Cell Size Critically Determines Initial Retention of Bone Marrow Mononuclear Cells in the Heart after Intracoronary Injection: Evidence from a Rat Model

Intracoronary injection of bone marrow mononuclear cells (BMMNC) is an emerging treatment for heart failure. Initial donor cell retention in the heart is the key to the success of this approach, but this process remains insufficiently characterized. Although it is assumed that cell size of injected cells may influence their initial retention, no scientific evidence has been reported. We developed a unique model utilizing an ex-vivo rat heart perfusion system, enabling quantitative assessment of retention of donor cells after intracoronary injection. The initial (5 minutes after intracoronary injection) retention rate of BMMNC was as low as approximately 20% irrespective of donor cell doses injected (1×10⁶, 8×10⁶, 4×10⁷). Quantitative cell-size assessment revealed a positive relationship between the size of BMMNC and retention ratio; larger subpopulations of BMMNC were more preferentially retained compared to smaller ones. Furthermore, a larger cell type—bone marrow-derived mesenchymal stromal cells (median size = 11.5μm versus 7.0μm for BMMNC)—had a markedly increased retention rate (77.5±1.8%). A positive relationship between the cell size and retention ratio was also seen in mesenchymal stromal cells. Flow-cytometric studies showed expression of cell-surface proteins, including integrins and selectin-ligands, was unchanged between pre-injection BMMNC and those exited from the heart, suggesting that biochemical interaction between donor cells and host coronary endothelium is not critical for BMMNC retention. Histological analyses showed that retained BMMNC and mesenchymal stromal cells were entrapped in the coronary vasculature and did not extravasate by 60 minutes after transplantation. Whilst BMMNC did not change coronary flow after intracoronary injection, mesenchymal stromal cells reduced it, suggesting coronary embolism, which was supported by the histological finding of intravascular cell-clump formation. These data indicate that cell-size dependent, passive (mechanical), intravascular entrapment is responsible for the initial donor cell retention after intracoronary injection of BMMNC in the heart having normal vasculatures (at least).

Allogeneic Mesenchymal Stromal Cells Transplanted Onto the Heart Surface Achieve Therapeutic Myocardial Repair Despite Immunologic Responses in Rats

Background

Transplantation of allogeneic mesenchymal stromal cells (MSCs) is a promising treatment for heart failure. We have shown that epicardial placement of cell sheets markedly increases donor cell survival and augments therapeutic effects compared with the current methods. Although immune rejection of intramyocardially injected allogeneic MSCs have been suggested, allogeneic MSCs transplanted on the heart surface (virtual space) may undergo different courses. This study aimed to elucidate immunologic response against epicardially placed allogeneic MSCs, rejection or acceptance of these cells, and their therapeutic effects for heart failure.

Methods and Results

At 4 weeks after coronary artery ligation, Lewis rats underwent epicardial placement of MSC sheets from syngeneic Lewis or allogeneic Fischer 344 rats or sham treatment. At days 3 and 10 after treatment, similar ratios (≈50% and 30%, respectively) of grafted MSCs survived on the heart surface in both MSC sheet groups. By day 28, survival of syngeneic MSCs was substantially reduced (8.9%); survival of allogeneic MSCs was more extensively reduced (0.2%), suggesting allorejection. Correspondingly, allogeneic MSCs were found to have evoked an immunologic response, albeit low level, as characterized by accumulation of CD4⁺ T cells and upregulation of interleukin 6. Despite this alloimmune response, the allogeneic MSC sheet achieved myocardial upregulation of reparative factors, enhanced repair of the failing myocardium, and improved cardiac function to the equivalent degree observed for the syngeneic MSC sheet.

Conclusions

Allogeneic MSCs placed on the heart surface evoked an immunologic response; however, this allowed sufficient early phase donor cell survival to induce equivalent therapeutic benefits to syngeneic MSCs. Further development of this approach toward clinical application is warranted.

Toll-like receptor 9 protects non-immune cells from stress by modulating mitochondrial ATP synthesis through the inhibition of SERCA2

Toll-like receptor 9 (TLR9) has a key role in the recognition of pathogen DNA in the context of infection and cellular DNA that is released from damaged cells. Pro-inflammatory TLR9 signalling pathways in immune cells have been well investigated, but we have recently discovered an alternative pathway in which TLR9 temporarily reduces energy substrates to induce cellular protection from stress in cardiomyocytes and neurons. However, the mechanism by which TLR9 stimulation reduces energy substrates remained unknown. Here, we identify the calcium-transporting ATPase, SERCA2 (also known as Atp2a2), as a key molecule for the alternative TLR9 signalling pathway. TLR9 stimulation reduces SERCA2 activity, modulating Ca(2+) handling between the SR/ER and mitochondria, which leads to a decrease in mitochondrial ATP levels and the activation of cellular protective machinery. These findings reveal how distinct innate responses can be elicited in immune and non-immune cells--including cardiomyocytes--using the same ligand-receptor system.

Extracellular high mobility group box 1 plays a role in the effect of bone marrow mononuclear cell transplantation for heart failure

Transplantation of unfractionated bone marrow mononuclear cells (BMCs) repairs and/or regenerates the damaged myocardium allegedly due to secretion from surviving BMCs (paracrine effect). However, donor cell survival after transplantation is known to be markedly poor. This discrepancy led us to hypothesize that dead donor BMCs might also contribute to the therapeutic benefits from BMC transplantation. High mobility group box 1 (HMGB1) is a nuclear protein that stabilizes nucleosomes, and also acts as a multi-functional cytokine when released from damaged cells. We thus studied the role of extracellular HMGB1 in the effect of BMC transplantation for heart failure. Four weeks after coronary artery ligation in female rats, syngeneic male BMCs (or PBS only as control) were intramyocardially injected with/without anti-HMGB1 antibody or control IgG. One hour after injection, ELISA showed that circulating extracellular HMGB1 levels were elevated after BMC transplantation compared to the PBS injection. Quantitative donor cell survival assessed by PCR for male-specific sry gene at days 3 and 28 was similarly poor. Echocardiography and catheterization showed enhanced cardiac function after BMC transplantation compared to PBS injection at day 28, while this effect was abolished by antibody-neutralization of HMGB1. BMC transplantation reduced post-infarction fibrosis, improved neovascularization, and increased proliferation, while all these effects in repairing the failing myocardium were eliminated by HMGB1-inhibition. Furthermore, BMC transplantation drove the macrophage polarization towards alternatively-activated, anti-inflammatory M2 macrophages in the heart at day 3, while this was abolished by HMGB1-inhibition. Quantitative RT-PCR showed that BMC transplantation upregulated expression of an anti-inflammatory cytokine IL-10 in the heart at day 3 compared to PBS injection. In contrast, neutralizing HMGB1 by antibody-treatment suppressed this anti-inflammatory expression. These data suggest that extracellular HMGB1 contributes to the effect of BMC transplantation to recover the damaged myocardium by favorably modulating innate immunity in heart failure.

The use of cell-sheet technique eliminates arrhythmogenicity of skeletal myoblast-based therapy to the heart with enhanced therapeutic effects

BACKGROUND

Clinical application of skeletal myoblast transplantation has been curtailed due to arrhythmogenicity and inconsistent therapeutic benefits observed in previous studies. However, these issues may be solved by the use of a new cell-delivery mode. It is now possible to generate "cell-sheets" using temperature-responsive dishes without artificial scaffolds. This study aimed to validate the safety and efficacy of epicardial placement of myoblast-sheets (myoblast-sheet therapy) in treating heart failure.

METHODS AND RESULTS

After coronary artery ligation in rats, the same numbers of syngeneic myoblasts were transplanted by intramyocardial injection or cell-sheet placement. Continuous radio-telemetry monitoring detected increased ventricular arrhythmias, including ventricular tachycardia, after intramyocardial injection compared to the sham-control, while these were abolished in myoblast-sheet therapy. This effect was conjunct with avoidance of islet-like cell-cluster formation that disrupts electrical conduction, and with prevention of increased arrhythmogenic substrates due to exaggerated inflammation. Persistent ectopic donor cells were found in the lung only after intramyocardial injection, strengthening the improved safety of myoblast-sheet therapy. In addition, myoblast-sheet therapy enhanced cardiac function, corresponding to a 9.2-fold increase in donor cell survival, compared to intramyocardial injection. Both methods achieved reduced infarct size, decreased fibrosis, attenuated cardiomyocyte hypertrophy, and increased neovascular formation, in association with myocardial upregulation of a group of relevant molecules. The pattern of these beneficial changes was similar between two methods, but the degree was more substantial after myoblast-sheet therapy.

CONCLUSION

The cell-sheet technique enhanced safety and therapeutic efficacy of myoblast-based therapy, compared to the current method, thereby paving the way for clinical application.

Quantitative assessment of initial retention of bone marrow mononuclear cells injected into the coronary arteries

BACKGROUND

Intracoronary injection of bone marrow mononuclear cells (BMMNC) is a common clinical protocol of cell transplantation for heart disease, but poor engraftment of donor cells in the heart, which will limit its therapeutic efficacy, is a major issue. Initial "retention" (endothelial adherence and/or extravasation) of BMMNC immediately after intracoronary injection is a key step toward successful engraftment; however, this event has not been fully characterized. The aim of this study is to quantitatively clarify the frequency of "retention" of BMMNC after intracoronary injection, determine the impact of prior induction of ischemia-reperfusion injury on "retention" efficiency, and elucidate the underlying mechanisms focusing on adhesion molecule-mediated cell-cell interactions.

METHODS

One million BMMNC collected from green fluorescent protein (GFP)-transgenic mice were injected into the coronary arteries of syngeneic wild-type mouse hearts under Langendorff perfusion. Retention efficiency was quantitatively estimated from the GFP-positive cell number flushed out into the coronary effluent.

RESULTS

Whereas only 13.3 ± 1.2% of injected BMMNC were retained into normal hearts, prior induction of 30-minute ischemia and 30-minute reperfusion increased the retention efficiency to 36.5 ± 1.6% (p < 0.05, n = 8). Immunoconfocal observation further confirmed this enhanced retention after ischemia-reperfusion. Noticeably, the enhanced retention efficiency after ischemia-reperfusion treatment was diminished by administration of anti-P-selectin antibody (8.3 ± 0.8%, p < 0.05), but was not affected by inhibiting intercellular adhesion molecule-1 (39.6 ± 3.3%) or vascular cell adhesion molecule-1 (43.9 ± 2.9%).

CONCLUSIONS

Retention efficiency of intracoronary-injected BMMNC was poor in a model of isolated, crystalloid-perfused murine hearts. An antecedent period of global ischemia-reperfusion increased the retention via P-selectin-dependent BMMNC-endothelial interaction.

Donor cell-type specific paracrine effects of cell transplantation for post-infarction heart failure

Cell transplantation is an emerging therapy for treating post-infarction heart failure. Although the paracrine effect has been proposed to be an important mechanism for the therapeutic benefits, details remain largely unknown. This study compared various aspects of the paracrine effect after transplantation of either bone marrow mononuclear cells (BMC) or skeletal myoblasts (SMB) into the post-infarction chronically failing heart. Three weeks after left coronary artery ligation, adult rats received intramyocardial injection of either BMC, SMB or PBS only. Echocardiography demonstrated that injection of either cell type improved cardiac function compared to PBS injection. Interestingly, BMC injection markedly improved neovascularization in the border areas surrounding infarcts, while SMB injection decreased fibrosis in both the border and remote areas. Injection of either cell type similarly reduced hypertrophy of cardiomyocytes as assessed by cell-size planimetry using isolated cardiomyocytes. Quantitative RT-PCR revealed that, among 15 candidate mediators of paracrine effects studied, Fgf2 and Hgf were upregulated only after BMC injection, while Mmp2 and Timp4 were modulated after SMB injection. Additional investigations of signalling pathways relevant to heart failure by western blotting showed that p38 and STAT3 were temporarily activated after BMC injection, in contrast, ERK1/2 and JNK were activated after SMB injection. There was no difference in activation of Akt, PKD or Smad3 among groups. These data suggest that paracrine effects observed after cell transplantation in post-infarction heart failure were noticeably different between cell types in terms of mediators, signal transductions and consequent effects.

Modulated inflammation by injection of high-mobility group box 1 recovers post-infarction chronically failing heart

BACKGROUND

Inflammation plays an important role in the progress of adverse ventricular remodeling after myocardial infarction. High-mobility group box 1 (HMGB1) is a nuclear protein, which has recently been uncovered to also act as a modifier of inflammation when released. We hypothesized that HMGB1 injection could preferentially modulate local myocardial inflammation, attenuate ventricular remodeling, and subsequently improve cardiac performance of postinfarction chronic heart failure.

METHODS AND RESULTS

Three weeks after left coronary artery ligation, HMGB1 (2.5 mug) or PBS was intramyocardially injected into rat hearts. At 28 days after injection, left ventricular ejection fraction was significantly improved after HMGB1 injection compared to PBS (39.3+/-1.4 versus 33.3+/-1.8%; P<0.01). Accumulation of CD45(+) inflammatory cells, two thirds of which were OX62(+) dendritic cells, in the peri-infarct area was significantly attenuated by HMGB1 injection. Dramatic changes in the expression of major proinflammatory cytokines were not detected by microarray or RT-PCR. Adverse ventricular remodeling including cardiomyocyte hypertrophy (cardiomyocyte cross-sectional area; 439+/-7 versus 458+/-6 mum(2); P<0.05) and extracellular collagen deposition (collagen volume fraction; 11.9+/-0.4 versus 15.2+/-0.6%; P<0.01) was attenuated by HMGB1 injection. Analyses of signal transduction pathways revealed that HMGB1 injection activated ERK1/2, but not p38, Akt, and Smad3. Cardiac regeneration and neovascularization were not observed.

CONCLUSIONS

HMGB1 injection modulated the local inflammation in the postinfarction chronically failing myocardium, particularly via reducing the accumulation of dendritic cells. This modulated inflammation resulted in attenuated fibrosis and cardiomyocyte hypertrophy, which thereby improved global cardiac function. These data suggest that HMGB1 may be valuable for the chronic heart failure treatment.

A factor underlying late-phase arrhythmogenicity after cell therapy to the heart: global downregulation of connexin43 in the host myocardium after skeletal myoblast transplantation

BACKGROUND

Arrhythmia occurrence is a variable but serious concern of cell therapy for treating heart failure. Using a rat postinfarction chronic heart failure model, we compared skeletal myoblast (SMB) with bone marrow cell (BMC) injection to highlight donor cell-specific, late-phase arrhythmogenesis and the underlying factors.

METHODS AND RESULTS

SMBs or BMCs derived from male GFP-transgenic rats, or PBS were injected intramyocardially into female rat hearts 3 weeks after coronary artery occlusion. At 28 days after injection, echocardiography showed that the left ventricular ejection fraction was significantly improved in both the SMB and BMC groups, compared to PBS control despite poor graft survival as assessed by PCR for the male-specific gene. Radio-telemetry analysis revealed that the SMB group displayed a higher occurrence of ventricular premature contractions with an elongation of the QRS complex and the hearts were more susceptible to isopreterenol-induced ventricular tachycardia compared to the BMC and PBS groups. Western blot and immunoconfocal analysis showed that the gap junction protein, connexin43, was widely and persistently decreased in the SMB group compared to the other groups. IL-1beta was shown to be upregulated in hearts after SMB injection, and in vitro experiments demonstrated that exposure to IL-1beta caused a decrease in connexin43 and intercellular communication in cultured cardiomyocytes.

CONCLUSIONS

Although cell therapy was capable of improving function of the postinfarction chronically failing heart, there was late-phase arrhythmogenicity specific to donor cell type. Global downregulation of connexin43 in the host myocardium was indicated to be an important factor underlying late-phase arrhythmogenicity after SMB transplantation.

Choice of cell-delivery route for skeletal myoblast transplantation for treating post-infarction chronic heart failure in rat

BACKGROUND

Intramyocardial injection of skeletal myoblasts (SMB) has been shown to be a promising strategy for treating post-infarction chronic heart failure. However, insufficient therapeutic benefit and occurrence of ventricular arrhythmias are concerns. We hypothesised that the use of a retrograde intracoronary route for SMB-delivery might favourably alter the behaviour of the grafted SMB, consequently modulating the therapeutic effects and arrhythmogenicity.

METHODS AND RESULTS

Three weeks after coronary artery ligation in female wild-type rats, 5x10(6) GFP-expressing SMB or PBS only (control) were injected via either the intramyocardial or retrograde intracoronary routes. Injection of SMB via either route similarly improved cardiac performance and physical activity, associated with reduced cardiomyocyte-hypertrophy and fibrosis. Grafted SMB via either route were only present in low numbers in the myocardium, analysed by real-time PCR for the Y-chromosome specific gene, Sry. Cardiomyogenic differentiation of grafted SMB was extremely rare. Continuous ECG monitoring by telemetry revealed that only intramyocardial injection of SMB produced spontaneous ventricular tachycardia up to 14 days, associated with local myocardial heterogeneity generated by clusters of injected SMB and accumulated inflammatory cells. A small number of ventricular premature contractions with latent ventricular tachycardia were detected in the late-phase of SMB injection regardless of the injection-route.

CONCLUSION

Retrograde intracoronary injection of SMB provided significant therapeutic benefits with attenuated early-phase arrhythmogenicity in treating ischaemic cardiomyopathy, indicating the promising utility of this route for SMB-delivery. Late-phase arrhythmogenicity remains a concern, regardless of the delivery route.

Heterogeneic nature of adult cardiac side population cells

Side population cells have been found in various types of adult tissue including heart and are presumed to be tissue-specific stem/progenitor cells. In the present study, we confirmed the presence of cardiac side population (cSP) cells, which showed both the Hoechst 33342 efflux ability and ABCG2 expression, in adult murine heart. Flow cytometric analysis showed that more than half of cSP cells expressed the endothelial marker VE-cadherin or the smooth muscle markers, alpha-smooth muscle actin and desmin. In addition, immunohistochemical analysis demonstrated that ABCG2(+) cells were mainly localized within vascular walls. Quantitative RT-PCR analysis demonstrated that VE-cadherin(-) cSP cells progressively expressed Nkx2.5 and cardiac troponin T with time in culture. VE-cadherin(-) cSP cells also expressed mesodermal-mesenchymal-associated markers and differentiated into osteocytes and adipocytes. These results highlight the heterogeneic nature of cSP cells, consisting of vascular endothelial cells, smooth muscle cells, and mesenchymal stem/progenitor cells including potential cardiomyogenic cells.

Direct intramyocardial but not intracoronary injection of bone marrow cells induces ventricular arrhythmias in a rat chronic ischemic heart failure model

BACKGROUND

Therapeutic efficacy of bone marrow (BM) cell injection for treating ischemic chronic heart failure has not been established. In addition, experimental data are lacking on arrhythmia occurrence after BM cell injection. We hypothesized that therapeutic efficacy and arrhythmia occurrence induced by BM cell injection may be affected by the cell delivery route.

METHODS AND RESULTS

Three weeks after left coronary artery ligation, wild-type female rats were injected with 1x10(7) mononuclear BM cells derived from green fluorescent protein-transgenic male rats through either a direct intramyocardial or a retrograde intracoronary route. Both intramyocardial and intracoronary injection of BM cells demonstrated similar improvement in left ventricular ejection fraction measured by echocardiography and a similar graft size analyzed by real-time polymerase chain reaction for the Y chromosome-specific Sry gene. Noticeably, intramyocardial injection of BM cells induced frequent ventricular premature contractions (108+/-73 per hour at 7 days after BM cell injection), including multiform, consecutive ventricular premature contractions and ventricular tachycardia for the initial 14 days; intracoronary injection of BM cells and intramyocardial injection of phosphate-buffered saline rarely induced arrhythmias. Immunohistochemistry demonstrated that intramyocardial BM cell injection formed distinct cell clusters containing donor-derived cells and accumulated host-derived inflammatory cells in the infarct border zone, whereas intracoronary BM cell injection provided more homogeneous donor cell dissemination with less inflammation and without disrupting the native myocardial structure.

CONCLUSIONS

BM cell injection is able to improve cardiac function in ischemic chronic heart failure but has a risk of arrhythmia occurrence when the intramyocardial route is used. Such arrhythmias may be prevented by using the intracoronary route.

Direct effects of apelin on cardiomyocyte contractility and electrophysiology

Apelin, the ligand for the angiotensin receptor like-1, has been implicated in the pathogenesis of atrial fibrillation and heart failure. However, it is unknown if apelin has direct effects on cardiomyocyte contractility and electrophysiology. APJ-like immunoreactivity was localized to T-tubules and intercalated disc area in isolated adult rat ventricular myocytes. Apelin (1 nM) significantly increased sarcomere shortening in normal as well as failing cardiomyocytes. The transient increase in shortening was not accompanied by increased [Ca(2+)] transient amplitude. Apelin significantly activated the sarcolemmal Na(+)/H(+) exchanger (NHE) and increased intracellular pH. Moreover, apelin (10 nM) increased conduction velocity in monolayers of cultured neonatal rat cardiac myocytes. Our results demonstrate for the first time that apelin has direct effects on the propagation of action potential and contractility in cardiomyocytes. One of the mechanisms involved in the inotropic effect may be an increased myofilament sensitivity to Ca(2+) as apelin enhanced the activity of NHE with consequent intracellular alkalinization.

A novel strategy for myocardial protection by combined antibody therapy inhibiting both P-selectin and intercellular adhesion molecule-1 via retrograde intracoronary route

BACKGROUND

Antibody therapy to inhibit either P-selectin or intercellular adhesion molecule-1 (ICAM-1) has been reported to provide myocardial protection against leukocyte-mediated reperfusion injury. Because these molecules play different roles in the leukocyte-endothelial interaction, co-inhibition of both may achieve further enhanced cardioprotection. In addition, the therapeutic efficacy of such antibody therapy may be affected by the delivery route used. Retrograde intracoronary infusion will offer an effective, direct access to the postcapillary venules, where the target event (leukocyte-endothelial interaction) takes place. We investigated the feasibility and efficiency of the combined antibody therapy targeting both P-selection and ICAM-1 via the retrograde intracoronary route to attenuate myocardial ischemia-reperfusion injury.

METHODS AND RESULTS

Lewis rats underwent 30-minute left coronary artery occlusion. Just before reperfusion, anti-P-selectin monoclonal antibody (150 microg/kg), anti-ICAM-1 monoclonal antibody (200 microg/kg), both antibodies together, or control antibody were retrogradely infused into the left cardiac vein. At 24 hours after reperfusion, administration of either anti-P-selectin or anti-ICAM-1 antibody significantly (P<0.05) improved left ventricular ejection fraction and attenuated infarct size (40.6+/-3.2% and 34.8+/-3.5%, respectively) compared with the control (56.8+/-3.4%). This was associated with reduced leukocyte accumulation and improved regional blood flow in the ischemic area. Noticeably, co-administration of both antibodies achieved a much greater reduction in infarct size (19.1+/-3.6%), associated with greater attenuation in leukocyte infiltration, compared with administration of either single antibody.

CONCLUSIONS

Combined antibody therapy inhibiting both P-selectin and ICAM-1 via the retrograde intracoronary route could be a promising new strategy for myocardial protection against ischemia-reperfusion injury.

Evaluation of frequency, type, and function of gap junctions between skeletal myoblasts overexpressing connexin43 and cardiomyocytes: relevance to cell transplantation

Cell transplantation of skeletal myoblasts (SMs) is one possible treatment for repairing cardiac tissue after myocardial injury. However, inappropriate electrical coupling between grafted SMs and host cardiomyocytes may be responsible for the arrhythmias observed in clinical trials of SM transplantation. Whether functional gap junctions occur between the two cell types remains controversial. We have studied the ability of SMs to electrically couple with isolated adult rat cardiomyocytes (CMs) and assessed whether connexin43 (Cx43) overexpression enhanced gap junctional conductance (Gj). C2C12 myoblast lines overexpressing Cx43 were generated by gene transfection and clonal selection. CMs were cocultured with either SMs overexpressing Cx43 (CM-SM(Cx43)) or control SMs (CM-SM(WT)) in vitro. Gj between pairs of SMs and CMs was quantified with dual whole cell patch clamping. Formation of Gj occurred between 22% of CM-SM(WT) pairs (n=73) and 48% of CM-SM(Cx43) pairs (n=71, P<0.001). The Gj of CM-SM(Cx43) pairs (29.7+/-4.3 nS, n=21) was greater than that of CM-SM(WT) pairs (14.8+/-2.0 nS, n=12, P<0.05). The overexpression of Cx43 in SMs increased the formation of electrical communication and the steady-state conductance between SMs and CMs. Enhanced gap junctional conductance may be useful to promote the integration of transplanted SMs into the myocardium.

Enhanced efficiency of superoxide dismutase-induced cardioprotection by retrograde intracoronary administration

OBJECTIVE

We hypothesized that modification of the infusion route may improve the efficiency of superoxide dismutase (SOD)-induced cardioprotection against reperfusion injury. The routes for SOD delivery previously examined were intravenous, via the left atrium, or by a combination of these, all of which can deliver SOD into the ischemic myocardium only after reperfusion. In contrast, retrograde intracoronary infusion may be able to deliver SOD before reperfusion. We investigated the feasibility and efficiency of the retrograde intracoronary infusion of SOD to attenuate reperfusion injury.

METHODS AND RESULTS

Lewis rats underwent 30-min left coronary artery occlusion followed by reperfusion for 24 h. Just before reperfusion, CuZn-SOD was administered intravenously (15,000 U/kg, V-SOD group) or by retrograde intracoronary infusion (1500 U/kg, R-SOD group) through a catheter inserted into left cardiac vein via left superior vena cava as we have previously reported. This method has been shown to perfuse the whole left ventricular free walls. Controls for each group were injected with phosphate buffer saline only via the same routes (V-PBS and R-PBS group). The R-SOD group demonstrated significantly preserved left ventricular ejection fraction (LVEF; 71.3+/-1.7% vs. 60.8+/-2.3%, p=0.028), reduced infarct size (23.3+/-2.3% vs. 42.4+/-3.5%, p<0.001), and attenuated polymorphonuclear leukocyte (PMNL) infiltration (11.8+/-0.4 vs. 14.8+/-0.2 10(3)/mm(2), p<0.001) compared to the V-SOD group. The V-SOD group demonstrated significantly improved reflow (64.3+/-2.1% vs. 53.4+/-2.4%, p=0.017) and attenuated PMNL infiltration (14.8+/-0.2 vs. 16.8+/-0.7 10(3)/mm(2), p=0.018) compared to the V-PBS group.

CONCLUSION

Retrograde intracoronary infusion is a promising, clinically applicable method to enhance the efficacy of SOD-induced myocardial protection against ischemia-reperfusion injury.

High-resolution en-face visualization of the cardiomyocyte plasma membrane reveals distinctive distributions of spectrin and dystrophin

The actin-binding proteins, spectrin and dystrophin, are key components of the plasma membrane-associated cytoskeleton of the cardiac muscle cell. From confocal immunofluorescence studies, the distribution of spectrin appears to overlap with that of dystrophin, but the precise functional differentiation, molecular distributions and spatial relationship of these two cytoskeletal systems remain unclear. Freeze-fracture replica immuno-electron microscopy, in parallel with immunofluorescence/confocal microscopy, were applied to examine at high resolution the spatial relationships between the spectrin and dystrophin membrane-associated cytoskeleton systems in cardiac muscle. Application of freeze-fracture replica cytochemistry, with single and double immunogold labeling, permitted simultaneous examination of the organization of spectrin and dystrophin in en-face views of the plasma membrane at high resolution. In contrast to the close spatial relationship previously demonstrated for dystrophin and beta-dystroglycan, no association between the gold label marking dystrophin and that marking spectrin was observed. Our freeze-fracture cytochemical results suggest that the two membrane skeletal networks formed by dystrophin and spectrin in cardiac muscle are independently organized, implying that whatever overlap of function (e.g., in structural support to the plasma membrane) may exist between them, the two systems may each have additional distinctive roles.

Role of interleukin-1beta in acute inflammation and graft death after cell transplantation to the heart

BACKGROUND

Poor survival of grafted cells is a major factor hindering the therapeutic effect of cell transplantation; however, the causes of cell death remain unclear. We hypothesized that interleukin-1beta (IL-1beta) might play a role in the acute inflammatory response and graft death after cell transplantation and that inhibition of IL-1beta might improve graft survival.

METHODS AND RESULTS

14C-labeled male skeletal muscle precursor cells were implanted into female mouse hearts by direct intramuscular injection. The amount of 14C-label provides an estimate of the surviving cell number, whereas the amount of male-specific Smcy gene measured by polymerase chain reaction indicates the total (surviving+proliferated) number of donor-derived cells. At 10 minutes after implantation, 44.8+/-2.4% of the grafted cells survived and this steadily decreased to 14.6+/-1.1% by 24 hours, and to 7.9+/-0.6% by 72 hours (n=6 in each point). Proliferation of the surviving cells, which began after 24 hours, resulted in an increase in the total cell number from 15.5+/-0.8% at 24 hours to 24.4+/-1.6% at 72 hours. Acute inflammation was prominent at 24 hours and was reduced by 72 hours, in parallel with IL-1beta expression. Administration of anti-IL-1beta antibody improved graft survival at both 24 (25.6+/-1.6%) and 72 hours (14.8+/-1.1%) and resulted in a 2-fold increase in the total cell number at 72 hours (45.8+/-2.4%). The effects of IL-1beta inhibition corresponded with a reduced inflammatory response.

CONCLUSIONS

IL-1beta is involved in acute inflammation and graft death after direct intramyocardial cell transplantation. Targeted inhibition of IL-1beta may be a useful strategy to improve graft survival.

Targeted cell delivery into infarcted rat hearts by retrograde intracoronary infusion: distribution, dynamics, and influence on cardiac function

BACKGROUND

Intracoronary infusion for cell transplantation has potential advantages in disseminating cells globally into the myocardium with less injury over direct intramuscular injection. Arterial route, however, has a risk of coronary embolism and a limitation in cell delivery into ischemic or infarcted areas. We assessed the efficiency of retrograde intracoronary cell implantation into infarcted hearts using a novel rat model.

METHODS AND RESULTS

After left coronary artery ligation in rat, a catheter was inserted into the left cardiac vein, which drains the left ventricular free wall. Through this, 1x10(6) skeletal muscle precursor cells expressing nuclear beta-galactosidase were infused retrogradely into the vein. In situ staining demonstrated that beta-galactosidase-expressing donor cells had disseminated throughout the left ventricular free wall, including both infarcted and surrounding border areas, at 10 minutes after infusion. At 28 days, in contrast, positively stained multinuclear myotubes were found in border zones, whereas no positive cells were seen in infarcted areas. Measurement of beta-galactosidase enzyme activity estimated that 29.8+/-6.9% of total infused cells were retained within the myocardium at 10 minutes and that this number decreased to 23.7+/-8.1% at 3 days but rapidly increased thereafter, reaching a plateau at 90.2+/-17.1% by 14 days. Echocardiography and Langendorff perfusion demonstrated that cell implantation improved cardiac function and dimensions by 28 days, compared with both sham-treated and phosphate-buffered saline-infused infarcted hearts, and this was associated with decreased collagen deposition.

CONCLUSIONS

Retrograde intracoronary cell transplantation could provide an effective cell delivery into infarcted hearts and could be a useful strategy for treating myocardial infarction.

Dynamics and mediators of acute graft attrition after myoblast transplantation to the heart

Survival and proliferation of skeletal myoblasts within the cardiac environment are crucial to the therapeutic efficacy of myoblast transplantation to the heart. We have analyzed the early dynamics of myoblasts implanted into the myocardium and investigated the mechanisms underlying graft attrition. At 10 min after implantation of [14C]thymidine-labeled male myoblasts into female mice hearts, 14C measurement showed that 39.2 +/- 3.0% of the grafted cells survived, and this steadily decreased to 16.0 +/- 1.7% by 24 h and to 7.4 +/- 0.9% by 72 h. PCR of male-specific Smcy gene calculated that the total (surviving plus proliferated) number of donor-derived cells was 18.3 +/- 1.6 and 23.3 +/- 1.3% at 24 and 72 h, respectively, indicating that proliferation of the surviving cells began after 24 h. Acute inflammation became prominent by 24 h and was reduced by 72 h as indicated by myeloperoxidase activity and histological findings. Multiplex RT-PCR revealed corresponding changes in IL-1beta, TGF-beta, IL-6, and TNF-alpha expression. Treatment with CuZn-superoxide dismutase attenuated the initial rapid death and resulted in enhanced cell numbers afterward, giving a twofold increased total number at 72 h compared with the nontreatment. This effect was associated with reduced inflammatory response, suggesting a causative role for superoxide in the initial rapid graft death and subsequent inflammation. These data describe the early dynamics of myoblasts implanted into the myocardium and suggest that initial oxidative stress and following inflammatory response may be important mechanisms contributing to acute graft attrition, both of which could be potential therapeutic targets to improve the efficiency of cell transplantation to the heart.

Remodelling of gap junctions and connexin expression in heart disease

Different combinations and relative quantities of three connexins-connexin43, connexin40 and connexin45-are expressed in different subsets of cardiomyocyte. In the healthy heart, gap junctions assembled from these different connexin combinations form the cell-to-cell pathways for the precisely orchestrated patterns of current flow that govern the normal heart rhythm. Remodelling of gap junction organization and connexin expression is a conspicuous feature of human heart disease in which there is an arrhythmic tendency. This remodelling may take the form of structural remodelling, involving disturbances in the distribution of gap junctions (i.e., disruption of the normal ordered pathways for cell-to-cell conduction), and remodelling of connexin expression, involving alteration in the amount or type of connexin(s) present. Most notable among quantitative alterations in connexin expression is a reduction in ventricular connexin43 levels in human congestive heart failure. By correlating data from studies in experimental animal models, gap junction and connexin remodelling emerges as a factor to be considered in understanding the pro-arrhythmic substrate characteristic of many forms of heart disease. However, our knowledge of the functional correlates of the specific patterns of multiple connexin expression found in different regions of the heart in health and disease remains rudimentary, and the development of new experimental cell models heralds advances in this area over the next few years.

Gap junction alterations in human cardiac disease

Gap junctions, assembled from connexins, form the cell-to-cell pathways for propagation of the precisely orchestrated patterns of current flow that govern the regular rhythm of the healthy heart. As in most tissues and organs, multiple connexin types are expressed in the heart; connexin43, connexin40 and connexin45 are found in distinctive combinations and relative quantities in different, functionally specialized subsets of cardiomyocyte. Alterations of gap junction organization and connexin expression are now well established as a consistent feature of human heart disease in which there is an arrhythmic tendency. These alterations may take the form of structural remodelling, involving disturbances in the distribution of gap junctions and/or alteration of the amount or type of connexin(s) expressed. In the diseased ventricles, the most consistent quantitative alteration involves heterogeneous reduction in connexin43 expression. In the atria, features of gap organization and connexin expression have been implicated in the initiation of atrial fibrillation and, once the condition becomes chronic, gap junction alterations associated with remodelling may contribute to persistence of the condition. By correlating data from studies on the human patient with those from animal and cell models, alterations in gap junctions and connexins have emerged as important factors to be considered in understanding the pro-arrhythmic substrate found in a variety of forms of heart disease.

Spatiotemporal pattern of commitment to slowed proliferation in the embryonic mouse heart indicates progressive differentiation of the cardiac conduction system

Patterns of DNA synthesis in the developing mouse heart between ED7.5-18.5 were studied by a combination of thymidine and bromodeoxyuridine labeling techniques. From earliest stages, we found zones of slow myocyte proliferation at both the venous and arterial poles of the heart, as well as in the atrioventricular region. The labeling index was distinctly higher in nonmyocardial populations (endocardium, epicardium, and cardiac cushions). Ventricular trabeculae showed lower proliferative activity than the ventricular compact layer after their appearance at ED9.5. Low labeling was observed in the pectinate muscles of the atria from ED11.5. The His bundle, bundle branches, and Purkinje fiber network likewise were distinguished by their lack of labeling. Thymidine birthdating (label dilution) showed that the cells in these emerging components of the cardiac conduction system terminally differentiated between ED8.5-13.5. These patterns of slowed proliferation correlate well with those in other species, and can serve as a useful marker for the forming conduction system.

Impedance measurements and connexin expression in human detrusor muscle from stable and unstable bladders

Three of this month's Scientific Discovery papers highlight the importance of collaboration in delivering high quality scientific research. As scientific technology increases in power and cost, and specific areas of interest become more specialized, it is becoming more difficult to cover all aspects of a completeresearch story. Collaborating with other experts in the field or other fields, including industry, allows strong scientific proof to be generated for the hypothesis and aims. Building strong collaborative,inter-disciplinary, multi-institutional, international groups with academic and industrial partners is the way forward for all discovery. We look forward to publishing more of these collaborative papersin future issues of the BJU International.

OBJECTIVES

To test the hypothesis that intercellular electrical coupling is altered in human detrusor smooth muscle from patients with unstable bladders.

MATERIALS AND METHODS

Human detrusor biopsy samples were obtained from patients with stable and unstable bladders. Intracellular electrical impedance was measured with alternating current (20 Hz-300 kHz) across the ends of detrusor strips in an oil-gap, after correcting for extracellular space resistance. Gap junctions were identified by localization of connexins (Cx), specifically Cx45, Cx43 and Cx40 transcripts, using immunoconfocal microscopy.

RESULTS

Total intracellular resistivity was greater in strips from unstable than from stable bladders (median 1246 vs 817 Omega.cm). The increase was attributed to an increase in junctional resistance; cytoplasmic resistance was unchanged. Cx43 was localized to a submucosal layer and to connective tissue; Cx40 label was confined to endothelial cells of blood vessels. Cx45 labelling was localized to detrusor bundles and appeared to be less marked in samples from unstable bladders. Semi-quantitative analysis of Northern blots showed that Cx45 expression in unstable was less than that in stable bladders.

CONCLUSIONS

These data suggest that intercellular coupling is reduced in detrusor from unstable bladders. Cx45 was localized to the detrusor layer, with Cx 43 more evident in the suburothelial mucosa. Cx45 labelling was less intense in detrusor samples from unstable bladders. These results are consistent with reduced gap junction coupling in detrusor from unstable bladders.

Development of a cell model for functional and structural analysis of connexin co-expression: achieving homogeneous and inducible expression of multiple connexins in stable transfectants

We set out to develop an in vitro cell model in which connexins 43, 40 and 45 are co-expressed in the same combinations as found in different sub-types of cardiomyocyte in vivo, using inducible promoters of the Tet-Off and Ecdysone systems. In initial studies, a heterogeneous pattern of gene expression was observed. To achieve homogeneous expression, an Internal Ribosome Entry Site (IRES) sequence was employed, ensuring that a single mRNA coded for connexin and antibiotic resistance. We then constructed plasmids that combine the inducibility of the Tet-Off and Ecdysone systems with the homogeneous expression given by the IRES constructs. These were demonstrated to give inducible and homogeneous expression. By using the reporter gene, Enhanced Green Fluorescent Protein (EGFP), it was further shown in the Tet-Off system that expression of the transfected gene was modulated homogeneously in all cells when induction was repressed. The cell model is now at a suitable stage of development for investigation of the functional correlates of the distinctive connexin co-expression found in different regions of the heart.

Comparison of connexin expression patterns in the developing mouse heart and human foetal heart

Heart muscle cells are electrically coupled by gap junctions, clusters of low-resistance transmembrane channels composed of connexins (Cx). The expression of the three major connexins (Cx43, Cx40 and Cx45) present in cardiac myocytes is known to be developmentally regulated but it is not clear how the patterns in the human heart compare with those found in the mouse. This issue is of importance given the wide use of transgenic mice to investigate gene function with the aim of extrapolating the results to human. In the present study we applied immunoconfocal microscopy to investigate the spatial distribution of the three connexins in the developing mouse heart and foetal human heart. Although Cx45 labelling was present at low levels throughout the developing mouse heart and human foetal (9-week) heart, it was most prominent in the conduction tissues. In the developing mouse heart, Cx40 was widely expressed at embryonic day 12.5 (E12.5) but at E17.5 expression was restricted to the conduction tissues and atria. In the 9-week human foetal heart, the Cx40 labelling pattern was similar to the E15 mouse heart, being far more abundant in conduction tissues (bundle branches to Purkinje fibres) and atria than in the ventricular muscle. Cx43 labelling became more apparent in the ventricular myocardium as development of the mouse heart progressed but was virtually undetectable in the central conduction system. In the human foetal heart Cx43 was virtually undetectable in the atria but was the predominant connexin in the ventricles. We conclude that, at least in some key aspects, the pattern of connexin expression in the developing mouse heart parallels that found in the human embryonic heart.

Alteration of gap junctions and connexins in the right atrial appendage during cardiopulmonary bypass

OBJECTIVES

We investigated the influence of cardiopulmonary bypass on cardiomyocyte gap junctions and connexins.

METHODS

Samples were collected at intervals during operation from the right atrial appendage in 21 patients (mean [+/- SD] age 55 +/- 21 years). Immunodetection of connexins was conducted by Western blotting and confocal microscopy with parallel electron microscopic examination of gap junctions.

RESULTS

Downregulation of connexin 43 during the course of operation occurred in more than half of the patients. The mean densitometric value of connexin 43 decreased by 23%, with samples from patients with coronary artery disease showing a greater reduction than seen in those from patients with other diseases (31% +/- 22% vs 10% +/- 24%, P =.04). Such alterations were confirmed by confocal microscopy, which also demonstrated reduced connexin 45 immunolabeling in most patients. Electron microscopy revealed a reduction in the dimensions of cell membrane-located gap junctions and more frequent intracytoplasmic gap junctional membrane in samples from later time points (P =.04).

CONCLUSIONS

Downregulation of connexins accompanied by a reduction in gap junctions is common in the cardiomyocytes of the right atrial appendage during cardiopulmonary bypass. The association of a marked reduction in connexin 43 with coronary artery disease may imply inadequate intraoperative cardiac protection in patients with this disease.

Heterogeneous expression of connexins in rabbit sinoatrial node cells: correlation between connexin isotype and cell size

OBJECTIVE

Intercellular coupling through gap junctions allows the morphologically and functionally heterogeneous sinoatrial node to synchronize and drive the atrial muscle. The purpose of this study was to identify the connexin isotypes expressed by sinoatrial node cells and to analyse the density of connexins in relation to cell size.

METHODS

Labeling for the different connexins using isotype-specific antibodies was assessed in cells isolated from the rabbit sinoatrial node by immunoconfocal microscopy.

RESULTS

Sinoatrial node cells with a cell projection area smaller than 800 microm(2) were devoid of immunolabeling for connexin43. Such small cells showed high levels of connexin45 labeling (compared to that in large cells) and low levels of connexin40 labeling. Sinoatrial node cells with a projection area between 800 and 1200 microm(2) had a lower amount of connexin45 label and again a small amount of connexin40 but an increased amount of connexin43 label. In the larger sinoatrial node cells, some colocalization of connexin45 and connexin43 immunolabeled spots was observed.

CONCLUSIONS

Rabbit sinoatrial node cells are heterogeneous in terms of connexin expression, and there is a clear cell size-dependence in pattern of connexin expression. Small (putative central) cells express connexin45 but not connexin43, whereas larger (putative peripheral) cells express both connexin45 and connexin43. The co-localization of connexin43 and connexin45 in larger cells raises the possibility that heterotypic or heteromeric connexin43/connexin45 channels could be present in gap junctions at the periphery of the sinoatrial node.

Multiple connexins localized to individual gap-junctional plaques in human myometrial smooth muscle

The synchronous contractions of the uterus in labour depend on electrical coupling of myometrial smooth muscle cells by gap junctions. In the human myometrium, gap junctions are scarce in the non-pregnant uterus, but become abundant at term in preparation for labour. We have previously demonstrated that in the human myometrium at term, three different gap-junctional proteins are expressed, connexins 43, 45, and 40. These connexins are known to have distinctive functional capacities in in vitro expression systems but whether, in the human myometrium in vivo, they are co-assembled into the same gap junction or form different types of gap junction has previously been unclear. By applying triple immunogold labelling to sections of Lowicryl-embedded tissue for electron microscopy, together with complementary immunoconfocal microscopy, we demonstrate here that connexins 43, 45, and 40 are commonly present as mixtures within the same gap-junctional plaque. While all gap junctions contain connexin43, the relative signal for each connexin type varies between individual junctions. The presence within single gap-junctional plaques of three different connexins, each with the potential for conferring distinctive channel properties, suggests an inherent versatility for modulation of smooth muscle cell intercellular communication properties during human parturition.

Regional differentiation of desmin, connexin43, and connexin45 expression patterns in rat aortic smooth muscle

The gap-junctional protein, connexin43, is differentially expressed in vascular smooth muscle cells (SMCs) according to phenotype. Previous studies suggest that desmin-negative SMCs are characterized by high levels of connexin43, whereas desmin-positive SMCs (of a more contractile phenotype) typically have low connexin43 levels. In this study, we examine systematically the inverse relationship between connexin43 and desmin in SMCs of defined regions of the rat aortic media and determine whether additional connexin isotypes are expressed and contribute to this relationship. Immunoconfocal microscopy demonstrated that (1) the inverse relationship between connexin43 and desmin expression holds true for the media of sequential aortic zones, with 1 exception, the ascending aorta, and (2) an additional vascular connexin, connexin45, is expressed by aortic SMCs. Examination of connexin43, connexin45, and desmin expression in sequential aortic zones reveals 3 SMC subpopulations. The first, predominating in the aortic arch and thoracic aorta, is desmin negative and contains high connexin43 levels; the second, predominating in the abdominal aorta and iliac artery, is desmin positive and contains low connexin43 levels; and the third, which is restricted to the ascending aorta, is desmin positive and expresses high connexin43 levels. Connexin45 levels are high in the ascending aorta but low in the other aortic segments. In para-aortic veins, a fourth SMC subpopulation appears, one that is desmin positive and contains connexin45 but not connexin43. These results demonstrate that a diversity of connexin expression patterns characterizes distinctive subpopulations of medial SMCs in situ with a potential to contribute to regional differentiation of vascular function.

The gap-junctional protein connexin40 is elevated in patients susceptible to postoperative atrial fibrillation

BACKGROUND

Atrial fibrillation (AF), a cardiac arrhythmia arising from atrial re-entrant circuits, is a common complication after cardiac surgery, but the proarrhythmic substrate underlying the development of postoperative AF remains unclear. This study investigated the hypothesis that altered expression of connexins, the component proteins of gap junctions, is a determinant of a predisposition to AF.

METHODS AND RESULTS

The expression of the 3 atrial connexins-connexins 43, 40, and 45-was analyzed at the mRNA and protein levels by Northern and Western blotting techniques and immunoconfocal microscopy in right atrial appendages from patients with ischemic heart disease who were undergoing coronary artery bypass surgery. Twenty percent of the patients subsequently developed AF, which allowed retrospective division of the samples into 2 groups, non-AF and AF. Connexin43 and connexin45 transcript and protein levels did not differ between the groups. However, connexin40 transcript and protein were expressed at significantly higher levels in the AF group. Connexin40 protein was markedly heterogeneous in distribution.

CONCLUSIONS

Atrial myocardium susceptible to AF is distinguished from its nonsusceptible counterpart by elevated connexin40 expression. The heterogeneity of connexin distribution could give rise to different resistive properties and conduction velocities in spatially adjacent regions of tissue, which become enhanced and, hence, proarrhythmic the higher the overall level of connexin40.

Immunocytochemical analysis of connexin expression in the healthy and diseased cardiovascular system

Gap junctions play essential roles in the normal function of the heart and arteries, mediating the spread of the electrical impulse that stimulates synchronized contraction of the cardiac chambers, and contributing to co-ordination of activities between cells of the arterial wall. In common with other multicellular systems, cardiovascular tissues express multiple connexin isotypes that confer distinctive channel properties. This review highlights how state-of-the-art immunocytochemical and cellular imaging techniques, as part of a multidisciplinary approach in gap junction research, have advanced our understanding of connexin diversity in cardiovascular cell function in health and disease. In the heart, spatially defined patterns of expression of three connexin isotypes-connexin43, connexin40, and connexin45-underlie the precisely orchestrated patterns of current flow governing the normal cardiac rhythm. Derangement of gap junction organization and/or reduced expression of connexin43 are associated with arrhythmic tendency in the diseased human ventricle, and high levels of connexin40 in the atrium are associated with increased risk of developing atrial fibrillation after coronary by-pass surgery. In the major arteries, endothelial gap junctions may simultaneously express three connexin isotypes, connexin40, connexin37, and connexin43; underlying medial smooth muscle, by contrast, predominantly expresses connexin43, with connexin45 additionally expressed at restricted sites. In normal arterial smooth muscle, the abundance of connexin43 gap junctions varies according to vascular site, and shows an inverse relationship with desmin expression and positive correlation with the quantity of extracellular matrix. Increased connexin43 expression between smooth muscle cells is closely linked to phenotypic transformation in early human coronary atherosclerosis and in the response of the arterial wall to injury. Current evidence thus suggests that gap junctions in both their guises, as pathways for cell-to-cell signaling in the vessel wall and as pathways for impulse conduction in the heart, contribute to the initial pathogenesis and eventual clinical manifestation of human cardiovascular disease.

Altered connexin expression in human congestive heart failure

Congestive heart failure is associated with a high risk of life-threatening ventricular re-entrant arrhythmias. Down-regulation of the principal gap-junctional protein of the ventricular myocytes, connexin43, has previously been implicated in arrhythmia in ischaemic heart disease, but it is not known whether connexin43 is similarly reduced in heart failure due to idiopathic dilated cardiomyopathy, whether disease-related connexin43 down-regulation occurs at the level of transcription or translation, or whether the expression of other connexin isotypes is altered in congestive heart failure. We therefore investigated the expression of the four connexins expressed in the heart-connexins 43, 40, 45 and 37-at the mRNA and protein levels in explanted hearts from transplant patients with end-stage heart failure (NYHA class 4) by immunoconfocal analysis, and northern and western blotting. Connexin43 mRNA and protein were markedly downregulated in the left ventricle in end-stage heart failure due both to ischaemic cardiomyopathy and idiopathic dilated cardiomyopathy. Connexin43 content was spatially heterogeneous in the diseased ventricle. Connexin40 mRNA was increased in the ischaemic group, more so in the left ventricle than the right. This correlated with an increased depth of connexin40 protein expression in myocytes at the endocardial surface. Connexin45 mRNA and protein, present only in very low quantities, followed a similar trend to connexin43, while connexin37 (exclusively expressed in endothelium) showed no change. Our findings show that congestive heart failure is associated with significantly reduced levels of the principal gap junction protein, connexin43, in the left ventricle, potentially contributing to enhanced arrhythmogenicity and contractile dysfunction. This down-regulation is due predominantly to a reduced transcript steady-state level. Elevated connexin40 may represent a compensatory response that improves the spread of depolarization in the otherwise compromised ischaemic ventricle.

Connexin45 is the first connexin to be expressed in the central conduction system of the mouse heart

OBJECTIVE

To examine the spatial pattern of labelling for the gap junctional protein, connexin45, in relation to that of the other two cardiac connexins, connexin40 and connexin43, during the development of the central conduction system in mouse heart.

ANIMALS AND METHODS

Hearts from Balb-c mice at stages from embryonic day (E) 12.5 to adult were frozen and sectioned. The sections were immunolabelled for connexins 45, 40 and 43 using fully characterized connexin-specific antibodies. Labelled sections were observed using confocal microscopy. Single, double and triple labelling were employed with sequential scanning to record images from multiple-labelled sections for the analysis of the spatial distribution of the three connexin types in relation to each other.

RESULTS

High levels of connexin45 label were detected in specific regions within the developing mouse heart. These regions corresponded to the conus myocardium, developing interatrial septum and other developing conduction tissues of the heart. Connexin40 label was initially absent from these tissues but by E15.5 was present in the more distal regions of the conduction system. However, by E17.5, connexin45 and 40 labelling was similar to the pattern observed in the adult heart, with both connexins present in most regions of the conduction system, though they were not completely colocalized. Connexin43 label was not observed in the regions of high connexin45 labelling.

CONCLUSIONS

These results show connexin45 to be the earliest detectable connexin in the central conduction system and to be the only connexin present throughout the whole conduction system. A distinct temporal pattern of connexin expression was also shown to occur during the development of the conduction tissues of the mouse heart.

Comparison of connexin 43, 40 and 45 expression patterns in the developing human and mouse hearts

The mouse is currently widely used as a model organism in the analysis of gene function but how developmentally regulated patterns of connexin gene expression in the mouse compare with those in the human is unclear. Here we compare the patterns of connexin expression in the heart during the development of the mouse (from embryonic day 12.5 to 6 weeks postpartum) and the human (at 9 weeks gestation and adult stage). The extent of connexin43 expression in the ventricles progressively increased during development of the mouse heart. The developmental pattern of expression for connexins 40 and 45 in the mouse heart was similar, but not identical, and in the ventricles showed a progressive and preferential expression in the conduction system. In general, these dynamic changes of connexins 43, 40 and 45 during mouse cardiac development appear to be mirrored in the human.

Presence of the Kv1.5 K(+) channel in the sinoatrial node

The aim of this study was to establish, using immunolabeling, whether the Kv1.5 K(+) channel is present in the pacemaker of the heart, the sinoatrial (SA) node. In the atrial muscle surrounding the SA node and in the SA node itself (from guinea pig and ferret), Western blotting analysis showed a major band of the expected molecular weight, approximately 64 kD. Confocal microscopy and immunofluorescence labeling showed Kv1.5 labeling clustered in atrial muscle but punctate in the SA node. In atrial muscle, Kv1.5 labeling was closely associated with labeling of Cx43 (gap junction protein) and DPI/II (desmosomal protein), whereas in SA node Kv1.5 labeling was closely associated with labeling of DPI/II but not labeling of Cx43 (absent in the SA node) or Cx45 (another gap junction protein present in the SA node). Electron microscopy and immunogold labeling showed that the Kv1.5 labeling in atrial muscle is preferentially associated with desmosomes rather than gap junctions.

Connexin45, a major connexin of the rabbit sinoatrial node, is co-expressed with connexin43 in a restricted zone at the nodal-crista terminalis border

The pacemaker of the heart, the sinoatrial (SA) node, is characterized by unique electrical coupling properties. To investigate the contribution of gap junction organization and composition to these properties, the spatial pattern of expression of three gap junctional proteins, connexin45 (Cx45), connexin40 (Cx40), and connexin43 (Cx43), was investigated by immunocytochemistry combined with confocal microscopy. The SA nodal regions of rabbits were dissected and rapidly frozen. Serial cryosections were double labeled for Cx45 and Cx43 and for Cx40 and Cx43, using pairs of antibody probes raised in different species. Dual-channel scanning confocal microscopy was applied to allow simultaneous visualization of the different connexins. Cx45 and Cx40, but not Cx43, were expressed in the central SA node. The major part of the SA nodal-crista terminalis border revealed a sharply demarcated boundary between Cx43-expressing myocytes of the crista terminalis and Cx45/Cx40-expressing myocytes of the node. On the endocardial side, however, a transitional zone between the crista terminalis and the periphery of the node was detected in which Cx43 and Cx45 expression merged. These distinct patterns of connexin compartmentation and merger identified suggest a morphological basis for minimization of contact between the tissues, thereby restricting the hyperpolarizing influence of the atrial muscle on the SA node while maintaining a communication route for directed exit of the impulse into the crista terminalis.

Chamber-related differences in connexin expression in the human heart

Electrical coupling in the heart is mediated by gap junctions, aggregates of cell-to-cell channels composed of connexins. The principal cardiac gap-junctional connexin, connexin43 (Cx43), is reduced in diseased human myocardium that is prone to arrhythmia. Three additional connexin isoforms, Cx40, Cx45 and Cx37, of distinctive functional capacities in vitro, are expressed in cardiovascular cells, but our knowledge of their expression patterns in the human heart is fragmentary. In the present study, we therefore applied Northern blotting, Western blotting and immunoconfocal microscopy to analyse and compare the expression of Cx43, Cx40, Cx37 and Cx45 mRNA and protein in the human left ventricle, right ventricle, left atrium and right atrium of the human heart. Cx43 was confirmed to be abundantly expressed at similar levels by myocytes in all four chambers. Cx40 levels varied between chambers in the order right atrium >left atrium >/= right ventricle approximately left ventricle. Cx37 (exclusively expressed in the endothelium) was expressed at similar overall levels in all chambers (as judged from Northern blots). Cx45 was detectable only at very low levels, with a trend toward higher levels in the atria than the ventricles in a pattern similar to Cx40. The results indicate that in humans, the ventricles and atria have distinctive connexin expression profiles, and that the atrial-type connexin profile is more pronounced in the right atrium than the left atrium. While the ventricular connexin expression pattern resembles that of other mammalian species, atrial connexin expression shows greater species variation. These differences contribute to the interpretative framework for examining the potential role of altered connexin expression in ventricular and atrial arrhythmia in the human heart.

Connexin make-up of endothelial gap junctions in the rat pulmonary artery as revealed by immunoconfocal microscopy and triple-label immunogold electron microscopy

Integration of vascular endothelial function relies on multiple signaling mechanisms, including direct cell-cell communication through gap junctions. Gap junction proteins expressed in the endothelium include connexin37, connexin40, and connexin43. To investigate whether individual endothelial cells in vivo express all three connexin types and, if so, whether multiple connexins are assembled into the same gap junction plaque, we used affinity-purified connexin-specific antibodies raised in three different species to permit multiple-label immunoconfocal and immunoelectron microscopy in the rat main pulmonary artery. Immunoconfocal microscopy showed a high incidence of co-localization between connexin43 and connexin40, but lower incidences of co-localization between connexin37 and connexin40 or connexin43. Immunoelectron microscopy revealed that 83% of gap junction profiles contained all three connexins, with the proportion of connexin40 labeling being significantly higher than that of connexin37 or connexin43. The presence of three different connexin types of distinct properties in vitro provides potential for complex regulation and functional differentiation of endothelial intercellular communication properties in vivo.