Arrestin Pathways as Drug Targets - Progress in Molecular Biology and Translational Science - page 462

conformational shifts at C265 (helix VI) and C327 (helix VII) upon binding
full agonists such as isoproterenol and formoterol, whereas the neutral antag-
onist albuterol and the inverse agonist carazolol either fail to perturb the
conformational equilibrium or shift helix VI independently of helix VII.
The partial agonists norepinephrine, tolubuterol, and clenbuterol, that lack
or have weaker polar features on their aromatic heads, produce smaller
effects on helix VI but shift helix VII as effectively as full agonists. Compar-
ing this with the arrestin-selective 2 adrenergic receptor agonist carvedilol

and the arrestin-enhanced agonist isoetharine, they find that G protein acti-
vation correlates with movement of helix VI, whereas

b

-arrestin-biased
ligands predominantly impact the conformational state of helix VII. Impor-
tantly, helix VI and helix VII were found to move independently, providing
a physical basis for biased agonism and a conformational “signature” predic-
tive of arrestin selectivity.
The initial transfer of information within the cell occurs when the
ligand-bound GPCR physically touches its proximal effectors, for example,
G proteins and arrestins. The development of bioluminescence resonance
energy transfer (BRET)-based technologies to measure protein-protein
interactions has enabled these events to be monitored in real time in live
cells. 35,36 The most comprehensive study of biased agonism reported to date
using these approaches is by Sauliere et al. , who compared the native angio-
tensin AT 1A receptor agonist, angiotensin II (AngII) to the arrestin-biased
agonist, [Sar 1 ,Ile 4 ,Ile 8 ]-AngII (SII), using a panel of BRET-based sensors
encompassing all heterotrimeric G protein families and arrestins. 37 With
respect to heterotrimeric G protein activation, they found that SII behaves
as a weak partial agonist, activating all G protein species affected by AngII,
albeit with much lower efficacy than the native ligand. SII did not qualita-
tively change the G protein coupling of the receptor; the SII response was
proportional to AngII across the board. Reflective of its arrestin signaling
bias, SII recruited arrestin3 to the AT 1A receptor with nearly comparable
efficacy as the native ligand. These data support the model that biased ligands
select receptor conformations that couple the receptor to downstream effec-
tors with different efficiency that the native ligand, but do not drive the
receptor to engage otherwise unrecognized effectors.
The next level of signal propagation occurs as G proteins, arrestins, and
potentially other non-G protein effectors, activate second messenger path-
ways, and protein phosphorylation cascades. This is the level at which much
high-throughput pharmaceutical screening occurs. To date, most studies of
short term downstream signaling (e.g., Refs. 24,38,39 ) have failed to find

b