A role for novel lipid interactions in the dynamic recruitment of SNX27 to the T-cell immune synapse

Ez-Metastasizing: The Crucial Roles of Ezrin in Metastasis

Ezrin is the cytoskeletal organizer and functions in the modulation of membrane-cytoskeleton interaction, maintenance of cell shape and structure, and regulation of cell-cell adhesion and movement, as well as cell survival. Ezrin plays a critical role in regulating tumor metastasis through interaction with other binding proteins. Notably, Ezrin has been reported to interact with immune cells, allowing tumor cells to escape immune attack in metastasis. Here, we review the main functions of Ezrin, the mechanisms through which it acts, its role in tumor metastasis, and its potential as a therapeutic target.

Endosomal Sorting Protein SNX27 and Its Emerging Roles in Human Cancers

SNX27 belongs to the sorting nexin (SNX) family of proteins that play a critical role in protein sorting and trafficking in the endocytosis pathway. This protein family is characterized by the presence of a Phox (PX) domain; however, SNX27 is unique in containing an additional PDZ domain. Recently, SNX27 has gained popularity as an important sorting protein that is associated with the retromer complex and mediates the recycling of internalized proteins from endosomes to the plasma membrane in a PDZ domain-dependent manner. Over 100 cell surface proteins have been identified as binding partners of the SNX27-retromer complex. However, the roles and underlying mechanisms governed by SNX27 in tumorigenesis remains to be poorly understood. Many of its known binding partners include several G-protein coupled receptors, such as β2-andrenergic receptor and parathyroid hormone receptor, are associated with multiple pathways implicated in oncogenic signaling and tumorigenesis. Additionally, SNX27 mediates the recycling of GLUT1 and the activation of mTORC1, both of which can regulate intracellular energy balance and promote cell survival and proliferation under conditions of nutrient deprivation. In this review, we summarize the structure and fundamental roles of SNX proteins, with a focus on SNX27, and provide the current evidence indicating towards the role of SNX27 in human cancers. We also discuss the gap in the field and future direction of SNX27 research. Insights into the emerging roles and mechanism of SNX27 in cancers will provide better development strategies to prevent and treat tumorigenesis.

F-Actin Dynamics in the Regulation of Endosomal Recycling and Immune Synapse Assembly

Membrane proteins endocytosed at the cell surface as vesicular cargoes are sorted at early endosomes for delivery to lysosomes for degradation or alternatively recycled to different cellular destinations. Cargo recycling is orchestrated by multimolecular complexes that include the retromer, retriever, and the WASH complex, which promote the polymerization of new actin filaments at early endosomes. These endosomal actin pools play a key role at different steps of the recycling process, from cargo segregation to specific endosomal subdomains to the generation and mobility of tubulo-vesicular transport carriers. Local F-actin pools also participate in the complex redistribution of endomembranes and organelles that leads to the acquisition of cell polarity. Here, we will present an overview of the contribution of endosomal F-actin to T-cell polarization during assembly of the immune synapse, a specialized membrane domain that T cells form at the contact with cognate antigen-presenting cells.

SNX27 regulates DRA activity and mediates its direct recycling by PDZ-interaction in early endosomes at the apical pole of Caco2 cells

DRA (downregulated in adenoma, SLC26A3) and NHE3 (Na⁺/H⁺ exchanger 3, SLC9A3) together mediate intestinal electroneutral NaCl absorption. Both transporters contain PDZ (postsynaptic density 95, disc large, zonula occludens 1) binding motifs and interact with PDZ adaptor proteins regulating their activity and recycling. SNX27 (sorting nexin 27) contains a PDZ domain and is involved in the recycling of cargo proteins including NHE3. The interaction of SNX27 with DRA and its potential role for the activity and recycling of DRA have been evaluated in this study. SNX27 specifically interacts with DRA via its PDZ domain. The knockdown (KD) of SNX27 reduced DRA activity by 50% but was not accompanied by a decrease of DRA surface expression. This indicates that DRA is trafficked to specific functional domains in the plasma membrane in which DRA is particularly active. Consistently, the disruption of lipid raft integrity by methyl-β-cyclodextrin has an inhibitory effect on DRA activity that was strongly reduced after SNX27 KD. In differentiated intestinal Caco2 cells, superresolution microscopy and a novel quantitative axial approach revealed that DRA and SNX27 colocalize in rab5-positive early endosomes at the apical pole. SNX27 regulates the activity of DRA in the apical plasma membrane through binding with its PDZ domain. This interaction occurs in rab5-positive early endosomes at the apical pole of differentiated intestinal Caco2 cells. SNX27 is involved in the direct recycling of DRA to the plasma membrane where it is inserted into lipid rafts facilitating increased activity.NEW & NOTEWORTHY SNX27 has a PDZ domain and is involved in the regulation and recycling of transmembrane proteins. The role of SNX27 on the activity and recycling of the intestinal Cl^-/HCO3- exchanger DRA has not yet been studied. This study shows that SNX27 directly interacts with DRA in early endosomes at the apical pole of intestinal Caco2 cells and mediates its direct recycling to facilitate high activity in lipid rafts in the apical plasma membrane.

Retromer-Mediated Protein Sorting and Vesicular Trafficking

Retromer is an evolutionarily conserved multimeric protein complex that mediates intracellular transport of various vesicular cargoes and functions in a wide variety of cellular processes including polarized trafficking, developmental signaling and lysosome biogenesis. Through its interaction with the Rab GTPases and their effectors, membrane lipids, molecular motors, the endocytic machinery and actin nucleation promoting factors, retromer regulates sorting and trafficking of transmembrane proteins from endosomes to the trans-Golgi network (TGN) and the plasma membrane. In this review, I highlight recent progress in the understanding of retromer-mediated protein sorting and vesicle trafficking and discuss how retromer contributes to a diverse set of developmental, physiological and pathological processes.