Cell adhesion: Ushering in a new understanding of myosin VII

The shaker-1 mouse myosin VIIa deafness mutation results in a severely reduced rate of the ATP hydrolysis step

Mutations in the MYO7A gene, encoding the motor protein myosin VIIa, can cause Usher 1B, a deafness/blindness syndrome in humans, and the shaker-1 phenotype, characterized by deafness, head tossing, and circling behavior, in mice. Myosin VIIa is responsible for tension bearing and the transduction mechanism in the stereocilia and for melanosome transport in the retina, in line with the phenotypic outcomes observed in mice. However, the effect of the shaker-1 mutation, a R502P amino acid substitution, on the motor function is unclear. To explore this question, we determined the kinetic properties and the effect on the filopodial tip localization of the recombinant mouse myosin VIIa-5IQ-SAH R502P (myoVIIa-sh1) construct. Interestingly, although residue 502 is localized to a region thought to be involved in interacting with actin, the kinetic parameters for actin binding changed only slightly for the mutant construct. However, the rate constant for ATP hydrolysis (k_+H + k_-H) was reduced by ∼200-fold from 12 s^-1 to 0.05 s^-1, making the hydrolysis step the rate-limiting step of the ATPase cycle in the presence and absence of actin. Given that wild-type mouse myosin VIIa is a slow, high-duty ratio, monomeric motor, this altered hydrolysis rate would reduce activity to extremely low levels. Indeed, the translocation to the filopodial tips was hampered by the diminished motor function of a dimeric construct of the shaker-1 mutant. We conclude that the diminished motor activity of this mutant is most likely responsible for impaired hearing in the shaker-1 mice.

Over-expression of myosin7A in cochlear hair cells of circling mice

Circling mouse (C57BL/6J-cir/cir) deleted the transmembrane inner ear (Tmie) gene is an animal model for human non-syndromic recessive deafness, DFNB6. In circling mouse, hair cells in the cochlea have degenerated and hair bundles have become irregularity as time goes on. Tmie protein carries out a function of the mechanoelectrical transduction channel in cochlear hair cells. Myosin7a (MYO7A) protein has key roles in development of the cochlear hair bundles as well as in the function of cochlear hair cells. To find whether Tmie protein interacts with MYO7A proteins in the cochlea postnatal developmental stage, we investigated expression of the MYO7A proteins in the cochlear hair cells of circling mice by western blot analysis and whole mount immunofluorescence at postnatal day 5 (P5). The expression of MYO7A showed statistically significant increase in the cochlea of C57BL/6J-+/cir and C57BL/6J-cir/cir mice than that of C57BL/6J-+/+ mice. The MYO7A intensity of the cochlear hair cells also increased in C57BL/6J-+/cir and C57BL/6J-cir/cir mice compared with those of C57BL/6J-+/+ mice. Taken together, the results indicate that Tmie protein may have an important role with MYO7A protein in the development and maintenance of the stereociliary bundles during postnatal developmental stage of the cochlea.

Non-muscle myosins in tumor progression, cancer cell invasion, and metastasis

The actin cytoskeleton, which regulates cell polarity, adhesion, and migration, can influence cancer progression, including initial acquisition of malignant properties by normal cells, invasion of adjacent tissues, and metastasis to distant sites. Actin-dependent molecular motors, myosins, play key roles in regulating tumor progression and metastasis. In this review, we examine how non-muscle myosins regulate neoplastic transformation and cancer cell migration and invasion. Members of the myosin superfamily can act as either enhancers or suppressors of tumor progression. This review summarizes the current state of knowledge on how mutations or epigenetic changes in myosin genes and changes in myosin expression may affect tumor progression and patient outcomes and discusses the proposed mechanisms linking myosin inactivation or upregulation to malignant phenotype, cancer cell migration, and metastasis.

Structures of usher syndrome 1 proteins and their complexes

Usher syndrome 1 (USH1) is the most common and severe form of hereditary loss of hearing and vision. Genetic, physiological, and cell biological studies, together with recent structural investigations, have not only uncovered the physiological functions of the five USH1 proteins but also provided mechanistic explanations for the hearing and visual deficiencies in humans caused by USH1 mutations. This review focuses on the structural basis of the USH1 protein complex organization.

crinkled reveals a new role for Wingless signaling in Drosophila denticle formation

The specification of the body plan in vertebrates and invertebrates is controlled by a variety of cell signaling pathways, but how signaling output is translated into morphogenesis is an ongoing question. Here, we describe genetic interactions between the Wingless (Wg) signaling pathway and a nonmuscle myosin heavy chain, encoded by the crinkled (ck) locus in Drosophila. In a screen for mutations that modify wg loss-of-function phenotypes, we isolated multiple independent alleles of ck. These ck mutations dramatically alter the morphology of the hook-shaped denticles that decorate the ventral surface of the wg mutant larval cuticle. In an otherwise wild-type background, ck mutations do not significantly alter denticle morphology, suggesting a specific interaction with Wg-mediated aspects of epidermal patterning. Here, we show that changing the level of Wg activity changes the structure of actin bundles during denticle formation in ck mutants. We further find that regulation of the Wg target gene, shaven-baby (svb), and of its transcriptional targets, miniature (m) and forked (f), modulates this ck-dependent process. We conclude that Ck acts in concert with Wg targets to orchestrate the proper shaping of denticles in the Drosophila embryonic epidermis.

The kinetic mechanism of mouse myosin VIIA

Myosin VIIa is crucial in hearing and visual processes. We examined the kinetic and association properties of the baculovirus expressed, truncated mouse myosin VIIa construct containing the head, all 5IQ motifs and the putative coiled coil domain (myosin VIIa-5IQ). The construct appears to be monomeric as determined by analytical ultracentrifugation experiments, and only single headed molecules were detected by negative stain electron microscopy. The relatively high basal steady-state rate of 0.18 s(-1) is activated by actin only by ∼3.5-fold resulting in a V(max) of 0.7 s(-1) and a K(ATPase) of 11.5 μM. There is no single rate-limiting step of the ATP hydrolysis cycle. The ATP hydrolysis step (M·T M·D·P) is slow (12 s(-1)) and the equilibrium constant (K(H)) of 1 suggests significant reversal of hydrolysis. In the presence of actin ADP dissociates with a rate constant of 1.2 s(-1). Phosphate dissociation is relatively fast (>12 s(-1)), but the maximal rate could not be experimentally obtained at actin concentrations ≤ 50 μM because of the weak binding of the myosin VIIa-ADP-P(i) complex to actin. At higher actin concentrations the rate of attached hydrolysis (0.4 s(-1)) becomes significant and partially rate-limiting. Our findings suggest that the myosin VIIa is a "slow", monomeric molecular motor with a duty ratio of 0.6.

Nonmuscle myosin II is required for cell proliferation, cell sheet adhesion and wing hair morphology during wing morphogenesis

Metazoan development involves a myriad of dynamic cellular processes that require cytoskeletal function. Nonmuscle myosin II plays essential roles in embryonic development; however, knowledge of its role in post-embryonic development, even in model organisms such as Drosophila melanogaster, is only recently being revealed. In this study, truncation alleles were generated and enable the conditional perturbation, in a graded fashion, of nonmuscle myosin II function. During wing development they demonstrate novel roles for nonmuscle myosin II, including in adhesion between the dorsal and ventral wing epithelial sheets; in the formation of a single actin-based wing hair from the distal vertex of each cell; in forming unbranched wing hairs; and in the correct positioning of veins and crossveins. Many of these phenotypes overlap with those observed when clonal mosaic analysis was performed in the wing using loss of function alleles. Additional requirements for nonmuscle myosin II are in the correct formation of other actin-based cellular protrusions (microchaetae and macrochaetae). We confirm and extend genetic interaction studies to show that nonmuscle myosin II and an unconventional myosin, encoded by crinkled (ck/MyoVIIA), act antagonistically in multiple processes necessary for wing development. Lastly, we demonstrate that truncation alleles can perturb nonmuscle myosin II function via two distinct mechanisms--by titrating light chains away from endogenous heavy chains or by recruiting endogenous heavy chains into intracellular aggregates. By allowing myosin II function to be perturbed in a controlled manner, these novel tools enable the elucidation of post-embryonic roles for nonmuscle myosin II during targeted stages of fly development.

Spatiotemporal analysis of cell response to a rigidity gradient: a quantitative study using multiple optical tweezers

We investigate the dynamic response of single cells to weak and local rigidities, applied at controlled adhesion sites. Using multiple latex beads functionalized with fibronectin, and each trapped in its own optical trap, we study the reaction in real time of single 3T3 fibroblast cells to asymmetrical tensions in the tens of pN x microm(-1) range. We show that the cell feels a rigidity gradient even at this low range of tension, and over time develops an adapted change in the force exerted on each adhesion site. The rate at which force increases is proportional to trap stiffness. Actomyosin recruitment is regulated in space and time along the rigidity gradient, resulting in a linear relationship between the amount of recruited actin and the force developed independently in trap stiffness. This time-regulated actomyosin behavior sustains a constant and rigidity-independent velocity of beads inside the traps. Our results show that the strengthening of extracellular matrix-cytoskeleton linkages along a rigidity gradient is regulated by controlling adhesion area and actomyosin recruitment, to maintain a constant deformation of the extracellular matrix.

F-actin-based extensions of the head cyst cell adhere to the maturing spermatids to maintain them in a tight bundle and prevent their premature release in Drosophila testis

Background

In Drosophila, all the 64 clonally derived spermatocytes differentiate in syncytium inside two somatic-origin cyst cells. They elongate to form slender spermatids, which are individualized and then released into the seminal vesicle. During individualization, differentiating spermatids are organized in a tight bundle inside the cyst, which is expected to play an important role in sperm selection. However, actual significance of this process and its underlying mechanism are unclear.

Results

We show that dynamic F-actin-based processes extend from the head cyst cell at the start of individualization, filling the interstitial space at the rostral ends of the maturing spermatid bundle. In addition to actin, these structures contained lamin, beta-catenin, dynamin, myosin VI and several other filopodial components. Further, pharmacological and genetic analyses showed that cytoskeletal stability and dynamin function are essential for their maintenance. Disruption of these F-actin based processes was associated with spermatid bundle disassembly and premature sperm release inside the testis.

Conclusion

Altogether, our data suggests that the head cyst cell adheres to the maturing spermatid heads through F-actin-based extensions, thus maintaining them in a tight bundle. This is likely to regulate mature sperm release into the seminal vesicle. Overall, this process bears resemblance to mammalian spermiation.

The SH3 domain of a M7 interacts with its C-terminal proline-rich region

Myosins play essential roles in migration, cytokinesis, endocytosis, and adhesion. They are composed of a large N-terminal motor domain with ATPase and actin binding sites and C-terminal neck and tail regions, whose functional roles and structural context in the protein are less well characterized. The tail regions of myosins I, IV, VII, XII, and XV each contain a putative SH3 domain that may be involved in protein-protein interactions. SH3 domains are reported to bind proline-rich motifs, especially "PxxP" sequences, and such interactions serve regulatory functions. The activity of Src, PI3, and Itk kinases, for example, is regulated by intramolecular interactions between their SH3 domain and internal proline-rich sequences. Here, we use NMR spectroscopy to reveal the structure of a protein construct from Dictyostelium myosin VII (DdM7) spanning A1620-T1706, which contains its SH3 domain and adjacent proline-rich region. The SH3 domain forms the signature beta-barrel architecture found in other SH3 domains, with conserved tryptophan and tyrosine residues forming a hydrophobic pocket known to bind "PxxP" motifs. In addition, acidic residues in the RT or n-Src loops are available to interact with the basic anchoring residues that are typically found in ligands or proteins that bind SH3 domains. The DdM7 SH3 differs in the hydrophobicity of the second pocket formed by the 3(10) helix and following beta-strand, which contains polar rather than hydrophobic side chains. Most unusual, however, is that this domain binds its adjacent proline-rich region at a surface remote from the region previously identified to bind "PxxP" motifs. The interaction may affect the orientation of the tail without sacrificing the availability of the canonical "PxxP"-binding surface.

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.

Biochemical and molecular characterization of diseases linked to motor proteins

Recent studies have revealed that kinesin, dynein and myosin each form large superfamilies and participate in many different intracellular transport systems. Importantly, these motor proteins play significant roles in the pathogenesis of a variety of diseases. Studies using knockout mice for kinesin KIF1B have led to the identification of the cause of a human hereditary neuropathy, Charcot-Marie-Tooth disease type 2A. The function of members of the dynein superfamily whose existence has previously only been confirmed through genome databases, has been revealed by studies of immotile cilia syndrome. Unconventional myosins have been shown to function in the inner-ear cells by examination of hereditary human hearing impairment and studies using mouse models. In addition, some diseases are caused by mutations, not in the motor itself, but in the proteins associated with the motor proteins. Here, we discuss the relationship of these motor proteins and how they contribute to disease in molecular terms.

Conserved features of endocytosis in Dictyostelium

Endocytosis in protozoa is often regarded as largely different from the pathways operating in mammalian cells. Experiments in the amoeba Dictyostelium, one of the genetically tractable single-celled organisms, have allowed us to manipulate the flow through endocytic compartments and to study the dynamic distribution of molecules by means of green fluorescent protein fusions. This review attempts to compile the molecular data available from Dictyostelium and assign them to specific steps of internalization by phagocytosis or macropinocytosis and to subsequent stages of the endocytic pathway. Parallels to phagocytes of the mammalian immune system are emphasized. The major distinctive feature between mammalian phagocytes and free-living cells is the need for osmoregulation. Therefore Dictyostelium cells possess a contractile vacuole that has occasionally obscured analysis of endocytosis but is now found to be entirely separate from endocytic organelles. In conclusion, the potential of Dictyostelium amoebas to provide a model system of mammalian phagocytes is ever increasing.

Myosin light chain kinase inhibitors induce retraction of mature oligodendrocyte processes

Mature oligodendrocytes emit numerous myelinating processes. Force generating molecules are required for process outgrowth and spreading. We have analyzed the effect of the myosin II light chain kinase inhibitors ML-7 and ML-9 in cultured oligodendrocytes. Both drugs affect oligodendrocyte cell shape, provoking a retraction of high order processes. Our results suggest that the adhesion of the myelinating processes to the substrate depends on MLC phosphorylation, thus likely implicating myosin IIA.

Molecular architecture of adherens junctions

Adherens junctions are composed of a cadherin-catenin complex and its associated proteins. Recently, an increasing number of novel members of adherens junctions, including membrane and PDZ proteins, have been reported. Interactions among these components in adherens junctions seem to be dynamically regulated during the formation of adherens junction complexes in epithelial cells.