Effects of the regulatory light chain phosphorylation of myosin II on mitosis and cytokinesis of mammalian cells

Phosphoproteomic analysis identifies supervillin as an ERK3 substrate regulating cytokinesis and cell ploidy

Extracellular signal-regulated kinase 3 (ERK3) is a poorly characterized member of the mitogen-activated protein (MAP) kinase family. Functional analysis of the ERK3 signaling pathway has been hampered by a lack of knowledge about the substrates and downstream effectors of the kinase. Here, we used large-scale quantitative phosphoproteomics and targeted gene silencing to identify direct ERK3 substrates and gain insight into its cellular functions. Detailed validation of one candidate substrate identified the gelsolin/villin family member supervillin (SVIL) as a bona fide ERK3 substrate. We show that ERK3 phosphorylates SVIL on Ser245 to regulate myosin II activation and cytokinesis completion in dividing cells. Depletion of SVIL or ERK3 leads to increased cytokinesis failure and multinucleation, a phenotype rescued by wild type SVIL but not by the non-phosphorylatable S245A mutant. Our results unveil a new function of the atypical MAP kinase ERK3 in cell division and the regulation of cell ploidy.

MRLC controls apoptotic cell death and functions to regulate epidermal development during planarian regeneration and homeostasis

Adult stem cells (ASCs) are pluripotent cells with the capacity to self-renew and constantly replace lost cells due to physiological turnover or injury. Understanding the molecular mechanisms of the precise coordination of stem cell proliferation and proper cell fate decision is important to regeneration and organismal homeostasis. The planarian epidermis provides a highly tractable model to study ASC complex dynamic due to the distinct spatiotemporal differentiation stages during lineage development. Here, we identified the myosin regulatory light chain (MRLC) homologue in the Dugesia japonica transcriptome. We found high expression levels of MRLC in wound region during regeneration and also expressed in late epidermal progenitors as an essential regulator of the lineage from neoblasts to mature epidermal cells. We investigated the function of MRLC using in situ hybridization, real-time polymerase chain reaction and double fluorescent and uncovered the potential mechanism. Knockdown of MRLC leads to a remarkable increase in cell death, causes severe abnormalities during regeneration and homeostasis and eventually leads to animal death. The global decrease in epidermal cell in MRLC RNAi animals induces accelerated epidermal proliferation and differentiation. Additionally, we find that MRLC is co-expressed with cdc42 and acts cooperatively to control the epidermal lineage development by affecting cell death. Our results uncover an important role of MRLC, as an inhibitor of apoptosis, involves in epidermal development.

Myosin II regulatory light chain phosphorylation and formin availability modulate cytokinesis upon changes in carbohydrate metabolism

Cytokinesis, the separation of daughter cells at the end of mitosis, relies in animal cells on a contractile actomyosin ring (CAR) composed of actin and class II myosins, whose activity is strongly influenced by regulatory light chain (RLC) phosphorylation. However, in simple eukaryotes such as the fission yeast Schizosaccharomyces pombe, RLC phosphorylation appears dispensable for regulating CAR dynamics. We found that redundant phosphorylation at Ser35 of the S. pombe RLC homolog Rlc1 by the p21-activated kinases Pak1 and Pak2, modulates myosin II Myo2 activity and becomes essential for cytokinesis and cell growth during respiration. Previously, we showed that the stress-activated protein kinase pathway (SAPK) MAPK Sty1 controls fission yeast CAR integrity by downregulating formin For3 levels (Gómez-Gil et al., 2020). Here, we report that the reduced availability of formin For3-nucleated actin filaments for the CAR is the main reason for the required control of myosin II contractile activity by RLC phosphorylation during respiration-induced oxidative stress. Thus, the restoration of For3 levels by antioxidants overrides the control of myosin II function regulated by RLC phosphorylation, allowing cytokinesis and cell proliferation during respiration. Therefore, fine-tuned interplay between myosin II function through Rlc1 phosphorylation and environmentally controlled actin filament availability is critical for a successful cytokinesis in response to a switch to a respiratory carbohydrate metabolism.

Phosphatidylinositol transfer protein/planar cell polarity axis regulates neocortical morphogenesis by supporting interkinetic nuclear migration

The neocortex expands explosively during embryonic development. The earliest populations of neural stem cells (NSCs) form a thin pseudostratified epithelium whose contour determines that of the adult neocortex. Neocortical complexity is accompanied by disproportional expansion of the NSC layer in its tangential dimension to increase tissue surface area. How such disproportional expansion is controlled remains unknown. We demonstrate that a phosphatidylinositol transfer protein (PITP)/non-canonical Wnt planar cell polarity (ncPCP) signaling axis promotes tangential expansion of developing neocortex. PITP signaling supports trafficking of specific ncPCP receptors from the NSC Golgi system to potentiate actomyosin activity important for cell-cycle-dependent interkinetic nuclear migration (IKNM). In turn, IKNM promotes lateral dispersion of newborn NSCs and tangential growth of the cerebral wall. These findings clarify functional roles for IKNM in NSC biology and identify tissue dysmorphogenesis resulting from impaired IKNM as a factor in autism risk, developmental brain disabilities, and neural tube birth defects.

Xie and Bankaitis report that a phosphatidylinositol transfer protein/non-canonical planar cell polarity signaling axis supports interkinetic nuclear migration by promoting trafficking of specific non-canonical planar cell polarity receptors from the Golgi system to the plasma membrane, activating actomyosin, and supporting lateral expansion of the neocortex via a convergent extension mechanism.

Caveolin-1-Derived Peptide Reduces ER Stress and Enhances Gelatinolytic Activity in IPF Fibroblasts

Idiopathic pulmonary fibrosis (IPF) is a fatal disease characterized by an excess deposition of extracellular matrix in the pulmonary interstitium. Caveolin-1 scaffolding domain peptide (CSP) has been found to mitigate pulmonary fibrosis in several animal models. However, its pathophysiological role in IPF is obscure, and it remains critical to understand the mechanism by which CSP protects against pulmonary fibrosis. We first studied the delivery of CSP into cells and found that it is internalized and accumulated in the Endoplasmic Reticulum (ER). Furthermore, CSP reduced ER stress via suppression of inositol requiring enzyme1α (IRE1α) in transforming growth factor β (TGFβ)-treated human IPF lung fibroblasts (hIPF-Lfs). Moreover, we found that CSP enhanced the gelatinolytic activity of TGFβ-treated hIPF-Lfs. The IRE1α inhibitor; 4µ8C also augmented the gelatinolytic activity of TGFβ-treated hIPF-Lfs, supporting the concept that CSP induced inhibition of the IRE1α pathway. Furthermore, CSP significantly elevated expression of MMPs in TGFβ-treated hIPF-Lfs, but conversely decreased the secretion of collagen 1. Similar results were observed in two preclinical murine models of PF, bleomycin (BLM)- and adenovirus expressing constitutively active TGFβ (Ad-TGFβ)-induced PF. Our findings provide new insights into the mechanism by which lung fibroblasts contribute to CSP dependent protection against lung fibrosis.

Megakaryocyte migration defects due to nonmuscle myosin IIA mutations underlie thrombocytopenia in MYH9-related disease

Megakaryocytes (MKs), the precursor cells for platelets, migrate from the endosteal niche of the bone marrow (BM) toward the vasculature, extending proplatelets into sinusoids, where circulating blood progressively fragments them into platelets. Nonmuscle myosin IIA (NMIIA) heavy chain gene (MYH9) mutations cause macrothrombocytopenia characterized by fewer platelets with larger sizes leading to clotting disorders termed myosin-9-related disorders (MYH9-RDs). MYH9-RD patient MKs have proplatelets with thicker and fewer branches that produce fewer and larger proplatelets, which is phenocopied in mouse Myh9-RD models. Defective proplatelet formation is considered to be the principal mechanism underlying the macrothrombocytopenia phenotype. However, MYH9-RD patient MKs may have other defects, as NMII interactions with actin filaments regulate physiological processes such as chemotaxis, cell migration, and adhesion. How MYH9-RD mutations affect MK migration and adhesion in BM or NMIIA activity and assembly prior to proplatelet production remain unanswered. NMIIA is the only NMII isoform expressed in mature MKs, permitting exploration of these questions without complicating effects of other NMII isoforms. Using mouse models of MYH9-RD (NMIIAR702C+/-GFP+/-, NMIIAD1424N+/-, and NMIIAE1841K+/-) and in vitro assays, we investigated MK distribution in BM, chemotaxis toward stromal-derived factor 1, NMIIA activity, and bipolar filament assembly. Results indicate that different MYH9-RD mutations suppressed MK migration in the BM without compromising bipolar filament formation but led to divergent adhesion phenotypes and NMIIA contractile activities depending on the mutation. We conclude that MYH9-RD mutations impair MK chemotaxis by multiple mechanisms to disrupt migration toward the vasculature, impairing proplatelet release and causing macrothrombocytopenia.

Impairment of cytokinesis by cancer-associated DAPK3 mutations

Death-associated protein kinase 3 (DAPK3), a member of the DAPK family, contributes to cytokinesis by phosphorylating myosin II regulatory light chain (MRLC). Missense mutations in DAPK3, T112M, D161N, and P216S, were observed in the lung, colon, and cervical cancers, respectively, but the effects of these mutations on cytokinesis remain unclear. Here, we show that cells expressing EGFP-DAPK3-T112M, -D161N, or -P216S exhibited reduced rates of cytokinesis, with an increased ratio of multinucleated cells. In addition, these cells exhibited reduced levels of phosphorylated MRLC at the contractile ring. Collectively, our data demonstrates that cancer-associated DAPK3 mutations impair cytokinesis by reducing phosphorylated MRLC.

ZIP kinase phosphorylated and activated by Rho kinase/ROCK contributes to cytokinesis in mammalian cultured cells

Cytokinesis of animal cells requires contraction of a contractile ring, composed of actin filaments and myosin II filaments. Phosphorylation of myosin II regulatory light chain (MRLC) promotes contraction of the actomyosin ring by activating myosin II motor activity. Both Rho-associated coiled-coil kinase (Rho kinase/ROCK) and Zipper-interacting protein kinase (ZIP kinase/ZIPK) have been reported to phosphorylate MRLC at the contractile ring. However, it remains unclear whether these kinases function independently of each other. Here, we clarified that ROCK colocalizes and forms a complex with ZIPK at telophase. As ROCK is reported to phosphorylate and activate ZIPK in vitro, we hypothesized that ZIPK phosphorylated by ROCK contributes to control cytokinesis. To address this, we expressed EGFP-ZIPK wild type (WT), a non-phosphorylatable mutant (T265A) or a phosphorylation-mimicking mutant (T265D) in HeLa cells and treated these cells with a ROCK inhibitor. Decrease in phosphorylated MRLC and a delay of furrow ingression by the ROCK inhibitor were rescued by the expression of EGFP-ZIPK-T265D, but not EGFP-ZIPK-WT or -T265A. This suggests that ROCK regulates MRLC phosphorylation followed by furrow ingression, through ZIPK phosphorylation.

p116Rip promotes myosin phosphatase activity in airway smooth muscle cells

Myosin phosphatase-Rho interacting protein (p116^Rip ) was originally found as a RhoA-binding protein. Subsequent studies by us and others revealed that p116^Rip facilitates myosin light chain phosphatase (MLCP) activity through direct and indirect manners. However, it is unclear how p116^Rip regulates myosin phosphatase activity in cells. To elucidate the role of p116^Rip in cellular contractile processes, we suppressed the expression of p116^Rip by RNA interference in human airway smooth muscle cells (HASMCs). We found that knockdown of p116^Rip in HASMCs led to increased di-phosphorylated MLC (pMLC), that is phosphorylation at both Ser19 and Thr18. This was because of a change in the interaction between MLCP and myosin, but not an alteration of RhoA/ROCK signaling. Attenuation of Zipper-interacting protein kinase (ZIPK) abolished the increase in di-pMLC, suggesting that ZIPK is involved in this process. Moreover, suppression of p116^Rip expression in HASMCs substantially increased the histamine-induced collagen gel contraction. We also found that expression of the p116^Rip was decreased in the airway smooth muscle tissue from asthmatic patients compared with that from non-asthmatic patients, suggesting a potential role of p116^Rip expression in asthma pathogenesis.

Using the SpyTag SpyCatcher system to label smooth muscle myosin II filaments with a quantum dot on the regulatory light chain

The regulatory light chain (RLC) of myosin is commonly tagged to monitor myosin behavior in vitro, in muscle fibers, and in cells. The goal of this study was to prepare smooth muscle myosin (SMM) filaments containing a single head labeled with a quantum dot (QD) on the RLC. We show that when the RLC is coupled to a QD at Cys-108 and exchanged into SMM, subsequent filament assembly is severely disrupted. To address this, we used a novel approach for myosin by implementing the SpyTag002 SpyCatcher002 system to prepare SMM incorporated with RLC constructs fused to SpyTag or SpyCatcher. We show that filament assembly, actin-activated steady-state ATPase activities, ability to be phosphorylated, and selected enzymatic and mechanical properties were essentially unaffected if either SpyTag or SpyCatcher were fused to the C-terminus of the RLC. Crucially for our application, we also show that a QD coupled to SpyCatcher can be covalently attached to a RLC-Spy incorporated into a SMM filament without disrupting the filament, and that the filaments can move along actin in vitro.

Visualization of stimulus-specific heterogeneous activation of individual vascular smooth muscle cells in aortic tissues

Intercellular communication among autonomic nerves, endothelial cells (ECs), and vascular smooth muscle cells (VSMCs) plays a central role in an uninterrupted regulation of blood flow through vascular contractile machinery. Impairment of this communication is linked to development of vascular diseases such as hypertension, cerebral/coronary vasospasms, aortic aneurism, and erectile dysfunction. Although the basic concept of the communication as a whole has been studied, the spatiotemporal correlation of ECs/VSMCs in tissues at the cellular level is unknown. Here, we show a unique VSMC response to ECs during contraction and relaxation of isolated aorta tissues through visualization of spatiotemporal activation patterns of smooth muscle myosin II. ECs in the intimal layer dictate the stimulus-specific heterogeneous activation pattern of myosin II in VSMCs within distinct medial layers. Myosin light chain (MLC) phosphorylation (active form of myosin II) gradually increases towards outer layers (approximately threefold higher MLC phosphorylation at the outermost layer than that of the innermost layer), presumably by release of an intercellular messenger, nitric oxide (NO). Our study also demonstrates that the MLC phosphorylation at the outermost layer in spontaneously hypertensive rats (SHR) during NO-induced relaxation is quite high and approximately 10-fold higher than that of its counterpart, the Wister-Kyoto rats (WKY), suggesting that the distinct pattern of myosin II activation within tissues is important for vascular protection against elevated blood pressure.

Tension regulates myosin dynamics during Drosophila embryonic wound repair

Embryos repair epithelial wounds rapidly in a process driven by collective cell movements. Upon wounding, actin and the molecular motor non-muscle myosin II are redistributed in the cells adjacent to the wound, forming a supracellular purse string around the lesion. Purse string contraction coordinates cell movements and drives rapid wound closure. By using fluorescence recovery after photobleaching in Drosophila embryos, we found that myosin turns over as the purse string contracts. Myosin turnover at the purse string was slower than in other actomyosin networks that had a lower level of contractility. Mathematical modelling suggested that myosin assembly and disassembly rates were both reduced by tension at the wound edge. We used laser ablation to show that tension at the purse string increased as wound closure progressed, and that the increase in tension was associated with reduced myosin turnover. Reducing purse string tension by laser-mediated severing resulted in increased turnover and loss of myosin. Finally, myosin motor activity was necessary for its stabilization around the wound and for rapid wound closure. Our results indicate that mechanical forces regulate myosin dynamics during embryonic wound repair.

Proteomic and phosphoproteomic analysis of renal cortex in a salt-load rat model of advanced kidney damage

Salt plays an essential role in the progression of chronic kidney disease and hypertension. However, the mechanisms underlying pathogenesis of salt-induced kidney damage remain largely unknown. Here, Sprague-Dawley rats, that underwent 5/6 nephrectomy (5/6Nx, a model of advanced kidney damage) or sham operation, were treated for 2 weeks with a normal or high-salt diet. We employed aTiO₂ enrichment, iTRAQ labeling and liquid-chromatography tandem mass spectrometry strategy for proteomic and phosphoproteomic profiling of the renal cortex. We found 318 proteins differentially expressed in 5/6Nx group relative to sham group, and 310 proteins significantly changed in response to salt load in 5/6Nx animals. Totally, 1810 unique phosphopeptides corresponding to 550 phosphoproteins were identified. We identified 113 upregulated and 84 downregulated phosphopeptides in 5/6Nx animals relative to sham animals. Salt load induced 78 upregulated and 91 downregulated phosphopeptides in 5/6Nx rats. The differentially expressed phospholproteins are important transporters, structural molecules, and receptors. Protein-protein interaction analysis revealed that the differentially phosphorylated proteins in 5/6Nx group, Polr2a, Srrm1, Gsta2 and Pxn were the most linked. Salt-induced differential phosphoproteins, Myh6, Lmna and Des were the most linked. Altered phosphorylation levels of lamin A and phospholamban were validated. This study will provide new insight into pathogenetic mechanisms of chronic kidney disease and salt sensitivity.

Myosin light chains: Teaching old dogs new tricks

The myosin holoenzyme is a multimeric protein complex consisting of heavy chains and light chains. Myosin light chains are calmodulin family members which are crucially involved in the mechanoenzymatic function of the myosin holoenzyme. This review examines the diversity of light chains within the myosin superfamily, discusses interactions between the light chain and the myosin heavy chain as well as regulatory and structural functions of the light chain as a subunit of the myosin holoenzyme. It covers aspects of the myosin light chain in the localization of the myosin holoenzyme, protein-protein interactions and light chain binding to non-myosin binding partners. Finally, this review challenges the dogma that myosin regulatory and essential light chain exclusively associate with conventional myosin heavy chains while unconventional myosin heavy chains usually associate with calmodulin.

Four things to know about myosin light chains as reporters for non-muscle myosin-2 dynamics in live cells

The interplay between non-muscle myosins-2 and filamentous actin results in cytoplasmic contractility which is essential for eukaryotic life. Concomitantly, there is tremendous interest in elucidating the physiological function and temporal localization of non-muscle myosin-2 in cells. A commonly used method to study the function and localization of non-muscle myosin-2 is to overexpress a fluorescent protein (FP)-tagged version of the regulatory light chain (RLC) which binds to the myosin-2 heavy chain by mass action. Caveats about this approach include findings from recent studies indicating that the RLC does not bind exclusively to the non-muscle myosin-2 heavy chain. Rather, it can also associate with the myosin heavy chains of several other classes as well as other targets than myosin. In addition, the presence of the FP moiety may compromise myosin's enzymatic and mechanical performance. This and other factors to be discussed in this commentary raise questions about the possible complications in using FP-RLC as a marker for the dynamic localization and regulatory aspects of non-muscle myosin-2 motor functions in cell biological experiments.

RhoA-mediated MLC2 regulates actin dynamics for cytokinesis in meiosis

During oocyte meiosis, the bipolar spindle forms in the central cytoplasm and then migrates to the cortex. Subsequently, the oocyte extrudes the polar body through two successive asymmetric divisions, which are regulated primarily by actin filaments. Myosin light chain2 (MLC2) phosphorylation plays pivotal roles in smooth muscle contraction, stress fiber formation, cell motility and cytokinesis. However, whether MLC2 phosphorylation participates in the oocyte polarization and asymmetric division has not been clarified. The present study investigated the expression and functions of MLC2 during mouse oocyte meiosis. Our result showed that p-MLC2 was localized in the oocyte cortex, with a thickened cap above the chromosomes. Meanwhile, p-MLC2 was also localized in the poles of spindle. Disruption of MLC2 activity by MLC2 knock down (KD) caused the failure of polar body extrusion. Immunofluorescent staining showed that a large proportion of oocytes arrested in telophase stage and failed to undergo cytokinesis after culturing for 12 hours. In the meantime, actin filament staining at oocyte membrane and cytoplasm were reduced in MLC2 KD oocytes. Finally, we found that the phosphorylation of MLC2 protein levels was decreased after disruption of RhoA activity. Above all, our data indicated that the RhoA-mediated MLC2 regulates the actin organization for cytokinesis during mouse oocyte maturation.

Phosphorylation of myosin II regulatory light chain by ZIP kinase is responsible for cleavage furrow ingression during cell division in mammalian cultured cells

Zipper-interacting protein kinase (ZIPK) is known to regulate several functions such as apoptosis, smooth muscle contraction, and cell migration. While exogenously expressed GFP-ZIPK localizes to the cleavage furrow, role of ZIPK in cytokinesis is obscure. Here, we show that ZIPK is a major MRLC kinase during mitosis. Moreover, ZIPK siRNA-mediated knockdown causes delay of cytokinesis. The delay in cytokinesis of ZIPK-knockdown cells was rescued by the exogenous diphosphorylation-mimicking MRLC mutant. Taken together, these findings suggest that ZIPK plays a role in the progression and completion of cytokinesis through MRLC phosphorylation.

Relocation of CLIC1 promotes tumor cell invasion and colonization of fibrin

Chloride intracellular channel 1 (CLIC1) has been shown to be upregulated in various malignancies but its exact function remains unclear. Here, it is revealed that CLIC1 is critical for the stability of invadopodia in endothelial and tumor cells embedded in a 3-dimensional (3D) matrix of fibrin. Invadopodia stability was associated with the capacity of CLIC1 to induce stress fiber and fibronectin matrix formation following its β3 integrin (ITGB3)-mediated recruitment into invadopodia. This pathway, in turn, was relevant for fibrin colonization as well as slug (SNAI2) expression and correlated with a significant role of CLIC1 in metastasis in vivo. Mechanistically, a reduction of myosin light chain kinase (MYLK) in CLIC1-depleted as well as β3 integrin-depleted cells suggests an important role of CLIC1 for integrin-mediated actomyosin dynamics in cells embedded in fibrin. Overall, these results indicate that CLIC1 is an important contributor to tumor invasion, metastasis, and angiogenesis.

IMPLICATIONS

This study uncovers an important new function of CLIC1 in the regulation of cell-extracellular matrix interactions and ability of tumor cells to metastasize to distant organs.

Core binding factor β (CBFβ) is retained in the midbody during cytokinesis

Core Binding Factor β (CBFβ) is complexed with the RUNX family of transcription factors in the nucleus to support activation or repression of genes related to bone (RUNX2), hematopoiesis (RUNX1) and gastrointestinal (RUNX3) development. Furthermore, RUNX proteins contribute to the onset and progression of different types of cancer. Although CBFβ localizes to cytoskeletal architecture, its biological role in the cytoplasmic compartment remains to be established. Additionally, the function and localization of CBFβ during the cell cycle are important questions relevant to its biological role. Here we show that CBFβ dynamically distributes in different stages of cell division and importantly is present during telophase at the midbody, a temporal structure important for successful cytokinesis. A functional role for CBFβ localization at the midbody is supported by striking defects in cytokinesis that include polyploidy and abscission failure following siRNA-mediated downregulation of endogenous CBFβ or overexpression of the inv(16) fusion protein CBFβ-SMMHC. Our results suggest that CBFβ retention in the midbody during cytokinesis reflects a novel function that contributes to epigenetic control.

Chaperone-enhanced purification of unconventional myosin 15, a molecular motor specialized for stereocilia protein trafficking

Unconventional myosin 15 is a molecular motor expressed in inner ear hair cells that transports protein cargos within developing mechanosensory stereocilia. Mutations of myosin 15 cause profound hearing loss in humans and mice; however, the properties of this motor and its regulation within the stereocilia organelle are unknown. To address these questions, we expressed a subfragment 1-like (S1) truncation of mouse myosin 15, comprising the predicted motor domain plus three light-chain binding sites. Following unsuccessful attempts to express functional myosin 15-S1 using the Spodoptera frugiperda (Sf9)-baculovirus system, we discovered that coexpression of the muscle-myosin-specific chaperone UNC45B, in addition to the chaperone heat-shock protein 90 (HSP90) significantly increased the yield of functional protein. Surprisingly, myosin 15-S1 did not bind calmodulin with high affinity. Instead, the IQ domains bound essential and regulatory light chains that are normally associated with class II myosins. We show that myosin 15-S1 is a barbed-end-directed motor that moves actin filaments in a gliding assay (∼ 430 nm · s(-1) at 30 °C), using a power stroke of 7.9 nm. The maximum ATPase rate (k(cat) ∼ 6 s(-1)) was similar to the actin-detachment rate (k(det) = 6.2 s(-1)) determined in single molecule optical trapping experiments, indicating that myosin 15-S1 was rate limited by transit through strongly actin-bound states, similar to other processive myosin motors. Our data further indicate that in addition to folding muscle myosin, UNC45B facilitates maturation of an unconventional myosin. We speculate that chaperone coexpression may be a simple method to optimize the purification of other myosin motors from Sf9 insect cells.

Spatiotemporal control of epithelial remodeling by regulated myosin phosphorylation

Spatiotemporally regulated actomyosin contractility generates the forces that drive epithelial cell rearrangements and tissue remodeling. Phosphorylation of the myosin II regulatory light chain (RLC) promotes the assembly of myosin monomers into active contractile filaments and is an essential mechanism regulating the level of myosin activity. However, the effects of phosphorylation on myosin localization, dynamics, and function during epithelial remodeling are not well understood. In Drosophila, planar polarized myosin contractility is required for oriented cell rearrangements during elongation of the body axis. We show that regulated myosin phosphorylation influences spatial and temporal properties of contractile behavior at molecular, cellular, and tissue length scales. Expression of myosin RLC variants that prevent or mimic phosphorylation both disrupt axis elongation, but have distinct effects at the molecular and cellular levels. Unphosphorylatable RLC produces fewer, slower cell rearrangements, whereas phosphomimetic RLC accelerates rearrangement and promotes higher-order cell interactions. Quantitative live imaging and biophysical approaches reveal that both phosphovariants reduce myosin planar polarity and mechanical anisotropy, altering the orientation of cell rearrangements during axis elongation. Moreover, the localized myosin activator Rho-kinase is required for spatially regulated myosin activity, even when the requirement for phosphorylation is bypassed by the expression of phosphomimetic myosin RLC. These results indicate that myosin phosphorylation influences both the level and the spatiotemporal regulation of myosin activity, linking molecular properties of myosin activity to tissue morphogenesis.

Rho GTPases as regulators of mitosis and cytokinesis in mammalian cells

Rho GTPases regulate a diverse range of cellular functions primarily through their ability to modulate microtubule dynamics and the actin-myosin cytoskeleton. Both of these cytoskeletal structures are crucial for a mitotic cell division. Specifically, their assembly and disassembly is tightly regulated in a temporal manner to ensure that each mitotic stage occurs in the correct sequential order and not prematurely until the previous stage is completed. Thus, it is not surprising that the Rho GTPases, RhoA, and Cdc42, have reported roles in several stages of mitosis: cell cortex stiffening during cell rounding, mitotic spindle formation, and bi-orient attachment of the spindle microtubules to the kinetochore and during cytokinesis play multiple roles in establishing the division plane, assembly, and activation of the contractile ring, membrane ingression, and abscission. Here, I review the molecular mechanisms regulating the spatial and temporal activation of RhoA and Cdc42 during mitosis, and how this is critical for mitotic progression and completion.

ZIPK is critical for the motility and contractility of VSMCs through the regulation of nonmuscle myosin II isoforms

Migration of medial vascular smooth muscle cells (VSMCs) into the intimal layer contributes to pathological remodeling of the blood vessel in arterial hypertension and atherosclerosis. It is well established that reorganization of cytoskeletal networks is an essential component of cellular motile events. However, there is currently a lack of insight into the cellular characteristics of VSMC migration under three-dimensional environments. Here, we investigated the mechanisms of VSMC migration and remodeling using two different collagen matrix assays as in vitro models: migration of VSMCs within a collagen matrix for VSMC invasion and contraction of a collagen gel by VSMCs for VSMC remodeling and contraction. We found that nonmuscle myosin IIA (NMIIA) and nonmuscle myosin IIB (NMIIB) differentially contribute to the migratory capacity of VSMCs via NMII isoform-dependent cytoskeletal reorganization. Depletion of NMIIA by short hairpin RNA revealed a unique interplay between actomyosin and microtubules during VSMC migration. On the other hand, NMIIB was required for the structural maintenance of migrating VSMC. Interestingly, there was a significant difference between NMIIA and NMIIB knockdown in the VSMC migration but not in the VSMC-mediated collagen gel contraction. Furthermore, depletion of zipper-interacting protein kinase by short hairpin RNA resulted in an impairment of VSMC migration and a substantial decrease of VSMC-mediated collagen gel contraction. These results suggest that NMIIA and NMIIB uniquely control VSMC migration and may contribute to vascular remodeling, which are both regulated by zipper-interacting protein kinase.

The role of PLK1-phosphorylated SVIL in myosin II activation and cytokinetic furrowing

Polo-like kinase 1 (PLK1) is a widely conserved serine/threonine kinase that regulates progression of multiple stages of mitosis. Although extensive studies about PLK1 functions during cell division have been performed, it is still not known how PLK1 regulates myosin II activation at the equatorial cortex and ingression of the cleavage furrow. In this report, we show that an actin/myosin-II-binding protein, supervillin (SVIL), is a substrate of PLK1. PLK1 phosphorylates Ser238 of SVIL, which can promote the localization of SVIL to the central spindle and association with PRC1. Expression of a PLK1 phosphorylation site mutant, S238A-SVIL, inhibited myosin II activation at the equatorial cortex and induced aberrant furrowing. SVIL has both actin- and myosin-II-binding regions in the N-terminus. Expression of ΔMyo-SVIL (deleted of the myosin-II-binding region), but not of ΔAct-SVIL (deleted of actin-binding region), reduced myosin II activation and caused defects in furrowing. Our study indicates a possible role of phosphorylated SVIL as a molecular link between the central spindle and the contractile ring to coordinate the activation of myosin II for the ingression of the cleavage furrow.

ROS-mediated downregulation of MYPT1 in smooth muscle cells: a potential mechanism for the aberrant contractility in atherosclerosis

Reactive oxygen species (ROS) mediates the aberrant contractility in hypertension. Abnormal contractility occurs in atherosclerotic vessels but changes in proteins that regulate contractility remain poorly understood. Myosin phosphatase (MP) activity, which regulates smooth muscle relaxation, is regulated by the phosphorylation of its regulatory subunit, MP targeting subunit 1 (MYPT1). In the present study, we examined the roles of ROS in MP subunit expression both in cultured human aortic smooth muscle cells (HASMCs) and during atherosclerosis progression in apolipoprotein E-knockout (apoE-KO) mice. Furthermore, the effect of decreased MYPT1 on actin cytoskeleton and cell migration activity was assessed in HASMCs. Short hairpin RNA-mediated knockdown of MYPT1 increased stress fibers and attenuated platelet-derived growth factor-induced cell migration in HASMCs. Superoxide anion-inducing agent LY83583 downregulated MYPT1 mRNA and protein levels, but did not affect the phosphorylation of MYPT1 and catalytic subunit of MP, PP1δ. The LY83583-induced decrease in MYPT1 was abolished by co-treating with superoxide dismutase or by inhibiting NADPH oxidase with diphenyleneiodonium. Treatment of peroxynitrite, but not hydrogen peroxide (H2O2), downregulated MYPT1 protein expression and induced MYPT1 phosphorylation without affecting mRNA levels. Co-treatment with a proteasome inhibitor, MG-132, eliminated peroxynitrite-induced MYPT1 downregulation. In apoE-KO mice, MYPT1 protein, but not mRNA, levels were markedly decreased in 16-week- and 24-week-old mice. Oral estrogen treatment, which was previously shown to decrease aortic ROS levels, upregulated aortic MYPT1 expression. Moreover, reduction in MYPT1 expression correlated with increased aortic sensitivity toward vasoconstrictors. These results suggested that during atherosclerosis progression oxidative stress mediates the downregulation of MYPT1, which may inhibit smooth muscle cell migration and contribute to the aberrant contractility.

The role of myosin phosphorylation in anaphase chromosome movement

This work deals with the role of myosin phosphorylation in anaphase chromosome movement. Y27632 and ML7 block two different pathways for phosphorylation of the myosin regulatory light chain (MRLC). Both stopped or slowed chromosome movement when added to anaphase crane-fly spermatocytes. To confirm that the effects of the pharmacological agents were on the presumed targets, we studied cells stained with antibodies against mono- or bi-phosphorylated myosin. For all chromosomes whose movements were affected by a drug, the corresponding spindle fibres of the affected chromosomes had reduced levels of 1P- and 2P-myosin. Thus the drugs acted on the presumed target and myosin phosphorylation is involved in anaphase force production. Calyculin A, an inhibitor of MRLC dephosphorylation, reversed and accelerated the altered movements caused by Y27632 and ML-7, suggesting that another phosphorylation pathway is involved in phosphorylation of spindle myosin. Staurosporine, a more general phosphorylation inhibitor, also reduced the levels of MRLC phosphorylation and caused anaphase chromosomes to stop or slow. The effects of staurosporine on chromosome movements were not reversed by Calyculin A, confirming that another phosphorylation pathway is involved in phosphorylation of spindle myosin.

Mammalian myosin-18A, a highly divergent myosin

The Mus musculus myosin-18A gene is expressed as two alternatively spliced isoforms, α and β, with reported roles in Golgi localization, in maintenance of cytoskeleton, and as receptors for immunological surfactant proteins. Both myosin-18A isoforms feature a myosin motor domain, a single predicted IQ motif, and a long coiled-coil reminiscent of myosin-2. The myosin-18Aα isoform, additionally, has an N-terminal PDZ domain. Recombinant heavy meromyosin- and subfragment-1 (S1)-like constructs for both myosin-18Aα and -18β species were purified from the baculovirus/Sf9 cell expression system. These constructs bound both essential and regulatory light chains, indicating an additional noncanonical light chain binding site in the neck. Myosin-18Aα-S1 and -18Aβ-S1 molecules bound actin weakly with Kd values of 4.9 and 54 μm, respectively. The actin binding data could be modeled by assuming an equilibrium between two myosin conformations, a competent and an incompetent form to bind actin. Actin binding was unchanged by presence of nucleotide. Both myosin-18A isoforms bound N-methylanthraniloyl-nucleotides, but the rate of ATP hydrolysis was very slow (<0.002 s(-1)) and not significantly enhanced by actin. Phosphorylation of the regulatory light chain had no effect on ATP hydrolysis, and neither did the addition of tropomyosin or of GOLPH3, a myosin-18A binding partner. Electron microscopy of myosin-18A-S1 showed that the lever is strongly angled with respect to the long axis of the motor domain, suggesting a pre-power stroke conformation regardless of the presence of ATP. These data lead us to conclude that myosin-18A does not operate as a traditional molecular motor in cells.

Crosstalk between adenosine A1 and β1-adrenergic receptors regulates translocation of PKCε in isolated rat cardiomyocytes

Adenosine A(1) receptor (A(1)R)-induced translocation of PKCε to transverse (t) tubular membranes in isolated rat cardiomyocytes is associated with a reduction in β(1)-adrenergic-stimulated contractile function. The PKCε-mediated activation of protein kinase D (PKD) by endothelin-1 is inhibited by β(1)-adrenergic stimulated protein kinase A (PKA) suggesting a similar mechanism of A(1)R signal transduction modulation by adrenergic agonists may exist in the heart. We have investigated the influence of β(1)-adrenergic stimulation on PKCε translocation elicited by A(1)R. Immunofluorescence imaging and Western blotting with PKCε and β-COP antibodies were used to quantify the co-localization of PKCε and t-tubular structures in isolated rat cardiomyocytes. The A(1)R agonist CCPA increased the co-localization of PKCε and t-tubules as detected by imaging. The β(1)-adrenergic receptor agonist isoproterenol (ISO) inhibited this effect of CCPA. Forskolin, a potent activator of PKA, mimicked, and H89, a pharmacological PKA inhibitor, and PKI, a membrane-permeable PKA peptide PKA inhibitor, attenuated the negative effect of ISO on the A(1)R-mediated PKCε translocation. Western blotting with isolated intact hearts revealed an increase in PKCε/β-COP co-localization induced by A(1)R. This increase was attenuated by the A(1)R antagonist DPCPX and ISO. The ISO-induced attenuation was reversed by H89. It is concluded that adrenergic stimulation inhibits A(1)R-induced PKCε translocation to the PKCε anchor site RACK2 constituent of a coatomer containing β-COP and associated with the t-tubular structures of the heart. In that this translocation has been previously associated with the antiadrenergic property of A(1)R, it is apparent that the interactive effects of adenosine and β(1)-adrenergic agonists on function are complex in the heart.

SNX9, SNX18 and SNX33 are required for progression through and completion of mitosis

Mitosis involves considerable membrane remodelling and vesicular trafficking to generate two independent cells. Consequently, endocytosis and endocytic proteins are required for efficient mitotic progression and completion. Several endocytic proteins also participate in mitosis in an endocytosis-independent manner. Here, we report that the sorting nexin 9 (SNX9) subfamily members - SNX9, SNX18 and SNX33 - are required for progression and completion of mitosis. Depletion of any one of these proteins using siRNA induces multinucleation, an indicator of cytokinesis failure, as well as an accumulation of cytokinetic cells. Time-lapse microscopy on siRNA-treated cells revealed a role for SNX9 subfamily members in progression through the ingression and abscission stages of cytokinesis. Depletion of these three proteins disrupted MRLC(S19) localization during ingression and recruitment of Rab11-positive recycling endosomes to the intracellular bridge between nascent daughter cells. SNX9 depletion also disrupted the localization of Golgi during cytokinesis. Endocytosis of transferrin was blocked during cytokinesis by depletion of the SNX9 subfamily members, suggesting that these proteins participate in cytokinesis in an endocytosis-dependent manner. In contrast, depletion of SNX9 did not block transferrin uptake during metaphase but did delay chromosome alignment and segregation, suggesting that SNX9 plays an additional non-endocytic role at early mitotic stages. We conclude that SNX9 subfamily members are required for mitosis through both endocytosis-dependent and -independent processes.

Nonmuscle myosin-2: mix and match

Members of the nonmuscle myosin-2 (NM-2) family of actin-based molecular motors catalyze the conversion of chemical energy into directed movement and force thereby acting as central regulatory components of the eukaryotic cytoskeleton. By cyclically interacting with adenosine triphosphate and F-actin, NM-2 isoforms promote cytoskeletal force generation in established cellular processes like cell migration, shape changes, adhesion dynamics, endo- and exo-cytosis, and cytokinesis. Novel functions of the NM-2 family members in autophagy and viral infection are emerging, making NM-2 isoforms regulators of nearly all cellular processes that require the spatiotemporal organization of cytoskeletal scaffolding. Here, we assess current views about the role of NM-2 isoforms in these activities including the tight regulation of NM-2 assembly and activation through phosphorylation and how NM-2-mediated changes in cytoskeletal dynamics and mechanics affect cell physiological functions in health and disease.

Ramping up mitosis: an AMPKα2-regulated signaling network promotes mitotic progression

In the December 23rd issue of Molecular Cell, Banko et al. (2011) describe a chemical genetic screen that identified 28 novel AMPKα2 direct substrates. A subset of these substrates comprise a signaling network by which AMPK, seemingly independent of cellular energy status, promotes mitotic progression.

Chemical genetic screen for AMPKα2 substrates uncovers a network of proteins involved in mitosis

The energy-sensing AMP-activated protein kinase (AMPK) is activated by low nutrient levels. Functions of AMPK, other than its role in cellular metabolism, are just beginning to emerge. Here we use a chemical genetics screen to identify direct substrates of AMPK in human cells. We find that AMPK phosphorylates 28 previously unidentified substrates, several of which are involved in mitosis and cytokinesis. We identify the residues phosphorylated by AMPK in vivo in several substrates, including protein phosphatase 1 regulatory subunit 12C (PPP1R12C) and p21-activated protein kinase (PAK2). AMPK-induced phosphorylation is necessary for PPP1R12C interaction with 14-3-3 and phosphorylation of myosin regulatory light chain. Both AMPK activity and PPP1R12C phosphorylation are increased in mitotic cells and are important for mitosis completion. These findings suggest that AMPK coordinates nutrient status with mitosis completion, which may be critical for the organism's response to low nutrients during development, or in adult stem and cancer cells.

And the dead shall rise: actin and myosin return to the spindle

The spindle directs chromosome partitioning in eukaryotes and, for the last three decades, has been considered primarily a structure based on microtubules, microtubule motors, and other microtubule binding proteins. However, a surprisingly large body of both old and new studies suggests roles for actin filaments (F-actin) and myosins (F-actin-based motor proteins) in spindle assembly and function. Here we review these data and conclude that in several cases the evidence for the participation of F-actin and myosins in spindle function is very strong, and in the situations where it is less strong, there is nevertheless enough evidence to warrant further investigation.

Myosin light chain kinases and phosphatase in mitosis and cytokinesis

At mitosis, cells undergo drastic alterations in morphology and cytoskeletal organization including cell rounding during prophase, mitotic spindle assembly during prometaphase and metaphase, chromatid segregation in anaphase, and cytokinesis during telophase. It is well established that myosin II is a motor responsible for cytokinesis. Recent reports have indicated that myosin II is also involved in spindle assembly and karyokinesis. In this review, we summarize current understanding of the functions of myosin II in mitosis and cytokinesis of higher eukaryotes, and discuss the roles of possible upstream molecules that control myosin II in these mitotic events.

MPF governs the assembly and contraction of actomyosin rings by activating RhoA and MAPK during chemical-induced cytokinesis of goat oocytes

The interplay between maturation-promoting factor (MPF), mitogen-activated protein kinase (MAPK) and Rho GTPase during actin-myosin interactions has yet to be determined. The mechanism by which microtubule disrupters induce the formation of ooplasmic protrusion during chemical-assisted enucleation of mammalian oocytes is unknown. Moreover, a suitable model is urgently needed for the study of cytokinesis. We have established a model of chemical-induced cytokinesis and have studied the signaling events leading to cytokinesis using this model. The results suggested that microtubule inhibitors activated MPF, which induced actomyosin assembly (formation of ooplasmic protrusion) by activating RhoA and thus MAPK. While MAPK controlled actin recruitment on its own, MPF promoted myosin enrichment by activating RhoA and MAPK. A further chemical treatment of oocytes with protrusions induced constriction of the actomyosin ring by inactivating MPF while activating RhoA. In conclusion, the present data suggested that the assembly and contraction of the actomyosin ring were two separable steps: while an increase in MPF activity promoted the assembly through RhoA-mediated activation of MAPK, a decrease in MPF activity triggered contraction of the ring by activating RhoA.

Nonmuscle myosin IIA with a GFP fused to the N-terminus of the regulatory light chain is regulated normally

Nonmuscle myosin II plays a crucial role in a variety of cellular processes (e.g., polarity formation, cell motility, and cytokinesis). It is composed of two heavy chains, two regulatory light chains and two essential light chains. The ATPase activity of the myosin II motor domain is regulated through phosphorylation of the regulatory light chain (RLC) by myosin light chain kinase. To study myosin function and localization in cellular processes, GFP-fused RLCs are widely used; however, the exact kinetic properties of myosins with bound GFP-RLC are poorly described. More importantly, it has not been shown that a regulatory light chain fused at its N-terminus with GFP can maintain the normal phosphorylation-dependent regulation of nonmuscle myosin or serve as a substrate for myosin light chain kinase. We coexpressed N-terminal GFP-RLC with a heavy meromyosin (HMM)-like fragment of nonmuscle myosin IIA and essential light chain to characterize the phosphorylation dynamics and in vitro kinetic properties of the resulting HMM. Myosin light chain kinase phosphorylates the GFP-RLC bound to HMM IIA with the same V(max) as it does the wild type RLC bound to HMM IIA, but the K(m) is about two fold higher for the GFP fusion protein, meaning that it is a somewhat poorer substrate. The steady-state actin-activated MgATPase activity of the GFP-RLC HMM is very low in the absence of phosphorylation demonstrating that the GFP moiety does not prevent formation of the off state. The actin-activated MgATPase activity of phosphorylated GFP-RLC-HMM and is about half that of wild type phosphorylated HMM. The ability of phosphorylated GFP-RLC-HMM to move actin filaments in the actin gliding assay is also slightly compromised. These data indicate that despite some kinetic differences the N-terminal GFP fusion to the regulatory light chain is a reasonable model system for studying myosin function in vivo.

Duration of fusion pore opening and the amount of hormone released are regulated by myosin II during kiss-and-run exocytosis

Since the fusion pore of the secretory vesicle is resealed before complete dilation during 'kiss-and-run' exocytosis, their cargoes are not completely released. Although the transient fusion pore is kept open for several seconds, the precise mechanisms that control fusion pore maintenance, and their physiological significance, are not well understood. Using dual-colour TIRF (total internal reflection fluorescence) microscopy in neuroendocrine PC12 cells, we show that myosin II regulates the fusion pore dynamics during kiss-and-run exocytosis. The release kinetics of mRFP (monomeric red fluorescent protein)-tagged tPA (tissue plasminogen activator) and Venus-tagged BDNF (brain-derived neurotrophic factor), which show slower release kinetics than NPY (neuropeptide Y)-mRFP and insulin-mRFP, were prolonged by the overexpression of a wild-type form of the RLC (myosin II regulatory light chain). In contrast, overexpression of a dominant-negative form of RLC shortened the release kinetics. Using spH (synapto-pHluorin), a green fluorescent protein-based pH sensor inside the vesicles, we confirmed that the modulation of the release kinetics by myosin II is due to changes in the duration of fusion pore opening. In addition, we revealed that the amount of hormone released into the extracellular space upon stimulation was increased by overexpression of wild-type RLC. We propose that the duration of fusion pore opening is regulated by myosin II to control the amount of hormone released from a single vesicle.

Cytokinesis and cancer: Polo loves ROCK'n' Rho(A)

Cytokinesis is the last step of the M (mitosis) phase, yet it is crucial for the faithful division of one cell into two. Cytokinesis failure is often associated with cancer. Cytokinesis can be morphologically divided into four steps: cleavage furrow initiation, cleavage furrow ingression, midbody formation and abscission. Molecular studies have revealed that RhoA as well as its regulators and effectors are important players to ensure a successful cytokinesis. At the same time, Polo-like kinase 1 (Plk1) is an important kinase that can target many substrates and carry out different functions during mitosis, including cytokinesis. Recent studies are beginning to unveil a closer tie between Plk1 and RhoA networks. More specifically, Plk1 phosphorylates the centralspindlin complex Cyk4 and MKLP1/CHO1, thus recruiting RhoA guanine nucleotide-exchange factor (GEF) Ect2 through its phosphopeptide-binding BRCT domains. Ect2 itself can be phosphorylated by Plk1 in vitro. Plk1 can also phosphorylate another GEF MyoGEF to regulate RhoA activity. Once activated, RhoA-GTP will activate downstream effectors, including ROCK1 and ROCK2. ROCK2 is among the proteins that associate with Plk1 Polo-binding domain (PBD) in a large proteomic screen, and Plk1 can phosphorylate ROCK2 in vitro. We review current understandings of the interplay between Plk1, RhoA proteins and other proteins (e.g., NudC, MKLP2, PRC1, CEP55) involved in cytokinesis, with particular emphasis of its clinical implications in cancer.

Deficiency in myosin light-chain phosphorylation causes cytokinesis failure and multipolarity in cancer cells

Cancer cells often have unstable genomes and increased centrosome and chromosome numbers, which are an important part of malignant transformation in the most recent model of tumorigenesis. However, very little is known about divisional failures in cancer cells that may lead to chromosomal and centrosomal amplifications. In this study, we show that cancer cells often failed at cytokinesis because of decreased phosphorylation of the myosin regulatory light chain (MLC), a key regulatory component of cortical contraction during division. Reduced MLC phosphorylation was associated with high expression of myosin phosphatase and/or reduced myosin light-chain kinase levels. Furthermore, expression of phosphomimetic MLC largely prevented cytokinesis failure in the tested cancer cells. When myosin light-chain phosphorylation was restored to normal levels by phosphatase knockdown, multinucleation and multipolar mitosis were markedly reduced, resulting in enhanced genome stabilization. Furthermore, both overexpression of myosin phosphatase or inhibition of the myosin light-chain kinase in nonmalignant cells could recapitulate some of the mitotic defects of cancer cells, including multinucleation and multipolar spindles, indicating that these changes are sufficient to reproduce the cytokinesis failures we see in cancer cells. These results for the first time define the molecular defects leading to divisional failure in cancer cells.

Regulation of fission yeast myosin-II function and contractile ring dynamics by regulatory light-chain and heavy-chain phosphorylation

We investigated the role of regulatory light-chain (Rlc1p) and heavy-chain phosphorylation in controlling fission yeast myosin-II (Myo2p) motor activity and function during cytokinesis. Phosphorylation of Rlc1p leads to a fourfold increase in Myo2p's in vitro motility rate, which ensures effective contractile ring constriction and function. Surprisingly, unlike with smooth muscle and nonmuscle myosin-II, RLC phosphorylation does not influence the actin-activated ATPase activity of Myo2p. A truncated form of Rlc1p lacking its extended N-terminal regulatory region (including phosphorylation sites) supported maximal Myo2p in vitro motility rates and normal contractile ring function. Thus, the unphosphorylated N-terminal extension of Rlc1p can uncouple the ATPase and motility activities of Myo2p. We confirmed the identity of one out of two putative heavy-chain phosphorylation sites previously reported to control Myo2p function and cytokinesis. Although in vitro studies indicated that phosphorylation at Ser-1444 is not needed for Myo2p motor activity, phosphorylation at this site promotes the initiation of contractile ring constriction.

Direct evidence for roles of phosphorylated regulatory light chain of myosin II in furrow ingression during cytokinesis in HeLa cells

Phosphorylation of myosin II is thought to play an important role in cytokinesis. Although it is well known that phosphorylated regulatory light chain of myosin II (P-MRLC) localizes along the contractile ring, it is not clear how P-MRLC controls myosin II and F-actin in furrow ingression during cytokinesis. To elucidate roles of P-MRLC in furrow ingression, HeLa cells transfected with EGFP-tagged wild-type or each MRLC mutant were observed using a live-imaging microscope. Time-lapse observation revealed that a delay of furrow ingression was observed in the nonphosphorylatable form of MRLC (AA-MRLC)-expressing cell but not in the wild-type or phospho-mimic MRLC-expressing cell. Among each form of MRLC-expressing cell, the total amount of P-MRLC including phospho-mimic MRLCs was smallest in the cell expressing AA-MRLC. However, the amount of F-actin and myosin II at the contractile ring in the AA-MRLC-expressing cell was the same as that in the normal cell. Interestingly, delay of furrow ingression by a Rho-kinase inhibitor, Y27632, was rescued by phospho-mimic MRLCs. These results suggest that the P-MRLC is essential for the progress of furrow ingression but not the retainment of F-actin and myosin II in the contractile ring of dividing HeLa cells.

Mitosis: spindle evolution and the matrix model

Current spindle models explain "anaphase A" (movement of chromosomes to the poles) in terms of a motility system based solely on microtubules (MTs) and that functions in a manner unique to mitosis. We find both these propositions unlikely. An evolutionary perspective suggests that when the spindle evolved, it should have come to share not only components (e.g., microtubules) of the interphase cell but also the primitive motility systems available, including those using actin and myosin. Other systems also came to be involved in the additional types of motility that now accompany mitosis in extant spindles. The resultant functional redundancy built reliability into this critical and complex process. Such multiple mechanisms are also confusing to those who seek to understand how chromosomes move. Narrowing this commentary down to just anaphase A, we argue that the spindle matrix participates with MTs in anaphase A and that this matrix may contain actin and myosin. The diatom spindle illustrates how such a system could function. This matrix may be motile and work in association with the MT cytoskeleton, as it does with the actin cytoskeleton during cell ruffling and amoeboid movement. Instead of pulling the chromosome polewards, the kinetochore fibre's role might be to slow polewards movement to allow correct chromosome attachment to the spindle. Perhaps the earliest eukaryotic cell was a cytoplast organised around a radial MT cytoskeleton. For cell division, it separated into two cytoplasts via a spindle of overlapping MTs. Cytokinesis was actin-based cleavage. As chromosomes evolved into individual entities, their interaction with the dividing cytoplast developed into attachment of the kinetochore to radial (cytoplast) MTs. We believe it most likely that cytoplasmic motility systems participated in these events.

PKU-beta/TLK1 regulates myosin II activities, and is required for accurate equaled chromosome segregation

Tousled-like kinase 1 (or protein kinase ubiquitous, PKU-beta/TLK1) is a serine/threonine protein kinase that is implicated in chromatin remodeling, DNA replication and mitosis. RNAi-mediated PKU-beta/TLK1-depleted human cells showed aneuploidy, and immunofluorescence analysis of these cells revealed the unequal segregation of daughter chromosomes. Immunoblots indicated a substantial reduction in the phosphorylation level of Ser19/Thr18 on the myosin II regulatory light chain (MRLC) in PKU-beta/TLK1-depleted cells, with no change in total MRLC protein. To confirm the relationship between mitotic aberration and MRLC dysfunction, we expressed wild type MRLC or DD-MRLC (mimics diphosphorylation; substitution of both Thr18 and Ser19 with aspartate) in PKU-beta/TLK1-depleted cells. DD-MRLC expression dramatically reduced the unequal segregation of chromosomes. Our data suggest that human PKU-beta/TLK1 plays an important role in chromosome integrity via the regulation of myosin II dynamics by phosphorylating MRLC during mitosis.

Regulation of myosin light chain kinase expression by angiotensin II in hypertension

BACKGROUND

Increased growth and contraction of vascular smooth muscle cells (VSMCs) are major abnormalities in many vascular disorders. To investigate the signaling pathways that mediate these processes, we studied the expression of smooth muscle myosin light chain kinase (smMLCK) in VSMCs.

METHODS

Primary cultured VSMCs isolated from normotensive Wistar-Kyoto (WKY) rats were treated with angiotensin II (Ang II). smMLCK expression was examined in the cells using western blot analysis. In vivo, a specific inhibitor of smMLCK or MAP kinase kinase (MEK) was delivered to spontaneously hypertensive rats (SHRs) using an osmotic pump, and their blood pressures were measured using tail-cuff sphygmomanometry.

RESULTS

Expression of smMLCK protein is rapidly increased by Ang II, an important agonist responsible for increased vasoconstriction and vascular remodeling, in concert with increased myosin light chain phosphorylation. Inhibiting Ang II type 1 (AT1) receptor, Ras, or MEK blocked the Ang II-induced increase in smMLCK expression. In vivo, inhibiting MEK decreased smMLCK expression, blood pressure, and vascular thickening in SHRs. Moreover, inhibiting smMLCK activity decreased blood pressure and smooth muscle mass in arteries in SHRs.

CONCLUSIONS

The regulation of smMLCK expression by Ang II via Ras signaling is important in the regulation of vascular remodeling and blood pressure. Targeting this pathway could be an effective strategy for developing novel therapeutics to treat hypertension.