A ryanodine receptor-dependent Cai2+ asymmetry at Hensen's node mediates avian lateral identity

In mouse, the establishment of left-right (LR) asymmetry requires intracellular calcium (Ca-i(2+)) enrichment on the left of the node. The use of Ca-i(2+) asymmetry by other vertebrates, and its origins and relationship to other laterality effectors are largely unknown. Additionally, the architecture of Hensen's node raises doubts as to whether Ca-i(2+) asymmetry is a broadly conserved mechanism to achieve laterality. We report here that the avian embryo uses a left-side enriched Ca-i(2+) asymmetry across Hensen's node to govern its lateral identity. Elevated Ca-i(2+) was first detected along the anterior node at early HH4, and its emergence and left-side enrichment by HH5 required both ryanodine receptor (RyR) activity and extracellular calcium, implicating calcium-induced calcium release (CICR) as the novel source of the Ca-i(2+). Targeted manipulation of node Ca-i(2+) randomized heart laterality and affected nodal expression. Bifurcation of the Ca-i(2+) field by the emerging prechordal plate may permit the independent regulation of LR Ca-i(2+) levels. To the left of the node, RyR/CICR and H+V-ATPase activity sustained elevated Ca-i(2+). On the right, Ca-i(2+) levels were actively repressed through the activities of H+K+ ATPase and serotonin-dependent signaling, thus identifying a novel mechanism for the known effects of serotonin on laterality. Vitamin A-deficient quail have a high incidence of situs inversus hearts and had a reversed calcium asymmetry. Thus, Ca-i(2+) asymmetry across the node represents a more broadly conserved mechanism for laterality among amniotes than had been previously believed.