Unconventional secretion: an extracellular trap for export of fibroblast growth factor 2

Abstract

Andrei et al., 2004) and by plasma membrane shedding (MacKenzie et al., 2001). Here, I discuss a new model for the molecular mechanism of FGF-2 secretion based on recent data. Key aspects of this model are: (1) direct translocation of FGF-2 from the cytoplasm across the plasma membrane in the absence of transport vesicles; (2) the independence of membrane translocation from ATP hydrolysis or a membrane potential; (3) diffusion-controlled membrane translocation process; and (4) an extracellular molecular trap formed by membrane-proximal heparan sulfates that ensures directional transport of FGF-2 into the extracellular space. Passive versus active mechanisms of translocationCentral to the molecular mechanism of FGF-2 secretion is the question of what actually drives translocation of FGF-2 across the plasma membrane in terms of energy requirements. Studies employing an intact cell model system led to the proposal that the overall process of FGF-2 secretion in living cells depends on ATP hydrolysis (Florkiewicz et al., 1995). However, the early data did not really demonstrate that membrane translocation itself is driven by ATP hydrolysis. Depletion of ATP from intact cells affects many fundamental cellular functions and, therefore, the inefficient FGF-2 secretion observed could well be an indirect effect. An in vitro approach using plasma-membrane-derived inside-out vesicles shows that FGF-2 can translocate directly across the plasma membrane but that neither ATP hydrolysis nor a membrane potential is required (Schäfer et al., 2004). FGF-2 thus appears to traverse the plasma membrane by passive diffusion.At first glance, passive diffusion seems unusual compared with other membrane translocation processes, such as import into the mitochondrial matrix (driven by ATP hydrolysis) (Neupert and Herrmann, 2007) or the bacterical twin arginine secretion system (driven by a membrane potential) (Lee et al., Several secretory proteins are released from cells by mechanisms that are distinct from the classical endoplasmic reticulum (ER)/Golgi-mediated secretory pathway. Recent studies unexpectedly revealed that the interaction between one such protein, fibroblast growth factor 2 (FGF-2), and cell surface heparan sulfate proteoglycans (HSPGs) is essential for secretion. FGF-2 mutants that cannot bind to heparan sulfates are not secreted, and cells that do not express functional HSPGs cannot secrete wild-type FGF-2. FGF-2 appears to be secreted by direct translocation across the plasma membrane in an ATP-and membrane-potentialindependent manner. I propose that its translocation across the membrane is a diffusion-controlled process in which cell surface HSPGs function as an extracellular molecular trap that drives directional transport of FGF-2.

Phosphatidylinositol 4,5-Bisphosphate (PI(4,5)P2)-dependent Oligomerization of Fibroblast Growth Factor 2 (FGF2) Triggers the Formation of a Lipidic Membrane Pore Implicated in Unconventional Secretion

“…7). At this stage, heparan sulfate proteoglycan-dependent translocation of FGF2 into the extracellular space can occur (23,24) resulting in the disassembly of the membrane pore. Because FGF2 has about a hundredfold higher affinity to heparan sulfate proteoglycans compared with PI(4,5)P 2 (22) and binding of FGF2 to either PI(4,5)P 2 or heparan sulfates is mutually exclusive, we propose that heparan sulfate proteoglycans liberate FGF2 from the membrane-associated state resulting in its exposure on cell surfaces.…”

“…At the outer leaflet, membrane-proximal heparan sulfate proteoglycans are involved in a late step of FGF2 secretion resulting in its exposure on cell surfaces (23,24). These sequential interactions are essential, as experimental conditions that prevent the ability of FGF2 to interact with either PI(4,5)P 2 or heparan sulfates cause inhibition of FGF2 secretion (22,24,25).…”

Small Molecule Inhibitors Targeting Tec Kinase Block Unconventional Secretion of Fibroblast Growth Factor 2

“…FGF2 exits cells by direct translocation across the plasma membrane (18,19). This process involves (i) membrane recruitment at the inner leaflet mediated by the phosphoinositide PI(4,5)P 2 (1, 2, 21, 22), (ii) FGF2 oligomerization and membrane pore formation (9,11,23,24,30), and (iii) extracellular trapping mediated by membraneproximal heparan sulfate proteoglycans (25,26). In addition, Tec kinase (9, 31) and ATP1A1 (12,(32)(33)(34), two additional factors physically associated with the plasma membrane, have been shown to play critical roles in the unconventional secretory pathway of FGF2 (12).…”

“…It is based upon direct translocation of folded species of FGF2 across plasma membranes (9,(17)(18)(19)(20). Hallmarks of this process are (i) FGF2 recruitment at the inner leaflet of the plasma membrane mediated by the phosphoinositide PI(4,5)P 2 2 (9, 21, 22), (ii) FGF2 oligomerization and membrane pore formation (11,12,23,24), and (iii) extracellular trapping of FGF2 mediated by membrane-proximal heparan sulfate proteoglycans (1,2,25,26). Recently, these hallmarks of FGF2 secretion have also been implicated in unconventional secretion of HIV-Tat (HIV trans-activator of transcription) (12, 27-30).…”

Quest for Cancer Biomarkers: Assaying Mutant Proteins and RNA that Provides the Much Needed Specificity

“…Proteolysis on the exosomes, SMVs and cell surface, releases ectodomains of membrane proteins into the extracellular microenvironment [29,30]. In an unconventional manner, proteins such as fibroblast growth factors-1 and -2 are capable of translocating from the cytoplasm directly through the PM into the extracellular space [31][32][33]. Finally, the flip flop mechanism mediates secretion of proteins (e.g., HASPB) anchored to membrane through dual acylation in the N-terminus [34,35].…”