Progress toward new function and design of extracellular G protein-coupled receptor nanobodies.

Antibodies have played a pivotal role in G protein-coupled receptor (GPCR) research and drug development. Nanobodies, or variable domain heavy chain-only antibodies, have emerged as a next-generation antibody with unique advantages in targeting GPCRs. The first generation of intracellular nanobodies have been instrumental in stabilizing GPCR structures for crystallography and in enabling in vitro GPCR imaging. More recently, extracellular-targeted nanobodies have demonstrated diverse pharmacological profiles, with the ability to modulate GPCR activity, localization, and downstream signaling. With these newly uncovered functional properties, nanobodies can be viewed not only as structural tools but also as modulators of receptor pharmacology. We highlight recent innovations in extracellular GPCR-targeting nanobodies and assess several approaches to accelerate their development as versatile research tools and therapeutics. SIGNIFICANCE STATEMENT: Nanobodies have emerged as a next-generation antibody platform with distinct advantages for targeting G protein-coupled receptors. This review highlights recent advances in extracellular G protein-coupled receptor-targeting nanobodies and explores innovative strategies to accelerate their development as powerful research tools and therapeutic agents.