BACKGROUND The EP4 prostanoid receptor is one of four GPCRs that mediate the diverse actions of prostaglandin E2 (PGE2). Novel selective EP4 receptor agonists would assist to further elucidate receptor sub-type function and promote development of therapeutics for bone healing, heart failure, and other receptor associated conditions. The rat EP4 (rEP4) receptor has been used as a surrogate for the human EP4 (hEP4) receptor in multiple SAR studies. To better understand the validity of this traditional approach, homology models were generated by threading for both receptors using the RaptorX server. These models were fit to an implicit membrane using the PPM server and OPM database with refinement of intra and extracellular loops by Prime (Schrödinger). To understand the interaction between the receptors and known agonists, induced-fit docking experiments were performed using Glide and Prime (Schrödinger), with both endogenous agonists and receptor sub-type selective, small-molecule agonists. The docking scores and observed interactions were compared with radioligand displacement experiments and receptor (rat & human) activation assays monitoring cAMP. RESULTS Rank-ordering of in silico compound docking scores aligned well with in vitro activity assay EC50 and radioligand binding Ki. We observed variations between rat and human EP4 binding pockets that have implications in future small-molecule receptor-modulator design and SAR, specifically a S103G mutation within the rEP4 receptor. Additionally, these models helped identify key interactions between the EP4 receptor and ligands including PGE2 and several known sub-type selective agonists while serving as a marked improvement over the previously reported models. CONCLUSIONS This work has generated a set of novel homology models of the rEP4 and hEP4 receptors. The homology models provide an improvement upon the previously reported model, largely due to improved solvation. The hEP4 docking scores correlates best with the cAMP activation data, where both data sets rank order Rivenprost>CAY10684 > PGE1 ≈ PGE2 > 11-deoxy-PGE1 ≈ 11-dexoy-PGE2 > 8-aza-11-deoxy-PGE1. This rank-ordering matches closely with the rEP4 receptor as well. Species-specific differences were noted for the weak agonists Sulprostone and Misoprostol, which appear to dock more readily within human receptor versus rat receptor.

中文翻译：

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改进的人类和大鼠EP4前列腺素受体的同源性建模。

背景技术EP4前列腺素受体是介导前列腺素E2（PGE2）多种作用的四种GPCR之一。新型选择性EP4受体激动剂将有助于进一步阐明受体亚型功能并促进骨愈合，心力衰竭和其他受体相关疾病的治疗方法的开发。在多个SAR研究中，大鼠EP4（rEP4）受体已被用作人类EP4（hEP4）受体的替代物。为了更好地了解这种传统方法的有效性，使用RaptorX服务器通过对两个受体进行穿线生成了同源性模型。这些模型使用PPM服务器和OPM数据库拟合到隐式膜上，Prime（Schrödinger）完善了细胞内和细胞外环。要了解受体与已知激动剂之间的相互作用，使用Glide和Prime（Schrödinger）进行内含激动剂和受体亚型选择性小分子激动剂的诱导拟合对接实验。将对接得分和观察到的相互作用与放射性配体置换实验和监测cAMP的受体（大鼠和人类）活化试验进行了比较。结果计算机模拟化合物对接分数的等级排序与体外活性测定EC50和放射性配体结合Ki吻合良好。我们观察到大鼠和人的EP4结合口袋之间的变异对未来的小分子受体调节剂设计和SAR有影响，特别是rEP4受体内的S103G突变。此外，这些模型有助于确定EP4受体与包括PGE2在内的配体和几种已知的亚型选择性激动剂之间的关键相互作用，同时与以前报道的模型相比有显着改进。结论这项工作已经产生了一组新的rEP4和hEP4受体同源模型。同源性模型提供了对先前报道的模型的改进，这在很大程度上归因于改进的溶剂化。hEP4对接分数与cAMP激活数据最相关，其中两个数据集的排列顺序依次为Rivenprost> CAY10684> PGE1≈PGE2> 11-脱氧-PGE1≈11-dexoy-PGE2> 8-aza-11-deoxy-PGE1。该等级顺序也与rEP4受体紧密匹配。注意到弱激动剂舒普司通和米索前列醇的物种特异性差异，