Myo1c regulates lipid raft recycling to control cell spreading, migration and Salmonella invasion

Dances with Membranes: Breakthroughs from Super-resolution Imaging

Biological membrane organization mediates numerous cellular functions and has also been connected with an immense number of human diseases. However, until recently, experimental methodologies have been unable to directly visualize the nanoscale details of biological membranes, particularly in intact living cells. Numerous models explaining membrane organization have been proposed, but testing those models has required indirect methods; the desire to directly image proteins and lipids in living cell membranes is a strong motivation for the advancement of technology. The development of super-resolution microscopy has provided powerful tools for quantification of membrane organization at the level of individual proteins and lipids, and many of these tools are compatible with living cells. Previously inaccessible questions are now being addressed, and the field of membrane biology is developing rapidly. This chapter discusses how the development of super-resolution microscopy has led to fundamental advances in the field of biological membrane organization. We summarize the history and some models explaining how proteins are organized in cell membranes, and give an overview of various super-resolution techniques and methods of quantifying super-resolution data. We discuss the application of super-resolution techniques to membrane biology in general, and also with specific reference to the fields of actin and actin-binding proteins, virus infection, mitochondria, immune cell biology, and phosphoinositide signaling. Finally, we present our hopes and expectations for the future of super-resolution microscopy in the field of membrane biology.

Control of the initiation and termination of kinesin-1-driven transport by myosin-Ic and nonmuscle tropomyosin

Intracellular transport is largely driven by processive microtubule- and actin-based molecular motors. Nonprocessive motors have also been localized to trafficking cargos, but their roles are not well understood. Myosin-Ic (Myo1c), a nonprocessive actin motor, functions in a variety of exocytic events, although the underlying mechanisms are not yet clear. To investigate the interplay between myosin-I and the canonical long-distance transport motor kinesin-1, we attached both motor types to lipid membrane-coated bead cargo, using an attachment strategy that allows motors to actively reorganize within the membrane in response to the local cytoskeletal environment. We compared the motility of kinesin-1-driven cargos in the absence and presence of Myo1c at engineered actin/microtubule intersections. We found that Myo1c significantly increases the frequency of kinesin-1-driven microtubule-based runs that begin at actin/microtubule intersections. Myo1c also regulates the termination of processive runs. Beads with both motors bound have a significantly higher probability of pausing at actin/microtubule intersections, remaining tethered for an average of 20 s, with some pauses lasting longer than 200 s. The actin-binding protein nonmuscle tropomyosin (Tm) provides spatially specific regulation of interactions between myosin motors and actin filaments in vivo; in the crossed-filament in vitro assay, we found that Tm2-actin abolishes Myo1c-specific effects on both run initiation and run termination. Together, these observations suggest Myo1c is important for the selective initiation and termination of kinesin-1-driven runs along microtubules at specific actin filament populations within the cell.

Selective expression of myosin IC Isoform A in mouse and human cell lines and mouse prostate cancer tissues

Myosin IC is a single headed member of the myosin superfamily. We recently identified a novel isoform and showed that the MYOIC gene in mammalian cells encodes three isoforms (isoforms A, B, and C). Furthermore, we demonstrated that myosin IC isoform A but not isoform B exhibits a tissue specific expression pattern. In this study, we extended our analysis of myosin IC isoform expression patterns by analyzing the protein and mRNA expression in various mammalian cell lines and in various prostate specimens and tumor tissues from the transgenic mouse prostate (TRAMP) model by immunoblotting, qRT-PCR, and by indirect immunohistochemical staining of paraffin embedded prostate specimen. Analysis of a panel of mammalian cell lines showed an increased mRNA and protein expression of specifically myosin IC isoform A in a panel of human and mouse prostate cancer cell lines but not in non-cancer prostate or other (non-prostate-) cancer cell lines. Furthermore, we demonstrate that myosin IC isoform A expression is significantly increased in TRAMP mouse prostate samples with prostatic intraepithelial neoplasia (PIN) lesions and in distant site metastases in lung and liver when compared to matched normal tissues. Our observations demonstrate specific changes in the expression of myosin IC isoform A that are concurrent with the occurrence of prostate cancer in the TRAMP mouse prostate cancer model that closely mimics clinical prostate cancer. These data suggest that elevated levels of myosin IC isoform A may be a potential marker for the detection of prostate cancer.

Class I myosins in B-cell physiology: functions in spreading, immune synapses, motility, and vesicular traffic

Myosins comprise a family of motor proteins whose role in muscle contraction and motility in a large range of eukaryotic cells has been widely studied. Although these proteins have been characterized extensively and much is known about their function in different cellular compartments, little is known about these molecules in hematopoietic cells. Myosins expressed by cells from the immune response are involved in maintaining plasma membrane tension, moving and secreting vesicles, endo- and exocytotic processes, and promoting the adhesion and motility of cells. Herein, we summarize our current understanding of class I myosins in B cells, with an emphasis on the emerging roles of these molecular motors in immune functions.

Crystal structure of human myosin 1c--the motor in GLUT4 exocytosis: implications for Ca2+ regulation and 14-3-3 binding

Myosin 1c (Myo1c) plays a key role in supporting motile events that underlie cell migration, vesicle trafficking, insulin-stimulated glucose uptake and hearing. Here, we present the crystal structure of the human Myo1c motor in complex with its light chain calmodulin. Our structure reveals tight interactions of the motor domain with calmodulin bound to the first IQ motif in the neck region. Several of the calmodulin residues contributing to this interaction are also involved in Ca(2+) binding. Contact residues in the motor domain are linked to the central β-sheet and the HO helix, suggesting a mechanism for communicating changes in Ca(2+) binding in the neck region to the actin and nucleotide binding regions of the motor domain. The structural context and the chemical environment of Myo1c mutations that are involved in sensorineural hearing loss in humans are described and their impact on motor function is discussed. We show that a construct consisting of the motor domain of Myo1c and the first IQ motif is sufficient to establish a tight interaction with 14-3-3β (KD=0.9 μM) and present the model of a double-headed Myo1c-14-3-3 complex. This complex has been implicated in the exocytosis of glucose transporter 4 storage vesicles during insulin-stimulated glucose uptake.

Tissue specific expression of myosin IC isoforms

Background

Myosin IC is a single headed member of the myosin superfamily that localizes to the cytoplasm and the nucleus and is implicated in a variety of processes in both compartments. We recently identified a novel isoform of myosin IC and showed that the MYOIC gene in mammalian cells encodes three isoforms (isoforms A, B, and C) that differ only in the addition of short isoform-specific N-terminal peptides. The expression pattern of the isoforms and the mechanisms of expression regulation remain unknown.

Results

To determine the expression patterns of myosin IC isoforms, we performed a comprehensive expression analysis of the two myosin IC isoforms (isoform A and B) that contain isoform-specific sequences. By immunoblotting with isoform-specific antibodies and by qRT-PCR with isoform-specific primer we demonstrate that myosin IC isoforms A and B have distinct expression patterns in mouse tissues. Specifically, we show that myosin IC isoform A is expressed in a tissue specific pattern, while myosin IC isoform B is ubiquitously expressed at comparable levels in mouse tissues.

Conclusions

The differences in the expression profile of the myosin IC isoforms indicate a tissue-specific MYOIC gene regulation and further suggest that the myosin IC isoforms, despite their high sequence homology, might have tissue-specific and isoform-specific functions.

Interaction of crk with Myosin-1c participates in fibronectin-induced cell spreading

We previously reported a novel interaction between v-Crk and myosin-1c, and demonstrated that this interaction is essential for cell migration, even in the absence of p130CAS. We here demonstrate a role for Crk-myosin-1c interaction in cell adhesion and spreading. Crk-knockout (Crk^‑/‑) mouse embryo fibroblasts (MEFs) exhibited significantly decreased cell spreading and reduced Rac1 activity. A stroboscopic analysis of cell dynamics during cell spreading revealed that the cell-spreading deficiency in Crk^‑/‑ MEFs was due to the short protrusion/retraction distances and long persistence times of membrane extensions. The low activity of Rac1 in Crk^‑/‑ MEFs, which led to delayed cell spreading in these cells, is consistent with the observed defects in membrane dynamics. Reintroduction of v-Crk into Crk^‑/‑ MEFs rescued these defects, restoring cell-spreading activity and membrane dynamics to Crk^+/+ MEF levels, and normalizing Rac1 activity. Knockdown of myosin-1c by introduction of small interfering RNA resulted in a delay in cell spreading and reduced Rac1 activity to low levels, suggesting that myosin-1c also plays an essential role in cell adhesion and spreading. In addition, deletion of the v-Crk SH3 domain, which interacts with the myosin-1c tail, led to defects in cell spreading. Overexpression of the GFP-myosin-1c tail domain effectively inhibited the v-Crk-myosin-1c interaction and led to a slight decrease in cell spreading and cell surface area. Collectively, these findings suggest that the v-Crk-myosin-1c interaction, which modulates membrane dynamics by regulating Rac1 activity, is crucial for cell adhesion and spreading.

Clathrin-independent endocytosis: a cargo-centric view

Clathrin-independent endocytosis occurs in all cells and interest in this mode of cellular entry has grown. Although this form of endocytosis was first described for entry of bacterial toxins, here we focus our attention on the endogenous cell surface "cargo" proteins that enter cells by this mechanism. The cargo proteins entering by this mechanism are varied and include nutrient transporters, ion channels, cell adhesion molecules and proteins associated with the immune system. Despite the apparent lack of selection at the cell surface, we provide some examples of specific sorting of these cargo proteins after entry, leading to distinct itineraries and cellular fates.

Myosin-1c regulates the dynamic stability of E-cadherin-based cell-cell contacts in polarized Madin-Darby canine kidney cells

Myo1c knockdown causes defects in E-cadherin localization, E-cadherin binding, and cell–cell contact of Madin–Darby canine kidney cells. Expression of wild-type Myo1c, but not motor-dead mutants or those unable to bind membrane, reverses the phenotype, evidence that Myo1c modulates the assembly/maintenance of adherens junctions.

Cooperation between cadherins and the actin cytoskeleton controls the formation and maintenance of cell–cell adhesions in epithelia. We find that the molecular motor protein myosin-1c (Myo1c) regulates the dynamic stability of E-cadherin–based cell–cell contacts. In Myo1c-depleted Madin–Darby canine kidney cells, E-cadherin localization was dis­organized and lateral membranes appeared less vertical with convoluted edges versus control cells. In polarized monolayers, Myo1c-knockdown (KD) cells were more sensitive to reduced calcium concentration. Myo1c separated in the same plasma membrane fractions as E-cadherin, and Myo1c KD caused a significant reduction in the amount of E-cadherin recovered in one peak fraction. Expression of green fluorescent protein (GFP)–Myo1c mutants revealed that the phosphatidylinositol-4,5-bisphosphate–binding site is necessary for its localization to cell–cell adhesions, and fluorescence recovery after photobleaching assays with GFP-Myo1c mutants revealed that motor function was important for Myo1c dynamics at these sites. At 18°C, which inhibits vesicle recycling, Myo1c-KD cells accumulated more E-cadherin–positive vesicles in their cytoplasm, suggesting that Myo1c affects E-cadherin endocytosis. Studies with photoactivatable GFP–E-cadherin showed that Myo1c KD reduced the stability of E-cadherin at cell–cell adhesions. We conclude that Myo1c stabilizes E-cadherin at adherens junctions in polarized epithelial cells and that the motor function and ability of Myo1c to bind membrane are critical.

Actin acting at the Golgi

The organization, assembly and remodeling of the actin cytoskeleton provide force and tracks for a variety of (endo)membrane-associated events such as membrane trafficking. This review illustrates in different cellular models how actin and many of its numerous binding and regulatory proteins (actin and co-workers) participate in the structural organization of the Golgi apparatus and in trafficking-associated processes such as sorting, biogenesis and motion of Golgi-derived transport carriers.

Molecular roles of Myo1c function in lipid raft exocytosis

Lipid rafts are highly dynamic membrane subdomains enriched in specific protein and lipid components that create specialized ‘organizing’ platforms essential for an array of important cellular functions. The role of lipid rafts in membrane trafficking involves the constant remodelling of the plasma membrane through membrane uptake and balanced exocytosis of intracellular membranes. Our lab has identified the first motor protein, myosin 1c (Myo1c) involved in driving the recycling of lipid-raft enriched membranes from the perinuclear recycling compartment to the cell surface. This newly discovered role for Myo1c in lipid raft exocytosis is crucial for cell spreading, migration and pathogen entry; key cellular processes that require cell surface expansion and plasticity. Here we present a model suggesting Myo1c’s possible molecular functions in lipid raft recycling and discuss its wider implications for important cellular functions.

Myo1c is an unconventional myosin required for zebrafish glomerular development

The targeting and organization of podocyte slit diaphragm proteins nephrin and neph1 is critical for development and maintenance of a functional glomerular filtration barrier. Myo1c is a non-muscle myosin motor protein that interacts directly with nephrin and neph1 and mediates their intracellular transport to the podocyte intercellular junction. Here we investigated the necessity of Myo1c in podocyte development using zebrafish as a model system. Immunofluorescence microscopy and in situ RNA hybridization analysis of zebrafish embryos showed that Myo1c is widely expressed in various tissues including the zebrafish glomerulus. Knockdown of the Myo1c gene in zebrafish using antisense morpholino derivatives resulted in an abnormal developmental phenotype that included pericardial edema and dilated renal tubules. Ultra-structural analysis of the glomerulus in Myo1c depleted zebrafish showed abnormal podocyte morphology and absence of the slit diaphragm. Consistent with these observations, the glomerular filter permeability appeared altered in zebrafish in which Myo1c expression was attenuated. The specificity of Myo1c knockdown was confirmed by a rescue experiment in which co-injection of Myo1c morpholino derivatives with orthologous Myo1c mRNA prepared from mouse cDNA lessened phenotypic abnormalities including edema in Myo1c morphants. Thus, our results demonstrate that Myo1c is necessary for podocyte morphogenesis.

Retromer-mediated endosomal protein sorting: all WASHed up!

Endosomal protein sorting governs the fate of many physiologically important proteins involved in a panoply of cellular functions. Recent discoveries have revealed a vital role for endosomally localised branched actin patches in facilitating protein sorting. The formation of the actin patches has been shown to require the function of the WASH complex - the major endosomal actin polymerisation-promoting complex - which stimulates the activity of the ubiquitously expressed Arp2/3 complex. Another key component of the endosomal protein-sorting machinery is the retromer complex. Studies now show that retromer mediates the recruitment of the WASH complex and its regulators to endosomes. In this review, recent progress in understanding the role of the WASH complex along with retromer in endosomal protein sorting is discussed.

LMX1B is part of a transcriptional complex with PSPC1 and PSF

The LIM homeodomain transcription factor Lmx1b is essential for the development of the isthmic organizer and mesodiencephalic dopaminergic neurons. The uncoupling of Pitx3 and Th expression, in the Lmx1b null mutant, suggests that Lmx1b may act as a positional activator of the mdDA domain, eventually leading to properly differentiating mdDA neurons. In this study, we aimed to elucidate how Lmx1b functions mechanistically in this developmental process, by searching for molecular interactors of Lmx1b at the protein level. Initially, affinity-purification of LMX1B-HIS overexpressed protein in MN9D dopaminergic cells followed by mass-spectrometry analysis, resulted in the identification of PSPC1 protein as a possible binding partner of LMX1B. Subsequent immunoprecipitation experiments revealed an interaction between LMX1B and PSPC1 in a larger protein complex also containing PSF. This complex was observed in vitro and in vivo, and we hypothesize that, via PSF and PSPC1, LMX1B may be part of the previously identified Nurr1 transcriptional complex wherein interaction with the co-repressor PSF and the transcription factor Pitx3 is needed to drive expression of Nurr1 target genes in specifying the dopaminergic phenotype. Furthermore, we identified GRLF1, DHX9, MYO1C, HSP70 and TMPO as potential LMX1B interactors. DHX9 and GRLF1 are highly expressed in the developing mdDA neuronal field, and GRLF1 and MYO1C have both been linked to neurite outgrowth. The identification of these proteins suggests that Lmx1b may act directly in the transcriptional activation of Nurr1 target genes and be involved in other processes like neurite outgrowth as well.

Functional roles for myosin 1c in cellular signaling pathways

Cellular signaling pathways underlie the transfer of information throughout the cell and to adjoining cells and so govern most critical cellular functions. Increasing evidence points to the molecular motor myosin 1c as a prominent player in many signaling cascades, from the integrin-dependent signaling involved in cell migration to the signaling events underlying insulin resistance. Myosin 1c functions on these pathways both via an important role in regulating lipid raft recycling and also via direct involvement in signaling cascades. This review provides an overview of the functional involvement of myosin 1c in cellular signaling and discusses the possible potential for myosin 1c as a target for drug-based treatments for human diseases.

The myosin motor Myo1c is required for VEGFR2 delivery to the cell surface and for angiogenic signaling

Vascular endothelial growth factor receptor-2 (VEGFR2) is a receptor tyrosine kinase that is expressed in endothelial cells and regulates angiogenic signal transduction under both physiological and pathological conditions. VEGFR2 turnover at the plasma membrane (PM) is regulated by its transport through endocytic and secretory transport pathways. Short-range cargo trafficking along actin filaments is commonly regulated by motor proteins of myosin superfamily. In the current study, performed in primary human endothelial cells, we demonstrate that unconventional myosin 1c (Myo1c; class I family member) regulates the localization of VEGFR2 at the PM. We further demonstrate that the recruitment of VEGFR2 to the PM and its colocalization with Myo1c and caveolin-1 occur in response to VEGF-A (VEGF) stimulation. In addition, VEGF-induced delivery of VEGFR2 to the cell surface requires Myo1c; surface VEGFR2 levels are reduced in the absence of Myo1c and, more importantly, are restored by the overexpression of wild-type but not mutant Myo1c. Subcellular density gradient fractionation revealed that partitioning of VEGFR2 into caveolin-1- and Myo1c-enriched membrane fractions is dependent on VEGF stimulation. Myo1c depletion resulted in increased VEGF-induced VEGFR2 transport to the lysosomes for degradation and was rescued by applying either brefeldin A, which blocks trafficking between the endoplasmic reticulum and the Golgi complex, or dynasore, an inhibitor of dynamin-mediated endocytosis. Myo1c depletion also reduced VEGF-induced VEGFR2 phosphorylation at Y1175 and phosphorylation-dependent activation of ERK1/2 and c-Src kinase, leading to reduced cell proliferation and cell migration. This is the first report demonstrating that Myo1c is an important mediator of VEGF-induced VEGFR2 delivery to the cell surface and plays a role in angiogenic signaling.

Regulation and control of myosin-I by the motor and light chain-binding domains

Members of the myosin-I family of molecular motors are expressed in many eukaryotes, where they are involved in a multitude of critical processes. Humans express eight distinct members of the myosin-I family, making it the second largest family of myosins expressed in humans. Despite the high degree of sequence conservation in the motor and light chain-binding domains (LCBDs) of these myosins, recent studies have revealed surprising diversity of function and regulation arising from isoform-specific differences in these domains. Here we review the regulation of myosin-I function and localization by the motor and LCBDs.

Membrane-bound myo1c powers asymmetric motility of actin filaments

Class I myosins are molecular motors that link cellular membranes to the actin cytoskeleton and play roles in membrane tension generation, membrane dynamics, and mechanosignal transduction. The widely expressed myosin-Ic (myo1c) isoform binds tightly to phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] via a pleckstrin homology domain located in the myo1c tail, which is important for its proper cellular localization. In this study, we found that myo1c can power actin motility on fluid membranes composed of physiological concentrations of PtdIns(4,5)P(2) and that this motility is inhibited by high concentrations of anionic phospholipids. Strikingly, this motility occurs along curved paths in a counterclockwise direction (i.e., the actin filaments turn in leftward circles). A biotinylated myo1c construct containing only the motor domain and the lever arm anchored via streptavidin on a membrane containing biotinylated lipid can also generate asymmetric motility, suggesting that the tail domain is not required for the counterclockwise turning. We found that the ability to produce counterclockwise motility is not a universal characteristic of myosin-I motors, as membrane-bound myosin-Ia (myo1a) and myosin-Ib (myo1b) are able to power actin gliding, but the actin gliding has no substantial turning bias. This work reveals a possible mechanism for establishing asymmetry in relationship to the plasma membrane.

Myosin IC generates power over a range of loads via a new tension-sensing mechanism

Myosin IC (myo1c), a widely expressed motor protein that links the actin cytoskeleton to cell membranes, has been associated with numerous cellular processes, including insulin-stimulated transport of GLUT4, mechanosensation in sensory hair cells, endocytosis, transcription of DNA in the nucleus, exocytosis, and membrane trafficking. The molecular role of myo1c in these processes has not been defined, so to better understand myo1c function, we utilized ensemble kinetic and single-molecule techniques to probe myo1c's biochemical and mechanical properties. Utilizing a myo1c construct containing the motor and regulatory domains, we found the force dependence of the actin-attachment lifetime to have two distinct regimes: a force-independent regime at forces < 1 pN, and a highly force-dependent regime at higher loads. In this force-dependent regime, forces that resist the working stroke increase the actin-attachment lifetime. Unexpectedly, the primary force-sensitive transition is the isomerization that follows ATP binding, not ADP release as in other slow myosins. This force-sensing behavior is unique amongst characterized myosins and clearly demonstrates mechanochemical diversity within the myosin family. Based on these results, we propose that myo1c functions as a slow transporter rather than a tension-sensitive anchor.