Myosin II activity is not essential for recruitment of myosin II to the furrow in dividing HeLa cells

LIS1 determines cleavage plane positioning by regulating actomyosin-mediated cell membrane contractility

Heterozygous loss of human PAFAH1B1 (coding for LIS1) results in the disruption of neurogenesis and neuronal migration via dysregulation of microtubule (MT) stability and dynein motor function/localization that alters mitotic spindle orientation, chromosomal segregation, and nuclear migration. Recently, human- induced pluripotent stem cell (iPSC) models revealed an important role for LIS1 in controlling the length of terminal cell divisions of outer radial glial (oRG) progenitors, suggesting cellular functions of LIS1 in regulating neural progenitor cell (NPC) daughter cell separation. Here, we examined the late mitotic stages NPCs in vivo and mouse embryonic fibroblasts (MEFs) in vitro from Pafah1b1-deficient mutants. Pafah1b1-deficient neocortical NPCs and MEFs similarly exhibited cleavage plane displacement with mislocalization of furrow-associated markers, associated with actomyosin dysfunction and cell membrane hyper-contractility. Thus, it suggests LIS1 acts as a key molecular link connecting MTs/dynein and actomyosin, ensuring that cell membrane contractility is tightly controlled to execute proper daughter cell separation.

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.

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.

RUNX1-induced silencing of non-muscle myosin heavy chain IIB contributes to megakaryocyte polyploidization

Megakaryocytes are unique mammalian cells that undergo polyploidization (endomitosis) during differentiation, leading to an increase in cell size and protein production that precedes platelet production. Recent evidence demonstrates that endomitosis is a consequence of a late failure in cytokinesis associated with a contractile ring defect. Here we show that the non-muscle myosin IIB heavy chain (MYH10) is expressed in immature megakaryocytes and specifically localizes in the contractile ring. MYH10 downmodulation by short hairpin RNA increases polyploidization by inhibiting the return of 4N cells to 2N, but other regulators, such as of the G1/S transition, might regulate further polyploidization of the 4N cells. Conversely, re-expression of MYH10 in the megakaryocytes prevents polyploidization and the transition of 2N to 4N cells. During polyploidization, MYH10 expression is repressed by the major megakaryocyte transcription factor RUNX1. Thus, RUNX1-mediated silencing of MYH10 is required for the switch from mitosis to endomitosis, linking polyploidization with megakaryocyte differentiation.

Phosphorylation of myosin II regulatory light chain controls its accumulation, not that of actin, at the contractile ring in HeLa cells

During cytokinesis in eukaryotic cells, an actomyosin-based contractile ring (CR) is assembled along the equator of the cell. Myosin II ATPase activity is stimulated by the phosphorylation of the myosin II regulatory light chain (MRLC) in vitro, and phosphorylated MRLC localizes at the CR in various types of cells. Previous studies have determined that phosphorylated MRLC plays an important role in CR furrowing. However, the role of phosphorylated MRLC in CR assembly remains unknown. Here, we have used confocal microscopy to observe dividing HeLa cells expressing fluorescent protein-tagged MRLC mutants and actin during CR assembly near the cortex. Di-phosphomimic MRLC accumulated at the cell equator earlier than non-phosphorylatable MRLC and actin. Interestingly, perturbation of myosin II activity by non-phosphorylatable MRLC expression or treatment with blebbistatin, a myosin II inhibitor, did not alter the time of actin accumulation at the cell equator. Furthermore, inhibition of actin polymerization by treatment with latrunculin A had no effect on MRLC accumulation at the cell equator. Taken together, these data suggest that phosphorylated MRLC temporally controls its own accumulation, but not that of actin, in cultured mammalian cells.

Diphosphorylated but not monophosphorylated myosin II regulatory light chain localizes to the midzone without its heavy chain during cytokinesis

Myosin II is activated by the monophosphorylation of its regulatory light chain (MRLC) at Ser19 (1P-MRLC). Its ATPase activity is further enhanced by MRLC diphosphorylation at Thr18/Ser19 (2P-MRLC). As these phosphorylated MRLCs are colocalized with their heavy chains at the contractile ring in dividing cells, we believe that the phosphorylated MRLC acts as a subunit of the activated myosin II during cytokinesis. However, the distinct role(s) of 1P- and 2P-MRLC during cytokinesis has not been elucidated. In this study, a monoclonal antibody (4F12) specific for 2P-MRLC was raised and used to examine the roles of 2P-MRLC in cultured mammalian cells. Our confocal microscopic observations using 4F12 revealed that 2P-MRLC localized to the contractile ring, and, unexpectedly, to the midzone also. Interestingly, 2P-MRLC did not colocalize with 1P-MRLC, myosin II heavy chain, and F-actin at the midzone. These results suggest that 2P-MRLC has a role different from that of 1P-MRLC at the midzone, and is not a subunit of myosin II.

Enhancement of myosin II/actin turnover at the contractile ring induces slower furrowing in dividing HeLa cells

Myosin II ATPase activity is enhanced by the phosphorylation of MRLC (myosin II regulatory light chain) in non-muscle cells. It is well known that pMRLC (phosphorylated MRLC) co-localizes with F-actin (filamentous actin) in the CR (contractile ring) of dividing cells. Recently, we reported that HeLa cells expressing non-phosphorylatable MRLC show a delay in the speed of furrow ingression, suggesting that pMRLC plays an important role in the control of furrow ingression. However, it is still unclear how pMRLC regulates myosin II and F-actin at the CR to control furrow ingression during cytokinesis. In the present study, to clarify the roles of pMRLC, we measured the turnover of myosin II and actin at the CR in dividing HeLa cells expressing fluorescent-tagged MRLCs and actin by FRAP (fluorescence recovery after photobleaching). A myosin II inhibitor, blebbistatin, caused an enhancement of the turnover of MRLC and actin at the CR, which induced a delay in furrow ingression. Furthermore, only non-phosphorylatable MRLC and a Rho-kinase inhibitor, Y-27632, accelerated the turnover of MRLC and actin at the CR. Interestingly, the effect of Y-27632 was cancelled in the cell expressing phosphomimic MRLCs. Taken together, these results reveal that pMRLC reduces the turnover of myosin II and also actin at the CR. In conclusion, we show that the enhancement of myosin II and actin turnover at the CR induced slower furrowing in dividing HeLa cells.

Myosin II recruitment during cytokinesis independent of centralspindlin-mediated phosphorylation

During cell division, the mechanisms by which myosin II is recruited to the contractile ring are not fully understood. Much recent work has focused on a model in which spatially restricted de novo filament assembly occurs at the cell equator via localized myosin II regulatory light chain (RLC) phosphorylation, stimulated by the RhoA-activating centralspindlin complex. Here, we show that a recombinant myosin IIA protein that assembles constitutively and is incapable of binding RLC still displays strong localization to the furrow in mammalian cells. Furthermore, this RLC-deficient myosin II efficiently drives cytokinesis, demonstrating that centralspindlin-based RLC phosphorylation is not necessary for myosin II localization during furrowing. Myosin II truncation analysis further reveals two distinct myosin II tail properties that contribute to furrow localization: a central tail domain mediating cortical furrow binding to heterologous binding partners and a carboxyl-terminal region mediating co-assembly with existing furrow myosin IIA or IIB filaments.

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.

The small molecule tool (S)-(-)-blebbistatin: novel insights of relevance to myosin inhibitor design

The small molecule blebbistatin is now a front line tool in the study of myosin function. Chemical modification of the tricyclic core of blebbistatin could deliver the next generation of myosin inhibitors and to help address this we report here on the impact of structural changes in the methyl-substituted aromatic ring of blebbistatin on its biological activity. Chemical methods for the preparation of isomeric methyl-containing analogues are reported and a series of co-crystal structures are used to rationalise the observed variations in their biological activity. These studies further support the view that the previously identified binding mode of blebbistatin to Dictyostelium discoideum myosin II is of relevance to its mode of action. A discussion of the role that these observations have on planning the synthesis of focused libraries of blebbistatin analogues is also provided including an assessment of possibilities by computational methods. These studies are ultimately directed at the development of novel myosin inhibitors with improved affinity and different selectivity profiles from blebbistatin itself.

mDia2 induces the actin scaffold for the contractile ring and stabilizes its position during cytokinesis in NIH 3T3 cells

mDia proteins are mammalian homologues of Drosophila diaphanous and belong to the formin family proteins that catalyze actin nucleation and polymerization. Although formin family proteins of nonmammalian species such as Drosophila diaphanous are essential in cytokinesis, whether and how mDia proteins function in cytokinesis remain unknown. Here we depleted each of the three mDia isoforms in NIH 3T3 cells by RNA interference and examined this issue. Depletion of mDia2 selectively increased the number of binucleate cells, which was corrected by coexpression of RNAi-resistant full-length mDia2. mDia2 accumulates in the cleavage furrow during anaphase to telophase, and concentrates in the midbody at the end of cytokinesis. Depletion of mDia2 induced contraction at aberrant sites of dividing cells, where contractile ring components such as RhoA, myosin, anillin, and phosphorylated ERM accumulated. Treatment with blebbistatin suppressed abnormal contraction, corrected localization of the above components, and revealed that the amount of F-actin at the equatorial region during anaphase/telophase was significantly decreased with mDia2 RNAi. These results demonstrate that mDia2 is essential in mammalian cell cytokinesis and that mDia2-induced F-actin forms a scaffold for the contractile ring and maintains its position in the middle of a dividing cell.