Superior Induced Pluripotent Stem Cell Generation through Phactr3-Driven Mechanomodulation of Both Early and Late Phases of Cell Reprogramming.

Human cell reprogramming traditionally involves time-intensive, multistage, costly tissue culture polystyrene-based cell culture practices that ultimately produce low numbers of reprogrammed cells of variable quality. Previous studies have shown that very soft 2- and 3-dimensional hydrogel substrates/matrices (of stiffnesses ≤ 1 kPa) can drive ~2× improvements in human cell reprogramming outcomes. Unfortunately, these similarly complex multistage protocols lack intrinsic scalability, and, furthermore, the associated underlying molecular mechanisms remain to be fully elucidated, limiting the potential to further maximize reprogramming outcomes. In screening the largest range of polyacrylamide (pAAm) hydrogels of varying stiffness to date (1 kPa to 1.3 MPa), we have found that a medium stiffness gel (~100 kPa) increased the overall number of reprogrammed cells by up to 10-fold (10×), accelerated reprogramming kinetics, improved both early and late phases of reprogramming, and produced induced pluripotent stem cells (iPSCs) having more naïve characteristics and lower remnant transgene expression, compared to the gold standard tissue culture polystyrene practice. Functionalization of these pAAm hydrogels with poly-l-dopamine enabled, for the first-time, continuous, single-step reprogramming of fibroblasts to iPSCs on hydrogel substrates (noting that even the tissue culture polystyrene practice is a 2-stage process). Comparative RNA sequencing analyses coupled with experimental validation revealed that a novel reprogramming regulator, protein phosphatase and actin regulator 3, up-regulated under the gel condition at a very early time point, was responsible for the observed enhanced reprogramming outcomes. This study provides a novel culture protocol and substrate for continuous hydrogel-based cell reprogramming and previously unattained clarity of the underlying mechanisms via which substrate stiffness modulates reprogramming kinetics and iPSC quality outcomes.

Tailored integrin-extracellular matrix interactions to direct human mesenchymal stem cell differentiation.

Integrins provide the primary link between mesenchymal stem cells (MSCs) and their surrounding extracellular matrix (ECM), with different integrin pairs having specificity for different ECM molecules or peptide sequences contained within them. It is widely acknowledged that the type of ECM present can influence MSC differentiation; however, it is yet to be determined how specific integrin-ECM interactions may alter this or how they change during differentiation. We determined that human bone marrow-derived mesenchymal stem cells (hMSCs) express a broad range of integrins in their undifferentiated state and show a dramatic, but transient, increase in the level of α5 integrin on day 7 of osteogenesis and an increase in α6 integrin expression throughout adipogenesis. We used a nonfouling polystyrene-block-poly(ethylene oxide)-copolymer (PS-PEO) surface to present short peptides with defined integrin-binding capabilities (RGD, IKVAV, YIGSR, and RETTAWA) to hMSCs and investigate the effects of such specific integrin-ECM contacts on differentiation. hMSCs cultured on these peptides displayed different morphologies and had varying abilities to differentiate along the osteogenic and adipogenic lineages. The peptide sequences most conducive to differentiation (IKVAV for osteogenesis and RETTAWA and IKVAV for adipogenesis) were not necessarily those that were bound by those integrin subunits seen to increase during differentiation. Additionally, we also determined that presentation of RGD, which is bound by multiple integrins, was required to support long-term viability of hMSCs. Overall we confirm that integrin-ECM contacts change throughout hMSC differentiation and show that surfaces presenting defined peptide sequences can be used to target specific integrins and ultimately influence hMSC differentiation. This platform also provides information for the development of biomaterials capable of directing hMSC differentiation for use in tissue engineering therapies.

Development of myocardial constructs using modulus-matched acrylated polypropylene glycol triol substrate and different nonmyocyte cell populations.

Tissue engineering approaches are currently being investigated for the restoration of myocardial function in heart failure patients, most commonly by combining cells with a substrate to form myocardial-like constructs (MCs). The final properties of these constructs are dependant on the characteristics of both the substrate and the cells used for fabrication. To create a construct with the appropriate mechanical properties required for any future therapeutic, we tailored an acrylated polypropylene glycol triol (aPPGT) substrate to the elastic modulus of heart tissue and then investigated the fabrication of MCs. We first assessed the aPPGT substrate alone in vivo, both under normal conditions and in an infarct model in mice, and found that there was a mild foreign body response with good integration of the substrate into the epicardial surface in mice hearts. We next studied the fabrication and properties of MCs by culturing mouse embryonic cardiomyocytes on the aPPGT substrate. To achieve myocardial-like concentrically contractile constructs, cocultures with supportive stromal cells were found to be essential and both mouse heart-derived stromal cells or bone-derived mouse mesenchymal stromal progenitor cells (mMSCs) could be used. These different stromal cell types produced MCs with different properties. The average beating rate of the constructs formed from mouse heart-derived stromal cells was significantly higher those constructs formed using mMSCs. Conversely, the constructs formed using mMSCs had reduced fibrotic extracellular matrix secretion and increased hepatocyte growth factor expression. Both of these mMSC construct properties may enhance integration and therapeutic efficacy of the construct postimplantation on the surface of the infarcted heart. This study thus demonstrates the formation of MCs using mechanically tailored aPPGT substrate and also demonstrates the effects of different stromal cell populations have on the properties of the resultant MCs, both of which are critical for future applications of tissue engineering in heart failure patients.

Increasing electrospun scaffold pore size with tailored collectors for improved cell penetration.

This study investigates the use of patterned collectors to increase the pore size of electrospun scaffolds for enhanced cell infiltration. The morphology of the patterned scaffolds was investigated by scanning electron microscopy, which showed that the collector pattern was accurately mimicked by the electrospun fibres. We observed an enlargement in the pore size and in the pore size distribution compared with conventional electrospinning. Mechanical testing revealed that the mechanical properties could be tailored, to some extent, according to the patterning and that the patterned scaffolds were softer than standard electrospun scaffolds. When NIH 3T3 fibroblasts were seeded onto patterned collectors improved cell infiltration was observed. Cells were able to penetrate up to 250 µm into the scaffolds, compared with 30 µm for the standard scaffolds. This increase in the depth of infiltration occurred as early as 24 h post-seeding and remained constant over 7 days.

A cell migration device that maintains a defined surface with no cellular damage during wound edge generation.

Studying the rate of cell migration provides insight into fundamental cell biology as well as a tool to assess the functionality of synthetic surfaces and soluble environments used in tissue engineering. The traditional tools used to study cell migration include the fence and wound healing assays. In this paper we describe the development of a microchannel based device for the study of cell migration on defined surfaces. We demonstrate that this device provides a superior tool, relative to the previously mentioned assays, for assessing the propagation rate of cell wave fronts. The significant advantage provided by this technology is the ability to maintain a virgin surface prior to the commencement of the cell migration assay. Here, the device is used to assess rates of mouse fibroblasts (NIH 3T3) and human osteosarcoma (SaOS2) cell migration on surfaces functionalized with various extracellular matrix proteins as a demonstration that confining cell migration within a microchannel produces consistent and robust data. The device design enables rapid and simplistic assessment of multiple repeats on a single chip, where surfaces have not been previously exposed to cells or cellular secretions.