Arrestin Pathways as Drug Targets - Progress in Molecular Biology and Translational Science - page 461

activation. 20 As shown, the reference agonist hPTH(1-34), which repro-
duces the response profile of the native 84 amino acid hormone, 26 behaves
as a full agonist in all three assays. In this system, the hPTH(1-31) frag-
ment 27,28 appears as a classical partial agonist, that is, its RA i is less than that
of the full agonist, but similar in all assays. The N-terminal truncated frag-
ment hPTH(7-34) 29 lacks any demonstrable efficacy and thus appears as a
classical “neutral” antagonist. The other three ligands exhibit signal bias.
Trp 1 -PTHrp(1-36) 24 appears selective for G s -cAMP signaling; bPTH
(3-34) is selective for arrestin-dependent ERK1/2 activation; and
(D-Trp 12 ,Tyr 34 )-bPTH(7-34) 24,30 exhibits frank reversal of efficacy,
appearing as an inverse agonist for cAMP production, neutral for calcium
signaling, and a partial agonist for ERK1/2 activation. 24,31

3. THE MANY FACES OF ARRESTIN-DEPENDENT
BIASED AGONISM

The cellular response to an extracellular GPCR ligand originates at a
single point of contact between the ligand and its receptor on the plasma
membrane. Agonist binding changes the distribution of receptors occupying
each of the component microstates that comprise the overall conformational
ensemble, shifting the proportion of receptors existing in one or more
“active” states. 9,10 The nature of the information that is transmitted across
the membrane is specified not only by the receptor, but also by the structure
of the ligand and the cellular environment in which the receptor is
expressed. Inside the cell, the flow of information proceeds in ever widening
circles, like ripples on the surface of a pond that emanate outward from a
dropped pebble. Ligand effects can be assayed at many steps along the
way, with each measurement providing some, more, or less, useful informa-
tion about the ultimate biological phenotype.

3.1. characterization of arrestin-selective ligand bias

The most immediate measurable consequence of ligand binding is a change
in receptor conformation, and intramolecular fluorescence probes can be
used to monitor how orthosteric ligand binding affects receptor conforma-
tion at different points within the molecule. 32,33 A recent study by Liu et al.
using site-specific 1D 19 F NMR spectroscopy offers insight into how con-
ventional and biased ligands affect the conformation of a population of
receptors. 34 They found that the

In vitro

b 2 adrenergic receptor undergoes major