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BACKGROUND: Shortly after birth, cardiomyocytes exit the cell cycle and cease proliferation. At present, the regulatory mechanisms for this loss of proliferative capacity are poorly understood. CBX7 (chromobox 7), a polycomb group (PcG) protein, regulates the cell cycle, but its role in cardiomyocyte proliferation is unknown. METHODS: We profiled CBX7 expression in the mouse hearts through quantitative real-time polymerase chain reaction, Western blotting, and immunohistochemistry. We overexpressed CBX7 in neonatal mouse cardiomyocytes through adenoviral transduction. We knocked down CBX7 by using constitutive and inducible conditional knockout mice ( Tnnt2-Cre;Cbx7 fl/+ and Myh6-MCM;Cbx7 fl/fl , respectively). We measured cardiomyocyte proliferation by immunostaining of proliferation markers such as Ki67, phospho-histone 3, and cyclin B1. To examine the role of CBX7 in cardiac regeneration, we used neonatal cardiac apical resection and adult myocardial infarction models. We examined the mechanism of CBX7-mediated repression of cardiomyocyte proliferation through coimmunoprecipitation, mass spectrometry, and other molecular techniques. RESULT: We explored Cbx7 expression in the heart and found that mRNA expression abruptly increased after birth and was sustained throughout adulthood. Overexpression of CBX7 through adenoviral transduction reduced proliferation of neonatal cardiomyocytes and promoted their multinucleation. On the other hand, genetic inactivation of Cbx7 increased proliferation of cardiomyocytes and impeded cardiac maturation during postnatal heart growth. Genetic ablation of Cbx7 promoted regeneration of neonatal and adult injured hearts. Mechanistically, CBX7 interacted with TARDBP (TAR DNA-binding protein 43) and positively regulated its downstream target, RBM38 (RNA Binding Motif Protein 38), in a TARDBP-dependent manner. Overexpression of RBM38 inhibited the proliferation of CBX7-depleted neonatal cardiomyocytes. CONCLUSIONS: Our results demonstrate that CBX7 directs the cell cycle exit of cardiomyocytes during the postnatal period by regulating its downstream targets TARDBP and RBM38. This is the first study to demonstrate the role of CBX7 in regulation of cardiomyocyte proliferation, and CBX7 could be an important target for cardiac regeneration.

Introduction: Cardiomyocyte (CM) proliferation notably decreases during the perinatal period. Regulatory mechanisms for this loss of proliferative capacity are poorly understood. CBX7, a polycomb group (PcG) protein, regulates the cell cycle but its role in CM proliferation is unknown. Methods: We profiled CBX7 expression in the mouse hearts via qRT-PCR, western blotting, and immunohistochemistry. We overexpressed CBX7 in neonatal mouse CMs via adenoviral transduction. We knocked down CBX7 by using constitutive and inducible conditional knockout mice (Myh6-cre;Cbx7 fl/+ and Myh6-MCM;Cbx7 fl/fl , respectively). We measured CM proliferation by immunostaining of proliferation markers such as Ki67, phospho-histone 3, and cyclin B1. We examined the mechanism of CBX7-mediated repression of CM proliferation via co-immunoprecipitation, mass spectrometry, and other molecular techniques. Results: The mRNA expression of Cbx7 was increased at the perinatal stage and sustained in the postnatal heart (fold increase of adult vs. prenatal: 32.1 ± 3.3, P < 0.01). Overexpression of CBX7 in neonatal mouse CMs reduced CM proliferation (% of Ki67 + CMs: Ad-Mock, 22.3 ± 1.7 vs. Ad-CBX7, 7.7 ± 0.7, P < 0.001). The haplodeficiency of CBX7 (Tnnt2-Cre;Cbx7 fl/+ ) enhanced proliferation of neonatal CMs in vivo (% of Ki67 + CMs: wild-type, 7.9 ± 0.9% vs. mutants, 24.7 ± 1.2%, P < 0.01), leading to increased myocardial wall thickness, cardiomegaly and neonatal lethality. Genetic deletion of CBX7 in CMs (Myh6-MCM;Cbx7 fl/fl ) at P1 resulted in increased CM proliferation (% of Ki67 + CMs: vehicle, 9.5 ± 1.4% vs. tamoxifen, 18.3 ± 1.8%, P < 0.01), leading to cardiomegaly at 3 months. Mechanistically, CBX7 interacted with TAR DNA-binding protein 43 (TARDBP) and positively regulated its downstream target, RNA Binding Motif Protein 38 (RBM38). Rbm38 was perinatally upregulated in the mouse hearts and overexpression of RBM38 reduced proliferation of neonatal CMs. Conclusions: CBX7 expression is perinatally increased in the mouse hearts and inhibits proliferation of CMs by controlling TARDBP/Rbm38 pathway. This is the first study to demonstrate the role of CBX7 in regulation of CM proliferation and CBX7 could be an important target for cardiac regeneration.

Background: Human pluripotent stem cell (hPSC)–derived endothelial cells (ECs) have limited clinical utility because of undefined components in the differentiation system and poor cell survival in vivo. Here, we aimed to develop a fully defined and clinically compatible system to differentiate hPSCs into ECs. Furthermore, we aimed to enhance cell survival, vessel formation, and therapeutic potential by encapsulating hPSC-ECs with a peptide amphiphile (PA) nanomatrix gel. Methods: We induced differentiation of hPSCs into the mesodermal lineage by culturing on collagen-coated plates with a glycogen synthase kinase 3β inhibitor. Next, vascular endothelial growth factor, endothelial growth factor, and basic fibroblast growth factor were added for endothelial lineage differentiation, followed by sorting for CDH5 (VE-cadherin). We constructed an extracellular matrix–mimicking PA nanomatrix gel (PA-RGDS) by incorporating the cell adhesive ligand Arg-Gly-Asp-Ser (RGDS) and a matrix metalloproteinase-2–degradable sequence. We then evaluated whether the encapsulation of hPSC-CDH5 + cells in PA-RGDS could enhance long-term cell survival and vascular regenerative effects in a hind-limb ischemia model with laser Doppler perfusion imaging, bioluminescence imaging, real-time reverse transcription–polymerase chain reaction, and histological analysis. Results: The resultant hPSC-derived CDH5 + cells (hPSC-ECs) showed highly enriched and genuine EC characteristics and proangiogenic activities. When injected into ischemic hind limbs, hPSC-ECs showed better perfusion recovery and higher vessel-forming capacity compared with media-, PA-RGDS–, or human umbilical vein EC–injected groups. However, the group receiving the PA-RGDS–encapsulated hPSC-ECs showed better perfusion recovery, more robust and longer cell survival (> 10 months), and higher and prolonged angiogenic and vascular incorporation capabilities than the bare hPSC-EC–injected group. Surprisingly, the engrafted hPSC-ECs demonstrated previously unknown sustained and dynamic vessel-forming behavior: initial perivascular concentration, a guiding role for new vessel formation, and progressive incorporation into the vessels over 10 months. Conclusions: We generated highly enriched hPSC-ECs via a clinically compatible system. Furthermore, this study demonstrated that a biocompatible PA-RGDS nanomatrix gel substantially improved long-term survival of hPSC-ECs in an ischemic environment and improved neovascularization effects of hPSC-ECs via prolonged and unique angiogenic and vessel-forming properties. This PA-RGDS–mediated transplantation of hPSC-ECs can serve as a novel platform for cell-based therapy and investigation of long-term behavior of hPSC-ECs.

Background— Although methods for generating cardiomyocytes from pluripotent stem cells have been reported, current methods produce heterogeneous mixtures of cardiomyocytes and noncardiomyocyte cells. Here, we report an entirely novel system in which pluripotent stem cell–derived cardiomyocytes are purified by cardiomyocyte-specific molecular beacons (MBs). MBs are nanoscale probes that emit a fluorescence signal when hybridized to target mRNAs. Method and Results— Five MBs targeting mRNAs of either cardiac troponin T or myosin heavy chain 6/7 were generated. Among 5 MBs, an MB that targeted myosin heavy chain 6/7 mRNA (MHC1-MB) identified up to 99% of HL-1 cardiomyocytes, a mouse cardiomyocyte cell line, but Conclusions— We developed a novel cardiomyocyte selection system that allows production of highly purified cardiomyocytes. These purified cardiomyocytes and this system can be valuable for cell therapy and drug discovery.

Background: Ventricular cardiomyocytes (CMs) are an ideal cell type for cardiac cell therapy since they are the main cells generating cardiac forces. However, isolating them from differentiating pluripotent stem cells (PSCs) has been challenging due to the lack of specific surface markers. Here we show that ventricular CMs can be purified from differentiating mouse embryonic stem cells (mESCs) using molecular beacons (MBs) targeting specific intracellular mRNAs. MBs are dual-labeled oligonucleotide hairpin probes that emit a fluorescence signal when hybridized to target mRNAs, allowing isolation of specific target cells by fluorescence activated cell sorting (FACS) with high specificity and sensitivity. Methods and Results: We generated three different MBs (IRX4-1, -2, -3) designed to target specific regions of mRNAs of iroquois homeobox protein 4 (Irx4), a specific transcription factor for ventricular CMs. Among three IRX4 MBs, IRX4-2 MB demonstrated the highest sensitivity and specificity, thus IRX4-2 MB was selected to purify mESC-derived ventricular CMs. Subsequently, IRX4-2 MBs were delivered into cardiomyogenically differentiating mESC cultures and cells showing strong signals from IRX4-2 MBs were FACS-sorted. Flow cytometry demonstrated that 92~97% of IRX4-2 MB-positive cells expressed a marker for ventricular CMs myosin light chain 2 ventricular isoform (Myl2) as well as cardiac troponin 2 (Tnnt2). Importantly, higher than 98% of IRX4-2 MB-positive cells displayed ventricular CM-like action potentials during electrophysiological analyses. These IRX4-2 MB-based purified ventricular CMs continuously maintained their CM characteristics verified by synchronous beating, Ca2+ transient, and expression of ventricular CM-specific proteins. Conclusions: We established a novel MB-based cell sorting system targeting a transcription factor that is specific for ventricular CM to generate homogeneous and functional ventricular CMs. This is the first report to show the feasibility of isolating pure ventricular CMs without modifying host genes, and this platform will be useful for therapeutic applications, disease modeling, and drug discovery.

Background— Peripheral neuropathy is a frequent and major complication of diabetes. Methods and Results— Severe peripheral neuropathy developed in type II diabetic mice, characterized by significant slowing of motor and sensory nerve conduction velocities. Rosuvastatin restored nerve vascularity, including vessel size, and nerve function also recovered to the levels of nondiabetic mice. Neuronal nitric oxide synthase expression in sciatic nerves was reduced in diabetic mice but was preserved by rosuvastatin. Coadministration of a nitric oxide synthase inhibitor with rosuvastatin attenuated the beneficial effects of rosuvastatin on nerve function and limited the recovery of vasa nervorum and nerve function. In vitro, rosuvastatin inhibited downregulation of neuronal nitric oxide synthase expression induced by high-glucose conditions in cultured Schwann cells. Furthermore, Akt phosphorylation in Schwann cells, downregulated by high-glucose conditions, was also restored by rosuvastatin, consistent with the change of neuronal nitric oxide synthase expression. Akt inhibition independently reduced neuronal nitric oxide synthase expression in Schwann cells in low-glucose cultures. Conclusions— These data indicate that the HMG-CoA reductase inhibitor rosuvastatin has a favorable effect on diabetic neuropathy independent of its cholesterol-lowering effect. Our data provide evidence that this effect may be mediated in part via neuronal nitric oxide synthase/nitric oxide and phosphatidylinositol 3-kinase/Akt-signaling pathways and also suggest that restoration or preservation of the microcirculation of the sciatic nerve may be involved.

Background— Toxic neuropathies induced by cisplatin and other chemotherapeutic agents are important clinical problems because of their high incidence, their lack of effective treatment, and the fact that neuropathy represents a dose-limiting factor for these therapies. The pathogenic basis for toxic neuropathies induced by chemotherapeutic agents has not been completely elucidated. Methods and Results— We investigated the hypothesis that experimental toxic neuropathy results from an antiangiogenic effect of these drugs, resulting in destruction of the vasa nervorum, and accordingly that the neuropathy could be prevented or reversed by locally administered VEGF gene transfer without augmenting tumor growth. In an animal model of cisplatin-induced neuropathy, nerve blood flow was markedly attenuated, and there was a profound reduction in the number of vasa nervorum associated with marked endothelial cell apoptosis, resulting in a severe peripheral neuropathy with focal axonal degeneration characteristic of ischemic neuropathy. After intramuscular gene transfer of naked plasmid DNA encoding VEGF-1 in animals with an established neuropathy, vascularity and blood flow returned to levels similar to those of control rats, peripheral nerve function was restored, and histological nerve architecture was normalized. Gene therapy administered in parallel with cisplatin chemotherapy completely attenuated endothelial cell apoptosis and inhibited destruction of nerve vasculature, deterioration of nerve function, and axonal degeneration. In a rat tumor model, VEGF gene transfer administered locally did not alter tumor growth or vascularity. Conclusions— These findings implicate microvascular damage as the basis for toxic neuropathy induced by cisplatin and suggest that local angiogenic gene therapy may constitute a novel prevention or treatment for this disorder without augmenting tumor growth or vascularization.

Background— Diabetic cardiomyopathy (DCM) is characterized by microvascular pathology and interstitial fibrosis, which leads to progressive heart failure; however, the pathogenesis of DCM remains uncertain. Methods and Results— Using the streptozotocin-induced diabetic rat model, we evaluated the natural course of DCM over a period of 1 year by serial echocardiography, Western blot analysis for vascular endothelial growth factor (VEGF), endothelial progenitor cell assays, myocardial blood flow measurements, and histopathologic analysis that included terminal dUTP nick end-labeling (TUNEL), capillary and cardiomyocyte density, and fibrosis area. Downregulation of myocardial VEGF expression preceded all other features of DCM and was followed by increased apoptosis of endothelial cells, decreased numbers of circulating endothelial progenitor cells, decreased capillary density, and impaired myocardial perfusion. Apoptosis and necrosis of cardiomyocytes ensued, along with fibrosis and progressive diastolic and then systolic dysfunction. To provide further evidence of the central role of VEGF in the pathophysiology of DCM, we replenished myocardial VEGF expression using naked DNA gene therapy via direct intramyocardial injection of plasmid DNA encoding VEGF (phVEGF 165 ). VEGF-replenished rats showed increased capillary density, decreased endothelial cell and cardiomyocyte apoptosis, and in situ differentiation of bone marrow–derived endothelial progenitor cells into endothelial cells. These anatomic findings were accompanied by significant improvements in cardiac function. Conclusions— These findings suggest that downregulation of VEGF may compromise microvascular homeostasis in the myocardium and thereby play a central role in the pathogenesis of DCM.

Background— Drug-eluting stents represent a useful strategy for the prevention of restenosis using various antiproliferative drugs. These strategies share the liability of impairing endothelial recovery, thereby altering the natural biology of the vessel wall and increasing the associated risk of stent thrombosis. Accordingly, we tested the hypothesis that local delivery via gene-eluting stent of naked plasmid DNA encoding for human vascular endothelial growth factor (VEGF)-2 could achieve similar reductions in neointima formation while accelerating, rather than inhibiting, reendothelialization. Methods and Results— phVEGF 2-plasmid (100 or 200 μg per stent)–coated BiodivYsio phosphorylcholine polymer stents versus uncoated stents were deployed in a randomized, blinded fashion in iliac arteries of 40 normocholesterolemic and 16 hypercholesterolemic rabbits. Reendothelialization was nearly complete in the VEGF stent group after 10 days and was significantly greater than in control stents (98.7±1% versus 79.0±6%, P 2 , P P Conclusions— Acceleration of reendothelialization via VEGF-2 gene–eluting stents provides an alternative treatment strategy for the prevention of restenosis. VEGF-2 gene–eluting stents may be considered as a stand-alone or combination therapy.

Background— We investigated whether catheter-based, intramyocardial transplantation of autologous endothelial progenitor cells can enhance neovascularization in myocardial ischemia. Methods and Results— Myocardial ischemia was induced by placement of an ameroid constrictor around swine left circumflex artery. Four weeks after constrictor placement, CD31+ mononuclear cells (MNCs) were freshly isolated from the peripheral blood of each animal. After overnight incubation of CD31+ MNCs in noncoated plates, nonadhesive cells (NA/CD31+ MNCs) were harvested as the endothelial progenitor cell–enriched fraction. Nonadhesive CD31− cells (NA/CD31− MNCs) were also prepared. Autologous transplantation of 10 7 NA/CD31+ MNCs, 10 7 NA/CD31− MNCs, or PBS was performed with a NOGA mapping injection catheter to target ischemic myocardium. In a parallel study, 10 5 human CD34+ MNCs, 10 5 human CD34− MNCs, or PBS was transplanted into ischemic myocardium of nude rats 10 minutes after ligation of the left anterior descending coronary artery. In the swine study, ischemic area by NOGA mapping, Rentrop grade angiographic collateral development, and echocardiographic left ventricular ejection fraction improved significantly 4 weeks after transplantation of NA/CD31+ MNCs but not after injection of NA/CD31− MNCs or PBS. Capillary density in ischemic myocardium 4 weeks after transplantation was significantly greater in the NA/CD31+ MNC group than the control groups. In the rat study, echocardiographic left ventricular systolic function and capillary density were significantly better preserved in the CD34+ MNC group than in the control groups 4 weeks after myocardial ischemia. Conclusions— These favorable outcomes encourage future clinical trials of catheter-based, intramyocardial transplantation of autologous CD34+ MNCs in the setting of chronic myocardial ischemia.

Background: Bone marrow (BM)-derived mononuclear cells (MNCs) have been shown to effectively treat ischemic cardiovascular diseases. Recent evidence suggested that diabetic neuropathy (DN) is causally related to impaired angio-vasculogenesis in vasa nervorum and deficiency of angiogenic and neurotrophic factors. Accordingly, we sought to investigate whether DN can be ameliorated by local injection of BM-derived MNCs. Methods and Results: A severe peripheral neuropathy, characterized by significant slowing of motor nerve conduction velocities (MCV) developed 12 wks after the induction of diabetes with streptozotocin in Fisher rats (vs normal rats; 46.6±2.6 vs 32.0±2.5 m/s, P < 0.05). These rats were randomly assigned to MNC or saline injection groups (n = 9, each group) and received either 5x106 MNCs or saline intramuscularly around the sciatic nerves. In the MNC group, MCV were significantly improved within 4 wks after treatment (MNC vs Saline, 41.9±3.2 vs 32.7±2.8 m/s, P < 0.01). Microvascular circulation of sciatic nerve, measured by laser Doppler perfusion imaging was markedly increased only in the MNC group. Capillary density at 4 wks was significantly higher in the MNC group than in the saline group (68±5.9 vs 37±4.4 /cross section, P < 0.01). Robust engraftment of MNCs were observed in sciatic nerves, which sustained over 4 wks. A fraction of engrafted MNCs expressed endothelial markers suggestive of transdifferentiation into endothelial cells in the vasa nervorum. Intriguingly, a large number of the engrafted MNCs are following the course of vasa nervorum in close proximity. Real-time RT-PCR on sciatic nerves revealed that the expression of angio-neurotrophic factors were significantly increased in the MNC group compared to the saline group: VEGF (2.1 fold), FGF-2 (2.4), eNOS (18.1), Brain-derived neurotrophic factor (35.1), IGF-1 (25.5) (all P < 0.05). The protein levels were well correlated with mRNA expression levels. Conclusion: Local transplantation of BM-derived MNCs could improve experimental DN by augmenting neovascularization and increasing angiogenic and neurotrophic factors in peripheral nerves. These findings suggest that BM-MNC transplantation may represent a novel therapeutic option for treating DN.

Background: Recently, we reported that murine bone marrow derived CD31 + cells have hemangioblastic activities. In this study, we hypothesized that human peripheral blood derived CD31 + cells (hCD31 + ) have robust angio-vasculogenic potential and can induce therapeutic neovascularization in hindlimb ischemia. Methods and Results: hCD31 + cells were isolated using a magnetic bead separation technique. By FACS analysis, we confirmed that more than 98% of hCD31 + cells are positive for CD45, suggesting that hCD31 + cells are not circulating endothelial cells. In endothelial progenitor cell (EPC) culture assay, hCD31 + cells almost exclusively gave rise to EPCs and produced significantly higher colony forming activity than hCD31 - cells (n=5, each) (25.2 ± 2.2 vs. 3.6 ± 0.5/mm 2 , P+ cells revealed that hCD31 + cells robustly contributed to network formation with HUVECs suggestive of angio-vasculogenic activity. Real-time PCR showed that hCD31 + cells express higher levels of angiogenic genes (fold increase: angiopoietin-1, 5.6 ± 0.8; HGF, 2.7 ± 0.6; VEGF-A, 3.8 ± 0.5; FGF-2, 2.0 ± 0.5) and lower levels of inflammatory genes (fold decrease: Interferonγ, 13.2 ± 2.5; TNFα, 2.3 ± 0.9) than hCD31 − cells. To investigate therapeutic potential, we surgically induced hindlimb ischemia in athymic nude mice, and injected hCD31 + cells, hCD31 − cells and PBS into the ischemic limbs (n=9, each). The hCD31 + treated group showed a higher limb salvage rate than other groups [total salvage/tip necrosis/amputation; hCD31 + cells 5/4/0, hCD31 − cells 2/5/2, PBS control 0/5/4] (P+ treated group compared to the hCD31 − and PBS treated groups. Histologic analysis demonstrated that a fraction of injected DiI-labeled hCD31 + cells exhibit endothelial phenotypes during the follow-up period of 8 weeks. Conclusion: The present study demonstrated that hCD31 + cells have robust angio-vasculogenic potential with low inflammatory activity, and enhanced post-ischemia recovery. These data suggested that hCD31 + cell transplantation may represent a novel therapeutic option for treating ischemic cardiovascular diseases.

Background— There has been a rapid increase in the number of clinical trials using unselected bone marrow (BM) cells or the mononuclear fraction of BM cells for treating ischemic heart diseases. Thus far, no significant deleterious effects or complications have been reported in any studies using BM-derived cells for treatment of various cardiac diseases. Methods and Results— Seven-week-old female Fisher-344 rats underwent surgery to induce acute myocardial infarction and were randomized into 3 groups of 16 rats, each receiving intramyocardial injection of either 7×10 5 DiI-labeled total BM cells (TBMCs), the same number of DiI-labeled, clonally expanded BM multipotent stem cells, or the same volume of phosphate-buffered saline in the peri-infarct area. Echocardiography 2 weeks after cell transplantation indicated intramyocardial calcification in 4 of 14 surviving rats (28.5%) in the TBMC group. Histological examination with hematoxylin and eosin staining and von Kossa staining confirmed the presence of extensive intramyocardial calcification. Alkaline phosphatase staining revealed strong positivity surrounding the calcified area suggestive of ongoing osteogenic activity. Fluorescent microscopic examination revealed that acellular calcific areas were surrounded by DiI-labeled TBMCs, suggesting the direct involvement of transplanted TBMCs in myocardial calcification. In contrast, in hearts receiving equal volumes of saline or BM multipotent stem cells delivered in the same manner, there was no evidence of calcification. Conclusions— These results demonstrate that direct transplantation of unselected BM cells into the acutely infarcted myocardium may induce significant intramyocardial calcification.