Transcript Journal Club pack

Circulation Research May 11, 2012 Journal Club
Localization of Islet-1–Positive Cells in the Healthy and Infarcted Adult
Murine Heart
Florian Weinberger, Dennis Mehrkens, Felix W. Friedrich, Mandy Stubbendorff, Xiaoqin Hua,
Jana Christina Müller, Sonja Schrepfer, Sylvia M. Evans, Lucie Carrier, Thomas Eschenhagen
Circ Res. 2012;110:1303-1310.
PDF [with Online Supplement]: http://circres.ahajournals.org/content/110/10/1303.full.pdf+html
Related Editorial by Mark Sussman [PDF]: Myocardial Isl+land: A Place With Lots of Rhythm,
but No Beat
Included in the Journal Club pack: Abstract, Novelty & Significance section, and all figures.
Localization of Islet-1–Positive Cells in the Healthy and Infarcted Adult
Murine Heart
Abstract
Rationale: The transcription factor Islet-1 is a marker of cardiovascular progenitors during
embryogenesis. The isolation of Islet-1–positive (Islet-1+) cells from early postnatal hearts suggested
that Islet-1 also marks cardiac progenitors in adult life.
Objective: We investigated the distribution and identity of Islet-1+ cells in adult murine heart and
evaluated whether their number or distribution change with age or after myocardial infarction.
Methods and Results: Distribution of Islet-1+ cells in adult heart was investigated using gene
targeted mice with nuclear β-galactosidase inserted into the Islet-1 locus. nLacZ-positive cells were
only present in 3 regions of the adult heart: clusters in the interatrial septum and around the
pulmonary veins, scattered within the wall of the great vessels, and a strictly delimited cluster
between the right atrium and superior vena cava. Islet-1+ cells in the first type of clusters coexpressed
markers for parasympathetic neurons. Positive cells in the great arteries coexpressed smooth muscle
actin and myosin heavy chain, indicating a smooth muscle cell identity. Very few Islet-1+ cells within
the outflow tract expressed the cardiomyocyte marker α-actinin. Islet-1+ cells in the right atrium
coexpressed the sinoatrial node pacemaker cell marker HCN4. Cell number and localization remained
unchanged between 1 to 18 months of age. Consistently Islet-1 mRNA was detected in human
sinoatrial node. Islet-1+ cells could not be detected in the infarct zone 2 to 28 days after myocardial
infarction, aside from 10 questionable cells in 1/13 hearts.
Conclusions: Our results identify Islet-1 as a novel marker of the adult sinoatrial node and do not
provide evidence for Islet-1+ cells to serve as cardiac progenitors.
Novelty and Significance
What Is Known?
In early cardiac development, cells expressing the transcription factor Islet-1 form the right ventricle, the outflow
tract, parts of the atria (the “second heart field”) and the sinoatrial node (SAN) and atrioventricular node (AVN).
 Islet-1+ cells from fetal hearts or embryonic stem cells form cardiac myocytes, smooth muscle and endothelial cells
when placed in cell culture, suggesting that Islet-1+ cells may also serve a cardiovascular progenitor role in the adult
heart.
 In the adult mouse heart, Islet-1 positivity was detected by antibody staining in distinct clusters in the heart basis
whose identity remained unclear.

What New Information Does This Article Contribute?
Genetic
labeling experiments showed that Islet-1 in the adult mouse heart is expressed in HCN4+ pacemaker cells of
the SAN, but not the AVN, parasympathetic nervous ganglia, smooth muscle cells of the proximal part of the great
arteries, and few cardiac myocytes of the outflow tract area.
The expression pattern was highly reproducible and stable for up to 18 months.
No Islet-1+ cells were found in healthy ventricular myocardium. These cells were found only in an atypical form in
10/≈100 000 000 cells in >4000 sections of hearts 2 to 28 days after myocardial infarction.
Islet-1 is an established marker of early cardiovascular development; however, its role in the adult heart, specifically its
progenitor role, remains obscure. Given the technical difficulties in studying Islet-1 distribution in the heart by
immunologic means, we used a genetic approach, that is, a mouse in which Islet-1 expression drives a nuclearly
localized β-galactosidase. This technique allows not only histological identification of Islet-1+ cells with high sensitivity
and specificity but even the macroscopic identification in the whole heart, facilitating the analysis of regional
distribution. Islet-1+ cells were undetectable in >5000 sections from normal or injured myocardium. This is important
because it provides strong evidence against the idea that Islet-1+ cells could serve a progenitor role in the normal or
injured postnatal heart. Stable Islet-1+ expression in very distinct and functionally specialized cells suggest that Islet-1+
expression in the postnatal heart is not a spurious “left-over” from development but serves an important role that
warrants further studies. The findings of this study also establish Islet-1 as a new and, compared with HCN4, a specific
marker of a subpopulation of SAN pacemaker cells
nLacZ+ cells in an Isl-1–nLacZ adult mouse heart.
Weinberger F et al. Circulation Research 2012;110:13031310
Copyright © American Heart Association
Characterization of nLacZ+ cells in the interatrial septum and around the pulmonary veins.
Weinberger F et al. Circulation Research 2012;110:13031310
Copyright © American Heart Association
Characterization of nLacZ+ cells in the proximal aorta and pulmonary artery.
Weinberger F et al. Circulation Research 2012;110:13031310
Copyright © American Heart Association
nLacZ+ cells in the outflow tract and the aortic valve.
Weinberger F et al. Circulation Research 2012;110:13031310
Copyright © American Heart Association
nLacZ+ cells in the sinoatrial node.
Weinberger F et al. Circulation Research 2012;110:13031310
Copyright © American Heart Association
Isolated sinoatrial node cells after X-gal staining.
Weinberger F et al. Circulation Research 2012;110:13031310
Copyright © American Heart Association
Islet-1 mRNA in murine and human sinoatrial node tissue.
Weinberger F et al. Circulation Research 2012;110:13031310
Copyright © American Heart Association
Isl-1–nLacZ-hearts after myocardial infarction.
Weinberger F et al. Circulation Research 2012;110:13031310
Copyright © American Heart Association