An Orthologue of the Retinoic Acid Receptor (RAR) Is Present in the Ecdysozoa Phylum Priapulida

Abstract

Signalling molecules and their cognate receptors are central components of the Metazoa endocrine system. Defining their presence or absence in extant animal lineages is critical to accurately devise evolutionary patterns, physiological shifts and the impact of endocrine disrupting chemicals. Here, we address the evolution of retinoic acid (RA) signalling in the Priapulida worm, Priapulus caudatus Lamarck, 1816, an Ecdysozoa. RA signalling has been shown to be central to chordate endocrine homeostasis, participating in multiple developmental and physiological processes. Priapulids, with their slow rate of molecular evolution and phylogenetic position, represent a key taxon to investigate the early phases of Ecdysozoa evolution. By exploring a draft genome assembly, we show, by means of phylogenetics and functional assays, that an orthologue of the nuclear receptor retinoic acid receptor (RAR) subfamily, a central mediator of RA signalling, is present in Ecdysozoa, contrary to previous perception. We further demonstrate that the Priapulida RAR displays low-affinity for retinoids (similar to annelids), and is not responsive to common endocrine disruptors acting via RAR. Our findings provide a timeline for RA signalling evolution in the Bilateria and give support to the hypothesis that the increase in RA affinity towards RAR is a late acquisition in the evolution of the Metazoa.

Nuclear Receptors and Development of Marine Invertebrates

“…During vertebrate development, RA signaling is, for example, required for axial patterning, nervous system development and organogenesis, with HOX genes being amongst the major targets of this signaling pathway [ 99 , 100 ]. Most bilaterian genomes contain a single RAR gene [ 17 , 27 , 36 , 49 , 50 , 101 , 102 , 103 ]. However, RARs have been lost in most ecdysozoans as well as in appendicularian (larvacean) tunicates ( Figure 3 ) ( Table 3 ) [ 27 , 101 , 103 ].…”

“…Similarly, although the genomes of most ecdysozoans do not encode a RAR gene, there are indications for active roles of RA signaling in ecdysozoans, for example, during nervous system regeneration and in tissue repair of insects [ 108 , 109 ]. As a matter of fact, RAR has been lost very early during ecdysozoan evolution, after the split of the priapulid lineage [ 103 ]. Notably, the RAR of the priapulid Priapulus caudatus binds all- trans -RA and 9- cis -RA with affinities similar to those of the P. dumerilii RAR [ 49 , 103 ].…”

“…As a matter of fact, RAR has been lost very early during ecdysozoan evolution, after the split of the priapulid lineage [ 103 ]. Notably, the RAR of the priapulid Priapulus caudatus binds all- trans -RA and 9- cis -RA with affinities similar to those of the P. dumerilii RAR [ 49 , 103 ]. In cnidarians, whose genomes do not encode RARs, RA has been implicated in the regulation of metamorphosis and neural development [ 55 , 64 , 99 , 110 ].…”

Of Retinoids and Organotins: The Evolution of the Retinoid X Receptor in Metazoa

“…Cell culture and transactivation assays were performed as previously described in [27,54,55]. Briefly, Cos-1 cells (Sigma-Aldrich) were maintained at 37 • C with 5% CO 2 (humidified atmosphere) in Dulbecco's modified Eagle's medium (DMEM) (PAN-Biotech, Aidenbach, Bayern, Germany) supplemented with 10% and 1% of fetal bovine serum (PAN-Biotech, Aidenbach, Bayern, Germany) and penicillin/streptomycin (PAN-Biotech, Aidenbach, Bayern, Germany), respectively.…”

Whole mount in situ hybridization and immunohistochemistry for studying retinoic acid signaling in developing amphioxus

“…RA signaling controlled by RAR evolved early in animal evolution, likely at the base of bilaterians, as a low-affinity sensor system. RARs with high affinity for RA, as seen in mammals, probably evolved only later and specifically in the chordate lineage (Fonseca et al, 2019;Handberg-Thorsager et al, 2018). This marks the origin of chordates as a milestone in the evolution of RA signaling.…”