Biomedical Engineering Reference
In-Depth Information
difficult and diffusion of
I
K1
suppression throughout the ventricle may entail pro-
arrhythmic risk.
With regard to over-expression of depolarizing currents, much attention has been
given to the main depolarizing current that induces spontaneous activity in the SA
node, the funny current
I
f
, which is mediated by a family of hyperpolarization
activated cyclic nucleotidegated (HCN) channels. This area has been pioneered by
Rosen et al. [13], whose review in this issue summarizes the evolution of the concept
and the results obtained. These authors used HCN2 as the
I
f
encoding isoform because
the resulting current kinetics are more favorable than with HCN4 and its cAMP
responsiveness is greater than that of HCN1. These investigators initially showed
suitability of
I
f
over-expression by injecting HCN2 encoding adenoviral vectors into
the left atrium or the left bundle branch of intact dog hearts. Both injection sites
proved to be successful in generating an ectopic rhythm. In addition, the experiments
also proved that pacemaker activity generated by expression of HCN2 was
autonomically regulated. To overcome the problems related to the adenoviral
infection method, the same group developed a cell-based approach. Human
mesenchymal stem cells (hMSC), loaded with the HCN2 gene, were injected
epicardially into the left ventricular free wall and resulted into an idioventricular
rhythm at the injection site. This rhythm was significantly faster than the escape
rhythm following AV nodal ablation, thus providing efficient pacemaker activity.
Recent studies also explored the feasibility to convert quiescent ventricular myocytes
into pacemakers using somatic cell fusion [4, 8]. Chemically induced fusion between
myocytes and syngeneic fibroblasts that had been engineered to express pacemaker
ion channels, has been attempted. The advantage of this approach, with respect to
classical cell-based therapy, is that the gapjunctional coupling between donor cells
and host myocardium, which might be suboptimal or unstable in time, is avoided.
Interestingly, a cell-based approach has also been proposed as a mean to down
regulate heart rate. De Boer et al. [2] reduced beating rate of spontaneously active
neonatal rat cardiomyocytes by coculturing them with
I
K1
overexpressing human
embryonic kidney cells (HEK, transduced with Kir2.1 gene). These investigators also
showed that the influence of Kir2.1 expressing cells on beating rate could be lessened
by the application of BaCl
2
, that blocks
I
K1
. Since pacemaker down-regulation
occurred through electronic interaction between the two cell types, this result also
implies that efficient connexin-mediated cell-to-cell coupling spontaneously develops
between HEK cells and ventricular myocytes.
Recent evolutions in bio-pacemaking techniques involve the expression of
''synthetic'' pacemaker channels, obtained by modification of genes originally
encoding non-pacemaker currents. The rationale of this approach is the concern that
coassembly of added HCN proteins with those naturally expressed by the cell may
result in unpredictable channel properties. To generate a synthetic pacemaker channel,
Kashiwakura et al. [7] converted the depolarization activated potassium channel
Kv1.4 into a hyperpolarization-activated non-selective channel by 4 point-mutations.
The properties of the synthetic channel were similar to those of HCN ones, but
co-assembly between endogenous and added proteins was avoided.
A requirement for successful propagation of pacemaker activity is an appropriate
match between the pacemaker generator properties and the electrical load imposed by
the tissue to be excited. Electrical-coupling is required for propagation between
