Temporal resolution of stimulation threshold: a tool for electrophysiologic analysis

A device for separated and reversible co-culture of cardiomyocytes

A novel technique is introduced for patterning and controllably merging two cultures of adherent cells on a microelectrode array (MEA) by separation with a removable physical barrier. The device was first demonstrated by separating two cardiomyocyte populations, which upon merging synchronized electrical activity. Next, two applications of this co-culture device are presented that demonstrate its flexibility as well as outline different metrics to analyze co-cultures. In a differential assay, the device contained two distinct cell cultures of neonatal wild-type and beta-adrenergic receptor (beta-AR) knockout cardiomyocytes and simultaneously exposed them with the beta-AR agonist isoproterenol. The beat rate and action potential amplitude from each cell type displayed different characteristic responses in both unmerged and merged states. This technique can be used to study the role of beta-receptor signaling and how the corresponding cellular response can be modulated by neighboring cells. In the second application, action potential propagation between modeled host and graft cell cultures was shown through the analysis of conduction velocity across the MEA. A co-culture of murine cardiomyocytes (host) and murine skeletal myoblasts (graft) demonstrated functional integration at the boundary, as shown by the progression of synchronous electrical activity propagating from the host into the graft cell populations. However, conduction velocity significantly decreased as the depolarization waves reached the graft region due to a mismatch of inherent cell properties that influence conduction.

Conduction analysis in mixed cardiomyocytes-fibroblasts cultures using microelectrode arrays

Models for cardiac arrhythmia currently exist primarily in in-vivo and computer simulation form. Towards the development of such a model in-vitro, a better understanding of electrical conduction in heterogeneous cultures is required. Increasing ratios of cardiomyocytes and fibroblasts were cultured on 500x500 microm arrays of 36 microelectrodes to study the emergence and properties of action potential propagation in mixed cultures. A minimum ratio of 70% cardiomyocytes to 30% fibroblasts was found to be necessary for detection of electrical activity. However, the establishment of a continuous, homogeneous depolarization wave across the culture required a higher proportion of cardiomyocytes; even a 90:10 ratio was unable to consistently produce a unidirectional, uniform depolarization wave as is seen in controls. This model underlines the importance and sensitivity of tissue homogeneity in supporting electrical conduction, and is especially relevant to studies of arrhythmia (reentry) and stem cell grafts.

Modeling conduction in host-graft interactions between stem cell grafts and cardiomyocytes

Cell therapy has recently made great strides towards aiding heart failure. However, while transplanted cells may electromechanically integrate into host tissue, there may not be a uniform propagation of a depolarization wave between the heterogeneous tissue boundaries. A model using microelectrode array technology that maps the electrical interactions between host and graft tissues in co-culture is presented and sheds light on the effects of having a mismatch of conduction properties at the boundary. Skeletal myoblasts co-cultured with cardiomyocytes demonstrated that conduction velocity significantly decreases at the boundary despite electromechanical coupling. In an attempt to improve the uniformity of conduction with host cells, differentiating human embryonic stem cells (hESC) were used in co-culture. Over the course of four to seven days, synchronous electrical activity was observed at the hESC boundary, implying differentiation and integration. Activity did not extend far past the boundary, and conduction velocity was significantly greater than that of the host tissue, implying the need for other external measures to properly match the conduction properties between host and graft tissue.

Mammalian cell-based sensor system

Use of living cells or cellular components in biosensors is receiving increased attention and opens a whole new area of functional diagnostics. The term "mammalian cell-based biosensor" is designated to biosensors utilizing mammalian cells as the biorecognition element. Cell-based assays, such as high-throughput screening (HTS) or cytotoxicity testing, have already emerged as dependable and promising approaches to measure the functionality or toxicity of a compound (in case of HTS); or to probe the presence of pathogenic or toxigenic entities in clinical, environmental, or food samples. External stimuli or changes in cellular microenvironment sometimes perturb the "normal" physiological activities of mammalian cells, thus allowing CBBs to screen, monitor, and measure the analyte-induced changes. The advantage of CBBs is that they can report the presence or absence of active components, such as live pathogens or active toxins. In some cases, mammalian cells or plasma membranes are used as electrical capacitors and cell-cell and cell-substrate contact is measured via conductivity or electrical impedance. In addition, cytopathogenicity or cytotoxicity induced by pathogens or toxins resulting in apoptosis or necrosis could be measured via optical devices using fluorescence or luminescence. This chapter focuses mainly on the type and applications of different mammalian cell-based sensor systems.

Cardiac differentiation of embryonic stem cells with point-source electrical stimulation

The use of pluripotent stem cells as a means to repair damaged heart tissue has recently emerged as a promising, yet controversial therapy. Despite the different approaches and the variety of cell types used, many of these procedures have been met with mixed success. The lack of understanding of the differentiation and integration process, notably with respect to electrical signaling, significantly hampers the development of these therapies. A system was thus developed allowing the use of point source electrical stimulation on embryonic stem (ES) cells to study the effect of physiologically-relevant electrical stimulus. When modulating the amplitude of the stimulus over various differentiation stages of embryonic stem cells, differences in the proportions of cardiomyocytes to embryonic stem cells were observed through quantitative PCR. The use of this technique might have larger applications in understanding molecular pathways towards the regeneration process.