Based on this evidence future research should be directed to
Based on this evidence, future research should be directed towards the identification of actual receptor oligomeric states at the analyzed cellular surfaces. The macromolecular organization of these oligomeric receptors might establish informational hubs, which relay ligand/receptor interactions in different maybe even cell line specific ways. In this light, the classification of ligands as agonists or antagonists and the invariable association of receptor agonists with common, group defining characteristics according to elicited signals is difficult and has to be reconsidered (see 2.3). The FPR interactome is likely very dynamic and differs according to cell type and activation. In agreement, it is well known that FPR1 expression levels increase in (various) pro-inflammatory settings, for example after LPS (lipopolysaccharide) stimulation . This will not only change receptor surface densities but in turn, might also shift the equilibrium towards distinct oligomeric receptor aggregates. In summary, oligomeric receptor gallic acid australia might severely impact on the ligand-mediated signaling and thus, render the formation of these high order structures and their signaling potential in the shaping of the integrated systemic response upon FPR activation (Fig. 3) a central future research question. Apart from receptor oligomerization, the concept of biased agonism and its molecular base require further analysis. As mentioned, chemical interactions between ligands and a given GPCR alter the conformational equilibrium of the receptor (see 2 Biased agonism, 3 FPR ligands, 4 , 5 Summary and future perspectives, Transparency document, , 2.3 Biased agonism and drug development) [, , ]. Receptor agonists, acting on GPCRs reduce the actual activation enthalpy, which is required to yield active receptor conformer state; in essence the occurrence of certain, active conformers is more probable for ligand-bound receptors. These conformational changes are transduced in the cellular interior and alter the efficiency of transducer recruitment to the C-terminal domain of the receptor . Ultimately these structural rearrangements are the molecular basis for ligand-selected receptor efficacies or biased agonism. However, it turned out that not only the regulation of transducer recruitment but even its altered activities might be (in part) controlled via functional selectivity. For instance, in case of GPCR/arrestin signaling ligand-induced conformational switches regulate the interaction of arrestins with its corresponding binding partners. These alterations might then impact on cellular signal transduction. Similarly, even the interaction of G-proteins with GTP is regulated by ligand-dependent conformational change of the receptor molecule [14,, , ]. In summary, receptor/agonist interactions not only alter the selection/recruitment of intracellular transducers but also might alter the activity of selected transducer and in turn their downstream signaling. These findings add a further layer of complexity to biased agonism (for review  and references therein). There is no doubt that the FPR family of PRRs is of great importance in many clinically relevant scenarios, but their therapeutic potential has yet not been fully exploited. The diversity and multitude of agonists (in particular in case of FPR2), which are associated with specific effects is very promising and underlines the potential of FPRs as drug targets (see 3 FPR ligands, 4 , 5 Summary and future perspectives, Transparency document, , 3.5.2 Endogenous lipids with pro-resolving functions) [43,80,81,154]. What, however, remains to be established is the systematic comparative assignment of individual ligands and their effective actions on a given FPR in a specified cellular context; similar to the familiar concept of “benchmarking” in order to increase quality and comparability of FPR/GPCR-centered research. This requires the determination of agonist responses in relation to reference agonists for a given pathway and allows the classification of ligands in terms of their agonist potential. Comparison across different pathways would then identify biased agonists that would evoke only subsets of all (possible) cellular responses (see 2.3 Biased agonism and drug development, 2.4 Receptor oligomers – general considerations, 3.1 Synthetic W-peptides, 3.2 Pathogen-derived ligands, 3.3 Endogenous agonists, 3.4 FPR2 receptor agonists and neurodegeneration, 3.5 Anti-inflammatory signaling, 4.1 FPR KO models and bacterial/viral infection, 4.2 FPR KO models and tissue protection, 2.3.1 Biased agonism and drug development – theoretical models and operating figures) [62,, , ]. Ideally, such analysis is facilitated by high-throughput assays for selection of those biased agonists that cause a response in the desired direction. The resulting molecules could ultimately serve as lead structures for further modification/derivatization leading to compounds with optimized pharmacological characteristics and profiles. Most research on FPRs utilizes multiple cellular systems for the analysis of agonist-mediated effects. However, this praxis hampers the comparability of ligand activity across (different) experiments. For instance, EC50 values for the same pathway and ligand, e.g., the adenylate cyclase activity, highly depend on the presence and availability of G-proteins and, therefore, might deviate depending on the actual cellular context. Inclusion of a benchmarking agonist as a reference for system response is, therefore, a necessary prerequisite, yet often missing.