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US & Belgium research team discover a novel “nanobody” (a “mini-antibody”) potentially applicable to retinitis pigmentosa (RP) treatment.

Researchers at the Department of Ophthalmology, University of California, Irvine and the Structural Biology Research Centre, Vrije Universiteit Brussels, Belgium, have reported the use of llama-derived nanobodies (Nbs) capable to bind to the intracellular surface of rhodopsin, allosterically modulating the G protein-coupled receptor (GPCR).  The researchers believe that their data show “the power of nanobodies to modulate the photoactivation of rhodopsin and potentially serve as therapeutic agents for disease-associated rhodopsin misfolding”.  Nanobodies, also known as a “single-domain antibody”, are capable to bind selectively to a specific antigen, but are considerably smaller (>15kDa), compared to other antibodies, such as aflibercept (115kDa) or bevacizumab (149kDa).  The low molecular weight of nanobodies should improve permeability, higher solubility, stability and lower production costs and potentially avoid triggering the complement system and thereby avoid cytotoxicity.

The discovery of nanobodies date to 1989, developed at the then Free University in Brussels (now Vrije Universiteit Brussels), researching on antibody structures – the two “heavy” protein chains that form the “Y” shape, and two “light” protein chains that adjoin at the arms of the Y structure.  While humans (and mice) have antibodies with the heavy and light chains, certain species have a pared version without the light chains and this suggested a number of applications, not least of all, versatility, solubility and effectivity.  Animals such as dromedary camels, llamas, alpacas and sharks, have this facility, highlighted in the Annual Review of Animal Biosciences (2021; 9:401-21), stating: “the small size, strict monomeric state, robustness, and easy tailoring of these nanobodies have inspired many groups to design innovative nanobody-based multi-domain constructs to explore novel applications. As such, nanobodies have been employed as an exquisite research tool in structural, cell, and developmental biology”. These “mini-antibodies” are now actively in progress in the pipelines of both academic and commercial entities, especially following the FDA approval for the first nanobody “Cablivi”, treating a rare blood disorder.

In the current research from Irvine and Brussels, the study was aimed to understand the conformational landscape of photoactivated rhodopsin using a series of camelid nanobodies that can modulate the rhodopsin molecule through the binding of its extracellular loop II (EL2) and the N-terminus.  This provided biochemical, structural, spectroscopic and cellular studies identifying a region of the rhodopsin structure that plays a pivotal role in the process of switching between different conformational states.  The data indicated that nanobodies can allosterically modulate rhodopsin and stabilize the molecule and, when co-expressed with a misfolding-prone P23H-bOpsin mutant, the nanobody (Nb) may act as a chaperone to partially restore proteostasis. Concluding in the their report, the researchers stated that: “the Nb-stabilization strategy shown here is likely to facilitate a long-term goal in the field by structurally resolving the key intermediate states of Rho, from the inactive state to the fully ligand-activated state, as well as the apoprotein and other states such as meta III. In addition, this research opens the door to the development of biological allosteric modulators that bind to the extracellular domain of other GPCRs and stabilize different activation states”.