.

Arp2/3 complex interactions and actin network turnover in lamellipodia.

Cell migration is initiated by lamellipodia-membrane-enclosed sheets of cytoplasm containing densely packed actin filament networks. Although the molecular details of network turnover remain obscure, recent work points towards key roles in filament nucleation for Arp2/3 complex and its activator WAVE complex. Here, we combine fluorescence recovery after photobleaching (FRAP) of different lamellipodial components with a new method of data analysis to shed light on the dynamics of actin assembly/disassembly. We show that Arp2/3 complex is incorporated into the network exclusively at the lamellipodium tip, like actin, at sites coincident with WAVE complex accumulation. Capping protein likewise showed a turnover similar to actin and Arp2/3 complex, but was confined to the tip. In contrast, cortactin-another prominent Arp2/3 complex regulator-and ADF/cofilin-previously implicated in driving both filament nucleation and disassembly-were rapidly exchanged throughout the lamellipodium. These results suggest that Arp2/3- and WAVE complex-driven actin filament nucleation at the lamellipodium tip is uncoupled from the activities of both cortactin and cofilin. Network turnover is additionally regulated by the spatially segregated activities of capping protein at the tip and cofilin throughout the mesh.

Reconstructing the orientation distribution of actin filaments in the lamellipodium of migrating keratocytes from electron microscopy tomography data.

Migration of motile cells on flat substrates is usually driven by the polymerization of a flat actin filament network. Theoretical models have made different predictions regarding the distribution of the filament orientation in the lamellipodium with respect to the direction of motion. Here we show how one can automatically reconstruct the orientation distribution of actin filaments in the lamellipodium of migrating keratocytes from electron microscopy tomography data. We use two different image analysis methods, an algorithm which explicitly extracts an abstract network representation and an analysis of the gray scale information based on the structure tensor. We show that the two approaches give similar results, both for simulated data and for electron microscopy tomography data from migrating keratocytes. For the lamellipodium at the leading edge of fast moving cells, we find an orientation distribution that is peaked at +35/-35 degrees. For the lamellipodium at the leading edge of slow moving cells as well as for the lamellipodium at the flanks of fast moving cells, one broad peak around 0 degree dominates the distribution.

The comings and goings of actin: coupling protrusion and retraction in cell motility.

Cells utilize actin filaments to produce protrusive and contractile arrays that cooperate to drive cell motility. The generation of the two arrays and the coupling between them result from the unique properties of the lamellipodium, a protrusive leaflet of cytoplasm at the cell edge. From the lamellipodium into the lamella behind, there is a transition from a fast retrograde flow of actin polymer driven by polymerization to a slow flow driven by the interaction of anti-parallel arrays of actin with myosin. In addition to driving protrusion, the lamellipodium appears to play a role in supplying filaments to the lamella for the assembly of the contractile network required for traction.

Microtubules meet substrate adhesions to arrange cell polarity.

Cell movement is driven by the regulated and polarised turnover of the actin cytoskeleton and of the adhesion complexes that link it to the extracellular matrix. For most cells, polarisation requires the engagement of microtubules, which exert their effect by mediating changes in the activity of the Rho GTPases. Evidence suggests that these changes are effected in a very localised fashion at sites of substrate adhesion, via specific microtubule-targeting interactions. Targeting serves to bring molecular complexes bound at the tips and along microtubules in close proximity with adhesion complexes, to promote adhesion disassembly and remodelling of the actin cytoskeleton.

Functional design in the actin cytoskeleton.

Changes in cell shape, anchorage and motility are all associated with the dynamic reorganisation of the architectural arrays of actin filaments that make up the actin cytoskeleton. The relative expression of these functionally different actin filament arrays is intimately linked to the pattern of contacts that a cell develops with its extracellular substrate. Cell polarity is acquired by the development of an asymmetric pattern of substrate contacts, effected in a specific, site-directed manner by the delivery of adhesion-site modulators along microtubules.

CTA Evaluation of Basilar Septations: An Entity Better Characterized as Aberrant Basilar Fenestrations.

BACKGROUND AND PURPOSE: A basilar artery intraluminal septation is an exceedingly rarely reported, presumed congenital abnormality. In our clinical practice, we have occasionally noticed an intraluminal band within the inferior aspect of the basilar artery on CTA. Furthermore, we have noticed, at times, the presence of a punctate calcification associated with this finding. We hypothesized that what previous studies have called "basilar septations" in fact represent miniature and thus aberrant basilar fenestrations. MATERIALS AND METHODS: We retrospectively reviewed CTA studies obtained between January 1, 2017, and August 31, 2019. Identified intraluminal basilar abnormalities were classified as either basilar septations or basilar fenestrations. Association with other posterior circulation abnormalities was documented. RESULTS: A total of 3509 studies were examined. A basilar intraluminal abnormality was evident in 80 patients (2.3%). Of these 80 patients, 59 were classified as having a basilar fenestration (1.7%) and 21 were classified as having basilar septations (0.6%). Associated calcification was evident in 3 of the basilar fenestration cases and 13 of the basilar septation cases. CONCLUSIONS: Basilar septations most likely represent and should be referred to as aberrant basilar fenestrations. They should be interpreted as benign congenital incidental findings and should not be misinterpreted as focal dissections or arterial webs. Important variations in the morphology of aberrant basilar fenestrations exist, including areas of thinning, varying thickness, and nodularity. Therefore, when associated with calcification or nodularity, aberrant basilar fenestrations should not be confused with focal intraluminal thrombi or calcified or noncalcified emboli.