Measuring the contractile forces of human induced pluripotent stem cell-derived cardiomyocytes with arrays of microposts.

Human stem cell-derived cardiomyocytes hold promise for heart repair, disease modeling, drug screening, and for studies of developmental biology. All of these applications can be improved by assessing the contractility of cardiomyocytes at the single cell level. We have developed an in vitro platform for assessing the contractile performance of stem cell-derived cardiomyocytes that is compatible with other common endpoints such as microscopy and molecular biology. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) were seeded onto elastomeric micropost arrays in order to characterize the contractile force, velocity, and power produced by these cells. We assessed contractile function by tracking the deflection of microposts beneath an individual hiPSC-CM with optical microscopy. Immunofluorescent staining of these cells was employed to assess their spread area, nucleation, and sarcomeric structure on the microposts. Following seeding of hiPSC-CMs onto microposts coated with fibronectin, laminin, and collagen IV, we found that hiPSC-CMs on laminin coatings demonstrated higher attachment, spread area, and contractile velocity than those seeded on fibronectin or collagen IV coatings. Under optimized conditions, hiPSC-CMs spread to an area of approximately 420 µm2, generated systolic forces of approximately 15 nN/cell, showed contraction and relaxation rates of 1.74 µm/s and 1.46 µm/s, respectively, and had a peak contraction power of 29 fW. Thus, elastomeric micropost arrays can be used to study the contractile strength and kinetics of hiPSC-CMs. This system should facilitate studies of hiPSC-CM maturation, disease modeling, and drug screens as well as fundamental studies of human cardiac contraction.

Evidence that gene activation and silencing during stem cell differentiation requires a transcriptionally paused intermediate state.

A surprising portion of both mammalian and Drosophila genomes are transcriptionally paused, undergoing initiation without elongation. We tested the hypothesis that transcriptional pausing is an obligate transition state between definitive activation and silencing as human embryonic stem cells (hESCs) change state from pluripotency to mesoderm. Chromatin immunoprecipitation for trimethyl lysine 4 on histone H3 (ChIP-Chip) was used to analyze transcriptional initiation, and 3' transcript arrays were used to determine transcript elongation. Pluripotent and mesodermal cells had equivalent fractions of the genome in active and paused transcriptional states (∼48% each), with ∼4% definitively silenced (neither initiation nor elongation). Differentiation to mesoderm changed the transcriptional state of 12% of the genome, with roughly equal numbers of genes moving toward activation or silencing. Interestingly, almost all loci (98-99%) changing transcriptional state do so either by entering or exiting the paused state. A majority of these transitions involve either loss of initiation, as genes specifying alternate lineages are archived, or gain of initiation, in anticipation of future full-length expression. The addition of chromatin dynamics permitted much earlier predictions of final cell fate compared to sole use of conventional transcript arrays. These findings indicate that the paused state may be the major transition state for genes changing expression during differentiation, and implicate control of transcriptional elongation as a key checkpoint in lineage specification.

RGS5, RGS4, and RGS2 expression and aortic contractibility are dynamically co-regulated during aortic banding-induced hypertrophy.

Overexpression of regulator of G protein signaling 5 (RGS5) in arteries over veins is the most striking difference observed using microarray analysis. The obvious question is what arterial function might require RGS5. Based on functions of homologous proteins in regulating cardiac mass and G-protein-coupled receptor (GPCR) signaling, we proposed that RGS5 and vascular expressed RGS2 and RGS4 could participate in regulating arterial hypertrophy. We used the suprarenal abdominal aorta banding model to induce hypertension and hypertrophy. All 3 RGS messages were expressed in unmanipulated aorta with RGS5 predominating. After 2 days, thoracic aorta lost expression of RGS5, 4, and 2. At 1 week, all three returned to normal, and at 28 days, they increased many fold above normal. Valsartan blockade of angiotensin II (angII)/angII type 1 receptor signaling prevented upregulation of RGS messages but only delayed mass increases, implying wall mass regulation involves both angII-dependent and angII-independent pathways. The abdominal aorta showed less dramatic expression changes in RGS5 and 4, but not 2. Again, those changes were delayed by valsartan treatment with no mass changes. Thoracic aorta contraction to GPCR agonists was examined in aortic explant rings to identify vessel wall physiological changes. In 2-day aorta, the response to Galphaq/i agonists increased above normal, while 28-day aorta had attenuated induced contraction via Galphaq/i agonist, implicating a connection between RGS message levels and changes in GPCR-induced contraction. In vitro overexpression studies showed RGS5 inhibits angII-induced signaling in smooth muscle cells. This study is the first experimental evidence that changes in RGS expression and function correlate with vascular remodeling.

Selective control of endothelial cell proliferation with a synthetic dimerizer of FGF receptor-1.

Basic fibroblast growth factor (bFGF) is a potent angiogenic molecule, but its therapeutic use is limited by mitogenic effects on multiple cell types. To specifically activate FGF signaling in endothelial cells, a chimeric FGF receptor was generated that contained a modified FK506 drug-binding domain (F36V) fused to the FGF receptor-1 (FGFR1) cytoplasmic domain. Human umbilical vein endothelial cells (HUVECs) and human microvascular endothelial cells were retrovirally transduced with this chimeric receptor, and the effects of administering synthetic receptor-dimerizing ligands were studied. As expected, both control and transduced cells proliferated in response to bFGF treatment; however, only transduced endothelial cells exhibited dose-dependent proliferative responses to dimerizer treatment. Dimerizer-induced proliferation was MEK-dependent and was accompanied by MAP kinase phosphorylation, indicating that the chimeric receptor utilizes signaling pathways similar to endogenous FGFR1. Although bFGF stimulated wound re-epithelialization in HUVECs (which natively express FGFR1 and FGFR4), chemical dimerization of FGFR1 did not; this suggests FGFR4 may control migration in these cells. The ability to selectively activate receptor subtypes should facilitate the study of signaling pathways in vitro and in vivo beyond what can be accomplished with nonselective natural ligands, and it may eventually permit stimulation of graft cell angiogenesis without driving overgrowth of host cells.

Regeneration gaps: observations on stem cells and cardiac repair.

Substantial evidence indicates that cell transplantation can improve function of the infarcted heart. A surprisingly wide range of non-myogenic cell types improves ventricular function, suggesting that benefit may result in part from mechanisms that are distinct from true myocardial regeneration. While clinical trials explore cells derived from skeletal muscle and bone marrow, basic researchers are investigating sources of new cardiomyocytes, such as resident myocardial progenitors and embryonic stem cells. In this commentary, we briefly review the evolution of cell-based cardiac repair, discuss the current state of clinical research, and offer some thoughts on how newcomers can critically evaluate this emerging field.