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Introduction: After myocardial infarction, necrosed cardiomyocytes are replaced with fibrotic tissue (FT) which dampens electrical activity and impedes impulse conduction in the heart, increasing the risk of cardiac arrhythmias. Polypyrrole-chitosan (PPY:CHI) hydrogel is a conductive biomaterial designed to restore the heart's conduction pathways to improve function and limit arrhythmias. Previously, we demonstrated that an injection of PPY:CHI into rat FT after an infarct shortened QRS interval duration. However, the underlying mechanisms were not investigated. In the current study, we injected PPY:CHI into FT to increase conductive potential and velocity towards the conduction seen in healthy myocardium. Methods: Gelatin, a resistive material, was used to model FT. We used three approaches to understand how PPY:CHI electrically works within FT. Groups of increasing concentrations of PPY:CHI and chitosan (CHI) control were used in this model: 5, 10, 20, 40, 60, 80 and 100%. 1) Impedance spectroscopy determined the effect PPY:CHI has on the impedance of the model. 2) Multi-electrode array (MEA) identified electrical potentials traveling through PPY:CHI when in contact with contracting cardiac tissue. 3) MEA and multi-channel systems were used to identify the conduction velocity of PPY:CHI, model concentrations, and healthy and infarct scar tissue. Results: 1) At concentrations ≥20%, PPY:CHI reduced the impedance of the model to a level similar to that of healthy myocardium. At a concentration of 10%, PPY:CHI reduced the impedance of the model by half compared to 40% CHI. 2) When in contact with contracting myocardium at concentrations ≥20%, PPY:CHI influenced the model to transmit field potentials >0.5 mV, mimicking healthy myocardium. The control and model had a potential of <0.5 mV, which resembles cardiac FT. 3) At concentrations >40%, PPY:CHI influenced the model to generate a velocity of 0.4-0.6 m/s, which resembles healthy myocardium. The control and model generated a velocity of <0.15 m/s, which is very similar to cardiac FT. Conclusions: PPY:CHI influenced our FT model to electrically resemble healthy cardiac tissue. Electrically coupling the left ventricle may not only induce synchronous contraction, but also limit fatal arrhythmias. [ABSTRACT FROM AUTHOR]

Background—Efficient cardiac function requires synchronous ventricular contraction. After myocardial infarction, the nonconductive nature of scar tissue contributes to ventricular dysfunction by electrically uncoupling viable cardiomyocytes in the infarct region. Injection of a conductive biomaterial polymer that restores impulse propagation could synchronize contraction and restore ventricular function by electrically connecting isolated cardiomyocytes to intact tissue, allowing them to contribute to global heart function. Methods and Results—We created a conductive polymer by grafting pyrrole to the clinically tested biomaterial chitosan to create a polypyrrole (PPy)-chitosan hydrogel. Cyclic voltammetry showed that PPy-chitosan had semiconductive properties lacking in chitosan alone. PPy-chitosan did not reduce cell attachment, metabolism, or proliferation in vitro. Neonatal rat cardiomyocytes plated on PPy-chitosan showed enhanced Ca2+ signal conduction in comparison with chitosan alone. PPy-chitosan plating also improved electric coupling between skeletal muscles placed 25 mm apart in comparison with chitosan alone, demonstrating that PPy-chitosan can electrically connect contracting cells at a distance. In rats, injection of PPy-chitosan 1 week after myocardial infarction decreased the QRS interval and increased the transverse activation velocity in comparison with saline or chitosan, suggesting improved electric conduction. Optical mapping showed increased activation in the border zone of PPy-chitosan-treated rats. Echocardiography and pressure-volume analysis showed improvement in load-dependent (ejection fraction, fractional shortening) and load-independent (preload recruitable stroke work) indices of heart function 8 weeks after injection. Conclusions—We synthesized a biocompatible conductive biomaterial (PPy-chitosan) that enhances biological conduction in vitro and in vivo. Injection of PPy-chitosan better maintained heart function after myocardial infarction than a nonconductive polymer. [ABSTRACT FROM AUTHOR]

Background--Allogeneic mesenchymal stem cells (MSCs) were immunoprivileged early after cardiac implantation and improved heart function in preclinical and clinical studies. However, long-term preclinical studies demonstrated that allogeneic MSCs lost their immunoprivilege and were rejected in the injured myocardium, resulting in recurrent ventricular dysfunction. This study identifies some of the mechanisms responsible for the immune switch in MSCs and suggests a new treatment to maintain immunoprivilege and preserve heart function. Methods and Results--Rat MSC immunoprivilege was mediated by prostaglandin E2 (PGE2)-induced secretion of 2 critical chemokines, CCL12 and CCL5. These chemokines stimulated the chemoattraction of T cells toward MSCs, suppressed cytotoxic T-cell proliferation, and induced the production of T regulatory cells. MSCs treated with 5-azacytidine for 24 hours differentiated into myogenic cells after 2 weeks, which was associated with decreased PGE2 and chemokine production and the loss of immunoprivilege. Treatment of differentiated MSCs with PGE2 restored chemokine levels and preserved MSC immunoprivilege. In a rat myocardial infarction model, allogeneic MSCs (3x106 cells/rat) were injected into the infarct region with or without a biodegradable hydrogel that slowly released PGE2. Five weeks later, the transplanted MSCs expressed myogenic lineage markers and were rejected in the control group, but in the PGE2-treated group, the transplanted cells survived and heart function improved. Conclusions--Allogeneic MSCs maintained immunoprivilege by PGE2-induced secretion of chemokines CCL12 and CCL5. Differentiation of MSCs decreased PGE2 levels, and immunoprivilege was lost. Maintaining PGE2 levels preserved immunoprivilege after differentiation, prevented rejection of implanted MSCs, and restored cardiac function. [ABSTRACT FROM AUTHOR]

Introduction: Cardiovascular disease is the leading cause of death worldwide and its prevalence increases with age. Replacing the bone marrow (BM) stem cells of aged mice with young BM stem cell antigen-1+ (Y-Sca-1+) cells rejuvenates the response of the aged heart to injury. Autophagy, a critical cellular recycling process, decreases with age. We hypothesized that the high rate of autophagy in Y-Sca-1+ BM cells contributes to the rejuvenation of the aged heart and improves tissue repair. Methods: In vitro: Young (Y) and old (O) Sca-1+ BM cells were isolated from Y and O mice and cultured in normoxia and hypoxia conditions to examine the rate of autophagy. To determine the effect of secretory factors, conditioned medium (CM) was produced from Y-Sca-1+ and O-Sca-1+ cells and used to culture old primary cardiac fibroblasts (O-Fs). In vivo: Old mice were lethally irradiated and reconstituted with Y-Sca-1+ or O-Sca-1+ BM cells from GFP mice to generate YP and OP chimeras, respectively. After 3 months, mice were sacrificed and hearts collected for molecular and histological analyses. Results: In vitro: Cultured Y-Sca-1+ cells exhibited a higher rate of autophagy and were more responsive to hypoxia-induced autophagy than O-Sca-1+ cells. Thus, Y-Sca-1+ cells may pre-condition and rejuvenate the aged heat. In support of this theory, factors secreted from Y-Sca-1+, but not O-Sca-1+ BM cells, enhanced several functions of O-Fs, including autophagy, cell migration, proliferation and trans-differentiation. This phenotype was not observed after boiling the CM, suggesting that rejuvenation is mediated by a secreted factor. In vivo: We found that reconstitution of aged mice with Y-Sca-1+ BM cells improved autophagy of the aged heart. The autophagy rate was increased in YP relative to OP hearts as demonstrated by an increase in the LC3b II/I ratio. Immunostaining of the reconstituted heart showed that the number of LC3b+ cells was 3-fold higher in YP than OP hearts. These LC3b+ cells were both donor and recepiant cells. Conclusions: The increase in autophagy rate in the YP heart suggests that autophagy is one of the mechanisms involved in rejuvenation of the aged heart after BM reconstitution. Y-Sca-1+ BM cells secret factors that improve the autophagy of recepient cardiac cells. [ABSTRACT FROM AUTHOR]

Introduction: Clinical trials of cell therapy for post-injury conditioning of the myocardium have shown only modest improvements, perhaps because the majority of donor cells die in the harsh post-ischemic environment. Hypothesis: We hypothesize that pre-conditioning the myocardium by delivering human CD34+ (hCD34+) cells through bone marrow (BM) engraftment may prevent adverse remodeling and heart failure post myocardial infarct (MI). BM engraftment may be especially important in elderly patients because of their reduced stem cell function. Methods: BM aspirates from 40 patients undergoing open-heart surgery were collected. Mononuclear cells were isolated and stem cell subpopulations were quantified using flow cytometry and magnetically sorted. Function of hCD34+ cells was analyzed by the colony forming unit (CFU) assay and their ability to reconstitute irradiated immune-deficient NSG mice. Engraftment was quantified as hCD45 chimerism by flow cytometry. To evaluate the benefits of hCD34+ cells in cardiac repair, coronary artery ligation was performed 12 weeks post-reconstitution. Left ventricular (LV) function and histology were assessed by echocardiography and immunohistochemistry. Results: Older patients (>60 years old, n=24) had lower numbers (p=0.0008) and reduced function (p<0.0001) of hCD34+ cells. We found a significant difference in the homing ability of hCD34+ (n=7) vs. hCD34- (n=6) cells to the bone marrow, blood, spleen and heart (p<0.05, all groups) 12 weeks post-reconstitution. hCD34+ mice engrafted with young patients' cells had higher reconstitution in blood, spleen and heart (p<0.01, all groups). At 4 weeks post-ligation, hCD34+ mice (n=8) had improved LV function, with greater fractional shortening (p=0.003), reduced LV dilation (p=0.02) and reduced scar size (p=0.0002). Immunohistochemistry of the peri-infarct region of CD34+ mice showed increased hCD34-derived vessels, indicating their role in infarct healing. Conclusions: hCD34+ cells have reduced frequency and function in older patients, but engraft and protect the myocardium from functional decline post-MI. Cardiac pre-conditioning by restoration of aged hCD34+ cells is a promising avenue for preventing cardiac dysfunction post-MI. [ABSTRACT FROM AUTHOR]