Biomedical Engineering Reference
In-Depth Information
implantation of an electronic cardiac pacemaker becomes necessary [15]. Potential
alternative approaches may derive from cell or gene therapy. In a first study we were
able to show that transplanted fetal cardiomyocytes can function as a biological
cardiac pacemaker [25]. Miake et al. [17] postulated latent pacemaker capability of
working myocardium. This potential ability is inhibited by the inward-rectifier
potassium current
I
K1
encoded by the gene Kir2 which is not expressed in pacemaker
cells. Following dominant-negative suppression of
I
K1
, ventricular myocytes exhibited
spontaneous activity with their action potentials resembling typical patterns of
genuine pacemaker cells. Qu et al. [22] were able to show that adenovirus-mediated
overexpression of the hyperpolarization-activated, cyclic nucleotidegated pacemaker
current HCN2 provides an
I
f
-based pacemaker sufficient to drive the heart when
injected into a localized region of the atrium. Finally, Potapova et al. [21]
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.
Transplanting cardiomyocytes as cardiac pacemaker may open a new perspective
for the treatment of cardiac arrhythmia such as sinus node dysfunction or
atrioventricular block (AV-block), ranging from infants and premature babies with
congenital AV-block (incidence: one in 20,000-25,000 live births) who might be
too small for the treatment with artificial pacemakers, to patients with acquired
disease [11, 18]. However, so far the number of transplantable cells of the cardiac
conduction system (CCS), regardless of their origin (fetal, adult and stem cell
derived) is limited. On the way to a biological cardiac pacemaker it is therefore
imperative to enrich pacemaker-like cells and, thus, to develop a method enabling
transformation of a heterogeneous population of cardiomyocytes into cells of the
CCS. Recently, Gassanov et al. [10] demonstrated that endothelin-1 directed
differentiation of embryonic stem cells towards a pacemaker phenotype. Moreover,
neuregulin-1 was shown to induce ectopic expression of the lac-Z conduction
marker in vivo in CCS-lac-Z reporter mice within a short period of 8.5-10.5 days
post coitum (dpc) [16, 23]. This indicated that neuregulin-1 might promote
formation of the murine CCS. However, so far no studies have been performed of
any inductive factors on cultured murine embryonic cardiomyocytes, which might
prove useful for cell transplantation.
Therefore, the aim of the present study was to possibly transform a heterogenous
population of cultured mouse embryonic cardiomyocytes (13 dpc) into cells with
pacemaker-like characteristics by the separate exposure to the Į-and
ȕ
-isoforms of
neuregulin-1, and cAMP. To determine the extent of enrichment of cells with a
pacemaker-like phenotype we evaluated the expression of connexin 40 (Cx-40), a
marker of the early differentiating conduction system in mice [5, 19], and secondly
recorded the hyperpolarization-activated inward current
I
f
, a characteristic ion current
of pacemaker cells [7, 10]. Our results show a significant enhancement of Cx-40
mRNA expression and increase of
I
f
current density upon neuregulin and cAMP
treatment, indicating direction of differentiation towards pacemaker-like cells in a
mixed population of embryonic cardiomyocytes.
