Neuropilin 2 Signaling Mediates Corticostriatal Transmission, Spine Maintenance, and Goal-Directed Learning in Mice.

The striatum represents the main input structure of the basal ganglia, receiving massive excitatory input from the cortex and the thalamus. The development and maintenance of cortical input to the striatum is crucial for all striatal function including many forms of sensorimotor integration, learning, and action control. The molecular mechanisms regulating the development and maintenance of corticostriatal synaptic transmission are unclear. Here we show that the guidance cue, Semaphorin 3F and its receptor Neuropilin 2 (Nrp2), influence dendritic spine maintenance, corticostriatal short-term plasticity, and learning in adult male and female mice. We found that Nrp2 is enriched in adult layer V pyramidal neurons, corticostriatal terminals, and in developing and adult striatal spiny projection neurons (SPNs). Loss of Nrp2 increases SPN excitability and spine number, reduces short-term facilitation at corticostriatal synapses, and impairs goal-directed learning in an instrumental task. Acute deletion of Nrp2 selectively in adult layer V cortical neurons produces a similar increase in the number of dendritic spines and presynaptic modifications at the corticostriatal synapse in the Nrp2-/- mouse, but does not affect the intrinsic excitability of SPNs. Furthermore, conditional loss of Nrp2 impairs sensorimotor learning on the accelerating rotarod without affecting goal-directed instrumental learning. Collectively, our results identify Nrp2 signaling as essential for the development and maintenance of the corticostriatal pathway and may shed novel insights on neurodevelopmental disorders linked to the corticostriatal pathway and Semaphorin signaling.SIGNIFICANCE STATEMENT The corticostriatal pathway controls sensorimotor, learning, and action control behaviors and its dysregulation is linked to neurodevelopmental disorders, such as autism spectrum disorder (ASD). Here we demonstrate that Neuropilin 2 (Nrp2), a receptor for the axon guidance cue semaphorin 3F, has important and previously unappreciated functions in the development and adult maintenance of dendritic spines on striatal spiny projection neurons (SPNs), corticostriatal short-term plasticity, intrinsic physiological properties of SPNs, and learning in mice. Our findings, coupled with the association of Nrp2 with ASD in human populations, suggest that Nrp2 may play an important role in ASD pathophysiology. Overall, our work demonstrates Nrp2 to be a key regulator of corticostriatal development, maintenance, and function, and may lead to better understanding of neurodevelopmental disease mechanisms.

Accelerated cardiomyocyte senescence contributes to late-onset doxorubicin-induced cardiotoxicity.

Children surviving cancer and chemotherapy are at risk for adverse health events including heart failure that may be delayed by years. Although the early effects of doxorubicin-induced cardiotoxicity may be attributed to a direct effect on the cardiomyocytes, the mechanisms underlying the delayed or late effects (8-20 yr) are unknown. The goal of this project was to develop a model of late-onset doxorubicin-induced cardiotoxicity to better delineate the underlying pathophysiology responsible. The underlying hypothesis was that doxorubicin-induced "late-onset cardiotoxicity" was the result of mitochondrial dysfunction leading to cell failure and death. Wistar rats, 3-4 wk of age, were randomly assigned to vehicle or doxorubicin injection groups (1-45 mg/kg). Cardiovascular function was unaltered at the lower dosages (1-15 kg/mg), but beginning at 6 mo after injection significant cardiac degradation was observed in the 45 mg/kg group. Doxorubicin significantly increased myocardial mitochondrial DNA (mtDNA) damage. In contrast, in isolated c-kit left ventricular (LV) cells, doxorubicin treatment did not increase mtDNA damage. Biomarkers of senescence within the LV were significantly increased, suggesting accelerated aging of the LV. Doxorubicin also significantly increased LV histamine content suggestive of mast cell activation. With the use of flow cytometry, a significant expansion of the c-kit and stage-specific embryonic antigen 1 cell populations within the LV were concomitant with significant decreases in the circulating peripheral blood population of these cells. These results are consistent with the concept that doxorubicin induced significant damage to the cardiomyocyte population and that although the heart attempted to compensate it eventually succumbed to an inability for self-repair.

Notch signaling modulates the electrical behavior of cardiomyocytes.

Notch receptor signaling is active during cardiac development and silenced in myocytes after birth. Conversely, outward K+ Kv currents progressively appear in postnatal myocytes leading to shortening of the action potential (AP) and acquisition of the mature electrical phenotype. In the present study, we tested the possibility that Notch signaling modulates the electrical behavior of cardiomyocytes by interfering with Kv currents. For this purpose, the effects of Notch receptor activity on electrophysiological properties of myocytes were evaluated using transgenic mice with inducible expression of the Notch1 intracellular domain (NICD), the functional fragment of the activated Notch receptor, and in neonatal myocytes after inhibition of the Notch transduction pathway. By patch clamp, NICD-overexpressing cells presented prolonged AP duration and reduced upstroke amplitude, properties that were coupled with reduced rapidly activating Kv and fast Na+ currents, compared with cells obtained from wild-type mice. In cultured neonatal myocytes, inhibition of the proteolitic release of NICD with a γ-secretase antagonist increased transcript levels of the Kv channel-interacting proteins 2 (KChIP2) and enhanced the density of Kv currents. Collectively, these results indicate that Notch signaling represents an important regulator of the electrophysiological behavior of developing and adult myocytes by repressing, at least in part, repolarizing Kv currents. NEW & NOTEWORTHY We investigated the effects of Notch receptor signaling on the electrical properties of cardiomyocytes. Our results indicate that the Notch transduction pathway interferes with outward K+ Kv currents, critical determinants of the electrical repolarization of myocytes.

Dysregulation of Neuropilin-2 Expression in Inhibitory Neurons Impairs Hippocampal Circuit Development Leading to Autism-Epilepsy Phenotype.

Dysregulation of development, migration, and function of interneurons, collectively termed interneuronopathies, have been proposed as a shared mechanism for autism spectrum disorders (ASDs) and childhood epilepsy. Neuropilin-2 (Nrp2), a candidate ASD gene, is a critical regulator of interneuron migration from the median ganglionic eminence (MGE) to the pallium, including the hippocampus. While clinical studies have identified Nrp2 polymorphisms in patients with ASD, whether dysregulation of Nrp2-dependent interneuron migration contributes to pathogenesis of ASD and epilepsy has not been tested. We tested the hypothesis that the lack of Nrp2 in MGE-derived interneuron precursors disrupts the excitation/inhibition balance in hippocampal circuits, thus predisposing the network to seizures and behavioral patterns associated with ASD. Embryonic deletion of Nrp2 during the developmental period for migration of MGE derived interneuron precursors (iCKO) significantly reduced parvalbumin, neuropeptide Y, and somatostatin positive neurons in the hippocampal CA1. Consequently, when compared to controls, the frequency of inhibitory synaptic currents in CA1 pyramidal cells was reduced while frequency of excitatory synaptic currents was increased in iCKO mice. Although passive and active membrane properties of CA1 pyramidal cells were unchanged, iCKO mice showed enhanced susceptibility to chemically evoked seizures. Moreover, iCKO mice exhibited selective behavioral deficits in both preference for social novelty and goal-directed learning, which are consistent with ASD-like phenotype. Together, our findings show that disruption of developmental Nrp2 regulation of interneuron circuit establishment, produces ASD-like behaviors and enhanced risk for epilepsy. These results support the developmental interneuronopathy hypothesis of ASD epilepsy comorbidity.

Dysregulation of Neuropilin-2 Expression in Inhibitory Neurons Impairs Hippocampal Circuit Development Leading to Autism-Epilepsy Phenotype.

Dysregulation of development, migration, and function of interneurons, collectively termed interneuronopathies, have been proposed as a shared mechanism for autism spectrum disorders (ASDs) and childhood epilepsy. Neuropilin-2 (Nrp2), a candidate ASD gene, is a critical regulator of interneuron migration from the median ganglionic eminence (MGE) to the pallium, including the hippocampus. While clinical studies have identified Nrp2 polymorphisms in patients with ASD, whether dysregulation of Nrp2-dependent interneuron migration contributes to pathogenesis of ASD and epilepsy has not been tested. We tested the hypothesis that the lack of Nrp2 in MGE-derived interneuron precursors disrupts the excitation/inhibition balance in hippocampal circuits, thus predisposing the network to seizures and behavioral patterns associated with ASD. Embryonic deletion of Nrp2 during the developmental period for migration of MGE derived interneuron precursors (iCKO) significantly reduced parvalbumin, neuropeptide Y, and somatostatin positive neurons in the hippocampal CA1. Consequently, when compared to controls, the frequency of inhibitory synaptic currents in CA1 pyramidal cells was reduced while frequency of excitatory synaptic currents was increased in iCKO mice. Although passive and active membrane properties of CA1 pyramidal cells were unchanged, iCKO mice showed enhanced susceptibility to chemically evoked seizures. Moreover, iCKO mice exhibited selective behavioral deficits in both preference for social novelty and goal-directed learning, which are consistent with ASD-like phenotype. Together, our findings show that disruption of developmental Nrp2 regulation of interneuron circuit establishment, produces ASD-like behaviors and enhanced risk for epilepsy. These results support the developmental interneuronopathy hypothesis of ASD epilepsy comorbidity.

Conditioned serum-free medium from umbilical cord mesenchymal stem cells has anti-photoaging properties.

Chronic exposure to solar radiation is the primary cause of photoaging and benign and malignant skin tumors. A conditioned serum-free medium (SFM) was prepared from umbilical cord mesenchymal stem cells (UC-MSCs) and its anti-photoaging effect, following chronic UV irradiation in vitro and in vivo, was evaluated. UC-MSC SFM had a stimulatory effect on human dermal fibroblast proliferation and reduced UVA-induced cell death. In addition, UC-MSC SFM blocked UVA inhibition of superoxide dismutase activity. Topical application of UC-MSC SFM to mouse skin prior to UV irradiation blocked the inhibition of superoxide dismutase and glutathione peroxidase activities, and prevented the upregulation of malonaldehyde. UC-MSC SFM thus protects against photoaging induced by UVA and UVB radiation and is a promising candidate for skin anti-photoaging treatments.

Inhibition of heart formation by lithium is an indirect result of the disruption of tissue organization within the embryo.

Lithium is a commonly used drug for the treatment of bipolar disorder. At high doses, lithium becomes teratogenic, which is a property that has allowed this agent to serve as a useful tool for dissecting molecular pathways that regulate embryogenesis. This study was designed to examine the impact of lithium on heart formation in the developing frog for insights into the molecular regulation of cardiac specification. Embryos were exposed to lithium at the beginning of gastrulation, which produced severe malformations of the anterior end of the embryo. Although previous reports characterized this deformity as a posteriorized phenotype, histological analysis revealed that the defects were more comprehensive, with disfigurement and disorganization of all interior tissues along the anterior-posterior axis. Emerging tissues were poorly segregated and cavity formation was decreased within the embryo. Lithium exposure also completely ablated formation of the heart and prevented myocardial cell differentiation. Despite the complete absence of cardiac tissue in lithium treated embryos, exposure to lithium did not prevent myocardial differentiation of precardiac dorsal marginal zone explants. Moreover, precardiac tissue freed from the embryo subsequent to lithium treatment at gastrulation gave rise to cardiac tissue, as demonstrated by upregulation of cardiac gene expression, display of sarcomeric proteins, and formation of a contractile phenotype. Together these data indicate that lithium's effect on the developing heart was not due to direct regulation of cardiac differentiation, but an indirect consequence of disrupted tissue organization within the embryo.

GATA6 reporter gene reveals myocardial phenotypic heterogeneity that is related to variations in gap junction coupling.

This study examined transgenic mice whose expression of a ß-galactosidase (lacZ) reporter is driven by a GATA6 gene enhancer. Previous investigations established that transcription of the transgene was associated with precardiac mesoderm and primary heart tube myocardium, which decreased progressively, so that its expression was no longer observed within ventricular myocardium by midgestation. Expression of this reporter in the adult was investigated for insights into myocyte homeostasis and cardiovascular biology. Morphometric analysis determined that <1% of myocytes, often found in small clusters, express this GATA6-associated reporter in the adult heart. LacZ expression was also found in the ascending aorta. Myocardial expression of the transgene was not associated with a proliferative phenotype or new myocyte formation, as lacZ-positive myocytes neither labeled with cell division markers nor following 5-bromodeoxyuridine pulse-chase experimentation. Despite exhibiting normal adherens junctions, these myocytes appeared to exhibit decreased connexin 43 gap junctions. Treatment with the gap junctional blocker heptanol both in vivo and in culture elevated myocardial ß-galactosidase activity, suggesting that deficient gap junctional communication underlies expression of the transgenic reporter. LacZ expression within the myocardium was also enhanced in response to cryoinjury and isoproterenol-induced hypertrophy. These results reveal a previously uncharacterized phenotypic heterogeneity in the myocardium and suggest that decreased gap junctional coupling leads to induction of a signaling pathway that utilizes a unique GATA6 enhancer. Upregulation of lacZ reporter gene expression following cardiac injury indicates this transgenic mouse may serve as a model for examining the transition of the heart from healthy to pathological states.

Reduced hippocampal inhibition and enhanced autism-epilepsy comorbidity in mice lacking neuropilin 2.

The neuropilin receptors and their secreted semaphorin ligands play key roles in brain circuit development by regulating numerous crucial neuronal processes, including the maturation of synapses and migration of GABAergic interneurons. Consistent with its developmental roles, the neuropilin 2 (Nrp2) locus contains polymorphisms in patients with autism spectrum disorder (ASD). Nrp2-deficient mice show autism-like behavioral deficits and propensity to develop seizures. In order to determine the pathophysiology in Nrp2 deficiency, we examined the hippocampal numbers of interneuron subtypes and inhibitory regulation of hippocampal CA1 pyramidal neurons in mice lacking one or both copies of Nrp2. Immunostaining for interneuron subtypes revealed that Nrp2-/- mice have a reduced number of parvalbumin, somatostatin, and neuropeptide Y cells, mainly in CA1. Whole-cell recordings identified reduced firing and hyperpolarized shift in resting membrane potential in CA1 pyramidal neurons from Nrp2+/- and Nrp2-/- mice compared to age-matched wild-type controls indicating decrease in intrinsic excitability. Simultaneously, the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs) are reduced in Nrp2-deficient mice. A convulsive dose of kainic acid evoked electrographic and behavioral seizures with significantly shorter latency, longer duration, and higher severity in Nrp2-/- compared to Nrp2+/+ animals. Finally, Nrp2+/- and Nrp2-/- but not Nrp2+/+, mice have impaired cognitive flexibility demonstrated by reward-based reversal learning, a task associated with hippocampal circuit function. Together these data demonstrate a broad reduction in interneuron subtypes and compromised inhibition in CA1 of Nrp2-/- mice, which could contribute to the heightened seizure susceptibility and behavioral deficits consistent with an ASD/epilepsy phenotype.

Toward cell therapy for vascular calcification: osteoclast-mediated demineralization of calcified elastin.

BACKGROUND: Elastin-oriented vascular calcification is a clinically significant feature, which involves formation of ectopic bone-like structures. Taking advantage of the similarities between arterial calcification and bone regulation, our hypothesis was that therapeutic approaches for limitation of vascular calcification could be developed using site-specific delivery of autologous osteoclasts. In the present paper, we tested the hypothesis that bone-marrow-derived osteoclasts have the ability to demineralize calcified elastin, without significant alterations in elastin integrity. METHODS: Active, multinucleated osteoclasts were obtained by in vitro maturation of rat bone-marrow-derived progenitor cells in the presence of vitamin D(3) and retinoic acid. Cell phenotype was validated by staining for tartrate-resistant acid phosphatase, formation of resorption pits on hydroxyapatite-coated disks, and RT-PCR for identification of cathepsin K gene expression. Calcified aortic elastin was seeded with osteoclasts and calcium, and phosphorous levels were monitored in gels and culture media to detect demineralization of elastin. Soluble elastin peptides were also monitored in culture media for elastin degradation. For in vivo experiments, pure aortic elastin was coimplanted with allogenic osteoclasts subdermally into rats, and the degree of elastin calcification and degradation was evaluated using mineral analysis and desmosine quantitation. RESULTS: Bone-marrow-derived osteoclasts reduced mineral content of calcified elastin in vitro by 80%. Moreover, in vivo implantation of allogenic osteoclasts in the vicinity of calcifying elastin limited elastin mineralization by almost 50%, in the absence of detectable elastin degradation. CONCLUSIONS: Osteoclasts have the ability to demineralize calcified elastin, without significant alterations in elastin integrity.

Evaluating the role of Wnt signal transduction in promoting the development of the heart.

Wnts are a family of secreted signaling proteins that are encoded by 19 distinct genes in the vertebrate genome. These molecules initiate several signal transduction pathways: the canonical Wnt, Wnt/Ca2+, and Wnt/planar cell polarity pathways. Wnt proteins have major impact on embryonic development, tumor progression, and stem cell differentiation. Wnt signal transduction also influences the formation of the heart, yet many issues concerning the involvement of Wnt regulation in initiating cardiac development remain unresolved. In this review, we will examine the published record to discern (a) what has been shown by experimental studies on the participation of Wnt signaling in cardiogenesis, and (b) what are the important questions that need to be addressed to understand the importance and function of Wnt signal transduction in facilitating the development of the heart.

Inhibition of Histone Methyltransferase, Histone Deacetylase, and ß-Catenin Synergistically Enhance the Cardiac Potential of Bone Marrow Cells.

Previously, we reported that treatment with the G9a histone methyltransferase inhibitor BIX01294 causes bone marrow mesenchymal stem cells (MSCs) to exhibit a cardiocompetent phenotype, as indicated by the induction of the precardiac markers Mesp1 and brachyury. Here, we report that combining the histone deacetylase inhibitor trichostatin A (TSA) with BIX01294 synergistically enhances MSC cardiogenesis. Although TSA by itself had no effect on cardiac gene expression, coaddition of TSA to MSC cultures enhanced BIX01294-induced levels of Mesp1 and brachyury expression 5.6- and 7.2-fold. Moreover, MSCs exposed to the cardiogenic stimulus Wnt11 generated 2.6- to 5.6-fold higher levels of the cardiomyocyte markers GATA4, Nkx2.5, and myocardin when pretreated with TSA in addition to BIX01294. MSC cultures also showed a corresponding increase in the prevalence of sarcomeric protein-positive cells when treated with these small molecule inhibitors. These results correlated with data showing synergism between (1) TSA and BIX01294 in promoting acetylation of lysine 27 on histone H3 and (2) BIX01294 and Wnt11 in decreasing ß-catenin accumulation in MSCs. The implications of these findings are discussed in light of observations in the early embryo on the importance of ß-catenin signaling and histone modifications for cardiomyocyte differentiation and heart development.

Embryonic myocardium shows increased longevity as a functional tissue when cultured in the presence of a noncardiac tissue layer.

A major aim of regenerative medicine is the construction of bioengineered organs and tissue for transplantation into human patients; yet living tissue is dynamic, and thus arranging cellular and extracellular constituents into an architecture resembling normal adult organs may not be sufficient to maintain tissue stability. In this study, we used cultures of embryonic chick heart tissue as a model to explore how newly formed cardiac tissue constructs can sustain their morphological structure and functional capabilities over extended periods. During the initial days of incubation, embryonic cardiac explants will thrive as beating three-dimensional tissue aggregates. However, within the first week of culture, cardiac aggregates lose their contractile function and flatten. After 2 weeks of incubation, the cardiac cells will have spread out into a homogeneous monolayer and dedifferentiated to a noncardiac phenotype. In contrast, when the embryonic heart tissue was co-cultured with a noncardiac cell layer obtained from adult bone marrow, the cardiac aggregates maintained their contractile function, three-dimensional tissue morphology, and myocyte phenotype for a full month of incubation. The capacity of this noncardiac cell layer to sustain the phenotype and morphology of the cardiac explants was partially replicated by treatment of the heart tissue with conditioned media from bone marrow cells. These findings are discussed in regard to the importance of adjacent cell layers for facilitating organogenesis in the developing embryo and having potential utility in producing stable bioengineered tissue constructs.

Fabrication of tubular tissue constructs by centrifugal casting of cells suspended in an in situ crosslinkable hyaluronan-gelatin hydrogel.

Achieving the optimal cell density and desired cell distribution in scaffolds is a major goal of cell seeding technologies in tissue engineering. In order to reach this goal, a novel centrifugal casting technology was developed using in situ crosslinkable hyaluronan-based (HA) synthetic extracellular matrix (sECM). Living cells were suspended in a viscous solution of thiol-modified HA and thiol-modified gelatin, a polyethyleneglycol diacrylate crosslinker was added, and a hydrogel was formed during rotation. The tubular tissue constructs consisting of a densely packed cell layer were fabricated with the rotation device operating at 2000 rpm for 10 min. The majority of cells suspended in the HA mixture before rotation were located inside the layer after centrifugal casting. Cells survived the effect of the centrifugal forces experienced under the rotational regime employed. The volume cell density (65.6%) approached the maximal possible volume density based on theoretical sphere packing models. Thus, centrifugal casting allows the fabrication of tubular constructs with the desired redistribution, composition and thickness of cell layers that makes the maximum efficient use of available cells. Centrifugal casting in this sECM would enable rapid fabrication of tissue-engineered vascular grafts, as well as other tubular and planar tissue-engineered constructs.

Inhibition of G9a Histone Methyltransferase Converts Bone Marrow Mesenchymal Stem Cells to Cardiac Competent Progenitors.

The G9a histone methyltransferase inhibitor BIX01294 was examined for its ability to expand the cardiac capacity of bone marrow cells. Inhibition of G9a histone methyltransferase by gene specific knockdown or BIX01294 treatment was sufficient to induce expression of precardiac markers Mesp1 and brachyury in bone marrow cells. BIX01294 treatment also allowed bone marrow mesenchymal stem cells (MSCs) to express the cardiac transcription factors Nkx2.5, GATA4, and myocardin when subsequently exposed to the cardiogenic stimulating factor Wnt11. Incubation of BIX01294-treated MSCs with cardiac conditioned media provoked formation of phase bright cells that exhibited a morphology and molecular profile resembling similar cells that normally form from cultured atrial tissue. Subsequent aggregation and differentiation of BIX01294-induced, MSC-derived phase bright cells provoked their cardiomyogenesis. This latter outcome was indicated by their widespread expression of the primary sarcomeric proteins muscle α-actinin and titin. MSC-derived cultures that were not initially treated with BIX01294 exhibited neither a commensurate burst of phase bright cells nor stimulation of sarcomeric protein expression. Collectively, these data indicate that BIX01294 has utility as a pharmacological agent that could enhance the ability of an abundant and accessible stem cell population to regenerate new myocytes for cardiac repair.

An in vitro analysis of myocardial potential indicates that phenotypic plasticity is an innate property of early embryonic tissue.

Explants from gastrula-stage avian embryos have provided an important culture model for examining the formation of the vertebrate heart. Explants harvested from anterior regions containing the precardiac mesoderm faithfully recapitulate cardiogenesis and generate contractile tissue in culture. Posterior regions of the early embryo do not supply cellular material to the developing heart in situ, and thus have been commonly employed as negative control tissues for studying cardiogenic induction. To begin to understand the cellular mechanisms that account for the distinct cell fates of precardiac and posterior tissue within the embryo, we undertook a comprehensive investigation on the myocardial potential of presumptive noncardiac tissue. Myocardial differentiation was assayed by expression of the myocardium-associated transcription factor gene Nkx2.5 and positive immunostaining for sarcomeric myosin, muscle alpha-actinin, and smooth muscle alpha-actin. Our results demonstrate that regions of the early embryo that do not provide a cellular contribution to the myocardium in situ are capable of generating myocardial tissue when removed from their normal embryonic environment and placed in culture under nontreated conditions. Although treatment with the presumptive cardiac inducer Dickkopf-1 increased the frequency that cardiac tissue appeared within cultures of posterior tissue, no difference was observed in either the size or morphology of the myocardium-positive areas among treated and nontreated explants. These findings suggest that progenitor cells within the early embryo possess an innate phenotypic plasticity and that presumptive cardiac inducing signals do not induce cardiac differentiation but instead augment a pre-existing cardiac potential of embryonic tissue.

Adult stem cells and their cardiac potential.

Adult cardiac muscle is unable to repair itself following severe disease or injury. Because of this fundamental property of the myocardium, it was long believed that the adult myocardium is a postmitotic tissue. Yet, recent studies have indicated that new cardiac myocytes are generated throughout the life span of an adult and that extracardiac cells can contribute to the renewal of individual cells within the myocardium. In addition, investigations of the phenotypic capacity of adult stem cells have suggested that their potential is not solely restricted to the differentiated cell phenotypes of the source tissue. These observations have great implications for cardiac biology, as stem cells obtained from the bone marrow and other readily accessible adult tissues may serve as a source of replacement cardiac myocytes. In this review, we describe the evidence for these new findings and discuss their implications in context of the continuing controversy over stem cell plasticity.

Hematopoietic cells from bone marrow have the potential to differentiate into cardiomyocytes in vitro.

Recent studies have indicated that hematopoietic progenitor cells (HPCs) have the capacity to form cardiomyocytes. In the present study, we further examined the cardiac competence of HPCs by asking whether these cells by themselves can be provoked to undergo cardiac differentiation. Our data indicate that in response to growth factor treatment, HPCs from avian bone marrow (BM) can undergo cardiac differentiation, as indicated by their expression of multiple cardiac transcription factors and sarcomeric proteins. Furthermore, coculture experiments with adult mouse BM cells and embryonic heart tissue confirmed that HPCs are able to both integrate into cardiac tissue and differentiate into cardiomyocytes. In an additional set of experiments, we investigated whether other hematopoietic populations might possess cardiac potential by examining whether blood cells that normally are recruited to damaged tissue might act as a source of newly generated cardiomyocytes. Remarkably, macrophages cocultured with cardiac explants also demonstrated an ability to integrate into contractile heart tissue and undergo cardiac differentiation. Thus, our data suggest that the capacity of blood cells to transdifferentiate into cardiomyocytes is not limited to classically defined hematopoietic progenitors.

Stem cells and the formation of the myocardium in the vertebrate embryo.

A major goal in cardiovascular biology is to repair diseased or damaged hearts with newly generated myocardial tissue. Stem cells offer a potential source of replacement myocytes for restoring cardiac function. Yet little is known about the nature of the cells that are able to generate myocardium and the conditions they require to form heart tissue. A source of information that may be pertinent to addressing these issues is the study of how the myocardium arises from progenitor cells in the early vertebrate embryo. Accordingly, this review will examine the initial events of cardiac developmental biology for insights into the identity and characteristics of the stem cells that can be used to generate myocardial tissue for therapeutic purposes.