Department of Medicine (D.P.S., L.M.W., R.T.S., S.A., R.J.L.), Department of Biochemistry (R.J.L.), and Howard Hughes Medical Institute (R.J.L.), Duke University Medical Center, Durham, North Carolina; Department of Neuroscience and Pharmacology, The Panum Institute, University of Copenhagen, Copenhagen, Denmark (S.G.F.R.); Structural Biology Brussels and Structural Biology Research Institute, Vrije Universiteit Brussel, Brussels, Belgium (E.P., J.S.); and Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, California (B.K.K.).
Mol Pharmacol. 2014 Mar;85(3):472-81. doi: 10.1124/mol.113.089516. Epub 2013 Dec 6.
The biologic activity induced by ligand binding to orthosteric or allosteric sites on a G protein-coupled receptor (GPCR) is mediated by stabilization of specific receptor conformations. In the case of the β2 adrenergic receptor, these ligands are generally small-molecule agonists or antagonists. However, a monomeric single-domain antibody (nanobody) from the Camelid family was recently found to allosterically bind and stabilize an active conformation of the β2-adrenergic receptor (β2AR). Here, we set out to study the functional interaction of 18 related nanobodies with the β2AR to investigate their roles as novel tools for studying GPCR biology. Our studies revealed several sequence-related nanobody families with preferences for active (agonist-occupied) or inactive (antagonist-occupied) receptors. Flow cytometry analysis indicates that all nanobodies bind to epitopes displayed on the intracellular receptor surface; therefore, we transiently expressed them intracellularly as "intrabodies" to test their effects on β2AR-dependent signaling. Conformational specificity was preserved after intrabody conversion as demonstrated by the ability for the intracellularly expressed nanobodies to selectively bind agonist- or antagonist-occupied receptors. When expressed as intrabodies, they inhibited G protein activation (cyclic AMP accumulation), G protein-coupled receptor kinase (GRK)-mediated receptor phosphorylation, β-arrestin recruitment, and receptor internalization to varying extents. These functional effects were likely due to either steric blockade of downstream effector (Gs, β-arrestin, GRK) interactions or stabilization of specific receptor conformations which do not support effector coupling. Together, these findings strongly implicate nanobody-derived intrabodies as novel tools to study GPCR biology.
配体与 G 蛋白偶联受体 (GPCR) 的变构或正位结合所诱导的生物活性是通过稳定特定受体构象来介导的。在β2 肾上腺素能受体的情况下，这些配体通常是小分子激动剂或拮抗剂。然而，最近发现来自骆驼科的单域抗体（纳米抗体）可以变构结合并稳定β2 肾上腺素能受体 (β2AR) 的活性构象。在这里，我们着手研究 18 种相关纳米抗体与β2AR 的功能相互作用，以研究它们作为研究 GPCR 生物学的新型工具的作用。我们的研究揭示了几个与序列相关的纳米抗体家族，它们对活性（激动剂占据）或非活性（拮抗剂占据）受体具有偏好性。流式细胞术分析表明，所有纳米抗体都结合在细胞内受体表面显示的表位上；因此，我们将它们作为“内抗体”瞬时表达在细胞内，以测试它们对β2AR 依赖性信号的影响。构象特异性在体内抗体转化后得以保留，这表现为细胞内表达的纳米抗体能够选择性结合激动剂或拮抗剂占据的受体。当作为内抗体表达时，它们在不同程度上抑制 G 蛋白激活（环腺苷酸积累）、G 蛋白偶联受体激酶 (GRK) 介导的受体磷酸化、β-arrestin 募集和受体内化。这些功能效应可能是由于下游效应物（Gs、β-arrestin、GRK）相互作用的空间位阻或不支持效应物偶联的特定受体构象的稳定。总之，这些发现强烈表明纳米抗体衍生的内抗体是研究 GPCR 生物学的新型工具。
