The phospholipase A₂ enzyme complex PAFAH Ib mediates endosomal membrane tubule formation and trafficking

Previous studies have shown that membrane tubule-mediated export from endosomal compartments requires a cytoplasmic phospholipase A(2) (PLA(2)) activity. Here we report that the cytoplasmic PLA(2) enzyme complex platelet-activating factor acetylhydrolase (PAFAH) Ib, which consists of α1, α2, and LIS1 subunits, regulates the distribution and function of endosomes. The catalytic subunits α1 and α2 are located on early-sorting endosomes and the central endocytic recycling compartment (ERC) and their overexpression, but not overexpression of their catalytically inactive counterparts, induced endosome membrane tubules. In addition, overexpression α1 and α2 altered normal endocytic trafficking; transferrin was recycled back to the plasma membrane directly from peripheral early-sorting endosomes instead of making an intermediate stop in the ERC. Consistent with these results, small interfering RNA-mediated knockdown of α1 and α2 significantly inhibited the formation of endosome membrane tubules and delayed the recycling of transferrin. In addition, the results agree with previous reports that PAFAH Ib α1 and α2 expression levels affect the distribution of endosomes within the cell through interactions with the dynein regulator LIS1. These studies show that PAFAH Ib regulates endocytic membrane trafficking through novel mechanisms involving both PLA(2) activity and LIS1-dependent dynein function.     

Influenza Virus Induces Cholesterol-Enriched Endocytic Recycling Compartments for Budozone Formation via Cell Cycle-Independent Centrosome Maturation

Influenza virus particles are assembled at the plasma membrane in concert with incorporation of the virus genome, but the details of its spatio-temporal regulation are not understood. Here we showed that influenza virus infection induces the assembly of pericentrosomal endocytic recycling compartment (ERC) through the activation of Rab11a GTPase and cell cycle-independent maturation of centrosome by YB-1, a multifunctional protein that is involved in mitotic division, RNA metabolism and tumorigenesis. YB-1 is recruited to the centrosome in infected cells and is required for anchoring microtubules to the centrosome. We also found that viral infection accumulates cholesterol in ERC and is dependent on YB-1. Depletion of YB-1 shows reduced cholesterol-enriched ERC and prevented budozone formation at the plasma membrane. These results suggest that cholesterol in recycling endosomes, which are emanated from ERC, may trigger the virus assembly concomitantly with the packaging of the virus genome. We propose that the virus genome is transported to the plasma membrane by cholesterol-enriched recycling endosomes through cell cycle-independent activation of the centrosome by YB-1.     

ARF-GEF cytohesin-2/ARNO regulates R-Ras and α5-integrin recycling through an EHD1-positive compartment

When expressed in epithelial cells, cytohesin-2/ARNO, a guanine nucleotide exchange factor (GEF) for ARF small GTPases, causes a robust migration response. Recent evidence suggests that cytohesin-2/ARNO acts downstream of small the GTPase R-Ras to promote spreading and migration. We hypothesized that cytohesin-2/ARNO could transmit R-Ras signals by regulating the recycling of R-Ras through ARF activation. We found that Eps15-homology domain 1 (EHD1), a protein that associates with the endocytic recycling compartment (ERC), colocalizes with active R-Ras in transiently expressed HeLa cells. In addition, we show that EHD1-positive recycling endosomes are a novel compartment for cytohesin-2/ARNO. Knockdown or expression of GEF-inactive (E156K) cytohesin-2/ARNO causes R-Ras to accumulate on recycling endosomes containing EHD1 and inhibits cell spreading. E156K-ARNO also causes a reduction in focal adhesion size and number. Finally, we demonstrate that R-Ras/ARNO signaling is required for recycling of α5-integrin and R-Ras to the plasma membrane. These data establish a role for cytohesin-2/ARNO as a regulator of R-Ras and integrin recycling and suggest that ARF-regulated trafficking of R-Ras is required for R-Ras–dependent effects on spreading and adhesion formation.     

PKCzeta is required for microtubule-based motility of vesicles containing the ntcp transporter

Intracellular trafficking regulates the abundance and therefore activity of transporters present at the plasma membrane. The transporter, Na+-taurocholate co-transporting polypeptide (ntcp), is increased at the plasma membrane upon treatment of cells with cAMP, for which microtubules (MTs) are required and the PI3K pathway and PKCzeta have been implicated. However, trafficking of ntcp on MTs has not been demonstrated directly and the regulation and intracellular localization of ntcp is not well understood. Here, we utilize in vitro and whole-cell immunofluorescence microscopy assays to demonstrate that ntcp is present on intracellular vesicles that bind MTs and move bidirectionally, using kinesin-1 and dynein. These vesicles co-localize with markers for recycling endosomes and early but not late endosomes. They frequently undergo fission, providing a mechanism for the exclusion of ntcp from late endosomes. PI(3,4,5)P3 activates PKCzeta and enhances motility of the ntcp vesicles and overcomes the partial inhibition produced by a PI3-kinase inhibitor. Specific inhibition of PKCzeta blocks the motility of ntcp-containing vesicles but has no effect on late vesicles as shown both in vitro and in living cells transfected with ntcp-GFP. These data indicate that PKCzeta is required specifically for the intracellular movement of vesicles that contain the ntcp transporter.     

G protein-coupled receptor-promoted trafficking of Gbeta1gamma2 leads to AKT activation at endosomes via a mechanism mediated by Gbeta1gamma2-Rab11a interaction

G-protein coupled receptors activate heterotrimeric G proteins at the plasma membrane in which most of their effectors are intrinsically located or transiently associated as the external signal is being transduced. This paradigm has been extended to the intracellular compartments by studies in yeast showing that trafficking of Gα activates phosphatidylinositol 3-kinase (PI3K) at endosomal compartments, suggesting that vesicle trafficking regulates potential actions of Gα and possibly Gβγ at the level of endosomes. Here, we show that Gβγ interacts with Rab11a and that the two proteins colocalize at early and recycling endosomes in response to activation of lysophosphatidic acid (LPA) receptors. This agonist-dependent association of Gβγ to Rab11a-positive endosomes contributes to the recruitment of PI3K and phosphorylation of AKT at this intracellular compartment. These events are sensitive to the expression of a dominant-negative Rab11a mutant or treatment with wortmannin, suggesting that Rab11a-dependent Gβγ trafficking promotes the activation of the PI3K/AKT signaling pathway associated with endosomal compartments. In addition, RNA interference-mediated Rab11a depletion, or expression of a dominant-negative Rab11a mutant attenuated LPA-dependent cell survival and proliferation, suggesting that endosomal activation of the PI3K/AKT signaling pathway in response to Gβγ trafficking, via its interaction with Rab11, is a relevant step in the mechanism controlling these fundamental events.     

Essential Role of Class II Phosphatidylinositol-3-kinase-C2α in Sphingosine 1-Phosphate Receptor-1-mediated Signaling and Migration in Endothelial Cells

The phosphatidylinositol (PtdIns) 3-kinase (PI3K) family regulates diverse cellular processes, including cell proliferation, migration, and vesicular trafficking, through catalyzing 3′-phosphorylation of phosphoinositides. In contrast to class I PI3Ks, including p110α and p110β, functional roles of class II PI3Ks, comprising PI3K-C2α, PI3K-C2β, and PI3K-C2γ, are little understood. The lysophospholipid mediator sphingosine 1-phosphate (S1P) plays the important roles in regulating vascular functions, including vascular formation and barrier integrity, via the G-protein-coupled receptors S1P_1–3. We studied the roles of PI3K-C2α in S1P-induced endothelial cell (EC) migration and tube formation. S1P stimulated cell migration and activation of Akt, ERK, and Rac1, the latter of which acts as a signaling molecule essential for cell migration and tube formation, via S1P₁ in ECs. Knockdown of either PI3K-C2α or class I p110β markedly inhibited S1P-induced migration, lamellipodium formation, and tube formation, whereas that of p110α or Vps34 did not. Only p110β was necessary for S1P-iduced Akt activation, but both PI3K-C2α and p110β were required for Rac1 activation. FRET imaging showed that S1P induced Rac1 activation in both the plasma membrane and PtdIns 3-phosphate (PtdIns(3)P)-enriched endosomes. Knockdown of PI3K-C2α but not p110β markedly reduced PtdIns(3)P-enriched endosomes and suppressed endosomal Rac1 activation. Also, knockdown of PI3K-C2α but not p110β suppressed S1P-induced S1P₁ internalization into PtdIns(3)P-enriched endosomes. Finally, pharmacological inhibition of endocytosis suppressed S1P-induced S1P₁ internalization, Rac1 activation, migration, and tube formation. These observations indicate that PI3K-C2α plays the crucial role in S1P₁ internalization into the intracellular vesicular compartment, Rac1 activation on endosomes, and thereby migration through regulating vesicular trafficking in ECs.     

Hepatocyte growth factor-regulated tyrosine kinase substrate (HRS) mediates post-endocytic trafficking of protease-activated receptor 2 and calcitonin receptor-like receptor

The E3 ligase c-Cbl ubiquitinates protease-activated receptor 2 (PAR(2)), which is required for post-endocytic sorting of PAR(2) to lysosomes, where degradation arrests signaling. The mechanisms of post-endocytic sorting of ubiquitinated receptors are incompletely understood. Here, we investigated the role of hepatocyte growth factor-regulated tyrosine kinase substrate (HRS), in post-endocytic sorting and signaling of PAR(2). In HEK-PAR(2) cells, PAR(2) activating peptide (PAR(2)-AP) induced PAR(2) trafficking from the cell surface to early endosomes containing endogenous HRS, and then to lysosomes. HRS overexpression or knockdown with small interfering RNA caused formation of enlarged HRS-positive endosomes, where activated PAR(2) and c-Cbl accumulated, and PAR(2) failed to traffic to lysosomes. Overexpression of HRS prevented PAR(2)-AP-induced degradation of PAR(2), as determined by Western blotting. Overexpression of HRS mutant lacking an ubiquitin-binding motif similarly caused retention of PAR(2) in enlarged endosomes. Moreover, HRS overexpression or knockdown caused retention of ubiquitin-resistant PAR(2)Delta14K/R in enlarged HRS-containing endosomes, preventing recycling and resensitization of PAR(2)Delta14K/R. HRS overexpression or knockdown similarly prevented lysosomal trafficking and recycling of calcitonin receptor-like receptor, a non-ubiquitinated receptor that traffics to lysosomes after sustained activation and recycles after transient activation. Thus, HRS plays a critically important role in the post-endocytic sorting of single receptors, PAR(2) and CLR, to both degradative and recycling pathways. This sorting role for HRS is independent of its ubiquitin-interacting motif, and it can regulate trafficking of both ubiquitinated and non-ubiquitinated PAR(2) and non-ubiquitinated CLR. The ultimate sorting decision to degradative or recycling pathways appears to occur downstream from HRS.     

Functional symmetry of endomembranes

In higher eukaryotic cells pleiomorphic compartments composed of vacuoles, tubules and vesicles move from the endoplasmic reticulum (ER) and the plasma membrane to the cell center, operating in early biosynthetic trafficking and endocytosis, respectively. Besides transporting cargo to the Golgi apparatus and lysosomes, a major task of these compartments is to promote extensive membrane recycling. The endocytic membrane system is traditionally divided into early (sorting) endosomes, late endosomes and the endocytic recycling compartment (ERC). Recent studies on the intermediate compartment (IC) between the ER and the Golgi apparatus suggest that it also consists of peripheral ("early") and centralized ("late") structures, as well as a third component, designated here as the biosynthetic recycling compartment (BRC). We propose that the ERC and the BRC exist as long-lived "mirror compartments" at the cell center that also share the ability to expand and become mobilized during cell activation. These considerations emphasize the functional symmetry of endomembrane compartments, which provides a basis for the membrane rearrangements taking place during cell division, polarization, and differentiation.     

Endosomal cargo recycling mediated by Gpa1 and phosphatidylinositol 3-kinase is inhibited by glucose starvation

Cell surface protein trafficking is regulated in response to nutrient availability, with multiple pathways directing surface membrane proteins to the lysosome for degradation in response to suboptimal extracellular nutrients. Internalized protein and lipid cargoes recycle back to the surface efficiently in glucose-replete conditions, but this trafficking is attenuated following glucose starvation. We find that cells with either reduced or hyperactive phosphatidylinositol 3-kinase (PI3K) activity are defective for endosome to surface recycling. Furthermore, we find that the yeast Gα subunit Gpa1, an endosomal PI3K effector, is required for surface recycling of cargoes. Following glucose starvation, mRNA and protein levels of a distinct Gα subunit Gpa2 are elevated following nuclear translocation of Mig1, which inhibits recycling of various cargoes. As Gpa1 and Gpa2 interact at the surface where Gpa2 concentrates during glucose starvation, we propose that this disrupts PI3K activity required for recycling, potentially diverting Gpa1 to the surface and interfering with its endosomal role in recycling. In support of this model, glucose starvation and overexpression of Gpa2 alter PI3K endosomal phosphoinositide production. Glucose deprivation therefore triggers a survival mechanism to increase retention of surface cargoes in endosomes and promote their lysosomal degradation.     

ORP2 couples LDL‐cholesterol transport to FAK activation by endosomal cholesterol/PI(4,5)P2 exchange

Low‐density lipoprotein (LDL)‐cholesterol delivery from late endosomes to the plasma membrane regulates focal adhesion dynamics and cell migration, but the mechanisms controlling it are poorly characterized. Here, we employed auxin‐inducible rapid degradation of oxysterol‐binding protein‐related protein 2 (ORP2/OSBPL2) to show that endogenous ORP2 mediates the transfer of LDL‐derived cholesterol from late endosomes to focal adhesion kinase (FAK)‐/integrin‐positive recycling endosomes in human cells. In vitro, cholesterol enhances membrane association of FAK to PI(4,5)P₂‐containing lipid bilayers. In cells, ORP2 stimulates FAK activation and PI(4,5)P₂ generation in endomembranes, enhancing cell adhesion. Moreover, ORP2 increases PI(4,5)P₂ in NPC1‐containing late endosomes in a FAK‐dependent manner, controlling their tubulovesicular trafficking. Together, these results provide evidence that ORP2 controls FAK activation and LDL‐cholesterol plasma membrane delivery by promoting bidirectional cholesterol/PI(4,5)P₂ exchange between late and recycling endosomes.     

Pre-sorting endosomal transport of the GPI-anchored protein, CD59, is regulated by EHD1

EHD1 regulates the trafficking of multiple receptors from the endocytic recycling compartment (ERC) to the plasma membrane. However, the potential role of EHD1 in regulating the family of glycosylphosphatidylinositol-anchored proteins (GPI-APs) has not been determined. Here we demonstrate a novel role for EHD1 in regulating the trafficking of CD59, an endogenous GPI-AP, at early stages of trafficking through the endocytic pathway. EHD1 displays significant colocalization with newly internalized CD59. Upon EHD1 depletion, there is a rapid Rab5-independent coalescence of CD59 in the ERC region. However, expression of an active Arf6 mutant (Q67L), which traps internalized pre-sorting endosomal cargo in phosphatidylinositol(4,5)-bisphosphate enriched vacuoles, prevents this coalescence. It is of interest that sustained PKC activation leads to a similar coalescence of CD59 at the ERC, and treatment of EHD1-depleted cells with a PKC inhibitor (Go6976) blocked this rapid relocation of CD59. However, unlike sustained PKC activation, EHD1 depletion does not induce the translocation of PKCα to ERC. The results presented herein provide evidence that EHD1 is involved in the control of CD59 transport from pre-sorting endosomes to the ERC in a PKC-dependent manner. However, the mechanisms of EHD1-induced coalescence of CD59 at the ERC differ from those induced by sustained PKC activation.     

Plasticity-induced growth of dendritic spines by exocytic trafficking from recycling endosomes

Dendritic spines are micron-sized membrane protrusions receiving most excitatory synaptic inputs in the mammalian brain. Spines form and grow during long-term potentiation (LTP) of synaptic strength. However, the source of membrane for spine formation and enlargement is unknown. Here we report that membrane trafficking from recycling endosomes is required for the growth and maintenance of spines. Using live-cell imaging and serial section electron microscopy, we demonstrate that LTP-inducing stimuli promote the mobilization of recycling endosomes and vesicles into spines. Preventing recycling endosomal transport abolishes LTP-induced spine formation. Using a pH-sensitive recycling cargo, we show that exocytosis from recycling endosomes occurs locally in spines, is triggered by activation of synaptic NMDA receptors, and occurs concurrently with spine enlargement. Thus, recycling endosomes provide membrane for activity-dependent spine growth and remodeling, defining a novel membrane trafficking mechanism for spine morphological plasticity and providing a mechanistic link between structural and functional plasticity during LTP.     

Kinetic imaging of NPC1L1 and sterol trafficking between plasma membrane and recycling endosomes in hepatoma cells

Niemann-Pick C1-like 1 (NPC1L1) is a recently identified protein that mediates intestinal cholesterol absorption and regulates biliary cholesterol excretion. The itineraries and kinetics of NPC1L1 trafficking remain uncertain. In this study, we have visualized movement of NPC1L1-enhanced green fluorescent protein (NPC1L1-EGFP) and cholesterol analogs in hepatoma cells. At steady state, about 42% of NPC1L1 resided in the transferrin (Tf)-positive, sterol-enriched endocytic recycling compartment (ERC), whereas time-lapse microscopy demonstrated NPC1L1 traffic between the plasma membrane and the ERC. Fluorescence recovery after photobleaching revealed rapid recovery (half-time approximately 2.5 min) of about 35% of NPC1L1 in the ERC, probably replenished from peripheral sorting endosomes. Acute cholesterol depletion blocked internalization of NPC1L1-EGFP and Tf and stimulated recycling of NPC1L1-EGFP from the ERC to the plasma membrane. NPC1L1-EGFP facilitated transport of fluorescent sterols from the plasma membrane to the ERC. Insulin induced translocation of vesicles containing NPC1L1 and fluorescent sterol from the ERC to the cell membrane. Upon polarization of hepatoma cells, NPC1L1 resided almost exclusively in the canalicular membrane, where the protein is highly mobile. Our study demonstrates dynamic trafficking of NPC1L1 between the cell surface and intracellular compartments and suggests that this transport is involved in NPC1L1-mediated cellular sterol uptake.     

Spatial and temporal regulation of 3-phosphoinositides by PI 3-kinase and PTEN mediates chemotaxis

We have investigated the mechanisms of leading edge formation in chemotaxing Dictyostelium cells. We demonstrate that while phosphatidylinositol 3-kinase (PI3K) transiently translocates to the plasma membrane in response to chemoattractant stimulation and to the leading edge in chemotaxing cells, PTEN, a negative regulator of PI3K pathways, exhibits a reciprocal pattern of localization. By uniformly localizing PI3K along the plasma membrane, we show that chemotaxis pathways are activated along the lateral sides of cells and PI3K can initiate pseudopod formation, providing evidence for a direct instructional role of PI3K in leading edge formation. These findings provide evidence that differential subcellular localization and activation of PI3K and PTEN is required for proper chemotaxis.     

Endosomal acidification by Na+/H+ exchanger NHE5 regulates TrkA cell-surface targeting and NGF-induced PI3K signaling

To facilitate polarized vesicular trafficking and signal transduction, neuronal endosomes have evolved sophisticated mechanisms for pH homeostasis. NHE5 is a member of the Na⁺/H⁺ exchanger family and is abundantly expressed in neurons and associates with recycling endosomes. Here we show that NHE5 potently acidifies recycling endosomes in PC12 cells. NHE5 depletion by plasmid-based short hairpin RNA significantly reduces cell surface abundance of TrkA, an effect similar to that observed after treatment with the V-ATPase inhibitor bafilomycin. A series of cell-surface biotinylation experiments suggests that anterograde trafficking of TrkA from recycling endosomes to plasma membrane is the likeliest target affected by NHE5 depletion. NHE5 knockdown reduces phosphorylation of Akt and Erk1/2 and impairs neurite outgrowth in response to nerve growth factor (NGF) treatment. Of interest, although both phosphoinositide 3-kinase–Akt and Erk signaling are activated by NGF-TrkA, NGF-induced Akt-phosphorylation appears to be more sensitively affected by perturbed endosomal pH. Furthermore, NHE5 depletion in rat cortical neurons in primary culture also inhibits neurite formation. These results collectively suggest that endosomal pH modulates trafficking of Trk-family receptor tyrosine kinases, neurotrophin signaling, and possibly neuronal differentiation.     

Sorting nexin 27 regulates basal and stimulated brush border trafficking of NHE3

Sorting nexin 27 (SNX27) contains a PDZ domain that is phylogenetically related to the PDZ domains of the NHERF proteins. Studies on nonepithelial cells have shown that this protein is located in endosomes, where it regulates trafficking of cargo proteins in a PDZ domain–dependent manner. However, the role of SNX27 in trafficking of cargo proteins in epithelial cells has not been adequately explored. Here we show that SNX27 directly interacts with NHE3 (C-terminus) primarily through the SNX27 PDZ domain. A combination of knockdown and reconstitution experiments with wild type and a PDZ domain mutant (GYGF → GAGA) of SNX27 demonstrate that the PDZ domain of SNX27 is required to maintain basal NHE3 activity and surface expression of NHE3 in polarized epithelial cells. Biotinylation-based recycling and degradation studies in intestinal epithelial cells show that SNX27 is required for the exocytosis (not endocytosis) of NHE3 from early endosome to plasma membrane. SNX27 is also required to regulate the retention of NHE3 on the plasma membrane. The findings of the present study extend our understanding of PDZ-mediated recycling of cargo proteins from endosome to plasma membrane in epithelial cells.     

Convergence of non-clathrin- and clathrin-derived endosomes involves Arf6 inactivation and changes in phosphoinositides

The trafficking of two plasma membrane (PM) proteins that lack clathrin internalization sequences, major histocompatibility complex class I (MHCI), and interleukin 2 receptor alpha subunit (Tac) was compared with that of PM proteins internalized via clathrin. MHCI and Tac were internalized into endosomes that were distinct from those containing clathrin cargo. At later times, a fraction of these internalized membranes were observed in Arf6-associated, tubular recycling endosomes whereas another fraction acquired early endosomal autoantigen 1 (EEA1) before fusion with the "classical" early endosomes containing the clathrin-dependent cargo, LDL. After convergence, cargo molecules from both pathways eventually arrived, in a Rab7-dependent manner, at late endosomes and were degraded. Expression of a constitutively active mutant of Arf6, Q67L, caused MHCI and Tac to accumulate in enlarged PIP(2)-enriched vacuoles, devoid of EEA1 and inhibited their fusion with clathrin cargo-containing endosomes and hence blocked degradation. By contrast, trafficking and degradation of clathrin-cargo was not affected. A similar block in transport of MHCI and Tac was reversibly induced by a PI3-kinase inhibitor, implying that inactivation of Arf6 and acquisition of PI3P are required for convergence of endosomes arising from these two pathways.     

Arf6 and microtubules in adhesion-dependent trafficking of lipid rafts

Integrin-mediated adhesion regulates membrane binding sites for Rac1 within lipid rafts. Detachment of cells from the substratum triggers the clearance of rafts from the plasma membrane through caveolin-dependent internalization. The small GTPase Arf6 and microtubules also regulate Rac-dependent cell spreading and migration, but the mechanisms are poorly understood. Here we show that endocytosis of rafts after detachment requires F-actin, followed by microtubule-dependent trafficking to recycling endosomes. When cells are replated on fibronectin, rafts exit from recycling endosomes in an Arf6-dependent manner and return to the plasma membrane along microtubules. Both of these steps are required for the plasma membrane targeting of Rac1 and for its activation. These data therefore define a new membrane raft trafficking pathway that is crucial for anchorage-dependent signalling.     

The dileucine motif within the tail of MPR46 is required for sorting of the receptor in endosomes

The cytoplasmic tail of MPR46 carries a leucine-based motif that is required for the sorting of lysosomal enzymes by the receptor. In addition, it is one of three independent, but functionally redundant, internalization signals present in the cytoplasmic tail of MPR46. We have analyzed a mutant of MPR46, in which the dileucine pair was replaced by alanines (MPR46 LL/AA) with respect to its intracellular distribution and trafficking. Ultrastructural analysis of cells expressing the MPR46 LL/AA mutant revealed that the substitution of the dileucine pair causes a shift of the receptor distribution from the TGN, where it is packaged into AP1-containing vesicles, to vesicular structures distributed throughout the cytoplasm. The vesicles could be identified as early endosomes with internalized BSA-gold and rab5 as markers. By analyzing the receptor trafficking biochemically, we found that return of the LL/AA mutant receptor from the plasma membrane/endosome pool back to the TGN was impaired, while recycling from endosomes to the plasma membrane was enhanced. In conclusion, our data indicate that the dileucine motif in the MPR46 tail is required for a sorting event in endosomes.     

The endocytic recycling compartment maintains cargo segregation acquired upon exit from the sorting endosome

The endocytic recycling compartment (ERC) is a series of perinuclear tubular and vesicular membranes that regulates recycling to the plasma membrane. Despite evidence that cargo is sorted at the early/sorting endosome (SE), whether cargo mixes downstream at the ERC or remains segregated is an unanswered question. Here we use three-dimensional (3D) structured illumination microscopy and dual-channel and 3D direct stochastic optical reconstruction microscopy (dSTORM) to obtain new information about ERC morphology and cargo segregation. We show that cargo internalized either via clathrin-mediated endocytosis (CME) or independently of clathrin (CIE) remains segregated in the ERC, likely on distinct carriers. This suggests that no further sorting occurs upon cargo exit from SE. Moreover, 3D dSTORM data support a model in which some but not all ERC vesicles are tethered by contiguous “membrane bridges.” Furthermore, tubular recycling endosomes preferentially traffic CIE cargo and may originate from SE membranes. These findings support a significantly altered model for endocytic recycling in mammalian cells in which sorting occurs in peripheral endosomes and segregation is maintained at the ERC.