Jaarboek no. 89. 2010/2011 - Koninklijke Maatschappij voor ...

Staying simple
An important question is why some areas of the
embryonic heart do not participate in the formation
of atrial or ventricular working myocardium and
mature in a nodal direction. To gain insight into this
process we studied the regulation of the ANFgene
in more detail. ANFis never expressed in the nodal
tissues from fish to human. In the embryonic heart
it marks the developing atrial and ventricular working
myocardium. Studies in transgenic mice showed
that the upstream ca. 500 base pairs of the Anf promoter
region are sufficient to recapitulate the spatiotemporal
pattern of expression of the endogenous
gene. We generated a series of chimeric regulatory
constructs containing part of the promoter region
of the Anf gene coupled to other cardiac promoters.
When placed upstream of a small Cardiac troponin
I promoter fragment that is predominantly active
in the atrioventricular canal, transcription was specifically
repressed in the atrioventricular canal.
When placed upstream of the Mlc2v promoter that
is active in the outflow tract and right ventricle,
repression of transcriptional activity in the outflow
tract was observed. In both cases the Anf regulatory
sequences were able to confer expression to the
atria and ventricles. Thus the 500 base-pair Anf promoter
fragment is able to stimulate transcriptional
activity in the chambers and to repress expression
in the atrioventricular canal and in the outflow tract.
Mutational analyses revealed two closely adjacent
sites, a T-box and an NK-2 element, that if mutated
relieved the repression in the atrioventricular canal.
In a search for T-box factors that could act as repressor
we observed that Tbx2 is expressed in inflow,
atrioventricular canal, inner curvature and outflow
myocardium. The pattern of expression is complementary
to that of Anf and Connexin40. Functional
studies showed that Anf and Connexin40 are the
functional targets of TBX2, which functions as a
repressor. Recent studies have revealed a similar
role for the transcriptional repressor TBX3 that
becomes confined to the nodal components of the
conduction system.
In a generalizing view one may envision that TBX2
Natuurkundige voordrachten I Nieuwe reeks 89
Ontwikkeling van het bouwplan van het hart
and/or other transcriptional repressors suppress
the chamber-specific programme of gene expression,
allowing the regions where these factors are
expressed to further mature in the nodal direction
(Fig. 2). Obviously, the spatiotemporal regulation of
these repressors is the next issue to be resolved.
Finally
Finally a few words have to be said on the development
of the ventricular conduction system and of
the internodal tracts. The ventricular conduction
system largely develops from the interiorly localized
trabecular ventricular component (bundle branches
and their ramifications) and the primary ring (atrioventricular
bundle) as we have reviewed previously.
This notion is entirely in line with the lineage study
of Burch and co-workers on the development of the
atrioventricular canal, atrioventricular node and
atrioventricular bundle and with the lineage studies
of Mikawa and co-workers regarding the development
of the bundle branches and their ramification.
The periarterial part of the ventricular conduction
system is a unique feature of chicken, not present
in mice and humans. Ventricular chamber myocardium
develops at the ventral side of the anterior
part of the heart tube. An intermediate stage of
its development is the so-called trabecular myocardium.
Compact myocardium proliferates exteriorly
whereas interiorly trabeculations display a low
proliferative activity and differentiates toward the
peripheral ventricular conduction system displaying
high abundance of connexin expression (Fig. 2).
At the end the highly controversial issue of the internodal
tracts is in place. The term ‘internodal tract’
has the same effect to some as a red rag to a bull.
In view of the cardiac building plan discussed above
the region of the internodal tracts would be localized
between the inflow or sinoatrial region and the
atrioventricular canal, and comprises the remaining
part of the linear heart tube that has not developed
into atrial chamber. Purposely, we have used
the words outflow and inflow tract rather than the
more conventional anatomical terms. The reason
for this is that functionally the heart tube always
has a myocardial inflow and outflow, which does
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