Mechanisms of Gβγ Release upon GPCR Activation

Growth factor-dependent phosphorylation of Gαi shapes canonical signaling by G protein-coupled receptors

A long-standing question in the field of signal transduction is how distinct signaling pathways interact with each other to control cell behavior. Growth factor receptors and G protein-coupled receptors (GPCRs) are the two major signaling hubs in eukaryotes. Given that the mechanisms by which they signal independently have been extensively characterized, we investigated how they may cross-talk with each other. Using linear ion trap mass spectrometry and cell-based biophysical, biochemical, and phenotypic assays, we found at least three distinct ways in which epidermal growth factor affected canonical G protein signaling by the Gi-coupled GPCR CXCR4 through the phosphorylation of Gαi. Phosphomimicking mutations in two residues in the αE helix of Gαi (tyrosine-154/tyrosine-155) suppressed agonist-induced Gαi activation while promoting constitutive Gβγ signaling. Phosphomimicking mutations in the P loop (serine-44, serine-47, and threonine-48) suppressed Gi activation entirely, thus completely segregating growth factor and GPCR pathways. As expected, most of the phosphorylation events appeared to affect intrinsic properties of Gαi proteins, including conformational stability, nucleotide binding, and the ability to associate with and to release Gβγ. However, one phosphomimicking mutation, targeting the carboxyl-terminal residue tyrosine-320, promoted mislocalization of Gαi from the plasma membrane, a previously uncharacterized mechanism of suppressing GPCR signaling through G protein subcellular compartmentalization. Together, these findings elucidate not only how growth factor and chemokine signals cross-talk through the phosphorylation-dependent modulation of Gαi but also how such cross-talk may generate signal diversity.

[G proteins: privileged transducers of 7-transmembrane spanning receptors]

G protein-coupled receptors or GPCR are the most abundant membrane receptors in our genome with around 800 members. They play an essential role in most physiological and pathophysiological phenomena. In addition, they constitute 30% of the targets of currently marketed drugs and remain an important reservoir for new innovative therapies. Their main effectors are heterotrimeric G proteins. These are composed of 3 subunits, α, β and γ, which, upon coupling with a GPCR, dissociate into G_α and G_βγ to activate numerous signaling pathways. This article describes some of the recent advances in understanding the function and role of heterotrimeric G proteins. After a short introduction to GPCRs, the history of the discovery of G proteins is briefly described. Then, the fundamental mechanisms of activation, signaling and regulation of G proteins are reviewed. New paradigms concerning intracellular signaling, specific recognition of G proteins by GPCRs as well as biased signaling are also discussed.

Modulation of RNA Splicing by Oligonucleotides: Mechanisms of Action and Therapeutic Implications

Dysregulation of RNA splicing causes many diseases and disorders. Several therapeutic approaches have been developed to correct aberrant alternative splicing events for the treatment of cancers and hereditary diseases, including gene therapy and redirecting splicing, using small molecules or splice switching oligonucleotides (SSO). Significant advances in the chemistry and pharmacology of nucleic acid have led to the development of clinically approved SSO drugs for the treatment of spinal muscular dystrophy and Duchenne muscular dystrophy (DMD). In this review, we discuss the mechanisms of SSO action with emphasis on "less common" approaches to modulate alternative splicing, including bipartite and bifunctional SSO, oligonucleotide decoys for splice factors and SSO-mediated mRNA degradation via AS-NMD and NGD pathways. We briefly discuss the current progress and future perspectives of SSO therapy for rare and ultrarare diseases.