FGF21
March 2022
Fibroblast growth factor 21
Fibroblast growth factor 21 (FGF21) is of increasing interest for the treatment of metabolic disorders including obesity, diabetes type 2, and non-alcoholic steatohepatitis (NASH).
It regulates glucose homeostasis, energy metabolism, insulin sensitivity and other metabolic pathways via FGF receptor (FGFR) signaling.
However, unlike other FGFs that bind the FGFR in the presence of heparin sulfate (HS) proteoglycan, the FGF21 and its subfamily members (FGF19 and FGF23) have a low affinity to HS and instead require β-Klotho (KLB) glycoprotein to achieve its function. Such a mechanism allows diffusion of FGF21 in the blood and explains its role as endocrine hormone.
Although preclinical studies have shown many benefits of FGF21 to treat metabolic complications, there are no available FGF21-based biologics so far. Their design and development have been challenging, mainly because human FGF21 shows poor pharmacokinetics, instability, and increased aggregation in solution. (Experts from Genentech summarized some of the efforts and hurdles in the development of the FGF21 analogs in their publication).
To facilitate the rational design of FGF21 therapeutics, scientists from academia determined the first structures of FGF21 using NMR (PDB IDs: 6M6E, 6M6F). The resolved structure guided the design of better FGF21 therapeutic variants that showed increased anti-diabetic activity in vivo compared to the native protein (ref here).
The especially significant finding from the NMR structure that aided mentioned design efforts is the very atypical conformation of FGF21 in the region where FGFs usually bind to HS (heparin binding site), compared to its subfamily members and other FGFs. This region is normally composed of 3 beta hairpins (β2‐β3, β6‐β7 and β10‐β11) oriented in a triangular way that makes the protein stable.
However, the β10‐β11 hairpin in FGF21 is very flexible and unstable, whichexplains the very low HS and FGFR binding affinity. At the same time, this highly dynamic conformation affects the neighboring β2‐β3 region, which causes the destabilization of the entire protein and consequently lowers its therapeutic activity.
Image: The superposition of the heparin binding domains in FGF21 (in white) and FGF19 (in orange). The β10‐β11 hairpin (left) is very unstable and affects the β2‐β3 hairpin (right). Superposition is done with the 3decision software.
With these structural features in mind, scientists developed a strategy to design a new variant alternating key residue in the loop (Gly43 was mutated to cysteine) and replacing the sequences of the unstable region with the ones from FGF19.
The produced variant named FGF21‐FGF19 chimera showed increased stability and anti-diabetic activity, raising hopes for successful clinical biologics to come.