Erik Kohlbrenner

Structure of adeno-associated virus serotype 8, a gene therapy vector.

ABSTRACT Adeno-associated viruses (AAVs) are being developed as gene therapy vectors, and their efficacy could be improved by a detailed understanding of their viral capsid structures. AAV serotype 8 (AAV8) shows a significantly greater liver transduction efficiency than those of other serotypes, which has resulted in efforts to develop this virus as a gene therapy vector for hemophilia A and familial hypercholesterolemia. Pseudotyping studies show that the differential tissue tropism and transduction efficiencies exhibited by the AAVs result from differences in their capsid viral protein (VP) amino acids. Towards identifying the structural features underpinning these disparities, we report the crystal structure of the AAV8 viral capsid determined to 2.6-Å resolution. The overall topology of its common overlapping VP is similar to that previously reported for the crystal structures of AAV2 and AAV4, with an eight-stranded β-barrel and long loops between the β-strands. The most significant structural differences between AAV8 and AAV2 (the best-characterized serotype) are located on the capsid surface at protrusions surrounding the two-, three-, and fivefold axes at residues reported to control transduction efficiency and antibody recognition for AAV2. In addition, a comparison of the AAV8 and AAV2 capsid surface amino acids showed a reduced distribution of basic charge for AAV8 at the mapped AAV2 heparin sulfate receptor binding region, consistent with an observed non-heparin-binding phenotype for AAV8. Thus, this AAV8 structure provides an additional platform for mutagenesis efforts to characterize AAV capsid regions responsible for differential cellular tropism, transduction, and antigenicity for these promising gene therapy vectors.

SERCA2a Gene Transfer Decreases Sarcoplasmic Reticulum Calcium Leak and Reduces Ventricular Arrhythmias in a Model of Chronic Heart Failure

Background—Sarcoplasmic reticulum calcium ATPase 2a (SERCA2a) gene therapy improves mechanical function in heart failure and is under evaluation in a clinical trial. A critical question is whether SERCA2a gene therapy predisposes to increased sarcoplasmic reticulum calcium (SR Ca2+) leak, cellular triggered activity, and ventricular arrhythmias in the failing heart. Methods and Results—We studied the influence of SERCA2a gene therapy on ventricular arrhythmogenesis in a rat chronic heart failure model. ECG telemetry studies revealed a significant antiarrhythmic effect of SERCA2a gene therapy with reduction of both spontaneous and catecholamine-induced arrhythmias in vivo. SERCA2a gene therapy also reduced susceptibility to reentry arrhythmias in ex vivo programmed electrical stimulation studies. Subcellular Ca2+ homeostasis and spontaneous SR Ca2+ leak characteristics were measured in failing cardiomyocytes transfected in vivo with a novel AAV9.SERCA2a vector. SR Ca2+ leak was reduced after SERCA2a gene therapy, with reversal of the greater spark mass observed in the failing myocytes, despite normalization of SR Ca2+ load. SERCA2a reduced ryanodine receptor phosphorylation, thereby resetting SR Ca2+ leak threshold, leading to reduced triggered activity in vitro. Both indirect effects of reverse remodeling and direct SERCA2a effects appear to underlie the antiarrhythmic action. Conclusions—SERCA2a gene therapy stabilizes SR Ca2+ load, reduces ryanodine receptor phosphorylation and decreases SR Ca2+ leak, and reduces cellular triggered activity in vitro and spontaneous and catecholamine-induced ventricular arrhythmias in vivo in failing hearts. SERCA2a gene therapy did not therefore predispose to arrhythmias and may represent a novel antiarrhythmic strategy in heart failure.

Neutralizing Antibodies Against AAV Serotypes 1, 2, 6, and 9 in Sera of Commonly Used Animal Models

Adeno-associated virus (AAV)-based vectors are promising gene delivery vehicles for human gene transfer. One significant obstacle to AAV-based gene therapy is the high prevalence of neutralizing antibodies in humans. Until now, it was thought that, except for nonhuman primates, pre-existing neutralizing antibodies are not a problem in small or large animal models for gene therapy. Here, we demonstrate that sera of several animal models of cardiovascular diseases harbor pre-existing antibodies against the cardiotropic AAV serotypes AAV1, AAV6, and AAV9 and against AAV2. The neutralizing antibody titers vary widely both between species and between serotypes. Of all species tested, rats displayed the lowest levels of neutralizing antibodies. Surprisingly, naive mice obtained directly from commercial vendors harbored neutralizing antibodies. Of the large animal models tested, the neutralization of AAV6 transduction by dog sera was especially pronounced. Sera of sheep and rabbits showed modest neutralization of AAV transduction whereas porcine sera strongly inhibited transduction by all AAV serotypes and displayed the largest variation between individual animals. Importantly, neutralizing antibody titers as low as 1/4 completely prevented in vivo transduction by AAV9 in rats. Our results suggest that prescreening of animals for neutralizing antibodies will be important for future gene transfer experiments in these animal models.

Plasticity of Surface Structures and β2-Adrenergic Receptor Localization in Failing Ventricular Cardiomyocytes During Recovery from Heart Failure

Background— Cardiomyocyte surface morphology and T-tubular structure are significantly disrupted in chronic heart failure, with important functional sequelae, including redistribution of sarcolemmal &bgr;2-adrenergic receptors (&bgr;2AR) and localized secondary messenger signaling. Plasticity of these changes in the reverse remodeled failing ventricle is unknown. We used AAV9.SERCA2a gene therapy to rescue failing rat hearts and measured z-groove index, T-tubule density, and compartmentalized &bgr;2AR-mediated cAMP signals, using a combined nanoscale scanning ion conductance microscopy-Förster resonance energy transfer technique. Methods and Results— Cardiomyocyte surface morphology, quantified by z-groove index and T-tubule density, was normalized in reverse-remodeled hearts after SERCA2a gene therapy. Recovery of sarcolemmal microstructure correlated with functional &bgr;2AR redistribution back into the z-groove and T-tubular network, whereas minimal cAMP responses were initiated after local &bgr;2AR stimulation of crest membrane, as observed in failing cardiomyocytes. Improvement of &bgr;2AR localization was associated with recovery of &bgr;AR-stimulated contractile responses in rescued cardiomyocytes. Retubulation was associated with reduced spatial heterogeneity of electrically stimulated calcium transients and recovery of myocardial BIN-1 and TCAP protein expression but not junctophilin-2. Conclusions— In summary, abnormalities of sarcolemmal structure in heart failure show plasticity with reappearance of z-grooves and T-tubules in reverse-remodeled hearts. Recovery of surface topology is necessary for normalization of &bgr;2AR location and signaling responses.

The carboxy-terminal fragment of inhibitor-2 of protein phosphatase-2A induces Alzheimer disease pathology and cognitive impairment.

Development of rational therapeutic treatments of Alzheimer disease (AD) requires the elucidation of the etiopathogenic mechanisms of neurofibrillary degeneration and β‐amyloidosis, the two hallmarks of this disease. Here we show, employing an adeno‐associated virus serotype 1 (AAV1)‐induced expression of the C‐terminal fragment (I2CTF)of I2PP2A, also called SET, in rat brain, decrease in protein phosphatase 2A (PP2A) activity, abnormal hyperphosphorylation of tau, and neurodegeneration; littermates treated identically but with vector only, i.e., AAV1‐enhanced green fluorescent protein (GFP), served as a control. Furthermore, there was an increase in the level of activated glycogen synthase kinase‐3β and enhanced expression of intraneuronal Aβ in AAV1‐I2CTF animals. Morris water maze behavioral test revealed that infection with AAV1‐I2CTF induced spatial reference memory and memory consolidation deficits and a decrease in the brain level of pSer133‐CREB. These findings suggest a novel etiopathogenic mechanism of AD, which is initiated by the cleavage of I2PP2A, producing I2CTF, and describe a novel disease‐relevant nontransgenic animal model of AD.—Wang, X., Blanchard, J., Kohlbrenner, E., Clement, N., Linden, R. M., Radu, A., Grundke‐Iqbal, I., Iqbal, K. The carboxy‐terminal fragment of inhibitor‐2 of protein phosphatase‐2A induces Alzheimer disease pathology and cognitive impairment. FASEB J. 24, 4420–4432 (2010). www.fasebj.org

SERCA2a Gene Transfer Decreases SR Calcium Leak and Reduces Ventricular Arrhythmias in a Model of Chronic Heart Failure

Background—Sarcoplasmic reticulum calcium ATPase 2a (SERCA2a) gene therapy improves mechanical function in heart failure and is under evaluation in a clinical trial. A critical question is whether SERCA2a gene therapy predisposes to increased sarcoplasmic reticulum calcium (SR Ca2+) leak, cellular triggered activity, and ventricular arrhythmias in the failing heart. Methods and Results—We studied the influence of SERCA2a gene therapy on ventricular arrhythmogenesis in a rat chronic heart failure model. ECG telemetry studies revealed a significant antiarrhythmic effect of SERCA2a gene therapy with reduction of both spontaneous and catecholamine-induced arrhythmias in vivo. SERCA2a gene therapy also reduced susceptibility to reentry arrhythmias in ex vivo programmed electrical stimulation studies. Subcellular Ca2+ homeostasis and spontaneous SR Ca2+ leak characteristics were measured in failing cardiomyocytes transfected in vivo with a novel AAV9.SERCA2a vector. SR Ca2+ leak was reduced after SERCA2a gene therapy, with reversal of the greater spark mass observed in the failing myocytes, despite normalization of SR Ca2+ load. SERCA2a reduced ryanodine receptor phosphorylation, thereby resetting SR Ca2+ leak threshold, leading to reduced triggered activity in vitro. Both indirect effects of reverse remodeling and direct SERCA2a effects appear to underlie the antiarrhythmic action. Conclusions—SERCA2a gene therapy stabilizes SR Ca2+ load, reduces ryanodine receptor phosphorylation and decreases SR Ca2+ leak, and reduces cellular triggered activity in vitro and spontaneous and catecholamine-induced ventricular arrhythmias in vivo in failing hearts. SERCA2a gene therapy did not therefore predispose to arrhythmias and may represent a novel antiarrhythmic strategy in heart failure.

Site-specific integration of adeno-associated virus involves partial duplication of the target locus

A variety of viruses establish latency by integrating their genome into the host genome. The integration event generally occurs in a nonspecific manner, precluding the prediction of functional consequences from resulting disruptions of affected host genes. The nonpathogenic adeno-associated virus (AAV) is unique in its ability to stably integrate in a site-specific manner into the human MBS85 gene. To gain a better understanding of the integration mechanism and the consequences of MBS85 disruption, we analyzed the molecular structure of AAV integrants in various latently infected human cell lines. Our study led to the observation that AAV integration causes an extensive but partial duplication of the target gene. Intriguingly, the molecular organization of the integrant leaves the possibility that a functional copy of the disrupted target gene could potentially be preserved despite the resulting rearrangements. A latently infected, Mbs85-targeted mouse ES cell line was generated to study the functional consequences of the observed duplication-based integration mechanism. AAV-modified ES cell lines continued to self-renew, maintained their multilineage differentiation potential and contributed successfully to mouse development when injected into blastocysts. Thus, our study reveals a viral strategy for targeted genome addition with the apparent absence of functional consequences.

Therapeutic Efficacy of AAV1.SERCA2a in Monocrotaline-Induced Pulmonary Arterial Hypertension

Background— Pulmonary arterial hypertension (PAH) is characterized by dysregulated proliferation of pulmonary artery smooth muscle cells leading to (mal)adaptive vascular remodeling. In the systemic circulation, vascular injury is associated with downregulation of sarcoplasmic reticulum Ca2+-ATPase 2a (SERCA2a) and alterations in Ca2+ homeostasis in vascular smooth muscle cells that stimulate proliferation. We, therefore, hypothesized that downregulation of SERCA2a is permissive for pulmonary vascular remodeling and the development of PAH. Methods and Results— SERCA2a expression was decreased significantly in remodeled pulmonary arteries from patients with PAH and the rat monocrotaline model of PAH in comparison with controls. In human pulmonary artery smooth muscle cells in vitro, SERCA2a overexpression by gene transfer decreased proliferation and migration significantly by inhibiting NFAT/STAT3. Overexpresion of SERCA2a in human pulmonary artery endothelial cells in vitro increased endothelial nitric oxide synthase expression and activation. In monocrotaline rats with established PAH, gene transfer of SERCA2a via intratracheal delivery of aerosolized adeno-associated virus serotype 1 (AAV1) carrying the human SERCA2a gene (AAV1.SERCA2a) decreased pulmonary artery pressure, vascular remodeling, right ventricular hypertrophy, and fibrosis in comparison with monocrotaline-PAH rats treated with a control AAV1 carrying &bgr;-galactosidase or saline. In a prevention protocol, aerosolized AAV1.SERCA2a delivered at the time of monocrotaline administration limited adverse hemodynamic profiles and indices of pulmonary and cardiac remodeling in comparison with rats administered AAV1 carrying &bgr;-galactosidase or saline. Conclusions— Downregulation of SERCA2a plays a critical role in modulating the vascular and right ventricular pathophenotype associated with PAH. Selective pulmonary SERCA2a gene transfer may offer benefit as a therapeutic intervention in PAH.

SUMO-1 Gene Transfer Improves Cardiac Function in a Large-Animal Model of Heart Failure

Cardiac gene delivery of small ubiquitin-related modifier 1 (SUMO-1) improved cardiac function and stabilized left ventricular volumes in a swine model of ischemic heart failure. Cardiac Gene Therapy to the Rescue Heart failure (HF) is one of the top reasons for hospitalization among the elderly and remains a leading cause of death in the Western world. Gene therapy has been proposed as a way to coerce the heart into being healthy by targeting cardiac-specific pathways. Replacing the gene sarcoplasmic reticulum Ca2+ adenosine triphosphatase (SERCA2a) in patients has made it to phase 2b/3 trials, with early signs pointing to an improvement in HF-related events. To boost the effects of SERCA2a, Tilemann et al. designed a large-animal study that also tests the delivery of small ubiquitin-related modifier 1 (SUMO-1)—an important regulator of SERCA2a. The authors compared the efficacy of SUMO-1 gene transfer to SERCA2a gene transfer alone and to the combined delivery of both genes in a pig model of HF. In addition to being safe, administering SUMO-1 directly to the heart of these animals showed improved cardiac contractility and prevented left ventricular dilatation (two major aspects of HF). According to the authors, the functional improvements in this model of heart failure are most likely the result of improved SR Ca2+ ATPase activity afforded by increased SUMO-1 protein levels. Delivery of both SUMO-1 and SERCA2a suggested additional beneficial effects, but more mechanistic studies will be needed to understand this potential synergy. With the precedent set by the SERCA2a clinical trials, moving SUMO-1 gene therapy from pigs to humans seems likely in the short-term. Recently, the impact of small ubiquitin-related modifier 1 (SUMO-1) on the regulation and preservation of sarcoplasmic reticulum calcium adenosine triphosphatase (SERCA2a) function was discovered. The amount of myocardial SUMO-1 is decreased in failing hearts, and its knockdown results in severe heart failure (HF) in mice. In a previous study, we showed that SUMO-1 gene transfer substantially improved cardiac function in a murine model of pressure overload–induced HF. Toward clinical translation, we evaluated in this study the effects of SUMO-1 gene transfer in a swine model of ischemic HF. One month after balloon occlusion of the proximal left anterior descending artery followed by reperfusion, the animals were randomized to receive either SUMO-1 at two doses, SERCA2a, or both by adeno-associated vector type 1 (AAV1) gene transfer via antegrade coronary infusion. Control animals received saline infusions. After gene delivery, there was a significant increase in the maximum rate of pressure rise [dP/dt(max)] that was most pronounced in the group that received both SUMO-1 and SERCA2a. The left ventricular ejection fraction (LVEF) improved after high-dose SUMO-1 with or without SERCA2a gene delivery, whereas there was a decline in LVEF in the animals receiving saline. Furthermore, the dilatation of LV volumes was prevented in the treatment groups. SUMO-1 gene transfer therefore improved cardiac function and stabilized LV volumes in a large-animal model of HF. These results support the critical role of SUMO-1 in SERCA2a function and underline the therapeutic potential of SUMO-1 for HF patients.

Potential Role of BNIP3 in Cardiac Remodeling, Myocardial Stiffness and Endoplasmic Reticulum-Mitochondrial Calcium Homeostasis in Diastolic and Systolic Heart Failure

Background— We have shown that BNIP3 expression is significantly increased in heart failure (HF). In this study, we tested the effects of BNIP3 manipulation in HF. Methods and Results— In a rat model of pressure overload HF, BNIP3 knockdown significantly decreased left ventricular (LV) volumes with significant improvement in LV diastolic and systolic function. There were significant decreases in myocardial apoptosis and LV interstitial fibrosis. Ultrastructurally, BNIP3 knockdown attenuated mitochondrial fragmentation and restored mitochondrial morphology and integrity. On the molecular level, there were significant decreases in endoplasmic reticulum (ER) stress and mitochondrial apoptotic markers. One of the mechanisms by which BNIP3 mediates mitochondrial dysfunction is via the oligomerization of the voltage-dependent anion channels causing a shift of calcium from the ER to mitochondrial compartments, leading to the decrease in ER calcium content, mitochondrial damage, apoptosis, and LV interstitial fibrosis, and hence contributes to both systolic and diastolic myocardial dysfunction, respectively. In systolic HF, the downregulation of SERCA2a (sarcoplasmic-endoplasmic reticulum calcium ATPase), along with an increased BNIP3 expression, further worsen myocardial diastolic and systolic function and contribute to the major remodeling seen in systolic HF as compared with diastolic HF with normal SERCA2a expression. Conclusions— The increase in BNIP3 expression contributes mainly to myocardial diastolic dysfunction through mitochondrial apoptosis, LV interstitial fibrosis, and to some extent to myocardial systolic dysfunction attributable to the shift of calcium from the ER to the mitochondria and to the decrease in ER calcium content. However, SERCA2a downregulation remains a prerequisite for the major LV remodeling seen in systolic HF.Background—We have shown that BNIP3 expression is significantly increased in heart failure (HF). In this study, we tested the effects of BNIP3 manipulation in HF. Methods and Results—In a rat model of pressure overload HF, BNIP3 knockdown significantly decreased left ventricular (LV) volumes with significant improvement in LV diastolic and systolic function. There were significant decreases in myocardial apoptosis and LV interstitial fibrosis. Ultrastructurally, BNIP3 knockdown attenuated mitochondrial fragmentation and restored mitochondrial morphology and integrity. On the molecular level, there were significant decreases in endoplasmic reticulum (ER) stress and mitochondrial apoptotic markers. One of the mechanisms by which BNIP3 mediates mitochondrial dysfunction is via the oligomerization of the voltage-dependent anion channels causing a shift of calcium from the ER to mitochondrial compartments, leading to the decrease in ER calcium content, mitochondrial damage, apoptosis, and LV interstitial fibrosis, and hence contributes to both systolic and diastolic myocardial dysfunction, respectively. In systolic HF, the downregulation of SERCA2a (sarcoplasmic-endoplasmic reticulum calcium ATPase), along with an increased BNIP3 expression, further worsen myocardial diastolic and systolic function and contribute to the major remodeling seen in systolic HF as compared with diastolic HF with normal SERCA2a expression. Conclusions—The increase in BNIP3 expression contributes mainly to myocardial diastolic dysfunction through mitochondrial apoptosis, LV interstitial fibrosis, and to some extent to myocardial systolic dysfunction attributable to the shift of calcium from the ER to the mitochondria and to the decrease in ER calcium content. However, SERCA2a downregulation remains a prerequisite for the major LV remodeling seen in systolic HF.