Download Myocardial Regeneration by Stem Cells

Journal of the American College of Cardiology
© 2006 by the American College of Cardiology Foundation
Published by Elsevier Inc.
EDITORIAL COMMENT
Myocardial Regeneration
by Stem Cells
Seeing the Unseeable*
James S. Forrester, MD,
Prediman K. Shah, MD, FACC,
Raj R. Makkar, MD
Los Angeles, California
The failure of a well-designed, randomized trial of granulocyte colony-stimulating factor (G-CSF) therapy to significantly improve cardiac function after acute myocardial
infarction, despite substantial bone marrow cell mobilization, is reported by Engelmann et al. (1) in this issue of the
Journal. In 44 revascularized subacute ST-segment elevation
myocardial infarction (STEMI) patients, 5 days of G-CSF
therapy resulted in a modest increase in ejection fraction
(5% to 6%) without significant change in regional function
in both subjects and controls, as assessed by magnetic
resonance imaging at 3 months. The study seems to signal
the end to an era that began just 5 years ago. In late 2001,
2 landmark publications (2,3) triggered a series of animal
laboratory studies showing that stem cells could engraft in
the damaged myocardium, decrease infarct size, improve
myocardial perfusion and function, and decrease mortality
(4 –7). The cells could be delivered by intramyocardial or
intravenous injection and even indirectly by cytokine stimulation. The end of this era is almost equally dramatic;
beyond small increases in ejection fraction and regional
perfusion (8), none of these strategies have produced similar
outcomes in man. It seems we have run into a wall, and need
to look around for a door; we need some great new ideas.
See page 1712
Such ideas commonly evolve from a simple observation
transformed by an intuitive leap to an entirely new concept.
Thus, Johannes Kepler’s astonishing leap to the laws of
planetary motion from Tycho Brahe’s decades-earlier recording of celestial data, the burrs stuck to a Swiss mountaineer’s wool pants that lead to velcro, or Alexander
Fleming’s leap from a spoiled Petri dish to the discovery of
penicillin. For cardiology’s greatest idea, myocardial regeneration, in the past 5 years, 3 such intuitive leaps from a
simple observation seem, as Daniel Quinn suggests in
Beyond Civilization, “to see that which is so evident as to be
*Editorials published in the Journal of the American College of Cardiology reflect the
views of the authors and do not necessarily represent the views of JACC or the
American College of Cardiology.
From the Cedars-Sinai Medical Center, Los Angeles, California.
Vol. 48, No. 8, 2006
ISSN 0735-1097/06/$32.00
doi:10.1016/j.jacc.2006.08.016
unseeable,” and to provide inspiration for a new generation
of clinical testing (9).
An observation: injury to the opossum’s pouch. The fetal
opossum’s pouch injured at day 9 heals without a scar with
normal hair follicle growth and epithelial maturation. The
same injury a few days later induces a permanent scar (10).
This fetal transition point in scarless healing, which occurs
in many species, appears coincident with appearance of the
inflammatory response. If ontogeny recapitulates phylogeny
(i.e., growth of the organism parallels the evolution of
species), then perhaps the essential elements of the nonscarring fetal environment could be re-created in an adult.
Indeed, this year the Kerr laboratory made that intuitive
leap in paralyzed rats by re-establishing spinal cord function
with stem cells, using a strategy that “overcomes repulsive
cues and/or provides attractive cues to stem cells” as occurs
in embryonic growth. The critical factor turned out to be
glial cell-derived neurotrophic factor (GDNF), which controls the development and survival of neurons in the
mesencephalon; addition of GDNF to the stem cells resulted in restoration of hind limb function; in its absence
paralysis persisted (11).
The challenge for myocardial regeneration seems similar:
to identify the factor(s) that might create this more hospitable fetal environment. Two classes of environmental
modifiers, prominent in embryonic development, are probably essential: cytokines that promote migration and proliferation, and inhibitors of apoptotic cell death. In the
developing heart, 2 such critical factors are thymosin ␤4 and
Akt-1. At 4 weeks after coronary artery ligation in mice,
treatment with thymosin ␤4 results in dramatic differences
in ejection fraction (58% vs. 28%) (12). This effect may be
mediated through activation of Akt-1, which has the dual
function of up-regulating expression of some familiar
growth factors (vascular endothelial growth factor, fibroblast
growth factor, insulin-like growth factor-1 [IGF-I], and
hepatocyte growth factor [HGF]) and inhibiting apoptotic
factors (Bcl-2 and caspases). In the rat infarct model, Akt-1
overexpression restores 80% to 90% of lost myocardial
volume and completely normalizes systolic and diastolic
cardiac function in a dose-dependent manner (13). Remarkably, just the culture medium itself, when taken from
Akt-overexpressing stem cells, significantly limits infarct
size and improves ventricular function (14). Clearly the cell’s
environment, ignored in the first era of clinical trials, seems
critical for myocardial regeneration.
Appealing environment-modifying candidates from embryologic development are not limited to thymosin ␤-4 and
Akt-1. Other growth factors like IGF-1 and HGF are both
angiogenic and antiapoptotic and, in addition, induce expression of collagen-degrading metalloproteinases that create an extracellular matrix receptive to cell migration.
The method for creating a more hospitable cell environment is also open to innovation. For instance, stem cells can
be induced to create their own more favorable environment
Forrester et al.
Editorial Comment
JACC Vol. 48, No. 8, 2006
October 17, 2006:1722–4
by up-regulating specific genes. Mesenchymal stem cells
pre-conditioned in a hypoxic environment up-regulate Akt
expression (13), and stem cells induced to overexpress HGF
increase capillary density, reduce collagen content, and
improve cardiac function in the rat infarct model (15). In
the upcoming era of clinical trials, this new idea, recapitulating ontogeny to create a cellular environment analogous
to the opossum’s pouch, seems certain to play a central role.
An observation: Y-chromosomes in transplanted female
hearts. In 2002, Quaini et al. (16) reported the interesting
observation that proliferating Y-chromosome cells appeared
in 7% to 10% of the myocytes and arterioles of female hearts
that had been transplanted into human male recipients. The
Anversa laboratory made the intuitive leap to the idea that
these cells came from residual cardiac tissue of the recipient.
Their search led to the discovery of previously unrecognized
multipotent self-renewing cardiac stem cells clustered in
interstitial niches of the atrium. Based on this discovery,
they have proposed that the heart is a continually selfrenewing organ (17). From this critical paradigm shift it is
a short step to a major new idea in myocardial regeneration:
use the heart’s own stem cells. Indeed, in the rat infarct
model, intra-aortic injection of cardiac stem cells decreased
infarct size by 29%, with significantly higher fractional
shortening, ejection fraction, and left ventricular systolic
wall thickness compared with controls (18). The problem of
myocardial regeneration is hardly solved, however, because
the regenerated myocytes were clustered together in foci
surrounded by collagenous scar. Thus, although the cardiac
stem cells engraft and improve cardiac performance, they do
not induce a return to normal myocardial histology. A
logical next step, testable in clinical trials, would seem to be
merging the new idea of cardiac stem cells with the new idea
of replicating the embryonic environment.
An observation: a hole punched in a mouse’s ear. In
1979, a new mouse line, termed MRL, was created for
studies of autoimmunity. For identification purposes, a
2-mm hole was routinely punched in the mouse ear. These
mice turned out to have an unanticipated, annoying characteristic: whereas normal mice retained the hole, the MRL
mouse ear healed completely within a month. Twenty-two
years later, Leferovich et al. (19) made the leap, freezeinfarcting a segment of the MRL heart. During the first
week after injury, the injury site showed cardiomyocytes
penetrating the wound site, and by day 60, the heart had
healed with little to no scar. In the healed myocardium, fully
20% of these cells were actively proliferating. When Ychromosome– containing bone marrow cells were injected
into lethally irradiated female recipients with no bone
marrow cells, the majority of proliferating myocytes in the
regenerated myocardium turned out to be of host origin.
Clearly, one possibility is that the source of proliferating
myocytes is the newly discovered host cardiac stem cell. An
additional fascinating observation in the MRL mouse
comes from differential gene expression studies. Matrix
metalloproteinase, the enzyme that digests collagen, is
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strikingly overexpressed in the MRL mouse wound, again
suggesting that in this animal the local environment may be
more receptive to stem cell migration and proliferation.
The “scarless heart” response in the MRL mouse, thus far
unique among mammals, is not, however, unique in the
animal kingdom. The newt, the zebrafish, and some other
amphibians are capable of regenerating limbs, spinal cord,
and retina without scar. In 2002, Poss et al. (20) in the
Keating laboratory demonstrated that zebrafish completely
regenerate myocardium within 2 months after a 20% resection. The Keating laboratory’s leap was to the idea that
unique genes responsible for this response could be identified, and then used to induce scarless healing in other
species that lacked this natural response. They identified
such genes; they seem to control the formation of the initial
mass of cells at the injury site, cell de-differentiation, and
rapid cell cycle proliferation (21). These data suggest an
entirely different idea. The source of cells for cardiac
regeneration might be mature cells at the injury site that
de-differentiate, and then proliferate as myocytes and endothelial cells. This idea also is entirely consistent with the
proliferation of host myocytes in the MRL mouse. Indeed,
an extract derived from regenerating newt limbs does cause
myotubes to de-differentiate (22). The tantalizing prospect
from this leap of reasoning is that if the proteins in this
extract can be identified and used to induce similar changes
in vivo, or if the responsible genes can be activated/
suppressed in mammals, the vision of myocardial regeneration in man might be realized.
Thus reading the G-CSF-STEMI (Granulocyte ColonyStimulating Factor ST-Segment Elevation Myocardial Infarction) trial results seems to provide us with a microcosm
of modern times: 2 entirely divergent interpretations. Concerning myocardial regeneration, cynics may see this trial as
marking an end to an era that began with a great vision in
2001, has had limited clinical success, and now confronts
more uncertainties than anyone could have imagined 5 years
ago. Yet the optimist may see in these same 5 years a new
era founded on 3 fascinating intuitive leaps that allow us to
see the unseeable, and that nature remains full of magical
things just waiting for us to discover them.
Reprint requests and correspondence: Dr. James S. Forrester,
Division of Cardiology, Cedars-Sinai Medical Center, 8700 Beverly Boulevard, Los Angeles, California 90048. E-mail:
[email protected].
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Editorial Comment
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