Glypican-1 drives unconventional secretion of Fibroblast Growth Factor 2

Mapping the FGF2 Interactome Identifies a Functional Proteoglycan Coreceptor

Fibroblast growth factor 2 (FGF2) is a multipotent growth factor and signaling protein that exhibits broad functions across multiple cell types. These functions are often initiated by binding to growth factor receptors and fine-tuned by glycosaminoglycan (GAG)-modified proteins called proteoglycans. The various outputs of FGF2 signaling and functions arise from a dynamic and cell type-specific set of binding partners. However, the interactome of FGF2 has yet to be comprehensively determined. Moreover, the identity of the proteoglycan proteins carrying GAG chains is often overlooked and remains unknown in most cell contexts. Here, we perform peroxidase-catalyzed live cell proximity labeling using an engineered APEX2-FGF2 fusion protein to map the interactome of FGF2. Across two cell lines with established and distinct FGF2-driven functions, we greatly expand upon the known FGF2 interactome, identifying >600 new putative FGF2 interactors. Notably, our results demonstrate a key role for the GAG binding capacity of FGF2 in modulating its interactome.

Uncovering key steps in FGF12 cellular release reveals a common mechanism for unconventional FGF protein secretion

FGF12 belongs to a subfamily of FGF proteins called FGF homologous factors (FHFs), which until recently were thought to be non-signaling intracellular proteins. Our recent studies have shown that although they lack a conventional signal peptide for secretion, they can reach the extracellular space, especially under stress conditions. Here, we unraveled that the long "a" isoform of FGF12 is secreted in a pathway involving the A1 subunit of Na(+)/K(+) ATPase (ATP1A1), Tec kinase and lipids such as phosphatidylinositol and phosphatidylserine. Further, we showed that the short "b" isoform of FGF12, which binds ATP1A1 and phosphatidylserine less efficiently, is not secreted from cells. We also indicated regions in the FGF12a protein sequence that are crucial for its secretion, including N-terminal fragment and specific residues, and proposed that liquid-liquid phase separation may be important in this process. Our results strongly suggest that the mechanism of this process is very similar for all unconventionally secreted FGF proteins.

Phosphoinositide switches in cell physiology - From molecular mechanisms to disease

Phosphoinositides are amphipathic lipid molecules derived from phosphatidylinositol that represent low abundance components of biological membranes. Rather than serving as mere structural elements of lipid bilayers, they represent molecular switches for a broad range of biological processes, including cell signaling, membrane dynamics and remodeling, and many other functions. Here, we focus on the molecular mechanisms that turn phosphoinositides into molecular switches and how the dysregulation of these processes can lead to disease.

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

Fibroblast growth factor 2 (FGF2) exits cells by direct translocation across the plasma membrane, a type I pathway of unconventional protein secretion. This process is initiated by phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2)-dependent formation of highly dynamic FGF2 oligomers at the inner plasma membrane leaflet, inducing the formation of lipidic membrane pores. Cell surface heparan sulfate chains linked to glypican-1 (GPC1) capture FGF2 at the outer plasma membrane leaflet, completing FGF2 membrane translocation into the extracellular space. While the basic steps of this pathway are well understood, the molecular mechanism by which FGF2 oligomerizes on membrane surfaces remains unclear. In the current study, we demonstrate the initial step of this process to depend on C95-C95 disulfide-bridge-mediated FGF2 dimerization on membrane surfaces, producing the building blocks for higher FGF2 oligomers that drive the formation of membrane pores. We find FGF2 with a C95A substitution to be defective in oligomerization, pore formation, and membrane translocation. Consistently, we demonstrate a C95A variant of FGF2 to be characterized by a severe secretion phenotype. By contrast, while also important for efficient FGF2 secretion from cells, a second cysteine residue on the molecular surface of FGF2 (C77) is not involved in FGF2 oligomerization. Rather, we find C77 to be part of the interaction interface through which FGF2 binds to the α1 subunit of the Na,K-ATPase, the landing platform for FGF2 at the inner plasma membrane leaflet. Using cross-linking mass spectrometry, atomistic molecular dynamics simulations combined with a machine learning analysis and cryo-electron tomography, we propose a mechanism by which disulfide-bridged FGF2 dimers bind with high avidity to PI(4,5)P2 on membrane surfaces. We further propose a tight coupling between FGF2 secretion and the formation of ternary signaling complexes on cell surfaces, hypothesizing that C95-C95-bridged FGF2 dimers are functioning as the molecular units triggering autocrine and paracrine FGF2 signaling.

Global impact of proteoglycan science on human diseases

In this comprehensive review, we will dissect the impact of research on proteoglycans focusing on recent developments involved in their synthesis, degradation, and interactions, while critically assessing their usefulness in various biological processes. The emerging roles of proteoglycans in global infections, specifically the SARS-CoV-2 pandemic, and their rising functions in regenerative medicine and biomaterial science have significantly affected our current view of proteoglycans and related compounds. The roles of proteoglycans in cancer biology and their potential use as a next-generation protein-based adjuvant therapy to combat cancer is also emerging as a constructive and potentially beneficial therapeutic strategy. We will discuss the role of proteoglycans in selected and emerging areas of proteoglycan science, such as neurodegenerative diseases, autophagy, angiogenesis, cancer, infections and their impact on mammalian diseases.

Proteomic Investigation of Differential Interactomes of Glypican 1 and a Putative Disease-Modifying Variant of Ataxia

In a currently 13-year-old girl of consanguineous Turkish parents, who developed unsteady gait and polyneuropathy at the ages of 3 and 6 years, respectively, we performed whole genome sequencing and identified a biallelic missense variant c.424C>T, p.R142W in glypican 1 (GPC1) as a putative disease-associated variant. Up to date, GPC1 has not been associated with a neuromuscular disorder, and we hypothesized that this variant, predicted as deleterious, may be causative for the disease. Using mass spectrometry-based proteomics, we investigated the interactome of GPC1 WT and the missense variant. We identified 198 proteins interacting with GPC1, of which 16 were altered for the missense variant. This included CANX as well as vacuolar ATPase (V-ATPase) and the mammalian target of rapamycin complex 1 (mTORC1) complex members, whose dysregulation could have a potential impact on disease severity in the patient. Importantly, these proteins are novel interaction partners of GPC1. At 10.5 years, the patient developed dilated cardiomyopathy and kyphoscoliosis, and Friedreich's ataxia (FRDA) was suspected. Given the unusually severe phenotype in a patient with FRDA carrying only 104 biallelic GAA repeat expansions in FXN, we currently speculate that disturbed GPC1 function may have exacerbated the disease phenotype. LC-MS/MS data are accessible in the ProteomeXchange Consortium (PXD040023).

Determining the Functional Oligomeric State of Membrane-Associated Protein Oligomers Forming Membrane Pores on Giant Lipid Vesicles

Several peripheral membrane proteins are known to form membrane pores through multimerization. In many cases, in biochemical reconstitution experiments, a complex distribution of oligomeric states has been observed that may, in part, be irrelevant to their physiological functions. This phenomenon makes it difficult to identify the functional oligomeric states of membrane lipid interacting proteins, for example, during the formation of transient membrane pores. Using fibroblast growth factor 2 (FGF2) as an example, we present a methodology applicable to giant lipid vesicles by which functional oligomers can be distinguished from nonspecifically aggregated proteins without functionality. Two distinct populations of fibroblast growth factor 2 were identified with (i) dimers to hexamers and (ii) a broad population of higher oligomeric states of membrane-associated FGF2 oligomers significantly distorting the original unfiltered histogram of all detectable oligomeric species of FGF2. The presented statistical approach is relevant for various techniques for characterizing membrane-dependent protein oligomerization.

Unconventional protein secretion (UPS): role in important diseases

Unconventional protein secretion (UPS) is the new secretion process discovered in liquid form over three decades ago. More recently, UPS has been shown to operate also in solid forms generated from four types of organelles: fractions of lysosomes and autophagy (APh) undergoing exocytosis; exosomes and ectosomes, with their extracellular vesicles (EVs). Recently many mechanisms and proteins of these solid forms have been shown to depend on UPS. An additional function of UPS is the regulation of diseases, often investigated separately from each other. In the present review, upon short presentation of UPS in healthy cells and organs, interest is focused on the mechanisms and development of diseases. The first reported are neurodegenerations, characterized by distinct properties. Additional diseases, including inflammasomes, inflammatory responses, glial effects and other diseases of various origin, are governed by proteins generated, directly or alternatively, by UPS. The diseases most intensely affected by UPS are various types of cancer, activated in most important processes: growth, proliferation and invasion, relapse, metastatic colonization, vascular leakiness, immunomodulation, chemoresistence. The therapy role of UPS diseases depends largely on exosomes. In addition to affecting neurodegenerative diseases, its special aim is the increased protection against cancer. Its immense relevance is due to intrinsic features, including low immunogenicity, biocompatibility, stability, and crossing of biological barriers. Exosomes, loaded with factors for pharmacological actions and target cell sensitivity, induce protection against various specific cancers. Further expansion of disease therapies is expected in the near future.

Cholesterol promotes clustering of PI(4,5)P2 driving unconventional secretion of FGF2

FGF2 is a cell survival factor involved in tumor-induced angiogenesis that is secreted through an unconventional secretory pathway based upon direct protein translocation across the plasma membrane. Here, we demonstrate that both PI(4,5)P2-dependent FGF2 recruitment at the inner plasma membrane leaflet and FGF2 membrane translocation into the extracellular space are positively modulated by cholesterol in living cells. We further revealed cholesterol to enhance FGF2 binding to PI(4,5)P2-containing lipid bilayers. Based on extensive atomistic molecular dynamics (MD) simulations and membrane tension experiments, we proposed cholesterol to modulate FGF2 binding to PI(4,5)P2 by (i) increasing head group visibility of PI(4,5)P2 on the membrane surface, (ii) increasing avidity by cholesterol-induced clustering of PI(4,5)P2 molecules triggering FGF2 oligomerization, and (iii) increasing membrane tension facilitating the formation of lipidic membrane pores. Our findings have general implications for phosphoinositide-dependent protein recruitment to membranes and explain the highly selective targeting of FGF2 toward the plasma membrane, the subcellular site of FGF2 membrane translocation during unconventional secretion of FGF2.

Exosome biogenesis: machinery, regulation, and therapeutic implications in cancer

Exosomes are well-known key mediators of intercellular communication and contribute to various physiological and pathological processes. Their biogenesis involves four key steps, including cargo sorting, MVB formation and maturation, transport of MVBs, and MVB fusion with the plasma membrane. Each process is modulated through the competition or coordination of multiple mechanisms, whereby diverse repertoires of molecular cargos are sorted into distinct subpopulations of exosomes, resulting in the high heterogeneity of exosomes. Intriguingly, cancer cells exploit various strategies, such as aberrant gene expression, posttranslational modifications, and altered signaling pathways, to regulate the biogenesis, composition, and eventually functions of exosomes to promote cancer progression. Therefore, exosome biogenesis-targeted therapy is being actively explored. In this review, we systematically summarize recent progress in understanding the machinery of exosome biogenesis and how it is regulated in the context of cancer. In particular, we highlight pharmacological targeting of exosome biogenesis as a promising cancer therapeutic strategy.

Serum exosomal and serum glypican-1 are associated with early recurrence of pancreatic ductal adenocarcinoma

Background

The diagnostic performance and prognostic value of serum exosomal glypican 1 (GPC-1) in pancreatic ductal adenocarcinoma (PDAC) remain controversial. In this study, we detected serum exosomal GPC-1 using enzyme-linked immunosorbent assay (ELISA) and determined whether it serves as a predictor of diagnosis and recurrence for early-stage PDAC.

Methods

Serum samples were obtained from patients with 50 PDAC, 6 benign pancreatic tumor (BPT), or 9 chronic pancreatitis (CP) and 50 healthy controls (HCs). Serum exosomes were isolated using an exosome isolation kit. Exosomal and serum GPC-1 levels were measured using ELISA. The freeze–thaw process was carried out to analyze the stability of GPC-1. Receiver operating characteristic (ROC) analysis was employed to assess the diagnostic value of GPC-1. Kaplan–Meier and multivariate Cox analyses were used to evaluate the prognostic value of GPC-1.

Results

The average concentrations of serum exosomal and serum GPC-1 were 1.5 and 0.8 ng/ml, respectively. GPC-1 expression levels were stable under repeated freezing and thawing (d1-5 freeze–thaw cycles vs. d0 P > 0.05). Serum exosomal and serum GPC-1 were significantly elevated in patients with PDAC compared with HCs (P < 0.0001) but were slightly higher compared with that in patients with CP and BPT (P > 0.05). The expression levels of exosomal and serum GPC-1 were elevated 5 days after surgery in patients with PDAC, CP, and BPT (P < 0.05). Patients with high levels of exosomal and serum GPC-1 had a shorter relapse-free survival (RFS) (P = 0.006, and P = 0.010). Multivariate analyses showed that serum exosomal and serum GPC-1 were independent prognostic indicators for early RFS (P = 0.008, and P = 0.041).

Conclusion

ELISA is an effective and sensitive method to detect exosomal and serum GPC-1. The detection of GPC-1 was stable under repeated freezing and thawing cycles and could distinguish early-stage PDAC from HCs but not CP and BPT. Exosomal and serum GPC-1 may be good independent predictors of early recurrence in early-stage PDAC.

Unconventional secretion mediated by direct protein self-translocation across the plasma membranes of mammalian cells

In recent years, a surprisingly complex picture emerged about endoplasmic reticulum (ER)/Golgi-independent secretory pathways, and several routes have been discovered that differ with regard to their molecular mechanisms and machineries. Fibroblast growth factor 2 (FGF2) is secreted by a pathway of unconventional protein secretion (UPS) that is based on direct self-translocation across the plasma membrane. Building on previous research, a component of this process has been identified to be glypican-1 (GPC1), a GPI-anchored heparan sulfate proteoglycan located on cell surfaces. These findings not only shed light on the molecular mechanism underlying this process but also reveal an intimate relationship between FGF2 and GPC1 that might be of critical relevance for the prominent roles they both have in tumor progression and metastasis.