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

Abstract

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