Elucidating Ciliopathies: The Role of Empty Spiracles Homeobox 2 and Cannabinoid Signaling in Cilia Development across Embryonic Tissues

Cilia are exquisite organelles with essential roles in ontogeny and physiology, and ciliogenesis defects underline a range of diseases ranging from rare genetic, chronic to highly infectious diseases. In our work, we elucidated the role of empty spiracles homeobox 2 (emx2) and cannabinoid receptor 1 (cnr1) in zebrafish embryonic cilia development across tissues, highlighting their roles in regulating renal MCCs. We report that emx2 is essential for cilia development across the ear, neuromasts and Kupffer’s vesicle (KV). emx2 deficient embryos manifest altered fluid homeostasis and kidney defects including decreased multiciliated cells (MCCs), determining that emx2 is essential to properly establish several renal lineages. Further, emx2 deficiency disrupted renal monociliated cells, MCCs and led to aberrant basal positioning. We reported that emx2 regulates prostaglandin biosynthesis in ciliogenesis and renal fate changes through key factors including ppargc1a, ptgs1 and PGE2. Our findings reveal essential roles of emx2 in tissue cilia development and identify emx2 as a critical regulator of prostaglandin biosynthesis during renal development and ciliogenesis. In our second study, we report that Cnr1 is essential for cilia development across tissues and regulates renal MCCs via cyclic AMP (cAMP) signaling. We found that the loss of function, agonism and antagonism of cnr1 all lead to reduced mature renal MCC populations. cnr1 deficiency also led to reduced cilia development across tissues, including the pronephros, ear, KV, and nasal placode. Interestingly, treatment with the cAMP activator Forskolin (FSK) restored renal MCC defects in agonist-treated embryos, suggesting cnr1 mediates cAMP signaling in renal MCC development. Meanwhile, treatment with the cAMP inhibitor SQ-22536 alone or with cnr1 deficiency both led to reduced MCC populations, suggesting cnr1 mediates renal MCC development independently of cAMP signaling. Our findings indicate that cnr1 has a critical role in controlling renal MCC development both via cAMP signaling and independent pathway.