Mechanical and Chemical Regulation of Arterial and Venous Specification - Mechanical and Chemical Signaling in Angiogenesis - page 7

3.1 Notch Signaling in Arterial Vessel Specification
The first insight into the mechanism underlying arterial specification came from a
study of a mutant zebrafish in which an arterial vessel is specifically missing
[ 16 , 17 ]. Zebrafish have served as a unique genetic model system in deciphering
molecular mechanisms underlying a variety of processes in development and
disease. This organism is a particularly powerful model system in studying the
developing vascular system. In other higher vertebrate models such as mice,
malformation or deficiency of the vascular system results in early embryonic
lethality, thus they are unsuitable for identifying genes involved in the formation
of the early vascular system. However, early development of zebrafish embryos is
not dependent on the formation of the vascular system (i.e., early stage zebrafish
embryos can survive without circulation), thus they provide a unique genetic
system for identifying genes that are critically involved in the differentiation and
formation of the vascular system.
Through the screening for a mutant that is defective in vascular development in
zebrafish embryos, Gridlock (grl) was identified [ 18 ]. In this mutant embryo, the
assembly of the first arterial vessel, the aorta, is specifically perturbed. A gene that
is responsible for this specific mutant phenotype was subsequently identified to
encode a basic-helix-loop-helix (bHLH) transcription factor belonging to the
Hairy/Enhancer of the split family of bHLH proteins, Hey2 [ 17 ]. The grl/Hey gene
is expressed in lateral plate mesoderm that is destined to become angioblast,
precursor cells for vascular endothelial cells. Its expression subsequently becomes
restricted to aorta (an artery) but undetectable in veins. Other studies by the same
and another groups indicated that grl/Hey2 is a downstream target of Notch
signaling by showing that the expression of grl/Hey2 can be induced by activating
Notch signaling and can be inhibited by inhibitors of this signaling pathway [ 16 ].
The critical involvement of Notch signaling in arterial specification was further
indicated by another zebrafish study where it was shown that the inhibition of
Notch signaling by either the injection of a dominant negative Su (H) (suppressor
of hairless) results in the failure of arterial marker, EphrinB2, expression and the
concomitant ectopic expression of venous markers, EphB4 and Flt4, a phenotype
that is also detected in a notch-deficient mutant, mindbomb (mib)[ 19 ]. Further-
more, the constitutive activation of Notch signaling by the forced expression of
NICD (Notch intracellular domain) resulted in reduced venous marker expression.
Several studies with mice also indicate an importance of Notch signaling in
arterial fate specification. The Notch1, Notch4, and the Notch ligands, Jag1, Jag2
and Dll4 are all expressed in arterial, but not in venous, endothelial cells [ 20 - 29 ].
Furthermore, Dll4 heterozygous mutant mice exhibit reduced expression of
EphrinB2 and increased expression of EphB4, a phenotype that is also observed in
Notch-deficient Rbpj (the recombination signal-binding protein for immunoglob-
ulin-j J region) and Mib mutant mice and Hey1/Hey2 double mutants [ 30 - 33 ].
These results are also consistent with the notion that Notch signaling is critical for
arterial differentiation.