High content image analysis of focal adhesion-dependent mechanosensitive stem cell differentiation.

Human mesenchymal stem cells (hMSCs) receive differentiation cues from a number of stimuli, including extracellular matrix (ECM) stiffness. The pathways used to sense stiffness and other physical cues are just now being understood and include proteins within focal adhesions. To rapidly advance the pace of discovery for novel mechanosensitive proteins, we employed a combination of in silico and high throughput in vitro methods to analyze 47 different focal adhesion proteins for cryptic kinase binding sites. High content imaging of hMSCs treated with small interfering RNAs for the top 6 candidate proteins showed novel effects on both osteogenic and myogenic differentiation; Vinculin and SORBS1 were necessary for stiffness-mediated myogenic and osteogenic differentiation, respectively. Both of these proteins bound to MAPK1 (also known as ERK2), suggesting that it plays a context-specific role in mechanosensing for each lineage; validation for these sites was performed. This high throughput system, while specifically built to analyze stiffness-mediated stem cell differentiation, can be expanded to other physical cues to more broadly assess mechanical signaling and increase the pace of sensor discovery.

Epigenetic Regulation of Phosphodiesterases 2A and 3A Underlies Compromised ß-Adrenergic Signaling in an iPSC Model of Dilated Cardiomyopathy.

ß-adrenergic signaling pathways mediate key aspects of cardiac function. Its dysregulation is associated with a range of cardiac diseases, including dilated cardiomyopathy (DCM). Previously, we established an iPSC model of familial DCM from patients with a mutation in TNNT2, a sarcomeric protein. Here, we found that the ß-adrenergic agonist isoproterenol induced mature ß-adrenergic signaling in iPSC-derived cardiomyocytes (iPSC-CMs) but that this pathway was blunted in DCM iPSC-CMs. Although expression levels of several ß-adrenergic signaling components were unaltered between control and DCM iPSC-CMs, we found that phosphodiesterases (PDEs) 2A and PDE3A were upregulated in DCM iPSC-CMs and that PDE2A was also upregulated in DCM patient tissue. We further discovered increased nuclear localization of mutant TNNT2 and epigenetic modifications of PDE genes in both DCM iPSC-CMs and patient tissue. Notably, pharmacologic inhibition of PDE2A and PDE3A restored cAMP levels and ameliorated the impaired ß-adrenergic signaling of DCM iPSC-CMs, suggesting therapeutic potential.

Interplay of matrix stiffness and protein tethering in stem cell differentiation.

Stem cells regulate their fate by binding to, and contracting against, the extracellular matrix. Recently, it has been proposed that in addition to matrix stiffness and ligand type, the degree of coupling of fibrous protein to the surface of the underlying substrate, that is, tethering and matrix porosity, also regulates stem cell differentiation. By modulating substrate porosity without altering stiffness in polyacrylamide gels, we show that varying substrate porosity did not significantly change protein tethering, substrate deformations, or the osteogenic and adipogenic differentiation of human adipose-derived stromal cells and marrow-derived mesenchymal stromal cells. Varying protein-substrate linker density up to 50-fold changed tethering, but did not affect osteogenesis, adipogenesis, surface-protein unfolding or underlying substrate deformations. Differentiation was also unaffected by the absence of protein tethering. Our findings imply that the stiffness of planar matrices regulates stem cell differentiation independently of protein tethering and porosity.

A co-culture device with a tunable stiffness to understand combinatorial cell-cell and cell-matrix interactions.

Cell behavior on 2-D in vitro cultures is continually being improved to better mimic in vivo physiological conditions by combining niche cues including multiple cell types and substrate stiffness, which are well known to impact cell phenotype. However, no system exists in which a user can systematically examine cell behavior on a substrate with a specific stiffness (elastic modulus) in culture with a different cell type, while maintaining distinct cell populations. We demonstrate the modification of a silicon reconfigurable co-culture system with a covalently linked hydrogel of user-defined stiffness. This device allows the user to control whether two separate cell populations are in contact with each other or only experience paracrine interactions on substrates of controllable stiffness. To illustrate the utility of this device, we examined the role of substrate stiffness combined with myoblast co-culture on adipose derived stem cell (ASC) differentiation and found that the presence of myoblasts and a 10 kPa substrate stiffness increased ASC myogenesis versus co-culture on stiff substrates. As this example highlights, this technology better controls the in vitro microenvironment, allowing the user to develop a more thorough understanding of the combined effects of cell-cell and cell-matrix interactions.

In situ mechanotransduction via vinculin regulates stem cell differentiation.

Human mesenchymal stem cell (hMSC) proliferation, migration, and differentiation have all been linked to extracellular matrix stiffness, yet the signaling pathway(s) that are necessary for mechanotransduction remain unproven. Vinculin has been implicated as a mechanosensor in vitro, but here we demonstrate its ability to also regulate stem cell behavior, including hMSC differentiation. RNA interference-mediated vinculin knockdown significantly decreased stiffness-induced MyoD, a muscle transcription factor, but not Runx2, an osteoblast transcription factor, and impaired stiffness-mediated migration. A kinase binding accessibility screen predicted a cryptic MAPK1 signaling site in vinculin which could regulate these behaviors. Indeed, reintroduction of vinculin domains into knocked down cells indicated that MAPK1 binding site-containing vinculin constructs were necessary for hMSC expression of MyoD. Vinculin knockdown does not appear to interfere with focal adhesion assembly, significantly alter adhesive properties, or diminish cell traction force generation, indicating that its knockdown only adversely affected MAPK1 signaling. These data provide some of the first evidence that a force-sensitive adhesion protein can regulate stem cell fate.

Mesenchymal stem cell durotaxis depends on substrate stiffness gradient strength.

Mesenchymal stem cells (MSCs) respond to the elasticity of their environment, which varies between and within tissues. Stiffness gradients within tissues can result from pathological conditions, but also occur through normal variation, such as in muscle. MSC migration can be directed by shallow stiffness gradients before differentiating. Gradients with fine control over substrate compliance - both in range and rate of change (strength) - are needed to better understand mechanical regulation of MSC migration in normal and diseased states. We describe polyacrylamide stiffness gradient fabrication using three distinct systems, generating stiffness gradients of physiological (1 Pa/µm), pathological (10 Pa/µm), and step change (≥ 100Pa/µm) strength. All gradients spanned a range of physiologically relevant elastic moduli for soft tissues (1-12 kPa). MSCs migrated to the stiffest region on each gradient. Time-lapse microscopy revealed that migration velocity correlated directly with gradient strength. Directed migration was reduced in the presence of the contractile agonist lysophosphatidic acid (LPA) and cytoskeleton-perturbing drugs nocodazole and cytochalasin. LPA- and nocodazole-treated cells remained spread and protrusive on the substrate, while cytochalasin-treated cells did not. Nocodazole-treated cells spread in a similar manner to untreated cells, but exhibited greatly diminished traction forces. These data suggest that a functional actin cytoskeleton is required for migration whereas microtubules are required for directed migration. The data also imply that, in vivo, MSCs may preferentially accumulate in regions of high elastic modulus and make a greater contribution to tissue repairs in these locations.

The alignment and fusion assembly of adipose-derived stem cells on mechanically patterned matrices.

Cell patterning is typically accomplished by selectively depositing proteins for cell adhesion only on patterned regions; however in tissues, cells are also influenced by mechanical stimuli, which can also result in patterned arrangements of cells. We developed a mechanically-patterned hydrogel to observe and compare it to extracellular matrix (ECM) ligand patterns to determine how to best regulate and improve cell type-specific behaviors. Ligand-based patterning on hydrogels was not robust over prolonged culture, but cells on mechanically-patterned hydrogels differentially sorted based on stiffness preference: myocytes and adipose-derived stem cells (ASCs) underwent stiffness-mediated migration, i.e. durotaxis, and remained on myogenic hydrogel regions. Myocytes developed aligned striations and fused on myogenic stripes of the mechanically-patterned hydrogel. ASCs aligned and underwent myogenesis, but their fusion rate increased, as did the number of cells fusing into a myotube as a result of their alignment. Conversely, neuronal cells did not exhibit durotaxis and could be seen on soft regions of the hydrogel for prolonged culture time. These results suggest that mechanically-patterned hydrogels could provide a platform to create tissue engineered, innervated micro-muscles of neural and muscle phenotypes juxtaposed next to each other in order better recreate a muscle niche.

Mechanical derivation of functional myotubes from adipose-derived stem cells.

Though reduced serum or myoblast co-culture alone can differentiate adipose-derived stem cells (ASCs) into mesenchymal lineages, efficiency is usually not sufficient to restore function in vivo. Often when injected into fibrotic muscle, their differentiation may be misdirected by the now stiffened tissue. Here ASCs are shown to not just simply reflect the qualitative stiffness sensitivity of bone marrow-derived stem cells (BMSCs) but to exceed BMSC myogenic capacity, expressing the appropriate temporal sequence of muscle transcriptional regulators on muscle-mimicking extracellular matrix in a tension and focal adhesion-dependent manner. ASCs formed multi-nucleated myotubes with a continuous cytoskeleton that was not due to misdirected cell division; microtubule depolymerization severed myotubes, but after washout, ASCs refused at a rate similar to pre-treated values. BMSCs never underwent stiffness-mediated fusion. ASC-derived myotubes, when replated onto non-permissive stiff matrix, maintained their fused state. Together these data imply enhanced mechanosensitivity for ASCs, making them a better therapeutic cell source for fibrotic muscle.

Microfluidic LIPS for serum antibody detection: demonstration of a rapid test for HSV-2 infection.

There is great interest in point-of-care antibody testing for the diagnosis of infectious and autoimmune diseases. As a first step in the development of self-contained and miniaturized devices for highly quantitative antibody detection, we demonstrate the application of Luciferase Immunoprecipitation Systems (LIPS) technology in a microfluidic format. Protein A/G was immobilized on the walls of PDMS-glass microchannels of 500 nL volume. The assay proceeds with the simultaneous introduction of plasma and Renilla luciferase-tagged antigens. Following washing, coelenterazine substrate was added and bound antigen-luciferase measured by chemiluminescence. Total assay time, including rinsing and detection, is under 10 min. Using these stable microfluidic devices, high diagnostic performance (100% sensitivity and 100% specificity) was achieved for the diagnosis of HSV-2 infection. Based on these findings, the LIPS microfluidic format should readily lend itself to automation and the transfer to portable instrumentation.