Mechanism for the selective interaction of C-terminal Eps15 homology domain proteins with specific Asn-Pro-Phe-containing partners

Cryo-electron tomography reveals structural insights into the membrane remodeling mode of dynamin-like EHD filaments

Eps15-homology domain containing proteins (EHDs) are eukaryotic, dynamin-related ATPases involved in cellular membrane trafficking. They oligomerize on membranes into filaments that induce membrane tubulation. While EHD crystal structures in open and closed conformations were previously reported, little structural information is available for the membrane-bound oligomeric form. Consequently, mechanistic insights into the membrane remodeling mechanism have remained sparse. Here, by using cryo-electron tomography and subtomogram averaging, we determined structures of nucleotide-bound EHD4 filaments on membrane tubes of various diameters at an average resolution of 7.6 Å. Assembly of EHD4 is mediated via interfaces in the G-domain and the helical domain. The oligomerized EHD4 structure resembles the closed conformation, where the tips of the helical domains protrude into the membrane. The variation in filament geometry and tube radius suggests a spontaneous filament curvature of approximately 1/70 nm^-1. Combining the available structural and functional data, we suggest a model for EHD-mediated membrane remodeling.

Coronin2A links actin-based endosomal processes to the EHD1 fission machinery

Fission of transport vesicles from endosomes is a crucial step in the recycling of lipids and receptors to the plasma membrane, but this process remains poorly understood. Although key components of the fission machinery, including the actin cytoskeleton and the ATPase Eps15 homology domain protein 1 (EHD1), have been implicated in endosomal fission, how this process is coordinately regulated is not known. We have identified the actin regulatory protein Coronin2A (CORO2A) as a novel EHD1 interaction partner. CORO2A localizes to stress fibers and actin microfilaments but also can be observed in partial overlap with EHD1 on endosomal structures. siRNA knockdown of CORO2A led to enlarged lamellae-like actin-rich protrusions, consistent with a role of other Coronin family proteins in attenuating actin-branching. Moreover, CORO2A depletion also caused a marked decrease in the internalization of clathrin-dependent cargo but had little impact on the uptake of clathrin-independent cargo, highlighting key differences in the role of branched actin for different modes of endocytosis. However, CORO2A was required for recycling of clathrin-independent cargo, and its depletion led to enlarged endosomes, supporting a role for CORO2A in the fission of endosomal vesicles. Our data support a novel role for CORO2A in coordinating endosomal fission and recycling with EHD1. [Media: see text].

Differential requirements for the Eps15 homology Domain Proteins EHD4 and EHD2 in the regulation of mammalian ciliogenesis

The endocytic protein EHD1 controls primary ciliogenesis by facilitating fusion of the ciliary vesicle and by removal of CP110 from the mother centriole. EHD3, the closest EHD1 paralog, has a similar regulatory role, but initial evidence suggested that the other two more distal paralogs, EHD2 and EHD4 may be dispensable for ciliogenesis. Herein, we define a novel role for EHD4, but not EHD2, in regulating primary ciliogenesis. To better understand the mechanisms and differential functions of the EHD proteins in ciliogenesis, we first demonstrated a requirement for EHD1 ATP‐binding to promote ciliogenesis. We then identified two sequence motifs that are entirely conserved between EH domains of EHD1, EHD3 and EHD4, but display key amino acid differences within the EHD2 EH domain. Substitution of either P446 or E470 in EHD1 with the aligning S451 or W475 residues from EHD2 was sufficient to prevent rescue of ciliogenesis in EHD1‐depleted cells upon reintroduction of EHD1. Overall, our data enhance the current understanding of the EHD paralogs in ciliogenesis, demonstrate a need for ATP‐binding and identify conserved sequences in the EH domains of EHD1, EHD3 and EHD4 that regulate EHD1 binding to proteins and its ability to rescue ciliogenesis in EHD1‐depleted cells.

circEPS15 Overexpression in Hepatocellular Carcinoma Modulates Tumor Invasion and Migration

Hepatocellular carcinoma (HCC) is one of the leading causes of cancer-related deaths worldwide. Recent evidence has shown that circular RNAs (circRNAs) play important roles in tissue development, gene transcription, signal regulation and tumorigenesis. However, whether circRNAs are involved in HCC progression and encode functional proteins remains largely unknown. In the present study, we aimed to explore the function and molecular mechanism of circRNAs in HCC. First, many circRNAs were found to be differentially expressed in HCC samples and paired adjacent normal liver tissues. The validation of dysregulated circRNAs by qRT-PCR revealed that circEPS15 expression was downregulated in HCC tissues, and the survival curves showed that low circEPS15 levels were associated with poor overall survival in HCC patients. Then, the overexpression of circEPS15 suppressed tumor cell invasion and migration by inhibiting the TJP1/CDH2/VIM signaling pathway and retarded cell cycle progression, which was confirmed by the Transwell culture system, wound healing assays, flow cytometry and western blot assays. After that, the spanning junction open reading frame in circEPS15 driven by IRES was shown to encode a novel protein, which was verified by western blotting with full-length, mutated, and truncated sequences of circEPS15 with a FLAG tag. Moreover, ceRNA analysis and qRT-PCR results suggest a possible circRNA (circEPS15)-miRNA-mRNA network in HCC. Collectively, our study reveals that endogenous circEPS15 plays a novel role in repressing HCC through the ceRNA network and encodes a functional protein.

Eps15 Homology Domain Protein 4 (EHD4) is required for Eps15 Homology Domain Protein 1 (EHD1)-mediated endosomal recruitment and fission

Upon internalization, receptors are trafficked to sorting endosomes (SE) where they undergo sorting and are then packaged into budding vesicles that undergo fission and transport within the cell. Eps15 Homology Domain Protein 1 (EHD1), the best-characterized member of the Eps15 Homology Domain Protein (EHD) family, has been implicated in catalyzing the fission process that releases endosome-derived vesicles for recycling to the plasma membrane. Indeed, recent studies suggest that upon receptor-mediated internalization, EHD1 is recruited from the cytoplasm to endosomal membranes where it catalyzes vesicular fission. However, the mechanism by which this recruitment occurs remains unknown. Herein, we demonstrate that the EHD1 paralog, EHD4, is required for the recruitment of EHD1 to SE. We show that EHD4 preferentially dimerizes with EHD1, and knock-down of EHD4 expression by siRNA, shRNA or by CRISPR/Cas9 gene-editing leads to impaired EHD1 SE-recruitment and enlarged SE. Moreover, we demonstrate that at least 3 different asparagine-proline-phenylalanine (NPF) motif-containing EHD binding partners, Rabenosyn-5, Syndapin2 and MICAL-L1, are required for the recruitment of EHD1 to SE. Indeed, knock-down of any of these SE-localized EHD interaction partners leads to enlarged SE, presumably due to impaired endosomal fission. Overall, we identify a novel mechanistic role for EHD4 in recruitment of EHD1 to SE, thus positioning EHD4 as an essential component of the EHD1-fission machinery at SE.

Structure and function of Pygo in organ development dependent and independent Wnt signalling

Pygo is a nuclear protein containing two conserved domains, NHD and PHD, which play important roles in embryonic development and carcinogenesis. Pygo was first identified as a core component of the Wnt/β-catenin signalling pathway. However, it has also been reported that the function of Pygo is not always Wnt/β-catenin signalling dependent. In this review, we summarise the functions of both domains of Pygo and show that their functions are synergetic. The PHD domain mainly combines with transcription co-factors, including histone 3 and Bcl9/9l. The NHD domain mainly recruits histone methyltransferase/acetyltransferase (HMT/HAT) to modify lysine 4 of the histone 3 tail (H3K4) and interacts with Chip/LIM-domain DNA-binding proteins (ChiLS) to form enhanceosomes to regulate transcriptional activity. Furthermore, we summarised chromatin modification differences of Pygo in Drosophila (dPygo) and vertebrates, and found that Pygo displayes a chromatin silencing function in Drosophila, while in vertebates, Pygo has a chromatin-activating function due to the two substitution of two amino acid residues. Next, we confirmed the relationship between Pygo and Bcl9/9l and found that Pygo-Bcl/9l are specifically partnered both in the nucleus and in the cytoplasm. Finally, we discuss whether transcriptional activity of Pygo is Wnt/β-catenin dependent during embryonic development. Available information indications that the transcriptional activity of Pygo in embryonic development is either Wnt/β-catenin dependent or independent in both tissue-specific and cell-specific-modes.

Sorting nexin 17 (SNX17) links endosomal sorting to Eps15 homology domain protein 1 (EHD1)-mediated fission machinery

Following endocytosis, receptors that are internalized to sorting endosomes are sorted to different pathways, in part by sorting nexin (SNX) proteins. Notably, SNX17 interacts with a multitude of receptors in a sequence-specific manner to regulate their recycling. However, the mechanisms by which SNX17-labeled vesicles that contain sorted receptors bud and undergo vesicular fission from the sorting endosomes remain elusive. Recent studies suggest that a dynamin-homolog, Eps15 homology domain protein 1, catalyzes fission and releases endosome-derived vesicles for recycling to the plasma membrane. However, the mechanism by which EHD1 is coupled to various receptors and regulates their recycling remains unknown. Here we sought to characterize the mechanism by which EHD1 couples with SNX17 to regulate recycling of SNX17-interacting receptors. We hypothesized that SNX17 couples receptors to the EHD1 fission machinery in mammalian cells. Coimmunoprecipitation experiments and in vitro assays provided evidence that EHD1 and SNX17 directly interact. We also found that inducing internalization of a SNX17 cargo receptor, low-density lipoprotein receptor-related protein 1 (LRP1), led to recruitment of cytoplasmic EHD1 to endosomal membranes. Moreover, surface rendering and quantification of overlap volumes indicated that SNX17 and EHD1 partially colocalize on endosomes and that this overlap further increases upon LRP1 internalization. Additionally, SNX17-containing endosomes were larger in EHD1-depleted cells than in WT cells, suggesting that EHD1 depletion impairs SNX17-mediated endosomal fission. Our findings help clarify our current understanding of endocytic trafficking, providing significant additional insight into the process of endosomal fission and connecting the sorting and fission machineries.

A nanobody-based molecular toolkit provides new mechanistic insight into clathrin-coat initiation

Besides AP-2 and clathrin triskelia, clathrin coat inception depends on a group of early-arriving proteins including Fcho1/2 and Eps15/R. Using genome-edited cells, we described the role of the unstructured Fcho linker in stable AP-2 membrane deposition. Here, expanding this strategy in combination with a new set of llama nanobodies against EPS15 shows an FCHO1/2–EPS15/R partnership plays a decisive role in coat initiation. A nanobody containing an Asn-Pro-Phe peptide within the complementarity-determining region 3 loop is a function-blocking pseudoligand for tandem EPS15/R EH domains. Yet, in living cells, EH domains gathered at clathrin-coated structures are poorly accessible, indicating residence by endogenous NPF-bearing partners. Forcibly sequestering cytosolic EPS15 in genome-edited cells with nanobodies tethered to early endosomes or mitochondria changes the subcellular location and availability of EPS15. This combined approach has strong effects on clathrin coat structure and function by dictating the stability of AP-2 assemblies at the plasma membrane.

The enigmatic endosome - sorting the ins and outs of endocytic trafficking

The early endosome (EE), also known as the sorting endosome (SE) is a crucial station for the sorting of cargoes, such as receptors and lipids, through the endocytic pathways. The term endosome relates to the receptacle-like nature of this organelle, to which endocytosed cargoes are funneled upon internalization from the plasma membrane. Having been delivered by the fusion of internalized vesicles with the EE or SE, cargo molecules are then sorted to a variety of endocytic pathways, including the endo-lysosomal pathway for degradation, direct or rapid recycling to the plasma membrane, and to a slower recycling pathway that involves a specialized form of endosome known as a recycling endosome (RE), often localized to the perinuclear endocytic recycling compartment (ERC). It is striking that 'the endosome', which plays such essential cellular roles, has managed to avoid a precise description, and its characteristics remain ambiguous and heterogeneous. Moreover, despite the rapid advances in scientific methodologies, including breakthroughs in light microscopy, overall, the endosome remains poorly defined. This Review will attempt to collate key characteristics of the different types of endosomes and provide a platform for discussion of this unique and fascinating collection of organelles. Moreover, under-developed, poorly understood and important open questions will be discussed.

Thioether-stapled macrocyclic inhibitors of the EH domain of EHD1

Recycling of receptors from the endosomal recycling compartment to the plasma membrane is a critical cellular process, and recycling is particularly important for maintaining invasiveness in solid tumors. In this work, we continue our efforts to inhibit EHD1, a critical adaptor protein involved in receptor recycling. We applied a diversity-oriented macrocyclization approach to produce cyclic peptides with varied conformations, but that each contain a motif that binds to the EH domain of EHD1. Screening these uncovered several new inhibitors for EHD1's EH domain, the most potent of which bound with a K_d of 3.1μM. Several of the most potent inhibitors were tested in a cellular assay that measures extent of vesicle recycling. Inhibiting EHD1 could potentially slow cancer invasiveness and metastasis, and these cyclic peptides represent the most potent inhibitors of EHD1 to date.

Molecular dynamics simulation of the interactions between EHD1 EH domain and multiple peptides

OBJECTIVE

To provide essential information for peptide inhibitor design, the interactions of Eps15 homology domain of Eps15 homology domain-containing protein 1 (EHD1 EH domain) with three peptides containing NPF (asparagine-proline-phenylalanine), DPF (aspartic acid-proline-phenylalanine), and GPF (glycine-proline-phenylalanine) motifs were deciphered at the atomic level. The binding affinities and the underlying structure basis were investigated.

METHODS

Molecular dynamics (MD) simulations were performed on EHD1 EH domain/peptide complexes for 60 ns using the GROMACS package. The binding free energies were calculated and decomposed by molecular mechanics/generalized Born surface area (MM/GBSA) method using the AMBER package. The alanine scanning was performed to evaluate the binding hot spot residues using FoldX software.

RESULTS

The different binding affinities for the three peptides were affected dominantly by van der Waals interactions. Intermolecular hydrogen bonds provide the structural basis of contributions of van der Waals interactions of the flanking residues to the binding.

CONCLUSIONS

van der Waals interactions should be the main consideration when we design peptide inhibitors of EHD1 EH domain with high affinities. The ability to form intermolecular hydrogen bonds with protein residues can be used as the factor for choosing the flanking residues.

EHD3 Protein Is Required for Tubular Recycling Endosome Stabilization, and an Asparagine-Glutamic Acid Residue Pair within Its Eps15 Homology (EH) Domain Dictates Its Selective Binding to NPF Peptides

An elaborate network of dynamic lipid membranes, termed tubular recycling endosomes (TRE), coordinates the process of endocytic recycling in mammalian cells. The C-terminal Eps15 homology domain (EHD)-containing proteins have been implicated in the bending and fission of TRE, thus regulating endocytic recycling. EHD proteins have an EH domain that interacts with proteins containing an NPF motif. We found that NPF-containing EHD1 interaction partners such as molecules interacting with CasL-like1 (MICAL-L1) and Syndapin2 are essential for TRE biogenesis. Also crucial for TRE biogenesis is the generation of phosphatidic acid, an essential lipid component of TRE that serves as a docking point for MICAL-L1 and Syndapin2. EHD1 and EHD3 have 86% amino acid identity; they homo- and heterodimerize and partially co-localize to TRE. Despite their remarkable identity, they have distinct mechanistic functions. EHD1 induces membrane vesiculation, whereas EHD3 supports TRE biogenesis and/or stabilization by an unknown mechanism. While using phospholipase D inhibitors (which block the conversion of glycerophospholipids to phosphatidic acid) to deplete cellular TRE, we observed that, upon inhibitor washout, there was a rapid and dramatic regeneration of MICAL-L1-marked TRE. Using this "synchronized" TRE biogenesis system, we determined that EHD3 is involved in the stabilization of TRE rather than in their biogenesis. Moreover, we identify the residues Ala-519/Asp-520 of EHD1 and Asn-519/Glu-520 of EHD3 as defining the selectivity of these two paralogs for NPF-containing binding partners, and we present a model to explain the atomic mechanism and provide new insight for their differential roles in vesiculation and tubulation, respectively.

The endocytic recycling regulatory protein EHD1 Is required for ocular lens development

The C-terminal Eps15 homology domain-containing (EHD) proteins play a key role in endocytic recycling, a fundamental cellular process that ensures the return of endocytosed membrane components and receptors back to the cell surface. To define the in vivo biological functions of EHD1, we have generated Ehd1 knockout mice and previously reported a requirement of EHD1 for spermatogenesis. Here, we show that approximately 56% of the Ehd1-null mice displayed gross ocular abnormalities, including anophthalmia, aphakia, microphthalmia and congenital cataracts. Histological characterization of ocular abnormalities showed pleiotropic defects that include a smaller or absent lens, persistence of lens stalk and hyaloid vasculature, and deformed optic cups. To test whether these profound ocular defects resulted from the loss of EHD1 in the lens or in non-lenticular tissues, we deleted the Ehd1 gene selectively in the presumptive lens ectoderm using Le-Cre. Conditional Ehd1 deletion in the lens resulted in developmental defects that included thin epithelial layers, small lenses and absence of corneal endothelium. Ehd1 deletion in the lens also resulted in reduced lens epithelial proliferation, survival and expression of junctional proteins E-cadherin and ZO-1. Finally, Le-Cre-mediated deletion of Ehd1 in the lens led to defects in corneal endothelial differentiation. Taken together, these data reveal a unique role for EHD1 in early lens development and suggest a previously unknown link between the endocytic recycling pathway and regulation of key developmental processes including proliferation, differentiation and morphogenesis.

An ancient Pygo-dependent Wnt enhanceosome integrated by Chip/LDB-SSDP

TCF/LEF factors are ancient context-dependent enhancer-binding proteins that are activated by β-catenin following Wnt signaling. They control embryonic development and adult stem cell compartments, and their dysregulation often causes cancer. β-catenin-dependent transcription relies on the NPF motif of Pygo proteins. Here, we use a proteomics approach to discover the Chip/LDB-SSDP (ChiLS) complex as the ligand specifically binding to NPF. ChiLS also recognizes NPF motifs in other nuclear factors including Runt/RUNX2 and Drosophila ARID1, and binds to Groucho/TLE. Studies of Wnt-responsive dTCF enhancers in the Drosophila embryonic midgut indicate how these factors interact to form the Wnt enhanceosome, primed for Wnt responses by Pygo. Together with previous evidence, our study indicates that ChiLS confers context-dependence on TCF/LEF by integrating multiple inputs from lineage and signal-responsive factors, including enhanceosome switch-off by Notch. Its pivotal function in embryos and stem cells explain why its integrity is crucial in the avoidance of cancer.

Role of the EHD2 unstructured loop in dimerization, protein binding and subcellular localization

The C-terminal Eps 15 Homology Domain proteins (EHD1-4) play important roles in regulating endocytic trafficking. EHD2 is the only family member whose crystal structure has been solved, and it contains an unstructured loop consisting of two proline-phenylalanine (PF) motifs: KPFRKLNPF. In contrast, despite EHD2 having nearly 70% amino acid identity with its paralogs, EHD1, EHD3 and EHD4, the latter proteins contain a single KPF or RPF motif, but no NPF motif. In this study, we sought to define the precise role of each PF motif in EHD2’s homo-dimerization, binding with the protein partners, and subcellular localization. To test the role of the NPF motif, we generated an EHD2 NPF-to-NAF mutant to mimic the homologous sequences of EHD1 and EHD3. We demonstrated that this mutant lost both its ability to dimerize and bind to Syndapin2. However, it continued to localize primarily to the cytosolic face of the plasma membrane. On the other hand, EHD2 NPF-to-APA mutants displayed normal dimerization and Syndapin2 binding, but exhibited markedly increased nuclear localization and reduced association with the plasma membrane. We then hypothesized that the single PF motif of EHD1 (that aligns with the KPF of EHD2) might be responsible for both binding and localization functions of EHD1. Indeed, the EHD1 RPF motif was required for dimerization, interaction with MICAL-L1 and Syndapin2, as well as localization to tubular recycling endosomes. Moreover, recycling assays demonstrated that EHD1 RPF-to-APA was incapable of supporting normal receptor recycling. Overall, our data suggest that the EHD2 NPF phenylalanine residue is crucial for EHD2 localization to the plasma membrane, whereas the proline residue is essential for EHD2 dimerization and binding. These studies support the recently proposed model in which the EHD2 N-terminal region may regulate the availability of the unstructured loop for interactions with neighboring EHD2 dimers, thus promoting oligomerization.

Endosome-based protein trafficking and Ca(2+) homeostasis in the heart

The ability to dynamically regulate, traffic, retain, and recycle proteins within the cell membrane is fundamental to life and central to the normal function of the heart. In the cardiomyocyte, these pathways are essential for the regulation of Ca(2+), both at the level of the plasma membrane, but also in local cellular domains. One intracellular pathway often overlooked in relation to cardiovascular Ca(2+) regulation and signaling is the endosome-based trafficking pathway. Highlighting its importance, this system and its molecular components are evolutionarily conserved across all metazoans. However, remarkably little is known of how endosome-based protein trafficking and recycling functions within mammalian cells systems, especially in the heart. As the endosomal system acts to regulate the expression and localization of membrane proteins central for cardiac Ca(2+) regulation, understanding the in vivo function of this system in the heart is critical. This review will focus on endosome-based protein trafficking in the heart in both health and disease with special emphasis for the role of endocytic regulatory proteins, C-terminal Eps15 homology domain-containing proteins (EHDs).

EHDs meet the retromer: Complex regulation of retrograde transport

Retrograde trafficking mediates the transport of endocytic membranes from endosomes to the trans-Golgi network (TGN). Dysregulation of these pathways can result in multiple ailments, including late-onset Alzheimer disease. One of the key retrograde transport regulators, the retromer complex, is tightly controlled by many factors, including the C-terminal Eps15 homology domain (EHD) proteins. This mini-review focuses on recent findings and discusses the regulation of the retromer complex by EHD proteins and the novel EHD1 interaction partner, Rabankyrin-5 (Rank-5).

Structured cyclic peptides that bind the EH domain of EHD1

EHD1 mediates long-loop recycling of many receptors by forming signaling complexes using its EH domain. We report the design and optimization of cyclic peptides as ligands for the EH domain of EHD1. We demonstrate that the improved affinity from cyclization allows fluorescence-based screening applications for EH domain inhibitors. The cyclic peptide is also unusually well-structured in aqueous solution, as demonstrated using nuclear magnetic resonance-based structural models. Because few EH domain inhibitors have been described, these more potent inhibitors will improve our understanding of the roles of EHD1 in the context of cancer invasion and metastasis.

GRAF1 forms a complex with MICAL-L1 and EHD1 to cooperate in tubular recycling endosome vesiculation

The biogenesis of tubular recycling endosomes (TREs) and their subsequent vesiculation after cargo-sorting has occurred, is essential for receptor and lipid recycling to the plasma membrane. Although recent studies have implicated the C-terminal Eps15 Homology Domain (EHD) protein, EHD1, as a key regulator of TRE vesiculation, additional proteins involved in this process have been largely uncharacterized. In the present study, we identify the GTPase Regulator Associated with Focal adhesion kinase-1 (GRAF1) protein in a complex with EHD1 and the TRE hub protein, Molecules Interacting with CasL-Like1 (MICAL-L1). Over-expression of GRAF1 caused vesiculation of MICAL-L1-containing TRE, whereas GRAF1-depletion led to impaired TRE vesiculation and delayed receptor recycling. Moreover, co-addition of purified EHD1 and GRAF1 in a semi-permeabilized cell vesiculation assay produced synergistic TRE vesiculation. Overall, based on our data, we suggest that in addition to its roles in clathrin-independent endocytosis, GRAF1 synergizes with EHD1 to support TRE vesiculation.

MICAL-family proteins: Complex regulators of the actin cytoskeleton

SIGNIFICANCE

The molecules interacting with CasL (MICAL) family members participate in a multitude of activities, including axonal growth cone repulsion, membrane trafficking, apoptosis, and bristle development in flies. An interesting feature of MICAL proteins is the presence of an N-terminal flavo-mono-oxygenase domain. This mono-oxygenase domain generates redox potential with which MICALs can either oxidize proteins or produce reactive oxygen species (ROS). Actin is one such protein that is affected by MICAL function, leading to dramatic cytoskeletal rearrangements. This review describes the MICAL-family members, and discusses their mechanisms of actin-binding and regulation of actin cytoskeleton organization.

RECENT ADVANCES

Recent studies show that MICALs directly induce oxidation of actin molecules, leading to actin depolymerization. ROS production by MICALs also causes oxidation of collapsin response mediator protein-2, a microtubule assembly promoter, which subsequently undergoes phosphorylation.

CRITICAL ISSUES

MICAL proteins oxidize proteins through two mechanisms: either directly by oxidizing methionine residues or indirectly via the production of ROS. It remains unclear whether MICAL proteins employ both mechanisms or whether the activity of MICAL-family proteins might vary with different substrates.

FUTURE DIRECTIONS

The identification of additional substrates oxidized by MICAL will shed new light on MICAL protein function. Additional directions include expanding studies toward the MICAL-like homologs that lack flavin adenine dinucleotide domains and oxidation activity.

Scratching the surface: actin' and other roles for the C-terminal Eps15 homology domain protein, EHD2

The C-terminal Eps15 homology domain-containing (EHD) proteins participate in multiple aspects of endocytic membrane trafficking. Of the four mammalian EHD proteins, EHD2 appears to be the most disparate, both in terms of sequence homology, and in subcellular localization/function. Since its initial description as a plasma membrane-associated protein, the precise function of EHD2 has remained enigmatic. Various reports have suggested roles for EHD2 at the plasma membrane, within the endocytic transport system, and even in the nucleus. For example, EHD2 facilitates membrane fusion/repair in muscle cells. Recently the focus has shifted to the role of EHD2 in regulating caveolae. Indeed, EHD2 is highly expressed in tissues rich in caveolae, including fat, muscle and blood vessels. This review highlights cumulative evidence linking EHD2 to actin-rich structures at the plasma membrane, where the plasma membrane-associated phospholipid phosphatidylinositol 4,5-bisphosphate controls EHD2 recruitment. Herein we examine the key pathways where EHD2 might function, and address its potential involvement in these processes.

Role of phosphatidylinositol 4,5-bisphosphate in regulating EHD2 plasma membrane localization

The four mammalian C-terminal Eps15 homology domain-containing proteins (EHD1-EHD4) play pivotal roles in endocytic membrane trafficking. While EHD1, EHD3 and EHD4 associate with intracellular tubular/vesicular membranes, EHD2 localizes to the inner leaflet of the plasma membrane. Currently, little is known about the regulation of EHD2. Thus, we sought to define the factors responsible for EHD2’s association with the plasma membrane. The subcellular localization of endogenous EHD2 was examined in HeLa cells using confocal microscopy. Although EHD partner proteins typically mediate EHD membrane recruitment, EHD2 was targeted to the plasma membrane independent of two well-characterized binding proteins, syndapin2 and EHBP1. Additionally, the EH domain of EHD2, which facilitates canonical EHD protein interactions, was not required to direct overexpressed EHD2 to the cell surface. On the other hand, several lines of evidence indicate that the plasma membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) plays a crucial role in regulating EHD2 subcellular localization. Pharmacologic perturbation of PIP2 metabolism altered PIP2 plasma membrane distribution (as assessed by confocal microscopy), and caused EHD2 to redistribute away from the plasma membrane. Furthermore, overexpressed EHD2 localized to PIP2-enriched vacuoles generated by active Arf6. Finally, we show that although cytochalasin D caused actin microfilaments to collapse, EHD2 was nevertheless maintained at the plasma membrane. Intriguingly, cytochalasin D induced relocalization of both PIP2 and EHD2 to actin aggregates, supporting a role of PIP2 in controlling EHD2 subcellular localization. Altogether, these studies emphasize the significance of membrane lipid composition for EHD2 subcellular distribution and offer new insights into the regulation of this important endocytic protein.

Cooperation of MICAL-L1, syndapin2, and phosphatidic acid in tubular recycling endosome biogenesis

MICAL-L1 and the BAR-domain protein syndapin2 bind to phosphatidic acid (PA), a novel lipid component of recycling endosomes (REs). Interactions between these proteins stabilize their association with membranes and allow nucleation of tubules by syndapin2. A new role is highlighted for PA in recycling, suggesting a mechanism for tubular RE formation.

Endocytic transport necessitates the generation of membrane tubules and their subsequent fission to transport vesicles for sorting of cargo molecules. The endocytic recycling compartment, an array of tubular and vesicular membranes decorated by the Eps15 homology domain protein, EHD1, is responsible for receptor and lipid recycling to the plasma membrane. It has been proposed that EHD dimers bind and bend membranes, thus generating recycling endosome (RE) tubules. However, recent studies show that molecules interacting with CasL-Like1 (MICAL-L1), a second, recently identified RE tubule marker, recruits EHD1 to preexisting tubules. The mechanisms and events supporting the generation of tubular recycling endosomes were unclear. Here, we propose a mechanism for the biogenesis of RE tubules. We demonstrate that MICAL-L1 and the BAR-domain protein syndapin2 bind to phosphatidic acid, which we identify as a novel lipid component of RE. Our studies demonstrate that direct interactions between these two proteins stabilize their association with membranes, allowing for nucleation of tubules by syndapin2. Indeed, the presence of phosphatidic acid in liposomes enhances the ability of syndapin2 to tubulate membranes in vitro. Overall our results highlight a new role for phosphatidic acid in endocytic recycling and provide new insights into the mechanisms by which tubular REs are generated.

A snapshot of the physical and functional wiring of the Eps15 homology domain network in the nematode

Protein interaction modules coordinate the connections within and the activity of intracellular signaling networks. The Eps15 Homology (EH) module, a protein-protein interaction domain that is a key feature of the EH-network, was originally identified in a few proteins involved in endocytosis and vesicle trafficking, and has subsequently also been implicated in actin reorganization, nuclear shuttling, and DNA repair. Here we report an extensive characterization of the physical connections and of the functional wirings of the EH-network in the nematode. Our data show that one of the major physiological roles of the EH-network is in neurotransmission. In addition, we found that the proteins of the network intersect, and possibly coordinate, a number of “territories” of cellular activity including endocytosis/recycling/vesicle transport, actin dynamics, general metabolism and signal transduction, ubiquitination/degradation of proteins, DNA replication/repair, and miRNA biogenesis and processing.

Trafficking cascades mediated by Rab35 and its membrane hub effector, MICAL-L1

Various receptors navigate through the endocytic recycling compartment (ERC) on route to the plasma membrane. They are transported through recycling endosomes that emanate from the ERC that display distinct tubular morphology. A key question in the field is how the trafficking via these endosomes is regulated and how regulatory proteins such as Rab35, Rab8, Arf6 and EHD1 control this trafficking. Recent studies point to the protein MICAL-L1 as a major scaffold for these regulators. MICAL-L1 not only localizes to these tubular recycling endosomes and regulates trafficking, but it also controls the localization of EHD1 and Rab8 to these structures. It also connects its associated membranes to the motor proteins dynein and kinesin through its binding partner, CRMP2. Our recent study promotes MICAL-L1 as a Rab35 effector, where Rab35, both directly and indirectly through Arf6, controls the localization of MICAL-L1 and Rab8 to tubular membranes. We find that MICAL-L1 is a multi-tasking scaffold connecting various proteins to recycling endosomes for efficient trafficking.

Molecular remodeling mechanisms of the neural somatodendritic compartment

Neuronal cells use the process of vesicle trafficking to manipulate the populations of neurotransmitter receptors and other membrane proteins. Long term potentiation (LTP) is a long-lived increase in synaptic strength between neurons and increases postsynaptic dendritic spine size and the concentration of the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate-type glutamate receptor (AMPAR) located in the postsynaptic density. AMPAR is removed from the cell surface via clathrin-mediated endocytosis. While the adaptor protein 2 (AP2) complex of endocytosis seems to have the components needed to allow temporal and spatial regulations of internalization, many accessory proteins are involved, such as epidermal growth factor receptor phosphorylation substrate 15 (Eps15). A sequence of repeats in the Eps15 protein is known as the Eps15 homology (EH) domain. It has affinity for asparagine-proline-phenylalanine (NPF) sequences that are contained within vesicle trafficking proteins such as epsin, Rab11 family interacting protein 2 (Rab11-FIP2), and Numb. After endocytosis, a pool of AMPAR is stored in the endosomal recycling compartment that can be transported to the dendritic spine surface upon stimulation during LTP for lateral diffusion into the postsynaptic density. Rab11 and the Eps15 homologue EHD1 are involved in receptor recycling. EHD family members are also involved in transcytosis of the neuronal cell adhesion molecule neuron-glia cell adhesion molecule (NgCAM) from the somatodendritic compartment to the axon. Neurons have a unique morphology comprising many projections of membrane that is constructed in part by the effects of the Eps15 homologue, intersectin. Morphogenesis in the somatodendritic compartment is becoming better understood, but there is still much exciting territory to explore, especially regarding the roles of various EH domain-NPF interactions in endocytic and recycling processes.

Secretory carrier membrane proteins

Secretory carrier membrane proteins (SCAMPs) are a family of integral membrane proteins that play roles in mediating exocytosis in animal cells. However, relatively little is known about the subcellular localization, trafficking, and function of SCAMPs in plants. Several recent studies in plant cells indicate that plant SCAMPs share many similarities with their mammalian homologs although there are differences. In this review, we will first summarize and compare animal and plant SCAMPs in terms of their subcellular localization, trafficking, and possible functions. We will then present a phylogenetic analysis of plant and animal SCAMPs. Finally, we will present expression analysis on selective Arabidopsis SCAMPs in the hope of pointing to directions for functional characterization of plant SCAMPs in the future.

Rabs and EHDs: alternate modes for traffic control

Endocytic trafficking is a highly organized process regulated by a network of proteins, including the Rab family of small GTP-binding proteins and the C-terminal EHDs (Eps15 homology-domain-containing proteins). Central roles for Rab proteins have been described in vesicle budding, delivery, tethering and fusion, whereas little is known about the functions of EHDs in membrane transport. Common effectors for these two protein families have been identified, and they facilitate regulation of sequential steps in transport. By comparing and contrasting key aspects in their modes of function, we shall promote a better understanding of how Rab proteins and EHDs regulate endocytic trafficking.

Rabankyrin-5 interacts with EHD1 and Vps26 to regulate endocytic trafficking and retromer function

Rabankyrin-5 (Rank-5) has been implicated as an effector of the small GTPase Rab5 and plays an important role in macropinocytosis. We have now identified Rank-5 as an interaction partner for the recycling regulatory protein, Eps15 homology domain 1 (EHD1). We have demonstrated this interaction by glutathione S-transferase-pulldown, yeast two-hybrid assay, isothermal calorimetry and co-immunoprecipitation, and found that the binding occurs between the EH domain of EHD1 and the NPFED motif of Rank-5. Similar to EHD1, we found that Rank-5 colocalizes and interacts with components of the retromer complex such as vacuolar protein sorting 26 (Vps26), suggesting a role for Rank-5 in retromer-based transport. Indeed, depletion of Rank-5 causes mislocalization of Vps26 and affects both the retrieval of mannose 6-phosphate receptor transport to the Golgi from endosomes and biosynthetic transport. Moreover, Rank-5 is required for normal retromer distribution, as overexpression of a wild-type Rank-5-small interfering RNA-resistant construct rescues retromer mislocalization. Finally, we show that depletion of either Rank-5 or EHD1 impairs secretion of vesicular stomatitis virus glycoprotein. Overall, our data identify a new interaction between Rank-5 and EHD1, and novel endocytic regulatory roles that include retromer-based transport and secretion.

EHD2 regulates caveolar dynamics via ATP-driven targeting and oligomerization

Eps15 homology domain-containing 2 (EHD2) belongs to the EHD-containing protein family of dynamin-related ATPases involved in membrane remodeling in the endosomal system. EHD2 dimers oligomerize into rings on highly curved membranes, resulting in stimulation of the intrinsic ATPase activity. In this paper, we report that EHD2 is specifically and stably associated with caveolae at the plasma membrane and not involved in clathrin-mediated endocytosis or endosomal recycling, as previously suggested. EHD2 interacts with pacsin2 and cavin1, and ordered membrane assembly of EHD2 is dependent on cavin1 and caveolar integrity. While the EHD of EHD2 is dispensable for targeting, we identified a loop in the nucleotide-binding domain that, together with ATP binding, is required for caveolar localization. EHD2 was not essential for the formation or shaping of caveolae, but high levels of EHD2 caused distortion and loss of endogenous caveolae. Assembly of EHD2 stabilized and constrained caveolae to the plasma membrane to control turnover, and depletion of EHD2, resulting in endocytic and more dynamic and short-lived caveolae. Thus, following the identification of caveolin and cavins, EHD2 constitutes a third structural component of caveolae involved in controlling the stability and turnover of this organelle.

MICAL-L1 is a tubular endosomal membrane hub that connects Rab35 and Arf6 with Rab8a

Endocytosis is a conserved process across species in which cell surface receptors and lipids are internalized from the plasma membrane. Once internalized, receptors can either be degraded or be recycled back to the plasma membrane. A variety of small GTP-binding proteins regulate receptor recycling. Despite our familiarity with many of the key regulatory proteins involved in this process, our understanding of the mode by which these proteins co-operate and the sequential manner in which they function remains limited. In this study, we identify two GTP-binding proteins as interaction partners of the endocytic regulatory protein molecule interacting with casl-like protein 1 (MICAL)-L1. First, we demonstrate that Rab35 is a MICAL-L1-binding partner in vivo. Over-expression of active Rab35 impairs the recruitment of MICAL-L1 to tubular recycling endosomes, whereas Rab35 depletion promotes enhanced MICAL-L1 localization to these structures. Moreover, we demonstrate that Arf6 forms a complex with MICAL-L1 and plays a role in its recruitment to tubular endosomes. Overall, our data suggest a model in which Rab35 is a critical upstream regulator of MICAL-L1 and Arf6, while both MICAL-L1 and Arf6 regulate Rab8a function.

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.

EHD proteins: key conductors of endocytic transport

Regulation of endocytic transport is controlled by an elaborate network of proteins. Rab GTP-binding proteins and their effectors have well-defined roles in mediating specific endocytic transport steps, but until recently less was known about the four mammalian dynamin-like C-terminal Eps15 homology domain (EHD) proteins that also regulate endocytic events. In recent years, however, great strides have been made in understanding the structure and function of these unique proteins. Indeed, a growing body of literature addresses EHD protein structure, interactions with binding partners, functions in mammalian cells, and the generation of various new model systems. Accordingly, this is now an opportune time to pause and review the function and mechanisms of action of EHD proteins, and to highlight some of the challenges and future directions for the field.

Collapsin response mediator protein-2 (Crmp2) regulates trafficking by linking endocytic regulatory proteins to dynein motors

Endocytosis is a conserved cellular process in which nutrients, lipids, and receptors are internalized and transported to early endosomes, where they are sorted and either channeled to degradative pathways or recycled to the plasma membrane. MICAL-L1 and EHD1 are important regulatory proteins that control key endocytic transport steps. However, the precise mechanisms by which they mediate transport, and particularly the mode by which they connect to motor proteins, have remained enigmatic. Here we have identified the collapsin response mediator protein-2 (Crmp2) as an interaction partner of MICAL-L1 in non-neuronal cells. Crmp2 interacts with tubulin dimers and kinesin and negatively regulates dynein-based transport in neuronal cells, but its expression and function in non-neuronal cells have remained poorly characterized. Upon Crmp2 depletion, we observed dramatic relocalization of internalized transferrin (Tf) from peripheral vesicles to the endocytic recycling compartment (ERC), similar to the effect of depleting either MICAL-L1 or EHD1. Moreover, Tf relocalization to the ERC could be inhibited by interfering with microtubule polymerization, consistent with a role for uncoupled motor protein-based transport upon depletion of Crmp2, MICAL-L1, or EHD1. Finally, transfection of dynamitin, a component of the dynactin complex whose overexpression inhibits dynein activity, prevented the relocalization of internalized Tf to the ERC upon depletion of Crmp2, MICAL-L1, or EHD1. These data provide the first trafficking regulatory role for Crmp2 in non-neuronal cells and support a model in which Crmp2 is an important endocytic regulatory protein that links MICAL-L1·EHD1-based vesicular transport to dynein motors.

Important relationships between Rab and MICAL proteins in endocytic trafficking

The internalization of essential nutrients, lipids and receptors is a crucial process for all eukaryotic cells. Accordingly, endocytosis is highly conserved across cell types and species. Once internalized, small cargo-containing vesicles fuse with early endosomes (also known as sorting endosomes), where they undergo segregation to distinct membrane regions and are sorted and transported on through the endocytic pathway. Although the mechanisms that regulate this sorting are still poorly understood, some receptors are directed to late endosomes and lysosomes for degradation, whereas other receptors are recycled back to the plasma membrane; either directly or through recycling endosomes. The Rab family of small GTP-binding proteins plays crucial roles in regulating these trafficking pathways. Rabs cycle from inactive GDP-bound cytoplasmic proteins to active GTP-bound membrane-associated proteins, as a consequence of the activity of multiple specific GTPase-activating proteins (GAPs) and GTP exchange factors (GEFs). Once bound to GTP, Rabs interact with a multitude of effector proteins that carry out Rab-specific functions. Recent studies have shown that some of these effectors are also interaction partners for the C-terminal Eps15 homology (EHD) proteins, which are also intimately involved in endocytic regulation. A particularly interesting example of common Rab-EHD interaction partners is the MICAL-like protein, MICAL-L1. MICAL-L1 and its homolog, MICAL-L2, belong to the larger MICAL family of proteins, and both have been directly implicated in regulating endocytic recycling of cell surface receptors and junctional proteins, as well as controlling cytoskeletal rearrangement and neurite outgrowth. In this review, we summarize the functional roles of MICAL and Rab proteins, and focus on the significance of their interactions and the implications for endocytic transport.