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Evolutionarily conserved developmental pathways
The understanding of pathways that induce mammalian cardiac and somatic muscle is incomplete, but in each case, elements common to both Drosophila and mammals suggest an evolutionarily conserved pathway. Induction of cardiac muscle in Drosophila requires tinman acting downstream of twist and decapentaplegic. The mammalian homolog of Tinman, Nkx2-5, is expressed in precardiac mesoderm and in the myocardium of embryonic and fetal hearts. A Xenopus homolog is likewise expressed in the heart region. A mammalian homolog of Bagpipe, involved in specification of visceral musculature in Drosophila, has been discovered in Xenopus. Xbap, like bagpipe is expressed in the developing musculature of the midgut. However, a second, novel role in development is suggested by the observation that Xbap is expressed in the region of the developing facial cartilage, a neural crest derivative.
The axial structures, consisting of the notochord and the neural tube, play an essential role in the dorsoventral patterning of somites and in the differentiation of their many cell lineages in vertebrates. Because Drosophila lacks a dorsal nervous cord and notochord, it is often thought that the roles of these tissues are not conserved. Nevertheless, expression of Nkx-3.1 (homologous to Drosophila bagpipe), functions downstream of Sonic hedgehog in vertebrates as it does in flies. The role of the axial structures was studied by using Nkx-3.1 as a medial somitic marker in explant in vitro assays. Nkx-3.1 is dynamically expressed during somitogenesis only in the youngest, most newly-formed somites at the caudal end of the embryo. The expression of Nkx-3.1 in pre-somitic tissue explants is induced by the notochord and maintained in newly-differentiated somites by the notochord and both ventral and dorsal parts of the neural tube. Sonic hedgehog is one of the signaling molecules that can reproduce the effect of the axial structures. Shh can induce and maintain Nkx-3.1 expression in pre-somitic mesoderm and young somites, but not in more mature, differentiated ones (Kos, 1998). In Drosophila, bagpipe expression is likewise regulated by Hedgehog signals, but in the case of Drosophila, HH signals emanate from the overlying ectoderm (Azpiazu, 1996).
A second example exists for a conserved role for external signals in mesoderm induction. Drosophila induction of the homeobox gene tinman, and subsequent heart formation, are both dependent on dpp signaling from overlying ectoderm. In order to define vertebrate heart-inducing signals dpp-homologs have been sought that are expressed in stage 4 chicken embryos. The majority of transcripts are found to be BMP-2 among several other members of the BMP family. From embryonic stage 4 onwards cardiogenic mesoderm appears to be in close contact to BMP-2 expressing cells that initially are present in lateral mesoderm and subsequently after headfold formation in the pharyngeal endoderm. In order to assess the role of BMP-2 for heart formation, gastrulating chick embryos in culture were implanted with BMP-2 producing cells. BMP-2 implantation results in ectopic cardiac mesoderm specification. BMP-2 is able to induce Nkx2-5 expression (Drosophila homolog: Tinman) ectopically within the anterior head domain, while GATA-4 (Drosophila homolog: Serpent) is induced more caudally. However, cardiogenic induction by BMP-2 remains incomplete, since neither Nkx2-8 nor the cardiac-restricted structural gene VMHC-1 become ectopically induced. These results suggest that BMP-2 is part of the complex of cardiogenic signals and is involved in the patterning of early mesoderm similar to the role of dpp in Drosophila (AndrŽe, 1998).
Induction of somatic musculature requires two components: (1) a basic helix-loop-helix transcription factors (bHLH) and (2) a MADS box transcription factor:
(1) The bHLH transcription factors involved are: Nautilus
in Drosophila, and MyoD homologs in vertebrates. Both Nautilus and MyoD
homologs require a cofactor: Daughterless
in Drosophila, and E12 in vertebrates. Nautilus acts downstream of twist
and decapentaplegic.
(2) Myocyte enhancer factor 2 is the MADS box transcription factor. For
Drosophila MEF2, there are two Xenopus homologs
and several mammalian homologs. Many muscle protein promoters have binding
sites for both bHLH heterodimers and MEF2, suggesting that the two components
integrate their action directly on target promoters.
REFERENCES
AndrŽe, B., et al. (1998). BMP-2 induces ectopic expression of cardiac lineage markers and interferes with somite formation in chicken embryos. Mech. Dev. 70(1-2): 119-131
Azpiazu, N., et al. (1996). Segmentation and specification of the Drosophila mesoderm. Genes Dev. 10: 3183-94
Kos, L., Chiang, C. and Mahon, K. A. (1998). Mediolateral patterning of somites: multiple
axial signals, including Sonic hedgehog, regulate Nkx-3.1 expression. Mech. Dev. 70(1-2): 25-34
date revised: 10 March 98
Developmental Pathways conserved in Evolution
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