Fibroblasts from human postmyocardial infarction scars acquire properties of cardiomyocytes after transduction with a recombinant myocardin gene

Identification of Functional Variant Enhancers Associated With Atrial Fibrillation

RATIONALE

Genome-wide association studies have identified a large number of common variants (single-nucleotide polymorphisms) associated with atrial fibrillation (AF). These variants are located mainly in noncoding regions of the genome and likely include variants that modulate the function of transcriptional regulatory elements (REs) such as enhancers. However, the actual REs modulated by variants and the target genes of such REs remain to be identified. Thus, the biological mechanisms by which genetic variation promotes AF has thus far remained largely unexplored.

OBJECTIVE

To identify REs in genome-wide association study loci that are influenced by AF-associated variants.

METHODS AND RESULTS

We screened 2.45 Mbp of human genomic DNA containing 12 strongly AF-associated loci for RE activity using self-transcribing active regulatory region sequencing and a recently generated monoclonal line of conditionally immortalized rat atrial myocytes. We identified 444 potential REs, 55 of which contain AF-associated variants (P<10^-8). Subsequently, using an adaptation of the self-transcribing active regulatory region sequencing approach, we identified 24 variant REs with allele-specific regulatory activity. By mining available chromatin conformation data, the possible target genes of these REs were mapped. To define the physiological function and target genes of such REs, we deleted the orthologue of an RE containing noncoding variants in the Hcn4 (potassium/sodium hyperpolarization-activated cyclic nucleotide-gated channel 4) locus of the mouse genome. Mice heterozygous for the RE deletion showed bradycardia, sinus node dysfunction, and selective loss of Hcn4 expression.

CONCLUSIONS

We have identified REs at multiple genetic loci for AF and found that loss of an RE at the HCN4 locus results in sinus node dysfunction and reduced gene expression. Our approach can be broadly applied to facilitate the identification of human disease-relevant REs and target genes at cardiovascular genome-wide association studies loci.

Pro-myogenic and low-oxygen culture increases expression of contractile smooth muscle markers in human fibroblasts

Smooth muscle cells (SMCs) are essential for tissue engineering strategies to fabricate organs such as blood vessels, the oesophagus and bladder, and to create disease models of these systems. In order for such therapies and models to be feasible, SMCs must be sourced effectively to enable production of large numbers of functional cells. In vitro, SMCs divide slowly and demonstrate short proliferative lifespans compared with other types of cells, including stem cells and fibroblasts, limiting the number of cells that can be derived from expansion in culture of a primary isolation. As such, it would be beneficial to better understand the factors underlying induction and maintenance of SMC phenotypes, in order to produce new sources of SMCs for tissue engineering and disease modelling. Here we report the ability of human dermal fibroblasts to display patterns of gene expression resembling contractile SMCs when cultured under conditions that are known to promote a contractile phenotype in SMCs, including culture on collagen IV, low-serum culture, TGF-β1 treatment and hypoxia. These factors drive expression of the myogenic transcription factor myocardin, as well as expression of several of its gene targets that are known contributors to contractile phenotype in SMCs, including smooth muscle alpha actin, calponin, and myosin heavy chain. Our results suggest that culture conditions associated with culture of SMCs may be sufficient to induce myogenic gene expression patterns and potential myogenic function in non-muscle cells.

Effects of transcytolemmal water exchange on the assessment of myocardial extracellular volume with cardiovascular MRI

Quantitative analysis of the myocardial interstitial space is gaining increased interest as a biomarker in the MRI and clinical cardiovascular communities. To investigate the effect of water exchange on the calculation of myocardial extracellular volume (ECV), we employed two tissue models: the standard ECV two-point model (SM) and the shutter speed model (SSM). Twenty individuals (18 men and two women; age 61.9 ± 10.3 years) underwent MRI at 1.5 T with pre-contrast and post-contrast dynamic T1 quantification. Means, standard deviations and ranges for SM and SSM model parameters were calculated. Infarct and viable myocardial model parameters as well as apparent ECV values calculated with the SM and SSM were statistically compared. Viable ECV(SM) remained temporally constant (27.3-28.0%: P = 0.5) and infarcted myocardial ECV(SM) changed significantly (49.3-58.8%; P < 0.001), reaching a steady-state value after 15 min. The intracellular lifetime of water was three times greater in infarcted myocardium when compared with viable myocardium (τi: 66.6 ± 115 versus 208.7 ± 72.7 ms) and accompanied a twofold increase in ECV (ECV(SSM) : 30.3 ± 11.1 versus 71.0 ± 13.1%; P < 0.001). There was a consistent significant difference in ECV values of infarcted myocardium at different timepoints between the SM and SSM, but not viable myocardium, presumably due to slower water exchange. In summary, we found a significant change in apparent ECV and water exchange in infarcted myocardium when compared with viable myocardium. This was visualized by changes in dynamic contrast enhanced curve shapes and quantified using the SSM as not only an increase in apparent ECV but also a decrease in water exchange.

A Novel Selectable Islet 1 Positive Progenitor Cell Reprogrammed to Expandable and Functional Smooth Muscle Cells

Disorders affecting smooth muscle structure/function may require technologies that can generate large scale, differentiated and contractile smooth muscle cells (SMC) suitable for cell therapy. To date no clonal precursor population that provides large numbers of differentiated SMC in culture has been identified in a rodent. Identification of such cells may also enhance insight into progenitor cell fate decisions and the relationship between smooth muscle precursors and disease states that implicate differentiated SMC.  In this study, we used classic clonal expansion techniques to identify novel self-renewing Islet 1 (Isl-1) positive primitive progenitor cells (PPC) within rat bone marrow that exhibited canonical stem cell markers and preferential differentiation towards a smooth muscle-like fate. We subsequently used molecular tagging to select Isl-1 positive clonal populations from expanded and de novo marrow cell populations. We refer to these previously undescribed cells as the PPC given its stem cell marker profile, and robust self-renewal capacity. PPC could be directly converted into induced smooth muscle cells (iSMC) using single transcription factor (Kruppel-like factor 4) knockdown or transactivator (myocardin) overexpression in contrast to three control cells (HEK 293, endothelial cells and mesenchymal stem cells) where such induction was not possible. iSMC exhibited immuno- and cytoskeletal-phenotype, calcium signaling profile and contractile responses similar to bona fide SMC. Passaged iSMC could be expanded to a scale sufficient for large scale tissue replacement.  PPC and reprogramed iSMC so derived may offer future opportunities to investigate molecular, structure/function and cell-based replacement therapy approaches to diverse cardiovascular, respiratory, gastrointestinal, and genitourinary diseases that have as their basis smooth muscle cell functional aberrancy or numerical loss. Stem Cells 2016;34:1354-1368.

Forced fusion of human ventricular scar cells with cardiomyocytes suppresses arrhythmogenicity in a co-culture model

AIMS

Fibrosis increases arrhythmogenicity in myocardial tissue by causing structural and functional disruptions in the cardiac syncytium. Forced fusion of fibroblastic cells with adjacent cardiomyocytes may theoretically resolve these disruptions. Therefore, the electrophysiological effects of such electrical and structural integration of fibroblastic cells into a cardiac syncytium were studied.

METHODS AND RESULTS

Human ventricular scar cells (hVSCs) were transduced with lentiviral vectors encoding enhanced green fluorescent protein alone (eGFP↑-hVSCs) or together with the fusogenic vesicular stomatitis virus G protein (VSV-G/eGFP↑-hVSCs) and subsequently co-cultured (1:4 ratio) with neonatal rat ventricular cardiomyocytes (NRVMs) in confluent monolayers yielding eGFP↑- and VSV-G/eGFP↑-co-cultures, respectively. Cellular fusion was induced by brief exposure to pH = 6.0 medium. Optical mapping experiments showed eGFP↑-co-cultures to be highly arrhythmogenic [43.3% early afterdepolarization (EAD) incidence vs. 7.7% in control NRVM cultures, P < 0.0001], with heterogeneous prolongation of action potential (AP) duration (APD). Fused VSV-G/eGFP↑-co-cultures displayed markedly lower EAD incidence (4.6%, P < 0.001) than unfused co-cultures, associated with decreases in APD, APD dispersion, and decay time of cytosolic Ca(2+) waves. Heterokaryons strongly expressed connexin43 (Cx43). Also, maximum diastolic potential in co-cultures was more negative after fusion, while heterokaryons exhibited diverse mixed NRVM/hVSC whole-cell current profiles, but consistently showed increased outward Kv currents compared with NRVMs or hVSCs. Inhibition of Kv channels by tetraethylammonium chloride abrogated the anti-arrhythmic effects of fusion in VSV-G/eGFP↑-co-cultures raising EAD incidence from 7.9 to 34.2% (P < 0.001).

CONCLUSION

Forced fusion of cultured hVSCs with NRVMs yields electrically functional heterokaryons and reduces arrhythmogenicity by preventing EADs, which is, at least partly, attributable to increased repolarization force.

The alarmin functions of high-mobility group box-1 and IL-33 in the pathogenesis of systemic lupus erythematosus

'Alarmins' are a group of endogenous proteins or molecules that are released from cells during cellular demise to alert the host innate immune system. Two of them, high-mobility group box-1 (HMGB1) and IL-33 shared many similarities of cellular localization, functions and involvement in various inflammatory diseases including systemic lupus erythematosus (SLE). The expressions of HMGB1 and IL-33, and their corresponding receptors RAGE (receptor for advanced glycation end products) and ST2, respectively, are substantially upregulated in patients with lupus nephritis (LN). This review highlights the emerging roles of alarmin proteins in various pathologies of LN, by focusing on classical HMGB1 and a newly discovered alarmin IL-33.

Overexpression of myocardin induces partial transdifferentiation of human-induced pluripotent stem cell-derived mesenchymal stem cells into cardiomyocytes

Mesenchymal stem cells (MSCs) derived from human‐induced pluripotent stem cells (iPSCs) show superior proliferative capacity and therapeutic potential than those derived from bone marrow (BM). Ectopic expression of myocardin further improved the therapeutic potential of BM‐MSCs in a mouse model of myocardial infarction. The aim was of this study was to assess whether forced myocardin expression in iPSC‐MSCs could further enhance their transdifferentiation to cardiomyocytes and improve their electrophysiological properties for cardiac regeneration. Myocardin was overexpressed in iPSC‐MSCs using viral vectors (adenovirus or lentivirus). The expression of smooth muscle cell and cardiomyocyte markers, and ion channel genes was examined by reverse transcription‐polymerase chain reaction (RT‐PCR), immunofluorescence staining and patch clamp. The conduction velocity of the neonatal rat ventricular cardiomyocytes cocultured with iPSC‐MSC monolayer was measured by multielectrode arrays recording plate. Myocardin induced the expression of α‐MHC, GATA4, α‐actinin, cardiac MHC, MYH11, calponin, and SM α‐actin, but not cTnT, β‐MHC, and MLC2v in iPSC‐MSCs. Overexpression of myocardin in iPSC‐MSC enhanced the expression of SCN9A and CACNA1C, but reduced that of KCa3.1 and Kir2.2 in iPSC‐MSCs. Moreover, BK_Ca, I_Kir, I_Cl, I_to and I_Na.TTX were detected in iPSC‐MSC with myocardin overexpression; while only BK_Ca, I_Kir, I_Cl, IK_DR, and IK_Ca were noted in iPSC‐MSC transfected with green florescence protein. Furthermore, the conduction velocity of iPSC‐MSC was significantly increased after myocardin overexpression. Overexpression of myocardin in iPSC‐MSCs resulted in partial transdifferentiation into cardiomyocytes phenotype and improved the electrical conduction during integration with mature cardiomyocytes.

Forced myocardin expression in human‐induced pluripotent stem cell (hiPSC)‐derived mesenchymal stem cells lead to partial transdifferentiation into cardiomyocytes and smooth muscle cells phenotypes through modification in ion channel expression profile and electrical conduction velocity.

Reprogramming and carcinogenesis--parallels and distinctions

Rapid progress made in various areas of regenerative medicine in recent years occurred both at the cellular level, with the Nobel prize-winning discovery of reprogramming (generation of induced pluripotent stem (iPS) cells) and also at the biomaterial level. The use of four transcription factors, Oct3/4, Sox2, c-Myc, and Klf4 (called commonly "Yamanaka factors") for the conversion of differentiated cells, back to the pluripotent/embryonic stage, has opened virtually endless and ethically acceptable source of stem cells for medical use. Various types of stem cells are becoming increasingly popular as starting components for the development of replacement tissues, or artificial organs. Interestingly, many of the transcription factors, key to the maintenance of stemness phenotype in various cells, are also overexpressed in cancer (stem) cells, and some of them may find the use as prognostic factors. In this review, we describe various methods of iPS creation, followed by overview of factors known to interfere with the efficiency of reprogramming. Next, we discuss similarities between cancer stem cells and various stem cell types. Final paragraphs are dedicated to interaction of biomaterials with tissues, various adverse reactions generated as a result of such interactions, and measures available, that allow for mitigation of such negative effects.

Brief report: Misinterpretation of coculture differentiation experiments by unintended labeling of cardiomyocytes through secondary transduction: delusions and solutions

Cardiomyogenic differentiation of stem cells can be accomplished by coculture with cardiomyocytes (CMCs). To facilitate their identification, stem cells are often labeled through viral transduction with a fluorescent protein. A second marker to distinguish stem cell-derived CMCs from native CMCs is rarely used. This study aimed to investigate the occurrence of secondary transduction of unlabeled neonatal rat (nr) CMCs after coculture with human cells that had been transduced 0, 7, or 14 days earlier with a vesicular stomatitis virus (VSV) G protein-pseudotyped lentiviral vector (LV) encoding enhanced green fluorescent protein (GFP). To reduce secondary LV transfer, GFP-labeled cells were incubated with non-heat-inactivated human serum (NHI) or with VSV-neutralizing rabbit serum (αVSV). Heat-inactivated human serum and normal rabbit serum were used as controls. Immunostaining showed substantial GFP gene transfer to nrCMCs in cocultures started at the day of transduction indicated by the presence of GFP-positive/human lamin A/C-negative nrCMCs. The extent of secondary transduction was significantly reduced in cocultures initiated 7 days after GFP transduction, while it was completely abolished when human cells were added to nrCMCs 14 days post-transduction. Both NHI and αVSV significantly reduced the occurrence of secondary transduction compared to their controls. However, under all circumstances, GFP-labeled human cells had to be passaged for 14 days prior to coculture initiation to prevent any horizontal GFP gene transfer to the nrCMCs. This study emphasizes that differentiation experiments involving the use of viral vector-marked donor cells should be interpreted with caution and describes measures to reduce/prevent secondary transduction.

Identification, selection, and enrichment of cardiomyocyte precursors

The large-scale production of cardiomyocytes is a key step in the development of cell therapy and tissue engineering to treat cardiovascular diseases, particularly those caused by ischemia. The main objective of this study was to establish a procedure for the efficient production of cardiomyocytes by reprogramming mesenchymal stem cells from adipose tissue. First, lentiviral vectors expressing neoR and GFP under the control of promoters expressed specifically during cardiomyogenesis were constructed to monitor cell reprogramming into precardiomyocytes and to select cells for amplification and characterization. Cellular reprogramming was performed using 5'-azacytidine followed by electroporation with plasmid pOKS2a, which expressed Oct4, Sox2, and Klf4. Under these conditions, GFP expression began only after transfection with pOKS2a, and less than 0.015% of cells were GFP(+). These GFP(+) cells were selected for G418 resistance to find molecular markers of cardiomyocytes by RT-PCR and immunocytochemistry. Both genetic and protein markers of cardiomyocytes were present in the selected cells, with some variations among them. Cell doubling time did not change after selection. Together, these results indicate that enrichment with vectors expressing GFP and neoR under cardiomyocyte-specific promoters can produce large numbers of cardiomyocyte precursors (CMPs), which can then be differentiated terminally for cell therapy and tissue engineering.

In search of novel targets for heart disease: myocardin and myocardin-related transcriptional cofactors

Growing evidence suggests that gene-regulatory networks, which are responsible for directing cardiovascular development, are altered under stress conditions in the adult heart. The cardiac gene regulatory network is controlled by cardioenriched transcription factors and multiple-cell-signaling inputs. Transcriptional coactivators also participate in gene-regulatory circuits as the primary targets of both physiological and pathological signals. Here, we focus on the recently discovered myocardin-(MYOCD) related family of transcriptional cofactors (MRTF-A and MRTF-B) which associate with the serum response transcription factor and activate the expression of a variety of target genes involved in cardiac growth and adaptation to stress via overlapping but distinct mechanisms. We discuss the involvement of MYOCD, MRTF-A, and MRTF-B in the development of cardiac dysfunction and to what extent modulation of the expression of these factors in vivo can correlate with cardiac disease outcomes. A close examination of the findings identifies the MYOCD-related transcriptional cofactors as putative therapeutic targets to improve cardiac function in heart failure conditions through distinct context-dependent mechanisms. Nevertheless, we are in support of further research to better understand the precise role of individual MYOCD-related factors in cardiac function and disease, before any therapeutic intervention is to be entertained in preclinical trials.

Use of atorvastatin to inhibit hypoxia-induced myocardin expression

BACKGROUND

Hypoxia induces the formation of reactive oxygen species (ROS), myocardin expression and cardiomyocyte hypertrophy. The 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins) have been demonstrated to have both antioxidant and antihypertrophic effects. We evaluated the pathways of atorvastatin in repressing ROS and myocardin after hypoxia to prevent cardiomyocyte hypertrophy.

MATERIALS AND METHODS

Cultured rat neonatal cardiomyocytes were subjected to hypoxia, and the expression of myocardin and ROS were evaluated. Different signal transduction inhibitors, atorvastatin and N-acetylcysteine (NAC) were used to identify the pathways that inhibited myocardin expression and ROS. Electrophoretic motility shift assay (EMSA) and luciferase assay were used to identify the binding of myocardin/serum response factor (SRF) and transcription to cardiomyocytes. Cardiomyocyte hypertrophy was assessed by (3)H-proline incorporation assay.

RESULTS

Myocardin expression after hypoxia was inhibited by atorvastatin, RhoA/Rho kinase inhibitor (Y27632), extracellular signal-regulated kinase (ERK) small interfering RNA (siRNA)/ERK pathway inhibitor (PD98059), myocardin siRNA and NAC. Bindings of myocardin/SRF, transcription of myocardin/SRF to cardiomyocytes, presence of myocardin in the nuclei of cardiomyocytes and protein synthesis after hypoxia were identified by EMSA, luciferase assay, confocal microscopy and (3)H-proline assay and were suppressed by atorvastatin, Y27632, PD98059 and NAC.

CONCLUSIONS

Hypoxia in neonatal cardiomyocytes increases myocardin expression and ROS to cause cardiomyocyte hypertrophy, which can be prevented by atorvastatin by suppressing ROS and myocardin expression.

Adenovirus-mediated expression of hypoxia-inducible factor 1α double mutant converts neonatal cardiac fibroblasts into (cardio)myocyte phenotype

Adenovirus-mediated expression of hypoxia-inducible factor 1α double mutant (pAd-HIF-1α-Ala564-Ala803) can be effectively transfected into bone marrow stem cells (MSCs) in the MSCs and cardiomyocytes co-culture system at normoxia to regulate the expression of downstream target genes of hypoxia-inducible factor 1α (HIF-1α), which in turn can promote MSC differentiation into cardiomyocytes. Fibroblasts share common characteristics with MSCs such as the morphology, phenotype and differentiation potential. Therefore, we further studied whether the pAd-HIF-1α-Ala564-Ala803 also can convert neonatal rat cardiac fibroblasts (NCFs) into (cardio)myocyte phenotype via regulating the downstream target genes of HIF-1α at normoxia. The immunostaining analysis showed that NCFs treated with pAd-HIF-1α-Ala564-Ala803 exhibited higher protein expression levels of smooth muscle α-actin (SMA, myocyte marker) and cardiac troponin T (cTnT, cardiomyocyte marker), compared with phosphate-buffered saline and pAd-LacZ treatments. The reverse transcription-polymerase chain reaction results showed that NCFs transfected with pAd-HIF-1α-Ala564-Ala803 augmented messenger RNA (mRNA) expression of transforming growth factor-β1 (TGF-β1), Smad4, NKx2.5, GATA4, myocardin, SMA and cTnT. The effects of HIF-1α-Ala564-Ala803 on NCFs were attenuated by pre-transfection of TGF-β1 or myocardin small interference RNAs. Adult CFs transfected with pAd-HIF-1α-Ala564-Ala803 showed a lower protein expression of SMA but not cTnT without any change in the mRNA expression level of NKx2.5, myocardin. Therefore, NCFs but not adult CFs possess a similar differentiation potential to MSCs as evidenced by the fact that pAd-HIF-1α-Ala564-Ala803 can convert NCFs into (cardio)myocyte phenotype via regulating its downstream target genes.

Transcription factor rational design improves directed differentiation of human mesenchymal stem cells into skeletal myocytes

There is great interest in transdifferentiating cells from one lineage into those of another and in dedifferentiating mature cells back into a stem/progenitor cell state by deploying naturally occurring transcription factors (TFs). Often, however, steering cellular differentiation pathways in a predictable and efficient manner remains challenging. Here, we investigated the principle of combining domains from different lineage-specific TFs to improve directed cellular differentiation. As proof-of-concept, we engineered the whole-human TF MyoDCD, which has the NH(2)-terminal transcription activation domain (TAD) and adjacent DNA-binding motif of MyoD COOH-terminally fused to the TAD of myocardin (MyoCD). We found via reporter gene and marker protein assays as well as by a cell fusion readout system that, targeting the TAD of MyoCD to genes normally responsive to the skeletal muscle-specific TF MyoD enforces more robust myogenic reprogramming of nonmuscle cells than that achieved by the parental, prototypic master TF, MyoD. Human mesenchymal stem cells (hMSCs) transduced with a codon-optimized microdystrophin gene linked to a synthetic striated muscle-specific promoter and/or with MyoD or MyoDCD were evaluated for complementing the genetic defect in Duchenne muscular dystrophy (DMD) myocytes through heterotypic cell fusion. Cotransduction of hMSCs with MyoDCD and microdystrophin led to chimeric myotubes containing the highest dystrophin levels.

Exploitation of herpesvirus immune evasion strategies to modify the immunogenicity of human mesenchymal stem cell transplants

BACKGROUND

Mesenchymal stem cells (MSCs) are multipotent cells residing in the connective tissue of many organs and holding great potential for tissue repair. In culture, human MSCs (hMSCs) are capable of extensive proliferation without showing chromosomal aberrations. Large numbers of hMSCs can thus be acquired from small samples of easily obtainable tissues like fat and bone marrow. MSCs can contribute to regeneration indirectly by secretion of cytokines or directly by differentiation into specialized cell types. The latter mechanism requires their long-term acceptance by the recipient. Although MSCs do not elicit immune responses in vitro, animal studies have revealed that allogeneic and xenogeneic MSCs are rejected.

METHODOLOGY/PRINCIPAL FINDINGS

We aim to overcome MSC immune rejection through permanent down-regulation of major histocompatibility complex (MHC) class I proteins on the surface of these MHC class II-negative cells through the use of viral immune evasion proteins. Transduction of hMSCs with a retroviral vector encoding the human cytomegalovirus US11 protein resulted in strong inhibition of MHC class I surface expression. When transplanted into immunocompetent mice, persistence of the US11-expressing and HLA-ABC-negative hMSCs at levels resembling those found in immunodeficient (i.e., NOD/SCID) mice could be attained provided that recipients' natural killer (NK) cells were depleted prior to cell transplantation.

CONCLUSIONS/SIGNIFICANCE

Our findings demonstrate the potential utility of herpesviral immunoevasins to prevent rejection of xenogeneic MSCs. The observation that down-regulation of MHC class I surface expression renders hMSCs vulnerable to NK cell recognition and cytolysis implies that multiple viral immune evasion proteins are likely required to make hMSCs non-immunogenic and thereby universally transplantable.

Integrin stimulation favors uptake of macromolecules by cardiomyocytes in vitro

Previously, our research group showed that integrin stimulation induces release of cardiac troponin I from viable neonatal rat ventricular cardiomyocytes (NRCMs), but would it also stimulate uptake of exogenous macromolecules? For this purpose, beating NRCMs were incubated without or with an RGD motif-containing peptide (GRGDS) to stimulate integrins in the presence of Texas Red-conjugated ovalbumin (OTR; 45 kDa) or dextran (DTR; 70 kDa). After incubation periods of 8, 16 and 24 hours endocytosis of red label was quantified by fluorescence microscopy. Uptake of OTR and DTR by NRCMs was intensified by GRGDS treatment (p for trend <0.001 and 0.019, respectively) and increased with duration of incubation (p<0.001 for both). The GRGDS-induced uptake of OTR by NRCMs correlated positively with OTR concentration (p<0.001). Experiments with pharmacological inhibitors of endocytosis indicated that in the absence of GRGDS, NRCMs take up OTR by the clathrin-mediated pathway of endocytosis while the GRGDS-dependent OTR uptake occurs by macropinocytosis. Cultures of NRCMs that were stretched cyclically showed ≍4-fold increased uptake of OTR compared to stationary NRCM cultures. Immunofluorescence microscopy revealed that the dysferlin-positive plasma membrane (PM) areas in beating GRGDS-treated NRCMs were ≍3-fold larger than in contracting NRCMs incubated with vehicle (p<0.001). However, in non-beating NRCMs exposure to GRGDS did not induce larger dysferlin-positive PM areas, nor did it stimulate uptake of OTR. After inhibition of dysferlin expression by short hairpin RNA-mediated RNA interference, OTR uptake by contracting NRCMs could no longer be stimulated via GRGDS treatment. We conclude that in NRCMs, stimulation of integrins by RGD motif-containing peptides or stretch cause uptake of labeled macromolecules. The latter process appears to depend on the contractile behavior of the NRCMs and on the PM repair protein dysferlin, probably because of its role in macropinocytosis.

Long-term contribution of human bone marrow mesenchymal stromal cells to skeletal muscle regeneration in mice

Mesenchymal stromal cells (MSCs) are attractive for cellular therapy of muscular dystrophies as they are easy to procure, can be greatly expanded ex vivo, and contribute to skeletal muscle repair in vivo. However, detailed information about the contribution of bone marrow (BM)-derived human MSCs (BM-hMSCs) to skeletal muscle regeneration in vivo is very limited. Here, we present the results of a comprehensive study of the fate of LacZ-tagged BM-hMSCs following implantation in cardiotoxin (CTX)-injured tibialis anterior muscles (TAMs) of immunodeficient mice. β-Galactosidase-positive (β-gal(+)) human-mouse hybrid myofibers (HMs) were counted in serial cross sections over the full length of the treated TAMs of groups of mice at monthly intervals. The number of human cells was estimated using chemiluminescence assays. While the number of human cells declined gradually to about 10% of the injected cells at 60 days after transplantation, the number of HMs increased from day 10 onwards, reaching 104 ± 39.1 per TAM at 4 months postinjection. β-gal(+) cells and HMs were distributed over the entire muscle, indicating migration of the former from the central injection site to the ends of the TAMs. The identification of HMs that stained positive for human spectrin suggests myogenic reprogramming of hMSC nuclei. In summary, our findings reveal that BM-hMSCs continue to participate in the regeneration/remodeling of CTX-injured TAMs, resulting in ±5% HMs at 4 months after damage induction. Moreover, donor-derived cells were shown to express genetic information, both endogenous and transgenic, in recipient myofibers.

Interleukin-1beta regulates cell proliferation and activity of extracellular matrix remodelling enzymes in cultured primary pig heart cells

After myocardial infarction, elevated levels of interleukins (ILs) are found within the myocardial tissue and IL-1beta is considered to play a major role in tissue remodelling events throughout the body. In the study presented, we have established a cell culture model of primary pig heart cells to evaluate the effects of different concentrations of IL-1beta on cell proliferation as well as expression and activity of enzymes typically involved in tissue remodelling. Primary pig heart cell cultures were derived from three different animals and stimulated with recombinant pig IL-1beta. RNA expression was detected by RT-PCR, protein levels were evaluated by Western blotting, activity of matrix metalloproteinases (MMPs) was quantified by gelatine zymography and cell proliferation was measured using colorimetric MTS assays. Pig heart cells express receptors for IL-1 and application of IL-1beta resulted in a dose-dependent increase of cell proliferation (P<0.05 vs. control; 100ng/ml; 24h). Gene expression of caspase-3 was increased by IL-1beta (P<0.05 vs. control; 100ng/ml; 3h), and pro-caspase-3 but not active caspase was detected in lysates of pig heart cells by Western blotting. MMP-2 gene expression as well as enzymatic activities of MMP-2 and MMP-9 were increased by IL-1beta (P<0.05 vs. control; 100ng/ml; 3h for gene expression, 48 and 72h for enzymatic activities of MMP-2 and MMP-9, respectively). Our in vitro data suggest that IL-1beta plays a major role in the events of tissue remodelling in the heart. Combined with our recently published in vivo data (Meybohm et al., PLoS One, 2009), the results presented here strongly suggest IL-1beta as a key molecule guiding tissue remodelling events after myocardial infarction.

Angiotensin II and the ERK pathway mediate the induction of myocardin by hypoxia in cultured rat neonatal cardiomyocytes

Hypoxic injury to cardiomyocytes is a stress that causes cardiac pathology through cardiac-restricted gene expression. SRF (serum-response factor) and myocardin are important for cardiomyocyte growth and differentiation in response to myocardial injuries. Previous studies have indicated that AngII (angiotensin II) stimulates both myocardin expression and cardiomyocyte hypertrophy. In the present study, we evaluated the expression of myocardin and AngII after hypoxia in regulating gene transcription in neonatal cardiomyocytes. Cultured rat neonatal cardiomyocytes were subjected to hypoxia, and the expression of myocardin and AngII were evaluated. Different signal transduction pathway inhibitors were used to identify the pathway(s) responsible for myocardin expression. An EMSA (electrophoretic mobility-shift assay) was used to identify myocardin/SRF binding, and a luciferase assay was used to identify transcriptional activity of myocardin/SRF in neonatal cardiomyocytes. Both myocardin and AngII expression increased after hypoxia, with AngII appearing at an earlier time point than myocardin. Myocardin expression was stimulated by AngII and ERK (extracellular-signal-regulated kinase) phosphorylation, but was suppressed by an ARB (AngII type 1 receptor blocker), an ERK pathway inhibitor and myocardin siRNA (small interfering RNA). AngII increased both myocardin expression and transcription in neonatal cardiomyocytes. Binding of myocardin/SRF was identified using an EMSA, and a luciferase assay indicated the transcription of myocardin/SRF in neonatal cardiomyocytes. Increased BNP (B-type natriuretic peptide), MHC (myosin heavy chain) and [³H]proline incorporation into cardiomyocytes was identified after hypoxia with the presence of myocardin in hypertrophic cardiomyocytes. In conclusion, hypoxia in cardiomyocytes increased myocardin expression, which is mediated by the induction of AngII and the ERK pathway, to cause cardiomyocyte hypertrophy. Myocardial hypertrophy was identified as an increase in transcriptional activities, elevated hypertrophic and cardiomyocyte phenotype markers, and morphological hypertrophic changes in cardiomyocytes.

Stimulation of homology-directed gene targeting at an endogenous human locus by a nicking endonuclease

Homologous recombination (HR) is a highly accurate mechanism of DNA repair that can be exploited for homology-directed gene targeting. Since in most cell types HR occurs very infrequently (approximately 10(-6) to 10(-8)), its practical application has been largely restricted to specific experimental systems that allow selection of the few cells that become genetically modified. HR-mediated gene targeting has nonetheless revolutionized genetics by greatly facilitating the analysis of mammalian gene function. Recent studies showed that generation of double-strand DNA breaks at specific loci by designed endonucleases greatly increases the rate of homology-directed gene repair. These findings opened new perspectives for HR-based genome editing in higher eukaryotes. Here, we demonstrate by using donor DNA templates together with the adeno-associated virus (AAV) Rep78 and Rep68 proteins that sequence- and strand-specific cleavage at a native, predefined, human locus can also greatly enhance homology-directed gene targeting. Our findings argue for the development of other strategies besides direct induction of double-strand chromosomal breaks to achieve efficient and heritable targeted genetic modification of cells and organisms. Finally, harnessing the cellular HR pathway through Rep-mediated nicking expands the range of strategies that make use of AAV elements to bring about stable genetic modification of human cells.

From fish to amphibians to mammals: in search of novel strategies to optimize cardiac regeneration

Different vertebrate species have different cardiac regeneration rates: high in teleost fish, moderate in urodele amphibians, and almost negligible in mammals. Regeneration may occur through stem and progenitor cell differentiation or via dedifferentiation with residual cardiomyocytes reentering the cell cycle. In this review, we will examine the ability of zebrafish and newts to respond to cardiac damage with de novo cardiogenesis, whereas rodents and humans respond with a marked fibrogenic response and virtually no cardiomyocyte regeneration. Concerted strategies are needed to overcome this evolutionarily imposed barrier and optimize cardiac regeneration in mammals.

Myocardin is sufficient for a smooth muscle-like contractile phenotype

BACKGROUND

Myocardin (Myocd) is a strong coactivator that binds the serum response factor (SRF) transcription factor over CArG elements embedded within smooth muscle cell (SMC) and cardiac muscle cyto-contractile genes. Here, we sought to ascertain whether Myocd-mediated gene expression confers a structural and physiological cardiac or SMC phenotype.

METHODS AND RESULTS

Adenoviral-mediated expression of Myocd in the BC(3)H1 cell line induces cardiac and SMC genes while suppressing both skeletal muscle markers and cell growth. Immunofluorescence microscopy shows that SRF and a SMC-like cyto-contractile apparatus are elevated with Myocd overexpression. A short hairpin RNA to Srf impairs BC(3)H1 cyto-architecture; however, cotransduction with Myocd results in complete restoration of the cyto-architecture. Electron microscopic studies demonstrate a SMC ultrastructural phenotype with no evidence for cardiac sarcomerogenesis. Biochemical and time-lapsed videomicroscopy assays reveal clear evidence for Myocd-induced SMC-like contraction.

CONCLUSIONS

Myocd is sufficient for the establishment of a SMC-like contractile phenotype.

Genetic complementation of human muscle cells via directed stem cell fusion

Duchenne muscular dystrophy (DMD) is caused by mutations in the X chromosome-linked DMD gene, which encodes the sarcolemma-stabilizing protein-dystrophin. Initial attempts at DMD therapy deployed muscle progenitor cells from healthy donors. The utilization of these cells is, however, hampered by their immunogenicity, while those from DMD patients are scarce and display limited ex vivo replication. Nonmuscle cells with myogenic capacity may offer valuable alternatives especially if, to allow autologous transplantation, they are amenable to genetic intervention. As a paradigm for therapeutic gene transfer by heterotypic cell fusion we are investigating whether human mesenchymal stem cells (hMSCs) can serve as donors of recombinant DMD genes for recipient human muscle cells. Here, we show that forced MyoD expression in hMSCs greatly increases their tendency to participate in human myotube formation turning them into improved DNA delivery vehicles. Efficient loading of hMSCs with recombinant DMD was achieved through a new tropism-modified high-capacity adenoviral (hcAd) vector directing striated muscle-specific synthesis of full-length dystrophin. This study introduces the principle of genetic complementation of gene-defective cells via directed cell fusion and provides an initial framework to test whether transient MyoD synthesis in autologous, gene-corrected hMSCs increases their potential for treating DMD and, possibly, other muscular dystrophies.

Forced myocardin expression enhances the therapeutic effect of human mesenchymal stem cells after transplantation in ischemic mouse hearts

Human mesenchymal stem cells (hMSCs) have only a limited differentiation potential toward cardiomyocytes. Forced expression of the cardiomyogenic transcription factor myocardin may stimulate hMSCs to acquire a cardiomyogenic phenotype, thereby improving their possible therapeutic potential. hMSCs were transduced with green fluorescent protein (GFP) and myocardin (hMSC(myoc)) or GFP and empty vector (hMSC). After coronary ligation in immune-compromised NOD/scid mice, hMSC(myoc) (n = 10), hMSC (n = 10), or medium only (n = 12) was injected into the infarct area. Sham-operated mice (n = 12) were used to determine baseline characteristics. Left ventricular (LV) volumes and ejection fraction (EF) were serially (days 2 and 14) assessed using 9.4-T magnetic resonance imaging. LV pressure-volume measurements were performed at day 15, followed by histological evaluation. At day 2, no differences in infarct size, LV volumes, or EF were observed among the myocardial infarction groups. At day 14, left ventricular ejection fraction in both cell-treated groups was preserved compared with the nontreated group; in addition, hMSC(myoc) injection also reduced LV volumes compared with medium injection (p < .05). Furthermore, pressure-volume measurements revealed a significantly better LV function after hMSC(myoc) injection compared with hMSC treatment. Immunohistochemistry at day 15 demonstrated that the engraftment rate was higher in the hMSC(myoc) group compared with the hMSC group (p < .05). Furthermore, these cells expressed a number of cardiomyocyte-specific markers not observed in the hMSC group. After myocardial infarction, injection of hMSC(myoc) improved LV function and limited LV remodeling, effects not observed after injection of hMSC. Furthermore, forced myocardin expression improved engraftment and induced a cardiomyocyte-like phenotype hMSC differentiation.