Department of Anatomy, Physiology and Pharmacology, College of Veterinary Medicine, Auburn University, Auburn, AL 36849, USA.
Charité-Universitätsmedizin Berlin, Corporate Member of Freie Universität Berlin and Humboldt-Universität zu Berlin, Institute of Medical Physics and Biophysics, Group Structural Biology of Cellular Signaling, D-10117 Berlin, Germany.
Biomolecules. 2024 Sep 4;14(9):1120. doi: 10.3390/biom14091120.
Functional melanocortin receptor (MCR) genes have been identified in the genomes of early chordates, e.g., the cyclostomata. Whether they appear in the most ancient chordates such as cephalochordate and urochordata, however, remains unclear due to missing genetic data. Herein, we studied five putative (from NCBI database), sequence-based predicted MCR-like receptors from urochordata and cephalochordate, including , , , and . The BLAST and phylogenetic analyses suggested a relationship between these specific receptors and vertebrate MCRs. However, several essential residues for MCR functions in vertebrates were missing in these putative chordata MCRs. To test receptor functionality, several experimental studies were conducted. Binding assays and functional analyses showed no specific binding and no ligand-induced cAMP or ERK1/2 signaling (with either endogenous α-MSH or synthetic ligands for MC4R), despite successfully expressing four receptors in HEK 293T cells. These four receptors showed high basal cAMP signaling, likely mediated by ligand-independent Gs coupling. In summary, our results suggest that the five predicted MCR-like receptors are, indeed, class A G protein-coupled receptors (GPCRs), which in four cases show high constitutive activity in the Gs-cAMP signaling pathway but are not MCR-like receptors in terms of ligand recognition of known MCR ligands. These receptors might be ancient G protein-coupled receptors with so far unidentified ligands.
功能性黑素皮质素受体 (MCR) 基因已在早期脊索动物（例如圆口类）的基因组中被鉴定出来。然而，由于缺少遗传数据，它们是否出现在最古老的脊索动物如头索动物和尾索动物中尚不清楚。在此，我们研究了来自尾索动物和头索动物的五个基于序列预测的假定（来自 NCBI 数据库）MCR 样受体，包括 、 、 、和 。BLAST 和系统发育分析表明，这些特定的受体与脊椎动物的 MCR 之间存在关系。然而，这些假定的脊索动物 MCR 中缺少脊椎动物 MCR 功能所必需的几个关键残基。为了测试受体的功能，进行了几项实验研究。结合测定和功能分析表明，尽管成功地在 HEK 293T 细胞中表达了这四个受体，但这些受体没有与内源性 α-MSH 或 MC4R 的合成配体结合，也没有配体诱导的 cAMP 或 ERK1/2 信号（与内源性 α-MSH 或合成配体）。这些受体表现出高基础 cAMP 信号，可能由配体非依赖性 Gs 偶联介导。总之，我们的结果表明，这五个预测的 MCR 样受体确实是 A 类 G 蛋白偶联受体 (GPCR)，其中在四种情况下，在 Gs-cAMP 信号通路中表现出高组成型活性，但在配体识别方面不是 MCR 样受体已知的 MCR 配体。这些受体可能是具有迄今为止未鉴定出的配体的古老 G 蛋白偶联受体。
