Out of the ESCPE room: Emerging roles of endosomal SNX-BARs in receptor transport and host-pathogen interaction

Morphodynamical adaptation of the endolysosomal system to stress

In eukaryotes, the spatiotemporal control of endolysosomal organelles is central to the maintenance of homeostasis. By providing an interface between the cytoplasm and external environment, the endolysosomal system is placed at the forefront of the response to a wide range of stresses faced by cells. Endosomes are equipped with a dedicated set of membrane-associated proteins that ensure endosomal functions as well as crosstalk with the secretory or the autophagy pathways. Morphodynamical processes operate through local spatialization of subdomains, enabling specific remodeling and membrane contact capabilities. Consequently, the plasticity of endolysosomal organelles can be considered a robust and flexible tool exploited by cells to cope with homeostatic deviations. In this review, we provide insights into how the cellular responses to various stresses (osmotic, UV, nutrient deprivation, or pathogen infections) rely on the adaptation of the endolysosomal system morphodynamics.

Mapping the global interactome of the ARF family reveals spatial organization in cellular signaling pathways

The ADP-ribosylation factors (ARFs) and ARF-like (ARL) GTPases serve as essential molecular switches governing a wide array of cellular processes. In this study, we used proximity-dependent biotin identification (BioID) to comprehensively map the interactome of 28 out of 29 ARF and ARL proteins in two cellular models. Through this approach, we identified ∼3000 high-confidence proximal interactors, enabling us to assign subcellular localizations to the family members. Notably, we uncovered previously undefined localizations for ARL4D and ARL10. Clustering analyses further exposed the distinctiveness of the interactors identified with these two GTPases. We also reveal that the expression of the understudied member ARL14 is confined to the stomach and intestines. We identified phospholipase D1 (PLD1) and the ESCPE-1 complex, more precisely, SNX1, as proximity interactors. Functional assays demonstrated that ARL14 can activate PLD1 in cellulo and is involved in cargo trafficking via the ESCPE-1 complex. Overall, the BioID data generated in this study provide a valuable resource for dissecting the complexities of ARF and ARL spatial organization and signaling.

Loss of the HOPS complex disrupts early-to-late endosome transition, impairs endosomal recycling and induces accumulation of amphisomes

The multisubunit HOPS tethering complex is a well-established regulator of lysosome fusion with late endosomes and autophagosomes. However, the role of the HOPS complex in other stages of endo-lysosomal trafficking is not well understood. To address this, we made HeLa cells knocked out for the HOPS-specific subunits Vps39 or Vps41, or the HOPS-CORVET-core subunits Vps18 or Vps11. In all four knockout cells, we found that endocytosed cargos were trapped in enlarged endosomes that clustered in the perinuclear area. By correlative light-electron microscopy, these endosomes showed a complex ultrastructure and hybrid molecular composition, displaying markers for early (Rab5, PtdIns3P, EEA1) as well as late (Rab7, CD63, LAMP1) endosomes. These "HOPS bodies" were not acidified, contained enzymatically inactive cathepsins and accumulated endocytosed cargo and cation-independent mannose-6-phosphate receptor (CI-MPR). Consequently, CI-MPR was depleted from the TGN, and secretion of lysosomal enzymes to the extracellular space was enhanced. Strikingly, HOPS bodies also contained the autophagy proteins p62 and LC3, defining them as amphisomes. Together, these findings show that depletion of the lysosomal HOPS complex has a profound impact on the functional organization of the entire endosomal system and suggest the existence of a HOPS-independent mechanism for amphisome formation.

Out of the ESCPE room: Emerging roles of endosomal SNX-BARs in receptor transport and host-pathogen interaction

Several functions of the human cell, such as sensing nutrients, cell movement and interaction with the surrounding environment, depend on a myriad of transmembrane proteins and their associated proteins and lipids (collectively termed "cargoes"). To successfully perform their tasks, cargo must be sorted and delivered to the right place, at the right time, and in the right amount. To achieve this, eukaryotic cells have evolved a highly organized sorting platform, the endosomal network. Here, a variety of specialized multiprotein complexes sort cargo into itineraries leading to either their degradation or their recycling to various organelles for further rounds of reuse. A key sorting complex is the Endosomal SNX-BAR Sorting Complex for Promoting Exit (ESCPE-1) that promotes the recycling of an array of cargos to the plasma membrane and/or the trans-Golgi network. ESCPE-1 recognizes a hydrophobic-based sorting motif in numerous cargoes and orchestrates their packaging into tubular carriers that pinch off from the endosome and travel to the target organelle. A wide range of pathogens mimic this sorting motif to hijack ESCPE-1 transport to promote their invasion and survival within infected cells. In other instances, ESCPE-1 exerts restrictive functions against pathogens by limiting their replication and infection. In this review, we discuss ESCPE-1 assembly and functions, with a particular focus on recent advances in the understanding of its role in membrane trafficking, cellular homeostasis and host-pathogen interaction.

Canonical and Non-Canonical Roles of SNX1 and SNX2 in Endosomal Membrane Dynamics

Sorting nexins (SNXs) are a family of membrane-binding proteins known to play a critical role in regulating endocytic pathway sorting and endosomal membrane trafficking. Among them, SNX1 and SNX2 are members of the SNX-BAR subfamily and possess a membrane-curvature domain and a phosphoinositide-binding domain, which enables their stabilization at the phosphatidylinositol-3-phosphate (PI3P)-positive surface of endosomes. While their binding to PI3P-positive platforms facilitates interaction with endosomal partners and stabilization at the endosomal membrane, their SNX-BAR region is pivotal for generating membrane tubulation from endosomal compartments. In this context, their primary identified biological roles-and their partnership-are tightly associated with the retromer and endosomal SNX-BAR sorting complex for promoting exit 1 complex trafficking, facilitating the transport of cargoes from early endosomes to the secretory pathway. However, recent literature indicates that these proteins also possess biological functions in other aspects of endosomal features and sorting processes. Notably, SNX2 has been found to regulate endosome-endoplasmic reticulum (ER) contact sites through its interaction with VAP proteins at the ER membrane. Furthermore, data from our laboratory show that SNX1 and SNX2 are involved in the tubulation of early endosomes toward ER sites associated with autophagy initiation during starvation. These findings shed light on a novel role of SNXs in inter-organelle tethering and communication. In this concise review, we will explore the non-retromer functions of SNX1 and SNX2, specifically focusing on their involvement in endosomal membrane dynamics during stress sensing and autophagy-associated processes.