Pennsylvania: The heart tissue of mammals has limited capacity to regenerate after
an injury such as a heart attack, in part due to the inability to
reactivate a cardiac muscle cell and proliferation program. Recent
studies have indicated a low level of cardiac muscle cell
(cardiomyocytes) proliferation in adult mammals, but it is insufficient
to repair damaged hearts.
A team led by Ed Morrisey, PhD,
a professor of Medicine and Cell and Developmental Biology and the
scientific director of the Institute for Regenerative Medicine in the Perelman School of Medicine at the University of Pennsylvania, has
now shown that a subset of RNA molecules, called microRNAs, is
important for cardiomyocyte cell proliferation during development and
is sufficient to induce proliferation in cardiomyocytes in the adult
heart. MicroRNAs, which do not generate proteins, repress gene
expression by binding messenger RNAs, which do generate proteins, and
promote their degradation. The findings appear this week in Science Translational Medicine.
The team found that the loss of the microRNA cluster miR302-367 in
mice led to decreased cardiomyocyte cell proliferation during
development. In contrast, increased expression of the microRNA cluster
in adult hearts led to a reactivation of proliferation in the normally
non-reproducing adult cardiomyocytes.
This reactivation occurred, in part, through repression of a pathway
called Hippo that governs cell proliferation and organ size. “The
Hippo pathway normally represses cell proliferation when it is turned
on. The cluster miR302-367 targets three of the major kinase components
in the Hippo pathway, reducing pathway activity, which allows
cardiomyocytes to re-enter the cell cycle and begin to regrow heart
muscle,” explains Morrisey. “This is a case of repressing a repressor.”
In adult mice, re-expression of the microRNA cluster reactivated the
cell cycle in cardiomyocytes, resulting in reduced scar formation
after an experimental myocardial infarction injury was induced in the
mice. There was also an increase in the number of heart muscle cells in
these same mice.
However, long-term expression of more than several months of the
microRNA cluster caused heart muscle cells to de-differentiation and
become less functional. “This suggested to us that persistent
reactivation of the cell cycle in adult cardiomyocytes could be harmful
and causes the heart to fail,” says Morrisey. The investigators
surmised that cardiomyocytes likely need to de-differentiate to divide,
but they may lose their ability to contract over time.
“We overcame this limitation by injecting synthetic microRNAs with a
short half-life called mimics into the mice,” says Morrisey. Mimic
treatment for seven days after cardiac infarction led to the desired
increase in cardiomyocyte proliferation and regrowth of new heart
muscle, which resulted in decreased fibrosis and improved heart
function after injury.
Importantly, the team found that the transient seven-day treatment
did not lead to the progressive loss of cardiac function as seen in the
genetic models of increased microRNA expression. Overall, these
results suggested that any treatment that promotes cardiomyocyte
proliferation to improve cardiac regeneration will likely need to be
transient to avoid the deleterious effects of maintaining a high level
of proliferation and de-differentiation in a tissue that is normally
non-proliferative.
“The next stage in this study is to determine whether miRNA mimics
will work in a larger animal model and to collaborate with bioengineers
to create a local delivery system for the heart, rather than giving it
systemically,” notes Morrisey.
Coauthors are Ying Tian (previously a postdoctoral
fellow in the Morrisey Lab who is now an assistant professor at Temple
University), Ying Liu, Tao Wang, Ning Zhou, Jun Kong, Li Chen, Melinda
Snitow, Michael Morley, Deqiang Li, Nataliya Petrenko, Su Zhou, Minmin
Lu, and Kathleen M. Stewart, all from Penn. Erhe Gao and Walter J. Koch
are from Temple University.
This study was funded by grants from National Heart, Lung, and Blood
Institute (R01-HL064632, R01-HL087825, U01-HL100405, K99/R00, and
K99/R00-HL111348).