Terminology summary
G protein selectivity (NOT ‘natural bias’) : The repertoire of G
proteins that a receptor can engage. The term ‘natural bias’ should not
be used as it is an oxymoron i.e., is self-contradictory.
Physiology-biased ligand/signaling: Ligand/signaling that is
biased relative to a receptor’s principal endogenous agonist, and
therefore bears the meaning that signaling is
non-physiological/unnatural.
Unambiguous description of ligand bias – experimental
evidence
Recommendation 5: Our foremost recommendation is to describe
ligand bias unambiguously by including the major information in an
initial statement, e.g.: “ligand L displays
[recruitment/signaling…] bias towards pathway P1 at time
point TP1 over pathway P2 at time point TP2 and relative to reference
ligand A at receptor R in cell line C” .
Of course, reference ligand A must be measured under identical
conditions as ligand L at each pathway. Like any definition, this should
be mentioned at the first mention of the biased ligand – also in
articles that reference data from another, original article.
Additionally, we recommend that authors tabulate experimental details
critical to the unambiguous description of ligand bias (a template is
provided in Table 2). The suggested tabulated experimental details
extend the above single-sentence statement with information about
temperatures, signal detection techniques and the measured molecules.
The time points refer to data collection times (see section Kinetics and
choosing measurement time points).
Reason: All the above information is necessary to interpret
ligand bias correctly and to separate it from system bias (Figure 1).
Hence, the mention of a ‘biased’ or ’unbiased’ ligand as short terms are
only meaningful after the prior definition of this
information. The recommended statement and tables will facilitate
meaningful comparisons of ligands and avoid different definitions
leading to different conclusions from different researchers. The more
consistent and comprehensive information provided in this statement will
also greatly improve clarity and transparency while easing the
annotation of biased ligands for review articles or databases.
Disclaimer: It follows from our recommendation 5 that assays
that do not use the same reference agonist for bias or the same
experimental conditions do not “disprove” biased agonism.
Unambiguous description of ligand bias – quantification
models and raw
data
Strategies used to quantify ligand-bias are all based on, and derived
from, the principles of classical theory of receptor action (Furchott,
1966; Stephenson, 1956). The common objective of all these methods is to
separate ligand-dependent parameters from the system-dependent ones in
observed responses. However, there is still different views of how to
best quantify bias and strategies for quantification are still being
developed/improved. Hence, there are numerous models that could be used
as long as pathways are measured with consistent pharmacological
parameters and in similar systems and assays with corresponding levels
of downstream processes and kinetics (see below sections). Of note, even
with the most detailed models it is not possible to directly compare
quantitative estimates of bias across different studies.