Distinct pathways control recruitment and maintenance of myosin II at the cleavage furrow during cytokinesis: Genes in the rho1 signaling pathway are required for the initial recruitment of myosin II to the equatorial cortex

The correct localization of myosin II to the equatorial cortex is crucial for proper cell division. A collection of genes was examined that causes defects in cytokinesis and revealed (with live cell imaging) two distinct phases of myosin II localization. Three genes in the rho1 signaling pathway, pebble (a Rho guanidine nucleotide exchange factor), rho1, and rho kinase, are required for the initial recruitment of myosin II to the equatorial cortex. This initial localization mechanism does not require F-actin or the two components of the centralspindlin complex, the mitotic kinesin pavarotti/MKLP1 and racGAP50c/CYK-4. However, F-actin, the centralspindlin complex, formin (diaphanous), and profilin (chickadee) are required to stably maintain myosin II at the furrow. In the absence of these latter genes, myosin II delocalizes from the equatorial cortex and undergoes highly dynamic appearances and disappearances around the entire cell cortex, sometimes associated with abnormal contractions or blebbing. These findings support a model in which a rho kinase-dependent event, possibly myosin II regulatory light chain phosphorylation, is required for the initial recruitment to the furrow, whereas the assembly of parallel, unbranched actin filaments, generated by formin-mediated actin nucleation, is required for maintaining myosin II exclusively at the equatorial cortex (Dean, 2005).

This study has discovered three steps in the myosin II localization/activation process that involve distinct groups of genes: (1) an initial recruitment of myosin II to the equatorial cortex that is independent of F-actin and centralspindlin but requires rho1 signaling; (2) a secondary stabilization of myosin II at the midzone that requires F-actin and a second set of genes that are likely involved in building a specific type of actin network, and (3) the activation of furrowing once myosin II is localized that depends on centralspindlin (Dean, 2005).

Rho1, its activating guanidine nucleotide exchange factor pebble, and rho kinase are each required for the initial recruitment of myosin II to the equatorial cortex. Rho1 has been implicated in two pathways that are important for cytokinesis. In the first pathway, rho1 signals to F-actin through the formin diaphanous. However, proteins on this F-actin pathway, including F-actin itself, are not essential for the initial myosin II recruitment to the equatorial cortex. However, rho kinase, another downstream target of rho1, is essential. Because rho kinase phosphorylates the myosin II RLC, it is possible that phosphorylation of the RLC is essential for myosin II recruitment to the furrow. This hypothesis could not be directly tested, because the myosin II heavy chain forms large aggregates when the RLC is depleted by RNAi (Dean, 2005).

Phosphorylation of the RLC both activates the motor domain and, in some myosins, increases bipolar thick filament formation. Because F-actin is not required for myosin II recruitment, activation of the motor is unlikely to be the mechanism by which phosphorylation of the RLC would cause recruitment of myosin II to the equatorial cortex. It is quite possible, however, that the rho kinase-mediated myosin II phosphorylation leads to thick filament assembly and that this assembly is important for localization of myosin to the equatorial cortex. Indeed, in Dictyostelium, it is clear that bipolar thick filament formation is sufficient for myosin II localization to the midzone of a mitotic cell. The nonactin-based mechanism of recruitment of myosin II filaments remains unknown (Dean, 2005).

In contrast to the lack of F-actin involvement in the early recruitment of myosin II to the equatorial cortex at anaphase, F-actin disruption by Latrunculin A results in a failure to maintain myosin II in the equatorial region. Interestingly, the downstream rho1 effectors diaphanous/formin and chickadee/profilin are also necessary for myosin II maintenance at the equatorial midzone. Although the loss of these genes could deplete F-actin, phalloidin staining has shown that F-actin is still present in all of the RNAi-treated cells. In addition, these RNAi-treated cells still contract, unlike when F-actin is completely disrupted with LatA. Thus, myosin II appears to be interacting with F-actin in the cortex as it disperses in dynamic patches throughout the cortex of these diaphanous- or chickadee-depleted cells (Dean, 2005).

It is suggested that the role of diaphanous/formin and chickadee/profilin in maintaining the myosin II contractile ring is through the creation of specific F-actin structures. In particular, formin- and profilin-mediated nucleation results in unbranched actin filaments because profilin promotes the barbed-end growth of formin-capped actin filaments. Indeed, electron microscopy has shown that F-actin in the cleavage furrow mainly consists of unbranched, bundled filaments. These parallel filaments contrast with Arp2/3-mediated nucleation, which creates a highly branched actin filament network. Indeed, Arp2/3, although essential for lamellipodia formation, is not required for cytokinesis in Drosophila cells. The hypothesis here is that once myosin II is recruited to the equatorial cortex of the cell by a rho kinase-dependent mechanism, possibly localized activation of RLC phosphorylation, it is retained there because of its higher affinity for parallel, unbranched actin filaments than to branched actin networks. Consistent with this hypothesis, myosin II is depleted from the lamellipodia in migrating cells where Arp2/3 is localized and branched F-actin networks are formed but is enriched in the lamella where F-actin filaments are more likely to be aligned in parallel bundles. Thus, it is proposed that high rho1 signaling to Diaphanous at the cleavage furrow maintains a higher concentration of parallel actin filaments in this region compared with the rest of the cortex, and these parallel filaments serve to selectively retain myosin II at the equator to form a stable contractile ring. In the absence of these parallel actin filaments, myosin II can bind branched F-actin throughout the cortex, perhaps occasionally organizing them into parallel bundles that cause increased myosin recruitment corresponding to the flashes of cortical myosin accumulation, but these interactions are unstable (Dean, 2005).

Live-cell imaging shows that when pavarotti or racGAP50c are depleted, the cells do not display significant contractions despite recruiting myosin II to the equatorial cortex. Although there is some modest membrane contractile activity in these cells, it is clear that significant contraction or furrowing requires both components of the centralspindlin complex. It is surprising that only these proteins were found to be necessary for cortical contraction at sites of myosin II localization. Data from fixed cells, as well as earlier studies, indicated that Drosophila cells do not undergo equatorial contractions during mitosis when Diaphanous or Chickadee is depleted. However, live-cell imaging shows that when either of these two genes is depleted in S2 cells, not only is myosin II transiently localized to the equatorial cortex before dispersing, but cells do indeed display transient equatorial contraction. It is difficult to recognize these events in fixed cells because of their transient nature and the somewhat irregular shapes of cells depleted of these proteins. This work highlights the importance of live-cell imaging in the study of dynamic processes such as cytokinesis (Dean, 2005).

In addition to the suppression of furrowing, depletion of centralspindlin also leads to an inability to retain F-actin exclusively at the equatorial cortex during cytokinesis. This similar phenotype of the centralspindlin complex and the F-actin affecting proteins suggests that centralspindlin may be an upstream regulator of F-actin filament formation. Indeed kinase-dead mutants of Pavarotti have been shown to accumulate at the spindle poles and are associated with an abnormal accumulation of F-actin near the centrosomes. Centralspindlin may be acting indirectly by helping to localize an important actin-affecting protein at the central spindle, or it may act more directly on the cortex. Because RacGAP50c has been shown to bind Pebble in vitro, it has been hypothesized that centralspindlin affects the F-actin cortex through rho1 signaling by the localization and/or activation of Pebble. However, RacGAP50c depletion does not lead to a lack of myosin II recruitment as does Pebble or Rho1 depletion, and, thus, centralspindlin must act in a rho1-independent manner. For instance, the racGAP activity of centralspindlin may itself be important for signaling to the F-actin cortex. Finally, centralspindlin cannot be the major actomyosin ring positioning signal because myosin II is properly recruited in its absence (Dean, 2005).

Protein Interactions

Mammalian Rho-kinase/ROK alpha, one of the targets of Rho, has been shown to bind to Rho in GTP-bound form and to phosphorylate the myosin light chain (MLC) and the myosin binding subunit (MBS: see Drosophila Myosin binding subunit) of myosin phosphatase, resulting in the activation of myosin. Thus, Rho-kinase/ROK alpha has been suggested to play essential roles in the formation of stress fibers and focal adhesions. A two-hybrid analysis demonstrates that Rok interacts with the GTP-bound form of the Drosophila Rho1 at the conserved Rho-binding site. Rok can phosphorylate MLC and MBS, preferable substrates for bovine Rho-kinase, in vitro. These results suggest that Rok is an effector of Rho1 (Mizuno, 1999).

To test the physical interaction between Rok and various Rho-GTPases, a pull-down assay using GST-GTPase fusion proteins and in vitro translated Rok was used. Rok binds to the constitutively active form of Drosophila RhoA, but not to constitutively active Rac1 or Cdc42. Mutating a key amino acid within the effector-binding domain (T37A) abolishes the interaction with Rok. These results suggest that Rok is an effector specific for Drosophila RhoA (Winter, 2001).

Studies in mammalian cells have identified several downstream substrates for Rho-kinase/ROCK (Amano, 2000). In particular, Rho-kinase regulates the phosphorylation of the nonmuscle myosin regulatory light chain (MRLC) primarily at Ser-19 and secondarily at the adjacent Thr-18 (Amano, 1996; Kimura, 1996). Phosphorylation of MRLC at these sites results in a conformational change that allows myosin II to form filaments and increases its actin-dependent ATPase activity (Winter, 2001).

The amino acid sequence around the phosphorylation site of MRLC is highly conserved between mammalian MRLC and the Drosophila homolog, encoded by spaghetti squash (sqh). Therefore the phosphorylation of the Drosophila MRLC was assessed using an antibody that recognizes mammalian MRLC only when Ser-19 is phosphorylated. Immunoblot analysis shows that this antibody specifically recognizes phosphorylated Sqh in larval extracts -- the single ~20 kDa band in wild-type extract is absent both in extracts of a sqh null, and when wild-type extract is treated with phosphatase. While phosphorylated Sqh is detectable in Drok2 mutant extracts, its level is greatly reduced, whereas the Sqh protein level is not affected in Drok2 mutants. Previous work with bovine Rho-kinase has established that expression of the N-terminal catalytic domain gives rise to a constitutively active kinase (Amano, 1997). Raising the level of Rok activity in vivo by transient expression of the catalytic domain of Rok (Drok-CAT) results in elevated phosphorylation of Sqh as compared to controls in which a kinase-dead form (Drok-CAT-KG) was used. Taken together, these experiments indicate that Rok is required for maintaining the proper level of MRLC phosphorylation in vivo, and that such regulation depends on its kinase activity (Winter, 2001).

The effect of loss of Rok function on MRLC phosphorylation was examined at the cellular level. In wild-type wing cells, phospho-MRLC is enriched at the cortex of the pupal wing cells, whereas in Drok2 mutant cells this perimembrane staining is reduced or absent. Thus, rok is cell autonomously required for maintaining the level of cortical phospho-MRLC in the pupal wing (Winter, 2001).

The next question considered was whether MRLC/Sqh is an effector for Rok in regulating hair number in response to Fz/Dsh signaling. Use was made of a series of mutant sqh transgenes with point mutations in the primary (Ser-21) and secondary (Thr-20) phosphorylation sites, changing them either to glutamic acid (phosphomimetic), or to nonphosphorylatable alanine. These sqh transgenes are under control of the endogenous promoter and are expressed at levels similar to the native protein. Remarkably, whereas 100% of Drok2 hemizygous animals die before the wandering third instar stage, introducing one copy of a sqh transgene carrying the E20E21 double mutation (mimicking phosphorylation on both sites) results in 4% hemizygous Drok2 survival to adulthood. Likewise, one copy of an analogous transgene expressing SqhE21 also results in Drok2 hemizygotes surviving to adulthood (albeit a lower percentage), with a large fraction surviving to late-stage pupae. No rescue was observed when transgenes expressing the alanine substituted forms (SqhA20A21 or SqhA21) were introduced into the Drok2 background. These observations support the notion that MRLC is a key target (either directly or indirectly) for Rok kinase in vivo, since mimicking its phosphorylation, even in an unregulated fashion, partially rescues Drok2 organismal lethality (Winter, 2001).

Moreover, the multiple hair defect resulting from rok loss of function is almost completely suppressed by the presence of the sqhE20E21 transgene in the rescued adults. Taken together with the modulation of MRLC phosphorylation by Rok, these results demonstrate that the regulation of MRLC phosphorylation is a principal function of Rok in regulating F-actin prehair number (Winter, 2001).

Mechanisms that regulate axon branch stability are largely unknown. Genome-wide analyses of Rho GTPase activating protein (RhoGAP) function in Drosophila using RNA interference has identified p190 RhoGAP as essential for axon stability in mushroom body neurons, the olfactory learning and memory center. RhoGAP inactivation leads to axon branch retraction, a phenotype mimicked by activation of GTPase RhoA and its effector kinase Drok and modulated by the level and phosphorylation of myosin regulatory light chain. Thus, there exists a retraction pathway from RhoA to myosin in maturing neurons, which is normally repressed by RhoGAP. Local regulation of RhoGAP could control the structural plasticity of neurons. Indeed, genetic evidence supports negative regulation of RhoGAP by integrin and Src, both implicated in neural plasticity (Billuart, 2001).

Drosophila Rho-kinase (Drok), an effector of RhoA, is essential for transducing signals to the actin cytoskeleton in wing cells (Winter, 2001). Since the effector domain mutant analysis of RhoA suggests that a cytoskeletal pathway is important for axon retraction, tests were performed to see if the Drok pathway is involved. Carboxy-terminal truncation of mammalian Rho-kinase/ROCK results in its constitutive activation. Expression of an analogous activated form (Drok-CAT; Winter, 2001) in MB neurons led to truncated dorsal lobes similar to the phenotypes of p190 RNAi and weak RhoA activation. A presumptive kinase-dead point mutation (Drok-CAT.KG; Winter, 2001) has no effect, indicating that Drok signaling is dependent on its kinase activity. Developmental studies indicate that the Drok-CAT phenotypes also results from axon retraction, as does the p190 RNAi phenotype (Billuart, 2001).

Neuroblast clones homozygous for Drok2 (Winter, 2001) do not show apparent defects in cell proliferation, because the adult clones contain dorsal axon lobes contributed by later born neurons. Close examination of Drok2 neuroblast clones reveals that 10 of 17 contain at least one axon that extends significantly further than the heterozygous neurons within the same MB. Although this phenotype is subtle, it is not seen in 19 control clones, the parental chromosome for the Drok2 mutant, nor in many other genotypes studied. Thus, it is concluded that Drok is required to limit dorsal axon extension (Billuart, 2001).

Biochemical and genetic evidence indicates that a key output for Drok signaling in vivo is the regulation of phosphorylation of myosin regulatory light chain (MRLC) encoded by spaghetti squash (sqh) (Winter, 2001). To test if endogenous MRLC is part of the axon retraction pathway regulated by p190, genetic interaction experiments were performed by reducing the dose of endogenous sqh in the context of the p190 dsRNA expression. Marked suppression of the phenotype was observed in flies heterozygous for a null mutation of sqh (sqhAX3). In contrast, expression of a phosphomimetic mutant, Sqh-E20E21, markedly enhanced the p190 phenotype, whereas analogous expression of a nonphosphorylable form (Sqh-A21) had no effect. Further, truncation of the medial lobe was frequently observed when Sqh-E20E21 was expressed with the intermediate p190 RNAi line. This is evident from the FasII staining, showing that the medial ß axons (strongly FasII positive) only extend a fraction of the length of the medial lobe. This phenotype was only observed in the strongest p190 RNAi lines, never in the intermediate line alone. Taken together, these results strongly suggest that Drok and phosphorylation of Drosophila MRLC participate in mediating axon retraction as a result of p190 inactivation (Billuart, 2001).

Drosophila Rho-kinase associates with the GTP-bound Drosophila Rho1 and can phosphorylate the vertebrate MRLC and myosin binding subunit (MBS). Rho-kinase is involved in the establishment of planar polarity in adult structures such as the compound eye and wing. Drosophila Myosin-binding subunit (MBS) has been characterized to elucidate the functions of myosin phosphatase in morphogenesis, revealing that MBS functions in dorsal closure and that it acts antagonistically to the Rho signaling cascade and its effector Rho-kinase (Mizuno, 2002).

Drosophila Rho-kinase physically interacts with Rho1 in the GTP form and phosphorylates the vertebrate MBS in vitro. The sequence at the putative phosphorylation site of vertebrate MBS is well conserved in Drosophila MBS, and tests were performed to see whether DRho-kinase phosphorylates MBS in vitro. The GST-fused DMBS-L was expressed and purified from Escherichia coli, and was found to be phosphorylated by wild-type Drosophila Rho-kinase but not by kinase-dead DRho-kinaseK116A (Mizuno, 2002).

Thr594 may correspond to the major phosphorylation site in vertebrate MBS. The threonine residue was replaced with an alanine, and this recombinant Drosophila MBS was used as a substrate. The level of phosphorylation was significantly reduced, indicating that Thr594 is the major site phosphorylated by Drosophila Rho-kinase. It has been reported that mammalian MBS is phosphorylated at several sites by Rho-kinase (Kawano, 1999), and there presumably are other phosphorylation sites in Drosophila MBS as well (Mizuno, 2002).

Drosophila Rho-kinase is thought to be responsible for the inactivation of myosin phosphatase through phosphorylation of MBS. If this inactivation turns out to have a considerable effect on the levels of phosphorylated MRLC, it can be expected that the phenotypes in the MBS mutant embryos and in the embryos overexpressing DRho-kinase would be similar. When wild-type Rho-kinase was expressed with the arm-GAL4 driver, about 80% of the embryos failed to hatch. A similar result was obtained with the 69B-GAL4 driver that induces the target gene in the ectoderm. Most of the lethal embryos show a dorsal open or dorsal hole phenotype, and the pattern of dorsal hairs is disturbed along the dorsal midline in the remaining embryos as observed in the MBS mutant embryos. Examination of the cell shape and the F-actin distribution reveals the same aberrations as those in the MBS mutant embryos (Mizuno, 2002).

Dynamic myosin phosphorylation regulates contractile pulses and tissue integrity during epithelial morphogenesis

Apical constriction is a cell shape change that promotes epithelial bending. Activation of nonmuscle myosin II (Myo-II) by kinases such as Rho-associated kinase (Rok) is important to generate contractile force during apical constriction. Cycles of Myo-II assembly and disassembly, or pulses, are associated with apical constriction during Drosophila melanogaster gastrulation. It is not understood whether Myo-II phosphoregulation organizes contractile pulses or whether pulses are important for tissue morphogenesis. This study shows that Myo-II pulses are associated with pulses of apical Rok. Mutants that mimic Myo-II light chain phosphorylation or depletion of myosin phosphatase inhibit Myo-II contractile pulses, disrupting both actomyosin coalescence into apical foci and cycles of Myo-II assembly/disassembly. Thus, coupling dynamic Myo-II phosphorylation to upstream signals organizes contractile Myo-II pulses in both space and time. Mutants that mimic Myo-II phosphorylation undergo continuous, rather than incremental, apical constriction. These mutants fail to maintain intercellular actomyosin network connections during tissue invagination, suggesting that Myo-II pulses are required for tissue integrity during morphogenesis (Vasquez, 2014).

Recent studies demonstrated that pulsatile Myo-II contractions drive diverse morphogenetic processes, including Caenorhabditis elegans embryo polarization, Drosophila gastrulation, dorsal closure, germband extension, oocyte elongation, and Xenopus laevis convergent extension. Although Rok, and likely Myo-II activation via Rok phosphorylation, is required for contraction, it was not clear whether Myo-II activation simply regulates cortical Myo-II levels or whether coupling between Myo-II activity and its regulators organizes contractile pulses in space and time. Furthermore, why cells undergo pulsatile, rather than continuous, contraction to drive tissue morphogenesis was unknown. This study was able to answer these questions by visualizing the consequences of uncoupling Myo-II activation from upstream signaling pathways on cell and tissue dynamics (Vasquez, 2014).

This study identified dynamic Myo-II phosphorylation as a key mechanism that regulates contractile pulses. Myo-II pulses are associated with dynamic medioapical Rok foci and myosin phosphatase. In addition, the phosphomimetic sqh-AE and sqh-EE mutants, which exhibited constitutive cytoplasmic Myo-II assembly in vivo, exhibited defects in two properties of contractile pulses. First, phosphomimetic mutants did not initially condense apical Myo-II or F-actin into medioapical foci, resulting in Myo-II accumulation across the apical domain and thus a defect in Myo-II radial cell polarity. Second, phosphomimetic mutants continuously accumulated Myo-II in the apical cortex, lacking clear cycles of Myo-II remodeling that are observed in wild-type embryos. Although the phosphomimetic alleles are predicted to partially activate the Myo-II motor’s ATPase activity compared with normal phosphorylation, the similarity of the Myosin binding subunit (MBS) (see Lee, 2004) knockdown phenotype suggests that the changes in Myo-II organization and dynamics in phosphomimetic mutants reflect defects in the control over Myo-II dynamics rather than a reduction in motor activity. The consequence of persistent Myo-II assembly across the apical surface in phosphomimetic mutants and MBS knockdown is a more continuous, rather than incremental, apical constriction, demonstrating that pulsatile cell shape change results from temporal and spatial regulation of Myo-II activity via a balance between kinase (Rok) and phosphatase (myosin phosphatase) activity (Vasquez, 2014).

Mutants that decrease Myo-II phosphorylation affected contractile pulses in a manner that was distinct from the phosphomimetic alleles. Both the sqh-AA and the sqh-TA mutants exhibited Myo-II assembly into apical foci, potentially mediated by phosphorylation of low levels of endogenous Sqh or phosphorylation of threonine-20, respectively. For the sqh-TA mutant, Myo-II assembly was correlated with constriction, suggesting that Myo-II motor activity is not rate limiting to initiate a contractile pulse. However, Myo-II foci in sqh-TA and sqh-AA mutants were not efficiently remodeled after assembly and coalescence. The persistence of cortical Myo-II foci in sqh-AA and sqh-TA mutants was surprising given that rok mutants and injection of Rok inhibitor reduce cortical localization of Myo-II. One explanation is that high levels of Myo-II activity induce actomyosin turnover and thus could be required to remodel the actomyosin network after contraction. Alternatively, apical recruitment of myosin phosphatase or proteins that negatively regulate Rok could depend on Myo-II phosphorylation or actomyosin contraction. Although future work is needed to address the role of Myo-II motor activity in contractile pulses, the phenotypes of alleles that constitutively reduce phosphorylation further suggest that cycling between high and low phosphorylation states is required for proper Myo-II pulses (Vasquez, 2014).

A model is proposed for contractile pulses in the ventral furrow where, in combination with unknown cortical cues that apically localize Myo-II, local pulses of apical Rok activity within the medioapical cortex polarize Myo-II assembly and coalescence. Rok foci could polarize actomyosin condensation by generating an intracellular gradient of minifilament assembly and tension that results in inward centripetal actomyosin network flow. In addition, local Myo-II activation by Rok foci combined with broader myosin phosphatase activity throughout the apical cytoplasm could generate a gradient of Myo-II turnover that will concentrate Myo-II into medioapical foci. MBS is required to restrict phosphorylated Myo-II to specific cell–cell interfaces during dorsal closure, demonstrating that the balance between Myo-II kinases and phosphatase can generate spatial patterns of Myo-II activation in epithelial cells. Myo-II remodeling after coalescence could result from local decreases in Rok activity and enrichment of apical myosin phosphatase with Myo-II structures. Thus, coupling Myo-II activation to dynamic signals that regulate Myo-II phosphorylation organizes contractile pulses in space and time to drive incremental apical constriction (Vasquez, 2014).

Polarized actomyosin contraction, pulses, and flows generate force and organize the actin cortex in a variety of cellular and developmental. In contrast to the ratchet-like constriction of ventral furrow cells, some cell types undergo extended periods of actomyosin pulsing and area fluctuations without net reduction in area. Furthermore, directional rearrangement of cell contacts, such as during convergent extension in the Drosophila germband, can be achieved through planar polarized accumulation of junctional Rok and Myo-II in conjunction with planar polarized medioapical actomyosin flows. Modulating the spatial and temporal regulation of Myo-II phosphorylation and dephosphorylation provides a possible mechanism to tune contractile dynamics and organization to generate diverse cell shape changes. Consistent with this organizational role, phosphomimetic RLC mutants also disrupt the planar polarized localization of junctional Myo-II in the Drosophila germband. Thus, it will be important to define the principles that control Myo-II activity and dynamics and how tuning Myo-II dynamics impacts force generation and tissue movement (Vasquez, 2014).

Myo-II phosphomutants resulted in a more continuous apical Myo-II assembly and apical constriction, enabling investigation of the role of pulsation during tissue morphogenesis. Continuous Myo-II assembly and contraction in the sqh-AE mutant resulted in a slower mean rate of apical constriction and thus delayed tissue invagination. This delay suggested that pulsing might be important for the efficiency of apical constriction. However, phosphomimetic mutants might not fully recapitulate the ATPase activity of phosphorylated Myo-II. The sqh-TA mutant, which also perturbs Myo-II remodeling, constricted ventral furrow cells at a rate that is only slightly slower than wild type. In addition, MBS knockdown, which disrupted Myo-II pulses, exhibited a more variable constriction rate, but with a mean rate comparable to control embryos. The current finding is distinct from studies in other cell types where loss of MBS results in excessive phosphorylated Myo-II accumulation and cell invagination. Thus, MBS can regulate Myo-II organization and dynamics without causing a significant increase in apical Myo-II levels. It is concluded that Myo-II pulses are not absolutely required for individual cell apical constriction (Vasquez, 2014).

Although phosphomimetic mutant cells constrict and undergo tissue invagination, the coordination of invagination and the stability of the supracellular actomyosin meshwork were perturbed. Continuous apical constriction was associated with abnormal separation events between Myo-II structures in adjacent cells, resulting in gaps or holes in the supracellular Myo-II meshwork. Thus, continuous Myo-II assembly and a lack of Myo-II dynamics during apical constriction appear to sensitize the tissue to loss of intercellular cytoskeletal integrity during morphogenesis. Although loss of cytoskeletal continuity in phosphomimetic mutants does not block tissue invagination, it is speculated that dynamic Myo-II pulses are important to make tissue invagination robust to changes in tensile stress. One possible function of Myo-II pulses is to attenuate tissue tension or stiffness during morphogenetic movements. Because pulsed Myo-II contractions are staggered between neighboring cells, pulsation could serve as a mechanism to coordinate contractile force generation across the tissue such that intercellular connections are buffered from high levels of tension. Indeed, reducing adherens junction proteins sensitizes the intercellular connections between cytoskeletal networks to tensile forces generated in ventral furrow cells. Alternatively, remodeling of actomyosin networks that occurs during pulses could be required to adapt the cytoskeletal organization such that forces transmitted between cells accommodate the changing pattern of tissue-scale forces during the course of morphogenesis. In either case, the current data suggest that Myo-II pulsing and remodeling are important for collective cell behavior by ensuring proper force transmission between cells in a tissue undergoing morphogenesis (Vasquez, 2014).

The Drosophila FHOD1-like formin Knittrig acts through Rok to promote stress fiber formation and directed macrophage migration during the cellular immune response

A tight spatiotemporal control of actin polymerization is important for many cellular processes that shape cells into a multicellular organism. The formation of unbranched F-actin is induced by several members of the formin family. Drosophila encodes six formin genes, representing six of the seven known mammalian subclasses. Knittrig (Formin homology 2 domain containing ortholog), the Drosophila homolog of mammalian FHOD1, is specifically expressed in the developing central nervous system midline glia, the trachea, the wing and in macrophages. knittrig mutants exhibit mild tracheal defects but survive until late pupal stages and mainly die as pharate adult flies. knittrig mutant macrophages are smaller and show reduced cell spreading and cell migration in in vivo wounding experiments. Rescue experiments further demonstrate a cell-autonomous function of Knittrig in regulating actin dynamics and cell migration. Knittrig localizes at the rear of migrating macrophages in vivo, suggesting a cellular requirement of Knittrig in the retraction of the trailing edge. Supporting this notion, this study found that Knittrig is a target of the Rho-dependent kinase Rok. Co-expression with Rok or expression of an activated form of Knittrig induces actin stress fibers in macrophages and in epithelial tissues. Thus, a model is proposed in which Rok-induced phosphorylation of residues within the basic region mediates the activation of Knittrig in controlling macrophage migration (Lammel, 2014).

Rho1 regulates adherens junction remodeling by promoting recycling endosome formation through activation of Myosin II

Once adherens junctions (AJs) are formed between polarized epithelial cells they must be maintained as AJ are constantly remodeled in dynamic epithelia. AJ maintenance involves endocytosis and subsequent recycling of E-cadherin to a precise location along the basolateral membrane. In the Drosophila pupal eye epithelium, Rho1 GTPase regulates AJ remodeling through DE-cadherin endocytosis by limiting the Cdc42/Par6/aPKC complex activity. This study demonstrates that Rho1 also influences AJ remodeling by regulating the formation of DE-cadherin containing Rab11-positive recycling endosomes in Drosophila post-mitotic pupal eye epithelia. This effect of Rho1 is mediated through Rok-dependent, but not MLCK-dependent, stimulation of myosin II activity yet independent of its effects upon actin remodeling. Both Rho1 and pMLC localize on endosomal vesicles, suggesting that Rho1 may regulate the formation of recycling endosomes thorough localized myosin II activation. This work identifies spatially distinct functions for Rho1 in the regulation of DE-cadherin containing vesicular trafficking during AJ remodeling in live epithelia (Yashiro, 2014).

Combover/CG10732, a novel PCP effector for Drosophila wing hair formation

The polarization of cells is essential for the proper functioning of most organs. Planar Cell Polarity (PCP), the polarization within the plane of an epithelium, is perpendicular to apical-basal polarity and established by the non-canonical Wnt/Fz-PCP signaling pathway. Within each tissue, downstream PCP effectors link the signal to tissue specific readouts such as stereocilia orientation in the inner ear and hair follicle orientation in vertebrates or the polarization of ommatidia and wing hairs in Drosophila melanogaster. Specific PCP effectors in the wing such as Multiple wing hairs (Mwh) and Rho kinase (Rok) are required to position the hair at the correct position and to prevent ectopic actin hairs. In a genome-wide screen in vitro, Combover (Cmb)/CG10732 was identified as a novel Rho kinase substrate. Overexpression of Cmb causes the formation of a multiple hair cell phenotype (MHC), similar to loss of rok and mwh. This MHC phenotype is dominantly enhanced by removal of rok or of other members of the PCP effector gene family. Furthermore, Cmb physically interacts with Mwh, and cmb null mutants suppress the MHC phenotype of mwh alleles. These data indicate that Cmb is a novel PCP effector that promotes to wing hair formation, a function that is antagonized by Mwh (Fagan, 2014).

Rho kinase, a member of the AGC kinase family which also includes PKC and Akt was originally identified as a RhoA effector reorganizing the cytoskeleton by promoting the formation of actin stress fibers. In Drosophila, Rok was shown to act downstream of Fz and Dsh in the non-canonical Wnt/Planar Cell Polarity pathway causing ommatidial rotation and structural defects in the eye and multiple hairs cells in the wing. This study has identified Combover/CG10732 as a novel substrate of Rok. A cmb protein null allele lacking both Cmb protein isoforms that is homozygous viable. Homozygous cmb mutants display no visible phenotype in the wing or in sections of the adult eyes. As a reduction or an excess of actin polymerization can cause MHCs, this study assessed the overexpression phenotype of Cmb. Indeed, overexpression of either Cmb isoform caused a multiple hair cell phenotype that is strongly dominantly enhanced by rok and the fy/in/mwh PCP effectors, validating the in vitro screening approach to identify PCP effectors. Importantly, the cmb mutation suppresses the MHC phenotype of mwh in double mutants. The data thus indicate that Cmb, while not essential for wing hair formation, nevertheless promotes trichome formation in vivo (Fagan, 2014).

It has been noted that known phosphorylation sites of Rok targets such as ERM proteins, Vimentin, Myosin regulatory light chain, or Adducin, often follow the consensus site [R/K]XX[S/T] or [R/K]X[S/T]. Of the five Rok sites identified in in vitro kinase assays followed by MS analysis, only S300 is preceded by a basic residue at position [-2] (RT[S]). In all other cases, no basic amino acid is found at position [-1] or [-2]. However, T46, T206, T368, and T370 are all followed by a Proline, more typical of MAP kinase phosphorylation sites. Nevertheless, mutation of these sites strongly reduced Cmb phosphorylation in vitro (Fagan, 2014).

In rok mutants, multiple hairs form at the distal end of wing cells. Similarly, overexpression of either Cmb isoform causes MHCs that originate at the distal end of cells, distinct from the in/fy group of PCP effectors and mwh, which form MHCs around the periphery of the cells (note that in mwh mutants, actin patches are initially even formed all over the apical cell surface). Importantly, reduction of rok activity by the removal of one gene dose (by two different alleles or a deficiency) increases the number of MHCs, suggesting an inhibitory effect of Rok on Cmb. It was suggested that Myosin II, which is concentrated at the site of prehair initiation and whose activity is regulated by Rok via phosphorylation of its regulatory light chain (MRLC), must be within an optimal range to properly bundle actin and to ensure the formation of a single hair. Consistent with the genetic interaction between cmb and rok, it is possible that in addition to regulating MRLC, Rok might also inhibit a potential hair promoting activity of Cmb, although it cannot be excluded that Rok/MRLC activity acts in parallel to the effect Cmb exerts on wing hair formation (Fagan, 2014).

mwh and the in/fy group of PCP effectors all have been implicated in restricting actin hair initiation to the distal vertex of the cells by inhibiting proximal hair assembly. Core PCP signaling ensures proper proximal localization of In, Frtz, and Fy proteins (and thus inhibition of prehair formation) to the proximal end of wing cells leading to the formation of a single wing hair on the distal side (Fagan, 2014).

cmb-RA or cmb-RB cause the formation of MHC phenotypes upon overexpression with several wing drivers. Importantly, this overexpression phenotype is enhanced by the removal of one gene dosage of the PCP effectors fy, frtz, in, and mwh as well as deficiencies uncovering those loci. These genetic interactions suggest cmb could exert a positive effect on hair initiation, although such a function would play a supportive or redundant role as neither a lack or ectopic trichomes are found in cmb mutants (Fagan, 2014).

Significantly, this study showed that Cmb physically interacts with Mwh in yeast two-hybrid and coimmunoprecipitation assays. Interestingly, while no vertebrate homologs of cmb have been identified, orthologs of both cmb and mwh were found outside of the insects in the genomes of the crustacean Daphnia magna and of the hard-bodied tick Ixodes scapularis. Ixodes is a member of the Chelicerata, the most basally-branching euarthropod clade that split from the remaining arthropod groups in the Cambrian. The presence of mwh and cmb in Ixodes may be indicative of an ancient protein-protein interaction that has been retained throughout arthropod evolution. Because both Ixodes and Daphnia lack wings, the Mwh/Cmb interaction likely performed different, possibly additional function in the ancestral arthropod. Consistent with this, mwh mutants cause other cuticular hair defects in other regions of the Drosophila body. Alternatively, the Mwh/Cmb interaction evolved much later than the appearance of both of these genes in the genome of the ancestral arthropod. The roles of and interactions between Cmb and Mwh proteins in more non-insect arthropods needs to be further explored (Fagan, 2014).

The presence of both mwh and cmb orthologs in the genomes of members of all holometabolous insect orders may indicate that the Mwh and Cmb interaction is also conserved in this insect clade. The retention of these two genes in members of the more basally-branching hemipteran orders, however, is less conserved. The conservation of mwh and cmb in Holometabolata may be due to their shared mode of wing development, i.e. via internal wing imaginal discs. This is in contrast to the mode of wing development in hemimetabolous insects by which the wings develop as buds outside of the body. Further study into the association of wing development and Mwh/Cmb interactions in other insect orders is needed to elucidate these findings (Fagan, 2014).

Interestingly, PCP effector mutations generally enhance each other. For example, the hypomorphic frtz3 allele is enhanced by weak alleles of in or fy in double mutants. Analogously, removal of a gene dosage of mwh in a fy or in background, enhances their MHC phenotype. In contrast, the MHC phenotype of mwh mutants was (partially) suppressed in mwh cmb double mutants, as significantly fewer cells formed additional hairs. This interaction is likely specific, because this study found it with the temperature sensitive mwh6 allele and with the spontaneous mwh1 allele, two alleles of independent origin unlikely to carry a similar second site mutation and thus further supporting the physiological function of Cmb as a PCP effector. cmb is the only gene reported so far to suppress mwh. Importantly, as mwh1 and cmb both are null alleles (cmbKO lacks expression of both protein isoforms), this result suggests that Mwh acts upstream of and normally antagonizes Cmb and that the derepression of Cmb thus may contribute to the MHC phenotype of mwh mutants (Fagan, 2014).

Unfortunately, tg Cmb antibodies do not detect endogenous Cmb protein in the developing pupal wing. Nevertheless, Cmb expressed in the posterior compartment of the wing under the control of en-Gal4 localizes apically in a punctate pattern. In cells that appear to express at a lower level, Cmb is enriched at the circumference of the cells, but shows no proximo-distal enrichment. Although it cannot be excluded that Cmb localization is an overexpression artifact, this appears unlikely, because it would be expected that Cmb would fill the cells rather than to localize specifically apically. Importantly, Cmb likely localizes to the area of wing cells where Mwh is present, as Mwh known to be initially enriched apically towards the proximal side, further supporting the model that a positive effect of Cmb as a novel PCP effector on wing hair formation may be restricted by Mwh (Fagan, 2014).

Cell polarity regulates biased myosin activity and dynamics during asymmetric cell division via Drosophila Rho kinase and Protein kinase N

Cell and tissue morphogenesis depends on the correct regulation of non-muscle Myosin II, but how this motor protein is spatiotemporally controlled is incompletely understood. This study shows that in asymmetrically dividing Drosophila neural stem cells, cell intrinsic polarity cues provide spatial and temporal information to regulate biased Myosin activity. Using live cell imaging and a genetically encoded Myosin activity sensor, Drosophila Rho kinase (Rok) was found to enrich for activated Myosin on the neuroblast cortex prior to nuclear envelope breakdown (NEB). After NEB, the conserved polarity protein Partner of Inscuteable (Pins) sequentially enriches Rok and Protein Kinase N (Pkn) on the apical neuroblast cortex. These data suggest that apical Rok first increases phospho-Myosin, followed by Pkn-mediated Myosin downregulation, possibly through Rok inhibition. It is proposed that polarity-induced spatiotemporal control of Rok and Pkn is important for unequal cortical expansion, ensuring correct cleavage furrow positioning and the establishment of physical asymmetry (Tsankova, 2017).

In asymmetrically dividing fly neural stem cells the protein kinases Rok and Pkn respond to cell polarity cues to regulate Myosin activity and dynamics in a stereotypical, spatiotemporal manner. It is proposed that the sequential regulation mediated by these two kinases is necessary to control Myosin activity and actomyosin dynamics, triggering stereotypic cell shape changes at various steps in the neuroblast cell cycle; first to induce cell rounding as neuroblasts enter mitosis, to permit cell elongation and unequal cortical expansion during anaphase, and finally to complete cytokinesis and the establishment of physical asymmetry (Tsankova, 2017).

Myosin recruitment before NEB is mediated by Rok. This kinase, implicated in Myosin phosphorylation, is already localized at the neuroblast cortex before NEB and in rok mutants, Myosin remains cytoplasmic. At NEB both Rok and Myosin enrich on the apical neuroblast cortex. This apical enrichment, but not cortical localization, depends on the polarity protein Pins since only apical Rok and Myosin enrichment is lost if Pins localization is compromised. Based on these data it is proposed that Rok responds to cell cycle cues, presumably through the small GTPase Rho1, to phosphorylate Myosin's regulatory subunit, enabling activated Myosin to engage with F-actin at the cell cortex prior to NEB. Subsequently, polarity cues enhance Rok on the apical cortex, resulting in the elevation of phosphorylated and, thus, activated Myosin on the apical neuroblast cortex at NEB (Tsankova, 2017).

With Pkn a second kinase has been identified, responding to polarity cues since its apical localization, starting at NEB and peaking by the end of metaphase, is dependent on Pins. Pkn is not absolutely necessary for cortical Myosin enrichment; pkn mutant neuroblasts still retain apical Myosin, although elsewhere on the cortex its localization is dramatically reduced. However, Pkn is required for Myosin's timely relocalization from the apical cortex. Wild-type neuroblasts clear Myosin from the apical cortex in early anaphase, creating an asymmetric distribution that is necessary for the unequal cortical expansion. In pkn mutants, however, both Rok and Myosin dynamics are changed, retaining both on the apical neuroblast cortex, causing aberrant cortical constrictions and concomitantly inverted polar expansion (Tsankova, 2017).

Based on these results, the following model is proposed. (1) Rok triggers cortical Myosin accumulation before NEB. (2) At NEB, apically localized Pins enriches Rok on the apical neuroblast cortex and concomitantly increases phospho-Myosin apically. (3) Pins also induces the apical enrichment of Pkn, which is necessary for the timely relocalization of Myosin from the apical neuroblast during metaphase. It is further proposed that Pkn is downregulating Myosin activity through inhibiting or downregulating apical Rok activity. Whether Pkn downregulates Rok activity by direct phosphorylation remains an attractive hypothesis, since vertebrate Rock2 has recently been identified as a Pkn target. Alternatively, Rok activity could be regulated independently of phosphorylation but governed by the length of its coiled-coil tether, linking the kinase domain with the membrane binding domain (Tsankova, 2017).

This sequential regulation of Myosin dynamics seems to be a key regulatory mechanism underlying physical asymmetric cell divisions. For instance, apical Myosin relocalization always precedes basal Myosin clearing in wild-type neuroblasts. Similarly, biasing the localization of activated Myosin affects cleavage furrow positioning and physical asymmetry (Tsankova, 2017).

It is hypothesized that polarity cues provide a cell-intrinsic timer, priming Myosin relocalization on the apical cortex, thereby ensuring the generation of physical asymmetry through unequal cortical extension. Polarity-induced enrichment of activated Myosin on the apical cortex could thus provide a symmetry breaking event, necessary for the subsequent induction of apical Myosin clearing. Consistent with this model is the finding that pins mutants, or uniform cortical localization of Pins, cause Myosin to clear from both poles at the same time and divide symmetrically by size (Tsankova, 2017).

Tissue and organ growth critically depends on the correct spatiotemporal regulation of cell division. This study provides a conceptual framework of how Rok and Pkn respond to both cell cycle and polarity cues. These cues, in conjunction with spindle-dependent signals, ensure correct physical asymmetric cell division that is necessary for stem cell homeostasis and cell differentiation. Spatial and temporal regulation of Myosin activity has also been shown to be important for pulsatile cell shape changes in the Drosophila embryo. Rok and Pkn play important roles during vertebrate development and morphogenesis , and it will be interesting to see how spatiotemporal cues, affecting local cell shape changes, are coordinated with overall tissue morphogenesis in flies and beyond (Tsankova, 2017).

Rho-kinase: Biological Overview | Evolutionary Homologs | Developmental Biology | Effects of Mutation | References

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