Aplnra/b Sequentially Regulate Organ Left-Right Patterning via Distinct Mechanisms

The G protein-coupled receptor APJ/Aplnr has been widely reported to be involved in heart and vascular development and disease, but whether it contributes to organ left-right patterning is largely unknown. Here, we show that in zebrafish, aplnra/b coordinates organ LR patterning in an apela/apln ligand-dependent manner using distinct mechanisms at different stages. During gastrulation and early somitogenesis, aplnra/b loss of function results in heart and liver LR asymmetry defects, accompanied by disturbed KV/cilia morphogenesis and disrupted left-sided Nodal/spaw expression in the LPM. In this process, only aplnra loss of function results in KV/cilia morphogenesis defect. In addition, only apela works as the early endogenous ligand to regulate KV morphogenesis, which then contributes to left-sided Nodal/spaw expression and subsequent organ LR patterning. The aplnra-apela cascade regulates KV morphogenesis by enhancing the expression of foxj1a, but not fgf8 or dnh9, during KV development. At the late somite stage, both aplnra and aplnrb contribute to the expression of lft1 in the trunk midline but do not regulate KV formation, and this role is possibly mediated by both endogenous ligands, apela and apln. In conclusion, our study is the first to identify a role for aplnra/b and their endogenous ligands apela/apln in LR patterning, and it clarifies the distinct roles of aplnra-apela and aplnra/b-apela/apln in orchestrating organ LR patterning.