During a severe heart attack many heart muscle cells die and are replaced by scar tissue to stabilize the heart wall. Connective tissue cells, known as fibroblasts (FB), are the dominant cell type in scar tissue. "Unfortunately this cells are not contractile and also delay the conduction of electrical impulses in the lesioned area” says Prof. Dr. Wilhelm Röll, senior physician at the UKB Heart Center and member of the Transdisciplinary Research Area (TRA) "Life & Health" at the University of Bonn. His co-corresponding author, Prof. Dr. Bernd K. Fleischmann from the Institute of Physiology I at the UKB and also a member of the TRA "Life & Health" at the University of Bonn, adds: "Fibroblasts only express small amounts of the gap junction protein connexin 43, which is essential for the conduction of stimuli between heart muscle cells." This is because they are directly electrically coupled to each other via the tunnel protein. The electrical signals can thus be transmitted extremely quickly from one heart muscle cell to the next. However, this is hardly the case between fibroblasts in the infarct scar, so that cardiac arrhythmias can occur, especially in the border zone to the vital myocardium. To date there are hardly any vectors that enable the targeted treatment of cardiac scar tissue in the heart, which severely limits therapeutic options.
Better electrical connection between the cells of the scar tissue
The goal of the Bonn research team was to find an effective vector to overexpress connexin 43 within fibroblasts of the cardiac scar. Therefore, the das Moloney Murine Leukemia Virus (MMLV), complexed with magnetic nanoparticles (MNP), was used as a gene shuttle and introduced into cardiac fibroblasts under application of a magnetic field. These connective tissue cells were then able to produce connexin 43. "The novelty of the approach was the use of the retrovirus MMLV, which introduces genetic information into fibroblasts in the heart in a relatively specific manner, but not into heart muscle cells, a process known as transduction," says co-first author Timo Mohr, a doctoral student of the University of Bonn at the UKB Heart Center. "In combination with magnetic nanoparticles, the gene ferry is concentrated in the target area of the magnetic field." The Bonn research team also combined the MMLV constructs with the fluorescent life-reporter mCherry. "With this concept, we were able to achieve substantial transduction of myofibroblasts in a relatively short time," says co-first author Dr. Miriam Schiffer, postdoctoral researcher at the University of Bonn in the Institute of Physiology I at the UKB. And not only outside an organism, but also in vivo in a mouse model: Three days after the initial myocardial infarction, the researchers injected the MMLV/MNP complexes directly into the lesion area of mice under application of a magnetic field. Two weeks later, they were able to demonstrate successful transduction of fibroblasts in the infarct scar by their mCherry expression. Subsequent functional tests showed that mice with connexin 43 in the infarct area developed dangerous cardiac arrhythmias only about half as often as the control animals, which were unable to produce the tunnel protein there.
"In principle, this therapy could be used in clinical applications at a later stage, possibly also for indications outside the cardiovascular system. However, suitable vectors to transduce human cardiac fibroblasts must first be identified and the clinical results confirmed in a large animal model," says Prof. Röll, who led the study together with Prof. Fleischmann.
Funding and participating institutes
The experimental work was carried out as part of the DFG-funded collaborative project SFB 1425 (University of Freiburg) in collaboration with the Institute of Pharmacology and Toxicology at the Technical University of Munich and the Institute for Experimental Cardiovascular Medicine at the University of Freiburg.