Biological Pacemakers: Still a Dream?
A biological pacemaker is one or more types of cellular components that, when implanted into certain regions of the heart, produce electrical stimuli that mimic that of the body’s natural pacemaker cells. Somatic gene transfer, cell fusion, or cell transplantation provide a way to realize it as somatic reprogramming strategies, which involve transfer of genes encoding transcription factors to transform working myocardium into a surrogate sinoatrial node, are furthest along in the possibilities. The idea, no doubt, is bright and appealing. The objective herein intends to dig into the subject trying to find out how realizable it really is.
Somatic gene transfer involves adding genes to cells other than egg or sperm cells, while horizontal gene transfer (HGT ) or lateral gene transfer (LGT ) is the movement of genetic material between unicellular and/or multicellular organisms differently than by the transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the evolution of many organisms. Cell fusion, in turn, constitutes a cellular process in which several mononuclear cells combine to form a multinuclear cell known as syncytium.
The most desirable feature of a biological pacemaker is the ability to generate a stable and spontaneous pacing rate that is physiological, nonarrythmogenic, and with optimal autonomic modulation. It should be self-sustaining, without any need of technology. Creating a biological pacemaker involves imparting a tissue with no or zero net current flow in diastole, a brief net inward current causing diastolic depolarization and generation of an action potential. This can be achieved by augmenting HCN channels, inhibiting Kir2.1 channels, or a combination of both. The first set, or the so-called Hyperpolarization-activated Cyclic Nucleotide- gated channels, are membrane proteins that serve as nonselective channels in the plasma membranes of heart and brain cells. HCN channels are sometimes referred to as pacemaker channels because they help to generate rhythmic activity within groups of heart and brain cells. HCN channels are encoded by four genes(1) (HCN1, 2, 3, 4) and are widely expressed throughout the heart and the central nervous system. The Kir2.1 inward-rectifier potassium ion channel is, instead, encoded by the KCNJ2 gene.
(1) Gene: Region of deoxyribonucleic acid (DNA), a molecule encoding an organism’s genetic blueprint. Chromosome: Long strand of DNA with many genes. Also responsible for genetic transmission.
The first successful experiment with biological pacemakers was carried out by Arjang Ruhparwar’s group at Hannover Medical School, in Germany, using transplanted fetal heart cells.(2) A few months later, Eduardo Marban’s group, from Johns Hopkins University, published the first successful gene-therapeutic approach towards the generation of pacemaking activity in otherwise non-pacemaking adult cardiomyocytes using guinea pigs. Marban’s current research areas are using biologically based therapies for cardiac regeneration and biological pacemakers. His group has several articles on the subject that are highly recommended. The research using cardiosphere-derived cells has demonstrated that such cells are well suited in their repair capacity and in their angiogenic ability. It is thus postulated that latent pacemaker capability exists in normal heart muscle cells. This potential ability is suppressed by the inward potassium current Ik1, encoded by the gene Kir2, which is not expressed in pacemaker cells. By specific inhibition of Ik1 below a certain level, spontaneous activity of cardiomyocytes was observed with resemblance to the action potential pattern of genuine pacemaker cells. All of the above briefly summarizes so far the current knowledge, which we must admit, looks relatively weak.
(2) Presented at the American Heart Association Congress, Anaheim, CA, USA, 2001.
Meanwhile, other genes and cells have been discovered, including heart muscle cells derived from embryonic stem cells and HCN genes that encode the pacemaker current If.3 Michael Rosen’s group demonstrated that transplantation of HCN2-transfected human mesenchymal stem cells (hMSCs) leads to expression of functional HCN2 channels in vitro and in vivo, mimicking overexpression of HCN2 genes in cardiac myocytes. In 2010, Ruhparwar’s group again demonstrated a type of biological pacemaker, this time showing that by injection of the adenylate cyclase gene into the heart muscle, a biological cardiac pacemaker can be created.
(3) “Funny” current, If, is originally described in sinoatrial node myocytes as an inward current activated on hyperpolarization to the diastolic range of voltages. Prof. Dario Di Francesco, at the University of Milan, discovered what he called “a very unusual current,” an inward current that activated on hyperpolarization. All other known inward currents in heart muscle activate on depolarization. “We termed it ‘funny’ because of its odd properties,” Di Francesco  said.
More recently, a gene called TBX18 has been non-invasively applied to speed up heart rates caused by heart block, and light-sensitive genes that react to blue light have been injected in order to activate the heart simultaneously from a number of sites.
Biological pacemakers, generated by somatic gene transfer, cell fusion, or cell transplantation, provide an alternative to electronic devices. Somatic reprogramming strategies, which involve transfer of genes encoding transcription factors to transform working myocardium into a surrogate sinoatrial node, are furthest along in the translational way. Even as electronic pacemakers become smaller and less invasive, biological pacemakers might expand the therapeutic armamentarium for conduction system disorders .
Another way of attacking this idea is a genetically engineered pacemaker by the so-called transfecting procedure (to transfect is to introduce genetic material by infecting a cell with free nucleic acid). Artificial biological pacemakers were developed and tested in canine ventricles. Oscillations should be sensitive to external regulations, and robust with respect to long-term drifts of expression levels of pacemaker currents and gap junctions. Besides, mathematical models were introduced, intended to be used in parallel with the experiments. Models were developed, intended to mimic experiments with oscillation induction in a cell pair, in cell culture, and in cardiac tissue . The latter article strongly called our attention for, in principle and at first sight, biological pacing appears as not tractable by theoretical models. However, almost everything seems possible in science!
It is difficult so far to advance any strong conclusion as all the collected information is still held by only fine threads. The most we dare say is that the idea is quite attractive with many references and investigators involved in it.
- E. Cingolani, J. I. Goldhaber, and E. Marban, “Next generation pacemakers: From small devices to biological pacemakers,” Nature Rev. Cardiol., vol. 15, no. 3, Nov. 2017. doi: 10.1038/nrcardio.2017.165
- D. Di Francesco, “The role of the ‘funny current’ in pacemaker activity,” Circul. Res., vol. 106, pp. 434–446, Feb. 2010. doi: 10.1161/ CIRCRESAHA.109.208041
- S. Kanani, V. Krinsky, and A. Pumir, “Stem cells transfected with HCN2 gene and myocytes—A model,” Phys. Lett. A, vol. 372, no. 2, Dec. 2005. doi: 10.1016/j.physleta.2007.07.075