Localization of unconventional myosins V and VI in neuronal growth cones

Herpes simplex virus-1 utilizes the host actin cytoskeleton for its release from axonal growth cones

Herpes simplex virus type 1 (HSV-1) has evolved mechanisms to exploit the host cytoskeleton during entry, replication and exit from cells. In this study, we determined the role of actin and the molecular motor proteins, myosin II and myosin V, in the transport and release of HSV-1 from axon termini, or growth cones. Using compartmentalized neuronal devices, we showed that inhibition of actin polymerization, but not actin branching, significantly reduced the release of HSV-1 from axons. Furthermore, we showed that inhibition of myosin V, but not myosin II, also significantly reduced the release of HSV-1 from axons. Using confocal and electron microscopy, we determined that viral components are transported along axons to growth cones, despite actin or myosin inhibition. Overall, our study supports the role of actin in virus release from axonal growth cones and suggests myosin V as a likely candidate involved in this process.

Herpes simplex virus type 1 (HSV-1) is a ubiquitous human pathogen causing cold sores and genital herpes. HSV-1 infects sensory neurons of the peripheral nervous system where it establishes a lifelong infection and cannot be cured. Reactivation is common, with the virus transported back along sensory nerves, forming new lesions, or is shed asymptomatically. Antiviral resistance is emerging to current antivirals that target viral replication, indicating the need to identify new targets for future treatment. The host cell cytoskeleton plays an important role during transport of the virus. HSV-1 is transported along axons via microtubules; however, how the virus is released from axon termini, where actin predominates, is unknown. Here we show that an intact actin cytoskeleton is required for efficient virus release from axon termini. Furthermore, we show that myosin V, an actin based molecular motor that drives transport, is essential in virus release from axon termini. Together, this study defines the mechanisms behind HSV-1 release from axon termini which will guide future directions in identifying possible therapeutic targets for HSV-1.

Expression of MYOSIN VIIA in developing mouse cochleovestibular ganglion neurons

Myosins make up a large super family of motor proteins responsible for actin-based motility in most eukaryotic cells. Myosin VIIA is essential for the development and function of sensory hair cells in the inner ear. The role of Myosin VIIA in the development of cochleovestibular ganglion (CVG) neurons in the mouse is largely unknown. Neurons of the CVG innervate sensory hair cells of the cochlea and vestibular organs to transmit hearing and balance information respectively to the brain. The aim of this study was to characterize the expression of MYOSIN VIIA in the CVG of mouse embryos. Spatiotemporal expression of MYOSIN VIIA was characterized in embryonic (E) mouse inner ear neurons from E9.5 to postnatal (P) day 0. At early stages, when otic neurons begin to delaminate to form the CVG, MYOSIN VIIA was co-expressed with TuJ1, ISLET1 and NEUROD in the otic epithelium and CVG. When CVG neurons were migrating and exiting mitosis, MYSOSIN VIIA was downregulated in a subset of neurons, which were NEUROD-negative and GATA3-positive. After segregation of the CVG, MYOSIN VIIA was observed in a subset of vestibular neurons marked by TUJ1 and absent in cochlear neurons, marked by GATA3. The differential expression of MYOSIN VIIA may indicate a role in inner ear neuron migration and specific labeling of vestibular neurons.

Identification of mutations in the MYO9A gene in patients with congenital myasthenic syndrome

Congenital myasthenic syndromes are a group of rare and genetically heterogenous disorders resulting from defects in the structure and function of the neuromuscular junction. Patients with congenital myasthenic syndrome exhibit fatigable muscle weakness with a variety of accompanying phenotypes depending on the protein affected. A cohort of patients with a clinical diagnosis of congenital myasthenic syndrome that lacked a genetic diagnosis underwent whole exome sequencing in order to identify genetic causation. Missense biallelic mutations in the MYO9A gene, encoding an unconventional myosin, were identified in two unrelated families. Depletion of MYO9A in NSC-34 cells revealed a direct effect of MYO9A on neuronal branching and axon guidance. Morpholino-mediated knockdown of the two MYO9A orthologues in zebrafish, myo9aa/ab, demonstrated a requirement for MYO9A in the formation of the neuromuscular junction during development. The morphants displayed shortened and abnormally branched motor axons, lack of movement within the chorion and abnormal swimming in response to tactile stimulation. We therefore conclude that MYO9A deficiency may affect the presynaptic motor axon, manifesting in congenital myasthenic syndrome. These results highlight the involvement of unconventional myosins in motor axon functionality, as well as the need to look outside traditional neuromuscular junction-specific proteins for further congenital myasthenic syndrome candidate genes.

Kinetic Adaptations of Myosins for Their Diverse Cellular Functions

Members of the myosin superfamily are involved in all aspects of eukaryotic life. Their function ranges from the transport of organelles and cargos to the generation of membrane tension, and the contraction of muscle. The diversity of physiological functions is remarkable, given that all enzymatically active myosins follow a conserved mechanoenzymatic cycle in which the hydrolysis of ATP to ADP and inorganic phosphate is coupled to either actin-based transport or tethering of actin to defined cellular compartments. Kinetic capacities and limitations of a myosin are determined by the extent to which actin can accelerate the hydrolysis of ATP and the release of the hydrolysis products and are indispensably linked to its physiological tasks. This review focuses on kinetic competencies that - together with structural adaptations - result in myosins with unique mechanoenzymatic properties targeted to their diverse cellular functions.

Netrin-1 induces local translation of down syndrome cell adhesion molecule in axonal growth cones

Down syndrome cell adhesion molecule (DSCAM) plays an important role in many neurodevelopmental processes such as axon guidance, dendrite arborization, and synapse formation. DSCAM is located in the Down syndrome trisomic region of human chromosome 21 and may contribute to the Down syndrome brain phenotype, which includes a reduction in the formation of long-distance connectivity. The local translation of a select group of mRNA transcripts within growth cones is necessary for the formation of appropriate neuronal connectivity. Interestingly, we have found that Dscam mRNA is localized to growth cones of mouse hippocampal neurons, and is dynamically regulated in response to the axon guidance molecule, netrin-1. Furthermore, netrin-1 stimulation results in an increase in locally translated DSCAM protein in growth cones. Deleted in colorectal cancer (DCC), a netrin-1 receptor, is required for the netrin-1-induced increase in Dscam mRNA local translation. We also find that two RNA-binding proteins-fragile X mental retardation protein (FMRP) and cytoplasmic polyadenylation element binding protein (CPEB)-colocalize with Dscam mRNA in growth cones, suggesting their regulation of Dscam mRNA localization and translation. Finally, overexpression of DSCAM in mouse cortical neurons results in a severe stunting of axon outgrowth and branching, suggesting that an increase in DSCAM protein results in a structural change having functional consequences. Taken together, these results suggest that netrin-1-induced local translation of Dscam mRNA during embryonic development may be an important mechanism to regulate axon growth and guidance in the developing nervous system. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 799-816, 2016.

Myosin lever arm directs collective motion on cellular actin network

The molecular motor myosin teams up to drive muscle contraction, membrane traffic, and cell division in biological cells. Myosin function in cells emerges from the interaction of multiple motors tethered to a scaffold, with surrounding actin filaments organized into 3D networks. Despite the importance of myosin function, the influence of intermotor interactions on collective motion remains poorly understood. In this study, we used precisely engineered myosin assemblies to examine emergence in collective myosin movement. We report that tethering multiple myosin VI motors, but not myosin V motors, modifies their movement trajectories on keratocyte actin networks. Single myosin V and VI dimers display similar skewed trajectories, albeit in opposite directions, when traversing the keratocyte actin network. In contrast, tethering myosin VI motors, but not myosin V motors, progressively straightens the trajectories with increasing myosin number. Trajectory shape of multimotor scaffolds positively correlates with the stiffness of the myosin lever arm. Swapping the flexible myosin VI lever arm for the relatively rigid myosin V lever increases trajectory skewness, and vice versa. A simplified model of coupled motor movement demonstrates that the differences in flexural rigidity of the two myosin lever arms is sufficient to account for the differences in observed behavior of groups of myosin V and VI motors. In accordance with this model trajectory, shapes for scaffolds containing both myosin V and VI are dominated by the myosin with a stiffer lever arm. Our findings suggest that structural features unique to each myosin type may confer selective advantages in cellular functions.

Myosins Are Differentially Expressed under Oxidative Stress in Chronic Streptozotocin-Induced Diabetic Rat Brains

Diabetes mellitus is a disease characterized by persistent hyperglycemia, which may lead to brain tissue damage due to oxidative stress and also contributes to neuronal death and changes in synaptic transmission. This study evaluated the effect of oxidative stress and the use of antioxidants supplementation on myosins expression levels in the brains of chronic diabetic rats induced by streptozotocin. Lipid peroxidation, antioxidant enzymes activities, and myosins-IIB and -Va expressions at transcriptional and translational levels were examined after 90 days induction. The chronic effect of the diabetes led to the upregulation of superoxide dismutase (SOD) and catalase (CAT) activities, and the downregulation of glutathione peroxidase (GPx), but there was no statistically significant increase in the malondialdehyde (MDA) levels. These alterations were accompanied by high myosin-IIB and low myosin-Va expressions. Although the antioxidant supplementation did not interfere on MDA levels, the oxidative stress caused by chronic hyperglycemia was reduced by increasing SOD and restoring CAT and GPx activities. Interestingly, after supplementation, diabetic rats recovered only myosin-Va protein levels, without interfering on myosins mRNA levels expressed in diabetic rat brains. Our results suggest that antioxidant supplementation reduces oxidative stress and also regulates the myosins protein expression, which should be beneficial to individuals with diabetes/chronic hyperglycemia.

The streptozotocin-induced rat model of diabetes mellitus evidences significant reduction of myosin-Va expression in the brain

Diabetes mellitus is a disease characterized by increased glucose levels in the blood. Hyperglycemia causes damage to the brain tissue, and induces significant changes in synaptic transmission. In this investigation, we have found a significant alteration in the expression of the molecular motor involved in the synaptic vesicles transport, myosin-Va, and its distribution in rat brains of streptozotocin-induced diabetes model. Brains were removed after 20 days, homogenized and analysed by Western blotting, qRT-PCR and immunohistochemistry. Myosin-Va presented significantly lower levels of both mRNA and protein in diabetic than those observed in non-diabetic animals. Moreover, neuronal and glial cells of the occipital and frontal cortex exhibited decreased myosin-Va immunostaining in diabetic rat brains. In conclusion, diabetic rat brains displayed altered expression and distribution of myosin-Va, and these finding may contribute to the basic understanding about this myosin role in brain function related to diabetes.

Myosins and DYNLL1/LC8 in the honey bee (Apis mellifera L.) brain

Honey bees have brain structures with specialized and developed systems of communication that account for memory, learning capacity and behavioral organization with a set of genes homologous to vertebrate genes. Many microtubule- and actin-based molecular motors are involved in axonal/dendritic transport. Myosin-Va is present in the honey bee Apis mellifera nervous system of the larvae and adult castes and subcastes. DYNLL1/LC8 and myosin-IIb, -VI and -IXb have also been detected in the adult brain. SNARE proteins, such as CaMKII, clathrin, syntaxin, SNAP25, munc18, synaptophysin and synaptotagmin, are also expressed in the honey bee brain. Honey bee myosin-Va displayed ATP-dependent solubility and was associated with DYNLL1/LC8 and SNARE proteins in the membrane vesicle-enriched fraction. Myosin-Va expression was also decreased after the intracerebral injection of melittin and NMDA. The immunolocalization of myosin-Va and -IV, DYNLL1/LC8, and synaptophysin in mushroom bodies, and optical and antennal lobes was compared with the brain morphology based on Neo-Timm histochemistry and revealed a distinct and punctate distribution. This result suggested that the pattern of localization is associated with neuron function. Therefore, our data indicated that the roles of myosins, DYNLL1/LC8, and SNARE proteins in the nervous and visual systems of honey bees should be further studied under different developmental, caste and behavioral conditions.

Myosin Va and myosin VI coordinate their steps while engaged in an in vitro tug of war during cargo transport

Myosin Va (myoV) and myosin VI (myoVI) are processive molecular motors that transport cargo in opposite directions on actin tracks. Because these motors may bind to the same cargo in vivo, we developed an in vitro "tug of war" to characterize the stepping dynamics of single quantum-dot-labeled myoV and myoVI motors linked to a common cargo. MyoV dominates its myoVI partner 79% of the time. Regardless of which motor wins, its stepping rate slows due to the resistive load of the losing motor (myoV, 2.1 pN; myoVI, 1.4 pN). Interestingly, the losing motor steps backward in synchrony with the winning motor. With ADP present, myoVI acts as an anchor to prevent myoV from stepping forward. This model system emphasizes the physical communication between opposing motors bound to a common cargo and highlights the potential for modulating this interaction by changes in the cell's ionic milieu.

Characterization and localization of dynein and myosins V and VI in the ovaries of queen bees

The presence of myosin and dynein in the ovaries of both Apis mellifera and Scaptotrigona postica was investigated in extracts and in histological sections. In the ovary extracts, motor proteins, myosins V, VI and dynein were detected by Western blot. In histological sections, they were detected by immunocytochemistry, using a mouse monoclonal antibody against the intermediary chain of dynein and a rabbit polyclonal antibody against the myosin V head domain. The myosin VI tail domain was recognized by a pig polyclonal antibody. The results show that these molecular motors are expressed in the ovaries of both bee species with few differences in location and intensity, in regions where movement of substances is expected during oogenesis. The fact that antibodies against vertebrate proteins recognize proteins of bee species indicates that the specific epitopes are evolutionarily well preserved.

Myosin motor proteins in the cell biology of axons and other neuronal compartments

Most neurons of both the central and peripheral nervous systems express multiple members of the myosin superfamily that include nonmuscle myosin II, and a number of classes of unconventional myosins. Several classes of unconventional myosins found in neurons have been shown to play important roles in transport processes. A general picture of the myosin-dependent transport processes in neurons is beginning to emerge, although much more work still needs to be done to fully define these roles and establish the importance of myosin for axonal transport. Myosins appear to contribute to three types of transport processes in neurons; recycling of receptors or other membrane components, dynamic tethering of vesicular components, and transport or tethering of protein translational machinery including mRNA. Defects in one or more of these functions have potential to contribute to disease processes.

Navigating the cell: UNC-53 and the navigators, a family of cytoskeletal regulators with multiple roles in cell migration, outgrowth and trafficking

Changes in cell shape are associated with a variety of processes including cell migration, axon outgrowth, cell division and vesicle trafficking. C. elegans UNC-53 and its vertebrate homologs, the Navigators, are required for the migration of cells and the outgrowth of neuronal processes. The identification of novel molecular interactions and live imaging studies have revealed that UNC-53/NAVs are signal transducers associated with actin filaments, microtubules and intermediate filaments. In addition to modulating cytoskeletal dynamics at the leading edge of migrating or outgrowing cells, both UNC-53 and the navigators are expressed in adult cells, conspicuously those with specialized roles in endocytosis or secretion. Collectively, these results suggest that UNC-53/NAVs may be a central regulator of cytoskeletal dynamics, responsible for integrating signaling cues to multiple components of the cytoskeleton to coordinate rearrangement during cell outgrowth or trafficking.

Localization of myosin-Va in subpopulations of cells in rat endocrine organs

Myosin-Va is a Ca(2+)/calmodulin-regulated unconventional myosin involved in the transport of vesicles, membranous organelles, and macromolecular complexes composed of proteins and mRNA. The cellular localization of myosin-Va has been described in great detail in several vertebrate cell types, including neurons, melanocytes, lymphocytes, auditory tissues, and a number of cultured cells. Here, we provide an immunohistochemical view of the tissue distribution of myosin-Va in the major endocrine organs. Myosin-Va is highly expressed in the pineal and pituitary glands and in specific cell populations of other endocrine glands, especially the parafollicular cells of the thyroid, the principal cells of the parathyroid, the islets of Langerhans of the pancreas, the chromaffin cells of the adrenal medulla, and a subpopulation of interstitial testicular cells. Weak to moderate staining has been detected in steroidogenic cells of the adrenal cortex, ovary, and Leydig cells. Myosin-Va has also been localized to non-endocrine cells, such as the germ cells of the seminiferous epithelium and maturing oocytes and in the intercalated ducts of the exocrine pancreas. These data provide the first systematic description of myosin-Va localization in the major endocrine organs of rat.

Impaired spatial memory and altered dendritic spine morphology in angiotensin II type 2 receptor-deficient mice

Mental retardation is the most frequent cause of serious handicap in children and young adults. Mutations in the human angiotensin II type 2 receptor (AT2) have been implicated in X-linked forms of mental retardation. We here demonstrate that mice lacking the AT2 receptor gene are significantly impaired in their performance in a spatial memory task and in a one-way active avoidance task. As no difference was observed between the genotypes in fear conditioning, the detected deficit in spatial memory may not relate to fear. Notably, receptor knockout mice showed increased motility in an activity meter and elevated plus maze. Importantly, these mice are characterized by abnormal dendritic spine morphology and length, both features also found to be associated with some cases of mental retardation. These findings suggest a crucial role of AT2 in normal brain function and that dysfunction of the receptor has impact on brain development and ultrastructural morphology with distinct consequences on learning and memory.

Expression of unconventional myosin genes during neuronal development in zebrafish

Neuronal migration and growth cone motility are essential aspects of the development and maturation of the nervous system. These cellular events result from dynamic changes in the organization and function of the cytoskeleton, in part due to the activity of cytoskeletal motor proteins such as myosins. Although specific myosins such as Myo2 (conventional or muscle myosin), Myo1, and Myo5 have been well characterized for roles in cell motility, the roles of the majority of unconventional (other than Myo2) myosins in cell motility events have not been investigated. To address this issue, we have undertaken an analysis of unconventional myosins in zebrafish, a premier model for studying cellular and growth cone motility in the vertebrate nervous system. We describe the characterization and expression patterns of several members of the unconventional myosin gene family. Based on available genomic sequence data, we identified 18 unconventional myosin- and 4 Myo2-related genes in the zebrafish genome in addition to previously characterized myosin (1, 2, 3, 5, 6, 7) genes. Phylogenetic analyses indicate that these genes can be grouped into existing classifications for unconventional myosins from mouse and man. In situ hybridization analyses using EST probes for 18 of the 22 identified genes indicate that 11/18 genes are expressed in a restricted fashion in the zebrafish embryo. Specific myosins are expressed in particular neuronal or neuroepithelial cell types in the developing zebrafish nervous system, spanning the periods of neuronal differentiation and migration, and of growth cone guidance and motility.

Diminished climbing fiber innervation of Purkinje cells in the cerebellum of myosin Va mutant mice and rats

Myosin Va is an actin-based molecular motor that is involved in organelle transport and membrane trafficking. Here, we explored the role of myosin Va in the formation of synaptic circuitry by examining climbing fiber (CF) innervation of Purkinje cells (PCs) in the cerebella of dilute-neurological (d-n) mice and dilute-opisthotonus (dop) rats that have mutations in dilute-encoded myosin Va. Anterograde labeling of CFs with biotinylated dextran amine (BDA) revealed that they arborized poorly and that their tips extended only half way through the thickness of the molecular layer (ML) in adult d-n mice. Using immunohistochemistry specific for vesicular glutamate transporter 2 (VGluT2) to visualize CF synaptic terminals, we found that during development and in adulthood, these terminals did not ascend as far along the proximal shaft dendrites of PCs in d-n mice and dop rats as they did in normal animals. An irregular distribution of BDA-labeled bulbous varicosities and VGluT2 spots along CF branches were also noted in these animals. Finally, VGluT2-positive CF terminals were occasionally localized on the PC somata of adult d-n cerebella. These phenotypes are consistent with our electrophysiological findings that CF-mediated excitatory postsynaptic currents (EPSCs) were significantly smaller in amplitude and faster in decay in adult d-n mice, and that the regression of multiple CFs was slightly delayed in developing d-n mice. Taken together, our results suggest that myosin Va is essential for terminal CF extension and for the establishment of CF synapses within the proper dendritic territories of PCs.

The axonal transport of mitochondria

Organelle transport is vital for the development and maintenance of axons, in which the distances between sites of organelle biogenesis, function, and recycling or degradation can be vast. Movement of mitochondria in axons can serve as a general model for how all organelles move: mitochondria are easy to identify, they move along both microtubule and actin tracks, they pause and change direction, and their transport is modulated in response to physiological signals. However, they can be distinguished from other axonal organelles by the complexity of their movement and their unique functions in aerobic metabolism, calcium homeostasis and cell death. Mitochondria are thus of special interest in relating defects in axonal transport to neuropathies and degenerative diseases of the nervous system. Studies of mitochondrial transport in axons are beginning to illuminate fundamental aspects of the distribution mechanism. They use motors of one or more kinesin families, along with cytoplasmic dynein, to translocate along microtubules, and bidirectional movement may be coordinated through interaction between dynein and kinesin-1. Translocation along actin filaments is probably driven by myosin V, but the protein(s) that mediate docking with actin filaments remain unknown. Signaling through the PI 3-kinase pathway has been implicated in regulation of mitochondrial movement and docking in the axon, and additional mitochondrial linker and regulatory proteins, such as Milton and Miro, have recently been described.

Localization of myosin II and V isoforms in cultured rat sympathetic neurones and their potential involvement in presynaptic function

While vesicle transport is one of the principal functions of myosin motors in neurones, the role played by specific myosin subtypes in discrete vesicle trafficking is poorly understood. We conducted electrophysiological and morphological experiments to determine whether myosin isoforms II and V might be involved in the transport of small synaptic vesicles in presynaptic nerve terminals of a model cholinergic synapse. Electron microscopy revealed the presence of normal synaptic architecture and synaptic vesicle density in presynaptic terminals of cultured superior cervical ganglion neurones (SCGNs) from myosin Va null rats (dilute-opisthotonus, dop). Similarly, electrophysiological analyses of synaptic transmission and synaptic vesicle cycling at paired SCGN synapses failed to uncover any significant differences in synaptic development and function between normal and dop rats. Immunocytochemistry and in situ localization of green fluorescent protein (GFP)-fusion proteins in wild-type synapses revealed that myosins IIB and Va were distributed throughout the cell soma and processes of SCGNs, while myosins IIA and Vb were not detected in SCGNs. Myosin Va was conspicuously absent in presynaptic nerve terminals, but myosin IIB alone was found to be expressed. Furthermore, synaptic transmission was inhibited by introduction of myosin IIB heavy chain fragments into presynaptic terminals of SCGNs. Together these results suggest that only myosin IIB isoform participates in vesicle trafficking in presynaptic nerve terminals of cultured SCGNs.

A role for myosin VI in postsynaptic structure and glutamate receptor endocytosis

Myosin VI (Myo6) is an actin-based motor protein implicated in clathrin-mediated endocytosis in nonneuronal cells, though little is known about its function in the nervous system. Here, we find that Myo6 is highly expressed throughout the brain, localized to synapses, and enriched at the postsynaptic density. Myo6-deficient (Snell's waltzer; sv/sv) hippocampus exhibits a decrease in synapse number, abnormally short dendritic spines, and profound astrogliosis. Similarly, cultured sv/sv hippocampal neurons display decreased numbers of synapses and dendritic spines, and dominant-negative disruption of Myo6 in wild-type hippocampal neurons induces synapse loss. Importantly, we find that sv/sv hippocampal neurons display a significant deficit in the stimulation-induced internalization of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid-type glutamate receptors (AMPARs), and that Myo6 exists in a complex with the AMPAR, AP-2, and SAP97 in brain. These results suggest that Myo6 plays a role in the clathrin-mediated endocytosis of AMPARs, and that its loss leads to alterations in synaptic structure and astrogliosis.

Guiding neuronal growth cones using Ca2+ signals

Pathfinding by growing axons in the developing or regenerating nervous system is guided by gradients of molecular guidance cues. The neuronal growth cone, located at the ends of axons, uses surface receptors to sense these cues and to transduce guidance information to cellular machinery that mediates growth and turning responses. Cytoplasmic Ca2+ signals have key roles in regulating this motility. Global growth cone Ca2+ signals can regulate cytoskeletal elements and membrane dynamics to control elongation, whereas Ca2+ signals localized to one side of the growth cone can cause asymmetric activation of effector enzymes to steer the growth cone. Modulating Ca2+ levels in the growth cone might overcome inhibitory signals that normally prevent regeneration in the central nervous system.

A role for myosin-1A in the localization of a brush border disaccharidase

To gain insight regarding myosin-1A (M1A) function, we expressed a dominant negative fragment of this motor in the intestinal epithelial cell line, CACO-2_BBE. Sucrase isomaltase (SI), a transmembrane disaccharidase found in microvillar lipid rafts, was missing from the brush border (BB) in cells expressing this fragment. Density gradient centrifugation, affinity purification, and immunopurification of detergent-resistant membranes isolated from CACO-2_BBE cells and rat microvilli (MV) all indicate that M1A and SI reside on the same population of low density (∼1.12 g/ml) membranes. Chemical cross-linking of detergent-resistant membranes from rat MV indicates that SI and M1A may interact in a lipid raft complex. The functional significance of such a complex is highlighted by expression of the cytoplasmic domain of SI, which results in lower levels of M1A and a loss of SI from the BB. Together, these studies are the first to assign a specific role to M1A and suggest that this motor is involved in the retention of SI within the BB.

Myosin-dependent transport in neurons

Axonal transport in neurons has been shown to be microtubule dependent, driven by the molecular motor proteins kinesin and dynein. However, organelles undergoing fast transport can often pause or rapidly change directions without apparent dissociation from their transport tracks. Cytoskeletal polymers such as neurofilaments and microtubules have also been shown to make infrequent but rapid movements in axons indicating that their transport is likely to involve molecular motors. In addition, neurons have multiple compartments that are devoid of microtubules where transport of organelles is still seen to occur. These areas are rich in other cytoskeletal polymers such as actin filaments. Transported organelles have been shown to associate with multiple motor proteins including myosins. This suggests that nonmicrotubule-based transport may be myosin driven. In this review we will focus our attention on myosin motors known to be present in neurons and evaluate the evidence that they contribute to transport or other functions in the different compartments of the neuron.

Myosin function in nervous and sensory systems

Development of the nervous system requires remarkable changes in cell structure that are dependent upon the cytoskeleton. The importance of specific components of the neuronal cytoskeleton, such as microtubules and neurofilaments, to neuronal function and development has been well established. Recently, increasing focus has been put on understanding the functional role of the actin cytoskeleton in neurons. Important modulators of the actin cytoskeleton are the large family of myosins, many of which (classes I, II, III, V, VI, VII, IX, and XV; Fig. 1) are expressed in developing neurons or sensory cells. Myosins are force-producing proteins that have been implicated in a wide variety of cellular functions in the developing nervous system, including neuronal migration, process outgrowth, and growth cone motility, as well as other aspects of morphogenesis, axonal transport, and synaptic and sensory functions. We review the roles that neuronal myosins play in these functions with particular focus on the first three events listed above, as well as sensory function.

Cytoskeletal dynamics and transport in growth cone motility and axon guidance

Recent studies indicate the actin and microtubule cytoskeletons are a final common target of many signaling cascades that influence the developing neuron. Regulation of polymer dynamics and transport are crucial for the proper growth cone motility. This review addresses how actin filaments, microtubules, and their associated proteins play crucial roles in growth cone motility, axon outgrowth, and guidance. We present a working model for cytoskeletal regulation of directed axon outgrowth. An important goal for the future will be to understand the coordinated response of the cytoskeleton to signaling cascades induced by guidance receptor activation.

Myosin IIA drives neurite retraction

Neuritic extension is the resultant of two vectorial processes: outgrowth and retraction. Whereas myosin IIB is required for neurite outgrowth, retraction is driven by a motor whose identity has remained unknown until now. Preformed neurites in mouse Neuro-2A neuroblastoma cells undergo immediate retraction when exposed to isoform-specific antisense oligonucleotides that suppress myosin IIB expression, ruling out myosin IIB as the retraction motor. When cells were preincubated with antisense oligonucleotides targeting myosin IIA, simultaneous or subsequent addition of myosin IIB antisense oligonucleotides did not elicit neurite retraction, both outgrowth and retraction being curtailed. Even during simultaneous application of antisense oligonucleotides against both myosin isoforms, lamellipodial spreading continued despite the complete inhibition of neurite extension, indicating an uncoupling of lamellipodial dynamics from movement of the neurite. Significantly, lysophosphatidate- or thrombin-induced neurite retraction was blocked not only by the Rho-kinase inhibitor Y27632 but also by antisense oligonucleotides targeting myosin IIA. Control oligonucleotides or antisense oligonucleotides targeting myosin IIB had no effect. In contrast, Y27632 did not inhibit outgrowth, a myosin IIB-dependent process. We conclude that the conventional myosin motor, myosin IIA, drives neurite retraction.

Myosin-Va binds to and mechanochemically couples microtubules to actin filaments

Myosin-Va was identified as a microtubule binding protein by cosedimentation analysis in the presence of microtubules. Native myosin-Va purified from chick brain, as well as the expressed globular tail domain of this myosin, but not head domain bound to microtubule-associated protein-free microtubules. Binding of myosin-Va to microtubules was saturable and of moderately high affinity (approximately 1:24 Myosin-Va:tubulin; Kd = 70 nM). Myosin-Va may bind to microtubules via its tail domain because microtubule-bound myosin-Va retained the ability to bind actin filaments resulting in the formation of cross-linked gels of microtubules and actin, as assessed by fluorescence and electron microscopy. In low Ca2+, ATP addition induced dissolution of these gels, but not release of myosin-Va from MTs. However, in 10 microM Ca2+, ATP addition resulted in the contraction of the gels into aster-like arrays. These results demonstrate that myosin-Va is a microtubule binding protein that cross-links and mechanochemically couples microtubules to actin filaments.

Eicosanoid activation of protein kinase C epsilon: involvement in growth cone repellent signaling

Exposure of growing neurons to thrombin or semaphorin 3A stimulates a receptor-mediated signaling cascade that results in collapse of their growth cones. This collapse response necessitates eicosanoid production, as we have shown earlier. The present report investigates whether and which protein kinase C (PKC) isoforms may be activated by such eicosanoids. To examine these questions, we isolated growth cones from fetal rat brain and tested whether thrombin or the eicosanoid, 12(S)-hydroxyeicosatetraenoic acid (12(S)-HETE), could activate endogenous growth cone PKC. We show that both thrombin and 12(S)-HETE stimulate the phosphorylation of the myristoylated alanine-rich protein kinase C substrate, an 87-kDa adhesion site protein. Furthermore, we show both with immunoprecipitated and with recombinant PKC that 12(S)-HETE activation is selective for the epsilon isoform and does not require accessory proteins. Last, we demonstrate that PKC activation is necessary for thrombin-induced growth cone collapse. These data indicate that eicosanoid-mediated repellent effects result from the direct and selective activation of PKCepsilon and suggest the involvement of myristoylated alanine-rich protein kinase C substrate phosphorylation in growth cone collapse.

Biochemical and immunohistochemical evidence for a non-muscle myosin at the neuromuscular junction in bovine skeletal muscle

We identified 220-kD protein in bovine skeletal muscle homogenate by affinity chromatography on an agarose column and subsequent SDS-PAGE. Peptide mass fingerprinting (MALDI mass spectrometry) and internal sequence analysis revealed that this protein has homology with several members of the myosin superfamily, particularly with human cardiac beta-myosin heavy chain (beta-MHC). A rabbit polyclonal antibody against the 220-kD protein specifically stained a 220-kD band in Western blots of skeletal muscle homogenate. Immunohistochemical experiments on cryostat sections demonstrated that in skeletal muscle this protein is exclusively localized at the neuromuscular junctions, no immunoreactivity being present at the myofibril level. Because of its relative homology with cardiac beta-MHC, we also investigated the distribution of the 220-kD protein in bovine heart. In cardiac fibers, 220-kD protein-related immunoreactivity was restricted to the intercalated disks, whereas myofibrils were completely devoid of specific immunoreactivity. This distribution pattern was completely different from that of cardiac beta-MHC, which involved myofibrils. Because of the above biochemical and immunohistochemical features, the 220-kD protein we have identified is suggested to be a novel member of the non-muscle (non-sarcomeric) myosin family.

Myosin 1c and myosin IIB serve opposing roles in lamellipodial dynamics of the neuronal growth cone

The myosin family of motor proteins is implicated in mediating actin-based growth cone motility, but the roles of many myosins remain unclear. We previously implicated myosin 1c (M1c; formerly myosin I beta) in the retention of lamellipodia (Wang et al., 1996). Here we address the role of myosin II (MII) in chick dorsal root ganglion neuronal growth cone motility and the contribution of M1c and MII to retrograde F-actin flow using chromophore-assisted laser inactivation (CALI). CALI of MII reduced neurite outgrowth and growth cone area by 25%, suggesting a role for MII in lamellipodial expansion. Micro-CALI of MII caused a rapid reduction in local lamellipodial protrusion in growth cones with no effects on filopodial dynamics. This is opposite to micro-CALI of M1c, which caused an increase in lamellipodial protrusion. We used fiduciary beads (Forscher et al., 1992) to observe retrograde F-actin flow during the acute loss of M1c or MII. Micro-CALI of M1c reduced retrograde bead flow by 76%, whereas micro-CALI of MII or the MIIB isoform did not. Thus, M1c and MIIB serve opposite and nonredundant roles in regulating lamellipodial dynamics, and M1c activity is specifically required for retrograde F-actin flow.

Constitutively active myosin light chain kinase alters axon guidance decisions in Drosophila embryos

Conventional myosin II activity provides the motile force for axon outgrowth, but to achieve directional movement during axon pathway formation, myosin activity should be regulated by the attractive and repulsive guidance cues that guide an axon to its target. Here, evidence for this regulation is obtained by using a constitutively active Myosin Light Chain Kinase (ctMLCK) to selectively elevate myosin II activity in Drosophila CNS neurons. Expression of ctMLCK pan-neurally or in primarily pCC/MP2 neurons causes these axons to cross the midline incorrectly. This occurs without altering cell fates and is sensitive to mutations in the regulatory light chains. These results confirm the importance of regulating myosin II activity during axon pathway formation. Mutations in the midline repulsive ligand Slit, or its receptor Roundabout, enhance the number of ctMLCK-induced crossovers, but ctMLCK expression also partially rescues commissure formation in commissureless mutants, where repulsive signals remain high. Overexpression of Frazzled, the receptor for midline attractive Netrins, enhances ctMLCK-dependent crossovers, but crossovers are suppressed when Frazzled activity is reduced by using loss-of-function mutations. These results confirm that proper pathway formation requires careful regulation of MLCK and/or myosin II activity and suggest that regulation occurs in direct response to attractive and repulsive cues.

Myosin light chain phosphorylation and growth cone motility

According to the treadmill hypothesis, the rate of growth cone advance depends upon the difference between the rates of protrusion (powered by actin polymerization at the leading edge) and retrograde F-actin flow, powered by activated myosin. Myosin II, a strong candidate for powering the retrograde flow, is activated by myosin light chain (MLC) phosphorylation. Earlier results showing that pharmacological inhibition of myosin light chain kinase (MLCK) causes growth cone collapse with loss of F-actin-based structures are seemingly inconsistent with the treadmill hypothesis, which predicts faster growth cone advance. These experiments re-examine this issue using an inhibitory pseudosubstrate peptide taken from the MLCK sequence and coupled to the fatty acid stearate to allow it to cross the membrane. At 5-25 microM, the peptide completely collapsed growth cones from goldfish retina with a progressive loss of lamellipodia and then filopodia, as seen with pharmacological inhibitors, but fully reversible. Lower concentrations (2.5 microM) both simplified the growth cone (fewer filopodia) and caused faster advance, doubling growth rates for many axons (51-102 microm/h; p <.025). Rhodamine-phalloidin staining showed reduced F-actin content in the faster growing growth cones, and marked reductions in collapsed ones. At higher concentrations, there was a transient advance of individual filopodia before collapse (also seen with the general myosin inhibitor, butanedione monoxime, which did not accelerate growth). The rho/rho kinase pathway modulates MLC dephosphorylation by myosin-bound protein phosphatase 1 (MPP1), and manipulations of MPP1 also altered motility. Lysophosphatidic acid (10 microM), which causes inhibition of MPP1 to accumulate activated myosin II, caused a contracted collapse (vs. that due to loss of F-actin) but was ineffective after treatment with low doses of peptide, demonstrating that the peptide acts via MLC phosphorylation. Inhibiting rho kinase with Y27632 (100 microM) to disinhibit the phosphatase increased the growth rate like the MLCK peptide, as expected. These results suggest that: varying the level of MLCK activity inversely affects the rate of growth cone advance, consistent with the treadmill hypothesis and myosin II powering of retrograde F-actin flow; MLCK activity in growth cones, as in fibroblasts, contributes strongly to controlling the amount of F-actin; and the phosphatase is already highly active in these cultures, because rho kinase inhibition produces much smaller effects on growth than does MLCK inhibition.

Interaction of SAP97 with minus-end-directed actin motor myosin VI. Implications for AMPA receptor trafficking

SAP97 is a modular protein composed of three PDZ domains, an SH3 domain, and a guanylate kinase-like domain. It has been implicated functionally in the assembly and structural stability of synaptic junctions as well as in the trafficking, recruitment, and localization of specific ion channels and neurotransmitter receptors. The N terminus of SAP97 (S97N) has been shown to play a key role in the selection of binding partners and the localization of SAP97 at adhesion sites, as well as the clustering of ion channels in heterologous cells. Using the S97N domain as bait in a yeast two-hybrid screen, we identified the minus-end-directed actin-based motor, myosin VI, as an S97N binding partner. Moreover, in light membrane fractions prepared from rat brain, we found that myosin VI and SAP97 form a trimeric complex with the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunit, GluR1. These data suggest that SAP97 may serve as a molecular link between GluR1 and the actin-dependent motor protein myosin VI during the dynamic translocation of AMPA receptors to and from the postsynaptic plasma membrane.

The yeast class V myosins, Myo2p and Myo4p, are nonprocessive actin-based motors

The motor properties of the two yeast class V myosins, Myo2p and Myo4p, were examined using in vitro motility assays. Both myosins are active motors with maximum velocities of 4.5 microm/s for Myo2p and 1.1 microm/s for Myo4p. Myo2p motility is Ca(2+) insensitive. Both myosins have properties of a nonprocessive motor, unlike chick myosin-Va (M5a), which behaves as a processive motor when assayed under identical conditions. Additional support for the idea that Myo2p is a nonprocessive motor comes from actin cosedimentation assays, which show that Myo2p has a low affinity for F-actin in the presence of ATP and Ca(2+), unlike chick brain M5a. These studies suggest that if Myo2p functions in organelle transport, at least five molecules of Myo2p must be present per organelle to promote directed movement.

Calmodulin and profilin coregulate axon outgrowth in Drosophila

Coordinated regulation of actin cytoskeletal dynamics is critical to growth cone movement. The intracellular molecules calmodulin and profilin actively regulate actin-based motility and participate in the signaling pathways used to steer growth cones. Here we show that in the developing Drosophila embryo, calmodulin and profilin convey complimentary information that is necessary for appropriate growth cone advance. Reducing calmodulin activity by expression of a dominant inhibitor (KA) stalls axon extension of pioneer neurons within the CNS, while a partial loss of profilin function decreases extension of motor axons in the periphery. Yet, surprisingly, when calmodulin and profilin are simultaneously reduced, the ability of both CNS pioneer axons and motor axons to extend beyond the choice points is restored. In the CNS, at the time when growth cones must decide whether to cross or not to cross the midline, a reduction in calmodulin and/or roundabout signaling causes axons to cross the midline inappropriately. These inappropriate crossings are suppressed when profilin activity is simultaneously reduced. Interestingly, the mutual suppression of calmodulin and profilin activity requires a minimal level of profilin. In KA combinations with profilin null alleles, defects in axon extension and midline guidance are synergistically enhanced rather than suppressed. Together, our data indicate that the growth cone must coordinate the activity of both calmodulin and profilin in order to advance past selected choice points, including those dictating midline crossovers.

The light chain composition of chicken brain myosin-Va: calmodulin, myosin-II essential light chains, and 8-kDa dynein light chain/PIN

Class V myosins are a ubiquitously expressed family of actin-based molecular motors. Biochemical studies on myosin-Va from chick brain indicate that this myosin is a two-headed motor with multiple calmodulin light chains associated with the regulatory or neck domain of each heavy chain, a feature consistent with the regulatory effects of Ca(2+) on this myosin. In this study, the identity of three additional low molecular weight proteins of 23-,17-, and 10 kDa associated with myosin-Va is established. The 23- and 17-kDa subunits are both members of the myosin-II essential light chain gene family, encoded by the chicken L23 and L17 light chain genes, respectively. The 10-kDa subunit is a protein originally identified as a light chain (DLC8) of flagellar and axonemal dynein. The 10-kDa subunit is associated with the tail domain of myosin-Va.

Nerve growth factor stimulates coupling of beta1 integrin to distinct transport mechanisms in the filopodia of growth cones

The cycling of membrane receptors for substrate-bound proteins via their interaction with the actin cytoskeleton at the leading edge of growth cones and other motile cells is important for neurite outgrowth and cell migration. Receptor delivered to the leading edge binds to its ligand, which induces coupling of the receptor to a rearward flowing network of actin filaments. This coupling is thought to facilitate advance. We show here that a soluble growth factor stimulates this cycling. We have used single particle tracking to monitor the effects of nerve growth factor (NGF) on the movements of beta1 integrin in the plane of the plasma membrane of the filopodia of growth cones. Beta1 integrin was visualized by its binding of 0.2 microm beads coated with a monoclonal Ab directed against an extracellular epitope distant from the binding site for extracellular matrix ligands. The beads were observed by video microscopy. Beads coated with a low concentration of antibody, and therefore bound to unliganded receptor with little cross-linking, showed an increase in both diffusion and directed forward transport in response to NGF. Transport had a net velocity of 37 microm/minute and was characterized by brief periods of sustained forward excursions with a velocity of 75–150 microm/minute. There was a 2-fold increase in the number of beads accumulated at the tips of filopodia after 10 minutes, indicating that NGF enhanced the delivery of beta1 integrin to the periphery. Forward transport was dependent on an intact actin cytoskeleton and myosin ATPase, since treatment with cytochalasin D or the myosin ATPase inhibitor butanedione monoxime inhibited the transport but not the diffusion of receptors. NGF also greatly increased the steady rearward migration of beads coated with a high density of (β)1 integrin antibody, indicating that coupling of cross-linked receptor to the retrograde flow of actin is also enhanced. The rate of the retrograde flow of actin was unaffected by NGF. These studies show that a soluble factor can stimulate the coupling of a receptor for substrate-bound factor to two actomyosin-based transport mechanisms and thus facilitate the response of the growth cone to the substrate-bound factor by increasing cycling of the receptor at the periphery.

A new direction for myosin

Members of the myosin superfamily of actin-based motor proteins were previously thought to move only towards the barbed end of the actin filament. In an extraordinary reversal of this dogma, an abundant and widespread unconventional myosin known as myosin VI has recently been shown to move towards the pointed end of the actin filament - the opposite direction of all other characterized myosins. This discovery raises novel and intriguing questions about the molecular mechanisms of reversal and the biological roles of this 'backwards' myosin.