Biomedical Engineering Reference
In-Depth Information
phase 4 depolarization and an automatic rhythm. The changes in membrane potential
result in current flow via the low resistance gap junctions such that the action
potential propagates from one cell to the next. Our view of the hMSC as a platform
was that it would be loaded with the HCN2 gene via electroporation, thereby avoiding
any viral component in the process [3, 8, 12, 13]. The hMSC would have to be
coupled effectively to the adjacent myocyte. If this occurred, then the high negative
membrane potential of coupled myocytes would hyperpolarize the hMSC, opening the
HCN channels and permitting inward current to flow. This current, in turn, would
propagate through the low resistance gap junctions, depolarize a coupled myocyte,
and bring it to threshold potential, resulting in an action potential that would then
propagate further in the conducting system. In other words, the hMSC and the
myocyte each would have to carry an essential piece of machinery: the myocyte
would bring the ionic components that generate an action potential, the hMSC would
carry the pacemaker current, and—if gap junctions were present—the two separate
structural entities would function as a single, seamless physiologic unit.
A key question then was: are gap junctions formed between hMSCs and myocytes?
The answer is yes, as is shown in Fig. 8. Note, using immunostaining connexins 43
and 40 are clearly demonstrable, while connexin 45 is not. Western blots also showed
the presence of both connexins 40 and 43. Moreover, injection of current into an
hMSC in close proximity to a myocyte results in current flow to the myocyte (Fig. 9)
[14], clearly indicating the existence of gap junctions and the occurrence of electrical
coupling. Another important question here is how critical the extent of coupling
between the engineered nodal cells and surrounding myocardium may be.
A
B
stem cell
V
1
V
2
canine ventricle cell
I
2
2
1
100 pA
1 s
Fig. 9.
Current flow demonstrating coupling between a human mesenchymal stem cell (hMSC)
and a canine myocyte.
a
Phase-contrast micrograph of a hMSC-canine ventricular myocyte
pair before (
upper
) and after (
lower
) impaling each with a micro- electrode.
Panel b
Monopolar
pulse protocol (
V
1 and
V
2) and associated macroscopic junctional currents (
I
) exhibiting
asymmetrical voltage dependence (reprinted by permission from reference [14]).
