Institute of Experimental Pediatric Endocrinology, Charité-Universitätsmedizin Berlin, Freie Universität Berlin, Humboldt-Universität zu Berlin, D-10117 Berlin, Germany.
Institute of Anatomy, Leipzig University, D-04103 Leipzig, Germany.
Int J Mol Sci. 2024 Jul 10;25(14):7565. doi: 10.3390/ijms25147565.
The melanocortin-4 receptor (MC4R) is a key player in the hypothalamic leptin-melanocortin pathway that regulates satiety and hunger. MC4R belongs to the G protein-coupled receptors (GPCRs), which are known to form heterodimers with other membrane proteins, potentially modulating receptor function or characteristics. Like MC4R, thyroid hormones (TH) are also essential for energy homeostasis control. TH transport across membranes is facilitated by the monocarboxylate transporter 8 (MCT8), which is also known to form heterodimers with GPCRs. Based on the finding in single-cell RNA-sequencing data that both proteins are simultaneously expressed in hypothalamic neurons, we investigated a putative interplay between MC4R and MCT8. We developed a novel staining protocol utilizing a fluorophore-labeled MC4R ligand and demonstrated a co-localization of MC4R and MCT8 in human brain tissue. Using in vitro assays such as BRET, IP1, and cAMP determination, we found that MCT8 modulates MC4R-mediated phospholipase C activation but not cAMP formation via a direct interaction, an effect that does not require a functional MCT8 as it was not altered by a specific MCT8 inhibitor. This suggests an extended functional spectrum of MCT8 as a GPCR signaling modulator and argues for the investigation of further GPCR-protein interactions with hitherto underrepresented physiological functions.
黑素皮质素 4 受体（MC4R）是调节饱腹感和饥饿感的下丘脑瘦素-黑素皮质素途径中的关键分子。MC4R 属于 G 蛋白偶联受体（GPCR），已知其与其他膜蛋白形成异二聚体，可能调节受体功能或特性。与 MC4R 类似，甲状腺激素（TH）对于能量平衡的控制也是必不可少的。单羧酸转运蛋白 8（MCT8）促进 TH 跨膜转运，该蛋白也已知与 GPCR 形成异二聚体。基于单细胞 RNA 测序数据的发现，这两种蛋白质同时在下丘脑神经元中表达，我们研究了 MC4R 和 MCT8 之间可能存在的相互作用。我们开发了一种利用荧光标记的 MC4R 配体的新染色方案，并在人脑组织中证明了 MC4R 和 MCT8 的共定位。通过 BRET、IP1 和 cAMP 测定等体外测定，我们发现 MCT8 通过直接相互作用调节 MC4R 介导的磷酯酶 C 激活，但不调节 cAMP 形成，这种作用不需要功能性 MCT8，因为特定的 MCT8 抑制剂没有改变它。这表明 MCT8 作为 GPCR 信号调节剂具有扩展的功能谱，并主张进一步研究与迄今代表性不足的生理功能相关的其他 GPCR-蛋白相互作用。
