Cell rheology: Stressed-out stem cells

Angular Dependence of Solar Cell Parameters in Crystalline Silicon Solar Cells Textured with Periodic Array of Microholes

Surface texturing is an indispensable way of increasing absorption in solar cells. In order to properly characterize the effect of texturing, the angular dependence of the incidence light should be addressed. This is particularly important when the actual application where the incidence angle of the sunlight varies during the day is considered. This study presents the angular dependence of solar cell parameters in the case of periodically textured crystalline silicon (c-Si) solar cells with microholes. A standard solar cell with pyramid texturing is also studied for comparison. It is shown that the incidence angle for the highest efficiency depends on the surface structure. While a standard pyramid-textured surface performs best at the zero angle of incidence, it is needed to tilt the sample with microholes textures 15° with respect to the surface normal. This is also confirmed by the simulation study performed for the structures presented in this study.

Regulating substrate mechanics to achieve odontogenic differentiation for dental pulp stem cells on TiO2 filled and unfilled polyisoprene

We have shown that materials other than hydrogels commonly used in tissue engineering can be effective in enabling differentiation of dental pulp stem cells (DPSC). Here we demonstrate that a hydrophobic elastomer, polyisoprene (PI), a component of Gutta-percha, normally used to obturate the tooth canal, can also be used to initiate differentiation of the pulp. We showed that PI substrates without additional coating promote cell adhesion and differentiation, while their moduli can be easily adjusted either by varying the coating thickness or incorporation of inorganic particles. DPSC plated on those PI substrates were shown, using SPM and hysitron indentation, to adjust their moduli to conform to differentially small changes in the substrate modulus. In addition, optical tweezers were used to separately measure the membrane and cytoplasm moduli of DPSC, with and without Rho kinase inhibitor. The results indicated that the changes in modulus were attributed predominantly to changes within the cytoplasm, rather than the cell membrane. CLSM was used to identify cell morphology. Differentiation, as determined by qRT-PCR, of the upregulation of OCN, and COL1α1 as well as biomineralization, characterized by SEM/EDAX, was observed on hard PI substrates in the absence of induction factors, i.e. dexamethasone, with moduli 3-4 MPa, regardless of preparation. SEM showed that even though biomineralization was deposited on both spun cast thin PI and filled thick PI substrates, the minerals were aggregated into large clusters on thin PI, and uniformly distributed on filled thick PI, where it was templated within banded collagen fibers. STATEMENT OF SIGNIFICANCE: This manuscript demonstrates the potential of polyisoprene (PI), an elastomeric polymer, for use in tissue engineering. We show how dental pulp stem cells adjust their moduli continuously to match infinitesimally small changes in substrate mechanics, till a critical threshold is reached when they will differentiate. The lineage of differentiation then becomes a sensitive function of both mechanics and morphology for a given chemical composition. Since PI is a major component of Gutta-percha, the FDA approved material commonly used for obturating the root canal, this work suggests that it can easily be adapted for in vivo use in dental regeneration.

A collagen based cryogel bioscaffold coated with nanostructured polydopamine as a platform for mesenchymal stem cell therapy

Cryo-hydrogels (cryogels) are polymer hydrogels formed at sub-zero temperatures. Bioscaffolds created from cryogels have interconnected macropores which allow for cell migration, tissue-ingrowth, unhindered diffusion of solutes and mass transport of therapeutics. In this study, we developed collagen based cryogel bioscaffolds and coated them with polydopamine using a simple two-step technique. Cryogel bioscaffolds were synthesized by collagen crosslinking at -20°C and exhibited a macroporous interconnected architecture with 75% ± 3% porosity. Two groups of pore sizes were observed: 300 ± 50 µm and 30 ± 10 µm in diameter. The addition of a polydopamine coating to cryogel bioscaffolds was confirmed using composition analysis. This resulted in a 41% ± 5% decrease in water uptake, 81% ± 10% decrease in swelling rate and 12% ± 3% decrease in their degree of dissolution (p < 0.05), with a 48% ± 2% increase in stiffness and 57% ± 5% increase in compressive strength (p < 0.05). Seeding adipose tissue-derived mesenchymal stem cells (AD-MSCs) into polydopamine coated-cryogel bioscaffolds resulted in cells demonstrating a 52% ± 4% increase in viability and 33% ± 3% increase in proliferation when compared to AD-MSCs seeded into uncoated-cryogel bioscaffolds (p < 0.05). In summary, our novel polydopamine coated-cryogel bioscaffold represents an efficient and low-cost bioscaffold platform to support MSC therapies. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 2213-2228, 2018.

Cells in the Non-Uniform Magnetic World: How Cells Respond to High-Gradient Magnetic Fields

Imagine cells that live in a high-gradient magnetic field (HGMF). Through what mechanisms do the cells sense a non-uniform magnetic field and how such a field changes the cell fate? We show that magnetic forces generated by HGMFs can be comparable to intracellular forces and therefore may be capable of altering the functionality of an individual cell and tissues in unprecedented ways. We identify the cellular effectors of such fields and propose novel routes in cell biology predicting new biological effects such as magnetic control of cell-to-cell communication and vesicle transport, magnetic control of intracellular ROS levels, magnetically induced differentiation of stem cells, magnetically assisted cell division, or prevention of cells from dividing. On the basis of experimental facts and theoretical modeling we reveal timescales of cellular responses to high-gradient magnetic fields and suggest an explicit dependence of the cell response time on the magnitude of the magnetic field gradient.

Receptor for Advanced Glycation End-Products Signaling Interferes with the Vascular Smooth Muscle Cell Contractile Phenotype and Function

Increased blood glucose concentrations promote reactions between glucose and proteins to form advanced glycation end-products (AGE). Circulating AGE in the blood plasma can activate the receptor for advanced end-products (RAGE), which is present on both endothelial and vascular smooth muscle cells (VSMC). RAGE exhibits a complex signaling that involves small G-proteins and mitogen activated protein kinases (MAPK), which lead to increased nuclear factor kappa B (NF-κB) activity. While RAGE signaling has been previously addressed in endothelial cells, little is known regarding its impact on the function of VSMC. Therefore, we hypothesized that RAGE signaling leads to alterations in the mechanical and functional properties of VSMC, which could contribute to complications associated with diabetes. We demonstrated that RAGE is expressed and functional in the A7r5 VSMC model, and its activation by AGE significantly increased NF-κB activity, which is known to interfere with the contractile phenotype of VSMC. The protein levels of the contraction-related transcription factor myocardin were also decreased by RAGE activation with a concomitant decrease in the mRNA and protein levels of transgelin (SM-22α), a regulator of VSMC contraction. Interestingly, we demonstrated that RAGE activation increased the overall cell rigidity, an effect that can be related to an increase in myosin activity. Finally, although RAGE stimulation amplified calcium signaling and slightly myosin activity in VSMC challenged with vasopressin, their contractile capacity was negatively affected. Overall, RAGE activation in VSMC could represent a keystone in the development of vascular diseases associated with diabetes by interfering with the contractile phenotype of VSMC through the modification of their mechanical and functional properties.

Modulation of monocytic leukemia cell function and survival by high gradient magnetic fields and mathematical modeling studies

The influence of spatially modulated high gradient magnetic fields on cellular functions of human THP-1 leukemia cells is studied. We demonstrate that arrays of high-gradient micrometer-sized magnets induce i) cell swelling, ii) prolonged increased ROS production, and iii) inhibit cell proliferation, and iv) elicit apoptosis of THP-1 monocytic leukemia cells in the absence of chemical or biological agents. Mathematical modeling indicates that mechanical stress exerted on the cells by high magnetic gradient forces is responsible for triggering cell swelling and formation of reactive oxygen species followed by apoptosis. We discuss physical aspects of controlling cell functions by focused magnetic gradient forces, i.e. by a noninvasive and nondestructive physical approach.

Mesoporous silica nanoparticle-based substrates for cell directed delivery of Notch signalling modulators to control myoblast differentiation

Biochemical cues are critical to control stem cell function and can be utilized to develop smart biomaterials for stem cell engineering. The challenge is to deliver these cues in a restricted manner with spatial and temporal control. Here we have developed bilayer films of mesoporous silica nanoparticles for delayed cellular delivery of Notch modulators to promote muscle stem cell differentiation. We demonstrate that drug-loaded particles are internalized from the particle-covered surface, which allows for direct delivery of the drug into the cell and a delayed and confined drug release. Substrates of particles loaded with γ-secretase-inhibitors, which block the Notch signalling pathway, promoted efficient differentiation of myoblasts. The particle substrates were fully biocompatible and did not interfere with the inherent differentiation process. We further demonstrate that impregnating commercially available, biocompatible polymer scaffolds with MSNs allows for a free standing substrate for cell directed drug delivery.

Life on magnets: stem cell networking on micro-magnet arrays

Interactions between a micro-magnet array and living cells may guide the establishment of cell networks due to the cellular response to a magnetic field. To manipulate mesenchymal stem cells free of magnetic nanoparticles by a high magnetic field gradient, we used high quality micro-patterned NdFeB films around which the stray field's value and direction drastically change across the cell body. Such micro-magnet arrays coated with parylene produce high magnetic field gradients that affect the cells in two main ways: i) causing cell migration and adherence to a covered magnetic surface and ii) elongating the cells in the directions parallel to the edges of the micro-magnet. To explain these effects, three putative mechanisms that incorporate both physical and biological factors influencing the cells are suggested. It is shown that the static high magnetic field gradient generated by the micro-magnet arrays are capable of assisting cell migration to those areas with the strongest magnetic field gradient, thereby allowing the build up of tunable interconnected stem cell networks, which is an elegant route for tissue engineering and regenerative medicine.