Endocytosis blocks the vesicular secretion of exosome marker proteins.

Exosomes are secreted vesicles of ~30 to 150 nm diameter that play important roles in human health and disease. To better understand how cells release these vesicles, we examined the biogenesis of the most highly enriched human exosome marker proteins, the exosomal tetraspanins CD81, CD9, and CD63. We show here that endocytosis inhibits their vesicular secretion and, in the case of CD9 and CD81, triggers their destruction. Furthermore, we show that syntenin, a previously described exosome biogenesis factor, drives the vesicular secretion of CD63 by blocking CD63 endocytosis and that other endocytosis inhibitors also induce the plasma membrane accumulation and vesicular secretion of CD63. Finally, we show that CD63 is an expression-dependent inhibitor of endocytosis that triggers the vesicular secretion of lysosomal proteins and the clathrin adaptor AP-2 mu2. These results suggest that the vesicular secretion of exosome marker proteins in exosome-sized vesicles occurs primarily by an endocytosis-independent pathway.

Proteomic characterization of hUC-MSC extracellular vesicles and evaluation of its therapeutic potential to treat Alzheimer's disease.

In recent years, human umbilical cord mesenchymal stem cell (hUC-MSC) extracellular vesicles (EVs) have been used as a cell replacement therapy and have been shown to effectively overcome some of the disadvantages of cell therapy. However, the specific mechanism of action of EVs is still unclear, and there is no appropriate system for characterizing the differences in the molecular active substances of EVs produced by cells in different physiological states. We used a data-independent acquisition (DIA) quantitative proteomics method to identify and quantify the protein composition of two generations EVs from three different donors and analysed the function and possible mechanism of action of the proteins in EVs of hUC-MSCs via bioinformatics. By comparative proteomic analysis, we characterized the different passages EVs. Furthermore, we found that adaptor-related protein complex 2 subunit alpha 1 (AP2A1) and adaptor-related protein complex 2 subunit beta 1 (AP2B1) in hUC-MSC-derived EVs may play a significant role in the treatment of Alzheimer's disease (AD) by regulating the synaptic vesicle cycle signalling pathway. Our work provides a direction for batch-to-batch quality control of hUC-MSC-derived EVs and their application in AD treatment.

The emerging roles of clathrin-mediated endocytosis in plant development and stress responses.

Clathrin-mediated endocytosis (CME) is a highly conserved pathway that plays a crucial role in the endocytosis of plasma membrane proteins in eukaryotic cells. The pathway is initiated when the adaptor protein complex 2 (AP2) and TPLATE complex (TPC) work together to recognize cargo proteins and recruit clathrin. This review provides a concise overview of the functions of each subunit of AP2 and TPC, and highlights the involvement of CME in various biological processes, such as pollen development, root development, nutrient transport, extracellular signal transduction, auxin polar transport, hyperosmotic stress, salinity stress, high ammonium stress, and disease resistance. Additionally, the review explores the regulation of CME by phytohormones, clathrin-mediated exocytosis (CMX), and AP2M phosphorylation. It also suggests potential future research directions for CME.

Adaptor protein complex 2 in the orbitofrontal cortex predicts alcohol use disorder.

Alcohol use disorder (AUD) is a life-threatening disease characterized by compulsive drinking, cognitive deficits, and social impairment that continue despite negative consequences. The inability of individuals with AUD to regulate drinking may involve functional deficits in cortical areas that normally balance actions that have aspects of both reward and risk. Among these, the orbitofrontal cortex (OFC) is critically involved in goal-directed behavior and is thought to maintain a representation of reward value that guides decision making. In the present study, we analyzed post-mortem OFC brain samples collected from age- and sex-matched control subjects and those with AUD using proteomics, bioinformatics, machine learning, and reverse genetics approaches. Of the 4,500+ total unique proteins identified in the proteomics screen, there were 47 proteins that differed significantly by sex that were enriched in processes regulating extracellular matrix and axonal structure. Gene ontology enrichment analysis revealed that proteins differentially expressed in AUD cases were involved in synaptic and mitochondrial function, as well as transmembrane transporter activity. Alcohol-sensitive OFC proteins also mapped to abnormal social behaviors and social interactions. Machine learning analysis of the post-mortem OFC proteome revealed dysregulation of presynaptic (e.g., AP2A1) and mitochondrial proteins that predicted the occurrence and severity of AUD. Using a reverse genetics approach to validate a target protein, we found that prefrontal Ap2a1 expression significantly correlated with voluntary alcohol drinking in male and female genetically diverse mouse strains. Moreover, recombinant inbred strains that inherited the C57BL/6J allele at the Ap2a1 interval consumed higher amounts of alcohol than those that inherited the DBA/2J allele. Together, these findings highlight the impact of excessive alcohol consumption on the human OFC proteome and identify important cross-species cortical mechanisms and proteins that control drinking in individuals with AUD.

Phosphorylation of ADAPTOR PROTEIN-2 µ-adaptin by ADAPTOR-ASSOCIATED KINASE1 regulates the tropic growth of Arabidopsis roots.

ADAPTOR-ASSOCIATED PROTEIN KINASE1 (AAK1) is a known regulator of clathrin-mediated endocytosis in mammals. Human AAK1 phosphorylates the µ2 subunit of the ADAPTOR PROTEIN-2 (AP-2) complex (AP2M) and plays important roles in cell differentiation and development. Previous interactome studies discovered the association of AAK1 with AP-2 in Arabidopsis (Arabidopsis thaliana), but its function was unclear. Here, genetic analysis revealed that the Arabidopsis aak1 and ap2m mutants both displayed altered root tropic growth, including impaired touch- and gravity-sensing responses. In Arabidopsis, AAK1-phosphorylated AP2M on Thr-163, and expression of the phospho-null version of AP2M in the ap2m mutant led to an aak1-like phenotype, whereas the phospho-mimic forms of AP2M rescued the aak1 mutant. In addition, we found that the AAK1-dependent phosphorylation state of AP2M modulates the frequency distribution of endocytosis. Our data indicate that the phosphorylation of AP2M on Thr-163 by AAK1 fine-tunes endocytosis in the Arabidopsis root to control its tropic growth.

Oligomer-to-monomer transition underlies the chaperone function of AAGAB in AP1/AP2 assembly.

Assembly of protein complexes is facilitated by assembly chaperones. Alpha and gamma adaptin-binding protein (AAGAB) is a chaperone governing the assembly of the heterotetrameric adaptor complexes 1 and 2 (AP1 and AP2) involved in clathrin-mediated membrane trafficking. Here, we found that before AP1/2 binding, AAGAB exists as a homodimer. AAGAB dimerization is mediated by its C-terminal domain (CTD), which is critical for AAGAB stability and is missing in mutant proteins found in patients with the skin disease punctate palmoplantar keratoderma type 1 (PPKP1). We solved the crystal structure of the dimerization-mediating CTD, revealing an antiparallel dimer of bent helices. Interestingly, AAGAB uses the same CTD to recognize and stabilize the γ subunit in the AP1 complex and the α subunit in the AP2 complex, forming binary complexes containing only one copy of AAGAB. These findings demonstrate a dual role of CTD in stabilizing resting AAGAB and binding to substrates, providing a molecular explanation for disease-causing AAGAB mutations. The oligomerization state transition mechanism may also underlie the functions of other assembly chaperones.

Palmitoyl Transferase FonPAT2-Catalyzed Palmitoylation of the FonAP-2 Complex Is Essential for Growth, Development, Stress Response, and Virulence in Fusarium oxysporum f. sp. niveum.

Protein palmitoylation, one of posttranslational modifications, is catalyzed by a group of palmitoyl transferases (PATs) and plays critical roles in the regulation of protein functions. However, little is known about the function and mechanism of PATs in plant pathogenic fungi. The present study reports the function and molecular mechanism of FonPATs in Fusarium oxysporum f. sp. niveum (Fon), the causal agent of watermelon Fusarium wilt. The Fon genome contains six FonPAT genes with distinct functions in vegetative growth, conidiation and conidial morphology, and stress response. FonPAT1, FonPAT2, and FonPAT4 have PAT activity and are required for Fon virulence on watermelon mainly through regulating in planta fungal growth within host plants. Comparative proteomics analysis identified a set of proteins that were palmitoylated by FonPAT2, and some of them are previously reported pathogenicity-related proteins in fungi. The FonAP-2 complex core subunits FonAP-2α, FonAP-2ß, and FonAP-2µ were palmitoylated by FonPAT2 in vivo. FonPAT2-catalyzed palmitoylation contributed to the stability and interaction ability of the core subunits to ensure the formation of the FonAP-2 complex, which is essential for vegetative growth, asexual reproduction, cell wall integrity, and virulence in Fon. These findings demonstrate that FonPAT1, FonPAT2, and FonPAT4 play important roles in Fon virulence and that FonPAT2-catalyzed palmitoylation of the FonAP-2 complex is critical to Fon virulence, providing novel insights into the importance of protein palmitoylation in the virulence of plant fungal pathogens. IMPORTANCE Fusarium oxysporum f. sp. niveum (Fon), the causal agent of watermelon Fusarium wilt, is one of the most serious threats for the sustainable development of the watermelon industry worldwide. However, little is known about the underlying molecular mechanism of pathogenicity in Fon. Here, we found that the palmitoyl transferase (FonPAT) genes play distinct and diverse roles in basic biological processes and stress response and demonstrated that FonPAT1, FonPAT2, and FonPAT4 have PAT activity and are required for virulence in Fon. We also found that FonPAT2 palmitoylates the core subunits of the FonAP-2 complex to maintain the stability and the formation of the FonAP-2 complex, which is essential for basic biological processes, stress response, and virulence in Fon. Our study provides new insights into the understanding of the molecular mechanism involved in Fon virulence and will be helpful in the development of novel strategies for disease management.

AP2S1 regulates APP degradation through late endosome-lysosome fusion in cells and APP/PS1 mice.

AP2S1 is the sigma 2 subunit of adaptor protein 2 (AP2) that is essential for endocytosis. In this study, we investigated the potential role of AP2S1 in intracellular processing of amyloid precursor protein (APP), which contributes to the pathogenesis of Alzheimer disease (AD) by generating the toxic ß-amyloid peptide (Aß). We found that knockdown or overexpression of AP2S1 decreased or increased the protein levels of APP and Aß in cells stably expressing human full-length APP695, respectively. This effect was unrelated to endocytosis but involved lysosomal degradation. Morphological studies revealed that silencing of AP2S1 promoted the translocalization of APP from RAB9-positive late endosomes (LE) to LAMP1-positive lysosomes, which was paralleled by the enhanced LE-lysosome fusion. In support, silencing of vacuolar protein sorting-associated protein 41 (VPS41) that is implicated in LE-lyso fusion prevented AP2S1-mediated regulation of APP degradation and translocalization. In APP/PS1 mice, an animal model of AD, AAV-mediated delivery of AP2S1 shRNA in the hippocampus significantly reduced the protein levels of APP and Aß, with the concomitant APP translocalization, LE-lyso fusion and the improved cognitive functions. Taken together, these data uncover a LE-lyso fusion mechanism in APP degradation and suggest a novel role for AP2S1 in the pathophysiology of AD.

MR1: An unconventional twist in the tail.

MR1 is a conserved molecule that binds microbial vitamin B metabolites and presents them to unconventional T cells. Lim and colleagues (2022. J. Cell Biol.https://doi.org/10.1083/jcb.202110125) uncover the role of AP2 in ensuring MR1 surface presentation, which relies on an atypical motif within the MR1 cytoplasmic tail.

A specialized tyrosine-based endocytosis signal in MR1 controls antigen presentation to MAIT cells.

MR1 is a highly conserved microbial immune-detection system in mammals. It captures vitamin B-related metabolite antigens from diverse microbes and presents them at the cell surface to stimulate MR1-restricted lymphocytes including mucosal-associated invariant T (MAIT) cells. MR1 presentation and MAIT cell recognition mediate homeostasis through host defense and tissue repair. The cellular mechanisms regulating MR1 cell surface expression are critical to its function and MAIT cell recognition, yet they are poorly defined. Here, we report that human MR1 is equipped with a tyrosine-based motif in its cytoplasmic domain that mediates low affinity binding with the endocytic adaptor protein 2 (AP2) complex. This interaction controls the kinetics of MR1 internalization from the cell surface and minimizes recycling. We propose MR1 uses AP2 endocytosis to define the duration of antigen presentation to MAIT cells and the detection of a microbial metabolic signature by the immune system.

The adaptor protein chaperone AAGAB stabilizes AP-4 complex subunits.

Adaptor protein 4 (AP-4) is a heterotetrameric complex composed of ε, ß4, µ4, and σ4 subunits that mediates export of a subset of transmembrane cargos, including autophagy protein 9A (ATG9A), from the trans-Golgi network (TGN). AP-4 has received particular attention in recent years because mutations in any of its subunits cause a complicated form of hereditary spastic paraplegia referred to as "AP-4-deficiency syndrome." The identification of proteins that interact with AP-4 has shed light on the mechanisms of AP-4-dependent cargo sorting and distribution within the cell. However, the mechanisms by which the AP-4 complex itself is assembled have remained unknown. Here, we report that the alpha- and gamma-adaptin-binding protein (AAGAB, also known as p34) binds to and stabilizes the AP-4 ε and σ4 subunits, thus promoting complex assembly. The physiological importance of these interactions is underscored by the observation that AAGAB-knockout cells exhibit reduced levels of AP-4 subunits and accumulation of ATG9A at the TGN like those in cells with mutations in AP-4-subunit genes. These findings demonstrate that AP-4 assembly is not spontaneous but AAGAB-assisted, further contributing to the understanding of an adaptor protein complex that is critically involved in development of the central nervous system.

Reconstituting and Purifying Assembly Intermediates of Clathrin Adaptors AP1 and AP2.

Clathrin-coated vesicles mediate membrane cargo transportation from the plasma membrane, the trans-Golgi network, the endosome, and the lysosome. Heterotetrameric adaptor complexes 1 and 2 (AP1 and AP2) are bridges that link cargo-loaded membranes to clathrin coats. Assembly of AP2 was previously considered to be spontaneous; however, a recent study found AP2 assembly is a highly orchestrated process controlled by alpha and gamma adaptin binding protein (AAGAB). Evidence shows that AAGAB controls AP1 assembly in a similar way. Insights into the orchestrated assembly process and three-dimensional structures of assembly intermediates are only emerging. Here, we describe a protocol for reconstitution and purification of the complexes containing AAGAB and AP1 or AP2 subunits, known as AP1 and AP2 hemicomplexes. Our purification routinely yields milligrams of pure complexes suitable for structural analysis by X-ray crystallography and electron microscopy.

OtDUB from the Human Pathogen Orientia tsutsugamushi Modulates Host Membrane Trafficking by Multiple Mechanisms.

Host cell membrane-trafficking pathways are often manipulated by bacterial pathogens to gain cell entry, avoid immune responses, or to obtain nutrients. The 1,369-residue OtDUB protein from the obligate intracellular human pathogen Orientia tsutsugamushi bears a deubiquitylase (DUB) and additional domains. Here we show that OtDUB ectopic expression disrupts membrane trafficking through multiple mechanisms. OtDUB binds directly to the clathrin adaptor-protein (AP) complexes AP-1 and AP-2, and the OtDUB275-675 fragment is sufficient for binding to either complex. To assess the impact of OtDUB interactions with AP-1 and AP-2, we examined trans-Golgi trafficking and endocytosis, respectively. Endocytosis is reduced by two separate OtDUB fragments: one contains the AP-binding domain (OtDUB1-675), and the other does not (OtDUB675-1369). OtDUB1-675 disruption of endocytosis requires its ubiquitin-binding capabilities. OtDUB675-1369 also fragments trans- and cis-Golgi structures. Using a growth-based selection in yeast, we identified viable OtDUB675-1369 point mutants that also no longer caused Golgi defects in human cells. In parallel, we found OtDUB675-1369 binds directly to phosphatidylserine, and this lipid binding is lost in the same mutants. Together these results show that OtDUB contains multiple activities capable of modulating membrane trafficking. We discuss how these activities may contribute to Orientia infections.

NRF1-mediated microglial activation triggers high-altitude cerebral edema.

High-altitude cerebral edema (HACE) is a potentially fatal encephalopathy associated with a time-dependent exposure to the hypobaric hypoxia of altitude. The formation of HACE is affected by both vasogenic and cytotoxic edema. The over-activated microglia potentiate the damage of blood-brain barrier (BBB) and exacerbate cytotoxic edema. In light with the activation of microglia in HACE, we aimed to investigate whether the over-activated microglia were the key turning point of acute mountain sickness to HACE. In in vivo experiments, by exposing mice to hypobaric hypoxia (7000 m above sea level) to induce HACE model, we found that microglia were activated and migrated to blood vessels. Microglia depletion by PLX5622 obviously relieved brain edema. In in vitro experiments, we found that hypoxia induced cultured microglial activation, leading to the destruction of endothelial tight junction and astrocyte swelling. Up-regulated nuclear respiratory factor 1 (NRF1) accelerated pro-inflammatory factors through transcriptional regulation on nuclear factor kappa B p65 (NF-κB p65) and mitochondrial transcription factor A (TFAM) in activated microglia under hypoxia. NRF1 also up-regulated phagocytosis by transcriptional regulation on caveolin-1 (CAV-1) and adaptor-related protein complex 2 subunit beta (AP2B1). The present study reveals a new mechanism in HACE: hypoxia over-activates microglia through up-regulation of NRF1, which both induces inflammatory response through transcriptionally activating NF-κB p65 and TFAM, and enhances phagocytic function through up-regulation of CAV-1 and AP2B1; hypoxia-activated microglia destroy the integrity of BBB and release pro-inflammatory factors that eventually induce HACE.

Induced nanoscale membrane curvature bypasses the essential endocytic function of clathrin.

During clathrin-mediated endocytosis (CME), flat plasma membrane is remodeled to produce nanometer-scale vesicles. The mechanisms underlying this remodeling are not completely understood. The ability of clathrin to bind membranes of distinct geometries casts uncertainty on its specific role in curvature generation/stabilization. Here, we used nanopatterning to produce substrates for live-cell imaging, with U-shaped features that bend the ventral plasma membrane of a cell into shapes resembling energetically unfavorable CME intermediates. This induced membrane curvature recruits CME proteins, promoting endocytosis. Upon AP2, FCHo1/2, or clathrin knockdown, CME on flat substrates is severely diminished. However, induced membrane curvature recruits CME proteins in the absence of FCHo1/2 or clathrin and rescues CME dynamics/cargo uptake after clathrin (but not AP2 or FCHo1/2) knockdown. Induced membrane curvature enhances CME protein recruitment upon branched actin assembly inhibition under elevated membrane tension. These data establish that membrane curvature assists in CME nucleation and that the essential function of clathrin during CME is to facilitate curvature evolution, rather than scaffold protein recruitment.

Structural basis of an endocytic checkpoint that primes the AP2 clathrin adaptor for cargo internalization.

Clathrin-mediated endocytosis (CME) is the main route of internalization from the plasma membrane. It is known that the heterotetrameric AP2 clathrin adaptor must open to simultaneously engage membrane and endocytic cargo, yet it is unclear how transmembrane cargos are captured to catalyze CME. Using cryogenic-electron microscopy, we discover a new way in which mouse AP2 can reorganize to expose membrane- and cargo-binding pockets, which is not observed in clathrin-coated structures. Instead, it is stimulated by endocytic pioneer proteins called muniscins, which do not enter vesicles. Muniscin-engaged AP2 is primed to rearrange into the vesicle-competent conformation on binding the tyrosine cargo internalization motif (YxxΦ). We propose adaptor priming as a checkpoint to ensure cargo internalization.

Clathrin-independent endocytic retrieval of SV proteins mediated by the clathrin adaptor AP-2 at mammalian central synapses.

Neurotransmission is based on the exocytic fusion of synaptic vesicles (SVs) followed by endocytic membrane retrieval and the reformation of SVs. Conflicting models have been proposed regarding the mechanisms of SV endocytosis, most notably clathrin/adaptor protein complex 2 (AP-2)-mediated endocytosis and clathrin-independent ultrafast endocytosis. Partitioning between these pathways has been suggested to be controlled by temperature and stimulus paradigm. We report on the comprehensive survey of six major SV proteins to show that SV endocytosis in mouse hippocampal neurons at physiological temperature occurs independent of clathrin while the endocytic retrieval of a subset of SV proteins including the vesicular transporters for glutamate and GABA depend on sorting by the clathrin adaptor AP-2. Our findings highlight a clathrin-independent role of the clathrin adaptor AP-2 in the endocytic retrieval of select SV cargos from the presynaptic cell surface and suggest a revised model for the endocytosis of SV membranes at mammalian central synapses.

AP2M1 mediates autophagy-induced CLDN2 (claudin 2) degradation through endocytosis and interaction with LC3 and reduces intestinal epithelial tight junction permeability.

The intestinal epithelial tight junctions (TJs) provide barrier against paracellular permeation of lumenal antigens. Defects in TJ barrier such as increased levels of pore-forming TJ protein CLDN2 (claudin-2) is associated with inflammatory bowel disease. We have previously reported that starvation-induced macroautophagy/autophagy enhances the TJ barrier by degrading pore-forming CLDN2. In this study, we examined the molecular mechanism underlying autophagy-induced CLDN2 degradation. CLDN2 degradation was persistent in multiple modes of autophagy induction. Immunolocalization, membrane fractionation, and pharmacological inhibition studies showed increased clathrin-mediated CLDN2 endocytosis upon starvation. Inhibition of clathrin-mediated endocytosis negated autophagy-induced CLDN2 degradation and enhancement of the TJ barrier. The co-immunoprecipitation studies showed increased association of CLDN2 with clathrin and adaptor protein AP2 (AP2A1 and AP2M1 subunits) as well as LC3 and lysosomes upon starvation, signifying the role of clathrin-mediated endocytosis in autophagy-induced CLDN2 degradation. The expression and phosphorylation of AP2M1 was increased upon starvation. In-vitro, in-vivo (mouse colon), and ex-vivo (human colon) inhibition of AP2M1 activation prevented CLDN2 degradation. AP2M1 knockout prevented autophagy-induced CLDN2 degradation via reduced CLDN2-LC3 interaction. Site-directed mutagenesis revealed that AP2M1 binds to CLDN2 tyrosine motifs (YXXФ) (67-70 and 148-151). Increased baseline expression of CLDN2 and TJ permeability along with reduced CLDN2-AP2M1-LC3 interactions in ATG7 knockout cells validated the role of autophagy in modulation of CLDN2 levels. Acute deletion of Atg7 in mice increased CLDN2 levels and the susceptibility to experimental colitis. The autophagy-regulated molecular mechanisms linking CLDN2, AP2M1, and LC3 may provide therapeutic tools against intestinal inflammation.Abbreviations: Amil: amiloride; AP2: adaptor protein complex 2; AP2A1: adaptor related protein complex 2 subunit alpha 1; AP2M1: adaptor related protein complex 2 subunit mu 1; ATG7: autophagy related 7; CAL: calcitriol; Cas9: CRISPR-associated protein 9; Con: control; CPZ: chlorpromazine; DSS: dextran sodium sulfate; EBSS: Earle's balanced salt solution; IBD: inflammatory bowel disease; TER: trans-epithelial resistance; KD: knockdown; KO: knockout; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MßCD: Methyl-ß-cyclodextrin; MET: metformin; MG132: carbobenzoxy-Leu-Leu-leucinal; MTOR: mechanistic target of rapamycin kinase; NT: non target; RAPA: rapamycin; RES: resveratrol; SMER: small-molecule enhancer 28; SQSTM1: sequestosome 1; ST: starvation; ULK1: unc-51 like autophagy activating kinase 1; WT: wild type.

De novo endocytic clathrin coats develop curvature at early stages of their formation.

Sculpting a flat patch of membrane into an endocytic vesicle requires curvature generation on the cell surface, which is the primary function of the endocytosis machinery. Using super-resolved live cell fluorescence imaging, we demonstrate that curvature generation by individual clathrin-coated pits can be detected in real time within cultured cells and tissues of developing organisms. Our analyses demonstrate that the footprint of clathrin coats increases monotonically during the formation of pits at different levels of plasma membrane tension. These findings are only compatible with models that predict curvature generation at the early stages of endocytic clathrin pit formation. We also found that CALM adaptors associated with clathrin plaques form clusters, whereas AP2 distribution is more homogenous. Considering the curvature sensing and driving roles of CALM, we propose that CALM clusters may increase the strain on clathrin lattices locally, eventually giving rise to rupture and subsequent pit completion at the edges of plaques.