Nodal signalling and the roles of the transcription factors SnR and Pitx2 in vertebrate left-right asymmetry

Left-specific mesodermal expression of the homeobox gene Pitx2 links a gast rula-stage intercellular signalling cascade to the later development of ver tebrate left-right organ asymmetry through the Nodal signalling pathway [1- 6]. SnR (Drosophila snail-related), a gene conserved in vertebrates and enc oding a zinc-finger protein, appears to function in a similar manner to Pit x2, but through right-specific mesodermal expression [7,8], Here, we presen t direct evidence for an earlier proposal [7] that Nodal signalling specifi cally represses expression of SnR in left lateral mesoderm, and thus ensure s its normal confinement to the right, while activating Pitx2 on the left. We addressed the relationship between Pitx2 and SnR using antisense disrupt ion of SnR, Relatively severe antisense-SnR treatment led to massive ectopi c expression of Pitx2 on the right, accompanied by randomisation of situs w ith embryos showing aspects of left-cardiac isomerism. This indicates a gen e cascade relationship in the propagation of left-right information, whereb y nodal activates Pitx2 on the left through a double-negative mechanism inv olving the repression of SnR's repressor role on Pitx2, Milder antisense-Sn R treatment reversed heart-loop direction and embryo torsion at significant incidence, although Pitx2 expression remained normally left-confined throu ghout, This reversed morphology following SnR disruption alone appeared dif ferent from that in which additional ectopic right-hand Pitx2 expression wa s seen. Therefore, in addition to their regulatory gene cascade relationshi p, these two transcription factors appear to have further, parallel but non -redundant, roles in directly controlling normally right and left-specific morphogenetic processes.