Poster Presentation The 47th Lorne Conference on Protein Structure and Function 2022

Structure and function of GPR88, an orphan GPCR target in psychiatric disorders (#214)

Yao Lu 1 2 , Rachel M Johnson 1 2 , Natalie A Diepenhorst 1 2 , Gregory D Stewart 2 , Yi-Lynn Liang 1 2 , Radostin Danev 3 , Matthew J Belousoff 1 2 , Denise L Wootten 1 2 , Patrick M Sexton 1 2 , Christopher J Langmead 1 2
  1. ARC Centre for Cryo-electron Microscopy of Membrane Proteins, Monash Institute of Pharmaceutical Sciences, Parkville, VIC, Australia
  2. Drug Discovery Biology, Monash Institute of Pharmaceutical Sciences, Parkville, VIC, Australia
  3. Graduate School of Medicine, University of Tokyo, Tokyo, S402, 7-3-1 Hongo, Bunkyo-ku, Japan

Many drugs on the market target G protein-coupled receptors (GPCRs). Orphan GPCRs, with no endogenous ligand identified, represent an untapped resource for therapeutic development. The orphan GPCR, GPR88, is a Gai/o coupled receptor that inhibits cAMP accumulation. It presents as a novel target for treating schizophrenia, and other psychological disorders due to its high expression in the striatum particularly in medium spiny neurons where it acts to dampen neuronal firing rates by modulating GABAergic neurons (Quintana, 2012).

Drug discovery efforts for GPR88 have identified a number of small molecule agonists, including (R,R)-2-PCCA (Jin, 2014). Whilst a potent GPR88 agonist, (R,R)-2-PCCA has a wide off target profile and poor lipophilicity and CNS penetration. Further pharmacological and structural insights are required to expand the chemical space around GPR88 agonists for drug discovery.

We sought to understand the activation requirements of GPR88 using structural biology. Utilising the receptor’s constitutive activity, we generated a ligand-free, G protein-bound structure by single particle cryo-EM, which we compared with the (R,R)-2-PCCA bound, G protein-coupled structure. We identified key activation features of the ligand-free receptor, including the presence of an aromatic Trp179 within extracellular loop 2, which partially occupies the binding pocket; this was replaced by the biphenyl group of (R,R)-2-PCCA in the ligand bound structure with extracellular loop 2 displaced from the pocket.

More generally, the (R,R)-2-PCCA binding site is largely hydrophobic and there is a cavity in the side of the receptor between TM4 and TM5, exposing the lower portion of the binding site. The structure permitted the design of a minimally modified, single heteroatom-substituted (R,R)-2-PCCA derivative, which maintains the aromatic interactions of the biphenyl group within the core of the receptor and retains similar GPR88 potency, but with significantly fewer off target effects, and improved lipophilicity and CNS penetration.

 

References:

Quintana, A., Sanz, E., Wang, W. et al. Lack of GPR88 enhances medium spiny neuron activity and alters motor- and cue-dependent behaviors. Nat Neurosci 15, 1547–1555 (2012).

Jin, C., Decker, A., Huang, X-P., Gilmour, B., Blough, B., Roth, B., Hu, Y., Gill, J., Zhang, X. ACS Chemical Neuroscience 2014 5 (7), 576-587