Wesley R. Legant

Degradation-mediated cellular traction directs stem cell fate in covalently crosslinked three-dimensional hydrogels

Although cell-matrix adhesive interactions are known to regulate stem cell differentiation, the underlying mechanisms, in particular for direct three-dimensional (3D) encapsulation within hydrogels, are poorly understood. Here, we demonstrate that in covalently crosslinked hyaluronic acid (HA) hydrogels, the differentiation of human mesenchymal stem cells (hMSCs) is directed by the generation of degradation-mediated cellular-traction, independent of cell morphology or matrix mechanics. hMSCs within HA hydrogels of equivalent elastic moduli that either permit (restrict) cell-mediated degradation exhibited high (low) degrees of cell spreading and high (low) tractions, and favoured osteogenesis (adipogenesis). In addition, switching the permissive hydrogel to a restrictive state via delayed secondary crosslinking reduced further hydrogel degradation, suppressed traction, and caused a switch from osteogenesis to adipogenesis in the absence of changes to the extended cellular morphology. Also, inhibiting tension-mediated signalling in the permissive environment mirrored the effects of delayed secondary crosslinking, whereas upregulating tension induced osteogenesis even in the restrictive environment.

Lattice light-sheet microscopy: Imaging molecules to embryos at high spatiotemporal resolution

Introduction In vivo imaging provides a window into the spatially complex, rapidly evolving physiology of the cell that structural imaging alone cannot. However, observing this physiology directly involves inevitable tradeoffs of spatial resolution, temporal resolution, and phototoxicity. This is especially true when imaging in three dimensions, which is essential to obtain a complete picture of many dynamic subcellular processes. Although traditional in vivo imaging tools, such as widefield and confocal microscopy, and newer ones, such as light-sheet microscopy, can image in three dimensions, they sacrifice substantial spatiotemporal resolution to do so and, even then, can often be used for only very limited durations before altering the physiological state of the specimen. Lattice light-sheet microscopy. An ultrathin structured light sheet (blue-green, center) excites fluorescence (orange) in successive planes as it sweeps through a specimen (gray) to generate a 3D image. The speed, noninvasiveness, and high spatial resolution of this approach make it a promising tool for in vivo 3D imaging of fast dynamic processes in cells and embryos, as shown here in five surrounding examples. Lattice light-sheet microscopy. An ultrathin structured light sheet (blue-green, center) excites fluorescence (orange) in successive planes as it sweeps through a specimen (gray) to generate a 3D image. The speed, noninvasiveness, and high spatial resolution of this approach make it a promising tool for in vivo 3D imaging of fast dynamic processes in cells and embryos, as shown here in five surrounding examples. Rationale To address these limitations, we developed a new microscope using ultrathin light sheets derived from two-dimensional (2D) optical lattices. These are scanned plane-by-plane through the specimen to generate a 3D image. The thinness of the sheet leads to high axial resolution and negligible photobleaching and background outside of the focal plane, while its simultaneous illumination of the entire field of view permits imaging at hundreds of planes per second even at extremely low peak excitation intensities. By implementing either superresolution structured illumination or by dithering the lattice to create a uniform light sheet, we imaged cells and small embryos in three dimensions, often at subsecond intervals, for hundreds to thousands of time points at the diffraction limit and beyond. Results We demonstrated the technique on 20 different biological processes spanning four orders of magnitude in space and time, including the binding kinetics of single Sox2 transcription factor molecules, 3D superresolution photoactivated localization microscopy of nuclear lamins, dynamic organelle rearrangements and 3D tracking of microtubule plus ends during mitosis, neutrophil motility in a collagen mesh, and subcellular protein localization and dynamics during embryogenesis in Caenorhabditis elegans and Drosophila melanogaster. Throughout, we established the performance advantages of lattice light-sheet microscopy compared with previous techniques and highlighted phenomena that, when seen at increased spatiotemporal detail, may hint at previously unknown biological mechanisms. Conclusion Photobleaching and phototoxicity are typically reduced by one to two orders of magnitude relative to that seen with a 1D scanned Bessel beam or the point array scanned excitation of spinning disk confocal microscopy. This suggests that the instantaneous peak power delivered to the specimen may be an even more important metric of cell health than the total photon dose and should enable extended 3D observation of endogenous levels of even sparsely expressed proteins produced by genome editing. Improvements of similar magnitude in imaging speed and a twofold gain in axial resolution relative to confocal microscopy yield 4D spatiotemporal resolution high enough to follow fast, nanoscale dynamic processes that would otherwise be obscured by poor resolution along one or more axes of spacetime. Last, the negligible background makes lattice light-sheet microscopy a promising platform for the extension of all methods of superresolution to larger and more densely fluorescent specimens and enables the study of signaling, transport, and stochastic self-assembly in complex environments with single-molecule sensitivity. From single molecules to embryos in living color Animation defines life, and the three-dimensional (3D) imaging of dynamic biological processes occurring within living specimens is essential to understand life. However, in vivo imaging, especially in 3D, involves inevitable tradeoffs of resolution, speed, and phototoxicity. Chen et al. describe a microscope that can address these concerns. They used a class of nondiffracting beams, known as 2D optical lattices, which spread the excitation energy across the entire field of view while simultaneously eliminating out-of-focus excitation. Lattice light sheets increase the speed of image acquisition and reduce phototoxicity, which expands the range of biological problems that can be investigated. The authors illustrate the power of their approach using 20 distinct biological systems ranging from single-molecule binding kinetics to cell migration and division, immunology, and embryonic development. Science, this issue 10.1126/science.1257998 A new microscope allows three-dimensional imaging of living systems at very high resolution in real time. Although fluorescence microscopy provides a crucial window into the physiology of living specimens, many biological processes are too fragile, are too small, or occur too rapidly to see clearly with existing tools. We crafted ultrathin light sheets from two-dimensional optical lattices that allowed us to image three-dimensional (3D) dynamics for hundreds of volumes, often at subsecond intervals, at the diffraction limit and beyond. We applied this to systems spanning four orders of magnitude in space and time, including the diffusion of single transcription factor molecules in stem cell spheroids, the dynamic instability of mitotic microtubules, the immunological synapse, neutrophil motility in a 3D matrix, and embryogenesis in Caenorhabditis elegans and Drosophila melanogaster. The results provide a visceral reminder of the beauty and the complexity of living systems.

Measurement of mechanical tractions exerted by cells in three-dimensional matrices

Quantitative measurements of cell-generated forces have heretofore required that cells be cultured on two-dimensional substrates. We describe a technique to quantitatively measure three-dimensional traction forces exerted by cells fully encapsulated in well-defined elastic hydrogel matrices. Using this approach we measured traction forces for several cell types in various contexts and revealed patterns of force generation attributable to morphologically distinct regions of cells as they extend into the surrounding matrix.

Single-molecule dynamics of enhanceosome assembly in embryonic stem cells.

Enhancer-binding pluripotency regulators (Sox2 and Oct4) play a seminal role in embryonic stem (ES) cell-specific gene regulation. Here, we combine in vivo and in vitro single-molecule imaging, transcription factor (TF) mutagenesis, and ChIP-exo mapping to determine how TFs dynamically search for and assemble on their cognate DNA target sites. We find that enhanceosome assembly is hierarchically ordered with kinetically favored Sox2 engaging the target DNA first, followed by assisted binding of Oct4. Sox2/Oct4 follow a trial-and-error sampling mechanism involving 84-97 events of 3D diffusion (3.3-3.7 s) interspersed with brief nonspecific collisions (0.75-0.9 s) before acquiring and dwelling at specific target DNA (12.0-14.6 s). Sox2 employs a 3D diffusion-dominated search mode facilitated by 1D sliding along open DNA to efficiently locate targets. Our findings also reveal fundamental aspects of gene and developmental regulation by fine-tuning TF dynamics and influence of the epigenome on target search parameters.

Microfabricated tissue gauges to measure and manipulate forces from 3D microtissues

Physical forces generated by cells drive morphologic changes during development and can feedback to regulate cellular phenotypes. Because these phenomena typically occur within a 3-dimensional (3D) matrix in vivo, we used microelectromechanical systems (MEMS) technology to generate arrays of microtissues consisting of cells encapsulated within 3D micropatterned matrices. Microcantilevers were used to simultaneously constrain the remodeling of a collagen gel and to report forces generated during this process. By concurrently measuring forces and observing matrix remodeling at cellular length scales, we report an initial correlation and later decoupling between cellular contractile forces and changes in tissue morphology. Independently varying the mechanical stiffness of the cantilevers and collagen matrix revealed that cellular forces increased with boundary or matrix rigidity whereas levels of cytoskeletal and extracellular matrix (ECM) proteins correlated with levels of mechanical stress. By mapping these relationships between cellular and matrix mechanics, cellular forces, and protein expression onto a bio-chemo-mechanical model of microtissue contractility, we demonstrate how intratissue gradients of mechanical stress can emerge from collective cellular contractility and finally, how such gradients can be used to engineer protein composition and organization within a 3D tissue. Together, these findings highlight a complex and dynamic relationship between cellular forces, ECM remodeling, and cellular phenotype and describe a system to study and apply this relationship within engineered 3D microtissues.

Bioactive Hydrogels Made from Step-Growth Derived PEG-Peptide Macromers

Synthetic hydrogels based on poly(ethylene glycol) (PEG) have been used as biomaterials for cell biology and tissue engineering investigations. Bioactive PEG-based gels have largely relied on heterobifunctional or multi-arm PEG precursors that can be difficult to synthesize and characterize or expensive to obtain. Here, we report an alternative strategy, which instead uses inexpensive and readily available PEG precursors to simplify reactant sourcing. This new approach provides a robust system in which to probe cellular interactions with the microenvironment. We used the step-growth polymerization of PEG diacrylate (PEGDA, 3400Da) with bis-cysteine matrix metalloproteinase (MMP)-sensitive peptides via Michael-type addition to form biodegradable photoactive macromers of the form acrylate-PEG-(peptide-PEG)(m)-acrylate. The molecular weight (MW) of these macromers is controlled by the stoichiometry of the reaction, with a high proportion of resultant macromer species greater than 500kDa. In addition, the polydispersity of these materials was nearly identical for three different MMP-sensitive peptide sequences subjected to the same reaction conditions. When photopolymerized into hydrogels, these high MW materials exhibit increased swelling and sensitivity to collagenase-mediated degradation as compared to previously published PEG hydrogel systems. Cell-adhesive acrylate-PEG-CGRGDS was synthesized similarly and its immobilization and stability in solid hydrogels was characterized with a modified Lowry assay. To illustrate the functional utility of this approach in a biological setting, we applied this system to develop materials that promote angiogenesis in an ex vivo aortic arch explant assay. We demonstrate the formation and invasion of new sprouts mediated by endothelial cells into the hydrogels from embedded embryonic chick aortic arches. Furthermore, we show that this capillary sprouting and three-dimensional migration of endothelial cells can be tuned by engineering the MMP-susceptibility of the hydrogels and the presence of functional immobilized adhesive ligands (CGRGDS vs. CGRGES peptide). The facile chemistry described and significant cellular responses observed suggest the usefulness of these materials in a variety of in vitro and ex vivo biologic investigations, and may aid in the design or refinement of material systems for a range of tissue engineering approaches.

Multidimensional traction force microscopy reveals out-of-plane rotational moments about focal adhesions

Recent methods have revealed that cells on planar substrates exert both shear (in-plane) and normal (out-of-plane) tractions against the extracellular matrix (ECM). However, the location and origin of the normal tractions with respect to the adhesive and cytoskeletal elements of cells have not been elucidated. We developed a high-spatiotemporal-resolution, multidimensional (2.5D) traction force microscopy to measure and model the full 3D nature of cellular forces on planar 2D surfaces. We show that shear tractions are centered under elongated focal adhesions whereas upward and downward normal tractions are detected on distal (toward the cell edge) and proximal (toward the cell body) ends of adhesions, respectively. Together, these forces produce significant rotational moments about focal adhesions in both protruding and retracting peripheral regions. Temporal 2.5D traction force microscopy analysis of migrating and spreading cells shows that these rotational moments are highly dynamic, propagating outward with the leading edge of the cell. Finally, we developed a finite element model to examine how rotational moments could be generated about focal adhesions in a thin lamella. Our model suggests that rotational moments can be generated largely via shear lag transfer to the underlying ECM from actomyosin contractility applied at the intracellular surface of a rigid adhesion of finite thickness. Together, these data demonstrate and probe the origin of a previously unappreciated multidimensional stress profile associated with adhesions and highlight the importance of new approaches to characterize cellular forces.

Actin Depletion Initiates Events Leading to Granule Secretion at the Immunological Synapse

Summary Cytotoxic T lymphocytes (CTLs) use polarized secretion to rapidly destroy virally infected and tumor cells. To understand the temporal relationships between key events leading to secretion, we used high-resolution 4D imaging. CTLs approached targets with actin-rich projections at the leading edge, creating an initially actin-enriched contact with rearward-flowing actin. Within 1 min, cortical actin reduced across the synapse, T cell receptors (TCRs) clustered centrally to form the central supramolecular activation cluster (cSMAC), and centrosome polarization began. Granules clustered around the moving centrosome within 2.5 min and reached the synapse after 6 min. TCR-bearing intracellular vesicles were delivered to the cSMAC as the centrosome docked. We found that the centrosome and granules were delivered to an area of membrane with reduced cortical actin density and phospholipid PIP2. These data resolve the temporal order of events during synapse maturation in 4D and reveal a critical role for actin depletion in regulating secretion.

High-density three-dimensional localization microscopy across large volumes

Extending three-dimensional (3D) single-molecule localization microscopy away from the coverslip and into thicker specimens will greatly broaden its biological utility. However, because of the limitations of both conventional imaging modalities and conventional labeling techniques, it is a challenge to localize molecules in three dimensions with high precision in such samples while simultaneously achieving the labeling densities required for high resolution of densely crowded structures. Here we combined lattice light-sheet microscopy with newly developed, freely diffusing, cell-permeable chemical probes with targeted affinity for DNA, intracellular membranes or the plasma membrane. We used this combination to perform high–localization precision, ultrahigh–labeling density, multicolor localization microscopy in samples up to 20 μm thick, including dividing cells and the neuromast organ of a zebrafish embryo. We also demonstrate super-resolution correlative imaging with protein-specific photoactivable fluorophores, providing a mutually compatible, single-platform alternative to correlative light-electron microscopy over large volumes.

Increased spatiotemporal resolution reveals highly dynamic dense tubular matrices in the peripheral ER

A dynamic view of the endoplasmic reticulum The endoplasmic reticulum (ER) is a complex membranous structure that extends from the nuclear envelope to the cell periphery. It has important roles in many cellular processes, and numerous proteins are involved in maintaining its structure. Nixon-Abell et al. used superresolution approaches to look at the ER at the periphery of the cell, where the ER contacts many other cellular organelles (see the Perspective by Terasaki). This peripheral ER has been thought to comprise tubules and sheets; however, the higher-resolution view revealed that most of the “sheets” consist of a dense clustering of tubules. This dynamic meshwork may allow the ER to change its conformation rapidly in response to cellular needs. Science, this issue p. 433; see also p. 415 Superresolution imaging shows that presumed sheets of endoplasmic reticulum comprise tightly packed, constantly changing tubules. INTRODUCTION The endoplasmic reticulum (ER) is a continuous, membrane-bound organelle, spanning from the nuclear envelope to the outer cell periphery, that contacts and influences nearly every other cellular organelle. In the peripheral ER, prevailing models propose a system of interconnected tubules and flattened sheets maintained by distinct proteins. Because mutations in these proteins and resultant ER irregularities coincide with various neurologic disorders, characterizing ER morphology is critical in understanding its roles in the basic biology of cells in both health and disease. Given limitations in imaging technologies, determining the dynamic rearrangements and fine ultrastructure of the peripheral ER has proven challenging. RATIONALE Previous work characterizing peripheral ER structure has relied extensively on diffraction-limited optical microscopy to describe gross morphology and dynamics, and electron microscopy (EM) for ultrastructural details. Regrettably, the respective spatial and temporal limitations of these techniques can obscure underlying cell processes where intricate morphology and/or rapid dynamism are important. Additionally, efforts to characterize protein distribution in the peripheral ER have presented confounding evidence regarding the localization of tubular junction-forming atlastin guanosine triphosphatases to sheets, and concerning the induction of sheet proliferation after atlastin overexpression. We exploited a variety of emerging superresolution (SR) microscopy techniques to collectively provide unprecedented spatiotemporal resolution that challenges prevailing models regarding peripheral ER morphology, dynamics, and protein distribution. RESULTS We used a combination of five SR technologies, with complementary strengths and weaknesses in the spatial and temporal domains, to image the peripheral ER in live and fixed cells. Using novel analytical approaches to study both protein and lipid components, we found that many structures previously proposed to be flat membrane sheets are instead densely packed tubular arrays—a previously undescribed structure we term an ER matrix. These matrices can become astoundingly compact, with spaces between the tubules far beneath the resolvable power of even most SR technologies. We observed dynamic oscillations of ER tubules and junctions, with matrices rapidly interconverting from tight to loose arrays, giving rise to different apparent morphologies dependent upon how closely their three-way junctions are clustered. We demonstrate how these ER matrices have been misinterpreted as a result of the spatiotemporal limitations of earlier imaging technologies. Finally, we account for the distribution of atlastin and other ER-shaping proteins within these structures. CONCLUSION The application of cutting-edge SR technologies to the peripheral ER has established a precedent for studying its dynamics and structural properties in living cells. The specific finding of dense tubular matrices in areas previously thought of as flat sheets provides a new model for maintaining and generating ER structure. Reorganization from tight to loose tubular network arrays may allow the ER to rapidly reach outward to the cell periphery during migration or other cell shape changes. Moreover, tight clusters of junctions may function as sites for sequestering excess membrane proteins and lipids or for contacting other organelles. Improved spatiotemporal resolution of ER structure and dynamics, as shown here, should help to address these and other key issues regarding ER function in healthy cells and during disease pathogenesis. Dense tubular matrices in the peripheral ER. New superresolution imaging modalities reveal that peripheral ER sheets are actually densely clustered tubules and interconnecting junctions. Shown is the distribution of an ER protein marker (3D-SIM) (upper right), internal cellular lipids (LLS-PAINT) (lower left), and an EM reconstruction (FIB-SEM) (upper left) demonstrating tubular matrices in the peripheral ER at high resolution. The endoplasmic reticulum (ER) is an expansive, membrane-enclosed organelle that plays crucial roles in numerous cellular functions. We used emerging superresolution imaging technologies to clarify the morphology and dynamics of the peripheral ER, which contacts and modulates most other intracellular organelles. Peripheral components of the ER have classically been described as comprising both tubules and flat sheets. We show that this system consists almost exclusively of tubules at varying densities, including structures that we term ER matrices. Conventional optical imaging technologies had led to misidentification of these structures as sheets because of the dense clustering of tubular junctions and a previously uncharacterized rapid form of ER motion. The existence of ER matrices explains previous confounding evidence that had indicated the occurrence of ER “sheet” proliferation after overexpression of tubular junction–forming proteins.