Structure-function relationship of FGF2 oligomers forming dynamic membrane pores in unconventional protein secretion

成纤维细胞生长因子2（FGF 2）是一种分泌性蛋白质，其通过非常规手段转运到细胞外空间。这种不寻常的分泌途径的核心过程是由PI（4，5）P2依赖性的FGF 2在内膜小叶处的寡聚化触发的。这反过来又引发了高度动态的膜重塑事件，导致形成具有环形结构的脂质膜孔。膜插入的FGF 2寡聚体被容纳在这些膜孔的中心亲水空间中。它们代表在外质膜小叶处被捕获和分解的瞬时易位中间体，这是由细胞表面硫酸乙酰肝素蛋白聚糖Glypican-1（GPC-1）介导的过程。因此，FGF 2跨质膜易位涉及以顺序方式的四种类型的膜重塑，其中（i）横向重组[PI（4，5）P2-依赖性FGF 2膜在内小叶处募集]，（ii）侧向分离[PI（4，5）P2分配到富含胆固醇的纳米结构域中]，（iii）形态学转化（作为环形孔的一部分的膜曲率）和（iv）拓扑学转化（当FGF 2易位完成时去除环形膜孔，恢复规则的双层结构）。 在这个项目中，我们将联合收割机的实验工作与分子动力学模拟相结合，以揭示分子机制，使FGF 2动态重塑质膜通过PI（4，5）P2依赖性寡聚化，产生一个易位中间体，通过它可以到达细胞外空间。对这一过程的深入理解对于开发阻断肿瘤细胞及其细胞微环境中FGF 2非常规分泌的抑制剂至关重要。这些抑制剂进一步发展为癌症治疗药物，具有治疗FGF 2产生的化学抗性的巨大潜力，例如在急性髓性白血病中。

Fibroblast Growth Factor 2 (FGF2) is a secretory protein that is transported into the extracellular space by unconventional means. The core process of this unusual secretory pathway is triggered by PI(4,5)P2-dependent oligomerization of FGF2 at the inner plasma membrane leaflet. This, in turn, initiates a highly dynamic membrane remodeling event that results in the formation of a lipidic membrane pore with a toroidal architecture. Membrane-inserted FGF2 oligomers are accommodated in the central hydrophilic space of these membrane pores. They represent transient translocation intermediates that are captured and disassembled at the outer plasma membrane leaflet, a process mediated by the cell surface heparan sulfate proteoglycan Glypican-1 (GPC-1). Thus, FGF2 translocation across the plasma membrane involves four types of membrane remodeling in a sequential manner with (i) transversal reorganization [PI(4,5)P2-dependent FGF2 membrane recruitment at the inner leaflet], (ii) lateral segregation [PI(4,5)P2 partitioning into cholesterol-enriched nanodomains], (iii) morphological transformation (membrane curvature as part of a toroidal pore) and (iv) topological transformation (removing toroidal membrane pores when FGF2 translocation is completed restoring the regular bilayer structure). In this project, we will combine experimental work with molecular dynamics simulations to reveal the molecular mechanism that enables FGF2 to dynamically remodel the plasma membrane through PI(4,5)P2-dependent oligomerization, generating a translocation intermediate through which it can reach the extracellular space. A deep understanding of this process is fundamental for developing inhibitors that block unconventional secretion of FGF2 from tumor cells and their cellular microenvironment. Developed further into drugs for cancer therapy, such inhibitors have great potential for treating chemoresistances that FGF2 exerts, for example, in acute myeloid leukemia.