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Eur J Cardiothorac Surg 2002;21:847-852
© 2002 Elsevier Science NL
a Department of Surgery, Duke University Medical Center, Durham, NC 27710, USA
b Department of Biochemistry, Duke University Medical Center, Durham, NC 27710, USA
Received 15 September 2001; received in revised form 22 December 2001; accepted 30 January 2002.
* Corresponding author. Present address: Department of Cardiac Surgery, Royal Victoria Hospital, Belfast BT12 6BA, UK. Tel.: +44-2890-263000; fax: +44-2890-894918
e-mail: mark{at}jmarkjones.freeserve.co.uk
| Abstract |
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Key Words: Cardiopulmonary bypass Gene therapy ß-Adrenergic receptor Myocardial protection Surgery
| 1. Introduction |
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However, transgenic models differ greatly from gene transfer models. Many existing methods of vector delivery have limited clinical applicability. Direct myocardial injection has confirmed the possibility of gene transfer to the myocardium [4]. Intracoronary delivery in rabbits has been performed during thoracotomy, with the aorta cross-clamped and the virus introduced by injection into the left ventricular cavity [57]. Due to the occluded aorta, left ventricular ejection forces the virus down the coronary arteries. However, the risk of ischaemia and acute pressure overload of the left ventricle limit the period of cross-clamping. A more clinically applicable method is direct catheterization of the right or left coronary artery [8]. Here too, exposure of the heart to the virus is limited as the adenovirus is rapidly washed out into the systemic circulation and other tissues have shown evidence of transgene expression [5,6,9].
By means of a larger animal model we have developed a method of gene delivery to the myocardium that could be more clinically applicable. During standard cardiopulmonary bypass (CPB) the heart was arrested, the aorta cross-clamped and the adenoviral vector administered after the cold cardioplegia. Global cardiac transgene expression was obtained with no evidence of extracardiac expression [10].
In a Langendorff perfused rabbit heart model it has been demonstrated that temperature, duration of exposure to vector and the solution in which the vector is delivered can all influence cardiac transgene overexpression [11]. Therefore, the current study was undertaken to investigate the effect which these factors have on adenoviral based gene transfer in the cardiac surgery model.
| 2. Material and methods |
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2.2. Animals
Twenty-six neonatal piglets (12 weeks old), weighing approximately 3 kg were used in these studies. Six-month old pigs weighing up to 40 kg were killed to provide fresh blood for priming of the CPB circuit. All animals received humane care in accordance with the Guide for the Care and Use of Laboratory Animals prepared by the National Academy of Sciences and published by the National Institutes of Health (NIH Publication No. 85-23, revised in 1985). The experimental protocol was approved by the institutional Animal Care and Use Committee.
2.3. Surgery, cardiopulmonary bypass and gene delivery
Animals were anaesthetized and CPB was established as previously described [10]. After stabilization, the aorta was cross-clamped and the heart arrested by infusion of cardioplegia into the aortic root or by induction of ventricular fibrillation. The cardioplegia consisted of lactated Ringer's solution with 40 mmol/l KCl added (Abbott Laboratories, North Chicago, IL). A volume of 30 ml/kg cardioplegia was injected via a hand-held syringe at a rate of approximately 50 ml/min through a 21-gauge butterfly cannula (Abbott Laboratories) inserted into the proximal aorta. Immediately following delivery of cardioplegia, or induction of fibrillatory arrest, the adenoviral vector, in 8 ml PBS, was injected through the cardioplegia cannula over a 10-s period. The catheter and proximal aorta were flushed with a further 2 ml of PBS. Saline slush was placed within the pericardial cavity to provide topical cooling and aid myocardial protection for those groups receiving cold myocardial protection techniques.
In order to test the effect of temperature, duration of arrest and type of myocardial protection on transgene expression, a total of four test groups (Adeno-ß2AR; Groups AD) were employed and one control group (Adeno-ßgal; Group E). For each group, 1x1012 total viral particles (tvp) were administered into the aortic root following cross-clamping of the aorta and arrest of the heart. Specifically, Group A (n=4) underwent cold crystalloid cardioplegia arrest for 10 min, Group B (n=6) underwent cold crystalloid cardioplegia arrest for 30 min, Group C (n=5) underwent warm crystalloid cardioplegia arrest for 10 min, and Group D (n=5) underwent warm fibrillatory arrest for 10 min. Group E (n=6) was treated with Adeno-ßgal following cold crystalloid cardioplegia arrest (30 min).
Therefore, the vector was allowed to dwell in the coronary circulation for 10 min in Groups A, C and D prior to release of the aortic cross-clamp. In Groups B and E a 30-min dwell-time was allowed before the cross-clamp was released. The heart was defibrillated as required, the animals were weaned off CPB and allowed to recover.
2.4. Collection of tissue samples
Two days following surgery, euthanasia was performed, using an intravenous injection of 16 mEq potassium chloride solution. The chest was reopened and the heart rapidly excised.
2.5. ß-Galactosidase staining
After excision of the heart, transverse cross-sections of myocardium at the level of the base of the papillary muscle were obtained. Frozen samples were mounted on a cryostat and 10-µm sections were cut, which were then mounted on a glass microscope slide. ß-Galactosidase staining was performed in 20 mM K4Fe3(CN)6, 20 mM K3Fe2(CN)6, 4 mM MgCl2, 0.5 mg/ml X-Gal (5-bromo-4-chloro-3-indoyl-ß-D-galactopyranoside) in 1 M PBS at 37 °C for 90 min [6].
2.6. Preparation of myocardial membranes
Tissue samples from the anterior left ventricle (LV) and right ventricle (RV) were homogenized in lysis buffer (5 mM TrisHCl (pH 7.4), 5 mM EDTA). Membrane fractions were resuspended in binding buffer (75 mM TrisHCl (pH 7.4), 12.5 mM MgCl2, and 2 mM EDTA) at a concentration of approximately 1 mg/ml of membrane protein as determined by the Bradford method [2,13].
2.7. Ligand binding assay
Ligand binding assays were performed in triplicate on membranes with saturating concentrations of the radiolabelled ß-adrenoceptor ligand, [125I]cyanopindolol. Non-specific binding was determined in the presence of alprenolol (20 µM). Assays were performed at 37 °C for 60 min [2,6]. The resulting specific binding (estimated Bmax) was reported as the receptor density (femtomoles) and was normalized to milligrams of membrane protein (fmol/mg membrane protein).
2.8. Statistics
Values are reported as mean±standard error of the mean (SEM). Comparisons of receptor density between groups were performed using the non-parametric MannWhitney U-test.
| 3. Results |
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| 4. Discussion |
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The optimal conditions for adenoviral based gene transfer to cultured rabbit ventricular myocytes in vitro have been investigated [11]. Delivery of Adeno-ßgal at 37 °C caused infection of approximately 50% of myocytes after 10 s exposure but this rose to 100% infection following 10 min of contact. When incubated at 37 °C the time to half-maximal infection was 19 min. Reduction of the temperature to 24 °C prolonged the time to half-maximal infection with no change in the final peak but when the temperature was reduced to 4 °C both the time to half-maximal infection and the final peak were reduced. The presence of heparinized rabbit blood caused a 60% reduction in the number of infected cells compared with incubating the myocytes and virus solution in M199 media or Krebs buffer. The optimal conditions for adenoviral based gene transfer were investigated further in a Langendorff perfused rabbit heart. A single pass of the virus solution containing 5x109 plaque forming units through the heart caused infection of 0.8% myocytes in contrast to 40% infection when the virus perfusate was recirculated for 60 min.
In view of these findings it was surprising that in an in vivo delivery system temperature did not seem to be important for improving gene transfer. LV ßAR density was actually lower for those groups that had warm conditions during the period that the virus solution was allowed to dwell in the coronary circulation, namely warm crystalloid cardioplegia and ventricular fibrillation, compared with a similar 10-min cross-clamp period following cold crystalloid cardioplegia. The absence of a significant influence of cold temperatures on transgene expression in an in vivo model may be due to the fact that attachment of adenovirus to cells occurs efficiently at 0 °C [14]. Furthermore, it is possible that while the viral vector is delivered to the interstitium under cold conditions it may only be taken up into myocytes after warm reperfusion. Myocardial temperature was not monitored in these experiments but it is clear that myocardial temperatures will quickly return towards the systemic body temperature of 37 °C following the return of coronary blood flow, after release of the aortic cross-clamp. Thus, while subsequent steps in adenoviral infection are inhibited at low temperatures, it seems that the temperature during the first 30 min does not limit infection [15].
The period of cross-clamping following adenoviral vector delivery may influence transgene expression. There was similar LV ßAR density in the group which had a 10-min cross-clamp period compared with the 30-min cross-clamp period after cold crystalloid cardiplegia. However, RV ßAR density was not increased in the 10-min cross-clamp period compared with the Adeno-ßgal group while the 30-min cross-clamp group did have increased RV ßAR density. In general, transgene expression in the RV was considerably lower relative to the LV. We have previously seen this pattern of reduced RV overexpression in this model [10]. For unclear reasons, RV delivery of the vector is impaired, or conversely wash-out of the vector from the RV is increased, relative to the LV.
The vector was resuspended in a crystalloid solution, namely PBS, in all groups. This was because adenovirus can agglutinate red blood cells and so potentially reduce the number of viral particles available for transfer through the endothelium and subsequent cell entry [14]. The additional presence of crystalloid cardioplegia compared with blood within the coronary circulation, as in the fibrillatory arrest group, had no discernable effect on transgene expression.
In all these cases the virus solution was administered in a bolus injection. Continuous circulation of an adenoviral vector solution through the heart has been suggested to be another potential method of gene delivery during CPB, although this requires the use of two separate circuits and oxygenators, unlike the standard approach [16]. Gene transfer has been documented using retrograde delivery of vector through the coronary sinus in a porcine beating heart model [17]. This is a further method that could also be utilized for vector delivery during cardiac surgery.
Additional work in the Langendorff perfused rabbit heart suggested that agents which increase endothelial permeability can greatly enhance the efficiency of gene transfer [18]. Cardioplegia has detrimental effects on endothelial structure and function [19,20]. Further experiments could be performed to assess if gene transfer is enhanced when the virus solution is delivered to the heart following a prolonged period of cardioplegic arrest. Contact with cardioplegia and the associated relative ischemia may increase endothelial permeability, thus overcoming one of the barriers to adenoviral infection of cardiac myocytes. Furthermore, if delivery of a gene that could enhance postoperative cardiac function was possible during CPB, this might be especially desirable towards the end of a prolonged cross-clamp period when impaired cardiac function was manifest. Although transgene expression was assessed 2 days after vector delivery in this study, we have previously documented that transgene expression is present 8 h after vector delivery [10]. Overexpression of the ß2AR has been shown to have functional consequences following gene transfer in other animal models [6,8]. This method of vector delivery could, therefore, be used to counteract postoperative ventricular dysfunction which can complicate cardiac surgical recovery [21].
This series of experiments demonstrate that in an in vivo model of simulated cardiac surgery, adenoviral based gene transfer is not impaired by the lower temperatures associated with cold crystalloid cardioplegic arrest. Shorter durations of exposure to the vector may play a limited role in the final level of transgene overexpression. The presence of blood versus crystalloid solution within the heart does not have a major effect. Thus, adenoviral based cardiac gene delivery is possible using various clinically utilized methods of myocardial protection in a cardiac surgical model.
| Acknowledgments |
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| Footnotes |
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| References |
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