Biomedical Engineering Reference
In-Depth Information
the tissue with a viral vector containing it. In practical terms, only the viral infection
provides adequate transduction efficiency and is universally adopted. Nonetheless, the
infection procedure involves a number of technical and safety problems. The
replication-deficient adenovirus is a safe and practical vector; however, because the
gene is not incorporated into the genome, its expression is only transient.
Retroviruses, like the widely used lentivirus, incorporate the added gene into the
genome, which results in stable gene expression. However, genomic transduction
carries potential carcinogenic risk, which might make this type of vector less suitable
for therapeutic use. Such problems have prompted the development of cell based
approaches, in which pacemaker function is intrinsic to the implanted cell, or can be
obtained by genetic modification prior to implant in the host myocardium.
In the cell-based approach, several strategies have been proposed. In one case
spontaneously beating clusters of myocytes derived from human embryonic stem cells
(hESCs) were directly used as pacemaker elements [15]. However, once implanted,
these cells could further differentiate into quiescent elements, thus compromising
pacemaker stability. Another, more promising, approach is based on in vitro genetic
modification of exogenous cells, originally devoid of pacemaker activity, which are
stably transduced with a gene encoding the current of interest. Once implanted, the
modified cells electrically couple to the surrounding myocardium, and modulate its
electrical activity [2, 12]. Cell-to-cell coupling is mediated by connexins, protein
channels that allow ionic current flow between adjacent cells. Connexins are at hand
in many cell types, including stem cells, which can successfully couple to cardiac
myocytes. Success of the cell-based approach depends on the possibility of avoiding
immunological rejection of the implant; thus an autologous origin of the implanted
cells is highly desirable. Stem cells may be particularly suitable for generating a bio-
pacemaker because they can be autologous and they replicate, thus allowing
amplification of the cell population. An alternative may be the development of
replicating cell-lines, engineered to achieve immunocompatibility.
To create a bio-pacemaker, the following strategies are currently followed: (1)
suppression of repolarizing currents to unmask latent pacemaker currents in normally
quiescent myocardial cells; (2) over-expression of a pacemaker (depolarizing) current
in electrically quiescent cells to convert them into pace-making elements; (3)
modulation of the expression of receptors involved in the regulation of pacemaker
currents [5].
The first approach, a pioneering one in the field of bio-pacemakers, relies on the
idea that ventricular ''working'' myocardium has latent pacemaker activity, but
spontaneous depolarization is normally suppressed by a large repolarizing
conductance, available at diastolic potential. Such a conductance is provided by the
''inward rectifier'' potassium current
I
K1
, known for its strong expression in
electrically quiescent cells of the atrial and ventricular working myocardium, but
virtually absent from the AV and SA node. Therefore, suppression of
I
K1
is a putative
approach for creating a bio-pacemaker. The group of Marban [9] provided a proof of
this concept by using a dominant-negative Kir2.1 construct, packaged into an
adenoviral vector. Once infected with the vector in vivo, ventricular myocardium
showed 80%
I
K1
suppression and developed automatic activity [9]. Although
conceptually innovative, such an approach is encumbered by the problems related to
all viral transduction methods; moreover, strict delimitation of the infection site is
