Материал: nm1394

© 2006 Nature Publishing Group http://www.nature.com/naturemedicine

© 2006 Nature Publishing Group http://www.nature.com/naturemedicine

g

engraftment were evaluated by three independent means including echocardiography, magnetic resonance imaging (MRI) and left heart catheterization; (iv) acquisition of echocardiography, MRI and catheterization data and their evaluation were carried out by independent investigators blinded to the study protocol.

T E C H N I C A L R E P O R T S

Figure 1 Construction of optimized EHTs. Effects of oxygen, load and insulin were analyzed by isometric contraction experiments in EHT rings. TT, twitch tension. Direct comparison of calcium response curves showed that EHTs benefited from oxygen supplementation (O2, n ¼ 11 at 21%; n ¼ 5 at 30%; n ¼ 10 at 40%; a), auxotonic (Aux.; n ¼ 8) in contrast to

static (Sta.; n ¼ 8) or phasic (Pha.; n ¼ 8) load (b), and addition of insulin (10 mg/ml; n ¼ 4) when compared to untreated controls (n ¼ 3; c). Stacking five single EHTs (d) on a custom-made device facilitated EHT fusion and allowed contractions under auxotonic load (e), resulting in synchronously contracting multiloop EHTs (f and Supplementary Video 1 online) ready for in vivo engraftment. (g) Six single-knot sutures served to fix multiloop EHTs on the recipients’ hearts. * P o 0.05 versus 21% O2, static load and culture in the absence of insulin (Ctr.), respectively, by repeated ANOVA. Scale bars, 10 mm.

contract against load-adjusted coils; Fig. 1b) and (iii) supplementation of the culture medium with insulin (Fig. 1c). The increase in force was paralleled by an increase in total and muscle-specific calsequestrin (CSQ2) protein (Supplementary Fig. 1 online). DNA content was not affected, indicating either an increased fraction of cardiomyocytes without a relevant change in total EHT cell number or advanced development of the sarcoplasmic reticulum in cardiomyocytes. To increase graft size and facilitate implantation, we stacked five circular EHTs (Fig. 1d) crosswise on a custom-made holding device after 7 d in culture (Fig. 1e). Under these conditions, circular EHTs fused and formed synchronously contracting multiloop EHTs (diameter, B15 mm; thickness, 1–4 mm; Fig. 1f and Supplementary Video 1 online). Multiloop EHTs consisted of longitudinally oriented, thoroughly interconnected muscle strands (Supplementary Fig. 2 online) and contracted spontaneously at 1–2 Hz.

h

*

*

*

Figure 3 Electrical integration of EHTs in vivo. Representative plots of epicardial activation times in sham-operated (a) and EHT-engrafted (b) hearts. Total activation time (c) and QRS-complex voltage (d) in right, anterior, lateral and posterior segments of the investigated hearts.

(e) Point stimulation of an implanted EHT with simultaneous recording of the propagated potential in the EHT and in remote myocardium showed retrograde coupling. EHT could be uncoupled after acidification of the hearts (f). Telemetric ECG recordings showed a similar frequency of arrhythmias in sham-operated and EHT-grafted groups (n ¼ 5 per group; g).

(h) Circadian rhythm in sham-operated and EHT groups. *P o 0.05 versus sham-operated rats by ANOVA with Mann-Whitney U-test.

fluorescence of the DAPI-labeled nuclei (Fig. 2d and Supplementary Fig. 4 online). EHT-derived cardiomyocytes developed a differentiated phenotype, including in registry organization of sarcomeres (Fig. 2e). We did not observe DAPI+ cardiomyocytes in the host myocardium or DAPI– cardiomyocytes within the EHT graft. Notably, blood vessels in the EHT grafts also contained DAPI+ cells (Fig. 2f–h) including endothelial cells (Fig. 2g) and smooth muscle cells (Fig. 2h), suggesting that they were of donor origin. Identification of erythrocytes in these vessels indicated that they were connected to the recipient’s vasculature (Fig. 2h). Whether DAPI+ macrophages detected in close vicinity to these vessels (Fig. 2h) have a role in de novo vascularization remains to be investigated.

EHT grafting on infarcted hearts

We implanted multiloop EHTs into male Wistar rats with large myocardial infarcts. Sham-operated (stitches only) rats served as negative controls, and implantation of either formaldehyde-fixed EHTs or noncardiomyocyte grafts (NCMGs) served as tests for effects of scar stabilization and supply of cardiac nonmyocyte–derived paracrine factors (Supplementary Table 1 online). We performed all interventions 14 d after left descending coronary artery (LAD) ligation in rats with compromised myocardial function (fractional area shortening (FAS) o40% by echocardiography; Fig. 1g and Supplementary Note and Supplementary Fig. 3 online).

Structural integration and integrity of EHT in vivo

Four weeks after grafting, EHTs could be identified by their distinctive appearance and location (Fig. 2a). The pale color indicated that EHTs retained a high connective-tissue content. Hematoxylin and eosin (H&E)-stained paraffin sections, however, showed compact and welldifferentiated EHT-derived heart muscle (diameter, 443 ± 32 mm; n ¼ 5) covering the infarcted myocardium (Fig. 2b,c). Noncontractile grafts did not show any cardiac muscle structures. To further evaluate whether cardiomyocytes within the reconstituted myocardium were of donor origin, we labeled EHTs with DAPI (1 mg/ml) before implantation (Supplementary Fig. 4 online) and searched for DAPI+ cells 4 weeks after EHT implantation (n ¼ 10). Engrafted EHTs could be clearly distinguished from the recipient’s myocardium by the blue

Electrical coupling of EHT grafts to host myocardium

We assessed electrical coupling of EHTs to the host myocardium 4 weeks after engraftment by high-resolution epicardial mapping28 (Fig. 3). Sham-operated rats (n ¼ 5) showed the expected delay of impulse propagation in infarcted anterior and lateral segments (Fig. 3a,c). In contrast, epicardial activation was normal in EHTgrafted hearts (n ¼ 8), indicating undelayed anterograde coupling of EHTs to the host myocardium (Fig. 3b,c). QRS amplitude in lateral segments was higher in the EHT group than in the sham-operated group, providing electrophysiological evidence for myocardial reconstitution (Fig. 3d). Hearts that received noncontractile grafts (n ¼ 4) showed similar conduction deficits as shamoperated hearts and showed no active responses in most areas of the grafts.

Pacing of implanted EHTs resulted in propagated electrical responses in remote myocardium, indicating retrograde coupling (n ¼ 2; Fig. 3e). Pacing through noncontractile grafts (formalde- hyde-fixed EHTs, n ¼ 3; NCMGs, n ¼ 2) did not provoke local or propagated potentials. Acidification of the perfusion buffer to pH 6.7 led to uncoupling of EHTs from the native myocardium, providing additional proof for direct but subnormal graft-host coupling (Fig. 3f). Maximum conduction velocity in engrafted EHTs was 0.55 ± 0.16 m/s longitudinally (VL) and 0.16 ± 0.07 m/s transversally (VT; n ¼ 3), comparable to conduction velocity in noninfarcted myocardium (VL, 0.69 ± 0.06 m/s; VT, 0.19 ± 0.05 m/s; n ¼ 3).

© 2006 Nature Publishing Group

Anisotropy (VL/VT) was similar in EHT-grafted and noninfarcted myocardium (4.15 ± 0.8 versus 4.07 ± 0.91).

Electrocardiographic (ECG) telemetry in sham-operated and EHTgrafted rats (n ¼ 5 each) showed that EHT engraftment did not increase total number, frequency or severity of arrhythmias (Fig. 3g and Supplementary Fig. 5 online) and that circadian rhythm was restored 4 d after EHT or sham surgery (Fig. 3h).

Functional consequences of EHT grafting

We performed echocardiographic examinations 14 d after LAD ligation in all surviving rats (n ¼ 172). Seventy rats had a FAS below 40%, survived the complete study and were subsequently subjected to echocardiography (70 of 70), MRI (59 of 70) and catheterization (50 of 70) 28 d after EHT or noncontractile graft implantation and sham surgery (Supplementary Tables 1–4 online). In contrast to echocardiography which had a longitudinal study design, we refrained

from repeated MRI and catheterization because of the high mortality during MRI (1–2 h under anesthesia) and the availability of only one defined access to the left ventricle through the right carotid artery (catheterization).

Evaluation 14 d after infarction (Figs. 4 and 5 and Supplementary Tables 2–4 online) showed enlargement of left ventricular dimension and volume (Figs. 4a,f and 5b), reduced FAS (Fig. 4c), decreased anterior wall thickening fraction (AWThF; Fig. 4g) without notable effects on the posterior wall thickening fraction (PWThF; Fig. 4h), increased left ventricular end-diastolic pressure (LVEDP; Fig. 5c) and tau (index of relaxation; Fig. 5d) and rightward-shifted pressurevolume loops (Fig. 5a).

Twenty-eight days later, echocardiography, MRI and catheterization data collectively indicated further deterioration in sham-operated rats (Figs. 4 and 5 and Supplementary Tables 2–4 online). In particular, left ventricular end-diastolic dimension (LVEDD) increased

a

Pressure (mmHg) Pressure (mmHg)

Volume ( l)

Pressure (mmHg) Pressure (mmHg)

Volume ( l)

P < 0.05

Healthy

MI baseline

Sham

EHT

© 2006 Nature Publishing Group http://www.nature.com/naturemedicine

(Fig. 4a,b) and FAS decreased (Fig. 4c,d). Moreover, LVEDP and tau increased markedly (Fig. 5c,d). In contrast, LVEDD and FAS remained unchanged in the EHT-grafted group; that is, LVEDD and FAS did not differ from baseline values 2 weeks after LAD ligation (Fig. 4a–d). In fact, retrospective stratification according to FAS (20–30% versus 30–40% at baseline) showed that FAS significantly (P ¼ 0.01) increased after EHT implantation in rats with lower baseline FAS (Supplementary Fig. 3 and Supplementary Tables 2–4 online).

MRI (Fig. 4e–h) supported the echocardiography data, showing smaller left ventricular volumes in EHT-grafted rats when compared to sham-operated rats (Fig. 4f). Moreover, MRI-recorded AWThF was markedly better in EHT-grafted rats when compared to sham-oper- ated rats (Fig. 4g). PWThF was not affected (Fig. 5h). In the group of rats that received noncontractile grafts (Supplementary Tables 2–4 online), AWThF was similar to sham-operated and myocardial infarction baseline values.

Pressure-volume loop analyses (Fig. 5a–d) supported these data, showing that left ventricular dilation was less (Fig. 5b) and LVEDP as well as tau were markedly decreased in EHT-grafted rats compared to sham-operated rats (Fig. 5c,d). Notably, EHT grafting resulted in a better catheterization-determined ejection fraction only in rats with a low baseline FAS (Supplementary Fig. 3 online).

DISCUSSION

We evaluated the hypothesis that myocardial function after infarction can be improved by EHT grafts. Our data show that EHTs integrate and electrically couple to host myocardium and exert beneficial effects on systolic and diastolic left ventricular function. Although we neither observed nor expected complete reversal of myocardial dysfunction after EHT engraftment, we believe that our study can serve as a proof of principle for a tissue engineering approach in repair of cardiac muscle.

A prerequisite for successful cardiac repair is that grafted cells electrically and mechanically couple to native myocardium and do not elicit arrhythmias29. Here, we observed normalization of epicardial impulse propagation after EHT engraftment and observed no evidence for arrhythmogenicity. This normalization occurs despite spontaneous electrical activity of EHTs, suggesting that with the establishment of electrical coupling, EHTs are likely to be overpaced by the recipient heart, beating at B5 Hz (EHTs, 1–2 Hz) and may then chronically adapt by electrical conditioning. Another reason for the apparent lack of EHT-induced arrhythmias despite electrical coupling may be that electrical contacts between EHT and recipient myocardium were subnormal, resulting in a low propensity of retrograde myocardial activation by EHT. This notion is supported by the finding that EHTs could be uncoupled from recipient myocardium by pH lowering. Proarrhythmic risks of EHT implantation, however, are likely to be of more relevance in larger, low-heart-rate species including humans and will need further investigation.

The observation that EHTs structurally and electrically integrate into host myocardium is not trivial given the fact that EHTs are not homogeneous heart muscles but organoids consisting of muscle strands, primitive capillaries, fibroblasts, smooth muscle cells and macrophages in a collagen matrix23. One may ask how cardiomyocytes and other structures in EHTs contact host myocardium. A possible explanation is that the implantation procedures injure EHTs and host myocardium and thereby destroy physiological barriers (epicardium of native hearts and nonmyocyte surface lining of EHTs). In addition, spontaneous fusion of single EHTs to multiloop configurations indicates a marked remodeling capacity of EHTs. The observation that blood vessels inside implanted EHTs were DAPI+ is intriguing and

implies that they are, at least in part, derived from EHTs. This may seem unexpected, but we have previously observed primitive capillaries in EHTs in vitro23. We have also documented that hostderived blood vessels start to invade EHTs early after implantation27. Thus, the most probable explanation is that host blood vessels contact primitive vascular structures in EHTs or that donor cells may be recruited during de novo vascularization. This may contribute to the survival and formation of thick muscle structures after implantation.

We took care to study functional consequences of EHT implantation under well-controlled conditions. These conditions include the well-validated rat LAD-ligation model, large groups of rats with predefined inclusion criteria, sham-operated and noncontractile graft controls, determination of heart function by three different high-end technologies and data evaluation by indepen-

This was mainly a consequence of high mortality after myocardial infarction (172 of 343 survived; 50%) as well as variability of infarct size and its functional consequences (80 of 172 had a FAS of 440%). We made the following observations: EHT-grafted hearts were smaller, had lower LVEDP and tau and better FAS and AWThF; that is, they showed better diastolic and systolic function. Our longitudinal echocardiography studies showed that part of this difference resulted from preservation of the myocardial infarction baseline state. We observed that rats with the worst myocardial baseline function (FAS, 20–30%), however, showed a substantial improvement after EHT engraftment.

The fact that we did not observe a similar effect in rats with noncontractile grafts suggests that the improvement of regional systolic function resulted from cardiomyocyte engraftment and not merely from scar stabilization or paracrine effects. Noncontractile grafts seemed to exert some beneficial effects on ventricular dimensions and ejection fraction, possibly resulting from ventricle-stabilizing effects. The absence of DAPI+ cardiomyocytes in recipient myocardium or DAPI– cardiomyocytes in engrafted DAPI-labeled EHTs argue against a substantial contribution of endogenous or exogenous cardiac progenitors.

Yet is improvement of cardiac function a realistic consequence of EHT implantation? After implantation, we observed the formation of new compact heart muscle with a thickness of 443 ± 32 mm. This thickness is close to the size of a rat papillary muscle and could indeed account for a relevant improvement of wall motion. Future experiments will have to evaluate whether further EHT conditioning in vitro or optimized implantation techniques will improve the functional effect of EHT grafting.

Several important questions remain in terms of a potential clinical

application. The most obvious relates to a suitable cell source. Autologous stem cells with a cardiogeneic potential13,14,19,20,30 would

be ideal, and we are currently evaluating whether they can be used for generating EHTs. The EHT culture format may in fact provide a milieu that supports cardiac differentiation23. Preliminary experiments with human embryonic stem cells support this notion, offering a perspective for the generation of human EHTs (W.-H.Z., I. Kehat & L. Gepstein, unpublished data). A second problem is the immunogenicity of EHTs in their present form27, and we are currently trying to establish serumand Matrigel-free culture conditions. A third caveat pertains to graft size, and it is not clear whether the functional outcome will improve with the number of EHTs implanted. Despite these and other open questions, our study provides a realistic therapeutic perspective for cardiac tissue engineering.