State of the art in cell-cell fusion

Fusion of Human Synovium-Derived Mesenchymal Stem/Stromal Cells with Primary Human Chondrocytes Using the Modified Adherence Method (MAM)

Cell fusion is a complex phenomenon that is key in maintaining tissue homeostasis, particularly in aiding tissue regeneration processes. Studies show that mesenchymal stem/stromal cells (MSCs) are capable of restoring damaged tissue by adopting the phenotype of various cell types via cell fusion. As cell fusion of MSCs with cells of different origin remains poorly researched, we have developed a protocol that allows successful electrofusion between human synovium-derived MSCs and human chondrocytes. Building on from our protocol can help researchers study the cell fusion processes in the in vitro environment and could set basis for development of fusion cell-based advanced therapy medicinal products (ATMPs). In our protocol, we provide a detailed description on how to culture both of the fusion partner cells, how to carry out the modified adherence method (MAM) to achieve a high yield of successfully fused cells, and how to determine the yield of cell fusion using either methyl violet or fluorescent cell trackers.

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.

Altered Phenotypes of Breast Epithelial × Breast Cancer Hybrids after ZEB1 Knock-Out

ZEB1 plays a pivotal role in epithelial-to-mesenchymal transition (EMT), (cancer) cell stemness and cancer therapy resistance. The M13HS tumor hybrids, which were derived from spontaneous fusion events between the M13SV1-EGFP-Neo breast epithelial cells and HS578T-Hyg breast cancer cells, express ZEB1 and exhibit prospective cancer stem cell properties. To explore a possible correlation between the ZEB1 and stemness/ EMT-related properties in M13HS tumor hybrids, ZEB1 was knocked-out by CRISPR/Cas9. Colony formation, mammosphere formation, cell migration, invasion assays, flow cytometry and Western blot analyses were performed for the characterization of ZEB1 knock-out cells. The ZEB1 knock-out in M13HS tumor cells was not correlated with the down-regulation of the EMT-related markers N-CADHERIN (CDH2) and VIMENTIN and up-regulation of miR-200c-3p. Nonetheless, both the colony formation and mammosphere formation capacities of the M13HS ZEB1 knock-out cells were markedly reduced. Interestingly, the M13HS-2 ZEB1-KO cells harbored a markedly higher fraction of ALDH1-positive cells. The Transwell/ Boyden chamber migration assay data indicated a reduced migratory activity of the M13HS ZEB1-knock-out tumor hybrids, whereas in scratch/ wound-healing assays only the M13SH-8 ZEB1-knock-out cells possessed a reduced locomotory activity. Similarly, only the M13HS-8 ZEB1-knock-out tumor hybrids showed a reduced invasion capacity. Although the ZEB1 knock-out resulted in only moderate phenotypic changes, our data support the role of ZEB1 in EMT and stemness.

Intrinsic signalling factors associated with cancer cell-cell fusion

Cellular fusion e.g. between cancer cells and normal cells represents a stepwise process that is tightly regulated. During a pre-hybrid preparation program somatic cells and/or cancer cells are promoted to a pro-fusogenic state as a prerequisite to prepare a fusion process. A pro-fusogenic state requires significant changes including restructure of the cytoskeleton, e.g., by the formation of F-actin. Moreover, distinct plasma membrane lipids such as phosphatidylserine play an important role during cell fusion. In addition, the expression of distinct fusogenic factors such as syncytins and corresponding receptors are of fundamental importance to enable cellular mergers. Subsequent hybrid formation and fusion are followed by a post-hybrid selection process. Fusion among normal cells is important and often required during organismal development. Cancer cells fusion appears more rarely and is associated with the generation of new cancer hybrid cell populations. These cancer hybrid cells contribute to an elevated tumour plasticity by altered metastatic behaviour, changes in therapeutic and apoptotic responses, and even in the formation of cancer stem/ initiating cells. While many parts within this multi-step cascade are still poorly understood, this review article predominantly focusses on the intracellular necessities for fusion among cancer cells or with other cell populations of the tumour microenvironment. Video Abstract.

Extracellular Events Involved in Cancer Cell-Cell Fusion

Fusion among different cell populations represents a rare process that is mediated by both intrinsic and extracellular events. Cellular hybrid formation is relayed by orchestrating tightly regulated signaling pathways that can involve both normal and neoplastic cells. Certain important cell merger processes are often required during distinct organismal and tissue development, including placenta and skeletal muscle. In a neoplastic environment, however, cancer cell fusion can generate new cancer hybrid cells. Following survival during a subsequent post-hybrid selection process (PHSP), the new cancer hybrid cells express different tumorigenic properties. These can include elevated proliferative capacity, increased metastatic potential, resistance to certain therapeutic compounds, and formation of cancer stem-like cells, all of which characterize significantly enhanced tumor plasticity. However, many parts within this multi-step cascade are still poorly understood. Aside from intrinsic factors, cell fusion is particularly affected by extracellular conditions, including an inflammatory microenvironment, viruses, pH and ionic stress, hypoxia, and exosome signaling. Accordingly, the present review article will primarily highlight the influence of extracellular events that contribute to cell fusion in normal and tumorigenic tissues.

The origins and formation of bone-resorbing osteoclasts

Osteoclasts (OCLs) are hematopoietic cells whose physiological function is to degrade bone. OCLs are key players in the processes that determine and maintain the mass, shape, and physical properties of bone. OCLs adhere to bone tightly and degrade its matrix by secreting protons and proteases onto the underlying surface. The combination of low pH and proteases degrades the mineral and protein components of the matrix and forms a resorption pit; the degraded material is internalized by the cell and then secreted into the circulation. Insufficient or excessive activity of OCLs can lead to significant changes in bone and either cause or exacerbate symptoms of diseases, as in osteoporosis, osteopetrosis, and cancer-induced bone lysis. OCLs are derived from monocyte-macrophage precursor cells whose origins are in two distinct embryonic cell lineages - erythromyeloid progenitor cells of the yolk sac, and hematopoietic stem cells. OCLs are formed in a multi-stage process that is induced by the cytokines M-CSF and RANKL, during which the cells differentiate, fuse to form multi-nucleated cells, and then differentiate further to become mature, bone-resorbing OCLs. Recent studies indicate that OCLs can undergo fission in vivo to generate smaller cells, called "osteomorphs", that can be "re-cycled" by fusing with other cells to form new OCLs. In this review we describe OCLs and discuss their cellular origins and the cellular and molecular events that drive osteoclastogenesis.

An interplay between cellular growth and atypical fusion defines morphogenesis of a modular glial niche in Drosophila

Neural stem cells (NSCs) live in an intricate cellular microenvironment supporting their activity, the niche. Whilst shape and function are inseparable, the morphogenetic aspects of niche development are poorly understood. Here, we use the formation of a glial niche to investigate acquisition of architectural complexity. Cortex glia (CG) in Drosophila regulate neurogenesis and build a reticular structure around NSCs. We first show that individual CG cells grow tremendously to ensheath several NSC lineages, employing elaborate proliferative mechanisms which convert these cells into syncytia rich in cytoplasmic bridges. CG syncytia further undergo homotypic cell-cell fusion, using defined cell surface receptors and actin regulators. Cellular exchange is however dynamic in space and time. This atypical cell fusion remodels cellular borders, restructuring the CG syncytia. Ultimately, combined growth and fusion builds the multi-level architecture of the niche, and creates a modular, spatial partition of the NSC population. Our findings provide insights into how a niche forms and organises while developing intimate contacts with a stem cell population.

The Role of MSCs and Cell Fusion in Tissue Regeneration

Regenerative medicine is concerned with the investigation of therapeutic agents that can be used to promote the process of regeneration after injury or in different diseases. Mesenchymal stem/stromal cells (MSCs) and their secretome—including extracellular vesicles (EVs) are of great interest, due to their role in tissue regeneration, immunomodulatory capacity and low immunogenicity. So far, clinical studies are not very conclusive as they show conflicting efficacies regarding the use of MSCs. An additional process possibly involved in regeneration might be cell fusion. This process occurs in both a physiological and a pathophysiological context and can be affected by immune response due to inflammation. In this review the role of MSCs and cell fusion in tissue regeneration is discussed.

Hybrid Formation and Fusion of Cancer Cells In Vitro and In Vivo

The generation of cancer hybrid cells by intra-tumoral cell fusion opens new avenues for tumor plasticity to develop cancer stem cells with altered properties, to escape from immune surveillance, to change metastatic behavior, and to broaden drug responsiveness/resistance. Genomic instability and chromosomal rearrangements in bi- or multinucleated aneuploid cancer hybrid cells contribute to these new functions. However, the significance of cell fusion in tumorigenesis is controversial with respect to the low frequency of cancer cell fusion events and a clonal advantage of surviving cancer hybrid cells following a post-hybrid selection process. This review highlights alternative processes of cancer hybrid cell development such as entosis, emperipolesis, cannibalism, therapy-induced polyploidization/endoreduplication, horizontal or lateral gene transfer, and focusses on the predominant mechanisms of cell fusion. Based upon new properties of cancer hybrid cells the arising clinical consequences of the subsequent tumor heterogeneity after cancer cell fusion represent a major therapeutic challenge.

Cell-Cell Fusion and the Roads to Novel Properties of Tumor Hybrid Cells

The phenomenon of cancer cell–cell fusion is commonly associated with the origin of more malignant tumor cells exhibiting novel properties, such as increased drug resistance or an enhanced metastatic capacity. However, the whole process of cell–cell fusion is still not well understood and seems to be rather inefficient since only a certain number of (cancer) cells are capable of fusing and only a rather small population of fused tumor hybrids will survive at all. The low survivability of tumor hybrids is attributed to post-fusion processes, which are characterized by the random segregation of mixed parental chromosomes, the induction of aneuploidy and further random chromosomal aberrations and genetic/epigenetic alterations in daughter cells. As post-fusion processes also run in a unique manner in surviving tumor hybrids, the occurrence of novel properties could thus also be a random event, whereby it might be speculated that the tumor microenvironment and its spatial habitats could direct evolving tumor hybrids towards a specific phenotype.

Sorting Nexin 10 as a Key Regulator of Membrane Trafficking in Bone-Resorbing Osteoclasts: Lessons Learned From Osteopetrosis

Bone homeostasis is a complex, multi-step process, which is based primarily on a tightly orchestrated interplay between bone formation and bone resorption that is executed by osteoblasts and osteoclasts (OCLs), respectively. The essential physiological balance between these cells is maintained and controlled at multiple levels, ranging from regulated gene expression to endocrine signals, yet the underlying cellular and molecular mechanisms are still poorly understood. One approach for deciphering the mechanisms that regulate bone homeostasis is the characterization of relevant pathological states in which this balance is disturbed. In this article we describe one such “error of nature,” namely the development of acute recessive osteopetrosis (ARO) in humans that is caused by mutations in sorting nexin 10 (SNX10) that affect OCL functioning. We hypothesize here that, by virtue of its specific roles in vesicular trafficking, SNX10 serves as a key selective regulator of the composition of diverse membrane compartments in OCLs, thereby affecting critical processes in the sequence of events that link the plasma membrane with formation of the ruffled border and with extracellular acidification. As a result, SNX10 determines multiple features of these cells either directly or, as in regulation of cell-cell fusion, indirectly. This hypothesis is further supported by the similarities between the cellular defects observed in OCLs form various models of ARO, induced by mutations in SNX10 and in other genes, which suggest that mutations in the known ARO-associated genes act by disrupting the same plasma membrane-to-ruffled border axis, albeit to different degrees. In this article, we describe the population genetics and spread of the original arginine-to-glutamine mutation at position 51 (R51Q) in SNX10 in the Palestinian community. We further review recent studies, conducted in animal and cellular model systems, that highlight the essential roles of SNX10 in critical membrane functions in OCLs, and discuss possible future research directions that are needed for challenging or substantiating our hypothesis.

Cell fusion in cancer hallmarks: Current research status and future indications

Cell fusion is involved in several physiological processes, such as reproduction, development and immunity. Although cell fusion in tumors was reported 130 years ago, it has recently attracted great interest, with recent progress in tumorigenesis research. However, the role of cell fusion in tumor progression remains unclear. The pattern of cell fusion and its role under physiological conditions are the basis for our understanding of the pathological role of cell fusion. However, the role of cell fusion in tumors and its functions are complicated. Cell fusion can directly increase tumor heterogeneity by forming polyploids or aneuploidies. Several studies have reported that cell fusion is associated with tumorigenesis, metastasis, recurrence, drug resistance and the formation of cancer stem cells. Given the diverse roles cell fusion plays in different tumor phenotypes, methods based on targeted cell fusion have been designed to treat tumors. Research on cell fusion in tumors may provide novel ideas for further treatment.

An SNX10-dependent mechanism downregulates fusion between mature osteoclasts

Homozygosity for the R51Q mutation in sorting nexin 10 (SNX10) inactivates osteoclasts (OCLs) and induces autosomal recessive osteopetrosis in humans and in mice. We show here that the fusion of wild-type murine monocytes to form OCLs is highly regulated, and that its extent is limited by blocking fusion between mature OCLs. In contrast, monocytes from homozygous R51Q SNX10 mice fuse uncontrollably, forming giant dysfunctional OCLs that can become 10- to 100-fold larger than their wild-type counterparts. Furthermore, mutant OCLs display reduced endocytotic activity, suggesting that their deregulated fusion is due to alterations in membrane homeostasis caused by loss of SNX10 function. This is supported by the finding that the R51Q SNX10 protein is unstable and exhibits altered lipid-binding properties, and is consistent with a key role for SNX10 in vesicular trafficking. We propose that OCL size and functionality are regulated by a cell-autonomous SNX10-dependent mechanism that downregulates fusion between mature OCLs. The R51Q mutation abolishes this regulatory activity, leading to excessive fusion, loss of bone resorption capacity and, consequently, to an osteopetrotic phenotype in vivo.

This article has an associated First Person interview with the joint first authors of the paper.

Summary: Fusion of monocytes to become bone-resorbing osteoclasts is limited by an SNX10-dependent cell-autonomous mechanism. Loss of SNX10 function deregulates fusion and generates giant inactive osteoclasts.

Human cell polyploidization: The good and the evil

Therapeutic resistance represents a major cause of death for most lethal cancers. However, the underlying mechanisms of such resistance have remained unclear. The polyploid cells are due to an increase in DNA content, commonly associated with cell enlargement. In human, they play a variety of roles in physiology and pathologic conditions and perform the specialized functions during development, inflammation, and cancer. Recent work shows that cancer cells can be induced into polyploid giant cancer cells (PGCCs) that leads to reprogramming of surviving cancer cells to acquire resistance. In this article, we will review the polyploidy involved in development and inflammation, and the process of PGCCs formation and propagation that benefits to cell survival. We will discuss the potential opportunities in fighting resistant cancers. The increased knowledge of PGCCs will offer a completely new paradigm to explore the therapeutic intervention for lethal cancers.

Mammalian Cell Fusion Assays for the Study of Cell Cycle Progression by Functional Complementation

Cell cycle progression, or its arrest upon checkpoint activation, is directed by a complex array of cellular processes dependent on the diffusion of chemical signals. These signals regulate the onset of each cell cycle phase and prevent undesired phase transitions. Functional complementation is a robust strategy to identify such signals, by which mutant phenotypes are rescued through complementation with candidate factors. Here we describe a method that reclaims a five-decade old mammalian cell-cell fusion strategy of functional complementation to study the molecular control of cell cycle progression. The generation of cell-cell fusions (heterokaryons) allows for the analysis, via immunofluorescence, of cell cycle regulator dynamics and evaluating the effective rescue of cell cycle progression in specific genetic settings.

Formation of Aberrant Myotubes by Myoblasts Lacking Myosin VI Is Associated with Alterations in the Cytoskeleton Organization, Myoblast Adhesion and Fusion

We have previously postulated that unconventional myosin VI (MVI) could be involved in myoblast differentiation. Here, we addressed the mechanism(s) of its involvement using primary myoblast culture derived from the hindlimb muscles of Snell’s waltzer mice, the natural MVI knockouts (MVI-KO). We observed that MVI-KO myotubes were formed faster than control heterozygous myoblasts (MVI-WT), with a three-fold increase in the number of myosac-like myotubes with centrally positioned nuclei. There were also changes in the levels of the myogenic transcription factors Pax7, MyoD and myogenin. This was accompanied by changes in the actin cytoskeleton and adhesive structure organization. We observed significant decreases in the levels of proteins involved in focal contact formation, such as talin and focal adhesion kinase (FAK). Interestingly, the levels of proteins involved in intercellular communication, M-cadherin and drebrin, were also affected. Furthermore, time-dependent alterations in the levels of the key proteins for myoblast membrane fusion, myomaker and myomerger, without effect on their cellular localization, were observed. Our data indicate that in the absence of MVI, the mechanisms controlling cytoskeleton organization, as well as myoblast adhesion and fusion, are dysregulated, leading to the formation of aberrant myotubes.

Dynamin regulates the dynamics and mechanical strength of the actin cytoskeleton as a multifilament actin-bundling protein

The dynamin GTPase is known to bundle actin filaments, but the underlying molecular mechanism and physiological relevance remain unclear. Our genetic analyses revealed a function of dynamin in propelling invasive membrane protrusions during myoblast fusion in vivo. Using biochemistry, total internal reflection fluorescence microscopy, electron microscopy and cryo-electron tomography, we show that dynamin bundles actin while forming a helical structure. At its full capacity, each dynamin helix captures 12-16 actin filaments on the outer rim of the helix. GTP hydrolysis by dynamin triggers disassembly of fully assembled dynamin helices, releasing free dynamin dimers/tetramers and facilitating Arp2/3-mediated branched actin polymerization. The assembly/disassembly cycles of dynamin promote continuous actin bundling to generate mechanically stiff actin super-bundles. Super-resolution and immunogold platinum replica electron microscopy revealed dynamin along actin bundles at the fusogenic synapse. These findings implicate dynamin as a unique multifilament actin-bundling protein that regulates the dynamics and mechanical strength of the actin cytoskeletal network.

Reprogramming Müller Glia to Regenerate Retinal Neurons

In humans, various genetic defects or age-related diseases, such as diabetic retinopathies, glaucoma, and macular degeneration, cause the death of retinal neurons and profound vision loss. One approach to treating these diseases is to utilize stem and progenitor cells to replace neurons in situ, with the expectation that new neurons will create new synaptic circuits or integrate into existing ones. Reprogramming non-neuronal cells in vivo into stem or progenitor cells is one strategy for replacing lost neurons. Zebrafish have become a valuable model for investigating cellular reprogramming and retinal regeneration. This review summarizes our current knowledge regarding spontaneous reprogramming of Müller glia in zebrafish and compares this knowledge to research efforts directed toward reprogramming Müller glia in mammals. Intensive research using these animal models has revealed shared molecular mechanisms that make Müller glia attractive targets for cellular reprogramming and highlighted the potential for curing degenerative retinal diseases from intrinsic cellular sources.

Cell Fusion-Mediated Tissue Regeneration as an Inducer of Polyploidy and Aneuploidy

The biological phenomenon of cell fusion plays a crucial role in several physiological processes, including wound healing and tissue regeneration. Here, it is assumed that bone marrow-derived stem cells (BMSCs) could adopt the specific properties of a different organ by cell fusion, thereby restoring organ function. Cell fusion first results in the production of bi- or multinucleated hybrid cells, which either remain as heterokaryons or undergo ploidy reduction/heterokaryon-to-synkaryon transition (HST), thereby giving rise to mononucleated daughter cells. This process is characterized by a merging of the chromosomes from the previously discrete nuclei and their subsequent random segregation into daughter cells. Due to extra centrosomes concomitant with multipolar spindles, the ploidy reduction/HST could also be associated with chromosome missegregation and, hence, induction of aneuploidy, genomic instability, and even putative chromothripsis. However, while the majority of such hybrids die or become senescent, aneuploidy and genomic instability appear to be tolerated in hepatocytes, possibly for stress-related adaption processes. Likewise, cell fusion-induced aneuploidy and genomic instability could also lead to a malignant conversion of hybrid cells. This can occur during tissue regeneration mediated by BMSC fusion in chronically inflamed tissue, which is a cell fusion-friendly environment, but is also enriched for mutagenic reactive oxygen and nitrogen species.

Drosophila Myoblast Fusion: Invasion and Resistance for the Ultimate Union

Cell-cell fusion is indispensable for creating life and building syncytial tissues and organs. Ever since the discovery of cell-cell fusion, how cells join together to form zygotes and multinucleated syncytia has remained a fundamental question in cell and developmental biology. In the past two decades, Drosophila myoblast fusion has been used as a powerful genetic model to unravel mechanisms underlying cell-cell fusion in vivo. Many evolutionarily conserved fusion-promoting factors have been identified and so has a surprising and conserved cellular mechanism. In this review, we revisit key findings in Drosophila myoblast fusion and highlight the critical roles of cellular invasion and resistance in driving cell membrane fusion.

Tumor Microenvironment and Cell Fusion

Cell fusion is a highly regulated biological process that occurs under both physiological and pathological conditions. The cellular and extracellular environment is critical for the induction of the cell-cell fusion. Aberrant cell fusion is initiated during tumor progression. Tumor microenvironment is a complex dynamic system formed by the interaction between tumor cells and their surrounding cells. Cell-cell fusion mediates direct interaction between tumor cells and their surrounding cells and is associated with tumor initiation and progression. Various microenvironmental factors affect cell fusion in tumor microenvironment and generate hybrids that acquire genomes of both parental cells and exhibit novel characteristics, such as tumor stem cell-like properties, radioresistance, drug resistance, immune evasion, and enhanced migration and invasion abilities, which are closely related to the initiation, invasion, and metastasis of tumor. The phenotypic characteristics of hybrids are based on the phenotypes of parental cells, and the fusion of tumor cells with diverse types of microenvironmental fusogenic cells is concomitant with phenotypic heterogeneity. This review highlights the types of fusogenic cells in tumor microenvironment that can fuse with tumor cells and their specific significance and summarizes the various microenvironmental factors affecting tumor cell fusion. This review may be used as a reference to develop strategies for future research on tumor cell fusion and the exploration of cell fusion-based antitumor therapies.

Cytotoxic stress induces transfer of mitochondria-associated human endogenous retroviral RNA and proteins between cancer cells

About 8 % of the human genome consists of human endogenous retroviruses (HERVs), which are relicts of ancient exogenous retroviral infections incurred during evolution. Although the majority of HERVs have functional gene defects or epigenetic modifications, many of them are still able to produce retroviral proteins that have been proposed to be involved in cellular transformation and cancer development.

We found that, in chemo-resistant U87^RETO glioblastoma cells, cytotoxic stress induced by etoposide promotes accumulation and large-scale fission of mitochondria, associated with the detection of HERV-WE₁ (syncytin-1) and HERV-FRD₁ (syncytin-2) in these organelles. In addition, mitochondrial preparations also contained the corresponding receptors, i.e. ASCT2 and MFSD2. We clearly demonstrated that mitochondria associated with HERV-proteins were shuttled between adjacent cancer cells not only via tunneling tubes, but also by direct cellular uptake across the cell membrane. Furthermore, anti-syncytin-1 and anti-syncytin-2 antibodies were able to specifically block this direct cellular uptake of mitochondria even more than antibodies targeting the cognate receptors.

Here, we suggest that the association of mitochondria with syncytin-1/syncytin-2 together with their respective receptors could represent a novel mechanism of cell-to-cell transfer. In chemotherapy-refractory cancer cells, this might open up attractive avenues to novel mitochondria-targeting therapies.

DC-STAMP Is an Osteoclast Fusogen Engaged in Periodontal Bone Resorption

Dendritic cell-specific transmembrane protein (DC-STAMP) plays a key role in the induction of osteoclast (OC) cell fusion, as well as DC-mediated immune regulation. While DC-STAMP gene expression is upregulated in the gingival tissue with periodontitis, its pathophysiological roles in periodontitis remain unclear. To evaluate the effects of DC-STAMP in periodontitis, anti-DC-STAMP-monoclonal antibody (mAb) was tested in a mouse model of ligature-induced periodontitis ( n = 6-7/group) where Pasteurella pneumotropica ( Pp)-reactive immune response activated T cells to produce receptor activator of nuclear factor kappa-B ligand (RANKL), which, in turn, promotes the periodontal bone loss via upregulation of osteoclastogenesis. DC-STAMP was expressed on the cell surface of mature multinuclear OCs, as well as immature mononuclear OCs, in primary cultures of RANKL-stimulated bone marrow cells. Anti-DC-STAMP-mAb suppressed the emergence of large, but not small, multinuclear OCs, suggesting that DC-STAMP is engaged in the late stage of cell fusion. Anti-DC-STAMP-mAb also inhibited pit formation caused by RANKL-stimulated bone marrow cells. Attachment of ligature to a second maxillary molar induced DC-STAMP messenger RNA and protein, along with elevated tartrate-resistant acid phosphatase-positive (TRAP+) OCs and alveolar bone loss. As we expected, systemic administration of anti-DC-STAMP-mAb downregulated the ligature-induced alveolar bone loss. Importantly, local injection of anti-DC-STAMP-mAb also suppressed alveolar bone loss and reduced the total number of multinucleated TRAP+ cells in mice that received ligature attachment. Attachment of ligature induced significantly elevated tumor necrosis factor-α, interleukin-1β, and RANKL in the gingival tissue compared with the control site without ligature ( P < 0.05), which was unaffected by local injection with either anti-DC-STAMP-mAb or control-mAb. Neither in vivo anti- Pp IgG antibody nor in vitro anti- Pp T-cell response and resultant production of RANKL was affected by anti-DC-STAMP-mAb. This study illustrated the roles of DC-STAMP in promoting local OC cell fusion without affecting adaptive immune responses to oral bacteria. Therefore, it is plausible that a novel therapeutic regimen targeting DC-STAMP could suppress periodontal bone loss.

Report on the National Eye Institute Audacious Goals Initiative: Replacement of Retinal Ganglion Cells from Endogenous Cell Sources

This report emerges from a workshop convened by the National Eye Institute (NEI) as part of the "Audacious Goals Initiative" (AGI). The workshop addressed the replacement of retinal ganglion cells (RGCs) from exogenous and endogenous sources, and sought to identify the gaps in our knowledge and barriers to progress in devising cellular replacement therapies for diseases where RGCs die. Here, we briefly review relevant literature regarding common diseases associated with RGC death, the genesis of RGCs in vivo, strategies for generating transplantable RGCs in vitro, and potential endogenous cellular sources to regenerate these cells. These topics provided the clinical and scientific context for the discussion among the workshop participants and are relevant to efforts that may lead to therapeutic approaches for replacing RGCs. This report also summarizes the content of the workshop discussion, which focused on: (1) cell sources for RGC replacement and regeneration, (2) optimizing integration, survival, and synaptogenesis of new RGCs, and (3) approaches for assessing the outcomes of RGC replacement therapies. We conclude this report with a summary of recommendations, based on the workshop discussions, which may guide vision scientists seeking to develop therapies for replacing RGCs in humans.

Anoctamins/TMEM16 Proteins: Chloride Channels Flirting with Lipids and Extracellular Vesicles

Anoctamin (ANO)/TMEM16 proteins exhibit diverse functions in cells throughout the body and are implicated in several human diseases. Although the founding members ANO1 (TMEM16A) and ANO2 (TMEM16B) are Ca²⁺-activated Cl^- channels, most ANO paralogs are Ca²⁺-dependent phospholipid scramblases that serve as channels facilitating the movement (scrambling) of phospholipids between leaflets of the membrane bilayer. Phospholipid scrambling significantly alters the physical properties of the membrane and its landscape and has vast downstream signaling consequences. In particular, phosphatidylserine exposed on the external leaflet of the plasma membrane functions as a ligand for receptors vital for cell-cell communication. A major consequence of Ca²⁺-dependent scrambling is the release of extracellular vesicles that function as intercellular messengers by delivering signaling proteins and noncoding RNAs to alter target cell function. We discuss the physiological implications of Ca²⁺-dependent phospholipid scrambling, the extracellular vesicles associated with this activity, and the roles of ANOs in these processes.