Terminology summary
Non-quantitative bias: Ligand with efficacy in only one
of compared pathways (neutral antagonist or inverse agonist in others).
As for all biased ligands, this refers to a particular system and in
another more sensitive system, there may be efficacy at several compared
pathways allowing the determination of concentration-response curves. We
discourage the use of the previous terms ‘full bias’ or ‘perfect bias’,
as they can misleadingly give the impression that the ligand has full
efficacy in the preferred pathway. In fact, a ligand with
non-quantitative bias may give a weaker activation than other agonists
for which bias can be quantified (as they induce agonism in at least two
pathways).
Conclusion and outlook
The scope of adoption of the recommendations presented herein will
ultimately depend on their utility across the wide GPCR field, which
will benefit from a common, standardized and therefore comparable
characterization of ligand bias. This precise characterization will also
help individual researchers, as the more comparable and reproducible
research data are, the more useful they will be for others, and the
fewer surprises encountered because of irreproducible experiments will
occur. Furthermore, the unambiguous and structured reporting of bias
data will be directly reflected in more relevant databases and
literature reviews.
Ligand-dependent biased function is also being investigated for other
protein classes e.g., receptor tyrosine kinases (Karl, Paul, Pasquale &
Hristova, 2020), nuclear receptors (Heidari et al., 2019), monoamine
transporters (Hasenhuetl, Bhat, Freissmuth & Sandtner, 2019), Toll-like
receptors (unpublished), and cytochrome P450 enzymes (Jensen et al.,
2021). Many of the recommendations brought forward in this paper are
also applicable to these protein families, which would in time benefit
from publication of additional dedicated guidelines.
To unlock the rational targeting of specific desired physiological
biases mediated by GPCRs, we must first map individual or combined
pathways to their distinct effects. This requires identification of many
more biased probes, pharmacological assays, animal models etc. This
represents a tremendous challenge, but one with extraordinary potential.
The recommendations herein can contribute to this massive endeavor by
better description of such probes and effects. We have deliberately left
the door open to include additional intracellular transducers, effectors
or modulators involved in GPCR signaling and our definitions should
therefore be future proof in the sense that they can be applied also to
signaling proteins that have yet to be discovered.
Acknowledgements
We acknowledge Kasper Harpsøe, Mette M. Rosenkilde and Nevin Lambert for
comments on this manuscript. D.E.G. received financial support from the
Lundbeck Foundation (R313-2019-526) and Novo Nordisk Foundation
(NNF18OC0031226). P.K. thanks the German Research Foundation DFG for
Heisenberg professorship KO4095/5-1. M.B. (Marcel Bermudez) thanks the
German Research Foundation DFG for funding DFG-407626949. M.E.S., D.G.,
P.K., XX, YY, and ZZ are members of COST Action CA18133 “ERNEST”. E.K.
was supported by the DFG-funded research unit FOR2372 with the grants
KO1582/10-1 and -2.
Author contributions
Conceptualization, D.E.G., P.K. and T.K.; Writing – Original Draft,
D.E.G., P.K. and T.K.; Writing – Review & Editing, all authors,
M.E.S.; Visualization, C.B., D.E.G.; Project Administration, D.E.G.;
Funding Acquisition, D.E.G., E.K., P.K., M.B.
Conflicts of interest
M.B. is the president of Domain Therapeutics scientific advisory board.