Hypertrophy, gene expression, and beating of neonatal cardiac myocytes are affected by microdomain heterogeneity in 3D

Decellularized extracellular matrix bioinks and the external stimuli to enhance cardiac tissue development in vitro

Engineered heart tissue (EHT) has ample potential as a model for in vitro tissue modeling or tissue regeneration. Using 3D cell printing technology, various hydrogels have been utilized as bioinks to fabricate EHT to date. However, its efficacy has remained limited due to poor functional properties of the cultured cardiomyocytes stemming from a lack of proper microenvironmental cues. Specifically, the surrounding matrix plays a key role in modulating cardiomyocyte differentiation and maturation. Recently, the use of heart tissue-derived extracellular matrix (hdECM) bioink has come to be seen as one of the most promising candidates due to its functional and structural similarities to native tissue. Here, we demonstrated a correlation between the synthesis of cardiomyocyte-specific proteins and the surrounding microenvironment irrespective of the similar material chemistry. Primary cardiomyocytes isolated from neonatal rats were encapsulated in different composition and concentration of bioinks (hdECM and collagen). The bioinks were sequentially printed using an extrusion-based 3D bioprinter and cultured either statically or dynamically. Qualitative and quantitative evaluation revealed enhanced maturation of cardiomyocytes in hdECM, unlike the collagen group under similar culture conditions. Specifically, 3D-printed EHT using a low concentration of hdECM promoted early differentiation of cardiomyocytes. Hence, the present study provides experimental insights regarding the establishment of a 3D-printed cardiac tissue model, highlighting that the matrix and the culture microenvironment can be decisive factors for cell-material interactions that affect cardiomyocyte maturation. STATEMENT OF SIGNIFICANCE: The regulation of signal transduction and responses to extracellular matrices (ECMs) is of particular relevance in tissue maturation. In particular, there is a clear need to understand the structural and phenotypical modulation in cardiomyocytes with respect to the surrounding microenvironment. Exploration of the key regulators, such as the compositional and the biophysical properties of bioinks associated directly with cell-cell and cell-matrix interactions would assist with the fabrication of cardiac tissue constructs with enhanced functionality. Hence, we documented the synergistic effects of surrounding matrices and culture conditions on the maturation of cardiomyocytes. Additionally, we highlighted the potential of using 3D bioprinting techniques to fabricate uniformly aligned cardiac constructs for mid- to high-throughput drug testing platforms that have great reproducibility and versatility.

Hang on tight: reprogramming the cell with microstructural cues

Cells interact intimately with complex microdomains in their extracellular matrix (ECM) and maintain a delicate balance of mechanical forces through mechanosensitive cellular components. Tissue injury results in acute degradation of the ECM and disruption of cell-ECM contacts, manifesting in loss of cytoskeletal tension, leading to pathological cell transformation and the onset of disease. Recently, microscale hydrogel constructs have been developed to provide cells with microdomains to form focal adhesion binding sites, which enable restoration of cytoskeletal tension. These synthetic anchors can recapitulate the complex 3D architecture of the native ECM to provide microtopographical cues. The mechanical deformation of proteins at the cell surface can activate signaling cascades to modulate downstream gene-level transcription, making this a unique materials-based approach for reprogramming cell behavior. An overview of the mechanisms underlying these mechanosensitive interactions in fibroblasts, stem and other cell types is provided to review their effects on cellular reprogramming. Recent investigations on the fabrication, functionalization and implementation of these materials and microtopographical features for drug testing and therapeutic applications are discussed.

Localized delivery of mechano-growth factor E-domain peptide via polymeric microstructures improves cardiac function following myocardial infarction

The Insulin like growth factor-I isoform mechano-growth factor (MGF), is expressed in the heart following myocardial infarction and encodes a unique E-domain region. To examine E-domain function, we delivered a synthetic peptide corresponding to the unique E-domain region of the human MGF (IGF-1Ec) via peptide eluting polymeric microstructures to the heart. The microstructures were made of poly (ethylene glycol) dimethacrylate hydrogel and bioengineered to be the same size as an adult cardiac myocyte (100 × 15 × 15 μm) and with a stiffness of 20 kPa. Peptide eluting microrods and empty microrods were delivered via intramuscular injection following coronary artery ligation in mice. To examine the physiologic consequences, we assessed the impact of peptide delivery on cardiac function and cardiovascular hemodynamics using pressure-volume loops and gene expression by quantitative RT-PCR. A significant decline in both systolic and diastolic function accompanied by pathologic hypertrophy occurred by 2 weeks which decompensated further by 10 weeks post-infarct in the untreated groups. Delivery of the E-domain peptide eluting microrods decreased mortality, ameliorated the decline in hemodynamics, and delayed decompensation. This was associated with the inhibition of pathologic hypertrophy despite increasing vascular impedance. Delivery of the empty microrods had limited effects on hemodynamics and while pathologic hypertrophy persisted there was a decrease in ventricular stiffness. Our data show that cardiac restricted administration of the MGF E-domain peptide using polymeric microstructures may be used to prevent adverse remodeling of the heart and improve function following myocardial infarction.

Novel functionalization of discrete polymeric biomaterial microstructures for applications in imaging and three-dimensional manipulation

Adapting ways to functionalize polymer materials is becoming increasingly important to their implementation in translational biomedical sciences. By tuning the mechanical, chemical, and biological qualities of these materials, their applications can be broadened, opening the door for more advanced integration into modern medical techniques. Here, we report on a method to integrate chemical functionalizations into discrete, microscale polymer structures, which are used for tissue engineering applications, for in vivo localization, and three-dimensional manipulation. Iron oxide nanoparticles were incorporated into the polymer matrix using common photolithographic techniques to create stably functional microstructures with magnetic potential. Using magnetic resonance imaging (MRI), we can promote visualization of microstructures contained in small collections, as well as facilitate the manipulation and alignment of microtopographical cues in a realistic tissue environment. Using similar polymer functionalization techniques, fluorine-containing compounds were also embedded in the polymer matrix of photolithographically fabricated microstructures. The incorporation of fluorine-containing compounds enabled highly sensitive and specific detection of microstructures in physiologic settings using fluorine MRI techniques ((19)F MRI). These functionalization strategies will facilitate more reliable noninvasive tracking and characterization of microstructured polymer implants as well as have implications for remote microstructural scaffolding alignment for three-dimensional tissue engineering applications.

Cellular heterogeneity profiling by hyaluronan probes reveals an invasive but slow-growing breast tumor subset

Tumor heterogeneity confounds cancer diagnosis and the outcome of therapy, necessitating analysis of tumor cell subsets within the tumor mass. Elevated expression of hyaluronan (HA) and HA receptors, receptor for HA-mediated motility (RHAMM)/HA-mediated motility receptor and cluster designation 44 (CD44), in breast tumors correlates with poor outcome. We hypothesized that a probe for detecting HA-HA receptor interactions may reveal breast cancer (BCa) cell heterogeneity relevant to tumor progression. A fluorescent HA (F-HA) probe containing a mixture of polymer sizes typical of tumor microenvironments (10-480 kDa), multiplexed profiling, and flow cytometry were used to monitor HA binding to BCa cell lines of different molecular subtypes. Formulae were developed to quantify binding heterogeneity and to measure invasion in vivo. Two subsets exhibiting differential binding (HA(-/low) vs. HA(high)) were isolated and characterized for morphology, growth, and invasion in culture and as xenografts in vivo. F-HA-binding amounts and degree of heterogeneity varied with BCa subtype, were highest in the malignant basal-like cell lines, and decreased upon reversion to a nonmalignant phenotype. Binding amounts correlated with CD44 and RHAMM displayed but binding heterogeneity appeared to arise from a differential ability of HA receptor-positive subpopulations to interact with F-HA. HA(high) subpopulations exhibited significantly higher local invasion and lung micrometastases but, unexpectedly, lower proliferation than either unsorted parental cells or the HA(-/low) subpopulation. Querying F-HA binding to aggressive tumor cells reveals a previously undetected form of heterogeneity that predicts invasive/metastatic behavior and that may aid both early identification of cancer patients susceptible to metastasis, and detection/therapy of invasive BCa subpopulations.

Spatially resolved microrheology of heterogeneous biopolymer hydrogels using covalently bound microspheres

Characterization of the rheological properties of heterogeneous biopolymers is important not only to understand the effect of substrate elasticity on cell behaviors, but also to provide insights into mechanical changes during cellular remodeling of the environment. Conventional particle-tracking microrheology (PTM) techniques are compromised by probe-network slippage and cage-hopping problems, and require a priori knowledge of network mesh size in order to determine a suitable probe size. We demonstrated here the usefulness of covalently bound probes for PTM of biopolymers to overcome the above limitations. We showed that, in a well-defined system like polyacrylamide gels, surface-modified probe particles using a zero-length crosslinker provided more reliable measurements of network mechanics as compared to standard carboxylated probes. We further demonstrated that appropriate surface modification of microspheres for PTM circumvented the requirement of using microspheres larger than the network mesh, an approach typically considered to be ideal. Using the method presented in this study, we found the local network at the leading edge of a typical C6 glioma cell to be stiffer as compared to the side. Our findings established that permanent interaction between the probe and network is crucial to reliably measure the local network mechanics in reconstituted, heterogeneous networks using PTM.

Microdomain heterogeneity in 3D affects the mechanics of neonatal cardiac myocyte contraction

Cardiac muscle cells are known to adapt to their physical surroundings, optimizing intracellular organization and contractile function for a given culture environment. A previously developed in vitro model system has shown that the inclusion of discrete microscale domains (or microrods) in three dimensions (3D) can alter long-term growth responses of neonatal ventricular myocytes. The aim of this work was to understand how cellular contact with such a domain affects various mechanical changes involved in cardiac muscle cell remodeling. Myocytes were maintained in 3D gels over 5 days in the presence or absence of 100-μm-long microrods, and the effect of this local heterogeneity on cell behavior was analyzed via several imaging techniques. Microrod abutment resulted in approximately twofold increases in the maximum displacement of spontaneously beating myocytes, as based on confocal microscopy scans of the gel xy-plane or the myocyte long axis. In addition, microrods caused significant increases in the proportion of aligned myofibrils (≤20° deviation from long axis) in fixed myocytes. Microrod-related differences in axial contraction could be abrogated by long-term interruption of certain signals of the RhoA-/Rho-associated kinase (ROCK) or protein kinase C (PKC) pathway. Furthermore, microrod-induced increases in myocyte size and protein content were prevented by ROCK inhibition. In all, the data suggest that microdomain heterogeneity in 3D appears to promote the development of axially aligned contractile machinery in muscle cells, an observation that may have relevance to a number of cardiac tissue engineering interventions.

Microtopographical assembly of cardiomyocytes

One of the central challenges in cardiac tissue engineering is the control of the assembly and organization of functional cardiac tissue. Maintenance of a three-dimensional tissue architecture is key to myocardial function in vivo, and a variety of studies hint that provision of topological cues within scaffolds can facilitate the engineering of functional myocardial tissue by promoting this architecture. To explore this possibility in an isolated and well-defined fashion, we have designed scaffolds of polydimethylsiloxane (PDMS) with microtopographic pillars ("micropegs") to provide cells with defined structures with which to interact in three dimensions. We show that these surfaces permit HL-1 cardiomyocytes to grow, form myofibrillar structures and cell-cell adhesions, and beat spontaneously. Additionally, the cells and their nuclei interact with the full length of the micropegs, indicating that the micropegs promote a three-dimensional cytoarchitecture in the context of a two-dimensional scaffold. We also show that the number of cells interacting with a micropeg can be controlled by manipulating incubation time, micropeg spatial arrangement, or micropeg diameter. Western blots reveal that the expression of the junctional markers N-cadherin and connexin 43 is upregulated in the presence of specific arrangements of micropegs, suggesting that micropegs can enhance cardiomyocyte function. Together, these data show that microtopography can be used to provide three-dimensional adhesion and control the assembly of functional cardiac tissue on a two-dimensional surface.

Integrin α3 blockade enhances microtopographical down-regulation of α-smooth muscle actin: role of microtopography in ECM regulation

Development of functional engineered matrices for regenerative therapies can benefit from an understanding of how physical cues at the microscale affect cell behavior. In this work, we use microfabricated systems to study how stiffness and microscale topographical cues in the form of "micropegs" affect extracellular matrix synthesis. Previous work from our lab has shown that microtopographical cues in 2D and 3D systems decrease cellular proliferation and regulate matrix synthesis. In this work, the combined role of stiffness and topography on ECM synthesis is investigated in a 2D micropeg system. These studies show that fibroblasts cultured on polydimethylsiloxane (PDMS) substrates with micropegs have reduced expression of collagen type I (Col I) and collagen type VI (Col VI) compared to fibroblasts cultured on flat substrates. In addition, cells on micropegged substrates exhibit down-regulation of other important regulators of ECM synthesis such as α-smooth muscle actin (α-SMA), and integrin α3 (Int α3). Interestingly, this effect is dependent on the contractility and adhesion of the cells. When cultured in the presence of RhoA kinase (ROCK) and myosin light chain kinase (MLCK) inhibitors, no significant differences in the expression of collagen, α-SMA, Int α3, and TGFB1 are observed. Additionally, disruptions in cell adhesion prevent microtopographical regulation of ECM synthesis. When using an antibody to block the extracellular domain of Int α3, no differences in the expression of collagen are observed and blocking Int α3 results in enhanced down-regulation of α-SMA on the stiffer micropegged substrates. These findings demonstrate that regulation of extracellular matrix production by cells on a synthetic substrate can be guided via physical cues at the microscale, and add to the body of knowledge on the role of integrin-mediated mechanotransduction.

Micromechanical regulation in cardiac myocytes and fibroblasts: implications for tissue remodeling

Cells of the myocardium are at home in one of the most mechanically dynamic environments in the body. At the cellular level, pulsatile stimuli of chamber filling and emptying are experienced as cyclic strains (relative deformation) and stresses (force per unit area). The intrinsic characteristics of tension-generating myocytes and fibroblasts thus have a continuous mechanical interplay with their extrinsic surroundings. This review explores the ways that the micromechanics at the scale of single cardiac myocytes and fibroblasts have been measured, modeled, and recapitulated in vitro in the context of adaptation. Both types of cardiac cells respond to externally applied strain, and many of the intracellular mechanosensing pathways have been identified with the careful manipulation of experimental variables. In addition to strain, the extent of loading in myocytes and fibroblasts is also regulated by cues from the microenvironment such as substrate surface chemistry, stiffness, and topography. Combinations of these structural cues in three dimensions are needed to mimic the micromechanical complexity derived from the extracellular matrix of the developing, healthy, or pathophysiologic heart. An understanding of cardiac cell micromechanics can therefore inform the design and composition of tissue engineering scaffolds or stem cell niches for future applications in regenerative medicine.