Dynamic exchange of myosin VI on endocytic structures

Endosomal sorting sorted - motors, adaptors and lessons from in vitro and cellular studies

Motor proteins are key players in exerting spatiotemporal control over the intracellular location of membrane-bound compartments, including endosomes containing cargo. In this Review, we focus on how motors and their cargo adaptors regulate positioning of cargoes from the earliest stages of endocytosis and through the two main intracellular itineraries: (1) degradation at the lysosome or (2) recycling back to the plasma membrane. In vitro and cellular (in vivo) studies on cargo transport thus far have typically focussed independently on either the motor proteins and adaptors, or membrane trafficking. Here, we will discuss recent studies to highlight what is known about the regulation of endosomal vesicle positioning and transport by motors and cargo adaptors. We also emphasise that in vitro and cellular studies are often performed at different scales, from single molecules to whole organelles, with the aim to provide a perspective on the unified principles of motor-driven cargo trafficking in living cells that can be learned from these differing scales.

Dynamic multimerization of Dab2-Myosin VI complexes regulates cargo processivity while minimizing cortical actin reorganization

Myosin VI ensembles on endocytic cargo facilitate directed transport through a dense cortical actin network. Myosin VI is recruited to clathrin-coated endosomes via the cargo adaptor Dab2. Canonically, it has been assumed that the interactions between a motor and its cargo adaptor are stable. However, it has been demonstrated that the force generated by multiple stably attached motors disrupts local cytoskeletal architecture, potentially compromising transport. In this study, we demonstrate that dynamic multimerization of myosin VI-Dab2 complexes facilitates cargo processivity without significant reorganization of cortical actin networks. Specifically, we find that Dab2 myosin interacting region (MIR) binds myosin VI with a moderate affinity (184 nM) and single-molecule kinetic measurements demonstrate a high rate of turnover (1 s⁻¹) of the Dab2 MIR–myosin VI interaction. Single-molecule motility shows that saturating Dab2-MIR concentration (2 μM) promotes myosin VI homodimerization and processivity with run lengths comparable with constitutive myosin VI dimers. Cargo-mimetic DNA origami scaffolds patterned with Dab2 MIR-myosin VI complexes are weakly processive, displaying sparse motility on single actin filaments and “stop-and-go” motion on a cellular actin network. On a minimal actin cortex assembled on lipid bilayers, unregulated processive movement by either constitutive myosin V or VI dimers results in actin remodeling and foci formation. In contrast, Dab2 MIR–myosin VI interactions preserve the integrity of a minimal cortical actin network. Taken together, our study demonstrates the importance of dynamic motor–cargo association in enabling cargo transportation without disrupting cytoskeletal organization.

Diverse functions of myosin VI in spermiogenesis

Spermiogenesis is the final stage of spermatogenesis, a differentiation process during which unpolarized spermatids undergo excessive remodeling that results in the formation of sperm. The actin cytoskeleton and associated actin-binding proteins play crucial roles during this process regulating organelle or vesicle delivery/segregation and forming unique testicular structures involved in spermatid remodeling. In addition, several myosin motor proteins including MYO6 generate force and movement during sperm differentiation. MYO6 is highly unusual as it moves towards the minus end of actin filaments in the opposite direction to other myosin motors. This specialized feature of MYO6 may explain the many proposed functions of this myosin in a wide array of cellular processes in animal cells, including endocytosis, secretion, stabilization of the Golgi complex, and regulation of actin dynamics. These diverse roles of MYO6 are mediated by a range of specialized cargo-adaptor proteins that link this myosin to distinct cellular compartments and processes. During sperm development in a number of different organisms, MYO6 carries out pivotal functions. In Drosophila, the MYO6 ortholog regulates actin reorganization during spermatid individualization and male KO flies are sterile. In C. elegans, the MYO6 ortholog mediates asymmetric segregation of cytosolic material and spermatid budding through cytokinesis, whereas in mice, this myosin regulates assembly of highly specialized actin-rich structures and formation of membrane compartments to allow the formation of fully differentiated sperm. In this review, we will present an overview and compare the diverse function of MYO6 in the specialized adaptations of spermiogenesis in flies, worms, and mammals.

Ultrastructural insights into pathogen clearance by autophagy

Autophagy defends cells against proliferation of bacteria such as Salmonella in the cytosol. After escape from a damaged Salmonella-containing vacuole (SCV) exposing luminal glycans that bind to Galectin-8, the host cell ubiquitination machinery deposits a dense layer of ubiquitin around the cytosolic bacteria. The nature and spatial distribution of this ubiquitin coat in relation to other autophagy-related membranes are unknown. Using transmission electron microscopy, we determined the exact localisation of ubiquitin, the ruptured SCV membrane and phagophores around cytosolic Salmonella. Ubiquitin was not predominantly present on the Salmonella surface, but enriched on the fragmented SCV. Cytosolic bacteria without SCVs were less efficiently targeted by phagophores. Single bacteria were contained in single phagophores but multiple bacteria could be within large autophagic vacuoles reaching 30 μm in circumference. These large phagophores followed the contour of the engulfed bacteria, they were frequently in close association with endoplasmic reticulum membranes and, within them, remnants of the SCV were seen associated with each engulfed particle. Our data suggest that the Salmonella SCV has a major role in the formation of autophagic phagophores and highlight evolutionary conserved parallel mechanisms between xenophagy and mitophagy with the fragmented SCV and the damaged outer mitochondrial membrane serving similar functions.

The adaptor protein melanophilin regulates dynamic myosin-Va:cargo interaction and dendrite development in melanocytes

The regulation of organelle transport by the cytoskeleton is fundamental for eukaryotic survival. Cytoskeleton motors are typically modular proteins with conserved motor and diverse cargo-binding domains. Motor:cargo interactions are often indirect and mediated by adaptor proteins, for example, Rab GTPases. Rab27a, via effector melanophilin (Mlph), recruits myosin-Va (MyoVa) to melanosomes and thereby disperses them into melanocyte dendrites. To better understand how adaptors regulate motor:cargo interaction, we used single melanosome fluorescence recovery after photobleaching (smFRAP) to characterize the association kinetics among MyoVa, its adaptors, and melanosomes. We found that MyoVa and Mlph rapidly recovered after smFRAP, whereas Rab27a did not, indicating that MyoVa and Mlph dynamically associate with melanosomes and Rab27a does not. This suggests that dynamic Rab27a:effector interaction rather than Rab27a melanosome:cytosol cycling regulates MyoVa:melanosome association. Accordingly, a Mlph-Rab27a fusion protein reduced MyoVa smFRAP, indicating that it stabilized melanosomal MyoVa. Finally, we tested the functional importance of dynamic MyoVa:melanosome interaction. We found that whereas a MyoVa-Rab27a fusion protein dispersed melanosomes in MyoVa-deficient cells, dendrites were significantly less elongated than in wild-type cells. Given that dendrites are the prime sites of melanosome transfer from melanocytes to keratinocytes, we suggest that dynamic MyoVa:melanosome interaction is important for pigmentation in vivo.

The MYO6 interactome reveals adaptor complexes coordinating early endosome and cytoskeletal dynamics

The intracellular functions of myosin motors requires a number of adaptor molecules, which control cargo attachment, but also fine-tune motor activity in time and space. These motor-adaptor-cargo interactions are often weak, transient or highly regulated. To overcome these problems, we use a proximity labelling-based proteomics strategy to map the interactome of the unique minus end-directed actin motor MYO6. Detailed biochemical and functional analysis identified several distinct MYO6-adaptor modules including two complexes containing RhoGEFs: the LIFT (LARG-Induced F-actin for Tethering) complex that controls endosome positioning and motility through RHO-driven actin polymerisation; and the DISP (DOCK7-Induced Septin disPlacement) complex, a novel regulator of the septin cytoskeleton. These complexes emphasise the role of MYO6 in coordinating endosome dynamics and cytoskeletal architecture. This study provides the first in vivo interactome of a myosin motor protein and highlights the power of this approach in uncovering dynamic and functionally diverse myosin motor complexes.

Myosin VI-Dependent Actin Cages Encapsulate Parkin-Positive Damaged Mitochondria

Mitochondrial quality control is essential to maintain cellular homeostasis and is achieved by removing damaged, ubiquitinated mitochondria via Parkin-mediated mitophagy. Here, we demonstrate that MYO6 (myosin VI), a unique myosin that moves toward the minus end of actin filaments, forms a complex with Parkin and is selectively recruited to damaged mitochondria via its ubiquitin-binding domain. This myosin motor initiates the assembly of F-actin cages to encapsulate damaged mitochondria by forming a physical barrier that prevents refusion with neighboring populations. Loss of MYO6 results in an accumulation of mitophagosomes and an increase in mitochondrial mass. In addition, we observe downstream mitochondrial dysfunction manifesting as reduced respiratory capacity and decreased ability to rely on oxidative phosphorylation for energy production. Our work uncovers a crucial step in mitochondrial quality control: the formation of MYO6-dependent actin cages that ensure isolation of damaged mitochondria from the network.

Disabled-2: A modular scaffold protein with multifaceted functions in signaling

Disabled-2 (Dab2) is a multimodular scaffold protein with signaling roles in the domains of cell growth, trafficking, differentiation, and homeostasis. Emerging evidences place Dab2 as a novel modulator of cell-cell interaction; however, its mode of action has remained largely elusive. In this review, we highlight the relevance of Dab2 function in cell signaling and development and provide the most recent and comprehensive analysis of Dab2's action as a mediator of homotypical and heterotypical interactions. Accordingly, Dab-2 controls the extent of platelet aggregation through various motifs within its N-terminus. Dab2 interacts with the cytosolic tail of the integrin receptor blocking inside-out signaling, whereas extracellular Dab2 competes with fibrinogen for integrin αIIb β3 receptor binding and, thus, modulates outside-in signaling. An additional level of regulation results from Dab2's association with cell surface lipids, an event that defines the extent of cell-cell interactions. As a multifaceted regulator, Dab2 acts as a mediator of endocytosis through its association with the [FY]xNPx[YF] motifs of internalized cell surface receptors, phosphoinositides, and clathrin. Other emerging roles of Dab2 include its participation in developmental mechanisms required for tissue formation and in modulation of immune responses. This review highlights the various novel mechanisms by which Dab2 mediates an array of signaling events with vast physiological consequences.

Widespread alternative exon usage in clinically distinct subtypes of Invasive Ductal Carcinoma

Cancer cells can have different patterns of exon usage of individual genes when compared to normal tissue, suggesting that alternative splicing may play a role in shaping the tumor phenotype. The discovery and identification of gene variants has increased dramatically with the introduction of RNA-sequencing technology, which enables whole transcriptome analysis of known, as well as novel isoforms. Here we report alternative splicing and transcriptional events among subtypes of invasive ductal carcinoma in The Cancer Genome Atlas (TCGA) Breast Invasive Carcinoma (BRCA) cohort. Alternative exon usage was widespread, and although common events were shared among three subtypes, ER+ HER2−, ER− HER2−, and HER2+, many events on the exon level were subtype specific. Additional RNA-seq analysis was carried out in an independent cohort of 43 ER+ HER2− and ER− HER2− primary breast tumors, confirming many of the exon events identified in the TCGA cohort. Alternative splicing and transcriptional events detected in five genes, MYO6, EPB41L1, TPD52, IQCG, and ACOX2 were validated by qRT-PCR in a third cohort of 40 ER+ HER2− and ER− HER2− patients, showing that these events were truly subtype specific.

Expression and localization of myosin VI in developing mouse spermatids

Myosin VI (MVI) is a versatile actin-based motor protein that has been implicated in a variety of different cellular processes, including endo- and exocytic vesicle trafficking, Golgi morphology, and actin structure stabilization. A role for MVI in crucial actin-based processes involved in sperm maturation was demonstrated in Drosophila. Because of the prominence and importance of actin structures in mammalian spermiogenesis, we investigated whether MVI was associated with actin-mediated maturation events in mammals. Both immunofluorescence and ultrastructural analyses using immunogold labeling showed that MVI was strongly linked with key structures involved in sperm development and maturation. During the early stage of spermiogenesis, MVI is associated with the Golgi and with coated and uncoated vesicles, which fuse to form the acrosome. Later, as the acrosome spreads to form a cap covering the sperm nucleus, MVI is localized to the acroplaxome, an actin-rich structure that anchors the acrosome to the nucleus. Finally, during the elongation/maturation phase, MVI is associated with the actin-rich structures involved in nuclear shaping: the acroplaxome, manchette, and Sertoli cell actin hoops. Since this is the first report of MVI expression and localization during mouse spermiogenesis and MVI partners in developing sperm have not yet been identified, we discuss some probable roles for MVI in this process. During early stages, MVI is hypothesized to play a role in Golgi morphology and function as well as in actin dynamics regulation important for attachment of developing acrosome to the nuclear envelope. Next, the protein might also play anchoring roles to help generate forces needed for spermatid head elongation. Moreover, association of MVI with actin that accumulates in the Sertoli cell ectoplasmic specialization and other actin structures in surrounding cells suggests additional MVI functions in spermatid movement across the seminiferous epithelium and in sperm release.

Loss of cargo binding in the human myosin VI deafness mutant (R1166X) leads to increased actin filament binding

Mutations in myosin VI have been associated with autosomal-recessive (DFNB37) and autosomal-dominant (DFNA22) deafness in humans. Here, we characterise an myosin VI nonsense mutation (R1166X) that was identified in a family with hereditary hearing loss in Pakistan. This mutation leads to the deletion of the C-terminal 120 amino acids of the myosin VI cargo-binding domain, which includes the WWY-binding motif for the adaptor proteins LMTK2, Tom1 as well as Dab2. Interestingly, compromising myosin VI vesicle-binding ability by expressing myosin VI with the R1166X mutation or with single point mutations in the adaptor-binding sites leads to increased F-actin binding of this myosin in vitro and in vivo As our results highlight the importance of cargo attachment for regulating actin binding to the motor domain, we perform a detailed characterisation of adaptor protein binding and identify single amino acids within myosin VI required for binding to cargo adaptors. We not only show that the adaptor proteins can directly interact with the cargo-binding tail of myosin VI, but our in vitro studies also suggest that multiple adaptor proteins can bind simultaneously to non-overlapping sites in the myosin VI tail. In conclusion, our characterisation of the human myosin VI deafness mutant (R1166X) suggests that defects in cargo binding may leave myosin VI in a primed/activated state with an increased actin-binding ability.

ΔF508 CFTR surface stability is regulated by DAB2 and CHIP-mediated ubiquitination in post-endocytic compartments

The ΔF508 mutant form of the cystic fibrosis transmembrane conductance regulator (ΔF508 CFTR) that is normally degraded by the ER-associated degradative pathway can be rescued to the cell surface through low-temperature (27°C) culture or small molecular corrector treatment. However, it is unstable on the cell surface, and rapidly internalized and targeted to the lysosomal compartment for degradation. To understand the mechanism of this rapid turnover, we examined the role of two adaptor complexes (AP-2 and Dab2) and three E3 ubiquitin ligases (c-Cbl, CHIP, and Nedd4-2) on low-temperature rescued ΔF508 CFTR endocytosis and degradation in human airway epithelial cells. Our results demonstrate that siRNA depletion of either AP-2 or Dab2 inhibits ΔF508 CFTR endocytosis by 69% and 83%, respectively. AP-2 or Dab2 depletion also increases the rescued protein half-life of ΔF508 CFTR by ~18% and ~91%, respectively. In contrast, the depletion of each of the E3 ligases had no effect on ΔF508 CFTR endocytosis, whereas CHIP depletion significantly increased the surface half-life of ΔF508 CFTR. To determine where and when the ubiquitination occurs during ΔF508 CFTR turnover, we monitored the ubiquitination of rescued ΔF508 CFTR during the time course of CFTR endocytosis. Our results indicate that ubiquitination of the surface pool of ΔF508 CFTR begins to increase 15 min after internalization, suggesting that CFTR is ubiquitinated in a post-endocytic compartment. This post-endocytic ubiquination of ΔF508 CFTR could be blocked by either inhibiting endocytosis, by siRNA knockdown of CHIP, or by treating cells with the CFTR corrector, VX-809. Our results indicate that the post-endocytic ubiquitination of CFTR by CHIP is a critical step in the peripheral quality control of cell surface ΔF508 CFTR.

Myosin VI must dimerize and deploy its unusual lever arm in order to perform its cellular roles

It is unclear whether the reverse-direction myosin (myosin VI) functions as a monomer or dimer in cells and how it generates large movements on actin. We deleted a stable, single-α-helix (SAH) domain that has been proposed to function as part of a lever arm to amplify movements without impact on in vitro movement or in vivo functions. A myosin VI construct that used this SAH domain as part of its lever arm was able to take large steps in vitro but did not rescue in vivo functions. It was necessary for myosin VI to internally dimerize, triggering unfolding of a three-helix bundle and calmodulin binding in order to step normally in vitro and rescue endocytosis and Golgi morphology in myosin VI-null fibroblasts. A model for myosin VI emerges in which cargo binding triggers dimerization and unfolds the three-helix bundle to create a lever arm essential for in vivo functions.

Myosin 6 is required for iris development and normal function of the outer retina

PURPOSE

To determine the molecular basis and the pathologic consequences of a chemically induced mutation in the translational vision research models 89 (tvrm89) mouse model with ERG defects.

METHODS

Mice from a G3 N-ethyl-N-nitrosourea mutagenesis program were screened for behavioral abnormalities and defects in retinal function by ERGs. The chromosomal position for the recessive tvrm89 mutation was determined in a genome-wide linkage analysis. The critical region was refined, and candidate genes were screened by direct sequencing. The tvrm89 phenotype was characterized by circling behavior, in vivo ocular imaging, detailed ERG-based studies of the retina and RPE, and histological analysis of these structures.

RESULTS

The tvrm89 mutation was localized to a region on chromosome 9 containing Myo6. Sequencing identified a T→C point mutation in the codon for amino acid 480 in Myo6 that converts a leucine to a proline. This mutation does not confer a loss of protein expression levels; however, mice homozygous for the Myo6(tvrm89) mutation display an abnormal iris shape and attenuation of both strobe-flash ERGs and direct-current ERGs by 4 age weeks, neither of which is associated with photoreceptor loss.

CONCLUSIONS

The tvrm89 phenotype mimics that reported for Myosin6-null mice, suggesting that the mutation confers a loss of myosin 6 protein function. The observation that homozygous Myo6(tvrm89) mice display reduced ERG a-wave and b-wave components, as well as components of the ERG attributed to RPE function, indicates that myosin 6 is necessary for the generation of proper responses of the outer retina to light.

Myosin VI contributes to maintaining epithelial barrier function

BACKGROUND

Epithelial barrier dysfunction is associated with the pathogenesis of a number of immune inflammations; the etiology is not fully understood. The fusion of endosome/lysosome is a critical process in the degradation of endocytic antigens in epithelial cells. Recent reports indicate that myosin VI (myo6) is involved in the activities of endosomes. The present study aims to investigate the role of myo6 in epithelial barrier dysfunction.

RESULTS

The endosome accumulation was observed in myo6-deficient Rmcs. More than 80% endosomes were fused with lysosomes in naïve Rmcs while less than 30% endosomes were fused with lysosomes in the myo6-deficient Rmcs. The myo6-deficient Rmc monolayers showed high permeability to a macromolecular antigen, ovalbumin, the latter still conserved the antigenicity, which induced strong T cell activation.

CONCLUSIONS

We conclude that myo6 plays a critical role in the fusion of endosome/lysosome in Rmc epithelial cells. Deficiency of myo6 compromises the epithelial barrier function.