Sangkyun Cho

All Publications

• Challenges and opportunities for the next generation of cardiovascular tissue engineering. Nature methods Cho, S., Discher, D. E., Leong, K. W., Vunjak-Novakovic, G., Wu, J. C. 2022

  Abstract

  Engineered cardiac tissues derived from human induced pluripotent stem cells offer unique opportunities for patient-specific disease modeling, drug discovery and cardiac repair. Since the first engineered hearts were introduced over two decades ago, human induced pluripotent stem cell-based three-dimensional cardiac organoids and heart-on-a-chip systems have now become mainstays in basic cardiovascular research as valuable platforms for investigating fundamental human pathophysiology and development. However, major obstacles remain to be addressed before the field can truly advance toward commercial and clinical translation. Here we provide a snapshot of the state-of-the-art methods in cardiac tissue engineering, with a focus on in vitro models of the human heart. Looking ahead, we discuss major challenges and opportunities in the field and suggest strategies for enabling broad acceptance of engineered cardiac tissues as models of cardiac pathophysiology and testbeds for the development of therapies.

  View details for DOI 10.1038/s41592-022-01591-3

  View details for PubMedID 36064773

• Single-cell RNA sequencing in cardiovascular development, disease and medicine. Nature reviews. Cardiology Paik, D. T., Cho, S., Tian, L., Chang, H. Y., Wu, J. C. 2020

  Abstract

  Advances in single-cell RNA sequencing (scRNA-seq) technologies in the past 10 years have had a transformative effect on biomedical research, enabling the profiling and analysis of the transcriptomes of single cells at unprecedented resolution and throughput. Specifically, scRNA-seq has facilitated the identification of novel or rare cell types, the analysis of single-cell trajectory construction and stem or progenitor cell differentiation, and the comparison of healthy and disease-related tissues at single-cell resolution. These applications have been critical in advances in cardiovascular research in the past decade as evidenced by the generation of cell atlases of mammalian heart and blood vessels and the elucidation of mechanisms involved in cardiovascular development and stem or progenitor cell differentiation. In this Review, we summarize the currently available scRNA-seq technologies and analytical tools and discuss the latest findings using scRNA-seq that have substantially improved our knowledge on the development of the cardiovascular system and the mechanisms underlying cardiovascular diseases. Furthermore, we examine emerging strategies that integrate multimodal single-cell platforms, focusing on future applications in cardiovascular precision medicine that use single-cell omics approaches to characterize cell-specific responses to drugs or environmental stimuli and to develop effective patient-specific therapeutics.

  View details for DOI 10.1038/s41569-020-0359-y

  View details for PubMedID 32231331

• Mechanosensing by the Lamina Protects against Nuclear Rupture, DNA Damage, and Cell-Cycle Arrest. Developmental cell Cho, S., Vashisth, M., Abbas, A., Majkut, S., Vogel, K., Xia, Y., Ivanovska, I. L., Irianto, J., Tewari, M., Zhu, K., Tichy, E. D., Mourkioti, F., Tang, H. Y., Greenberg, R. A., Prosser, B. L., Discher, D. E. 2019; 49 (6): 920-935.e5

  Abstract

  Whether cell forces or extracellular matrix (ECM) can impact genome integrity is largely unclear. Here, acute perturbations (∼1 h) to actomyosin stress or ECM elasticity cause rapid and reversible changes in lamin-A, DNA damage, and cell cycle. The findings are especially relevant to organs such as the heart because DNA damage permanently arrests cardiomyocyte proliferation shortly after birth and thereby eliminates regeneration after injury including heart attack. Embryonic hearts, cardiac-differentiated iPS cells (induced pluripotent stem cells), and various nonmuscle cell types all show that actomyosin-driven nuclear rupture causes cytoplasmic mis-localization of DNA repair factors and excess DNA damage. Binucleation and micronuclei increase as telomeres shorten, which all favor cell-cycle arrest. Deficiencies in lamin-A and repair factors exacerbate these effects, but lamin-A-associated defects are rescued by repair factor overexpression and also by contractility modulators in clinical trials. Contractile cells on stiff ECM normally exhibit low phosphorylation and slow degradation of lamin-A by matrix-metalloprotease-2 (MMP2), and inhibition of this lamin-A turnover and also actomyosin contractility are seen to minimize DNA damage. Lamin-A is thus stress stabilized to mechano-protect the genome.

  View details for DOI 10.1016/j.devcel.2019.04.020

  View details for PubMedID 31105008

  View details for PubMedCentralID PMC6581604

• Mechanosensing by the nucleus: From pathways to scaling relationships. The Journal of cell biology Cho, S., Irianto, J., Discher, D. E. 2017; 216 (2): 305-315

  Abstract

  The nucleus is linked mechanically to the extracellular matrix via multiple polymers that transmit forces to the nuclear envelope and into the nuclear interior. Here, we review some of the emerging mechanisms of nuclear mechanosensing, which range from changes in protein conformation and transcription factor localization to chromosome reorganization and membrane dilation up to rupture. Nuclear mechanosensing encompasses biophysically complex pathways that often converge on the main structural proteins of the nucleus, the lamins. We also perform meta-analyses of public transcriptomics and proteomics data, which indicate that some of the mechanosensing pathways relaying signals from the collagen matrix to the nucleus apply to a broad range of species, tissues, and diseases.

  View details for DOI 10.1083/jcb.201610042

  View details for PubMedID 28043971

  View details for PubMedCentralID PMC5294790

• Fractal heterogeneity in minimal matrix models of scars modulates stiff-niche stem-cell responses via nuclear exit of a mechanorepressor. Nature materials Dingal, P. C., Bradshaw, A. M., Cho, S., Raab, M., Buxboim, A., Swift, J., Discher, D. E. 2015; 14 (9): 951-60

  Abstract

  Scarring is a long-lasting problem in higher animals, and reductionist approaches could aid in developing treatments. Here, we show that copolymerization of collagen I with polyacrylamide produces minimal matrix models of scars (MMMS), in which fractal-fibre bundles segregate heterogeneously to the hydrogel subsurface. Matrix stiffens locally-as in scars-while allowing separate control over adhesive-ligand density. The MMMS elicits scar-like phenotypes from mesenchymal stem cells (MSCs): cells spread and polarize quickly, increasing nucleoskeletal lamin-A yet expressing the 'scar marker' smooth muscle actin (SMA) more slowly. Surprisingly, expression responses to MMMS exhibit less cell-to-cell noise than homogeneously stiff gels. Such differences from bulk-average responses arise because a strong SMA repressor, NKX2.5, slowly exits the nucleus on rigid matrices. NKX2.5 overexpression overrides rigid phenotypes, inhibiting SMA and cell spreading, whereas cytoplasm-localized NKX2.5 mutants degrade in well-spread cells. MSCs thus form a 'mechanical memory' of rigidity by progressively suppressing NKX2.5, thereby elevating SMA in a scar-like state.

  View details for DOI 10.1038/nmat4350

  View details for PubMedID 26168347

  View details for PubMedCentralID PMC4545733

• Heterozygous LMNA mutation-carrying iPSC lines from three cardiac laminopathy patients. Stem cell research Cho, S., Lee, C., Lai, C., Zhuge, Y., Haddad, F., Fowler, M., Sallam, K., Wu, J. C. 1800; 59: 102657

  Abstract

  LMNA-related dilated cardiomyopathy (LMNA-DCM) is caused by pathogenic variants in the LMNA gene and is characterized by left ventricular chamber enlargement, reduced systolic function, and arrhythmia. Here, we generated three human induced pluripotent stem cell (iPSC) lines from peripheral blood mononuclear cells (PBMCs) of three DCM patients carrying the same single heterozygous mutation, c.398 G>A, in LMNA. All lines exhibited normal iPSC morphology, expressed high levels of pluripotency markers, showed normal karyotypes, and could differentiate into the three germ layers. These patient-specific iPSC lines can serve as invaluable tools to model in vitro pathological mechanisms of LMNA-DCM.

  View details for DOI 10.1016/j.scr.2022.102657

  View details for PubMedID 34999423

• Generation of three iPSC lines from dilated cardiomyopathy patients carrying a pathogenic LMNA variant. Stem cell research Lee, C., Cho, S., Lai, C., Shenoy, S., Vagelos, R., Wu, J. C. 1800; 59: 102638

  Abstract

  LMNA-related dilated cardiomyopathy (DCM) is caused by pathogenic variants in LMNA and is characterized by left ventricular enlargement, reduced systolic function, and arrhythmia. Here, we generated three human induced pluripotent stem cell (iPSC) lines from peripheral blood mononuclear cells (PBMCs) of three DCM patients carrying the same single heterozygous mutation, c.1129C>T, in LMNA. All lines expressed normal iPSC morphology, high levels of pluripotent markers, normal karyotypes, and could differentiate into the three germ layers. These iPSC lines can serve as invaluable tools to model pathological mechanisms of DCM in vitro caused by LMNA mutations.

  View details for DOI 10.1016/j.scr.2021.102638

  View details for PubMedID 34954454

• Scaling concepts in 'omics: Nuclear lamin-B scales with tumor growth and often predicts poor prognosis, unlike fibrosis. Proceedings of the National Academy of Sciences of the United States of America Vashisth, M., Cho, S., Irianto, J., Xia, Y., Wang, M., Hayes, B., Wieland, D., Wells, R., Jafarpour, F., Liu, A., Discher, D. E. 2021; 118 (48)

  Abstract

  Physicochemical principles such as stoichiometry and fractal assembly can give rise to characteristic scaling between components that potentially include coexpressed transcripts. For key structural factors within the nucleus and extracellular matrix, we discover specific gene-gene scaling exponents across many of the 32 tumor types in The Cancer Genome Atlas, and we demonstrate utility in predicting patient survival as well as scaling-informed machine learning (SIML). All tumors with adjacent tissue data show cancer-elevated proliferation genes, with some genes scaling with the nuclear filament LMNB1, including the transcription factor FOXM1 that we show directly regulates LMNB1 SIML shows that such regulated cancers cluster together with longer overall survival than dysregulated cancers, but high LMNB1 and FOXM1 in half of regulated cancers surprisingly predict poor survival, including for liver cancer. COL1A1 is also studied because it too increases in tumors, and a pan-cancer set of fibrosis genes shows substoichiometric scaling with COL1A1 but predicts patient outcome only for liver cancer-unexpectedly being prosurvival. Single-cell RNA-seq data show nontrivial scaling consistent with power laws from bulk RNA and protein analyses, and SIML segregates synthetic from contractile cancer fibroblasts. Our scaling approach thus yields fundamentals-based power laws relatable to survival, gene function, and experiments.

  View details for DOI 10.1073/pnas.2112940118

  View details for PubMedID 34810266

  View details for PubMedCentralID PMC8640833

• Reconstructing the heart using iPSCs: Engineering strategies and applications. Journal of molecular and cellular cardiology Cho, S., Lee, C., Skylar-Scott, M. A., Heilshorn, S. C., Wu, J. C. 2021

  Abstract

  Induced pluripotent stem cells (iPSCs) have emerged as a key component of cardiac tissue engineering, enabling studies of cardiovascular disease mechanisms, drug responses, and developmental processes in human 3D tissue models assembled from isogenic cells. Since the very first engineered heart tissues were introduced more than two decades ago, a wide array of iPSC-derived cardiac spheroids, organoids, and heart-on-a-chip models have been developed incorporating the latest available technologies and materials. In this review, we will first outline the fundamental biological building blocks required to form a functional unit of cardiac muscle, including iPSC-derived cells differentiated by soluble factors (e.g., small molecules), extracellular matrix scaffolds, and exogenous biophysical maturation cues. We will then summarize the different fabrication approaches and strategies employed to reconstruct the heart in vitro at varying scales and geometries. Finally, we will discuss how these platforms, with continued improvements in scalability and tissue maturity, can contribute to both basic cardiovascular research and clinical applications in the future.

  View details for DOI 10.1016/j.yjmcc.2021.04.006

  View details for PubMedID 33895197

• An extracellular matrix paradox in myocardial scar formation. Signal transduction and targeted therapy Cho, S., Paik, D. T., Wu, J. C. 2020; 5 (1): 151

  View details for DOI 10.1038/s41392-020-00270-z

  View details for PubMedID 32788685

• Tension in fibrils suppresses their enzymatic degradation - A molecular mechanism for 'use it or lose it'. Matrix biology : journal of the International Society for Matrix Biology Saini, K., Cho, S., Dooling, L. J., Discher, D. E. 2019

  Abstract

  Tissue homeostasis depends on a balance of synthesis and degradation of constituent proteins, with turnover of a given protein potentially regulated by its use. Extracellular matrix (ECM) is predominantly composed of fibrillar collagens that exhibit tension-sensitive degradation, which we review here at different levels of hierarchy. Past experiments and recent proteomics measurements together suggest that mechanical strain stabilizes collagen against enzymatic degradation at the scale of tissues and fibrils whereas isolated collagen molecules exhibit a biphasic behavior that depends on load magnitude. Within a Michaelis-Menten framework, collagenases at constant concentration effectively exhibit a low activity on substrate fibrils when the fibrils are strained by tension. Mechanisms of such mechanosensitive regulation are surveyed together with relevant interactions of collagen fibrils with cells.

  View details for DOI 10.1016/j.matbio.2019.06.001

  View details for PubMedID 31201857

• Manipulating the mechanics of extracellular matrix to study effects on the nucleus and its structure. Methods (San Diego, Calif.) Xia, Y., Cho, S., Vashisth, M., Ivanovska, I. L., Dingal, P. C., Discher, D. E. 2019; 157: 3-14

  Abstract

  Tissues such as brain, muscle, and bone differ greatly not only in their biological functions but also in their mechanical properties. Brain is far softer than muscle while bone is the stiffest tissue. Stiffness of extracellular microenvironments affects fundamental cell biological processes such as polarization and DNA replication, which affect nuclear size, shape, and levels of nuclear proteins such as the lamins that modulate gene expression. Reductionist approaches have helped dissect the effects of matrix mechanics away from confounding biochemical signals. Here, we summarize materials and methods for synthesizing and characterizing soft and stiff synthetic hydrogels widely used for mechanobiological studies. Such gels are also easily made to mimic the mechanical heterogeneity of fibrotic tissues. We further describe a nano-thin collagen fiber system, which enables control of anisotropy in addition to stiffness. With the different systems, we illustrate the effects of matrix mechanics on nuclear size, shape, and proteins including the lamins.

  View details for DOI 10.1016/j.ymeth.2018.12.009

  View details for PubMedID 30593865

  View details for PubMedCentralID PMC6508970

• Nuclear rupture at sites of high curvature compromises retention of DNA repair factors. The Journal of cell biology Xia, Y., Ivanovska, I. L., Zhu, K., Smith, L., Irianto, J., Pfeifer, C. R., Alvey, C. M., Ji, J., Liu, D., Cho, S., Bennett, R. R., Liu, A. J., Greenberg, R. A., Discher, D. E. 2018; 217 (11): 3796-3808

  Abstract

  The nucleus is physically linked to the cytoskeleton, adhesions, and extracellular matrix-all of which sustain forces, but their relationships to DNA damage are obscure. We show that nuclear rupture with cytoplasmic mislocalization of multiple DNA repair factors correlates with high nuclear curvature imposed by an external probe or by cell attachment to either aligned collagen fibers or stiff matrix. Mislocalization is greatly enhanced by lamin A depletion, requires hours for nuclear reentry, and correlates with an increase in pan-nucleoplasmic foci of the DNA damage marker γH2AX. Excess DNA damage is rescued in ruptured nuclei by cooverexpression of multiple DNA repair factors as well as by soft matrix or inhibition of actomyosin tension. Increased contractility has the opposite effect, and stiff tumors with low lamin A indeed exhibit increased nuclear curvature, more frequent nuclear rupture, and excess DNA damage. Additional stresses likely play a role, but the data suggest high curvature promotes nuclear rupture, which compromises retention of DNA repair factors and favors sustained damage.

  View details for DOI 10.1083/jcb.201711161

  View details for PubMedID 30171044

  View details for PubMedCentralID PMC6219729

• Nuclear mechanosensing Emerging Topics in Life Sciences Xia, Y., Pfeifer, C. R., Cho, S., Discher, D. E., Irianto, J. 2018; 2 (5): 713-725
• Progerin phosphorylation in interphase is lower and less mechanosensitive than lamin-A,C in iPS-derived mesenchymal stem cells. Nucleus (Austin, Tex.) Cho, S., Abbas, A., Irianto, J., Ivanovska, I. L., Xia, Y., Tewari, M., Discher, D. E. 2018; 9 (1): 230-245

  Abstract

  Interphase phosphorylation of lamin-A,C depends dynamically on a cell's microenvironment, including the stiffness of extracellular matrix. However, phosphorylation dynamics is poorly understood for diseased forms such as progerin, a permanently farnesylated mutant of LMNA that accelerates aging of stiff and mechanically stressed tissues. Here, fine-excision alignment mass spectrometry (FEA-MS) is developed to quantify progerin and its phosphorylation levels in patient iPS cells differentiated to mesenchymal stem cells (MSCs). The stoichiometry of total A-type lamins (including progerin) versus B-type lamins measured for Progeria iPS-MSCs prove similar to that of normal MSCs, with total A-type lamins more abundant than B-type lamins. However, progerin behaves more like farnesylated B-type lamins in mechanically-induced segregation from nuclear blebs. Phosphorylation of progerin at multiple sites in iPS-MSCs cultured on rigid plastic is also lower than that of normal lamin-A and C. Reduction of nuclear tension upon i) cell rounding/detachment from plastic, ii) culture on soft gels, and iii) inhibition of actomyosin stress increases phosphorylation and degradation of lamin-C > lamin-A > progerin. Such mechano-sensitivity diminishes, however, with passage as progerin and DNA damage accumulate. Lastly, transcription-regulating retinoids exert equal effects on both diseased and normal A-type lamins, suggesting a differential mechano-responsiveness might best explain the stiff tissue defects in Progeria.

  View details for DOI 10.1080/19491034.2018.1460185

  View details for PubMedID 29619860

  View details for PubMedCentralID PMC5973135

• Stem Cell Differentiation is Regulated by Extracellular Matrix Mechanics. Physiology (Bethesda, Md.) Smith, L. R., Cho, S., Discher, D. E. 2018; 33 (1): 16-25

  Abstract

  Stem cells mechanosense the stiffness of their microenvironment, which impacts differentiation. Although tissue hydration anti-correlates with stiffness, extracellular matrix (ECM) stiffness is clearly transduced into gene expression via adhesion and cytoskeleton proteins that tune fates. Cytoskeletal reorganization of ECM can create heterogeneity and influence fates, with fibrosis being one extreme.

  View details for DOI 10.1152/physiol.00026.2017

  View details for PubMedID 29212889

  View details for PubMedCentralID PMC5866410

• Mechanosensing of matrix by stem cells: From matrix heterogeneity, contractility, and the nucleus in pore-migration to cardiogenesis and muscle stem cells in vivo. Seminars in cell & developmental biology Smith, L., Cho, S., Discher, D. E. 2017; 71: 84-98

  Abstract

  Stem cells are particularly 'plastic' cell types that are induced by various cues to become specialized, tissue-functional lineages by switching on the expression of specific gene programs. Matrix stiffness is among the cues that multiple stem cell types can sense and respond to. This seminar-style review focuses on mechanosensing of matrix elasticity in the differentiation or early maturation of a few illustrative stem cell types, with an intended audience of biologists and physical scientists. Contractile forces applied by a cell's acto-myosin cytoskeleton are often resisted by the extracellular matrix and transduced through adhesions and the cytoskeleton ultimately into the nucleus to modulate gene expression. Complexity is added by matrix heterogeneity, and careful scrutiny of the evident stiffness heterogeneity in some model systems resolves some controversies concerning matrix mechanosensing. Importantly, local stiffness tends to dominate, and 'durotaxis' of stem cells toward stiff matrix reveals a dependence of persistent migration on myosin-II force generation and also rigid microtubules that confer directionality. Stem and progenitor cell migration in 3D can be further affected by matrix porosity as well as stiffness, with nuclear size and rigidity influencing niche retention and fate choices. Cell squeezing through rigid pores can even cause DNA damage and genomic changes that contribute to de-differentiation toward stem cell-like states. Contraction of acto-myosin is the essential function of striated muscle, which also exhibit mechanosensitive differentiation and maturation as illustrated in vivo by beating heart cells and by the regenerative mobilization of skeletal muscle stem cells.

  View details for DOI 10.1016/j.semcdb.2017.05.025

  View details for PubMedID 28587976

  View details for PubMedCentralID PMC5659905

• SIRPA-Inhibited, Marrow-Derived Macrophages Engorge, Accumulate, and Differentiate in Antibody-Targeted Regression of Solid Tumors CURRENT BIOLOGY Alvey, C. M., Spinler, K. R., Irianto, J., Pfeifer, C. R., Hayes, B., Xia, Y., Cho, S., Dingal, P., Hsu, J., Smith, L., Tewari, M., Discher, D. E. 2017; 27 (14): 2065-+

  Abstract

  Marrow-derived macrophages are highly phagocytic, but whether they can also traffic into solid tumors and engulf cancer cells is questionable, given the well-known limitations of tumor-associated macrophages (TAMs). Here, SIRPα on macrophages from mouse and human marrow was inhibited to block recognition of its ligand, the "marker of self" CD47 on all other cells. These macrophages were then systemically injected into mice with fluorescent human tumors that had been antibody targeted. Within days, the tumors regressed, and single-cell fluorescence analyses showed that the more the macrophages engulfed, the more they accumulated within regressing tumors. Human-marrow-derived macrophages engorged on the human tumors, while TAMs were minimally phagocytic, even toward CD47-knockdown tumors. Past studies had opsonized tumors in situ with antibody and/or relied on mouse TAMs but had not injected SIRPα-inhibited cells; also, unlike past injections of anti-CD47, blood parameters remained normal and safe. Consistent with tumor-selective engorge-and-accumulate processes in vivo, phagocytosis in vitro inhibited macrophage migration through micropores that mimic features of dense 3D tissue. Accumulation of SIRPα-inhibited macrophages in tumors favored tumor regression for 1-2 weeks, but donor macrophages quickly differentiated toward non-phagocytic, high-SIRPα TAMs. Analyses of macrophages on soft (like marrow) or stiff (like solid tumors) collagenous gels demonstrated a stiffness-driven, retinoic-acid-modulated upregulation of SIRPα and the mechanosensitive nuclear marker lamin-A. Mechanosensitive differentiation was similarly evident in vivo and likely limited the anti-tumor effects, as confirmed by re-initiation of tumor regression by fresh injections of SIRPα-inhibited macrophages. Macrophage motility, phagocytosis, and differentiation in vivo are thus coupled.

  View details for DOI 10.1016/j.cub.2017.06.005

  View details for Web of Science ID 000406178400019

  View details for PubMedID 28669759

  View details for PubMedCentralID PMC5846676

• Cross-linked matrix rigidity and soluble retinoids synergize in nuclear lamina regulation of stem cell differentiation. Molecular biology of the cell Ivanovska, I. L., Swift, J., Spinler, K., Dingal, D., Cho, S., Discher, D. E. 2017; 28 (14): 2010-2022

  Abstract

  Synergistic cues from extracellular matrix and soluble factors are often obscure in differentiation. Here the rigidity of cross-linked collagen synergizes with retinoids in the osteogenesis of human marrow mesenchymal stem cells (MSCs). Collagen nanofilms serve as a model matrix that MSCs can easily deform unless the film is enzymatically cross-linked, which promotes the spreading of cells and the stiffening of nuclei as both actomyosin assembly and nucleoskeletal lamin-A increase. Expression of lamin-A is known to be controlled by retinoic acid receptor (RAR) transcription factors, but soft matrix prevents any response to any retinoids. Rigid matrix is needed to induce rapid nuclear accumulation of the RARG isoform and for RARG-specific antagonist to increase or maintain expression of lamin-A as well as for RARG-agonist to repress expression. A progerin allele of lamin-A is regulated in the same manner in iPSC-derived MSCs. Rigid matrices are further required for eventual expression of osteogenic markers, and RARG-antagonist strongly drives lamin-A-dependent osteogenesis on rigid substrates, with pretreated xenografts calcifying in vivo to a similar extent as native bone. Proteomics-detected targets of mechanosensitive lamin-A and retinoids underscore the convergent synergy of insoluble and soluble cues in differentiation.

  View details for DOI 10.1091/mbc.E17-01-0010

  View details for PubMedID 28566555

  View details for PubMedCentralID PMC5541850

• Matrix Mechanosensing: From Scaling Concepts in 'Omics Data to Mechanisms in the Nucleus, Regeneration, and Cancer. Annual review of biophysics Discher, D. E., Smith, L., Cho, S., Colasurdo, M., García, A. J., Safran, S. 2017; 46: 295-315

  Abstract

  Many of the most important molecules of life are polymers. In animals, the most abundant of the proteinaceous polymers are the collagens, which constitute the fibrous matrix outside cells and which can also self-assemble into gels. The physically measurable stiffness of gels, as well as tissues, increases with the amount of collagen, and cells seem to sense this stiffness. An understanding of this mechanosensing process in complex tissues, including fibrotic disease states with high collagen, is now utilizing 'omics data sets and is revealing polymer physics-type, nonlinear scaling relationships between concentrations of seemingly unrelated biopolymers. The nuclear structure protein lamin A provides one example, with protein and transcript levels increasing with collagen 1 and tissue stiffness, and with mechanisms rooted in protein stabilization induced by cytoskeletal stress. Physics-based models of fibrous matrix, cytoskeletal force dipoles, and the lamin A gene circuit illustrate the wide range of testable predictions emerging for tissues, cell cultures, and even stem cell-based tissue regeneration. Beyond the epigenetics of mechanosensing, the scaling in cancer of chromosome copy number variations and other mutations with tissue stiffness suggests that genomic changes are occurring by mechanogenomic processes that now require elucidation.

  View details for DOI 10.1146/annurev-biophys-062215-011206

  View details for PubMedID 28532215

  View details for PubMedCentralID PMC5444306

• Mechanical signaling coordinates the embryonic heartbeat PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Chiou, K. K., Rocks, J. W., Chen, C., Cho, S., Merkus, K. E., Rajaratnam, A., Robison, P., Tewari, M., Vogel, K., Majkut, S. F., Prosser, B. L., Discher, D. E., Liu, A. J. 2016; 113 (32): 8939–44

  Abstract

  In the beating heart, cardiac myocytes (CMs) contract in a coordinated fashion, generating contractile wave fronts that propagate through the heart with each beat. Coordinating this wave front requires fast and robust signaling mechanisms between CMs. The primary signaling mechanism has long been identified as electrical: gap junctions conduct ions between CMs, triggering membrane depolarization, intracellular calcium release, and actomyosin contraction. In contrast, we propose here that, in the early embryonic heart tube, the signaling mechanism coordinating beats is mechanical rather than electrical. We present a simple biophysical model in which CMs are mechanically excitable inclusions embedded within the extracellular matrix (ECM), modeled as an elastic-fluid biphasic material. Our model predicts strong stiffness dependence in both the heartbeat velocity and strain in isolated hearts, as well as the strain for a hydrogel-cultured CM, in quantitative agreement with recent experiments. We challenge our model with experiments disrupting electrical conduction by perfusing intact adult and embryonic hearts with a gap junction blocker, β-glycyrrhetinic acid (BGA). We find this treatment causes rapid failure in adult hearts but not embryonic hearts-consistent with our hypothesis. Last, our model predicts a minimum matrix stiffness necessary to propagate a mechanically coordinated wave front. The predicted value is in accord with our stiffness measurements at the onset of beating, suggesting that mechanical signaling may initiate the very first heartbeats.

  View details for DOI 10.1073/pnas.1520428113

  View details for Web of Science ID 000381293300042

  View details for PubMedID 27457951

  View details for PubMedCentralID PMC4987837

• Matrix Elasticity Regulates Lamin-A,C Phosphorylation and Turnover with Feedback to Actomyosin CURRENT BIOLOGY Buxboim, A., Swift, J., Irianto, J., Spinler, K. R., Dingal, P. P., Athirasala, A., Kao, Y. C., Cho, S., Harada, T., Shin, J., Discher, D. E. 2014; 24 (16): 1909–17

  Abstract

  Tissue microenvironments are characterized not only in terms of chemical composition but also by collective properties such as stiffness, which influences the contractility of a cell, its adherent morphology, and even differentiation. The nucleoskeletal protein lamin-A,C increases with matrix stiffness, confers nuclear mechanical properties, and influences differentiation of mesenchymal stem cells (MSCs), whereas B-type lamins remain relatively constant. Here we show in single-cell analyses that matrix stiffness couples to myosin-II activity to promote lamin-A,C dephosphorylation at Ser22, which regulates turnover, lamina physical properties, and actomyosin expression. Lamin-A,C phosphorylation is low in interphase versus dividing cells, and its levels rise with states of nuclear rounding in which myosin-II generates little to no tension. Phosphorylated lamin-A,C localizes to nucleoplasm, and phosphorylation is enriched on lamin-A,C fragments and is suppressed by a cyclin-dependent kinase (CDK) inhibitor. Lamin-A,C knockdown in primary MSCs suppresses transcripts predominantly among actomyosin genes, especially in the serum response factor (SRF) pathway. Levels of myosin-IIA thus parallel levels of lamin-A,C, with phosphosite mutants revealing a key role for phosphoregulation. In modeling the system as a parsimonious gene circuit, we show that tension-dependent stabilization of lamin-A,C and myosin-IIA can suitably couple nuclear and cell morphology downstream of matrix mechanics.

  View details for DOI 10.1016/j.cub.2014.07.001

  View details for Web of Science ID 000340686300031

  View details for PubMedID 25127216

  View details for PubMedCentralID PMC4373646

• Tight coupling between nucleus and cell migration through the perinuclear actin cap. Journal of cell science Kim, D. H., Cho, S., Wirtz, D. 2014; 127 (Pt 11): 2528-41

  Abstract

  Although eukaryotic cells are known to alternate between 'advancing' episodes of fast and persistent movement and 'hesitation' episodes of low speed and low persistence, the molecular mechanism that controls the dynamic changes in morphology, speed and persistence of eukaryotic migratory cells remains unclear. Here, we show that the movement of the interphase nucleus during random cell migration switches intermittently between two distinct modes - rotation and translocation - that follow with high fidelity the sequential rounded and elongated morphologies of the nucleus and cell body, respectively. Nuclear rotation and translocation mediate the stop-and-go motion of the cell through the dynamic formation and dissolution, respectively, of the contractile perinuclear actin cap, which is dynamically coupled to the nuclear lamina and the nuclear envelope through LINC complexes. A persistent cell movement and nuclear translocation driven by the actin cap are halted following the disruption of the actin cap, which in turn allows the cell to repolarize for its next persistent move owing to nuclear rotation mediated by cytoplasmic dynein light intermediate chain 2.

  View details for DOI 10.1242/jcs.144345

  View details for PubMedID 24639463

  View details for PubMedCentralID PMC4038945