Irina A. Potapova

Human Mesenchymal Stem Cells as a Gene Delivery System to Create Cardiac Pacemakers

Abstract— We tested the ability of human mesenchymal stem cells (hMSCs) to deliver a biological pacemaker to the heart. hMSCs transfected with a cardiac pacemaker gene, mHCN2, by electroporation expressed high levels of Cs+-sensitive current (31.1±3.8 pA/pF at −150 mV) activating in the diastolic potential range with reversal potential of −37.5±1.0 mV, confirming the expressed current as If-like. The expressed current responded to isoproterenol with an 11-mV positive shift in activation. Acetylcholine had no direct effect, but in the presence of isoproterenol, shifted activation 15 mV negative. Transfected hMSCs influenced beating rate in vitro when plated onto a localized region of a coverslip and overlaid with neonatal rat ventricular myocytes. The coculture beating rate was 93±16 bpm when hMSCs were transfected with control plasmid (expressing only EGFP) and 161±4 bpm when hMSCs were expressing both EGFP+mHCN2 (P <0.05). We next injected 10 6 hMSCs transfected with either control plasmid or mHCN2 gene construct subepicardially in the canine left ventricular wall in situ. During sinus arrest, all control (EGFP) hearts had spontaneous rhythms (45±1 bpm, 2 of right-sided origin and 2 of left). In the EGFP+mHCN2 group, 5 of 6 animals developed spontaneous rhythms of left-sided origin (rate=61±5 bpm; P <0.05). Moreover, immunostaining of the injected regions demonstrated the presence of hMSCs forming gap junctions with adjacent myocytes. These findings demonstrate that genetically modified hMSCs can express functional HCN2 channels in vitro and in vivo, mimicking overexpression of HCN2 genes in cardiac myocytes, and represent a novel delivery system for pacemaker genes into the heart or other electrical syncytia.

Connexin‐specific cell‐to‐cell transfer of short interfering RNA by gap junctions

The purpose of this study was to determine whether oligonucleotides the size of siRNA are permeable to gap junctions and whether a specific siRNA for DNA polymerase β (pol β) can move from one cell to another via gap junctions, thus allowing one cell to inhibit gene expression in another cell directly. To test this hypothesis, fluorescently labelled oligonucleotides (morpholinos) 12, 16 and 24 nucleotides in length were synthesized and introduced into one cell of a pair using a patch pipette. These probes moved from cell to cell through gap junctions composed of connexin 43 (Cx43). Moreover, the rate of transfer declined with increasing length of the oligonucleotide. To test whether siRNA for pol β was permeable to gap junctions we used three cell lines: (1) NRK cells that endogenously express Cx43; (2) Mβ16tsA cells, which express Cx32 and Cx26 but not Cx43; and (3) connexin‐deficient N2A cells. NRK and Mβ16tsA cells were each divided into two groups, one of which was stably transfected to express a small hairpin RNA (shRNA), which gives rise to siRNA that targets pol β. These two pol β knockdown cell lines (NRK‐kcdc and Mβ16tsA‐kcdc) were co‐cultured with labelled wild type, NRK‐wt or Mβ16tsA‐wt cells or N2A cells. The levels of pol β mRNA and protein were determined by semiquantitative RT‐PCR and immunoblotting. Co‐culture of Mβ16tsA‐kcdc cells with Mβ16tsA‐wt, N2A or NRK‐wt cells had no effect on pol β levels in these cells. Similarly, co‐culture of NRK‐kcdc with N2A cells had no effect on pol β levels in the N2A cells. In contrast, co‐culture of NRK‐kcdc with NRK‐wt cells resulted in a significant reduction in pol β in the wt cells. The inability of Mβ16tsA‐kcdc cells to transfer siRNA is consistent with the fact that oligonucleotides of the 12 nucleotide length were not permeable to Cx32/Cx26 channels. This suggested that Cx43 but not Cx32/Cx26 channels allowed the cell‐to‐cell movement of the siRNA. These results support the novel hypothesis that non‐hybridized and possible hybridized forms of siRNA can move between mammalian cells through connexin‐specific gap junctions.

Human mesenchymal stem cells make cardiac connexins and form functional gap junctions

Human mesenchymal stem cells (hMSCs) are a multipotent cell population with the potential to be a cellular repair or delivery system provided that they communicate with target cells such as cardiac myocytes via gap junctions. Immunostaining revealed typical punctate staining for Cx43 and Cx40 along regions of intimate cell‐to‐cell contact between hMSCs. The staining patterns for Cx45 rather were typified by granular cytoplasmic staining. hMSCs exhibited cell‐to‐cell coupling to each other, to HeLa cells transfected with Cx40, Cx43 and Cx45 and to acutely isolated canine ventricular myocytes. The junctional currents (Ij) recorded between hMSC pairs exhibited quasi‐symmetrical and asymmetrical voltage (Vj) dependence. Ij records from hMSC–HeLaCx43 and hMSC–HeLaCx40 cell pairs also showed symmetrical and asymmetrical Vj dependence, while hMSC–HeLaCx45 pairs always produced asymmetrical Ij with pronounced Vj gating when the Cx45 side was negative. Symmetrical Ij suggests that the dominant functional channel is homotypic, while the asymmetrical Ij suggests the activity of another channel type (heterotypic, heteromeric or both). The hMSCs exhibited a spectrum of single channels with transition conductances (γj) of 30–80pS. The macroscopic Ij obtained from hMSC–cardiac myocyte cell pairs exhibited asymmetrical Vj dependence, while single channel events revealed γj of the size range 40–100pS. hMSC coupling via gap junctions to other cell types provides the basis for considering them as a therapeutic repair or cellular delivery system to syncytia such as the myocardium.

Mesenchymal stem cells support migration, extracellular matrix invasion, proliferation, and survival of endothelial cells in vitro.

We investigated effects of the paracrine factors secreted by human mesenchymal stem cells (hMSCs) on endothelial cell migration, extracellular matrix invasion, proliferation, and survival in vitro. Human mesenchymal stem cells were cultured as a monolayer or as three‐dimensional aggregates in hanging drops (hMSC spheroids). We performed analysis of paracrine factors in medium conditioned by a monolayer of hMSCs and hMSC spheroids. Concentrations of vascular endothelial growth factor (VEGF), basic fibroblast growth factor, angiogenin, procathepsin B, interleukin (IL)‐11, and bone morphogenic protein 2 were increased 5–20 times in medium conditioned by hMSC spheroids, whereas concentrations of IL‐6, IL‐8, and monocyte hemoattractant protein‐1 were not increased. Concentrations of VEGF and angiogenin in medium conditioned by hMSC spheroids showed a weak dependence on the presence of serum, which allows serum‐free conditioned medium with elevated concentrations of angiogenic cytokines to be obtained. Medium conditioned by hMSC spheroids was more effective in stimulation of umbilical vein endothelial cell proliferation, migration, and basement membrane invasion than medium conditioned by a monolayer of hMSCs. This medium also promotes endothelial cell survival in vitro. We suggest that culturing of hMSCs as three‐dimensional cellular aggregates provides a method to concentrate proangiogenic factors secreted by hMSCs and allows for reduction of serum concentration in conditioned medium. Our data support the hypothesis that hMSCs serve as trophic mediators for endothelial cells.

Finding Fluorescent Needles in the Cardiac Haystack: Tracking Human Mesenchymal Stem Cells Labeled with Quantum Dots for Quantitative In Vivo Three‐Dimensional Fluorescence Analysis

Stem cells show promise for repair of damaged cardiac tissue. Little is known with certainty, however, about the distribution of these cells once introduced in vivo. Previous attempts at tracking delivered stem cells have been hampered by the autofluorescence of host tissue and limitations of existing labeling techniques. We have developed a novel loading approach to stably label human mesenchymal stem cells with quantum dot (QD) nanoparticles. We report the optimization and validation of this long‐term tracking technique and highlight several important biological applications by delivering labeled cells to the mammalian heart. The bright QD crystals illuminate exogenous stem cells in histologic sections for at least 8 weeks following delivery and permit, for the first time, the complete three‐dimensional reconstruction of the locations of all stem cells following injection into the heart.

Xenografted Adult Human Mesenchymal Stem Cells Provide a Platform for Sustained Biological Pacemaker Function in Canine Heart

Background— Biological pacemaking has been performed with viral vectors, human embryonic stem cells, and adult human mesenchymal stem cells (hMSCs) as delivery systems. Only with human embryonic stem cells are data available regarding stability for >2 to 3 weeks, and here, immunosuppression has been used to facilitate survival of xenografts. The purpose of the present study was to determine whether hMSCs provide stable impulse initiation over 6 weeks without the use of immunosuppression, the “dose” of hMSCs that ensures function over this period, and the catecholamine responsiveness of hMSC-packaged pacemakers. Methods and Results— A full-length mHCN2 cDNA subcloned in a pIRES2-EGFP vector was electroporated into hMSCs. Transfection efficiency was estimated by GFP expression. IHCN2 was measured with patch clamp, and cells were administered into the left ventricular anterior wall of adult dogs in complete heart block and with backup electronic pacemakers. Studies encompassed 6 weeks. IHCN2 for all cells was 32.1±1.3 pA/pF (mean±SE) at −150 mV. Pacemaker function in intact dogs required 10 to 12 days to fully stabilize and persisted consistently through day 42 in dogs receiving ≥700 000 hMSCs (≈40% of which carried current). Rhythms were catecholamine responsive. Tissues from animals killed at 42 days manifested neither apoptosis nor humoral or cellular rejection. Conclusions— hMSCs provide a means for administering catecholamine-responsive biological pacemakers that function stably for 6 weeks and manifest no cellular or humoral rejection at that time. Cell doses >700 000 are sufficient for pacemaking when administered to left ventricular myocardium.

Wild-Type and Mutant HCN Channels in a Tandem Biological-Electronic Cardiac Pacemaker

Background— Biological pacemakers (BPM) implanted in canine left bundle branch function competitively with electronic pacemakers (EPM). We hypothesized that BPM engineered with the use of mE324A mutant murine HCN2 (mHCN2) genes would improve function over mHCN2 and that BPM/EPM tandems confer advantage over either approach alone. Methods and Results— In cultured neonatal rat myocytes, activation midpoint was −46.9 mV in mE324A versus −66.1 mV in mHCN2 (P<0.05). mE324A manifested a positive shift of voltage dependence of gating kinetics of activation and deactivation compared with mHCN2 (P<0.05) in myocytes as well as Xenopus oocytes. In intact dogs in complete atrioventricular block, saline (control), mHCN2, or mE324A virus was injected into left bundle branch, and EPM were implanted (VVI 45 bpm). Twenty-four–hour ECGs were monitored for 14 days. With EPM discontinued, there was no difference in duration of overdrive suppression among groups. However, basal heart rates in controls were less than those in mHCN2, which did not differ from those in E324A (45 versus 57 versus 53 bpm; P<0.05). When spontaneous rate fell below 45 bpm, EPM intervened at that rate, triggering 83% of beats in control, contrasting (P<0.05) with 26% (mHCN2) and 36% (mE324A). On day 14, epinephrine (1 &mgr;g/kg per minute IV) induced a 50% heart rate increase in all mE324A, one third of mHCN2, and one fifth of control (P<0.05 mE324A versus control or mHCN2). Conclusions— mE324A induces faster, more positive pacemaker current activation than mHCN2 and stable, catecholamine-sensitive rhythms in situ that compete with EPM comparably but more catecholamine responsively than mHCN2. BPM/EPM tandems function reliably, reduce the number of EPM beats, and confer sympathetic responsiveness to the tandem.

Culturing of Human Mesenchymal Stem Cells as Three-dimensional Aggregates Induces Functional Expression of CXCR4 That Regulates Adhesion to Endothelial Cells

Culture-expanded human mesenchymal stem cells (hMSCs) are increasingly used in a variety of preclinical and clinical studies. However, these cells have a low rate of engraftment to bone marrow or damaged tissues. Several laboratories have shown that during isolation and subculturing mesenchymal stem cells quickly lose the expression of CXCR4, the key receptor responsible for lymphocytes and hematopoietic stem cell homing. Here we show that culturing of hMSCs as three-dimensional aggregates (hMSC spheroids) restores CXCR4 functional expression. Expression of CXCR4 inversely correlates with the secretion of SDF-1 by hMSCs. Cells from hMSC spheroids up-regulate expression of CD49b, the α2 integrin subunit, and suppress the expression of CD49d, the α4 integrin subunit. Transfer of cells from the spheroids back to a monolayer suppresses the expression of CXCR4 and CD49b and restores the expression of CD49d. Treatment of cells from the spheroids with SDF-1 leads to CXCR4 internalization and activation of ERK-1,2. Adhesion of hMSCs to human umbilical vein endothelial cells (HUVECs) was investigated. SDF-1, AMD-3100, or exposure of HUVECs to hypoxia did not affect adhesion of hMSCs from a monolayer to HUVECs. Adhesion of cells from hMSC spheroids to HUVECs was stimulated by SDF-1, AMD-3100, or by exposure of HUVECs to hypoxia. Stimulatory effects of hypoxia and addition of SDF-1 or AMD-3100 were not additive. Overall, our data indicate that the expression of CXCR4 by hMSCs regulates hMSC adhesion to endothelial cells.

MiRP1 modulates HCN2 channel expression and gating in cardiac myocytes.

MinK-related protein (MiRP1 or KCNE2) interacts with the hyperpolarization-activated, cyclic nucleotidegated (HCN) family of pacemaker channels to alter channel gating in heterologous expression systems. Given the high expression levels of MiRP1 and HCN subunits in the cardiac sinoatrial node and the contribution of pacemaker channel function to impulse initiation in that tissue, such an interaction could be of considerable physiological significance. However, the functional evidence for MiRP1/HCN interactions in heterologous expression studies has been accompanied by inconsistencies between studies in terms of the specific effects on channel function. To evaluate the effect of MiRP1 on HCN expression and function in a physiological context, we used an adenovirus approach to overexpress a hemagglutinin (HA)-tagged MiRP1 (HAMiRP1) and HCN2 in neonatal rat ventricular myocytes, a cell type that expresses both MiRP1 and HCN2 message at low levels. HA-MiRP1 co-expression with HCN2 resulted in a 4-fold increase in maximal conductance of pacemaker currents compared with HCN2 expression alone. HCN2 activation and deactivation kinetics also changed, being significantly more rapid for voltages between –60 and –95 mV when HA-MiRP1 was co-expressed with HCN2. However, the voltage dependence of activation was not affected. Co-immunoprecipitation experiments demonstrated that expressed HA-MiRP1 and HCN2, as well as endogenous MiRP1 and HCN2, co-assemble in ventricular myocytes. The results indicate that MiRP1 acts as a β subunit for HCN2 pacemaker channel subunits and alters channel gating at physiologically relevant voltages in cardiac cells.

Enhanced recovery of mechanical function in the canine heart by seeding an extracellular matrix patch with mesenchymal stem cells committed to a cardiac lineage.

The need to regenerate tissue is paramount, especially for the heart that lacks the ability to regenerate after injury. The urinary bladder extracellular matrix (ECM), when used to repair a right ventricular defect, successfully regenerated some mechanical function. The objective of the current study was to determine whether the regenerative effect of ECM could be improved by seeding the patch with human mesenchymal stem cells (hMSCs) enhanced to differentiate down a cardiac linage. hMSCs were used to form three-dimensional spheroids. The expression of cardiac proteins was determined in cells exposed to the spheroid formation and compared with nonmanipulated hMSCs. To determine whether functional calcium channels were present, the cells were patch clamped. To evaluate the ability of these cells to regenerate mechanical function, the spheroids were seeded on ECM and then implanted into the canine heart to repair a full-thickness right ventricular defect. As a result, many of the cells spreading from the spheroids expressed cardiac-specific proteins, including sarcomeric alpha-actinin, cardiotin, and atrial natriuretic peptide, as well as the cell cycle markers cyclin D1 and proliferating cell nuclear antigen. A calcium current similar in amplitude to that of ventricular myocytes was present in 16% of the cells. The cardiogenic cell-seeded scaffolds increased the regional mechanical function in the canine heart compared with the unmanipulated hMSC-seeded scaffolds. In addition, the cells prelabeled with fluorescent markers demonstrated myocyte-specific actinin staining with sarcomere spacing similar to that of normal myocytes. In conclusion, the spheroid-derived cells express cardiac-specific proteins and demonstrate a calcium current similar to adult ventricular myocytes. When these cells are implanted into the canine heart, some of these cells appear striated and mechanical function is improved compared with the unmanipulated hMSCs. Further investigation will be required to determine whether the increased mechanical function is due to a differentiation of the cardiogenic cells to myocytes or to other effects.