The cellular architecture and molecular determinants of the zebrafish fusogenic synapse

Spatiotemporal coordination of actin regulators generates invasive protrusions in cell-cell fusion

Invasive membrane protrusions play a central role in a variety of cellular processes. Unlike filopodia, invasive protrusions are mechanically stiff and propelled by branched actin polymerization. However, how branched actin filaments are organized to create finger-like invasive protrusions is unclear. Here, by examining the mammalian fusogenic synapse, where invasive protrusions are generated to promote cell membrane juxtaposition and fusion, we have uncovered the mechanism underlying invasive protrusion formation. We show that two nucleation-promoting factors for the Arp2/3 complex, WAVE and N-WASP, exhibit different localization patterns in the protrusions. Whereas WAVE is closely associated with the plasma membrane at the leading edge of the protrusive structures, N-WASP is enriched with WIP along the actin bundles in the shafts of the protrusions. During protrusion initiation and growth, the Arp2/3 complex nucleates branched actin filaments to generate low-density actin clouds in which the large GTPase dynamin organizes the new branched actin filaments into bundles, followed by actin-bundle stabilization by WIP, the latter functioning as an actin-bundling protein. Disruption of any of these components results in defective protrusions and failed myoblast fusion in cultured cells and mouse embryos. Together, our study has revealed the intricate spatiotemporal coordination between two nucleation-promoting factors and two actin-bundling proteins in building invasive protrusions at the mammalian fusogenic synapse and has general implications in understanding invasive protrusion formation in cellular processes beyond cell-cell fusion.

Ectopic expression of Myomaker and Myomixer in slow muscle cells induces slow muscle fusion and myofiber death

Zebrafish embryos possess two major types of myofibers, the slow and fast fibers, with distinct patterns of cell fusion. The fast muscle cells can fuse, while the slow muscle cells cannot. Here, we show that myomaker is expressed in both slow and fast muscle precursors, whereas myomixer is exclusive to fast muscle cells. The loss of Prdm1a, a regulator of slow muscle differentiation, results in strong myomaker and myomixer expression and slow muscle cell fusion. This abnormal fusion is further confirmed by the direct ectopic expression of myomaker or myomixer in slow muscle cells of transgenic models. Using the transgenic models, we show that the heterologous fusion between slow and fast muscle cells can alter slow muscle cell migration and gene expression. Furthermore, the overexpression of myomaker and myomixer also disrupts membrane integrity, resulting in muscle cell death. Collectively, this study identifies that the fiber-type-specific expression of fusogenic proteins is critical for preventing inappropriate fusion between slow and fast fibers in fish embryos, highlighting the need for precise regulation of fusogenic gene expression to maintain muscle fiber integrity and specificity.

Branched actin polymerization drives invasive protrusion formation to promote myoblast fusion during skeletal muscle regeneration

Skeletal muscle regeneration is a multistep process involving the activation, proliferation, differentiation, and fusion of muscle stem cells, known as satellite cells. The fusion of satellite cell-derived mononucleated muscle cells (SCMs) is indispensable for the generation of multinucleated, contractile myofibers during muscle repair. However, the molecular and cellular mechanisms underlying SCM fusion during muscle regeneration remain poorly understood. In this study, we uncovered an essential role for branched actin polymerization in SCM fusion. Using conditional knockouts of the Arp2/3 complex and its actin nucleation-promoting factors, N-WASP and WAVE, we demonstrated that branched actin polymerization is required for the SCM fusion, but not for satellite cell proliferation, differentiation, and migration. We showed that the N-WASP and WAVE complexes have partially redundant functions in regulating SCM fusion. Furthermore, we showed that branched actin polymerization is essential for generating invasive protrusions at the fusogenic synapses in SCMs. Taken together, our study has identified new components of the myoblast fusion machinery in skeletal muscle regeneration and demonstrated a critical role for branched actin-propelled invasive protrusions in this process.

Novel requirements for HAP2/GCS1-mediated gamete fusion in Tetrahymena

The ancestral gamete fusion protein, HAP2/GCS1, plays an essential role in fertilization in a broad range of taxa. To identify factors that may regulate HAP2/GCS1 activity, we screened mutants of the ciliate Tetrahymena thermophila for behaviors that mimic Δhap2/gcs1 knockout phenotypes in this species. Using this approach, we identified two new genes, GFU1 and GFU2, whose products are necessary for membrane pore formation following mating type recognition and adherence. GFU2 is predicted to be a single-pass transmembrane protein, while GFU1, though lacking obvious transmembrane domains, has the potential to interact directly with membrane phospholipids in the cytoplasm. Like Tetrahymena HAP2/GCS1, expression of GFU1 is required in both cells of a mating pair for efficient fusion to occur. To explain these bilateral requirements, we propose a model that invokes cooperativity between the fusion machinery on apposed membranes of mating cells and accounts for successful fertilization in Tetrahymena's multiple mating type system.

MTM1-mediated production of phosphatidylinositol 5-phosphate fuels the formation of podosome-like protrusions regulating myoblast fusion

Myogenesis is a multistep process that requires a spatiotemporal regulation of cell events resulting finally in myoblast fusion into multinucleated myotubes. Most major insights into the mechanisms underlying fusion seem to be conserved from insects to mammals and include the formation of podosome-like protrusions (PLPs) that exert a driving force toward the founder cell. However, the machinery that governs this process remains poorly understood. In this study, we demonstrate that MTM1 is the main enzyme responsible for the production of phosphatidylinositol 5-phosphate, which in turn fuels PI5P 4-kinase α to produce a minor and functional pool of phosphatidylinositol 4,5-bisphosphate that concentrates in PLPs containing the scaffolding protein Tks5, Dynamin-2, and the fusogenic protein Myomaker. Collectively, our data reveal a functional crosstalk between a PI-phosphatase and a PI-kinase in the regulation of PLP formation.

Molecular regulation of myocyte fusion

Myocyte fusion is a pivotal process in the development and regeneration of skeletal muscle. Failure during fusion can lead to a range of developmental as well as pathological consequences. This review aims to comprehensively explore the intricate processes underlying myocyte fusion, from the molecular to tissue scale. We shed light on key players, such as the muscle-specific fusogens - Myomaker and Myomixer, in addition to some lesser studied molecules contributing to myocyte fusion. Conserved across vertebrates, Myomaker and Myomixer play a crucial role in driving the merger of plasma membranes of fusing myocytes, ensuring the formation of functional muscle syncytia. Our multiscale approach also delves into broader cell and tissue dynamics that orchestrate the timing and positioning of fusion events. In addition, we explore the relevance of muscle fusogens to human health and disease. Mutations in fusogen genes have been linked to congenital myopathies, providing unique insights into the molecular basis of muscle diseases. We conclude with a discussion on potential therapeutic avenues that may emerge from manipulating the myocyte fusion process to remediate skeletal muscle disorders.

Machine learning inference of continuous single-cell state transitions during myoblast differentiation and fusion

Cells modify their internal organization during continuous state transitions, supporting functions from cell division to differentiation. However, tools to measure dynamic physiological states of individual transitioning cells are lacking. We combined live-cell imaging and machine learning to monitor ERK1/2-inhibited primary murine skeletal muscle precursor cells, that transition rapidly and robustly from proliferating myoblasts to post-mitotic myocytes and then fuse, forming multinucleated myotubes. Our models, trained using motility or actin intensity features from single-cell tracking data, effectively tracked real-time continuous differentiation, revealing that differentiation occurs 7.5-14.5 h post induction, followed by fusion ~3 h later. Co-inhibition of ERK1/2 and p38 led to differentiation without fusion. Our model inferred co-inhibition leads to terminal differentiation, indicating that p38 is specifically required for transitioning from terminal differentiation to fusion. Our model also predicted that co-inhibition leads to changes in actin dynamics. Mass spectrometry supported these in silico predictions and suggested novel fusion and maturation regulators downstream of differentiation. Collectively, this approach can be adapted to various biological processes to uncover novel links between dynamic single-cell states and their functional outcomes.

Germline and Somatic Cell Syncytia in Insects

Syncytia are common in the animal and plant kingdoms both under normal and pathological conditions. They form through cell fusion or division of a founder cell without cytokinesis. A particular type of syncytia occurs in invertebrate and vertebrate gametogenesis when the founder cell divides several times with partial cytokinesis producing a cyst (nest) of germ line cells connected by cytoplasmic bridges. The ultimate destiny of the cyst's cells differs between animal groups. Either all cells of the cyst become the gametes or some cells endoreplicate or polyploidize to become the nurse cells (trophocytes). Although many types of syncytia are permanent, the germ cell syncytium is temporary, and eventually, it separates into individual gametes. In this chapter, we give an overview of syncytium types and focus on the germline and somatic cell syncytia in various groups of insects. We also describe the multinuclear giant cells, which form through repetitive nuclear divisions and cytoplasm hypertrophy, but without cell fusion, and the accessory nuclei, which bud off the oocyte nucleus, migrate to its cortex and become included in the early embryonic syncytium.

Loss of Myomixer Results in Defective Myoblast Fusion, Impaired Muscle Growth, and Severe Myopathy in Zebrafish

The development and growth of fish skeletal muscles require myoblast fusion to generate multinucleated myofibers. While zebrafish fast-twitch muscle can fuse to generate multinucleated fibers, the slow-twitch muscle fibers remain mononucleated in zebrafish embryos and larvae. The mechanism underlying the fiber-type-specific control of fusion remains elusive. Recent genetic studies using mice identified a long-sought fusion factor named Myomixer. To understand whether Myomixer is involved in the fiber-type specific fusion, we analyzed the transcriptional regulation of myomixer expression and characterized the muscle growth phenotype upon genetic deletion of myomixer in zebrafish. The data revealed that overexpression of Sonic Hedgehog (Shh) drastically inhibited myomixer expression and blocked myoblast fusion, recapitulating the phenotype upon direct genetic deletion of myomixer from zebrafish. The fusion defect in myomixer mutant embryos could be faithfully rescued upon re-expression of zebrafish myomixer gene or its orthologs from shark or human. Interestingly, myomixer mutant fish survived to adult stage though were notably smaller than wildtype siblings. Severe myopathy accompanied by the uncontrolled adipose infiltration was observed in both fast and slow muscle tissues of adult myomixer mutants. Collectively, our data highlight an indispensable role of myomixer gene for cell fusion during both embryonic muscle development and post-larval muscle growth.

Phosphatidylserine orchestrates Myomerger membrane insertions to drive myoblast fusion

Muscle cell fusion is a multistep process where the final step of the reaction drives progression beyond early hemifusion events to complete fusion. This step requires activity of the muscle-specific fusogen Myomerger, a single-pass transmembrane protein containing 84 amino acids with an ectodomain that includes two α-helices. Previous studies have demonstrated that Myomerger acts by destabilizing membranes through generation of elastic stresses in the outer leaflet of the plasma membrane. An obvious question is how such destabilizing activity might be regulated to avoid membrane and cellular damage, and how the two juxtaposed helices cooperate in fusion. Using cellular fusion assays and in vitro liposome assays, we report that the two helices possess unique characteristics, both of which are needed for full activity of the protein. We demonstrate that externalized phosphatidylserine (PS), a lipid previously implicated in myoblast fusion, has a determinant role in the regulation of Myomerger activity. The membrane-proximal, amphipathic Helix-1 is normally disordered and its α-helical structure is induced by PS, making membrane interactions more efficacious. The distal, more hydrophobic Helix-2 is intrinsically ordered, possesses an ability to insert into membranes, and augments the membrane-stressing effects of Helix-1. These data reveal that Myomerger fusogenic activity is an exquisitely orchestrated event involving its two ectodomain helices, which are controlled by membrane lipid composition, providing an explanation as to how its membrane-stressing activity is spatially and temporally regulated during the final step of myoblast fusion.

Slow muscles guide fast myocyte fusion to ensure robust myotome formation despite the high spatiotemporal stochasticity of fusion events

Skeletal myogenesis is dynamic, and it involves cell-shape changes together with cell fusion and rearrangements. However, the final muscle arrangement is highly organized with striated fibers. By combining live imaging with quantitative analyses, we dissected fast-twitch myocyte fusion within the zebrafish myotome in toto. We found a strong mediolateral bias in fusion timing; however, at a cellular scale, there was heterogeneity in cell shape and the relationship between initial position of fast myocytes and resulting fusion partners. We show that the expression of the fusogen myomaker is permissive, but not instructive, in determining the spatiotemporal fusion pattern. Rather, we observed a close coordination between slow muscle rearrangements and fast myocyte fusion. In mutants that lack slow fibers, the spatiotemporal fusion pattern is substantially noisier. We propose a model in which slow muscles guide fast myocytes by funneling them close together, enhancing fusion probability. Thus, despite fusion being highly stochastic, a robust myotome structure emerges at the tissue scale.