Bone marrow-derived fibroblast precursors mediate ischemic cardiomyopathy in mice

We previously described a mouse model of fibrotic ischemia/reperfusion cardiomyopathy (I/RC) arising from daily, brief coronary occlusion. One characteristic of I/RC was the prolonged elevation of monocyte chemoattractant protein 1 (MCP-1), which was obligate to its phenotype and may contribute to the uptake of bloodborne cells. Here we describe in I/RC hearts a population of small spindle-shaped fibroblasts that were highly proliferative and expressed collagen I and alpha-smooth muscle actin (myofibroblast markers), CD34 (a precursor marker), and CD45 (a hematopoietic marker). These cells represented 3% of all nonmyocyte live cells. To confirm the cells' bone marrow origin, chimeric mice were created by the rescue of irradiated C57BL/6 mice with marrow from ROSA26, a congenic line expressing lacZ. I/RC resulted in a large population of spindle-shaped fibroblasts containing lacZ. We postulated that the fibroblast precursors represented a developmental path for a subset of monocytes, whose phenotype we have shown to be influenced by serum amyloid P (SAP). Thus, we administered SAP in vivo, which markedly reduced the number of proliferative spindle-shaped fibroblasts and completely prevented I/RC-induced fibrosis and global ventricular dysfunction. By contrast, SAP did not suppress the inflammation or chemokine expression seen in I/RC. SAP, a member of the pentraxin family, binds to Fcgamma receptors and modifies the pathophysiological function of monocytes. Our data suggest that SAP interferes with assumption of a fibroblast phenotype in a subset of monocytes and that SAP may be an important regulator in the linkage between inflammation and nonadaptive fibrosis in the heart.

The role of platelet-derived growth factor signaling in healing myocardial infarcts

OBJECTIVES

This study sought to examine the role of platelet-derived growth factor (PDGF) signaling in healing myocardial infarcts.

BACKGROUND

Platelet-derived growth factor isoforms exert potent fibrogenic effects through interactions with PDGF receptor (PDGFR)-alpha and PDGFR-beta. In addition, PDGFR-beta signaling mediates coating of developing vessels with mural cells, leading to the formation of a mature vasculature. We hypothesized that PDGFR activation may regulate fibrosis and vascular maturation in healing myocardial infarcts.

METHODS

Mice undergoing reperfused infarction protocols were injected daily with a neutralizing anti-PDGFR-beta antibody (APB5), an anti-PDGFR-alpha antibody (APA5), or control immunoglobulin G, and were killed after 7 days of reperfusion.

RESULTS

The PDGF-B, PDGFR-alpha, and PDGFR-beta mRNA expression was induced in reperfused mouse infarcts. Perivascular cells expressing phosphorylated PDGFR-beta were identified in the infarct after 7 days of reperfusion, indicating activation of the PDGF-BB/PDGFR-beta pathway. The PDGFR-beta blockade resulted in impaired maturation of the infarct vasculature, enhanced capillary density, and formation of dilated uncoated vessels. Defective vascular maturation in antibody-treated mice was associated with increased and prolonged extravasation of red blood cells and monocyte/macrophages, suggesting increased permeability. These defects resulted in decreased collagen content in the healing infarct. In contrast, PDGFR-alpha inhibition did not affect vascular maturation, but significantly decreased collagen deposition in the infarct.

CONCLUSIONS

Platelet-derived growth factor signaling critically regulates postinfarction repair. Both PDGFR-beta- and PDGFR-alpha-mediated pathways promote collagen deposition in the infarct. Activation of PDGF-B/PDGFR-beta is also involved in recruitment of mural cells by neovessels, regulating maturation of the infarct vasculature. Acquisition of a mural coat and maturation of the vasculature promotes resolution of inflammation and stabilization of the scar.

Signaling mechanism of renal fibrosis in unilateral ureteral obstructive kidney disease in ROCK1 knockout mice

It has been shown that blockade of Rho kinase with pharmacologic inhibitors inhibits renal fibrosis. This study examined the role of Rho kinase in renal fibrosis in the unilateral ureteral obstruction (UUO) model in mice that do not express the ROCK1 gene, a critical downstream mediator of Rho GTPase. Unexpected, real-time PCR, Western blot, and immunohistochemistry demonstrated that, compared with the wild-type mice, mice with ROCK1 knockout (KO) were not protected against renal fibrosis at both the early (day 5) and late (day 10) UUO, as determined by histology and expression of both mRNA and protein levels of alpha-smooth muscle actin, collagen types I and III, and fibronectin within the diseased kidney. Then the mechanisms of loss of protective effect on renal fibrosis in ROCK1 KO mice were investigated. It is interesting that mice that lacked ROCK1 did not have altered expression of ROCK2 but significantly increased TGF-beta expression and Smad2/3 activation (phosphorylation and nuclear translocation) in the diseased kidney at day 5, which remained high at day 10 of UUO. Similarly, primary cultures of kidney fibroblasts that were obtained from both ROCK1 wild-type and KO mice showed that deletion of ROCK1 did not prevent TGF-beta-induced activation of Smad2/3 and collagen I expression. This also was observed in the presence of Rho kinase inhibitor Y-27632. Taken together, results from this study suggest that Rho/Rho kinase may not be a necessary or a central pathway for renal fibrosis in the UUO model. The interplay between the Rho/Rho kinase pathway and the Smad signaling pathway may be a key mechanism by which loss of ROCK1 does not prevent renal fibrosis in the UUO model.

Activation of Rho-associated coiled-coil protein kinase 1 (ROCK-1) by caspase-3 cleavage plays an essential role in cardiac myocyte apoptosis

Rho-associated coiled-coil protein kinase 1 (ROCK-1) is a direct cleavage substrate of activated caspase-3, which is associated with heart failure. In the course of human heart failure, we found marked cleavage of ROCK-1 resulting in a 130-kDa subspecies, which was absent in normal hearts and in an equivalent cohort of patients with left ventricular assist devices. Murine cardiomyocytes treated with doxorubicin led to enhanced ROCK-1 cleavage and apoptosis, all of which was blocked by a caspase-3 inhibitor. In addition, a bitransgenic mouse model of severe cardiomyopathy, which overexpresses Gq protein and hematopoietic progenitor kinase-/germinal center kinase-like kinase, revealed the robust accumulation of the 130-kDa ROCK-1 cleaved fragment. This constitutively active ROCK-1 subspecies, when expressed in cardiomyocytes, led to caspase-3 activation, indicating a positive feed-forward regulatory loop. ROCK-1-dependent caspase-3 activation was coupled with the activation of PTEN and the subsequent inhibition of protein kinase B (Akt) activity, all of which was attenuated by siRNA directed against ROCK-1 expression. Similarly, ROCK-1-null mice (Rock-1(-/-)) showed a marked reduction in myocyte apoptosis associated with pressure overload. These data suggest an obligatory role for ROCK-1 cleavage in promoting apoptotic signals in myocardial hypertrophy and/or failure.

Determinants of cardiac electrophysiological properties in mice

INTRODUCTION

The transgenic mouse is a popular model for human inherited cardiac disease. Electrophysiology (EP) studies have recently been performed in transgenic mice to characterize the electrical phenotype of the heart. However, little is known regarding the impact of experimental conditions or model selection on the outcome of EP studies in mice.

METHODS AND RESULTS

We investigated the effects of experimental conditions on mouse cardiac EP by (1) comparing the findings of transesophageal pacing with those of invasive intracardiac pacing, (2) elucidating the effects of commonly used anesthetic agents, and (3) determining the impact of changes in body temperature. We also investigated the effects of model selection by (1) studying the dependence on mouse strain, and (2) exploring the effects of age. We found that EP parameters derived by both transesophageal and intracardiac pacing/recordings methods were similar. On the other hand, the anesthetic mixture of ketamine, xylazine, and acepromazine had profound effects on cardiac EP compared to sodium pentobarbital or isoflurane. Meanwhile, compared to normal body temperature (97-99 F), low body temperature (92-94 F) prolonged most cardiac EP parameters, while high body temperature (102-104 F) had little effect. Heart rate was a sensitive indicator of changes in body temperature. Significant differences were observed in specialized conduction system properties among the mouse strains studied (FVB, C57, and DBA). Furthermore, atrial electrical remodeling was evidently associated with age, while ventricular electrical properties were virtually unaltered. In comparison with corresponding invasive EP parameters, we found that the QT interval was not a reliable EP index in the mouse.

CONCLUSIONS

Cardiac EP variability may result from differences in experimental techniques including anesthesia and body temperature and from differences in mouse selection including strain and age. The influence of these factors should be considered when characterizing the electrical phenotype of transgenic mice in cardiovascular research.

The role of natural IgM in myocardial ischemia-reperfusion injury

Myocardial ischemia-reperfusion injury represents a combination of factors, namely the intrinsic cellular response to ischemia and the extrinsic acute inflammatory response. Recent studies in mesenteric and skeletal muscle reperfusion models identified natural IgM as a major initiator of pathology through the activation of the complement system and inflammatory cells. To determine whether a similar mechanism is involved in myocardial tissues, mice bearing an altered natural IgM repertoire (Cr2-/-) were examined in a murine model of coronary artery ischemia. Notably, these mice were significantly protected based on the reduced infarct size, limited apoptosis of cardiomyocytes, and decreased neutrophil infiltration. Protection was IgM-dependent as reconstitution of these mice with wild-type IgM restored myocardial reperfusion injury. These results support a model in which natural IgM initiates the acute inflammatory response in the myocardium following ischemia and reperfusion.

Induction of antioxidant gene expression in a mouse model of ischemic cardiomyopathy is dependent on reactive oxygen species

Ischemia and reperfusion (I/R) are characterized by oxidative stress as well as changes in the antioxidant enzymes of the heart. However, little is known about the transcriptional regulation of myocardial antioxidant enzymes in repetitive I/R and hibernating myocardium. In a mouse model of ischemic cardiomyopathy induced by repetitive I/R, we postulated that induction of antioxidant gene expression was dependent on reactive oxygen species (ROS). Repetitive closed-chest I/R (15 min) was performed daily in C57/BL6 mice and in mice overexpressing extracellular superoxide dismutase (EC-SOD). Antioxidant enzyme expression was measured at 3, 5, 7, and 28 days of repetitive I/R as well as 15 and 30 days after discontinuation of I/R. In order to determine whether ROS directly modulates antioxidant gene expression, transcript levels were measured in cardiomyocytes exposed to hydrogen peroxide. Repetitive I/R caused an early and sustained increase in glutathione peroxidase (GPX) transcript levels, while heme oxygenase-1 (HO-1) expression increased only after 7 days of repetitive I/R. Overexpression of EC-SOD prevented the upregulation of GPX and HO-1 transcript levels by repetitive I/R, suggesting that both genes are regulated by ROS. However, while HO-1 transcript levels increased in cardiomyocytes exposed to hydrogen peroxide, oxidative stress failed to induce the expression of GPX implying that ROS regulates GPX transcript levels only indirectly in repetitive I/R. In conclusion, repetitive I/R was associated with an early upregulation of GPX expression as well as a delayed increase of HO-1 transcript levels in the heart. The induction of both antioxidant genes was dependent on ROS, suggesting that alterations in redox balance mediate not only tissue injury but also components of "programmed cell survival" in hibernating myocardium.

Targeted deletion of ROCK1 protects the heart against pressure overload by inhibiting reactive fibrosis

Ventricular myocyte hypertrophy is an important compensatory growth response to pressure overload. However, pathophysiological cardiac hypertrophy is accompanied by reactive fibrosis and remodeling. The Rho kinase family, consisting of ROCK1 and ROCK2, has been implicated in cardiac hypertrophy and ventricular remodeling. However, these previous studies relied heavily on pharmacological inhibitors,and not on gene deletion. Here we used ROCK1knockout (ROCK1-/-) mice to investigate role of ROCK1 in the development of ventricular remodeling induced by transverse aortic banding. We observed that ROCK1 deletion did not impair compensatory hypertrophic response induced by pressure overload. However, ROCK1-/- mice exhibited reduced perivascular and interstitial fibrosis, which was observed at 3 wk but not at 1 wk after the banding. The reduced fibrosis in the myocardium of ROCK1-/- mice was closely associated with reduced expression of a variety of extracellular matrix (ECM) proteins and fibrogenic cytokines such as TGFbeta2 and connective tissue growth factor. This inhibitory effect of ROCK1 deletion on pathophysiological induction of fibrogenic cytokines was further confirmed in the myocardium of transgenic mice with cardiomyocyte-specific overexpression of Gq. Thus, these results indicate that ROCK1 contributes to the development of cardiac fibrosis and induction of fibrogenic cytokines in cardiomyocytes in response to pathological stimuli.

Effects of diet-induced obesity on inflammation and remodeling after myocardial infarction

Epidemiological studies indicate that obesity, insulin resistance, and diabetes are important comorbidities of patients with ischemic heart disease and increase mortality and development of congestive heart failure after myocardial infarction. Although ob/ob and db/db mice are commonly used to study obesity with insulin resistance or diabetes, mutations in the leptin gene or its receptor are rarely the cause of obesity in humans, which is, instead, primarily a consequence of dietary and lifestyle factors. Therefore, we used a murine model of diet-induced obesity to examine the physiological effects of obesity and the inflammatory and healing response of diet-induced obese (DIO) mice after myocardial ischemia-reperfusion injury. DIO mice developed hyperinsulinemia and insulin resistance and hepatic steatosis, with significant ectopic lipid deposition in the heart and cardiac hypertrophy in the absence of significant changes in blood pressure. The mRNA levels of chemokines at 24 h and cytokines at 24 and 72 h of reperfusion were higher in DIO than in lean mice. In granulation tissue at 72 h of reperfusion, macrophage density was significantly increased, whereas neutrophil density was reduced, in DIO mice compared with lean mice. At 7 days of reperfusion, collagen deposition in the scar was significantly reduced and left ventricular (LV) dilation and cardiac hypertrophy were increased, indicative of adverse LV remodeling, in infarcted DIO mice. Characterization of a murine diet-induced model of obesity and insulin resistance that satisfies many aspects commonly observed in human obesity allows detailed examination of the adverse cardiovascular effects of diet-induced obesity at the molecular level.

Extracellular matrix remodeling in canine and mouse myocardial infarcts

Extracellular matrix proteins not only provide structural support, but also modulate cellular behavior by activating signaling pathways. Healing of myocardial infarcts is associated with dynamic changes in the composition of the extracellular matrix; these changes may play an important role in regulating cellular phenotype and gene expression. We examined the time course of extracellular matrix deposition in a canine and mouse model of reperfused infarction. In both models, myocardial infarction resulted in fragmentation and destruction of the cardiac extracellular matrix, extravasation of plasma proteins, such as fibrinogen and fibronectin, and formation of a fibrin-based provisional matrix providing the scaffold for the infiltration of granulation tissue cells. Lysis of the plasma-derived provisional matrix was followed by the formation of a cell-derived network of provisional matrix composed of cellular fibronectin, laminin, and hyaluronic acid and containing matricellular proteins, such as osteopontin and osteonectin/SPARC. Finally, collagen was deposited in the infarct, and the wound matured into a collagen-based scar with low cellular content. Although the canine and mouse infarcts exhibited a similar pattern of extracellular matrix deposition, deposition of the provisional matrix was more transient in the mouse infarct and was followed by earlier formation of a mature collagen-based scar after 7-14 days of reperfusion; at the same timepoint, the canine infarct was highly cellular and evolving. In addition, mature mouse infarcts showed limited collagen deposition and significant tissue loss leading to the formation of a thin scar. In contrast, dogs exhibited extensive collagen accumulation in the infarcted area. These species-specific differences in infarct wound healing should be taken into account when interpreting experimental infarction studies and when attempting to extrapolate the findings to the human pathological process.

Oncostatin M differentially regulates CXC chemokines in mouse cardiac fibroblasts

Ischemia-reperfusion injury in the heart is characterized by marked infiltration of neutrophils in the myocardial interstitial space. Studies in human, canine, and murine models have revealed oncostatin M (OSM) expression in infiltrating leukocytes. In an effort to assess possible roles of OSM in the myocardium, we used cardiac fibroblasts (mCFs) isolated from adult mouse heart to determine whether recombinant murine OSM regulates the synthesis and release of MIP2/CXCL2, KC/CXCL1, and LIX/CXCL5, which are three potent neutrophil chemoattractants in the mouse. Our results demonstrate that mCFs express OSM receptors and that, within the IL-6 cytokine family, OSM uniquely induces significant release of KC and LIX in mCFs. In addition, although OSM activates the JAK-signal transducers and activators of transcription and MAPK pathways, we demonstrate that the OSM-mediated CXC chemokine release in mCFs is also dependent on the activation of the phosphatidylinositol 3-kinase pathway.

Identification of mast cells in the cellular response to myocardial infarction

Myocardial infarction is associated with an acute inflammatory response, leading to replacement of injured cardiomyocytes with granulation tissue. Mast cells are actively involved in postinfarction inflammation by releasing histamine and tumor necrosis factor-alpha, triggering a cytokine cascade. During the proliferative phase of healing, mast cells accumulate in the infarct and may regulate fibrous tissue deposition and angiogenesis by releasing growth factors, angiogenic mediators, and proteases. This chapter describes simple and reliable methods used to identify mast cells in control and infarcted canine hearts. Toluidine blue staining, labeling with conjugated avidin, and tryptase histochemistry are useful in the detection of mast cells in canine tissues. In the healing infarct, mast cells are associated with other cell types that are important for granulation tissue formation. We present immunohistochemical methods identifying monocytes, neutrophils, macrophages, endothelial cells, myofibroblasts, and smooth muscle cells in dog infarcts. These techniques are useful tools for pathological studies in canine models.

Pulsed Doppler Signal Processing for Use in Mice: Applications

We have developed a high-frequency, high-resolution Doppler spectrum analyzer (DSPW) and compared its performance against an adapted clinical Medasonics spectrum analyzer (MSA) and a zero-crossing interval histogram (ZCIH) used previously by us to evaluate cardiovascular physiology in mice. The aortic velocity (means +/- SE: 92.7 +/- 2.5 versus 82.2 +/- 1.8 cm/s) and aortic acceleration (8194 +/- 319 versus 5178 +/- 191 cm/s2) determined by the DSPW were significantly higher compared to those by the MSA. Aortic ejection time was shorter (48.3 +/- 0.9 versus 64.6 +/- 1.8 ms) and the isovolumic relaxation was longer (17.6 +/- 0.6 versus 13.5 +/- 0.6 ms) when determined by the DSPW because it generates shorter temporal widths in the velocity spectra when compared to the MSA. These data indicate that the performance of the DSPW in evaluating cardiovascular physiology was better than that of the MSA. There were no significant differences between the aortic pulse wave velocity determined by using the ZCIH (391 +/- 16 cm/s) and the DSPW (394 +/- 20 cm/s). Besides monitoring cardiac function, we have used the DSPW for studying peripheral vascular physiology in normal, transgenic, and surgical models of mice. Several applications such as the detection of high stenotic jet velocities (> 4 m/s), vortex shedding frequencies (250 Hz), and subtle changes in wave shapes in peripheral vessels which could not obtained with clinical Doppler systems are now made possible with the DSPW.

Regulatable atrial natriuretic peptide gene therapy for hypertension

Hypertension (HTN) is a disease that begins with dysfunctional renal-sodium excretion and progresses to a syndrome of highly elevated systolic, diastolic, and mean arterial pressures. Inadequacies in the therapy of HTN have led to the investigation of the gene therapy of this disease by using systemic overproduction of vasodilatory peptides, such as atrial natriuretic peptide (ANP). However, gene-therapy approaches to HTN using ANP are limited by the need for long-term ANP gene expression and, most important, control of ANP gene expression. Here, we introduce a helper-dependent adenoviral vector carrying the mifepristone (Mfp)-inducible gene-regulatory system to control in vivo ANP expression. In the BPH/2 mouse model of HTN, Mfp-inducible ANP expression was seen for a period of >120 days after administration of vector. Physiological effects of ANP, including decreased systolic blood pressure, increased urinary cGMP output, and decreases in heart weight as a percentage of body weight were also under the control of Mfp. Given these capabilities, this vector represents a paradigm for the gene therapy of HTN.

Downregulation of peroxisome proliferator-activated receptor-alpha gene expression in a mouse model of ischemic cardiomyopathy is dependent on reactive oxygen species and prevents lipotoxicity

BACKGROUND

The peroxisome proliferators-activated receptor-alpha (PPARalpha), a transcription factor that modulates fatty acid metabolism, regulates substrate preference in the heart. Although in acute ischemia there is a switch in substrate preference from fatty acids to glucose, metabolic gene expression in repetitive ischemia is not well described. In a mouse model of ischemic cardiomyopathy induced by repetitive ischemia/reperfusion (I/R), we postulated that downregulation of PPARalpha is regulated by reactive oxygen species and is necessary for maintaining contractile function in the heart.

METHODS AND RESULTS

Repetitive closed-chest I/R (15 minutes) was performed daily in C57/BL6 mice, mice overexpressing extracellular superoxide dismutase, and mice treated with the PPARalpha agonist-WY-14,643. Echocardiography, histology, and candidate gene expression were measured at 3, 5, 7, and 28 days of repetitive I/R and 15 and 30 days after discontinuation of I/R. Repetitive I/R was associated with a downregulation of PPARalpha-regulated genes and both myosin heavy chain isoform transcript levels, which was reversible on discontinuation of I/R. Overexpression of EC-SOD prevented the downregulation of PPARalpha-regulated genes and myosin iso-genes by repetitive I/R. Furthermore, reactivation of PPARalpha in mice exposed to repetitive I/R worsened contractile function, induced microinfarctions, and increased intramyocardial triglyceride deposition, features suggestive of cardiac lipotoxicity.

CONCLUSIONS

Metabolic and myosin isoform gene expression in repetitive I/R is mediated by reactive oxygen species. Furthermore, we suggest that downregulation of PPARalpha in repetitive I/R is an adaptive mechanism that is able to prevent lipotoxicity in the ischemic myocardium.

Chemokines in Myocardial Ischemia

Chemokine expression is markedly upregulated in healing myocardial infarcts and may play an important role in regulating leukocyte infiltration and activity and in modulating infarct angiogenesis as well as fibrous tissue deposition. The CC chemokine monocyte chemoattractant protein-1/CCL2 has important effects in infarct healing. Monocyte chemoattractant protein-1 -/- mice exhibit reduced macrophage infiltration and activation, suppressed cytokine synthesis, delayed phagocytotic removal of dead cardiomyocytes, diminished myofibroblast accumulation, and decreased ventricular remodeling after myocardial infarction. Monocyte chemoattractant protein-1 may also play an important role in the development of interstitial fibrosis in ischemic noninfarctive cardiomyopathy. CXC chemokines are also induced in healing infarcts. Interleukin-8/CXCL8 may mediate neutrophil recruitment and activation and may promote neovessel formation, whereas induction of the angiostatic and antifibrotic chemokine interferon-gamma-inducible protein-10/CXCL10 may serve to prevent premature wound angiogenesis and fibrous tissue deposition in the infarct, until the injured myocardium has been cleared from dead cells and debris and a fibrin-rich provisional matrix is formed. Understanding of the role of chemokines in myocardial ischemia may result in novel strategies in the treatment of patients with ischemic heart disease.

Critical Role of Endogenous Thrombospondin-1 in Preventing Expansion of Healing Myocardial Infarcts

Background— Matricellular proteins are extracellular matrix proteins that do not contribute directly to tissue integrity but are capable of modulating cell function. We hypothesized that the matricellular protein thrombospondin (TSP)-1, a potent inhibitor of angiogenesis and activator of transforming growth factor (TGF-β), is induced in healing myocardial infarcts and plays a role in suppressing the postinfarction inflammatory response, inhibiting local angiogenesis, and limiting expansion of granulation tissue into the noninfarcted area.

Methods and Results— We used a canine and a murine model of reperfused infarction. TSP-1 mRNA was induced in canine infarcts after 1 hour of ischemia and 3 to 7 days of reperfusion. TSP-1 protein showed a strikingly selective localization in the extracellular matrix, microvascular endothelium, and a subset of mononuclear cells of the infarct border zone after 5 to 28 days of reperfusion. Isolated canine venous endothelial cells showed low-level constitutive expression of TSP-1 mRNA, which was markedly induced by TGF-β, and basic fibroblast growth factor. Murine infarcts also had marked TSP-1 deposition in the border zone. Infarcted TSP-1 −/− mice exhibited sustained upregulation of the chemokines monocyte chemoattractant protein-1, macrophage inflammatory protein-1α, and interferon-γ–inducible protein-10/CXCL10 and the cytokines interleukin-1β, interleukin-6, and TGF-β, suggesting an enhanced and prolonged postinfarction inflammatory response. In addition, TSP-1 −/− mice had markedly increased macrophage and myofibroblast density in infarcts and in remodeling noninfarcted myocardial areas neighboring the myocardial scar, suggesting expansion of granulation tissue formation into the noninfarcted territory. TSP-1 −/− animals had more extensive postinfarction remodeling than wild-type mice, although infarct size was similar in both groups.

Conclusions— The infarct border zone may be capable of modulating the healing process through its unique extracellular matrix content. The selective endogenous expression of TSP-1 in the infarct border zone may serve as a “barrier,” limiting expansion of granulation tissue and protecting the noninfarcted myocardium from fibrotic remodeling.

CCL2/Monocyte Chemoattractant Protein-1 regulates inflammatory responses critical to healing myocardial infarcts

The CC chemokine Monocyte Chemoattractant Protein (MCP)-1/CCL2 has potent mononuclear cell chemo-attractant properties, modulates fibroblast and endothelial cell phenotype and may play an important role in wound healing. In order to examine whether MCP-1 critically regulates myocardial infarct healing, we studied the effects of MCP-1 gene disruption and antibody neutralization in a closed-chest model of reperfused murine myocardial infarction. MCP-1-/- mice had decreased and delayed macrophage infiltration in the healing infarct and demonstrated delayed replacement of injured cardiomyocytes with granulation tissue. In contrast, the time course and density of neutrophil infiltration was similar in MCP-1 null and wild-type animals. MCP-1-/- infarcts had decreased mRNA expression of the cytokines TNF-alpha, IL-1beta, TGF-beta2, -beta3, and IL-10 and demonstrated defective macrophage differentiation evidenced by decreased Osteopontin-1 expression. MCP-1 deficiency diminished myofibroblast accumulation but did not significantly affect infarct angiogenesis. Despite showing delayed phagocytotic removal of dead cardiomyocytes, MCP-1-/- mice had attenuated left ventricular remodeling, but similar infarct size when compared with wild-type animals. MCP-1 antibody inhibition resulted in defects comparable with the pathological findings noted in infarcted MCP-1-/- animals without an effect on macrophage recruitment. MCP-1 has important effects on macrophage recruitment and activation, cytokine synthesis and myofibroblast accumulation in healing infarcts. Absence of MCP-1 results in attenuated post-infarction left ventricular remodeling, at the expense of a prolonged inflammatory phase and delayed replacement of injured cardiomyocytes with granulation tissue.

A murine model of ischemic cardiomyopathy induced by repetitive ischemia and reperfusion

BACKGROUND

Repetitive brief myocardial ischemia has been implicated in the pathogenesis of the ventricular dysfunction associated with ischemic cardiomyopathy and myocardial hibernation. In this study we examine the effects of repetitive ischemia and reperfusion (I/R) on murine myocardium.

METHODS

C57/BL6 mice underwent daily 15 min left anterior descending coronary occlusions followed by reperfusion. After 3, 5, 7, 14, 21 and 28 days, echocardiographic studies were performed, and hearts of I/R and sham-operated animals were processed for histological examination.

RESULTS

Histological studies showed no evidence of myocardial necrosis in the ischemic region. Quantitative assessment of collagen revealed a marked persistent interstitial deposition of collagen after seven days I/R in the anterior left ventricular wall (sham 4.6 +/- 2.0 %, I/R 21.5 +/- 6.5 %, p < 0.05). Echocardiographic studies showed persistent regional anterior wall dysfunction in I/R animals. Histological evaluation showed absence of neovessel formation. After discontinuation of the I/R protocol, fibrosis and regional ventricular dysfunction decreased within 60 days.

CONCLUSIONS

Repetitive brief murine myocardial I/R induces reversible fibrotic remodeling and ventricular dysfunction, without myocardial infarction and necrosis, and may play a role in the pathogenesis of ischemic cardiomyopathy and myocardial hibernation.

Mast cell tryptase may modulate endothelial cell phenotype in healing myocardial infarcts

Mast cells and macrophages infiltrate healing myocardial infarcts and may play an important role in regulating fibrous tissue deposition and extracellular matrix remodelling. This study examined the time-course of macrophage and mast cell accumulation in healing infarcts and studied the histological characteristics and protease expression profile of mast cells in a canine model of experimental infarction. Although macrophages were more numerous than mast cells in infarct granulation tissue, macrophage density decreased during maturation of the scar, whereas mast cell numbers remained persistently elevated. During the inflammatory phase of infarction, newly recruited leucocytes infiltrated the injured myocardium and appeared to be clustered in close proximity to degranulating cardiac mast cells. During the proliferative phase of healing, mast cells had decreased granular content and were localized close to infarct neovessels. In contrast, macrophages showed no selective localization. Mast cells in healing canine infarcts were alcian blue/safranin-positive cells that expressed both tryptase and chymase. In order to explain the pro-inflammatory and angiogenic actions of tryptase--the major secretory protein of mast cells--its effects on endothelial chemokine expression were examined. Chemokines are chemotactic cytokines that play an important role in leucocyte trafficking and angiogenesis and are highly induced in infarcts. Tryptase, a proteinase-activated receptor (PAR)-2 agonist, induced endothelial expression of the angiogenic chemokines CCL2/MCP-1 and CXCL8/IL-8, but not the angiostatic chemokine CXCL10/IP-10. Endothelial PAR-2 stimulation with the agonist peptide SLIGKV induced a similar chemokine expression profile. Mast cell tryptase may exert its angiogenic effects in part through selective stimulation of angiogenic chemokines.

Noninvasive ultrasonic measurement of arterial wall motion in mice

Despite the extensive use of genetically altered mice to study cardiovascular physiology and pathology, it remains difficult to quantify arterial function noninvasively in vivo. We have developed a noninvasive Doppler method for quantifying vessel wall motion in anesthetized mice. A 20-MHz probe was held by an alligator clip and positioned over the carotid arteries of 16 mice, including six 3- to 5-mo-old wild-type (WT), four 30-mo-old senescent (old), two apolipoprotein E null (ApoE), and four α-smooth muscle actin null (α-SMA) mice. Doppler signals were obtained simultaneously from both vessel walls and from blood flow. The calculated displacement signals from the near and far walls were subtracted to generate a diameter signal from which the excursion and an augmentation index were calculated. The excursion ranged between 13 μm (in ApoE) and 95 μm (in α-SMA). The augmentation index was lowest in the WT mice (0.06) and highest in the old mice (0.29). We conclude that Doppler signal processing may be used to measure vessel wall motion in mice with high spatial and temporal resolution and that diameter signals can replace pressure signals for calculating the augmentation index. This noninvasive method is able to identify and confirm characteristic changes in arterial properties previously associated with age, atherosclerosis, and the absence of vascular tone.

Cardiac muscle plasticity in adult and embryo by heart-derived progenitor cells

The evidence of cardiomyocyte proliferation in damaged heart implied cardiac regeneration might occur by resident or extra cardiac stem cells. However, the specification and origin of these cells remain unknown. Here, we report using fluorescence-activated cell sorting that cardiac progenitor cells resided in adult heart and colocalized with small capillary vessels, within the stem cell antigen (Sca-1) population expressing high telomerase activity. Notably, hematopoietic stem cells capable of efflux Hoechst 33342, termed side population cells, also were identified within the heart-derived cells. The cardiac progenitor cells (CD45(-)/CD34(-)) express neither cardiac muscle nor endothelial cell markers at an undifferentiated stage. The exposure of 5-azacytidine induced cardiac differentiation, which depends, in part, on Bmpr1a, a type IA receptor for bone morphogenetic protein (BMP). The capability of adult Sca1(+) cells to adopt a cardiac muscle in embryogenesis was substantiated by blastocyst injection, using progenitors from the adult hearts of transgenic mice that harbor a bacterial artificial chromosome expressing GFP via the Nkx-2.5 locus. Intravenously injected progenitors, shortly after ischemic/reperfusion, homed and functionally differentiated 3.5% of total left ventricle in the host myocardium. Differentiation included both fusion-independent and fusion-associated components, proved by the Cre/loxP donor/recipient system. Our studies suggest that endogenous cardiac progenitors reside in the adult heart, regenerate cardiomyocytes functionally, and integrate into the existing heart circuitry.

Disruption of Rho signaling results in progressive atrioventricular conduction defects while ventricular function remains preserved

Recent studies suggest that RhoA and Rac1 mediate hypertrophic signals in cardiac myocyte hypertrophy. However, effects on cardiac function caused by inhibition of their activity in the heart have yet to be evaluated. Cardiac-specific inhibition of Rho family protein activities was achieved by expressing Rho GDIalpha, an endogenous specific GDP dissociation inhibitor for Rho family proteins, using the alpha-myosin heavy-chain promoter. Increased expression of Rho GDIalpha led to atrial arrhythmias and mild ventricular hypertrophy in adult mice (4-7 months). However, left ventricular systolic and diastolic function was largely preserved before and after the development of cardiac hypertrophy, indicating that Rho GTPases are not required to maintain ventricular contractile function under basal physiological condition. Electrocardiography and intracardiac electrophysiological studies revealed first-degree atrioventricular (AV) block in the transgenic heart at 1 week of age, which further progressed into second-degree AV block at 4 weeks of age before the development of cardiac hypertrophy. Expression of connexin 40 dramatically decreased from 1 week to 4 weeks of age in the transgenic heart, which may contribute in part to the conduction defects in the transgenic mice. This study provides novel evidence for an important role of Rho GTPases in regulating AV conduction.