Multivalent Tau/PSD-95 interactions arrest in vitro condensates and clusters mimicking the postsynaptic density.

Alzheimer's disease begins with mild memory loss and slowly destroys memory and thinking. Cognitive impairment in Alzheimer's disease has been associated with the localization of the microtubule-associated protein Tau at the postsynapse. However, the correlation between Tau at the postsynapse and synaptic dysfunction remains unclear. Here, we show that Tau arrests liquid-like droplets formed by the four postsynaptic density proteins PSD-95, GKAP, Shank, Homer in solution, as well as NMDA (N-methyl-D-aspartate)-receptor-associated protein clusters on synthetic membranes. Tau-mediated condensate/cluster arrest critically depends on the binding of multiple interaction motifs of Tau to a canonical GMP-binding pocket in the guanylate kinase domain of PSD-95. We further reveal that competitive binding of a high-affinity phosphorylated peptide to PSD-95 rescues the diffusional dynamics of an NMDA truncated construct, which contains the last five amino acids of the NMDA receptor subunit NR2B fused to the C-terminus of the tetrameric GCN4 coiled-coil domain, in postsynaptic density-like condensates/clusters. Taken together, our findings propose a molecular mechanism where Tau modulates the dynamic properties of the postsynaptic density.

Local membrane source gathering by p62 body drives autophagosome formation.

Autophagosomes are double-membrane vesicles generated intracellularly to encapsulate substrates for lysosomal degradation during autophagy. Phase separated p62 body plays pivotal roles during autophagosome formation, however, the underlying mechanisms are still not fully understood. Here we describe a spatial membrane gathering mode by which p62 body functions in autophagosome formation. Mass spectrometry-based proteomics reveals significant enrichment of vesicle trafficking components within p62 body. Combining cellular experiments and biochemical reconstitution assays, we confirm the gathering of ATG9 and ATG16L1-positive vesicles around p62 body, especially in Atg2ab DKO cells with blocked lipid transfer and vesicle fusion. Interestingly, p62 body also regulates ATG9 and ATG16L vesicle trafficking flux intracellularly. We further determine the lipid contents associated with p62 body via lipidomic profiling. Moreover, with in vitro kinase assay, we uncover the functions of p62 body as a platform to assemble ULK1 complex and invigorate PI3KC3-C1 kinase cascade for PI3P generation. Collectively, our study raises a membrane-based working model for multifaceted p62 body in controlling autophagosome biogenesis, and highlights the interplay between membraneless condensates and membrane vesicles in regulating cellular functions.

Biomolecular condensates in epithelial junctions.

Epithelial junctions are transmembrane protein complexes that regulate cell adhesion, cell polarity, tissue permeability, and tissue mechanics. Most junctional complexes contain membrane attached cytoplasmic plaques that regulate junction assembly and are composed of multivalent scaffold proteins. In this review, we discuss phase separation of multivalent proteins as a general process that drives assembly of many membrane-less cellular compartments. And we summarise recent evidence that phase separation of junctional scaffold proteins is involved in the assembly of tight junctions and focal adhesions.

Myosin 1D and the branched actin network control the condensation of p62 bodies.

Biomolecular condensation driven by liquid-liquid phase separation (LLPS) is key to assembly of membraneless organelles in numerous crucial pathways. It is largely unknown how cellular structures or components spatiotemporally regulate LLPS and condensate formation. Here we reveal that cytoskeletal dynamics can control the condensation of p62 bodies comprising the autophagic adaptor p62/SQSTM1 and poly-ubiquitinated cargos. Branched actin networks are associated with p62 bodies and are required for their condensation. Myosin 1D, a branched actin-associated motor protein, drives coalescence of small nanoscale p62 bodies into large micron-scale condensates along the branched actin network. Impairment of actin cytoskeletal networks compromises the condensation of p62 bodies and retards substrate degradation by autophagy in both cellular models and Myosin 1D knockout mice. Coupling of LLPS scaffold to cytoskeleton systems may represent a general mechanism by which cells exert spatiotemporal control over phase condensation processes.

Real-Time Study of Protein Phase Separation with Spatiotemporal Analysis of Single-Nanoparticle Trajectories.

Liquid-liquid phase separation (LLPS) underlies the formation mechanism of membraneless biomolecular condensates locally to perform important physiological functions such as selective autophagy, but little is known about the relationship between their dynamic structural organization and biophysical properties. Here, a dark-field microscopy based single plasmonic nanoparticle tracking (DFSPT) technique was introduced to simultaneously monitor the diffusion dynamics of multiple gold nanorod (AuNR) probes in a protein LLPS system and to quantitatively characterize the spatiotemporal heterogeneity of the LLPS condensates during their phase transformation. Based on spatially and temporally resolved analysis of the diffusional behavior of the AuNRs, structure and material properties of p62 condensates, such as the viscoelasticity, the compartmentalization, and the recruitment of protein-covered nanoparticles into the large droplet, have been observed. Moreover, the nonsmooth droplet interface, its solidification after further phase transition or maturation, and the size effect of the inner vacuoles have also been revealed. Our method can be potentially applied to in vitro investigation of different reconstituted membrane-free biomolecular condensates and in vivo study of their dynamic evolution.

GLIPR2 is a negative regulator of autophagy and the BECN1-ATG14-containing phosphatidylinositol 3-kinase complex.

A key mediator of macroautophagy/autophagy induction is the class III phosphatidylinositol 3-kinase complex I (PtdIns3K-C1) consisting of PIK3C3/VPS34, PIK3R4/VPS15, BECN1, and ATG14. Although several proteins are known to enhance or decrease PtdIns3K-C1 activity, our understanding of the molecular regulation of PtdIns3K-C1 is still incomplete. Previously, we identified a Golgi-associated protein, GLIPR2, in a screen for proteins that interact with amino acids 267-284 of BECN1, a region of BECN1 sufficient to induce autophagy when fused to a cell penetrating leader sequence. In this study, we used CRISPR-Cas9-mediated depletion of GLIPR2 in cells and mice to investigate the role of GLIPR2 in the regulation of autophagy and PtdIns3K-C1 activity. Depletion of GLIPR2 in HeLa cells increased autelophagic flux and generation of phosphatidylinositol 3-phosphate (PtdIns3P). GLIPR2 knockout resulted in less compact Golgi structures, which was also observed in autophagy-inducing conditions such as amino acid starvation or Tat-BECN1 peptide treatment. Importantly, the binding of GLIPR2 to purified PtdIns3K-C1 inhibited the in vitro lipid kinase activity of PtdIns3K-C1. Moreover, the tissues of glipr2 knockout mice had increased basal autophagic flux as well as increased recruitment of the PtdIns3P-binding protein, WIPI2. Taken together, our findings demonstrate that GLIPR2 is a negative regulator of PtdIns3K-C1 activity and basal autophagy.Abbreviations: ATG14: autophagy related 14; Baf A1: bafilomycin A1; BARA: ß-α repeated, autophagy-specific; CQ: chloroquine; GFP: green fluorescent protein; GLIPR2: GLI pathogenesis related 2; HBSS: Hanks' balanced salt solution; KO: knockout; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; PBS: phosphate-buffered saline; PtdIns3K-C1: phosphatidylinositol 3-kinase complex I; PtdIns3P: phosphatidylinositol-3-phosphate; SEM: standard error of the mean; WIPI2: WD repeat domain, phosphoinositide interacting 2.

Discovery of anti-inflammatory physiological peptides that promote tissue repair by reinforcing epithelial barrier formation.

Epithelial barriers that prevent dehydration and pathogen invasion are established by tight junctions (TJs), and their disruption leads to various inflammatory diseases and tissue destruction. However, a therapeutic strategy to overcome TJ disruption in diseases has not been established because of the lack of clinically applicable TJ-inducing molecules. Here, we found TJ-inducing peptides (JIPs) in mice and humans that corresponded to 35 to 42 residue peptides of the C terminus of alpha 1-antitrypsin (A1AT), an acute-phase anti-inflammatory protein. JIPs were inserted into the plasma membrane of epithelial cells, which promoted TJ formation by directly activating the heterotrimeric G protein G13. In a mouse intestinal epithelial injury model established by dextran sodium sulfate, mouse or human JIP administration restored TJ integrity and strongly prevented colitis. Our study has revealed TJ-inducing anti-inflammatory physiological peptides that play a critical role in tissue repair and proposes a previously unidentified therapeutic strategy for TJ-disrupted diseases.

Phase Separation in Regulation of Aggrephagy.

The selective degradation of protein aggregates is called aggrephagy. Misfolded proteins are thought to form aggregates, which are then surrounded by selective autophagy receptors and targeted to autophagosomes for degradation. Recent studies of p62 bodies, PGL granules, and stress granules indicate that proteins targeted for aggrephagy are not simple protein aggregates but rather form liquid-like protein condensates through liquid-liquid phase separation. The liquid-like properties of the condensates and hardening to a gel-like state may be crucial in the initiation of aggrephagy. Dysregulation of phase separation may cause human diseases. Here we review the potential roles of liquid-liquid phase separation in the process of aggrephagy.

Polyubiquitin chain-induced p62 phase separation drives autophagic cargo segregation.

Misfolded proteins can be degraded by selective autophagy. The prevailing view is that ubiquitin-tagged misfolded proteins are assembled into aggregates by the scaffold protein p62, and the aggregates are then engulfed and degraded by autophagosomes. Here we report that p62 forms droplets in vivo which have liquid-like properties such as high sphericity, the ability to undergo fusion, and recovery after photobleaching. Recombinant p62 does not undergo phase separation in vitro; however, adding a K63 polyubiquitin chain to p62 induces p62 phase separation, which results in enrichment of high-molecular weight ubiquitin signals in p62 droplets. Mixing recombinant p62 with cytosol from p62-/- cells also results in p62 phase separation in a polyubiquitination-dependent manner. Mechanistically, p62 phase separation is dependent on p62 polymerization, the interaction between p62 and ubiquitin, and the valence of the polyubiquitin chain. Moreover, p62 phase separation can be regulated by post-translational modifications such as phosphorylation. Finally, we demonstrate that disease-associated mutations in p62 can affect phase separation. We propose that polyubiquitin chain-induced p62 phase separation drives autophagic cargo concentration and segregation.

The general amino acid control pathway regulates mTOR and autophagy during serum/glutamine starvation.

Organisms have evolved elaborate mechanisms to adjust intracellular nutrient levels in response to fluctuating availability of exogenous nutrients. During starvation, cells can enhance amino acid uptake and synthesis through the general amino acid control (GAAC) pathway, whereas nonessential cellular contents are recycled by autophagy. How these two pathways are coordinated in response to starvation is currently unknown. Here we show that the GAAC pathway couples exogenous amino acid availability with autophagy. Starvation caused deactivation of mTOR, which then activated autophagy. In parallel, serum/glutamine starvation activated the GAAC pathway, which up-regulated amino acid transporters, leading to increased amino acid uptake. This elevated the intracellular amino acid level, which in turn reactivated mTOR and suppressed autophagy. Knockdown of activating transcription factor 4, the major transcription factor in the GAAC pathway, or of SLC7A5, a leucine transporter, caused impaired mTOR reactivation and much higher levels of autophagy. Thus, the GAAC pathway modulates autophagy by regulating amino acid uptake and mTOR reactivation during serum/glutamine starvation.