Context-dependent requirement of G protein coupling for Latrophilin-2 in target selection of hippocampal axons

The formation of neural circuits requires extensive interactions of cell-surface proteins to guide axons to their correct target neurons. Trans-cellular interactions of the adhesion G protein-coupled receptor latrophilin-2 (Lphn2) with its partner teneurin-3 instruct the precise assembly of hippocampal networks by reciprocal repulsion. Lphn2 acts as a repulsive receptor in distal CA1 neurons to direct their axons to the proximal subiculum, and as a repulsive ligand in the proximal subiculum to direct proximal CA1 axons to the distal subiculum. It remains unclear if Lphn2-mediated intracellular signaling is required for its role in either context. Here, we show that Lphn2 couples to Gα12/13 in heterologous cells; this coupling is increased by constitutive exposure of the tethered agonist. Specific mutations of Lphn2's tethered agonist region disrupt its G protein coupling and autoproteolytic cleavage, whereas mutating the autoproteolytic cleavage site alone prevents cleavage but preserves a functional tethered agonist. Using an in vivo misexpression assay, we demonstrate that wild-type Lphn2 misdirects proximal CA1 axons to the proximal subiculum and that Lphn2 tethered agonist activity is required for its role as a repulsive receptor in axons. By contrast, neither tethered agonist activity nor autoproteolysis were necessary for Lphn2's role as a repulsive ligand in the subiculum target neurons. Thus, tethered agonist activity is required for Lphn2-mediated neural circuit assembly in a context-dependent manner.

Disentangling autoproteolytic cleavage from tethered agonist-dependent activation of the adhesion receptor ADGRL3

Adhesion G protein-coupled receptor latrophilin 3 (ADGRL3), a cell adhesion molecule highly expressed in the central nervous system, acts in synapse formation through trans interactions with its ligands. It is largely unknown if these interactions serve a purely adhesive function or can modulate G protein signaling. To assess how different structural elements of ADGRL3 (e.g., the adhesive domains, autoproteolytic cleavage site, or tethered agonist (TA)) impact receptor function, we require constructs that disrupt specific receptor features without impacting others. While we showed previously that mutating conserved Phe and Met residues in the TA of ADGRL3-C-terminal fragment (CTF), a CTF truncated to the G protein-coupled receptor proteolysis site, abolishes receptor-mediated G protein activation, we now find that autoproteolytic cleavage is disrupted in the full-length version of this construct. To identify a construct that disrupts TA-dependent activity without impacting proteolysis, we explored other mutations in the TA. We found that mutating the sixth and seventh residues of the TA, Leu and Met, to Ala impaired activity in a serum response element activity assay for both full-length and CTF constructs. We confirmed this activity loss results from impaired G protein coupling using an assay that acutely exposes the TA through controlled proteolysis. The ADGRL3 mutant expresses normally at the cell surface, and immunoblotting shows that it undergoes normal autoproteolysis. Thus, we found a construct that disrupts tethered agonism while retaining autoproteolytic cleavage, providing a tool to disentangle these functions in vivo. Our approach and specific findings are likely to be broadly applicable to other adhesion receptors.

Short Arrestin-3-Derived Peptides Activate JNK3 in Cells

Arrestins were first discovered as suppressors of G protein-mediated signaling by G protein-coupled receptors. It was later demonstrated that arrestins also initiate several signaling branches, including mitogen-activated protein kinase cascades. Arrestin-3-dependent activation of the JNK family can be recapitulated with peptide fragments, which are monofunctional elements distilled from this multi-functional arrestin protein. Here, we use maltose-binding protein fusions of arrestin-3-derived peptides to identify arrestin elements that bind kinases of the ASK1-MKK4/7-JNK3 cascade and the shortest peptide facilitating JNK signaling. We identified a 16-residue arrestin-3-derived peptide expressed as a Venus fusion that leads to activation of JNK3α2 in cells. The strength of the binding to the kinases does not correlate with peptide activity. The ASK1-MKK4/7-JNK3 cascade has been implicated in neuronal apoptosis. While inhibitors of MAP kinases exist, short peptides are the first small molecule tools that can activate MAP kinases.

The two non-visual arrestins engage ERK2 differently

Arrestin binding to active phosphorylated G protein-coupled receptors terminates G protein coupling and initiates another wave of signaling. Among the effectors that bind directly to receptor-associated arrestins are extracellular signal-regulated kinases 1/2 (ERK1/2), which promote cellular proliferation and survival. Arrestins may also engage ERK1/2 in isolation in a pre- or post-signaling complex that is likely in equilibrium with the full signal initiation complex. Molecular details of these binary complexes remain unknown. Here, we investigate the molecular mechanisms whereby arrestin-2 and arrestin-3 (a.k.a. β-arrestin1 and β-arrestin2, respectively) engage ERK1/2 in pairwise interactions. We find that purified arrestin-3 binds ERK2 more avidly than arrestin-2. A combination of biophysical techniques and peptide array analysis demonstrates that the molecular basis in this difference of binding strength is that the two non-visual arrestins bind ERK2 via different parts of the molecule. We propose a structural model of the ERK2-arrestin-3 complex in solution using size-exclusion chromatography coupled to small angle X-ray scattering (SEC-SAXS). This binary complex exhibits conformational heterogeneity. We speculate that this drives the equilibrium either toward the full signaling complex with receptor-bound arrestin at the membrane or toward full dissociation in the cytoplasm. As ERK1/2 regulates cell migration, proliferation, and survival, understanding complexes that relate to its activation could be exploited to control cell fate.

Viewpoints on the First Transatlantic GPCR Symposium for Early-Career Investigators

In July 2021, we organized a virtual symposium aimed at early-career investigators (ECIs) in G protein-coupled receptor (GPCR) research: the first Transatlantic ECI GPCR Symposium. Here, we discuss the proceedings of this symposium and the unique networking events with GPCR leaders including the Nobel Laureates Dr. Robert Lefkowitz and Dr. Brian Kobilka.