A Role for Liquid-Ordered Plasma Membrane Nanodomains Coordinating the Unconventional Secretory Pathway of Fibroblast Growth Factor 2?

Abstract

Fibroblast growth factor 2 (FGF2) is a tumor cell survival factor that belongs to a subgroup of extracellular proteins lacking N-terminal signal peptides. Whereas this phenomenon was already recognized in the early 1990s, detailed insights into the molecular mechanisms underlying alternative pathways of protein secretion from eukaryotic cells were obtained only recently. Today, we know about a number of alternative secretory mechanisms, collectively termed unconventional protein secretion (UPS). FGF2 belongs to a subgroup of cargo proteins secreted by direct translocation across the plasma membrane. This feature has been classified as type I UPS and is shared with other unconventionally secreted proteins, such as HIV-Tat and Tau. FGF2 translocation across the membrane is initiated through sequential interactions with the Na,K-ATPase, Tec kinase, and phosphoinositide PI(4,5)P2 at the inner plasma membrane leaflet. Whereas the first two are auxiliary factors of this pathway, the interaction of FGF2 with PI(4,5)P2 triggers the core mechanism of FGF2 membrane translocation. It is based on a lipidic membrane pore that is formed by PI(4,5)P2-induced oligomerization of FGF2. Membrane-inserted FGF2 oligomers are recognized as translocation intermediates that are resolved at the outer plasma membrane leaflet by glypican-1, a heparan sulfate proteoglycan that captures and disassembles FGF2 oligomers on cell surfaces. Here, we discuss recent findings suggesting the molecular machinery mediating FGF2 membrane translocation to be highly organized in liquid-ordered plasma membrane nanodomains, the core process underlying this unusual pathway of protein secretion.

Canonical fibroblast growth factors in viral infection

“…FGF2 is secreted through a unique ER-independent process that requires phosphatidylinositol 4,5-bisphosphate (PI(4,5)P 2 ) on the inner leaflet of the plasma membrane and heparan sulfates (HS) on the outer leaflet where it binds to HS. [12][13][14][15] FGF2 is more stable than FGF1. 16 -FGF4 is produced in two isoforms with opposing effects: isoform 2 inhibits isoform 1-induced Erk1/2 phosphorylation.…”

“…There are four isoforms of FGF2, with only the lightest is secreted, while the heavier weight isoforms act in cells where they are produced. FGF2 is secreted through a unique ER‐independent process that requires phosphatidylinositol 4,5‐bisphosphate (PI(4,5)P 2 ) on the inner leaflet of the plasma membrane and heparan sulfates (HS) on the outer leaflet where it binds to HS 12–15 . FGF2 is more stable than FGF1 16 …”

Disulfide bridge-dependent dimerization triggers FGF2 membrane translocation into the extracellular space

“…The Na,K-ATPase has been demonstrated to be the first contact of FGF2 at the plasma membrane mediated by a direct physical interaction between its α1 subunit and FGF2 ( Legrand et al, 2020 ). It is believed to serve as a landing platform of FGF2 at the inner plasma membrane leaflet, however, it has also been hypothesized to play an additional regulatory role in coupling FGF2 membrane translocation to the maintenance of the plasma membrane potential ( Lolicato and Nickel, 2022 ; Sparn et al, 2022b ). In addition, Tec kinase has been shown to make direct physical contact with FGF2, resulting in tyrosine phosphorylation of FGF2 at Y81, an interaction that is likely to occur downstream of the Na,K-ATPase ( La Venuta et al, 2015 ; La Venuta et al, 2016 ; Lolicato and Nickel, 2022 ; Sparn et al, 2022b ).…”

“…It is believed to serve as a landing platform of FGF2 at the inner plasma membrane leaflet, however, it has also been hypothesized to play an additional regulatory role in coupling FGF2 membrane translocation to the maintenance of the plasma membrane potential ( Lolicato and Nickel, 2022 ; Sparn et al, 2022b ). In addition, Tec kinase has been shown to make direct physical contact with FGF2, resulting in tyrosine phosphorylation of FGF2 at Y81, an interaction that is likely to occur downstream of the Na,K-ATPase ( La Venuta et al, 2015 ; La Venuta et al, 2016 ; Lolicato and Nickel, 2022 ; Sparn et al, 2022b ). This modification has been proposed to regulate the overall efficiency of FGF2 secretion, in particular in the context of cancer development ( Ebert et al, 2010 ; Steringer et al, 2012 ; La Venuta et al, 2015 ; La Venuta et al, 2016 ; Sparn et al, 2022b ).…”

“…Thus, FGF2 hexamers may accumulate up to 30 PI(4,5)P 2 lipids within a highly confined membrane surface area of about 10 nm 2 . At these sites, given the cone-shaped structure of PI(4,5)P 2 , local accumulation of PI(4,5)P 2 will likely compromise the bilayer architecture and decrease the energy barrier for the formation of toroidal membrane pores ( Lolicato and Nickel, 2022 ; Sparn et al, 2022b ). Indeed, upon PI(4,5)P 2 -dependent FGF2 oligomerization on membrane surfaces, transbilayer diffusion of membrane lipids has been observed, suggesting a toroidal architecture of the membrane pores that were formed under these conditions ( Steringer et al, 2012 ; Müller et al, 2015 ; Steringer et al, 2017 ; Dimou and Nickel, 2018 ; Pallotta and Nickel, 2020 ).…”