The Arp1/11 minifilament of dynactin primes the endosomal Arp2/3 complex

An engineered trafficking biosensor reveals a role for DNAJC13 in DOR downregulation

Trafficking of G protein-coupled receptors (GPCRs) through the endosomal-lysosomal pathway is critical to homeostatic regulation of GPCRs following activation with agonist. Identifying the genes involved in GPCR trafficking is challenging due to the complexity of sorting operations and the large number of cellular proteins involved in the process. Here, we developed a high-sensitivity biosensor for GPCR expression and agonist-induced trafficking to the lysosome by leveraging the ability of the engineered peroxidase APEX2 to activate the fluorogenic substrate Amplex UltraRed (AUR). We used the GPCR-APEX2/AUR assay to perform a genome-wide CRISPR interference screen focused on identifying genes regulating expression and trafficking of the δ-opioid receptor (DOR). We identified 492 genes consisting of both known and new regulators of DOR function. We demonstrate that one new regulator, DNAJC13, controls trafficking of multiple GPCRs, including DOR, through the endosomal-lysosomal pathway by regulating the composition of the endosomal proteome and endosomal homeostasis.

DNAJC13 localization to endosomes is opposed by its J domain and its disordered C-terminal tail

Endosomes are a central sorting hub for membrane cargos. DNAJC13/RME-8 plays a critical role in endosomal trafficking by regulating the endosomal recycling or degradative pathways. DNAJC13 localizes to endosomes through its N-terminal Plekstrin Homology (PH)-like domain, which directly binds endosomal phosphoinositol-3-phosphate (PI(3)P). However, little is known about how DNAJC13 localization is regulated. Here, we show that two regions within DNAJC13, its J domain and disordered C-terminal tail, act as negative regulators of its PH-like domain. Using a structure-function approach combined with quantitative proteomics, we mapped these control points to a conserved YLT motif in the C-terminal tail as well as the catalytic HPD triad in its J domain. Mutation of either motif enhanced DNAJC13 endosomal localization in cells and increased binding to PI(3)P in vitro. Further, these effects required the N-terminal PH-like domain. We show that, similar to other PI(3)P binding domains, the N-terminal PH-like domain binds PI(3)P weakly in isolation and requires oligomerization for efficient PI(3)P binding and endosomal localization. Together, these results demonstrate that interaction between DNAJC13 and PI(3)P serves as a molecular control point for regulating DNAJC13 localization to endosomes.

Assembly and fission of tubular carriers mediating protein sorting in endosomes

Endosomes are central protein-sorting stations at the crossroads of numerous membrane trafficking pathways in all eukaryotes. They have a key role in protein homeostasis and cellular signalling and are involved in the pathogenesis of numerous diseases. Endosome-associated protein assemblies or coats collect transmembrane cargo proteins and concentrate them into retrieval domains. These domains can extend into tubular carriers, which then pinch off from the endosomal membrane and deliver the cargoes to appropriate subcellular compartments. Here we discuss novel insights into the structure of a number of tubular membrane coats that mediate the recruitment of cargoes into these carriers, focusing on sorting nexin-based coats such as Retromer, Commander and ESCPE-1. We summarize current and emerging views of how selective tubular endosomal carriers form and detach from endosomes by fission, highlighting structural aspects, conceptual challenges and open questions.

Structural basis for coupling of the WASH subunit FAM21 with the endosomal SNX27-Retromer complex

Endosomal membrane trafficking is mediated by specific protein coats and formation of actin-rich membrane domains. The Retromer complex coordinates with sorting nexin (SNX) cargo adaptors including SNX27, and the SNX27-Retromer assembly interacts with the Wiskott-Aldrich syndrome protein and SCAR homolog (WASH) complex which nucleates actin filaments establishing the endosomal recycling domain. Crystal structures, modeling, biochemical, and cellular validation reveal how the FAM21 subunit of WASH interacts with both Retromer and SNX27. FAM21 binds the FERM domain of SNX27 using acidic-Asp-Leu-Phe (aDLF) motifs similar to those found in the SNX1 and SNX2 subunits of the ESCPE-1 complex. Overlapping FAM21 repeats and a specific Pro-Leu containing motif bind three distinct sites on Retromer involving both the VPS35 and VPS29 subunits. Mutation of the major VPS35-binding site does not prevent cargo recycling; however, it partially reduces endosomal WASH association indicating that a network of redundant interactions promote endosomal activity of the WASH complex. These studies establish the molecular basis for how SNX27-Retromer is coupled to the WASH complex via overlapping and multiplexed motif-based interactions required for the dynamic assembly of endosomal membrane recycling domains.

Biogenesis of lysosome-related organelles complex-2 is an evolutionarily ancient proto-coatomer complex

Hermansky-Pudlak syndrome (HPS) is an inherited disorder of intracellular vesicle trafficking affecting the function of lysosome-related organelles (LROs). At least 11 genes underlie the disease, encoding four protein complexes, of which biogenesis of lysosome-related organelles complex-2 (BLOC-2) is the last whose molecular action is unknown. We find that the unicellular eukaryote Dictyostelium unexpectedly contains a complete BLOC-2, comprising orthologs of the mammalian subunits HPS3, -5, and -6, and a fourth subunit, an ortholog of the Drosophila LRO-biogenesis gene, Claret. Lysosomes from Dictyostelium BLOC-2 mutants fail to mature, similar to LROs from HPS patients, but for all endolysosomes rather than a specialized subset. They also strongly resemble lysosomes from WASH mutants. Dictyostelium BLOC-2 localizes to the same compartments as WASH, and in BLOC-2 mutants, WASH is inefficiently recruited, accounting for their impaired lysosomal maturation. BLOC-2 is recruited to endolysosomes via its HPS3 subunit. Structural modeling suggests that all four subunits are proto-coatomer proteins, with important implications for BLOC-2's molecular function. The discovery of Dictyostelium BLOC-2 permits identification of orthologs throughout eukaryotes. BLOC-2 and lysosome-related organelles, therefore, pre-date the evolution of Metazoa and have broader and more conserved functions than previously thought.

Inactivating negative regulators of cortical branched actin enhances persistence of single cell migration

The Rac1-WAVE-Arp2/3 pathway pushes the plasma membrane by polymerizing branched actin, thereby powering membrane protrusions that mediate cell migration. Here, using knockdown (KD) or knockout (KO), we combine the inactivation of the Arp2/3 inhibitory protein arpin, the Arp2/3 subunit ARPC1A and the WAVE complex subunit CYFIP2, all of which enhance the polymerization of cortical branched actin. Inactivation of the three negative regulators of cortical branched actin increases migration persistence of human breast MCF10A cells and of endodermal cells in the zebrafish embryo, significantly more than any single or double inactivation. In the triple KO cells, but not in triple KD cells, the 'super-migrator' phenotype was associated with a heterogenous downregulation of vimentin (VIM) expression and a lack of coordination in collective behaviors, such as wound healing and acinus morphogenesis. Re-expression of vimentin in triple KO cells largely restored normal persistence of single cell migration, suggesting that vimentin downregulation contributes to the maintenance of the super-migrator phenotype in triple KO cells. Constant excessive production of branched actin at the cell cortex thus commits cells into a motile state through changes in gene expression.

Dynein-Powered Cell Locomotion Guides Metastasis of Breast Cancer

The principal cause of death in cancer patients is metastasis, which remains an unresolved problem. Conventionally, metastatic dissemination is linked to actomyosin-driven cell locomotion. However, the locomotion of cancer cells often does not strictly line up with the measured actomyosin forces. Here, a complementary mechanism of metastatic locomotion powered by dynein-generated forces is identified. These forces arise within a non-stretchable microtubule network and drive persistent contact guidance of migrating cancer cells along the biomimetic collagen fibers. It is also shown that the dynein-powered locomotion becomes indispensable during invasive 3D migration within a tissue-like luminal network formed by spatially confining granular hydrogel scaffolds (GHS) made up of microscale hydrogel particles (microgels). These results indicate that the complementary motricity mediated by dynein is always necessary and, in certain instances, sufficient for disseminating metastatic breast cancer cells. These findings advance the fundamental understanding of cell locomotion mechanisms and expand the spectrum of clinical targets against metastasis.

Case report: A novel WASHC5 variant altering mRNA splicing causes spastic paraplegia in a patient

Background: Hereditary spastic paraplegia (HSP) is a progressive upper-motor neurodegenerative disease. Mutations in the WASHC5 gene are associated with autosomal dominant HSP, spastic paraplegia 8 (SPG8). However, due to the small number of reported cases, the exact mechanism remains unclear. Method: We report a Chinese family with HSP. The proband was referred to our hospital due to restless leg syndrome and insomnia. The preliminary clinical diagnosis of the proband was spastic paraplegia. Whole-exome sequencing (WES) and RNA splicing analysis were conducted to evaluate the genetic cause of the disease in this family. Results: A novel splice-altering variant (c.712-2A>G) in the WASHC5 gene was detected and further verified by RNA splicing analysis and Sanger sequencing. Real-time qPCR analysis showed that the expression of genes involved in the Wiskott-Aldrich syndrome protein and SCAR homolog (WASH) complex and endosomal and lysosomal systems was altered due to this variant. Conclusion: A novel heterozygous splice-altering variant (c.712-2A>G) in the WASHC5 gene was detected in a Chinese family with HSP. Our study provided data for genetic counseling to this family and offered evidence that this splicing variant in the WASHC5 gene is significant in causing HSP.

Septins Enable T Cell Contact Guidance via Amoeboid-Mesenchymal Switch

Lymphocytes exit circulation and enter in-tissue guided migration toward sites of tissue pathologies, damage, infection, or inflammation. By continuously sensing and adapting to the guiding chemo-mechano-structural properties of the tissues, lymphocytes dynamically alternate and combine their amoeboid (non-adhesive) and mesenchymal (adhesive) migration modes. However, which mechanisms guide and balance different migration modes are largely unclear. Here we report that suppression of septins GTPase activity induces an abrupt amoeboid-to-mesenchymal transition of T cell migration mode, characterized by a distinct, highly deformable integrin-dependent immune cell contact guidance. Surprisingly, the T cell actomyosin cortex contractility becomes diminished, dispensable and antagonistic to mesenchymal-like migration mode. Instead, mesenchymal-like T cells rely on microtubule stabilization and their non-canonical dynein motor activity for high fidelity contact guidance. Our results establish septin's GTPase activity as an important on/off switch for integrin-dependent migration of T lymphocytes, enabling their dynein-driven fluid-like mesenchymal propulsion along the complex adhesion cues.

Changes in cell surface excess are coordinated with protrusion dynamics during 3D motility

To facilitate rapid changes in morphology without endangering cell integrity, each cell possesses a substantial amount of cell surface excess (CSE) that can be promptly deployed to cover cell extensions. CSE can be stored in different types of small surface projections such as filopodia, microvilli, and ridges, with rounded bleb-like projections being the most common and rapidly achieved form of storage. We demonstrate that, similar to rounded cells in 2D culture, rounded cells in 3D collagen contain large amounts of CSE and use it to cover developing protrusions. Upon retraction of a protrusion, the CSE this produces is stored over the cell body similar to the CSE produced by cell rounding. We present high-resolution imaging of F-actin and microtubules (MTs) for different cell lines in a 3D environment and demonstrate the correlated changes between CSE and protrusion dynamics. To coordinate CSE storage and release with protrusion formation and motility, we expect cells to have specific mechanisms for regulating CSE, and we hypothesize that MTs play a substantial role in this mechanism by reducing cell surface dynamics and stabilizing CSE. We also suggest that different effects of MT depolymerization on cell motility, such as inhibiting mesenchymal motility and enhancing amoeboid, can be explained by this role of MTs in CSE regulation.

Cell-specific secretory granule sorting mechanisms: the role of MAGEL2 and retromer in hypothalamic regulated secretion

Intracellular protein trafficking and sorting are extremely arduous in endocrine and neuroendocrine cells, which synthesize and secrete on-demand substantial quantities of proteins. To ensure that neuroendocrine secretion operates correctly, each step in the secretion pathways is tightly regulated and coordinated both spatially and temporally. At the trans-Golgi network (TGN), intrinsic structural features of proteins and several sorting mechanisms and distinct signals direct newly synthesized proteins into proper membrane vesicles that enter either constitutive or regulated secretion pathways. Furthermore, this anterograde transport is counterbalanced by retrograde transport, which not only maintains membrane homeostasis but also recycles various proteins that function in the sorting of secretory cargo, formation of transport intermediates, or retrieval of resident proteins of secretory organelles. The retromer complex recycles proteins from the endocytic pathway back to the plasma membrane or TGN and was recently identified as a critical player in regulated secretion in the hypothalamus. Furthermore, melanoma antigen protein L2 (MAGEL2) was discovered to act as a tissue-specific regulator of the retromer-dependent endosomal protein recycling pathway and, by doing so, ensures proper secretory granule formation and maturation. MAGEL2 is a mammalian-specific and maternally imprinted gene implicated in Prader-Willi and Schaaf-Yang neurodevelopmental syndromes. In this review, we will briefly discuss the current understanding of the regulated secretion pathway, encompassing anterograde and retrograde traffic. Although our understanding of the retrograde trafficking and sorting in regulated secretion is not yet complete, we will review recent insights into the molecular role of MAGEL2 in hypothalamic neuroendocrine secretion and how its dysregulation contributes to the symptoms of Prader-Willi and Schaaf-Yang patients. Given that the activation of many secreted proteins occurs after they enter secretory granules, modulation of the sorting efficiency in a tissue-specific manner may represent an evolutionary adaptation to environmental cues.

Codependencies of mTORC1 signaling and endolysosomal actin structures

The mechanistic target of rapamycin complex 1 (mTORC1) is part of the amino acid sensing machinery that becomes activated on the endolysosomal surface in response to nutrient cues. Branched actin generated by WASH and Arp2/3 complexes defines endolysosomal microdomains. Here, we find mTORC1 components in close proximity to endolysosomal actin microdomains. We investigated for interactors of the mTORC1 lysosomal tether, RAGC, by proteomics and identified multiple actin filament capping proteins and their modulators. Perturbation of RAGC function affected the size of endolysosomal actin, consistent with a regulation of actin filament capping by RAGC. Reciprocally, the pharmacological inhibition of actin polymerization or alteration of endolysosomal actin obtained upon silencing of WASH or Arp2/3 complexes impaired mTORC1 activity. Mechanistically, we show that actin is required for proper association of RAGC and mTOR with endolysosomes. This study reveals an unprecedented interplay between actin and mTORC1 signaling on the endolysosomal system.

Dynein-Powered Cell Locomotion Guides Metastasis of Breast Cancer

Metastasis is a principal cause of death in cancer patients, which remains an unresolved fundamental and clinical problem. Conventionally, metastatic dissemination is linked to the actomyosin-driven cell locomotion. However, locomotion of cancer cells often does not strictly line up with the measured actomyosin forces. Here, we identify a complementary mechanism of metastatic locomotion powered by the dynein-generated forces. These forces that arise within a non-stretchable microtubule network drive persistent contact guidance of migrating cancer cells along the biomimetic collagen fibers. We also show that dynein-powered locomotion becomes indispensable during invasive 3D migration within a tissue-like luminal network between spatially confining hydrogel microspheres. Our results indicate that the complementary contractile system of dynein motors and microtubules is always necessary and in certain instances completely sufficient for dissemination of metastatic breast cancer cells. These findings advance fundamental understanding of cell locomotion mechanisms and expand the spectrum of clinical targets against metastasis.

Branching out in different directions: Emerging cellular functions for the Arp2/3 complex and WASP-family actin nucleation factors

The actin cytoskeleton impacts practically every function of a eukaryotic cell. Historically, the best-characterized cytoskeletal activities are in cell morphogenesis, motility, and division. The structural and dynamic properties of the actin cytoskeleton are also crucial for establishing, maintaining, and changing the organization of membrane-bound organelles and other intracellular structures. Such activities are important in nearly all animal cells and tissues, although distinct anatomical regions and physiological systems rely on different regulatory factors. Recent work indicates that the Arp2/3 complex, a broadly expressed actin nucleator, drives actin assembly during several intracellular stress response pathways. These newly described Arp2/3-mediated cytoskeletal rearrangements are coordinated by members of the Wiskott-Aldrich Syndrome Protein (WASP) family of actin nucleation-promoting factors. Thus, the Arp2/3 complex and WASP-family proteins are emerging as crucial players in cytoplasmic and nuclear activities including autophagy, apoptosis, chromatin dynamics, and DNA repair. Characterizations of the functions of the actin assembly machinery in such stress response mechanisms are advancing our understanding of both normal and pathogenic processes, and hold great promise for providing insights into organismal development and interventions for disease.

From WRC to Arp2/3: Collective molecular mechanisms of branched actin network assembly

Branched actin networks have emerged as major force-generating structures driving the protrusions in various distinct cell types and processes, ranging from lamellipodia operating in mesenchymal and epithelial cell migration or tails pushing intracellular pathogens and vesicles to developing spine heads on neurons. Many key molecular features are conserved among all those Arp2/3 complex-containing, branched actin networks. Here, we will review recent progress in our molecular understanding of the core biochemical machinery driving branched actin nucleation, from the generation of filament primers to Arp2/3 activator recruitment, regulation and turnover. Due to the wealth of information on distinct, Arp2/3 network-containing structures, we are largely focusing-in an exemplary fashion-on canonical lamellipodia of mesenchymal cells, which are regulated by Rac GTPases, their downstream effector WAVE Regulatory Complex and its target Arp2/3 complex. Novel insight additionally confirms that WAVE and Arp2/3 complexes regulate or are themselves tuned by additional prominent actin regulatory factors, including Ena/VASP family members and heterodimeric capping protein. Finally, we are considering recent insights into effects exerted by mechanical force, both at the branched network and individual actin regulator level.

Endosome biogenesis is controlled by ER and the cytoskeleton at tripartite junctions

The plasma membrane (PM) and its associated cargo are internalized into small vesicles via endocytosis funneling cargo into endosomes. The endosomal system must efficiently deliver cargos, as well as recycle cargo receptors and membrane to maintain homeostasis. In animal cells, endosome trafficking, maturation, and cargo recycling rely on the actin and microtubule cytoskeleton. Microtubules and their associated motor proteins provide the roads on which endosomes move and fuse during cargo sorting and delivery. In addition, highly dynamic assemblies of actin adjust the shape of the endosomal membrane to promote cargo segregation into budding domains allowing for receptor recycling. Recent work has revealed that the endoplasmic reticulum (ER) frequently acts as an intermediary between endosomes and their cytoskeletal regulators via membrane contact sites (MCSs). This review will discuss the factors which form these tripartite junction between the ER, endosomes, and the cytoskeleton as well as their function.

Biochemical and mechanical regulation of actin dynamics

Polymerization of actin filaments against membranes produces force for numerous cellular processes, such as migration, morphogenesis, endocytosis, phagocytosis and organelle dynamics. Consequently, aberrant actin cytoskeleton dynamics are linked to various diseases, including cancer, as well as immunological and neurological disorders. Understanding how actin filaments generate forces in cells, how force production is regulated by the interplay between actin-binding proteins and how the actin-regulatory machinery responds to mechanical load are at the heart of many cellular, developmental and pathological processes. During the past few years, our understanding of the mechanisms controlling actin filament assembly and disassembly has evolved substantially. It has also become evident that the activities of key actin-binding proteins are not regulated solely by biochemical signalling pathways, as mechanical regulation is critical for these proteins. Indeed, the architecture and dynamics of the actin cytoskeleton are directly tuned by mechanical load. Here we discuss the general mechanisms by which key actin regulators, often in synergy with each other, control actin filament assembly, disassembly, and monomer recycling. By using an updated view of actin dynamics as a framework, we discuss how the mechanics and geometry of actin networks control actin-binding proteins, and how this translates into force production in endocytosis and mesenchymal cell migration.

Analysis of functional surfaces on the actin nucleation promoting factor Dip1 required for Arp2/3 complex activation and endocytic actin network assembly

Arp2/3 complex nucleates branched actin filaments that drive processes like endocytosis and lamellipodial protrusion. WISH/DIP/SPIN90 (WDS) proteins form a class of Arp2/3 complex activators or nucleation promoting factors (NPFs) that, unlike WASP family NPFs, activate Arp2/3 complex without requiring preformed actin filaments. Therefore, activation of Arp2/3 complex by WDS proteins is thought to produce the initial actin filaments that seed branching nucleation by WASP-bound Arp2/3 complexes. However, whether activation of Arp2/3 complex by WDS proteins is important for the initiation of branched actin assembly in cells has not been directly tested. Here, we used structure-based point mutations of the Schizosaccharomyces pombe WDS protein Dip1 to test the importance of its Arp2/3-activating activity in cells. Six of thirteen Dip1 mutants caused severe defects in Arp2/3 complex activation in vitro, and we found a strong correlation between the ability of mutants to activate Arp2/3 complex and to rescue endocytic actin assembly defects caused by deleting Dip1. These data support a model in which Dip1 activates Arp2/3 complex to produce actin filaments that initiate branched actin assembly at endocytic sites. Dip1 mutants that synergized with WASP in activating Arp2/3 complex in vitro showed milder defects in cells compared to those that did not, suggesting that in cells the two NPFs may coactivate Arp2/3 complex to initiate actin assembly. Finally, the mutational data reveal important complementary electrostatic contacts at the Dip1-Arp2/3 complex interface and corroborate the previously proposed wedge model, which describes how Dip1 binding triggers structural changes that activate Arp2/3 complex.

FAM21C Promotes Hepatocellular Carcinoma Invasion and Metastasis by Driving Actin Cytoskeleton Remodeling via Inhibiting Capping Ability of CAPZA1

Hepatocellular carcinoma (HCC) is characterized by a high incidence of metastasis. The dynamic remodeling of the actin cytoskeleton plays an important role in the invasion and migration of HCC cells. In previous studies, we found that CAPZA1, a capping protein, can promote EMT of HCC cells by regulating the remodeling of the actin filament (F-actin) cytoskeleton, thus promoting the invasion and migration of HCC cells. In this study, we found that FAM21C may have a regulatory effect on CAPZA1, and we conducted an in-depth study on its potential regulatory mechanism. First, we found that FAM21C is highly expressed in HCC tissues and its high expression could promote the malignant progression of HCC. Meanwhile, the high expression of FAM21C promoted the invasion and migration of HCC cells in vitro and in vivo. Further, FAM21C interacted with CAPZA1, and their binding inhibited the capping capacity of CAPZA1, thus promoting the invasion and migration of HCC cells. This effect of FAM21C was abolished by mutating the CP-interacting (CPI) domain, the CAPZA1 binding site on FAM21C. In conclusion, high expression of FAM21C in HCC tissues can promote malignant progression of HCC and its potential mechanism involves FAM21C inhibition of CAPZA1 capping capacity by binding to CAPZA1, which drives F-actin cytoskeleton remodeling, and thus promotes invasion and migration of HCC cells.

Nucleation, stabilization, and disassembly of branched actin networks

Arp2/3 complex is an actin filament nucleation and branching machinery conserved in all eukaryotes from yeast to human. Arp2/3 complex branched networks generate pushing forces that drive cellular processes ranging from membrane remodeling to cell and organelle motility. Several molecules regulate these processes by directly inhibiting or activating Arp2/3 complex and by stabilizing or disassembling branched networks. Here, we review recent advances in our understanding of Arp2/3 complex regulation, including high-resolution cryoelectron microscopy (cryo-EM) structures that illuminate the mechanisms of Arp2/3 complex activation and branch formation, and novel cellular pathways of branch formation, stabilization, and debranching. We also identify major gaps in our understanding of Arp2/3 complex inhibition and branch stabilization and disassembly.

Mechanical Counterbalance of Kinesin and Dynein Motors in a Microtubular Network Regulates Cell Mechanics, 3D Architecture, and Mechanosensing

Microtubules (MTs) and MT motor proteins form active 3D networks made of unstretchable cables with rod-like bending mechanics that provide cells with a dynamically changing structural scaffold. In this study, we report an antagonistic mechanical balance within the dynein-kinesin microtubular motor system. Dynein activity drives the microtubular network inward compaction, while isolated activity of kinesins bundles and expands MTs into giant circular bands that deform the cell cortex into discoids. Furthermore, we show that dyneins recruit MTs to sites of cell adhesion, increasing the topographic contact guidance of cells, while kinesins antagonize it via retraction of MTs from sites of cell adhesion. Actin-to-microtubule translocation of septin-9 enhances kinesin-MT interactions, outbalances the activity of kinesins over that of dyneins, and induces the discoid architecture of cells. These orthogonal mechanisms of MT network reorganization highlight the existence of an intricate mechanical balance between motor activities of kinesins and dyneins that controls cell 3D architecture, mechanics, and cell-microenvironment interactions.

New twists in actin-microtubule interactions

Actin filaments and microtubules are cytoskeletal polymers that participate in many vital cell functions including division, morphogenesis, phagocytosis, and motility. Despite the persistent dogma that actin filament and microtubule networks are distinct in localization, structure, and function, a growing body of evidence shows that these elements are choreographed through intricate mechanisms sensitive to either polymer. Many proteins and cellular signals that mediate actin–microtubule interactions have already been identified. However, the impact of these regulators is typically assessed with actin filament or microtubule polymers alone, independent of the other system. Further, unconventional modes and regulators coordinating actin–microtubule interactions are still being discovered. Here we examine several methods of actin–microtubule crosstalk with an emphasis on the molecular links between both polymer systems and their higher-order interactions.

Multiple roles for actin in secretory and endocytic pathways

Actin filaments play multiple roles in the secretory pathway and in endosome dynamics in mammals, including maintenance of Golgi structure, release of membrane cargo from the trans-Golgi network (TGN), endocytosis, and endosomal sorting dynamics. In addition, TGN carrier transport and endocytosis both occur by multiple mechanisms in mammals. Actin likely plays a role in at least four mammalian endocytic pathways, five pathways for membrane release from the TGN, and three processes involving endosomes. Also, the mammalian Golgi structure is highly dynamic, and actin is likely important for these dynamics. One challenge for many of these processes is the need to deal with other membrane-associated structures, such as the cortical actin network at the plasma membrane or the matrix that surrounds the Golgi. Arp2/3 complex is a major actin assembly factor in most of the processes mentioned, but roles for formins and tandem WH2-motif-containing assembly factors are being elucidated and are anticipated to grow with further study. The specific role for actin has not been defined for most of these processes, but is likely to involve the generation of force for membrane dynamics, either by actin polymerization itself or by myosin motor activity. Defining these processes mechanistically is necessary for understanding membrane dynamics in general, as well as pathways that utilize these processes, such as autophagy.

Assembly and Activity of the WASH Molecular Machine: Distinctive Features at the Crossroads of the Actin and Microtubule Cytoskeletons

The Arp2/3 complex generates branched actin networks at different locations of the cell. The WASH and WAVE Nucleation Promoting Factors (NPFs) activate the Arp2/3 complex at the surface of endosomes or at the cell cortex, respectively. In this review, we will discuss how these two NPFs are controlled within distinct, yet related, multiprotein complexes. These complexes are not spontaneously assembled around WASH and WAVE, but require cellular assembly factors. The centrosome, which nucleates microtubules and branched actin, appears to be a privileged site for WASH complex assembly. The actin and microtubule cytoskeletons are both responsible for endosome shape and membrane remodeling. Motors, such as dynein, pull endosomes and extend membrane tubules along microtubule tracks, whereas branched actin pushes onto the endosomal membrane. It was recently uncovered that WASH assembles a super complex with dynactin, the major dynein activator, where the Capping Protein (CP) is exchanged from dynactin to the WASH complex. This CP swap initiates the first actin filament that primes the autocatalytic nucleation of branched actin at the surface of endosomes. Possible coordination between pushing and pulling forces in the remodeling of endosomal membranes is discussed.