Super-resolution microscopy reveals LINC complex recruitment at nuclear indentation sites

Cellular Microbiaxial Stretching to Measure a Single-Cell Strain Energy Density Function

The stress in a cell due to extracellular mechanical stimulus is determined by its mechanical properties, and the structural organization of many adherent cells suggests that their properties are anisotropic. This anisotropy may significantly influence the cells' mechanotransductive response to complex loads, and has important implications for development of accurate models of tissue biomechanics. Standard methods for measuring cellular mechanics report linear moduli that cannot capture large-deformation anisotropic properties, which in a continuum mechanics framework are best described by a strain energy density function (SED). In tissues, the SED is most robustly measured using biaxial testing. Here, we describe a cellular microbiaxial stretching (CμBS) method that modifies this tissue-scale approach to measure the anisotropic elastic behavior of individual vascular smooth muscle cells (VSMCs) with nativelike cytoarchitecture. Using CμBS, we reveal that VSMCs are highly anisotropic under large deformations. We then characterize a Holzapfel-Gasser-Ogden type SED for individual VSMCs and find that architecture-dependent properties of the cells can be robustly described using a formulation solely based on the organization of their actin cytoskeleton. These results suggest that cellular anisotropy should be considered when developing biomechanical models, and could play an important role in cellular mechano-adaptation.

Physical Signals May Affect Mesenchymal Stem Cell Differentiation via Epigenetic Controls

Marrow mesenchymal stem cells supply bone osteoblasts and adipocytes. Exercise effects to increase bone and decrease fat involve transfer of signals from the cytoplasm into the nucleus to regulate gene expression. We propose that exercise control of stem cell fate relies on structural connections that terminate in the nucleus and involve intranuclear actin structures that regulate epigenetic gene expression.

Nuclear lamin A/C harnesses the perinuclear apical actin cables to protect nuclear morphology

The distinct spatial architecture of the apical actin cables (or actin cap) facilitates rapid biophysical signaling between extracellular mechanical stimuli and intracellular responses, including nuclear shaping, cytoskeletal remodeling, and the mechanotransduction of external forces into biochemical signals. These functions are abrogated in lamin A/C-deficient mouse embryonic fibroblasts that recapitulate the defective nuclear organization of laminopathies, featuring disruption of the actin cap. However, how nuclear lamin A/C mediates the ability of the actin cap to regulate nuclear morphology remains unclear. Here, we show that lamin A/C expressing cells can form an actin cap to resist nuclear deformation in response to physiological mechanical stresses. This study reveals how the nuclear lamin A/C-mediated formation of the perinuclear apical actin cables protects the nuclear structural integrity from extracellular physical disturbances. Our findings highlight the role of the physical interactions between the cytoskeletal network and the nucleus in cellular mechanical homeostasis.

Nuclear positioning in migrating fibroblasts

The positioning and movement of the nucleus has recently emerged as an important aspect of cell migration. Understanding of nuclear positioning and movement has reached an apogee in studies of fibroblast migration. Specific nuclear positioning and movements have been described in the polarization of fibroblast for cell migration and in active migration in 2D and 3D environments. Here, we review recent studies that have uncovered novel molecular mechanisms that contribute to these events in fibroblasts. Many of these involve a connection between the nucleus and the cytoskeleton through the LINC complex composed of outer nuclear membrane nesprins and inner nuclear membrane SUN proteins. We consider evidence that appropriate nuclear positioning contributes to efficient fibroblast polarization and migration and the possible mechanism through which the nucleus affects cell migration.

Cellular mechanosensing of the biophysical microenvironment: A review of mathematical models of biophysical regulation of cell responses

Cells in vivo reside within complex microenvironments composed of both biochemical and biophysical cues. The dynamic feedback between cells and their microenvironments hinges upon biophysical cues that regulate critical cellular behaviors. Understanding this regulation from sensing to reaction to feedback is therefore critical, and a large effort is afoot to identify and mathematically model the fundamental mechanobiological mechanisms underlying this regulation. This review provides a critical perspective on recent progress in mathematical models for the responses of cells to the biophysical cues in their microenvironments, including dynamic strain, osmotic shock, fluid shear stress, mechanical force, matrix rigidity, porosity, and matrix shape. The review highlights key successes and failings of existing models, and discusses future opportunities and challenges in the field.

The Nuclear Option: Evidence Implicating the Cell Nucleus in Mechanotransduction

Biophysical stimuli presented to cells via microenvironmental properties (e.g., alignment and stiffness) or external forces have a significant impact on cell function and behavior. Recently, the cell nucleus has been identified as a mechanosensitive organelle that contributes to the perception and response to mechanical stimuli. However, the specific mechanotransduction mechanisms that mediate these effects have not been clearly established. Here, we offer a comprehensive review of the evidence supporting (and refuting) three hypothetical nuclear mechanotransduction mechanisms: physical reorganization of chromatin, signaling at the nuclear envelope, and altered cytoskeletal structure/tension due to nuclear remodeling. Our goal is to provide a reference detailing the progress that has been made and the areas that still require investigation regarding the role of nuclear mechanotransduction in cell biology. Additionally, we will briefly discuss the role that mathematical models of cell mechanics can play in testing these hypotheses and in elucidating how biophysical stimulation of the nucleus drives changes in cell behavior. While force-induced alterations in signaling pathways involving lamina-associated polypeptides (LAPs) (e.g., emerin and histone deacetylase 3 (HDAC3)) and transcription factors (TFs) located at the nuclear envelope currently appear to be the most clearly supported mechanism of nuclear mechanotransduction, additional work is required to examine this process in detail and to more fully test alternative mechanisms. The combination of sophisticated experimental techniques and advanced mathematical models is necessary to enhance our understanding of the role of the nucleus in the mechanotransduction processes driving numerous critical cell functions.

Super-Resolution Microscopy in Studying the Structure and Function of the Cell Nucleus

In recent decades, novel microscopic methods commonly referred to as super- resolution microscopy have been developed. These methods enable the visualization of a cell with a resolution of up to 10 nm. The application of these methods is of great interest in studying the structure and function of the cell nucleus. The review describes the main achievements in this field.

Cell-Shaping Micropatterns for Quantitative Super-Resolution Microscopy Imaging of Membrane Mechanosensing Proteins

Patterning cells on microcontact-printed substrates is a powerful approach to control cell morphology and introduce specific mechanical cues on a cell's molecular organization. Although global changes in cellular architectures caused by micropatterns can easily be probed with diffraction-limited optical microscopy, studying molecular reorganizations at the nanoscale demands micropatterned substrates that accommodate the optical requirements of single molecule microscopy techniques. Here, we developed a simple micropatterning strategy that provides control of cellular architectures and is optimized for nanometer accuracy single molecule tracking and three-dimensional super-resolution imaging of plasma and nuclear membrane proteins in cells. This approach, based on fibronectin microcontact printing on hydrophobic organosilane monolayers, allows evanescent wave and light-sheet microscopy of cells whilst fulfilling the stringent optical demands of point reconstruction optical microscopy. By imposing steady-state mechanical cues on cells grown in these micropatterns, we reveal nanoscale remodeling in the dynamics and the structural organizations of the nuclear envelope mechanotransducing protein emerin and of the plasma membrane mechanosensing protein caveolin-1 using single particle tracking photoactivated localization microscopy and direct stochastic optical reconstruction microscopy imaging. In addition to allowing quantitative biophysical studies of mechanoresponsive membrane proteins, this approach provides an easy means to probe mechanical regulations in cellular membranes with high optical resolution and nanometer precision.

The nucleus is irreversibly shaped by motion of cell boundaries in cancer and non-cancer cells

Actomyosin stress fibers impinge on the nucleus and can exert compressive forces on it. These compressive forces have been proposed to elongate nuclei in fibroblasts, and lead to abnormally shaped nuclei in cancer cells. In these models, the elongated or flattened nuclear shape is proposed to store elastic energy. However, we found that deformed shapes of nuclei are unchanged even after removal of the cell with micro-dissection, both for smooth, elongated nuclei in fibroblasts and abnormally shaped nuclei in breast cancer cells. The lack of shape relaxation implies that the nuclear shape in spread cells does not store any elastic energy, and the cellular stresses that deform the nucleus are dissipative, not static. During cell spreading, the deviation of the nucleus from a convex shape increased in MDA-MB-231 cancer cells, but decreased in MCF-10A cells. Tracking changes of nuclear and cellular shape on micropatterned substrata revealed that fibroblast nuclei deform only during deformations in cell shape and only in the direction of nearby moving cell boundaries. We propose that motion of cell boundaries exert a stress on the nucleus, which allows the nucleus to mimic cell shape. The lack of elastic energy in the nuclear shape suggests that nuclear shape changes in cells occur at constant surface area and volume.

The assembly and function of perinuclear actin cap in migrating cells

Stress fibers are actin bundles encompassing actin filaments, actin-crosslinking, and actin-associated proteins that represent the major contractile system in the cell. Different types of stress fibers assemble in adherent cells, and they are central to diverse cellular processes including establishment of the cell shape, morphogenesis, cell polarization, and migration. Stress fibers display specific cellular organization and localization, with ventral fibers present at the basal side, and dorsal fibers and transverse actin arcs rising at the cell front from the ventral to the dorsal side and toward the nucleus. Perinuclear actin cap fibers are a specific subtype of stress fibers that rise from the leading edge above the nucleus and terminate at the cell rear forming a dome-like structure. Perinuclear actin cap fibers are fixed at three points: both ends are anchored in focal adhesions, while the central part is physically attached to the nucleus and nuclear lamina through the linker of nucleoskeleton and cytoskeleton (LINC) complex. Here, we discuss recent work that provides new insights into the mechanism of assembly and the function of these actin stress fibers that directly link extracellular matrix and focal adhesions with the nuclear envelope.

3D culture increases pluripotent gene expression in mesenchymal stem cells through relaxation of cytoskeleton tension

Three‐dimensional (3D) culture has been shown to improve pluripotent gene expression in mesenchymal stem cells (MSCs), but the underlining mechanisms were poorly understood. Here, we found that the relaxation of cytoskeleton tension of MSCs in 3D culture was critically associated with the expressional up‐regulation of Nanog. Cultured in spheroids, MSCs showed decreased integrin‐based cell–matrix adhesion but increased cadherin‐based cell–cell interaction. Different from that in 2D culture, where MSCs exhibited branched and multiple‐directed F‐actin stress bundles at the cell edge and strengthened stress fibres transversing the cell body, MSCs cultured in spheroids showed compact cell body, relaxed cytoskeleton tension with very thin cortical actin filament outlining the cell, and increased expression of Nanog along with reduced levels of Suv39h1 (H3K9 methyltransferase) and H3K9me3. Notably, pharmaceutical inhibition of actin polymerization with cytochalasin D or silencing Suv39h1 expression with siRNA in 2D‐cultured MSCs elevated the expression of Nanog via H3K9 demethylation. Thus, our data suggest that 3D culture increases the expression of Nanog through the relaxation of actin cytoskeleton, which mediates reduced Suv39h1 and H3K9me3 levels.

Mechanisms and functions of nuclear envelope remodelling

As a compartment border, the nuclear envelope (NE) needs to serve as both a protective membrane shell for the genome and a versatile communication interface between the nucleus and the cytoplasm. Despite its important structural role in sheltering the genome, the NE is a dynamic and highly adaptable boundary that changes composition during differentiation, deforms in response to mechanical challenges, can be repaired upon rupture and even rapidly disassembles and reforms during open mitosis. NE remodelling is fundamentally involved in cell growth, division and differentiation, and if perturbed can lead to devastating diseases such as muscular dystrophies or premature ageing.

Deep nuclear invaginations are linked to cytoskeletal filaments - integrated bioimaging of epithelial cells in 3D culture

The importance of context in regulation of gene expression is now an accepted principle; yet the mechanism by which the microenvironment communicates with the nucleus and chromatin in healthy tissues is poorly understood. A functional role for nuclear and cytoskeletal architecture is suggested by the phenotypic differences observed between epithelial and mesenchymal cells. Capitalizing on recent advances in cryogenic techniques, volume electron microscopy and super-resolution light microscopy, we studied human mammary epithelial cells in three-dimensional (3D) cultures forming growth-arrested acini. Intriguingly, we found deep nuclear invaginations and tunnels traversing the nucleus, encasing cytoskeletal actin and/or intermediate filaments, which connect to the outer nuclear envelope. The cytoskeleton is also connected both to other cells through desmosome adhesion complexes and to the extracellular matrix through hemidesmosomes. This finding supports a physical and/or mechanical link from the desmosomes and hemidesmosomes to the nucleus, which had previously been hypothesized but now is visualized for the first time. These unique structures, including the nuclear invaginations and the cytoskeletal connectivity to the cell nucleus, are consistent with a dynamic reciprocity between the nucleus and the outside of epithelial cells and tissues.

Vibratory stimulation enhances thyroid epithelial cell function

The tissues of the body are routinely subjected to various forms of mechanical vibration, the frequency, amplitude, and duration of which can contribute both positively and negatively to human health. The vocal cords, which are in close proximity to the thyroid, may also supply the thyroid with important mechanical signals that modulate hormone production via mechanical vibrations from phonation. In order to explore the possibility that vibrational stimulation from vocalization can enhance thyroid epithelial cell function, FRTL-5 rat thyroid cells were subjected to either chemical stimulation with thyroid stimulating hormone (TSH), mechanical stimulation with physiological vibrations, or a combination of the two, all in a well-characterized, torsional rheometer-bioreactor. The FRTL-5 cells responded to mechanical stimulation with significantly (p<0.05) increased metabolic activity, significantly (p<0.05) increased ROS production, and increased gene expression of thyroglobulin and sodium-iodide symporter compared to un-stimulated controls, and showed an equivalent or greater response than TSH only stimulated cells. Furthermore, the combination of TSH and oscillatory motion produced a greater response than mechanical or chemical stimulation alone. Taken together, these results suggest that mechanical vibrations could provide stimulatory cues that help maintain thyroid function.

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Thyroid epithelial cells responded to mechanical vibrations similar to those from vocalization.

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This response was equivalent or greater compared to chemical stimulation.

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The combination of mechanical and chemical stimulation was synergistic.

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It may be possible to influence thyroid function with mechanical vibrations.

Nesprin-2G, a Component of the Nuclear LINC Complex, Is Subject to Myosin-Dependent Tension

The nucleus of a cell has long been considered to be subject to mechanical force. Despite the observation that mechanical forces affect nuclear geometry and movement, how forces are applied onto the nucleus is not well understood. The nuclear LINC (linker of nucleoskeleton and cytoskeleton) complex has been hypothesized to be the critical structure that mediates the transfer of mechanical forces from the cytoskeleton onto the nucleus. Previously used techniques for studying nuclear forces have been unable to resolve forces across individual proteins, making it difficult to clearly establish if the LINC complex experiences mechanical load. To directly measure forces across the LINC complex, we generated a fluorescence resonance energy transfer-based tension biosensor for nesprin-2G, a key structural protein in the LINC complex, which physically links this complex to the actin cytoskeleton. Using this sensor we show that nesprin-2G is subject to mechanical tension in adherent fibroblasts, with highest levels of force on the apical and equatorial planes of the nucleus. We also show that the forces across nesprin-2G are dependent on actomyosin contractility and cell elongation. Additionally, nesprin-2G tension is reduced in fibroblasts from Hutchinson-Gilford progeria syndrome patients. This report provides the first, to our knowledge, direct evidence that nesprin-2G, and by extension the LINC complex, is subject to mechanical force. We also present evidence that nesprin-2G localization to the nuclear membrane is altered under high-force conditions. Because forces across the LINC complex are altered by a variety of different conditions, mechanical forces across the LINC complex, as well as the nucleus in general, may represent an important mechanism for mediating mechanotransduction.

Persistence of fan-shaped keratocytes is a matrix-rigidity-dependent mechanism that requires α5β1 integrin engagement

Despite the importance of matrix rigidity on cell functions, many aspects of the mechanosensing process in highly migratory cells remain elusive. Here, we studied the migration of highly motile keratocytes on culture substrates with similar biochemical properties and rigidities spanning the range between soft tissues (~kPa) and stiff culture substrates (~GPa). We show that morphology, polarization and persistence of motile keratocytes are regulated by the matrix stiffness over seven orders of magnitude, without changing the cell spreading area. Increasing the matrix rigidity leads to more F-actin in the lamellipodia and to the formation of mature contractile actomyosin fibers that control the cell rear retraction. Keratocytes remain rounded and form nascent adhesions on compliant substrates, whereas large and uniformly distributed focal adhesions are formed on fan-shaped keratocytes migrating on rigid surfaces. By combining poly-L-lysine, fibronectin and vitronectin coatings with selective blocking of α_vβ₃ or α₅β₁ integrins, we show that α_Vβ₃ integrins permit the spreading of keratocytes but are not sufficient for polarization and rigidity sensing that require the engagement of α₅β₁ integrins. Our study demonstrates a matrix rigidity-dependent regulation of the directional persistence in motile keratocytes and refines the role of α_vβ₃ and α₅β₁ integrins in the molecular clutch model.

Dorsal stress fibers, transverse actin arcs, and perinuclear actin fibers form an interconnected network that induces nuclear movement in polarizing fibroblasts

In polarized motile cells, stress fibers display specific three-dimensional organization. Ventral stress fibers, attached to focal adhesions at both ends, are restricted to the basal side of the cell and nonprotruding cell sides. Dorsal fibers, transverse actin arcs, and perinuclear actin fibers emanate from protruding cell front toward the nucleus and toward apical side of the cell. Perinuclear cap fibers further extend above the nucleus, associate with nuclear envelope through LINC (linker of nucleoskeleton and cytoskeleton) complex and terminate in focal adhesions at cell rear. How are perinuclear actin fibers formed is poorly understood. We show that the formation of perinuclear actin fibers requires dorsal stress fibers that polymerize from focal adhesions at leading edge, and transverse actin arcs that are interconnected with dorsal fibers in spots rich in α-actinin-1. During cell polarization, the interconnected dorsal fibers and transverse arcs move from leading edge toward dorsal side of the cell. As they move, transverse arcs associate with one end of stress fibers present at nonprotruding cell sides, move them above the nucleus thus forming perinuclear actin fibers. Furthermore, the formation of perinuclear actin fibers induces temporal rotational movement of the nucleus resulting in nuclear reorientation to the direction of migration. These results suggest that the network of dorsal fibers, transverse arcs, and perinuclear fibers transfers mechanical signal between the focal adhesions and nuclear envelope that regulates the nuclear reorientation in polarizing cells.

Cellular functions of the ADF/cofilin family at a glance

The actin depolymerizing factor (ADF)/cofilin family comprises small actin-binding proteins with crucial roles in development, tissue homeostasis and disease. They are best known for their roles in regulating actin dynamics by promoting actin treadmilling and thereby driving membrane protrusion and cell motility. However, recent discoveries have increased our understanding of the functions of these proteins beyond their well-characterized roles. This Cell Science at a Glance article and the accompanying poster serve as an introduction to the diverse roles of the ADF/cofilin family in cells. The first part of the article summarizes their actions in actin treadmilling and the main mechanisms for their intracellular regulation; the second part aims to provide an outline of the emerging cellular roles attributed to the ADF/cofilin family, besides their actions in actin turnover. The latter part discusses an array of diverse processes, which include regulation of intracellular contractility, maintenance of nuclear integrity, transcriptional regulation, nuclear actin monomer transfer, apoptosis and lipid metabolism. Some of these could, of course, be indirect consequences of actin treadmilling functions, and this is discussed.

In silico synchronization reveals regulators of nuclear ruptures in lamin A/C deficient model cells

The nuclear lamina is a critical regulator of nuclear structure and function. Nuclei from laminopathy patient cells experience repetitive disruptions of the nuclear envelope, causing transient intermingling of nuclear and cytoplasmic components. The exact causes and consequences of these events are not fully understood, but their stochastic occurrence complicates in-depth analyses. To resolve this, we have established a method that enables quantitative investigation of spontaneous nuclear ruptures, based on co-expression of a firmly bound nuclear reference marker and a fluorescent protein that shuttles between the nucleus and cytoplasm during ruptures. Minimally invasive imaging of both reporters, combined with automated tracking and in silico synchronization of individual rupture events, allowed extracting information on rupture frequency and recovery kinetics. Using this approach, we found that rupture frequency correlates inversely with lamin A/C levels, and can be reduced in genome-edited LMNA knockout cells by blocking actomyosin contractility or inhibiting the acetyl-transferase protein NAT10. Nuclear signal recovery followed a kinetic that is co-determined by the severity of the rupture event, and could be prolonged by knockdown of the ESCRT-III complex component CHMP4B. In conclusion, our approach reveals regulators of nuclear rupture induction and repair, which may have critical roles in disease development.

Focusing super resolution on the cytoskeleton

Super resolution imaging is becoming an increasingly important tool in the arsenal of methods available to cell biologists. In recognition of its potential, the Nobel Prize for chemistry was awarded to three investigators involved in the development of super resolution imaging methods in 2014. The availability of commercial instruments for super resolution imaging has further spurred the development of new methods and reagents designed to take advantage of super resolution techniques. Super resolution offers the advantages traditionally associated with light microscopy, including the use of gentle fixation and specimen preparation methods, the ability to visualize multiple elements within a single specimen, and the potential to visualize dynamic changes in living specimens over time. However, imaging of living cells over time is difficult and super resolution imaging is computationally demanding. In this review, we discuss the advantages/disadvantages of different super resolution systems for imaging fixed live specimens, with particular regard to cytoskeleton structures.

Moving Cell Boundaries Drive Nuclear Shaping during Cell Spreading

The nucleus has a smooth, regular appearance in normal cells, and its shape is greatly altered in human pathologies. Yet, how the cell establishes nuclear shape is not well understood. We imaged the dynamics of nuclear shaping in NIH3T3 fibroblasts. Nuclei translated toward the substratum and began flattening during the early stages of cell spreading. Initially, nuclear height and width correlated with the degree of cell spreading, but over time, reached steady-state values even as the cell continued to spread. Actomyosin activity, actomyosin bundles, microtubules, and intermediate filaments, as well as the LINC complex, were all dispensable for nuclear flattening as long as the cell could spread. Inhibition of actin polymerization as well as myosin light chain kinase with the drug ML7 limited both the initial spreading of cells and flattening of nuclei, and for well-spread cells, inhibition of myosin-II ATPase with the drug blebbistatin decreased cell spreading with associated nuclear rounding. Together, these results show that cell spreading is necessary and sufficient to drive nuclear flattening under a wide range of conditions, including in the presence or absence of myosin activity. To explain this observation, we propose a computational model for nuclear and cell mechanics that shows how frictional transmission of stress from the moving cell boundaries to the nuclear surface shapes the nucleus during early cell spreading. Our results point to a surprisingly simple mechanical system in cells for establishing nuclear shapes.

Probing cytoskeletal pre-stress and nuclear mechanics in endothelial cells with spatiotemporally controlled (de-)adhesion kinetics on micropatterned substrates

The mechanical properties of living cells reflect their propensity to migrate and respond to external forces. Both cellular and nuclear stiffnesses are strongly influenced by the rigidity of the extracellular matrix (ECM) through reorganization of the cyto- and nucleoskeletal protein connections. Changes in this architectural continuum affect cell mechanics and underlie many pathological conditions. In this context, an accurate and combined quantification of the mechanical properties of both cells and nuclei can contribute to a better understanding of cellular (dys-)function. To address this challenge, we have established a robust method for probing cellular and nuclear deformation during spreading and detachment from micropatterned substrates. We show that (de-)adhesion kinetics of endothelial cells are modulated by substrate stiffness and rely on the actomyosin network. We combined this approach with measurements of cell stiffness by magnetic tweezers to show that relaxation dynamics can be considered as a reliable parameter of cellular pre-stress in adherent cells. During the adhesion stage, large cellular and nuclear deformations occur over a long time span (>60 min). Conversely, nuclear deformation and condensed chromatin are relaxed in a few seconds after detachment. Finally, our results show that accumulation of farnesylated prelamin leads to modifications of the nuclear viscoelastic properties, as reflected by increased nuclear relaxation times. Our method offers an original and non-intrusive way of simultaneously gauging cellular and nuclear mechanics, which can be extended to high-throughput screens of pathological conditions and potential countermeasures.

Concise Review: Plasma and Nuclear Membranes Convey Mechanical Information to Regulate Mesenchymal Stem Cell Lineage

Numerous factors including chemical, hormonal, spatial, and physical cues determine stem cell fate. While the regulation of stem cell differentiation by soluble factors is well-characterized, the role of mechanical force in the determination of lineage fate is just beginning to be understood. Investigation of the role of force on cell function has largely focused on "outside-in" signaling, initiated at the plasma membrane. When interfaced with the extracellular matrix, the cell uses integral membrane proteins, such as those found in focal adhesion complexes to translate force into biochemical signals. Akin to these outside-in connections, the internal cytoskeleton is physically linked to the nucleus, via proteins that span the nuclear membrane. Although structurally and biochemically distinct, these two forms of mechanical coupling influence stem cell lineage fate and, when disrupted, often lead to disease. Here we provide an overview of how mechanical coupling occurs at the plasma and nuclear membranes. We also discuss the role of force on stem cell differentiation, with focus on the biochemical signals generated at the cell membrane and the nucleus, and how those signals influence various diseases. While the interaction of stem cells with their physical environment and how they respond to force is complex, an understanding of the mechanical regulation of these cells is critical in the design of novel therapeutics to combat diseases associated with aging, cancer, and osteoporosis. Stem Cells 2016;34:1455-1463.

An unresolved LINC in the nuclear envelope

The nuclear envelope segregates the nucleoplasm from the cytoplasm and is a key feature of eukaryotic cells. Nuclear envelope architecture is comprised of two concentric membrane shells which fuse at multiple sites and yet maintain a uniform separation of 30-50 nm over the rest of the membrane. Studies have revealed the roles for numerous nuclear proteins in forming and maintaining the architecture of the nuclear envelope. However, there is a lack of consensus on the fundamental forces and physical mechanisms that establish the geometry. The objective of this review is to discuss recent findings in the context of membrane mechanics in an effort to define open questions and possible answers.

Cell Mechanosensitivity is Enabled by the LINC Nuclear Complex

Mechanoresponses in mesenchymal stem cells (MSCs) guide both differentiation and function. In this review, we focus on advances in0 our understanding of how the cytoplasmic cytoskeleton, nuclear envelope and nucleoskeleton, which are connected via LINC (Linker of Nucleoskeleton and Cytoskeleton) complexes, are emerging as an integrated dynamic signaling platform to regulate MSC mechanobiology. This dynamic interconnectivity affects mechanical signaling and transfer of signals into the nucleus. In this way, nuclear and LINC-mediated cytoskeletal connectivity play a critical role in maintaining mechanical signaling that affects MSC fate by serving as both mechanosensory and mechanoresponsive structures. We review disease and age related compromises of LINC complexes and nucleoskeleton that contribute to the etiology of musculoskeletal diseases. Finally we invite the idea that acquired dysfunctions of LINC might be a contributing factor to conditions such as aging, microgravity and osteoporosis and discuss potential mechanical strategies to modulate LINC connectivity to combat these conditions.

New imaging methods and tools to study vascular biology

PURPOSE OF REVIEW

Throughout history, development of novel microscopy techniques has been of fundamental importance to advance the vascular biology field.This review offers a concise summary of the most recently developed imaging techniques and discusses how they can be applied to vascular biology. In addition, we reflect upon the most important fluorescent reporters for vascular research that are currently available.

RECENT FINDINGS

Recent advances in light sheet-based imaging techniques now offer the ability to live image the vascular system in whole organs or even in whole animals during development and in pathological conditions with a satisfactory spatial and temporal resolution. Conversely, super resolution microscopy now allows studying cellular processes at a near-molecular resolution.

SUMMARY

Major recent improvements in a number of imaging techniques now allow study of vascular biology in ways that could not be considered previously. Researchers now have well-developed tools to specifically examine the dynamic nature of vascular development during angiogenic sprouting, remodeling and regression as well as the vascular responses in disease situations in vivo. In addition, open questions in endothelial and lymphatic cell biology that require subcellular resolution such as actin dynamics, junctional complex formation and stability, vascular permeability and receptor trafficking can now be approached with high resolution.

Specification of Architecture and Function of Actin Structures by Actin Nucleation Factors

The actin cytoskeleton is essential for diverse processes in mammalian cells; these processes range from establishing cell polarity to powering cell migration to driving cytokinesis to positioning intracellular organelles. How these many functions are carried out in a spatiotemporally regulated manner in a single cytoplasm has been the subject of much study in the cytoskeleton field. Recent work has identified a host of actin nucleation factors that can build architecturally diverse actin structures. The biochemical properties of these factors, coupled with their cellular location, likely define the functional properties of actin structures. In this article, we describe how recent advances in cell biology and biochemistry have begun to elucidate the role of individual actin nucleation factors in generating distinct cellular structures. We also consider how the localization and orientation of actin nucleation factors, in addition to their kinetic properties, are critical to their ability to build a functional actin cytoskeleton.

Nuclear deformability and telomere dynamics are regulated by cell geometric constraints

Forces generated by the cytoskeleton can be transmitted to the nucleus and chromatin via physical links on the nuclear envelope and the lamin meshwork. Although the role of these active forces in modulating prestressed nuclear morphology has been well studied, the effect on nuclear and chromatin dynamics remains to be explored. To understand the regulation of nuclear deformability by these active forces, we created different cytoskeletal states in mouse fibroblasts using micropatterned substrates. We observed that constrained and isotropic cells, which lack long actin stress fibers, have more deformable nuclei than elongated and polarized cells. This nuclear deformability altered in response to actin, myosin, formin perturbations, or a transcriptional down-regulation of lamin A/C levels in the constrained and isotropic geometry. Furthermore, to probe the effect of active cytoskeletal forces on chromatin dynamics, we tracked the spatiotemporal dynamics of heterochromatin foci and telomeres. We observed increased dynamics and decreased correlation of the heterochromatin foci and telomere trajectories in constrained and isotropic cell geometry. The observed enhanced dynamics upon treatment with actin depolymerizing reagents in elongated and polarized geometry were regained once the reagent was washed off, suggesting an inherent structural memory in chromatin organization. We conclude that active forces from the cytoskeleton and rigidity from lamin A/C nucleoskeleton can together regulate nuclear and chromatin dynamics. Because chromatin remodeling is a necessary step in transcription control and its memory, genome integrity, and cellular deformability during migration, our results highlight the importance of cell geometric constraints as critical regulators in cell behavior.

ADF and Cofilin1 Control Actin Stress Fibers, Nuclear Integrity, and Cell Survival

Genetic co-depletion of the actin-severing proteins ADF and CFL1 triggers catastrophic loss of adult homeostasis in multiple tissues. There is impaired cell-cell adhesion in skin keratinocytes with dysregulation of E-cadherin, hyperproliferation of differentiated cells, and ultimately apoptosis. Mechanistically, the primary consequence of depleting both ADF and CFL1 is uncontrolled accumulation of contractile actin stress fibers associated with enlarged focal adhesions at the plasma membrane, as well as reduced rates of membrane protrusions. This generates increased intracellular acto-myosin tension that promotes nuclear deformation and physical disruption of the nuclear lamina via the LINC complex that normally connects regulated actin filaments to the nuclear envelope. We therefore describe a pathway involving the actin-severing proteins ADF and CFL1 in regulating the dynamic turnover of contractile actin stress fibers, and this is vital to prevent the nucleus from being damaged by actin contractility, in turn preserving cell survival and tissue homeostasis.

High-content imaging with micropatterned multiwell plates reveals influence of cell geometry and cytoskeleton on chromatin dynamics

Understanding the mechanisms underpinning cellular responses to microenvironmental cues requires tight control not only of the complex milieu of soluble signaling factors, extracellular matrix (ECM) connections and cell-cell contacts within cell culture, but also of the biophysics of human cells. Advances in biomaterial fabrication technologies have recently facilitated detailed examination of cellular biophysics and revealed that constraints on cell geometry arising from the cellular microenvironment influence a wide variety of human cell behaviors. Here, we create an in vitro platform capable of precise and independent control of biochemical and biophysical microenvironmental cues by adapting microcontact printing technology into the format of standard six- to 96-well plates to create MicroContact Printed Well Plates (μCP Well Plates). Automated high-content imaging of human cells seeded on μCP Well Plates revealed tight, highly consistent control of single-cell geometry, cytoskeletal organization, and nuclear elongation. Detailed subcellular imaging of the actin cytoskeleton and chromatin within live human fibroblasts on μCP Well Plates was then used to describe a new relationship between cellular geometry and chromatin dynamics. In summary, the μCP Well Plate platform is an enabling high-content screening technology for human cell biology and cellular engineering efforts that seek to identify key biochemical and biophysical cues in the cellular microenvironment.