Download The role of Tbx2 in the development of the - UvA-DARE

UvA-DARE (Digital Academic Repository)
The role of Tbx2 in the development of the atrioventriculair canal and conduction
system of the heart. `Making the beat go on and on`
Aanhaanen, W.T.J.
Link to publication
Citation for published version (APA):
Aanhaanen, W. T. J. (2011). The role of Tbx2 in the development of the atrioventriculair canal and conduction
system of the heart. `Making the beat go on and on`
General rights
It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s),
other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons).
Disclaimer/Complaints regulations
If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating
your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask
the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam,
The Netherlands. You will be contacted as soon as possible.
UvA-DARE is a service provided by the library of the University of Amsterdam (http://dare.uva.nl)
Download date: 17 Jun 2017
The Fate of the Tbx2+ Myocardial Lineage of the Atrioventricular Canal
Landmarks and Lineages in the Developing Heart
Richard P. Harvey, Sigolène M. Meilhac, Margaret E. Buckingham
Circulation Research 2009 June 5;104(11):1235-7
One of the overarching concepts underpinning mammalian embryonic development is that
of “regulation.” This implies that the lineage potential of a particular cell is broader than its
actual fate. Although the influential experiments in this area performed by Hans Driesch in the
late 1800s concerned the fate of individual embryonic blastomeres, the concept of regulative
development has pervaded all aspects of embryology, overlapping with the concept of
regenerative fields. In the context of the heart, a good example is that of frog embryo cells fated
to form the dorsal mesocardium and dorsal pericardial (splanchnic) mesoderm but which can
“regulate” and form myocardial tissue if the normal heart is injured or extirpated.1 As we delve
into the molecular mechanisms of developmental processes, we understand that limitations to
cell fate are often set, particularly in the embryo, by geographical or environmental parameters,
such as the source and strength of a secreted inductive signal, and these may change with time.
Anatomic patterns or landmarks provide vital clues to how we should think about
development and indeed disease. For example, segmentation of the embryonic paraxial mesoderm
into repetitive units called somites, which bear one of the main progenitor populations for the
musculoskeletal system, has been widely studied and it is known that somite segmentation
is controlled by a network of synchronized molecular oscillators coupled by the Delta-Notch
intercellular signaling system.2 The heart has also long been considered to have a segmental
prepattern based, in part, on the series of swellings and constrictions that become evident during
early heart tube formation and early fate-mapping experiments suggesting that these were the
precursor structures of the chambers and their flanking regions, including the atrioventricular canal
(AVC). This notion has been perpetuated as fact in embryology and medical textbooks with
very little experimental support, reflecting how alluring the concept of metamerism is to
developmental biologists, no doubt influenced by the Drosophila model. Indeed, in the fruit fly,
the dorsal vessel, equivalent to the heart, is clearly segmented and develops under the control of
homeotic (HOMC) genes (Figure, A).
So, how is the vertebrate heart patterned, how does it achieve its final size and form,
and how do heart chambers come into being? There has been great progress in this area
over the last decade and a half, to the extent that we now have a fairly clear notion of the
anatomic building blocks of the heart and the first insights into the conserved transcription
factor pathways that guide heart specification and morphogenesis. Growth of the early heart is
extremely dynamic, with a proliferating undifferentiated precursor pool contributing cells to
both inflow and outflow poles of the initial heart tube over a period of 2 days (in the mouse). Fate
mapping studies using Cre recombinase technology and a novel retrospective lineage technique
show that this dynamic behavior has, at its base, an early lineage segregation (Figure, B),
61
2
Chapter 2
dating to a time at or likely before the specification of heart cell types.3 These experiments
show that the initial heart tube forms from a precursor population that is quite distinct from the
cells subsequently added. This “first” lineage begins to differentiate early while still within its
2
crescent-shaped progenitor pool. Subsequently, this epithelial structure fuses to form the initial
myogenic heart tube, which adopts a lower proliferative index and contributes cells mainly
to the anatomic left (systemic) ventricle, with contributions also to other regions. Cells that
arise from a distinct “second” lineage are added progressively from a dorsal proliferative center
(the second heart field) to the poles of this early heart tube which serves as a scaffold. Within
the second lineage, different cellular and oriented proliferative behaviors, and gene expression
patterns, arise, depending on the stage of differentiation and site of deployment at the inflow or
outflow poles. This lineage contributes to most regions of the heart but not to the embryonic
left ventricle. Thus, although lineage restriction does play a role in heart development, myocyte
clonal growth patterns in the heart are largely inconsistent with a lineage-based segmental
prepattern for heart chamber formation.3 Recent evidence also suggests that some myocardial
cells of the embryonic heart derive from the proepicardium, a distinct progenitor organ that
separates early from the heart fields and gives rise to the outer epicardial layer of the heart and,
subsequently, the coronary blood vessels.4 However, such a myocardial contribution is still
controversial.5
In this issue of Circulation Research, Aanhaanen et al highlight an interesting complexity
in the way myocardial cells are patterned in the forming heart tube.6 At the heart of the issue
is how cardiac chamber and nonchamber myocardium come into being, if not by segmental
or clonal restriction mechanisms. Existing genetic and gene expression evidence suggests
that the specialized chamber myocardium of the atria and ventricles forms progressively from
“primary” myocardium in regional domains along the growing heart tube.7 Such specialization
involves activation of distinct genes, including Nppa/Anf, Smpx/Chisel, and Atp2a2/
Serca2a, not expressed in cardiomyocytes of the early heart tube, sometimes referred to as “less
differentiated” although clearly contractile. The heart tube is also less mature in that it has not
developed trabeculae. Specified chambers become trabeculated and come to dominate the heart
through selective growth only later. The remaining primary (nonchamber) myocardium adopts a
distinct fate, becoming the less-specialized myocardium of the inner curvature, elements of the
proximal conduction system (sinoatrial and AV node and His bundle), inflow and outflow vessel
myocardium, and fibrotic tissue of the AV junction. Homozygous mutation of several mouse
cardiac transcription factors, including the homeodomain factor Nkx2–5 and T-box factors
Tbx5 and Tbx20, lead to formation of a rudimentary heart tube in which formation of chamber
myocardium is blocked.
The family of T-box transcription factors plays key roles in establishing the initial pattern
of chamber and nonchamber myocardium, and different family members appear to work in an
antagonistic fashion. A synergistic activity between Tbx5 with Nkx2–5 promotes expression of
chamber genes, whereas the repressor Tbx2, expressed in nonchamber myocardium, competes
62
The Fate of the Tbx2+ Myocardial Lineage of the Atrioventricular Canal
with Tbx5 for interaction with Nkx2–5, forming a repressive complex that inhibits expression
of chamber genes. The apparent restriction in the expression of Tbx2 and its upstream
inducers Bmp2 and -4, to non chamber myocardium, suggests that these factors play a key role
in imposing nonchamber fate on the early heart tube.
In a careful study using Cre recombinase technology and mutant mice, Aanhaanen et
al have now discovered that the initial Tbx2 expression domain encompassing nonchamber
myocardium of the AVC can also give rise to a significant portion of the basal part of left
ventricular chamber myocardium6 (Figure, C). De la Cruz had also reported additional later
contributions to the inlet and outlet regions of the avian ventricles.8 Aanhaanen et al show in
the mouse that this contribution had not occurred in the heart tube at embryonic day 9.25,
but was evident at embryonic day 10.5 at the beginning of ventricular expansion. Cells that
had expressed Tbx2 also extend further into the right ventricle. The findings in this article
indicate that the primitive left ventricle mainly contributes to the apical part of the definitive left
ventricle, proximal to, and including part of, the intraventricular septum. Given that the primitive
left ventricle occupies a larger region than the primitive AVC, and has a higher proliferative
index once chamber specification has occurred, it is certainly surprising that its contribution
is ultimately smaller that that of AVC derived cells. To clearly assess the complementary
contribution of the primitive ventricles and the degree of overlap with AVC derived cells, it
would be interesting to systematically mark these cells in the early heart tube and follow their
descendants. As far as clonal considerations are concerned, the AVC is constituted by cells of
both first and second lineages, so that cells from this source that later invade the left ventricle
may be of either origin. There is no suggestion that the AVC deployment represents a special
progenitor pool. Rather, the work demonstrates that the initial Tbx2 expression domain does not
faithfully define nonchamber myocardium and, as suggested by the lineage studies discussed
above, that boundaries between chambers and their flanking regions are not clonally restricted,
nor are they rigidly defined by signaling mechanisms at this early time. Formation of chamber/
nonchamber boundaries in the heart is therefore regulative and may occur in equal part by
progressive restriction of cell dispersal and by changing signaling gradients. Indeed, transcription
factors Hesr1 and Hesr2, which in some circumstances can act downstream of Notch signaling,
have been implicated in the formation of the AVC boundary9 by repression of Tbx2 (Figure, C).
We do not know exactly when the left ventricle/AVC boundary becomes fixed, but these studies
suggest that the junctional region of the Tbx2-positive AVC at embryonic day 9.25 is coopted
to chamber myocardium. This implies a positive pathway for chamber induction. The fact that
the right ventricle was labeled also argues that specialization of chamber myocardium from
primary nonchamber myocardium (in this case the outflow region) is a positive and progressive
process and does not simply occur through imposition of a nonchamber state on a default
chamber state by expression of Tbx2. Other observations, based on the use of Cre recombinaseexpressing strains in which Cre is thought to mark only the second lineage, also showed more
extensive marking of the heart at later stages, possibly also reflecting a similar expansion
63
2
Chapter 2
into chamber myocardium.10-12 Aanhaanen et al. rightly suggest that errors in development of
the early AVC may affect a larger region of the heart than anticipated, and the findings of the
authors will be important for interpretation of congenital heart disease phenotypes and those
in mouse genetic models. A final reflection, then, is that the early AVC is an ill-defined entity,
in fact merely a constriction at best. It may be better to consider only that the definitive AVC is
formed from cells that lie within the initial Tbx2 expression domain, which at the earliest time
of heart tube formation covers, we now know from this study, much broader domain than that
of AVC precursors.
2
Reference List
(1)
Raffin M, Leong LM, Rones MS, Sparrow D, Mohun T, Mercola M. Subdivision of the cardiac Nkx2.5 expression domain into
myogenic and nonmyogenic compartments. Dev Biol 2000 February 15;218(2):326-40.
(2)
Riedel-Kruse IH, Muller C, Oates AC. Synchrony dynamics during initiation, failure, and rescue of the segmentation clock. Science
2007 September 28;317(5846):1911-5.
(3)
Buckingham M, Meilhac S, Zaffran S. Building the mammalian heart from two sources of myocardial cells. Nat Rev Genet 2005
November;6(11):826-37.
(4)
Zhou B, Ma Q, Rajagopal S, Wu SM, Domian I, Rivera-Feliciano J, Jiang D, von GA, Ikeda S, Chien KR, Pu WT. Epicardial
progenitors contribute to the cardiomyocyte lineage in the developing heart. Nature 2008 July 3;454(7200):109-13.
(5)
Christoffels VM, Grieskamp T, Norden J, Mommersteeg MT, Rudat C, Kispert A. Tbx18 and the fate of epicardial progenitors.
Nature 2009 April 16;458(7240):E8-E9.
(6)
Aanhaanen WT, Brons JF, Dominguez JN, Rana MS, Norden J, Airik R, Wakker V, de Gier-de Vries C, Brown NA, Kispert A,
Moorman AF, Christoffels VM. The Tbx2+ primary myocardium of the atrioventricular canal forms the atrioventricular node and
the base of the left ventricle. Circ Res 2009 May 7;104(11):1267.
(7)
Christoffels VM, Habets PEMH, Franco D, Campione M, de Jong F, Lamers WH, Bao ZZ, Palmer S, Biben C, Harvey RP, Moorman
AFM. Chamber formation and morphogenesis in the developing mammalian heart. Dev Biol 2000;223:266-78.
(8)
De la Cruz MV, Sánchez-Gómez C, Palomino M. The primitive cardiac regions in the straight tube heart (stage 9) and their
anatomical expression in the mature heart: an experimental study in the chick embryo. J Anat 1989;165:121-31.
(9)
Kokubo H, Tomita-Miyagawa S, Hamada Y, Saga Y. Hesr1 and Hesr2 regulate atrioventricular boundary formation in the developing
heart through the repression of Tbx2. Dev 2007 February;134(4):747-55.
(10)
Yang L, Cai CL, Lin L, Qyang Y, Chung C, Monteiro RM, Mummery CL, Fishman GI, Cogen A, Evans S. Isl1Cre reveals a
common Bmp pathway in heart and limb development. Dev 2006 April;133(8):1575-85.
(11)
Verzi MP, McCulley DJ, De VS, Dodou E, Black BL. The right ventricle, outflow tract, and ventricular septum comprise a restricted
expression domain within the secondary/anterior heart field. Dev Biol 2005 November 1;287(1):134-45.
(12)
Brown CB, Wenning JM, Lu MM, Epstein DJ, Meyers EN, Epstein JA. Cre-mediated excision of Fgf8 in the Tbx1 expression
domain reveals a critical role for Fgf8 in cardiovascular development in the mouse. Dev Biol 2004 March 1;267(1):190-202.
64
The Fate of the Tbx2+ Myocardial Lineage of the Atrioventricular Canal
2
Figure. A, Segments of the dorsal vessel in Drosophila, distinguished by the expression of homeotic (HOMC)
genes. B, Early lineage segregation of mouse myocardial cells. Arrows show the addition of cells at the inflow and
outflow poles of the heart tube. C, Novel model of chamber formation. The boundary between the atrioventricular
canal and the left ventricle is defined progressively. Early "primitive" territories of the heart tube defined by the
expression of indicated genes, overlap in the later heart. AVC indicates atrioventricular canal; LA, left atrium; LV,
left ventricle; OFT, outflow tract; pAVC, primitive atrioventricular canal; PhA, pharyngeal arches; pLV, primitive
left ventricle; pOFT, primitive outflow tract; pRV, primitive right ventricle; RA, right atrium; RV, right ventricle.
65