Myosin heavy chain-like localizes at cell contact sites during Drosophila myoblast fusion and interacts in vitro with Rolling pebbles 7

Tanc1/2 TPR domain interacts with Myo18a C-terminus and undergoes liquid-liquid phase separation

Tanc1 and its homologous protein Tanc2 are critical synaptic scaffold proteins which regulate synaptic spine densities and excitatory synapse strength. Recent studies indicated TANC1 and TANC2 are candidate genes of several neurodevelopmental disorders (NDDs). In this study, we identified and characterized a novel interaction between Tanc1/2 and Myo18a, mediated by the Tanc1/2 TPR domains and Myo18a coiled-coil domain and C-extension (CCex). Sequence analysis and size exclusion chromatography experiments reveal that high salt disrupts the interaction between Myo18a and Tanc1/2, indicating that the interaction is primarily driven by charge-charge interactions. More importantly, we found that the Tanc1-TPR/Myo18a CCex interaction could undergo liquid-liquid phase separation (LLPS) in both cultured cells and test tubes, which provides the biochemical basis and potential molecular mechanisms for the LLPS-mediated interactions between Myo18a and Tanc1/2.

Cutting Edge: Myosin 18A Is a Novel Checkpoint Regulator in B Cell Differentiation and Antibody-Mediated Immunity

We investigated the function of the newly discovered myosin family protein myosin 18A (Myo18A) in Ab-mediated immunity by generating B cell-conditional Myo18A-deficient mice. Myo18A deficiency led to expansion of bone marrow progenitor B cells and mature B cells in secondary lymphoid organs. Myo18A-deficient mice displayed serum IgM hyperglobulinemia and increased splenic IgM-secreting cells, with older mice switching to IgG1 hyperglobulinemia and autoantibody development. Immunization of Myo18A-deficient mice with inactivated influenza virus led to development of more potent neutralizing Abs against the major Ag hemagglutinin, associated with persistent accumulation of Ag-specific germinal center B cells and more Ag-specific bone marrow plasma cells. In vitro stimulation with TLR7 and BCR ligands revealed a greater ability of Myo18A-deficient B cells to differentiate into Ab-secreting cells, associated with higher AID and Blimp-1 expression. Overall, our study demonstrates that Myo18A is a novel negative regulator of B cell homeostasis, differentiation, and humoral immunity.

Multifaceted Function of Myosin-18, an Unconventional Class of the Myosin Superfamily

Myosin is a diverse superfamily of motor proteins responsible for actin-based motility and contractility in eukaryotic cells. Myosin-18 family, including myosin-18A and myosin-18B, belongs to an unconventional class of myosin, which lacks ATPase motor activity, and the investigations on their functions and molecular mechanisms in vertebrate development and diseases have just been initiated in recent years. Myosin-18A is ubiquitously expressed in mammalian cells, whereas myosin-18B shows strong enrichment in striated muscles. Myosin-18 family is important for cell motility, sarcomere formation, and mechanosensing, mostly by interacting with other cytoskeletal proteins and cellular apparatus. Myosin-18A participates in several intracellular transport processes, such as Golgi trafficking, and has multiple roles in focal adhesions, stress fibers, and lamellipodia formation. Myosin-18B, on the other hand, participates in actomyosin alignment and sarcomere assembly, thus relating to cell migration and muscle contractility. Mutations of either Myo18a or Myo18b cause cardiac developmental defects in mouse, emphasizing their crucial role in muscle development and cardiac diseases. In this review, we revisit the discovery history of myosin-18s and summarize the evolving understanding of the molecular functions of myosin-18A and myosin-18B, with an emphasis on their separate yet closely related functions in cell motility and contraction. Moreover, we discuss the diseases tightly associated with myosin-18s, especially cardiovascular defects and cancer, as well as highlight the unanswered questions and potential future research perspectives on myosin-18s.

Myosin 18Aα targets the guanine nucleotide exchange factor β-Pix to the dendritic spines of cerebellar Purkinje neurons and promotes spine maturation

Myosin 18Aα is a myosin 2-like protein containing unique N- and C-terminal protein interaction domains that co-assembles with myosin 2. One protein known to bind to myosin 18Aα is β-Pix, a guanine nucleotide exchange factor (GEF) for Rac1 and Cdc42 that has been shown to promote dendritic spine maturation by activating the assembly of actin and myosin filaments in spines. Here, we show that myosin 18A⍺ concentrates in the spines of cerebellar Purkinje neurons via co-assembly with myosin 2 and through an actin binding site in its N-terminal extension. miRNA-mediated knockdown of myosin 18A⍺ results in a significant defect in spine maturation that is rescued by an RNAi-immune version of myosin 18A⍺. Importantly, β-Pix co-localizes with myosin 18A⍺ in spines, and its spine localization is lost upon myosin 18A⍺ knockdown or when its myosin 18A⍺ binding site is deleted. Finally, we show that the spines of myosin 18A⍺ knockdown Purkinje neurons contain significantly less F-actin and myosin 2. Together, these data argue that mixed filaments of myosin 2 and myosin 18A⍺ form a complex with β-Pix in Purkinje neuron spines that promotes spine maturation by enhancing the assembly of actin and myosin filaments downstream of β-Pix's GEF activity.

Myosin XVIII

Class XVIII myosins represent a branch of the myosin family tree characterized by the presence of large N- and C-terminal extensions flanking a generic myosin core. These myosins display the highest sequence similarity to conventional class II muscle myosins and are compatible with but not restricted to myosin-2 contractile structures. Instead, they fulfill their functions at diverse localities, such as lamella, actomyosin bundles, the Golgi apparatus, focal adhesions, the cell membrane, and within sarcomeres. Sequence comparison of active-site residues and biochemical data available thus far indicate that this myosin class lacks active ATPase-driven motor activity, suggesting that its members function as structural myosins. An emerging body of evidence indicates that this structural capability is essential for the organization, maturation, and regulation of the contractile machinery in both muscle and nonmuscle cells. This is supported by the clear association of myosin-18A (Myo18A) and myosin-18B (Myo18B) dysregulation with diseases such as cancer and various myopathies.

MYO18A: An unusual myosin

MYO18A is a divergent member of the myosin family characterized by the presence of an amino-terminal PDZ domain. MYO18A has been found in a few different complexes involved in intracellular transport processes. MYO18A is found in a complex with LURAP1 and MRCK that functions in retrograde treadmilling of actin. It also has been found in a complex with PAK2, βPIX, and GIT1, functioning to transport that protein complex from focal adhesions to the leading edge. Finally, a high proportion of MYO18A is found in complex with GOLPH3 at the trans Golgi, where it functions to promote vesicle budding for Golgi-to-plasma membrane trafficking. Interestingly, MYO18A has been implicated as a cancer driver, as have other components of the GOLPH3 pathway. It remains uncertain as to whether or not MYO18A has intrinsic motor activity. While many questions remain, MYO18A is a fascinatingly unique myosin that is essential in higher organisms.

Surface apposition and multiple cell contacts promote myoblast fusion in Drosophila flight muscles

Transmission EM methods reveal that cell–cell fusion of individual myoblasts with growing Drosophila flight muscles is a stepwise process in which the cell adhesion and branched actin machineries mediate tight apposition and formation of multiple contacts and pores between the surfaces of the fusing cells.

Fusion of individual myoblasts to form multinucleated myofibers constitutes a widely conserved program for growth of the somatic musculature. We have used electron microscopy methods to study this key form of cell–cell fusion during development of the indirect flight muscles (IFMs) of Drosophila melanogaster. We find that IFM myoblast–myotube fusion proceeds in a stepwise fashion and is governed by apparent cross talk between transmembrane and cytoskeletal elements. Our analysis suggests that cell adhesion is necessary for bringing myoblasts to within a minimal distance from the myotubes. The branched actin polymerization machinery acts subsequently to promote tight apposition between the surfaces of the two cell types and formation of multiple sites of cell–cell contact, giving rise to nascent fusion pores whose expansion establishes full cytoplasmic continuity. Given the conserved features of IFM myogenesis, this sequence of cell interactions and membrane events and the mechanistic significance of cell adhesion elements and the actin-based cytoskeleton are likely to represent general principles of the myoblast fusion process.

A new level of plasticity: Drosophila smooth-like testes muscles compensate failure of myoblast fusion

The testis of Drosophila resembles an individual testis tubule of mammals. Both are surrounded by a sheath of smooth muscles, which in Drosophila are multinuclear and originate from a pool of myoblasts that are set aside in the embryo and accumulate on the genital disc later in development. These muscle stem cells start to differentiate early during metamorphosis and give rise to all muscles of the inner male reproductive system. Shortly before the genital disc and the developing testes connect, multinuclear nascent myotubes appear on the anterior tips of the seminal vesicles. Here, we show that adhesion molecules are distinctly localized on the seminal vesicles; founder cell (FC)-like myoblasts express Dumbfounded (Duf) and Roughest (Rst), and fusion-competent myoblast (FCM)-like cells mainly express Sticks and stones (Sns). The smooth but multinuclear myotubes of the testes arose by myoblast fusion. RNAi-mediated attenuation of Sns or both Duf and Rst severely reduced the number of nuclei in the testes muscles. Duf and Rst probably act independently in this context. Despite reduced fusion in all of these RNAi-treated animals, myotubes migrated onto the testes, testes were shaped and coiled, muscle filaments were arranged as in the wild type and spermatogenesis proceeded normally. Hence, the testes muscles compensate for fusion defects so that the myofibres encircling the adult testes are indistinguishable from those of the wild type and male fertility is guaranteed.

Summary:Drosophila testes muscles arise from stem cells and can compensate for fusion defects to safeguard fertility; this plasticity may compensate for the observed lack of satellite cells in Drosophila.

Mechanisms of myoblast fusion during muscle development

The development and regeneration of skeletal muscle require the fusion of mononucleated muscle cells to form multinucleated, contractile muscle fibers. Studies using a simple genetic model, Drosophila melanogaster, have discovered many evolutionarily conserved fusion-promoting factors in vivo. Recent work in zebrafish and mouse also identified several vertebrate-specific factors required for myoblast fusion. Here, we integrate progress in multiple in vivo systems and highlight conceptual advance in understanding how muscle cell membranes are brought together for fusion. We focus on the molecular machinery at the fusogenic synapse and present a three-step model to describe the molecular and cellular events leading to fusion pore formation.

Involvement of unconventional myosin VI in myoblast function and myotube formation

The important role of unconventional myosin VI (MVI) in skeletal and cardiac muscle has been recently postulated (Karolczak et al. in Histochem Cell Biol 139:873-885, 2013). Here, we addressed for the first time a role for this unique myosin motor in myogenic cells as well as during their differentiation into myotubes. During myoblast differentiation, the isoform expression pattern of MVI and its subcellular localization underwent changes. In undifferentiated myoblasts, MVI-stained puncti were seen throughout the cytoplasm and were in close proximity to actin filaments, Golgi apparatus, vinculin-, and talin-rich focal adhesion as well as endoplasmic reticulum. Colocalization of MVI with endoplasmic reticulum was enhanced during myotube formation, and differentiation-dependent association was also seen in sarcoplasmic reticulum of neonatal rat cardiomyocytes (NRCs). Moreover, we observed enrichment of MVI in myotube regions containing acetylcholine receptor-rich clusters, suggesting its involvement in the organization of the muscle postsynaptic machinery. Overexpression of the H246R MVI mutant (associated with hypertrophic cardiomyopathy) in myoblasts and NRCs caused the formation of abnormally large intracellular vesicles. MVI knockdown caused changes in myoblast morphology and inhibition of their migration. On the subcellular level, MVI-depleted myoblasts exhibited aberrations in the organization of actin cytoskeleton and adhesive structures as well as in integrity of Golgi apparatus and endoplasmic reticulum. Also, MVI depletion or overexpression of H246R mutant caused the formation of significantly wider or aberrant myotubes, respectively, indicative of involvement of MVI in myoblast differentiation. The presented results suggest an important role for MVI in myogenic cells and possibly in myoblast differentiation.

Myosin 18A coassembles with nonmuscle myosin 2 to form mixed bipolar filaments

Class-18 myosins are most closely related to conventional class-2 nonmuscle myosins (NM2). Surprisingly, the purified head domains of Drosophila, mouse, and human myosin 18A (M18A) lack actin-activated ATPase activity and the ability to translocate actin filaments, suggesting that the functions of M18A in vivo do not depend on intrinsic motor activity. M18A has the longest coiled coil of any myosin outside of the class-2 myosins, suggesting that it might form bipolar filaments similar to conventional myosins. To address this possibility, we expressed and purified full-length mouse M18A using the baculovirus/Sf9 system. M18A did not form large bipolar filaments under any of the conditions tested. Instead, M18A formed an ∼ 65-nm-long bipolar structure with two heads at each end. Importantly, when NM2 was polymerized in the presence of M18A, the two myosins formed mixed bipolar filaments, as evidenced by cosedimentation, electron microscopy, and single-molecule imaging. Moreover, super-resolution imaging of NM2 and M18A using fluorescently tagged proteins and immunostaining of endogenous proteins showed that NM2 and M18A are present together within individual filaments inside living cells. Together, our in vitro and live-cell imaging data argue strongly that M18A coassembles with NM2 into mixed bipolar filaments. M18A could regulate the biophysical properties of these filaments and, by virtue of its extra N- and C-terminal domains, determine the localization and/or molecular interactions of the filaments. Given the numerous, fundamental cellular and developmental roles attributed to NM2, our results have far-reaching biological implications.

Tethering membrane fusion: common and different players in myoblasts and at the synapse

Drosophila

Membrane fusion is essential for the communication of membrane-defined compartments, development of multicellular organisms and tissue homeostasis. Although membrane fusion has been studied extensively, still little is known about the molecular mechanisms. Especially the intercellular fusion of cells during development and tissue homeostasis is poorly understood. Somatic muscle formation in Drosophila depends on the intercellular fusion of myoblasts. In this process, myoblasts recognize each other and adhere, thereby triggering a protein machinery that leads to electron-dense plaques, vesicles and F-actin formation at apposing membranes. Two models of how local membrane stress is achieved to induce the merging of the myoblast membranes have been proposed: the electron-dense vesicles transport and release a fusogen and F-actin bends the plasma membrane. In this review, we highlight cell-adhesion molecules and intracellular proteins known to be involved in myoblast fusion. The cell-adhesion proteins also mediate the recognition and adhesion of other cell types, such as neurons that communicate with each other via special intercellular junctions, termed chemical synapses. At these synapses, neurotransmitters are released through the intracellular fusion of synaptic vesicles with the plasma membrane. As the targeting of electron-dense vesicles in myoblasts shares some similarities with the targeting of synaptic vesicle fusion, we compare molecules required for synaptic vesicle fusion to recently identified molecules involved in myoblast fusion.

Distinct genetic programs guide Drosophila circular and longitudinal visceral myoblast fusion

BACKGROUND

The visceral musculature of Drosophila larvae comprises circular visceral muscles tightly interwoven with longitudinal visceral muscles. During myogenesis, the circular muscles arise by one-to-one fusion of a circular visceral founder cell (FC) with a visceral fusion-competent myoblast (FCM) from the trunk visceral mesoderm, and longitudinal muscles arise from FCs of the caudal visceral mesoderm. Longitudinal FCs migrate anteriorly under guidance of fibroblast growth factors during embryogenesis; it is proposed that they fuse with FCMs from the trunk visceral mesoderm to give rise to syncytia containing up to six nuclei.

RESULTS

Using fluorescence in situ hybridization and immunochemical analyses, we investigated whether these fusion events during migration use the same molecular repertoire and cellular components as fusion-restricted myogenic adhesive structure (FuRMAS), the adhesive signaling center that mediates myoblast fusion in the somatic mesoderm. Longitudinal muscles were formed by the fusion of one FC with Sns-positive FCMs, and defects in FCM specification led to defects in longitudinal muscle formation. At the fusion sites, Duf/Kirre and the adaptor protein Rols7 accumulated in longitudinal FCs, and Blow and F-actin accumulated in FCMs. The accumulation of these four proteins at the fusion sites argues for FuRMAS-like adhesion and signaling centers. Longitudinal fusion was disturbed in rols and blow single, and scar wip double mutants. Mutants of wasp or its interaction partner wip had no defects in longitudinal fusion.

CONCLUSIONS

Our results indicated that all embryonic fusion events depend on the same cell-adhesion molecules, but that the need for Rols7 and regulators of F-actin distinctly differs. Rols7 was required for longitudinal visceral and somatic myoblast fusion but not for circular visceral fusion. Importantly, longitudinal fusion depended on Kette and SCAR/Wave but was independent of WASp-dependent Arp2/3 activation. Thus, the complexity of the players involved in muscle formation increases from binucleated circular muscles to longitudinal visceral muscles to somatic muscles.

Critical role of cortical vesicles in dissecting regulated exocytosis: overview of insights into fundamental molecular mechanisms

Regulated exocytosis is one of the defining features of eukaryotic cells, underlying many conserved and essential functions. Definitively assigning specific roles to proteins and lipids in this fundamental mechanism is most effectively accomplished using a model system in which distinct stages of exocytosis can be effectively separated. Here we discuss the establishment of sea urchin cortical vesicle fusion as a model to study regulated exocytosis-a system in which the docked, release-ready, and late Ca(2+)-triggered steps of exocytosis are isolated and can be quantitatively assessed using the rigorous coupling of functional and molecular assays. We provide an overview of the insights this has provided into conserved molecular mechanisms and how these have led to and integrate with findings from other regulated exocytotic cells.