AIMS: Optogenetics approaches, utilizing light-sensitive proteins, have emerged 
as unique experimental paradigms to modulate neuronal excitability. We aimed to 
evaluate whether a similar strategy could be used to control cardiac-tissue 
excitability.
METHODS AND RESULTS: A combined cell and gene therapy strategy was developed in 
which fibroblasts were transfected to express the light-activated depolarizing 
channel Channelrhodopsin-2 (ChR2). Patch-clamp studies confirmed the development 
of a robust inward current in the engineered fibroblasts following monochromatic 
blue-light exposure. The engineered cells were co-cultured with neonatal rat 
cardiomyocytes (or human embryonic stem cell-derived cardiomyocytes) and studied 
using a multielectrode array mapping technique. These studies revealed the 
ability of the ChR2-fibroblasts to electrically couple and pace the 
cardiomyocyte cultures at varying frequencies in response to blue-light flashes. 
Activation mapping pinpointed the source of this electrical activity to the 
engineered cells. Similarly, diffuse seeding of the ChR2-fibroblasts allowed 
multisite optogenetics pacing of the co-cultures, significantly shortening their 
electrical activation time and synchronizing contraction. Next, optogenetics 
pacing in an in vitro model of conduction block allowed the resynchronization of 
the tissue's electrical activity. Finally, the ChR2-fibroblasts were transfected 
to also express the light-sensitive hyperpolarizing proton pump 
Archaerhodopsin-T (Arch-T). Seeding of the ChR2/ArchT-fibroblasts allowed to 
either optogentically pace the cultures (in response to blue-light flashes) or 
completely suppress the cultures' electrical activity (following continuous 
illumination with 624 nm monochromatic light, activating ArchT).
CONCLUSIONS: The results of this proof-of-concept study highlight the unique 
potential of optogenetics for future biological pacemaking and resynchronization 
therapy applications and for the development of novel anti-arrhythmic 
strategies.