Living cells are constantly exposed to mechanical stimuli arising from the surrounding extracellular matrix (ECM) or from neighboring cells. The intracellular molecular processes through which such physical cues are transformed into a biological response are collectively dubbed as mechanotransduction and are of fundamental importance to help the cell timely adapt to the continuous dynamic modifications of the microenvironment. Local changes in ECM composition and mechanics are driven by a feed forward interplay between the cell and the matrix itself, with the first depositing ECM proteins that in turn will impact on the surrounding cells. As such, these changes occur regularly during tissue development and are a hallmark of the pathologies of aging. Only lately, though, the importance of mechanical cues in controlling cell function (e.g., proliferation, differentiation, migration) has been acknowledged. Here we provide a critical review of the recent insights into the molecular basis of cellular mechanotransduction, by analyzing how mechanical stimuli get transformed into a given biological response through the activation of a peculiar genetic program. Specifically, by recapitulating the processes involved in the interpretation of ECM remodeling by Focal Adhesions at cell-matrix interphase, we revise the role of cytoskeleton tension as the second messenger of the mechanotransduction process and the action of mechano-responsive shuttling proteins converging on stage and cell-specific transcription factors. Finally, we give few paradigmatic examples highlighting the emerging role of malfunctions in cell mechanosensing apparatus in the onset and progression of pathologies.

Hippo effectors YAP/TAZ act as on–off mechanosensing switches by sensing modifications in extracellular matrix (ECM) composition and mechanics. The regulation of their activity has been described by a hierarchical model in which elements of Hippo pathway are under the control of focal adhesions (FAs). Here we unveil the molecular mechanism by which cell spreading and RhoA GTPase activity control FA formation through YAP to stabilize the anchorage of the actin cytoskeleton to the cell membrane. This mechanism requires YAP co-transcriptional function and involves the activation of genes encoding for integrins and FA docking proteins. Tuning YAP transcriptional activity leads to the modification of cell mechanics, force development and adhesion strength, and determines cell shape, migration and differentiation. These results provide new insights into the mechanism of YAP mechanosensing activity and qualify this Hippo effector as the key determinant of cell mechanics in response to ECM cues.

Hepatocyte growth factor (HGF), a pleiotropic cytokine of mesenchymal origin promoting migration, proliferation, and survival in a wide spectrum of cells, can also modulate different biological responses in stem cells, but the mechanisms involved are not completely understood so far. In this context, we show that short-term exposure of mesenchymal stem cells (MSCs) to HGF can induce the activation of its cognate Met receptor and the downstream effectors ERK1/2, p38MAPK, and PI3K/Akt, while long-term exposure to HGF resulted in cytoskeletal rearrangement, cell migration, and marked inhibition of proliferation through the arrest in the G 1 -S checkpoint.When added to MSCs, the K252A tyrosine kinase inhibitor prevented HGF-induced responses. HGF's effect on MSC proliferation was reversed by p38 inhibitor SB203580, while the effects on cell migration were abrogated by PI3K inhibitor Wortmannin, suggesting that HGF acts through different pathways to determine its complex effects on MSCs. Prolonged treatment with HGF induced the expression of cardiac-specific markers (GATA-4, MEF2C, TEF1, desmin, ␣-MHC, ␤-MHC, and nestin) with the concomitant loss of the stem cell markers nucleostemin, c-kit, and CD105. STEM CELLS 2006;24: 23-33

Cardiac progenitor cells (CPCs) are a promising autologous source of cells for cardiac\ud regenerative medicine. However, CPC culture in vitro requires the presence of microenvironmental\ud conditions (a complex array of bioactive substance concentration, mechanostructural\ud factors, and physicochemical factors) closely mimicking the natural cell surrounding in vivo,\ud including the capability to uphold reactive oxygen species (ROS) within physiological levels\ud in vitro. Cerium oxide nanoparticles (nanoceria) are redox-active and could represent a potent\ud tool to control the oxidative stress in isolated CPCs. Here, we report that 24 h exposure to 5, 10,\ud and 50 !g/mL of nanoceria did not a!ect cell growth and function in cardiac progenitor cells,\ud while being able to protect CPCs from H2O2-induced cytotoxicity for at least 7 days, indicating\ud that nanoceria in an e!ective antioxidant. Therefore, these "ndings con"rm the great\ud potential of nanoceria for controlling ROS-induced cell damage

a b s t r a c tA novel (scalable) electrospinning process was developed to fabricate bio-inspired multiscale threedimensional scaffolds endowed with a controlled multimodal distribution of fiber diameters and geared towards soft tissue engineering. The resulting materials finely mingle nano-and microscale fibers together, rather than simply juxtaposing them, as is commonly found in the literature. A detailed proof of concept study was conducted on a simpler bimodal poly(e-caprolactone) (PCL) scaffold with modes of fiber distribution at 600 nm and 3.3 lm. Three conventional unimodal scaffolds with mean diameters of 300 nm and 2.6 and 5.2 lm, respectively, were used as controls to evaluate the new materials. Characterization of the microstructure (i.e. porosity, fiber distribution and pore structure) and mechanical properties (i.e. stiffness, strength and failure mode) indicated that the multimodal scaffold had superior mechanical properties (Young's modulus $40 MPa and strength $1 MPa) in comparison with the controls, despite the large porosity ($90% on average). A biological assessment was conducted with bone marrow stromal cell type (mesenchymal stem cells, mTERT-MSCs). While the new material compared favorably with the controls with respect to cell viability (on the outer surface), it outperformed them in terms of cell colonization within the scaffold. The latter result, which could neither be practically achieved in the controls nor expected based on current models of pore size distribution, demonstrated the greater openness of the pore structure of the bimodal material, which remarkably did not come at the expense of its mechanical properties. Furthermore, nanofibers were seen to form a nanoweb bridging across neighboring microfibers, which boosted cell motility and survival. Lastly, standard adipogenic and osteogenic differentiation tests served to demonstrate that the new scaffold did not hinder the multilineage potential of stem cells.

Hybrid 2D polymeric–ceramic biosupports are fabricated by mixing a nanostructured CeO2 powder with 85:15 poly(D,L‐lactic‐co‐glycolic acid) (PLGA)/dichloromethane solutions at specific concentrations, followed by solvent casting onto pre‐patterned molds. The mold patterning allows the orientation of ceramic nanoparticles into parallel lines within the composite scaffold. The ability of the produced films to host and address cell growth is evaluated after 1, 3, and 6 days of culturing with murine derived cardiac and mesenchymal stem cells (CSCs and MSCs), and compared with PLGA films without ceramics and loaded with nanostructured TiO2. Aligned cell growth is observed only for scaffolds that incorporate oriented ceramic nanoparticles, attributed to the nanoceramic ability to modulate the roughness pitch, thus improving cell sensitivity towards the host surface features. Better CSC and MSC proliferative activity is observed for CeO2 composites with respect to either TiO2‐added or unfilled PLGA films. This evidence may be related to the nanostructured CeO2 antioxidative properties.

The replacement of injured cardiac contractile cells with stem cell-derived functionally efficient cardiomyocytes has been envisaged as the resolutive treatment for degenerative heart diseases. Nevertheless, many technical issues concerning the optimal procedures to differentiate and engraft stem cells remain to be answered before heart cell therapy could be routinely used in clinical practice. So far, most studies have been focused on evaluating the differentiative potential of different growth factors without considering that only the synergistic cooperation of biochemical, topographic, chemical, and physical factors could induce stem cells to adopt the desired phenotype. The present study demonstrates that the differentiation of cardiac progenitor cells to cardiomyocytes does not occur when cells are challenged with soluble growth factors alone, but requires strictly controlled procedures for the isolation of a progenitor cell population and the artifactual recreation of a microenvironment critically featured by a fine-tuned combination of specific biological and physical factors. Indeed, the scaffold geometry and stiffness are crucial in enhancing growth factor differentiative effects on progenitor cells. The exploitation of this concept could be essential in setting up suitable procedures to fabricate functionally efficient engineered tissues.

T he Hippo pathway has been recently identified as a crucial axis in the regulation of organ size and shape during organogenesis and cancer. The paralog Yes-associated protein (YAP) and WW domain-containing transcription regulator protein 1 (WWTR1 or TAZ) are the downstream effectors of the Hippo pathway. These proteins have also been identified as mammalian proto-oncogenes. 1 Moreover, they perform as transcriptional coactivators in the nucleus, mainly in combination with transcription factors belonging to TEAD family. 2 In vitro, YAP/TAZ activity has been associated with mesenchymal stem cell (MSC) fate decision through the interaction with key determinants of osteogenic (Runx2) or adipogenic (PPARγ) differentiation. 3,4 YAP/TAZ axis has also emerged as a central regulator of human embryonic stem cell self-renewal through the control of SMAD complex shuttling to the nucleus, with TAZ knock-down resulting in the loss of cell pluripotency. 5 The same cofactors control intestinal 6 and neural progenitor cell number and differentiation 7 by targeting We identify a novel activity of YAP and TAZ in the regulation of tubulogenesis in 3D environments and highlight a role for YAP/TAZ in cardiac progenitor proliferation and differentiation. Furthermore, we show that YAP/TAZ expression is triggered in the heart cells located at the infarct border zone. Our results suggest a fundamental role for the YAP/TAZ axis in the response of resident progenitor cells to the modifications in microenvironment nanostructure and mechanics, thereby contributing to the maintenance of myocardial homeostasis in the adult heart. These proteins are indicated as potential targets to control cardiac progenitor cell fate by materials design.