Gene name - zipper
Synonyms - myosin II
Cytological map position - 60E9-F1
Function - myosin heavy chain
Symbol - zip
Genetic map position - 2-
Classification - nonmuscle myosin
Cellular location - cytoplasmic
Myosins are a functionally divergent group of molecular motors that are involved in various non-muscle cell motile activities. Zipper, hence referred to as Myosin II, or non-muscle myosin, is crucial to the viability of the Drosophila embryo throughout its development. The clearest example of nonmuscle-driven shape change is cytokinesis, in which an actin and myosin-rich contractile ring cleaves the cell during mitosis. Null alleles of zipper coding for the non-muscle heavy chain cause failure of dorsal closure, during which the lateral epidermal cells elongate to cover the dorsal surface of the embryo after germ band retraction. Axon pathfinding and head involution defects also contribute to the embryonic lethality of zipper mutants. Early dorsal closure is normal in these mutants, suggesting that maternal coded Zipper protein is sufficient for early stages of closure. By late closure however, nonmuscle myosin is either improperly localized in or absent from the leading edge of the lateral epithelium. By the time zip mutant embryos arrest during late dorsal closure stages, the leading edge is dramatically disorganized, the lateral epidermis is not fused along the dorsal midline, and the amnioserosa remains exposed. In normal flies, dorsal closure is accompanied by a dramatic cell shape change as the entire epidermal cell sheet thins out to spread over the amnioserosa. As dorsal closure proceeds, more and more cells throughout the epidermis are elongated. This change in epidermal cell shape, driven for the most part by maternal myosin II, is sufficient to account for the spreading of the lateral epithelium over the region occupied by the amnioserosa (Young, 1993).
Nonmuscle myosin II is also required for rapid cytoplasmic transport during oogenesis and for axial nuclear migration in early embryos. Developing oocytes containing germline mutant spaghetti squash(sqh) clones fail to attain full suze due to a defect in 'dumping,' the rapid phase of cytoplasmic transport in nurse cells. Spaghetti squash is the regulatory light chain of nonmuscle myosin II; along with the Essential light chain, it acts to regulate the function of Myosin II. Two subunits of the regulatory light chain, along with two subunits of the essential light chain, each combine with two subunits of Myosin II to form the functional Myosin II hexamer. spaghetti squash mutant egg chambers show no evidence of ring canal obstruction, and no obvious alteration in the actin network. However, the distribution of myosin II is abnormal. It is thought that the molecular motor responsible for cytoplasmic dumping is supplied largely, if not exclusively by nurse cell myosin II. Regulation of myosin activity is one means by which cytoplasmic transport is controlled during oocyte development. Fertilized eggs developed from spaghetti squash maternal mutant clones begin development by exhibiting an early defect in axial migration of cleavage nuclei toward the posterior pole of the embryo. This is similar to development seen in early cleavage zygotes, in which the actin cytoskeleton is disrupted. Thus both nurse cell dumping and axial migration of the zygotic nuclei require maternally supplied myosin II (Wheatley, 1995).
In order to study the role of myosin II during imaginal disc development, spaghetti squash was put under control of a heat shock promoter and induced at various times in sqh mutants. sqh mutants can be rescued to adulthood by daily induction of sqh from a hsp 70 promoter. When SQH is transiently depleted in larvae, the resulting adult phenotypes demonstrate that SQH is required in a stage-specific fashion for proper development of eye and leg imaginal discs. When SQH is depleted in adult females, oogenesis is reversibly disrupted. Without SQH induction, developing egg chambers display a succession of phenotypes that demonstrate roles for myosin II in morphogenesis of the interfollicular stalks, three morphologically and mechanistically distinct types of follicle cell migration, and completion of nurse cell cytoplasm transport (dumping) (Edwards, 1996).
Normally at stage 9 of oogenesis, approximately eight so-called border cells form at the anterior tip of the egg chamber, delaminate, and migrate past the phalanx of nurse cells until they reach the anterior tip of the oocyte. This migration of border cells between and through the nurse cells is abnormal in sqh mutants. Normal border cell myosin II staining is much brighter than that of the surrounding nurse cells, indicating a presence of myosin II in border cells. Centripetal cells, a second morphologically distinct type of follicle cell, is derived from the most anterior ring of oocyte follicle cells. Centripetal cells normally elongate and plunge inward toward the border cells, which have come to rest at the anterior face of the oocyte. These centripetal cells will eventually cover the anterior of the oocyte and build several specialized structures of the anterior chorion. Each centripetal cell specifically accumulates a bright bar of myosin II staining at the edge of the apical (inner) surface that leads the penetration between the nurse cells and the oocyte (Edwards, 1996).
sqh mutant ovaries also display major defects in migration of centripetal cells. After three days without SQH, centripetal cells fail to elongate. Follicle cells that construct the dorsal appendages also fail to migrate properly as the supply of SQH dwindles and eventually is depleted. A shortage of SQH in migrating cells appears to be the primary cause of the shortened dorsal appendage phenotype found in sqh mutants (Edwards, 1996).
In sqh mutant tissues, Myosin II heavy chain is abnormally localized in punctate structures that do not contain appreciable amounts of filamentous actin or the myosin tail binding protein p127. p127 is coded for by the gene lethal (2) giant larvae tumor suppressor gene. In follicle cells, p127 is normally concentrated at the lateral membranes. In sqh mutant tissue, p127 shows the same pattern, with no punctate cytoplasmic staining. Likewise F-actin is not mislocalized in sqh mutants. Thus sqh mutations cause mislocalization of myosin, but not the other major cytoskeletal proteins (Edwards, 1996).
Diverse types of epithelial morphogenesis drive development. Similar cytoskeletal and cell adhesion machinery orchestrate these changes, but it is unclear how distinct tissue types are produced. Thus, it is important to define and compare different types of morphogenesis. Cell flattening and elongation were investigated in the amnioserosa, a squamous epithelium formed at Drosophila gastrulation. Amnioserosa cells are initially columnar. Remarkably, they flatten and elongate autonomously by perpendicularly rotating the microtubule cytoskeleton - this is called 'rotary cell elongation'. Apical microtubule protrusion appears to initiate the rotation and microtubule inhibition perturbs the process. F-actin restrains and helps orient the microtubule protrusions. As amnioserosa cells elongate, they maintain their original cell-cell contacts and develop planar polarity. Myosin II localizes to anterior-posterior contacts, while the polarity protein Bazooka (PAR-3) localizes to dorsoventral contacts. Genetic analysis revealed that Myosin II and Bazooka cooperate to properly position adherens junctions. These results identify a specific cellular mechanism of squamous tissue morphogenesis and molecular interactions involved (Pope, 2008).
Amnioserosa tissue morphogenesis involves dramatic cell shape change. Before amnioserosa morphogenesis, cells are columnar with lateral MT bundles in a basket-like array along the apicobasal axis. With amnioserosa morphogenesis, the cells elongate and flatten. Amnioserosa cells could change shape by symmetrically re-positioning cellular contents (full cytoskeleton and/or membrane reorganization) or by perpendicularly rotating cellular components to reorient the long axis of the cell into the plane of tissue extension. To distinguish these possibilities, 3D cell organization was studied over time; it was discovered that the MT array plus the nucleus, centrosomes and ER rotate, apparently as a unit, into the plane of tissue extension. More symmetric adherens junction (AJ) and cortical reorganizations appear to accompany the rotation. MT arrays also reorient to polarize cells during chemotaxis and tissue migration, and to reposition cell contents as occurs during cortical rotation in early Xenopus embryos. These results reveal rotation of the MT cytoskeleton linked to cell shape change and amnioserosa morphogenesis. Similar mechanisms may underlie the development of other squamous epithelial monolayers (Pope, 2008).
Regulated apical MT protrusion appears to initiate amnioserosa rotary cell elongation. MT inhibition perturbs initial elongation, and the process normally begins with MTs protruding into the apical domain, bending perpendicularly and then extending in the axis of cell elongation. Pre-existing lateral MT bundles appear to protrude across the apical domain - they are mainly non-centrosomal and contain older (acetylated) MTs. However, EB1-GFP imaging also revealed bi-directional MT growth across the apical domain. This indicates that the bundles are dynamic, but argues against MT bundle protrusion through polarized individual MT polymerization. Instead, MT bundle protrusion may involve greater net renewal of bundles apically versus basally, motors sliding MTs past MTs in the bundles and/or motors moving bundles along the cell cortex. Distinguishing these models requires further study. As MTs extend apically the actin cytoskeleton appears to inhibit them, as weakening actin leads to excessively long and randomly oriented MT-based protrusions. Actin is normally found around the full apical circumference as amnioserosa cells elongate. By contrast, Myosin II becomes enriched at AP contacts and is gradually lost from the full cell cortex. Thus, different pools of actin may regulate MT protrusion. It is speculated that the gradual overall loss of cortical actin-myosin complexes permits, and may help orient, regulated MT protrusion. Actin also antagonizes cortical MTs in other systems. MT-based primary axons form where cortical actin is weakest. Actin inhibits cortical MT protrusion in neutrophils and Myosin IIA inhibits cortical MTs in mammalian cells. Actin might physically block MT protrusion, but direct or indirect molecular interactions may also be involved (Pope, 2008).
In Drosophila embryos, MT-actin interactions also affect germband cells. At stage 7-8, actin disruption enhances AJ planar polarity at DV contacts. MT disruption suppresses this, suggesting that actin inhibits MT-based AJ positioning in these cells - however, germband cells show minimal shape change with actin disruption at this stage. Remarkably, the same actin disruption causes stage 9-10 germband cells to rotate analogously to early amnioserosa cells. Their apical domains elongate and their lateral regions rotate perpendicularly, becoming exposed to the embryo surface. Implicating MTs in this change, lateral MT bundles run into the extended apical domains and simultaneous MT disruption suppresses the cell shape change. Thus, actin may inhibit apical MTs to regulate tissue structure in many parts of the embryo. This MT inhibition may also involve coordination with AJs, as disrupted germband cells in armm/z mutants also display MTs protruding into extended apical domains (Pope, 2008).
How do MTs elongate the apical domain and how is this linked to the rotation of the full MT cytoskeleton in the amnioserosa? It is proposed that rotary cell elongation occurs in two phases. In phase one, the MT imaging and inhibitor studies indicate that regulated MT protrusion elongates the apical domain. This may involve a combination of physical force, membrane delivery and/or relaxation of actin-myosin contractility. The AJ clustering observed at abnormal apical MT protrusions formed with actin inhibition in both early amnioserosa cells and the later germband suggests that MTs may apply force to AJs. Consistent with this idea, MT inhibition affected both initial amnioserosa cell elongation and later amnioserosa cell-cell interactions. However, amnioserosa cell elongation in armm/z mutants suggests that MTs may not necessarily engage AJs directly. Since Baz localizes apically in early armm/z mutants, and is enriched at DV amnioserosa cell contacts to which MTs rotate in wild type, it is a strong candidate for coordinating these interactions. However, severe early defects in bazm/z mutants would confound analysis of amnioserosa development - this may require conditional mutants. Phase two of rotary cell elongation requires full perpendicular rotation of the MT array, apical and basal membrane growth, and lateral membrane removal. Although it is unclear how full rotation occurs, MT rotation and cortical remodeling may occur in concert. For example, membrane remodeling may explain how amnioserosa cells remain elongated with MT disruption during later development (Pope, 2008).
For rotary cell elongation to translate into tissue extension, cell contacts and AJs must be remodeled. Remarkably, amnioserosa cells maintain their neighbor relationships as they elongate, and two contact types develop; highly elongated AP contacts and lesser elongated DV contacts. Each appears to involve unique AJ remodeling. Intriguingly, Myosin II and Baz localize to AP and DV contacts, respectively -- the same reciprocal planar polarized relationship displayed in the germband. In the amnioserosa, Myosin II and Baz synergize to control overall AJ positioning, a regulatory interaction that has not been shown elsewhere (Pope, 2008).
Myosin II and Baz may regulate specific AJ remodeling events occurring at AP and DV contacts, respectively. Amnioserosa cells increase their apical circumference 10-fold, initially doubling the length of their AP cell contacts every 5-10 minutes. Remarkably, AJs localize around the full circumference as this occurs. This contrasts elongating Drosophila follicle cells, which lose AJ continuity, suggesting specific mechanisms for maintaining AJ continuity during amnioserosa morphogenesis. amnioserosa AJs do lose continuity with actin disruption, suggesting a role for actin. More specifically, AJ fragmentation in baz zip double mutants suggests a role for Myosin II. In the neighboring ventral furrow and germband, actin-myosin contractility is coupled to AJs during apical constriction and cell intercalation, respectively. The actin-myosin complexes enriched along amnioserosa AP contacts may also be contractile, but here they may counterbalance MT protrusion. Slowing apical elongation may indirectly allow AJ remodeling. However, Myosin II may also have direct affects on AJs (Pope, 2008).
Baz may regulate distinct AJ re-modeling at DV contacts. It is hypothesized that MT protrusion applies force to the DV contact at the cell 'front', and that cell elongation may also pull the 'rear' contact. Either force could detach AJs and necessitate AJ remodeling. Dynamic looping of DE-CadGFP and ArmCFP was observed at D-V contacts, and BazGFP partly colocalized with these loops. Although further experiments (e.g., photobleaching) are needed to understand this and other amnioserosa AJ remodeling, Baz localization at DV contacts and abnormal AJ aggregation at DV contacts in baz zip double mutants suggests a role for Baz in AJ remodeling at these sites. Baz appears to interact with MTs and Dynein to initially position AJs during Drosophila cellularization, and Baz might re-position AJs at DV amnioserosa contacts in a similar way (Pope, 2008).
In four different cases, cells were observed elongating towards potential sources of pulling forces. First, wild-type amnioserosa cells elongate along the DV axis towards the germband (potential source of DV pulling forces during convergent extension) and the ventral furrow (potential source of DV pulling forces during invagination). Second, amnioserosa cells elongate along the DV axis of bcd nos tsl mutants, in which germband extension fails, but ventral furrow formation occurs. Third, in dl mutants in which the ventral furrow does not form and the amnioserosa forms a ring around the DV axis, amnioserosa cells reoriented along the AP axis towards ectopic contractile furrows. Fourth, in the stage 9-11 wild-type germband, cells artificially induced to flatten and elongate did so in coordinated groups oriented towards contractile regions of the germband. Thus, polarized pulling forces across a tissue may orient rotary cell elongation. In wild-type embryos, these forces may come from germband extension and/or ventral furrow formation. However, Zen must first trigger the amnioserosa cell shape change, while AP patterning may specifically regulate AJ remodeling (Pope, 2008).
Apical constriction facilitates epithelial sheet bending and invagination during morphogenesis. Apical constriction is conventionally thought to be driven by the continuous purse-string-like contraction of a circumferential actin and non-muscle myosin-II (myosin) belt underlying adherens junctions. However, it is unclear whether other force-generating mechanisms can drive this process. This study shows, with the use of real-time imaging and quantitative image analysis of Drosophila gastrulation, that the apical constriction of ventral furrow cells is pulsed. Repeated constrictions, which are asynchronous between neighbouring cells, are interrupted by pauses in which the constricted state of the cell apex is maintained. In contrast to the purse-string model, constriction pulses are powered by actin-myosin network contractions that occur at the medial apical cortex and pull discrete adherens junction sites inwards. The transcription factors Twist and Snail differentially regulate pulsed constriction. Expression of snail initiates actin-myosin network contractions, whereas expression of twist stabilizes the constricted state of the cell apex. These results suggest a new model for apical constriction in which a cortical actin-myosin cytoskeleton functions as a developmentally controlled subcellular ratchet to reduce apical area incrementally (Martin, 2009).
During Drosophila gastrulation, apical constriction of ventral cells facilitates the formation of a ventral furrow and the subsequent internalization of the presumptive mesoderm. Although myosin is known to localize to the apical cortex of constricting ventral furrow cells, it is not known how myosin produces force to drive constriction. Understanding this mechanism requires a quantitative analysis of cell and cytoskeletal dynamics. Methods were developed to reveal and quantify apical cell shape with Spider-GFP, a green fluorescent protein (GFP)-tagged membrane-associated protein that outlines individual cells. Ventral cells were constricted to about 50% of their initial apical area before the onset of invagination and continued to constrict during invagination. Although the average apical area steadily decreased at a rate of about 5 microm2 min-1, individual cells showed transient pulses of rapid constriction that exceeded 10-15 microm2 min-1. During the initial 2 min of constriction, weak constriction pulses were often interrupted by periods of cell stretching. However, at 2 min, constriction pulses increased in magnitude and cell shape seemed to be stabilized between pulses, leading to net constriction. These two phases probably correspond to the 'slow/apical flattening' and 'fast/stochastic' phases that have been described previously. Overall, cells underwent an average of 3.2 ± 1.2 constriction pulses over 6 min, with an average interval of 82.8 ± 48 s between pulses (mean ± s.d., n = 40 cells, 126 pulses). Constriction pulses were mostly asynchronous between adjacent cells. As a consequence, cell apices between constrictions seemed to be pulled by their constricting neighbours. Thus, apical constriction occurs by means of pulses of rapid constriction interrupted by pauses during which cells must stabilize their constricted state before reinitiating constriction (Martin, 2009).
To determine how myosin might generate force during pulsed constrictions, myosin and cell dynamics were simultaneously imaged by using myosin regulatory light chain (spaghetti squash, or squ) fused to mCherry (Myosin-mCherry) and Spider-GFP. Discrete myosin spots and fibres present on the apical cortex formed a network that extended across the tissue. These myosin structures were dynamic, with apical myosin spots repeatedly increasing in intensity and moving together (at about 40 nm s-1) to form larger and more intense myosin structures at the medial apical cortex. This process, which is referred to as myosin coalescence, resulted in bursts of myosin accumulation that were correlated with constriction pulses. The peak rate of myosin coalescence preceded the peak constriction rate by 5-10 s, suggesting that myosin coalescence causes apical constriction. Between myosin coalescence events, myosin structures, including fibres, remained present on the cortex, possibly maintaining cortical tension between constriction pulses. Contrary to the purse-string model, no significant myosin accumulation was seen at cell-cell junctions. To confirm that constriction involved medial myosin coalescence and not contraction of a circumferential purse-string, constriction rate was correlated with myosin intensity at either the medial or junctional regions of the cell. Apical constriction was correlated more significantly with medial myosin, suggesting that, in contrast to the purse-string model, constriction is driven by contractions at the medial apical cortex (Martin, 2009).
Myosin coalescence resembled contraction of a cortical actin-myosin network. Therefore, to determine whether apical constriction is driven by pulsed contractions of the actin-myosin network, the organization of the cortical actin cytoskeleton was examined. In fibroblasts and keratocytes, actin network contraction bundles actin filaments into fibre-like structures. Consistent with this expectation was the identification of an actin filament meshwork underlying the apical cortex in which prominent actin-myosin fibres spanning the apical cortex appeared specifically in constricting cells. An actin-myosin network contraction model would predict that myosin coalescence results from myosin spots exerting traction on each other through the cortical actin network. To test whether myosin coalescence requires an intact actin network, the actin network was disrupted with cytochalasin D (CytoD). Disruption of the actin network with CytoD resulted in apical myosin spots that localized together with actin structures and appeared specifically in ventral cells. Myosin spots in CytoD-injected embryos showed more rapid movement than those in control-injected embryos, suggesting that apical myosin spots in untreated embryos are constrained by the cortical actin network. Although myosin movement was uninhibited in CytoD-treated embryos, myosin spots failed to coalesce and cells failed to constrict. Because myosin coalescence requires an intact actin network, it is proposed that pulses of myosin coalescence represent contractions of the actin-myosin network (Martin, 2009).
Because actin-myosin contractions occurred at the medial apical cortex, it was unclear how the actin-myosin network was coupled to adherens junctions. Therefore E-Cadherin-GFP and Myosin-mCherry were imaged to examine the relationship between myosin and adherens junctions. Before apical constriction, adherens junctions are present about 4 microm below the apical cortex. As apical constriction initiated, these subapical adherens junctions gradually disappeared and adherens junctions simultaneously appeared apically at the same level as myosin. This apical redistribution of adherens junctions occurred at specific sites along cell edges (midway between vertices). As apical constriction initiated, these sites bent inwards. This bending depended on the presence of an intact actin network, which is consistent with contraction of the actin-myosin network generating force to pull junctions. Indeed, myosin spots undergoing coalescence were observed to lead adherens junctions as they transiently bent inwards. Thus, pulsed contraction of the actin-myosin network at the medial cortex seems to pull the cell surface inwards at discrete adherens junction sites, resulting in apical constriction (Martin, 2009).
The transcription factors Twist and Snail regulate the apical constriction of ventral furrow cells. Snail is a transcriptional repressor whose target or targets are currently unknown, whereas Twist enhances snail expression and activates the expression of fog and t48, which are thought to activate the Rho1 GTPase and promote myosin contractility. To examine the mechanism of pulsed apical constriction further, how Twist and Snail regulate myosin dynamics was tested. In contrast to wild-type ventral cells, in which myosin was concentrated on the apical cortex, twist and snail mutants accumulated myosin predominantly at cell junctions, similarly to lateral cells. These ventral cells failed to constrict productively, which supported the cortical actin-myosin network contraction model, rather than the purse-string model, for apical constriction. twist and snail mutants differentially affected the coalescence of the minimal myosin that did localize to the apical cortex. Although myosin coalescence was inhibited in snail mutants, it still occurred in twist mutants, as did pulsed constrictions. This difference was also observed when Snail or Twist activity was knocked down by RNA-mediated interference. However, the magnitude of constriction pulses in twistRNAi embryos was greater than that of twist mutant embryos, suggesting that the low level of Twist activity present in twistRNAi embryos enhances contraction efficiency by activating the expression of snail or other transcriptional targets. Myosin coalescence was inhibited in snail twist double mutants, demonstrating that the pulsed constrictions in twist mutants required snail expression. Thus, the expression of snail, not twist, initiates the actin-myosin network contractions that power constriction pulses (Martin, 2009).
Net apical constriction was inhibited in both snailRNAi and twistRNAi embryos. It was therefore asked why the pulsed contractions that were observed in twistRNAi embryos failed to constrict cells. Using Spider-GFP to visualize cell outlines, it was found that although constriction pulses were inhibited in snailRNAi embryos, constriction pulses still occurred in twistRNAi embryos. However, the constricted state of cells in twistRNAi embryos was not stabilized between pulses, resulting in fluctuations in apical area with little net constriction. This stabilization defect was not due to lower snail activity, because these fluctuations continued when snail expression was driven independently of twist by using the P[sna] transgene. Although the frequency and magnitude of constriction pulses in such embryos were similar to those in control embryos, stretching events were significantly higher in twistRNAi; P[sna] embryos, suggesting a defect in maintaining cortical tension. This defect might result from a failure to establish a dense actin meshwork, because both twist mutants and twistRNAi embryos had a more loosely arranged apical meshwork of actin spots and fibres than constricting wild-type cells did. twist expression therefore stabilizes the constricted state of cells between pulsed contractions (Martin, 2009).
Thus, a 'ratchet' model is proposed for apical constriction, in which phases of actin-myosin network contraction and stabilization are repeated to constrict the cell apex incrementally. In contrast to the purse-string model, it was found that apical constriction is correlated with pulses of actin-myosin network contraction that occur on the apical cortex. Pulsed cortical contractions could allow dynamic rearrangements of the actin network to optimize force generation as cells change shape. Because contractions are asynchronous, cells must resist pulling forces from adjacent cells between contractions. A cortical actin-myosin meshwork seems to provide the cortical tension necessary to stabilize apical cell shape and promote net constriction. The transcription factors Snail and Twist are critical for the contraction and stabilization phases of constriction, respectively. Thus, Snail and Twist activities are temporally coordinated to drive productive apical constriction. Despite the dynamic nature of the contractions in individual cells, the behaviour of the system at the tissue level is continuous, in a similar manner to convergent extension in Xenopus. Pulsed contraction may therefore represent a conserved cellular mechanism that drives precise tissue-level behaviour (Martin, 2009).
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, 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). Genetic interaction studies were confirmed and extended 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, it was demonstrated 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 (Franke, 2010).
The array of phenotypes caused by the directed expression of an allelic series of myosin II truncation constructs shows a variety of new roles for zip/MyoII in wing morphogenesis and confirms expected roles. Perturbing zip/MyoII function resulted in: ectopic and/or expanded vein and crossveins, the formation of multiple wing hairs instead of a single hair, the branching of individual wing hairs and the loss of adhesion between the dorsal and ventral wing epithelial cell sheets. An expected role of myosin II in cytokinesis was very likely the cause of reduced cell proliferation. Truncation allele expression in the thorax indicates these new roles for myosin II are not restricted to wing morphogenesis and likely extend to the correct morphogenesis of actin-based protrusions (hairs or setae; bristles or microchaetae and macrochaetae) throughout the fly (Franke, 2010).
During wing development, expression of these dominant negative truncation alleles cause wing blister phenotypes, which are also caused by mutations in integrin genes. Nonmuscle myosin II is known to participate in integrin-based adhesion of individual migrating cells to extracellular matrix and ablation of nonmuscle myosin IIA heavy chain caused cell-cell adhesion defects in early mouse embryogenesis. Nonmuscle myosin II also functions in the correct lateral arrangement and regulation of cells within a single epithelia. In Drosophila embryonic myofibril formation, correct nonmuscle myosin II localization requires a PS2 integrin. This study extends the role of nonmuscle myosin II in cell adhesion to include an essential role in the apical adhesion of two different epithelial cell sheets to one another. The loss of zip/MyoII function in one of the two wing epithelia sheets was sufficient to abolish adhesion, indicating that zip/MyoII function is necessary in both cell sheets. An accumulation of zip/MyoII at the dorsal-ventral compartment boundary in wing imaginal discs has been reported and may be important for myosin's role in the adhesion between these two cell sheets. Understanding how zip/MyoII functions and coordinates with known adhesion molecules in cell sheet adhesion is an important area for further investigation (Franke, 2010).
Multiple wing hair phenotypes were most easily quantified when GFP-zip/MyoII-Rod(ΔNterm58) was expressed. Like numerous other proteins, zip/MyoII appears to have a direct role in the production of a single hair from the distal vertex of each wing cell. Branching phenotypes were observed in wing hairs, setae and bristles (micro- and macrochaetae) when GFP-zip/MyoII-Rod(ΔNterm58), zip/MyoII-Rod or GFP-zip/MyoII-Neck-Rod was expressed. Branching phenotypes are not generally observed in planar cell polarity mutants and show that zip/MyoII plays a distinct, yet important role in the correct morphology of different actin-based cellular protrusions. Branching of individual hairs or bristles has been characterized in furry and tricornered mutants and can also result from drug treatments (cytochalasin D or latrunculin A) that affect the actin cytoskeleton. As bristles and wing hairs use different assembly strategies of actin bundles to generate their morphology, the simplest explanation of these results is that zip/MyoII plays an early role in these processes. Consistent with these findings, zip/MyoII mutants have defects in shaping and positioning of actin-based protrusions in the embryonic epidermis. Proteins known to contribute to planar polarity (e.g., the proximal and distal proteins Flamingo, Frizzled, Dishelveled, Diego, Strabismus, also known as Van Gogh, and Prickle) all appear to function upstream of the actin cytoskeleton and there are no reports of a direct physical interactions between these proteins and the actin cytoskeleton. As a consequence it is suspected that zip/MyoII functions downstream of planar polarity patterning. How it coordinates with proteins known to be involved in hair morphogenesis will require further investigation (Franke, 2010).
Ectopic and/or expanded vein and crossvein wing patterning phenotypes were observed with the expression of each truncation allele and in the absence of other phenotypes indicating that vein and crossvein positioning defects result directly from zip/MyoII perturbation. The most obvious defects observed were in the positioning of the posterior crossvein, resulting in both ectopic crossveins as well as expanded crossvein tissue. While several loci have been identified that affect vein and crossvein patterning, only a few give rise to ectopic or expanded tissue. Mutant forms of the Dachsous and fat protocadherins have been shown to shift the relative position of the anterior and posterior crossveins with respect to one another but do not cause ectopic tissue. Thus these findings suggest a potentially novel role for zip/MyoII in tissue patterning (Franke, 2010).
The truncation alleles developed in this study will be useful for the analysis of myosin function elsewhere in development. Indeed the current studies show roles for zip/MyoII in movements that contribute to other regions of the adult fly epidermis al well (Franke, 2010).
This study found that full-length zip/MyoII is required for correct function and localization in Drosophila, consistent with findings in S. pombe, which showed that truncations of its myosin heavy chains are not capable of rescue. This contrasts findings in S. cerevisiae where the tail region of myosin II can functionally substitute for full-length protein. The dependence of GFP-zip/MyoII-Neck-Rod on full-length zip/MyoII for localization could occur through two, not necessarily mutually exclusive, mechanisms. First, an individual GFP-zip/MyoII-Neck-Rod protein could heterodimerize with an endogenous, wild-type zip/MyoII heavy chain. Second, GFP-zip/MyoII-Neck-Rod proteins could form homodimers, which associate with endogenous zip/MyoII homodimers, and assemble into bipolar filaments. The results indicate that GFP-zip/MyoII-Neck-Rod heavy chains predominantly form homodimers (Franke, 2010).
The observation that zip/MyoII-HMM(ΔCterm407)-GFP does not induce a multiple wing hair or branching phenotype may provide insights into how zip/MyoII contributes to these processes. One possibility is that zip/MyoII rod-induced aggregates, which recruit endogenous zip/MyoII, may also recruit proteins important for the establishment of PCP thereby altering their subcellular localization. Immunostaining imaginal discs for known PCP proteins that express rod-containing zip/MyoII fragments could help address this (Franke, 2010).
The results demonstrate that nonmuscle myosin II function can be dominantly perturbed by two distinct mechanisms. Previous findings in D. discoidium, yeast, tissue culture cells and Drosophila have demonstrated that myosin II function can be perturbed by expression of truncation constructs. Expression of GFP-zip/MyoII-Rod(ΔNterm58) and zip/MyoII-Rod resulted in the formation of very large intracellular aggregates. Aggregation is consistent with in vitro findings that the light meromyosin region (LMM) is insoluble at physiological ionic strength. Consistent with this study, it has been shown that the formation of myosin II rod aggregates in D. discoidium were intracellular and could contain endogenous, full-length myosin II. Thus, expression of the rod domain recruits full-length nonmuscle myosin II into intracellular aggregates, thereby depleting the total cellular pool of functional nonmuscle myosin II (Franke, 2010).
The second mechanism results in the titration of free light chains from endogenous, full-length nonmuscle myosin II thereby depleting the total cellular pool of nonmuscle myosin II that can be regulated through light chains. A construct capable of function through both mechanisms (one containing both the tail and neck domains) is expected to be the most potent nonmuscle myosin II dominant negative. The results are consistent with this -- GFP-zip/MyoII-Neck-Rod consistently generated the most severe phenotypes. Moreover, these observations suggest that both mechanisms function to simply titrate endogenous, wild-type zip/MyoII heavy chain and are therefore comparable to loss of function alleles (Franke, 2010).
The C-terminal and N-terminal antisera enable semi-quantitative analysis of the expression of a truncation allele to endogenous zip/MyoII. Western blot analysis of whole 3rd instar larvae may be misleading as it does not provide a cellular context for comparing expression levels. Comparing the relative amount of fluorescence between a control region (endogenous zip/MyoII) and an experimental region (endogenous zip/MyoII and rod truncation allele; the current results likely provides a more accurate means for comparing expression -- expression of GFP-zip/MyoII-Neck-Rod is approximately two to three times that of zip/MyoII (Franke, 2010).
That GFP-zip/MyoII expression caused a mild wing phenotype is likely the consequence of heavy chain overexpression without comparable light chain expression. When GFP-zip/MyoII was placed in a heterozygous zipper background the penetrance of phenotypes decreased. The most parsimonious explanation is that the total amount of heavy chain (endogenous plus transgene) was reduced due to the heterozygous background, which helped alleviate the imbalance of heavy and light chains (Franke, 2010).
Having distinct truncation alleles that perturb zip/MyoII function to different extents enables one to screen for enhancers and suppressors of zip/MyoII in desired processes. Expressing of GFP-zip/MyoII-Rod(ΔNterm58) identified both genetic enhancers and suppressors of the multiple wing hair phenotype with components of the PCP pathway (Franke, 2010).
Previously, zip/MyoII was shown to genetically interact with dsh. The current results extend previous findings to include interactions with other PCP pathway genes (Fz and ck/MyoVIIA). The genetic interaction with Fz suggests that in addition to having a direct role in the production of a single wing hair, zip/MyoII may also participate in wing hair polarity. The genetic studies with ck/MyoVIIA show that it acts antagonistically to zip/MyoII with respect to the multiple wing hair phenotype and to other processes in wing morphogenesis resulting in more severe wing phenotypes (e.g., wing blisters). A role for myosin VIIs in cell adhesion has been demonstrated in different organisms. The mechanism(s) by which these heavy chains function in these different processes will require further investigation (Franke, 2010).
Each truncation allele fulfilled criteria of being specific zip/MyoII dominant-negatives: each caused phenotypes in a dose-dependent manner and these phenotypes were overlapping among the different truncation alleles. The allelic series of myosin II truncation constructs generated allow nonmuscle myosin II function to be variably perturbed in a cell, tissue and temporally specific, and therefore, conditional manner. Thus, these tools now make it possible to interrogate zip/MyoII function during any desired time interval or developmental process in Drosophila (Franke, 2010).
The nonmuscle myosin heavy chain from Drosophila contains an alternatively spliced exon at the 5' end which generates two distinct heavy-chain transcripts: the longer transcript inserts an additional start codon upstream of the primary translation start site and encodes a myosin heavy chain with a 45-residue extension at its amino terminus. The shorter transcript is 3.5 times more abundant than the longer one (Ketchum, 1990).
A second alternative exon (40aa) is close to the nucleotide binding pocket. The position, size and sequence of this exon is conserved in D. simulans and putative alternative exons of different size (7 to 16 aa), but identical positions have been reported for other myosins in many phyla. The functional significance of either alternative splice is not clear (Mansfield, 1996)
Exons - 14
Bases in 3' UTR - 188
The coding sequence of zipper reveals extensive homology with other conventional myosins, especially metazoan nonmuscle and smooth muscle myosin isoforms. Comparisons among available myosin heavy-chain sequences establish that characteristic differences in sequence throughout the length of both the globular myosin head and extended rod-like tail readily distinguish nonmuscle and smooth muscle myosins from striated muscle isoform,s and predict a basis for their functional diversity. The ATP binding region is in the first 200 amino acids. The actin binding region is within 30 amino acids centered on amino acid 670. The myosin tail includes the 1085 residues from leucine-843 to leucine-1927 and is characterized by a strong heptad repeat, common to conventional myosin tails and other alpha-helical coiled-coil proteins. These regions dimerize and fold into the extended rod-like tail of the native Drosophila myosin molecule. The hinge region is in the center of the tail sequence. The terminal 47 amino acids consist of the globular tail region. The tail region contains putative casein kinase II and protein kinase C phosphorylation sites (Ketchum, 1990 and Mansfield, 1996).
date revised: 22 Dec 96
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