Detection of electroporation-induced membrane permeabilization states in the brain using diffusion-weighted MRI.

BACKGROUND: Tissue permeabilization by electroporation (EP) is a promising technique to treat certain cancers. Non-invasive methods for verification of induced permeabilization are important, especially in deep-seated cancers. In this study we evaluated diffusion-weighted magnetic resonance imaging (DW-MRI) as a quantitative method for detecting EP-induced membrane permeabilization of brain tissue using a rat brain model. MATERIAL AND METHODS: Fifty-four anesthetized Sprague-Dawley male rats were electroporated in the right hemisphere, using different voltage levels to induce no permeabilization (NP), transient membrane permeabilization (TMP), and permanent membrane permeabilization (PMP), respectively. DW-MRI was acquired 5 minutes, 2 hours, 24 hours and 48 hours after EP. Histology was performed for validation of the permeabilization states. Tissue content of water, Na+, K+, Ca2+, and extracellular volume were determined. The Kruskal-Wallis test was used to compare the DW-MRI parameters, apparent diffusion coefficient (ADC) and kurtosis, at different voltage levels. The two-sample Mann- Whitney test with Holm's Bonferroni correction was used to identify pairs of significantly different groups. The study was approved by the Danish Animal Experiments Inspectorate. RESULTS AND CONCLUSION: Results showed significant difference in the ADC between TMP and PMP at 2 hours (p<0.001) and 24 hours (p<0.05) after EP. Kurtosis was significantly increased both at TMP (p<0.05) and PMP (p<0.001) 5 minutes after EP, compared to NP. Kurtosis was also significantly higher at 24 hours (p<0.05) and 48 hours (p<0.05) at PMP compared to NP. Physiological parameters indicated correlation with the permeabilization states, supporting the DW-MRI findings. We conclude that DW-MRI is capable of detecting EP-induced permeabilization of brain tissue and to some extent of differentiating NP, TMP and PMP using appropriate scan timing.

Preclinical validation of electrochemotherapy as an effective treatment for brain tumors.

Electrochemotherapy represents a strategy to enhance chemotherapeutic drug uptake by delivering electrical pulses which exceed the dielectric strength of the cell membrane, causing transient formation of structures that enhance permeabilization. Here we show that brain tumors in a rat model can be eliminated by electrochemotherapy with a novel electrode device developed for use in the brain. By using this method, the cytotoxicity of bleomycin can be augmented more than 300-fold because of increased permeabilization and more direct passage of drug to the cytosol, enabling highly efficient local tumor treatment. Bleomycin was injected intracranially into male rats inoculated with rat glia-derived tumor cells 2 weeks before the application of the electrical field (32 pulses, 100 V, 0.1 ms, and 1 Hz). In this model, where presence of tumor was confirmed by magnetic resonance imaging (MRI) before treatment, we found that 9 of 13 rats (69%) receiving electrochemotherapy displayed a complete elimination of tumor, in contrast to control rats treated with bleomycin only, pulses only, or untreated where tumor progression occurred in each case. Necrosis induced by electrochemotherapy was restricted to the treated area, which MRI and histology showed to contain a fluid-filled cavity. In a long-range survival study, treatment side effects seemed to be minimal, with normal rat behavior observed after electrochemotherapy. Our findings suggest that electrochemotherapy may offer a safe and effective new tool to treat primary brain tumors and brain metastases.

Diffusion-weighted MRI for verification of electroporation-based treatments.

Clinical electroporation (EP) is a rapidly advancing treatment modality that uses electric pulses to introduce drugs or genes into, e.g., cancer cells. The indication of successful EP is an instant plasma membrane permeabilization in the treated tissue. A noninvasive means of monitoring such a tissue reaction represents a great clinical benefit since, in case of target miss, retreatment can be performed immediately. We propose diffusion-weighted magnetic resonance imaging (DW-MRI) as a method to monitor EP tissue, using the concept of the apparent diffusion coefficient (ADC). We hypothesize that the plasma membrane permeabilization induced by EP changes the ADC, suggesting that DW-MRI constitutes a noninvasive and quick means of EP verification. In this study we performed in vivo EP in rat brains, followed by DW-MRI using a clinical MRI scanner. We found a pulse amplitude-dependent increase in the ADC following EP, indicating that (1) DW-MRI is sensitive to the EP-induced changes and (2) the observed changes in ADC are indeed due to the applied electric field.

Efficiency of cellular delivery of antisense peptide nucleic acid by electroporation depends on charge and electroporation geometry.

Electroporation is potentially a very powerful technique for both in vitro cellular and in vivo drug delivery, particularly relating to oligonucleotides and their analogs for genetic therapy. Using a sensitive and quantitative HeLa cell luciferase RNA interference mRNA splice correction assay with a functional luciferase readout, we demonstrate that parameters such as peptide nucleic acid (PNA) charge and the method of electroporation have dramatic influence on the efficiency of productive delivery. In a suspended cell electroporation system (cuvettes), a positively charged PNA (+8) was most efficiently transferred, whereas charge neutral PNA was more effective in a microtiter plate electrotransfer system for monolayer cells. Surprisingly, a negatively charged (-23) PNA did not show appreciable activity in either system. Findings from the functional assay were corroborated by pulse parameter variations, polymerase chain reaction, and confocal microscopy. In conclusion, we have found that the charge of PNA and electroporation system combination greatly influences the transfer efficiency, thereby illustrating the complexity of the electroporation mechanism.

Angiotensinogen and ACE gene polymorphisms and risk of atrial fibrillation in the general population.

OBJECTIVES: The renin-angiotensin system may play a role in the pathogenesis of atrial fibrillation, and renin-angiotensin system blockers reduce the risk of atrial fibrillation. We hypothesized that polymorphisms in the angiotensinogen and angiotensin-converting enzyme (ACE) genes encoding proteins in this system predict risk of atrial fibrillation. METHODS AND RESULTS: We genotyped 9235 individuals from the Danish general population, The Copenhagen City Heart Study, for the a-20c, g-6a, T174M, and M235T polymorphisms in the angiotensinogen gene and the insertion/deletion (I/D) polymorphism in the ACE gene; rare allele frequencies were 0.16, 0.40, 0.12, 0.41, and 0.49, respectively. Participants had sinus rhythm at inclusion. During 26 years of follow-up, 968 individuals developed atrial fibrillation. Multifactorially adjusted hazard ratios for atrial fibrillation for a-20c ac and cc versus aa genotype were 1.1(95% confidence interval: 1.0-1.3; P=0.05) and 1.5(1.1-2.1; P=0.01). Compared with double noncarriers (angiotensinogen -20aa and ACE II), double heterozygotes (ac-I/D genotype), and double homozygotes (cc-DD) had hazard ratios for atrial fibrillation of 1.2(0.9-1.6; P=0.06) and 2.4(1.4-4.1; P=0.001). a-20c cc homozygotes above 70 years of age who were overweight, severely hypertensive, and had heart failure, had an absolute 10-year risk of atrial fibrillation of 61%. CONCLUSION: Angiotensinogen a-20c genotype alone and in combination with ACE I/D genotype predicts an increased risk of atrial fibrillation. Therefore, genetic variation in the renin-angiotensin system may influence effect of renin-angiotensin system blockers on atrial fibrillation.

Phosphorylation of a pest sequence in ABCA1 promotes calpain degradation and is reversed by ApoA-I.

ATP-binding cassette transporter A1 (ABCA1), the defective molecule in Tangier disease, mediates the apoAI-dependent efflux of excess cholesterol from cells. We recently showed that ABCA1 proteolysis by calpain was dependent on a PEST sequence in the cytoplasmic region of ABCA1 and was reversed by apoA-I interaction with ABCA1. We show here that phosphorylation of ABCA1 in HEK293 cells was reduced by 63 +/- 2.4% after removal of the PEST sequence (ABCA1delPEST) or by incubation of cells with apoAI (58 +/- 3.3%). By contrast, ABCA1delPEST showed no further decrease of phosphorylation upon apoAI treatment. To assess the hypothesis that PEST sequence phosphorylation could regulate ABCA1 calpain proteolysis, we mutagenized S/T residues in the PEST sequence and identified Thr-1286 and Thr-1305 as constitutively phosphorylated residues. The ABCA1-T1286A/T1305A mutant was not degraded by calpain and was not further stabilized upon apoA-I treatment. The T1286A/T1305A mutant showed a 3.1-fold increase in cell surface expression and a 2.3-fold increase of apoAI-mediated cholesterol efflux compared with wild type ABCA1. In conclusion, we propose a mechanism of regulation of ABCA1 cell surface expression and function in which the interaction with apoA-I results in dephosphorylation of the ABCA1 PEST sequence and thereby inhibits calpain degradation leading to an increase of ABCA1 cell surface expression.

A PEST sequence in ABCA1 regulates degradation by calpain protease and stabilization of ABCA1 by apoA-I.

Cholesterol-loaded macrophage foam cells are a central component of atherosclerotic lesions. ABCA1, the defective molecule in Tangier disease, mediates the efflux of phospholipids and cholesterol from cells to apoA-I, reversing foam cell formation. In ABCA1, we identified a sequence rich in proline, glutamic acid, serine, and threonine (PEST sequence) that enhances the degradation of ABCA1 by calpain protease and thereby controls the cell surface concentration and cholesterol efflux activity of ABCA1. In an apparent positive feedback loop, apoA-I binds ABCA1, promotes lipid efflux, inhibits calpain degradation, and leads to increased levels of ABCA1. ApoA-I infusion also increases ABCA1 in vivo. These studies reveal a novel mode of regulation of ABCA1 by PEST sequence-mediated calpain proteolysis that appears to be reversed by apolipoprotein-mediated phospholipid efflux. Inhibition of ABCA1 degradation by calpain could represent a novel therapeutic approach to increasing macrophage cholesterol efflux and decreasing atherosclerosis.