Cell fusion in the brain: two cells forward, one cell back

The nuclei of human adult stem cells can move within the cell and generate cellular protrusions to contact other cells

BACKGROUND

The neuronal transdifferentiation of adult bone marrow cells (BMCs) is still considered an artifact based on an alternative explanation of experimental results supporting this phenomenon obtained over decades. However, recent studies have shown that following neural induction, BMCs enter an intermediate cellular state before adopting neural-like morphologies by active neurite extension and that binucleated BMCs can be formed independent of any cell fusion events. These findings provide evidence to reject the idea that BMC neural transdifferentiation is merely an experimental artifact. Therefore, understanding the intermediate states that cells pass through during transdifferentiation is crucial given their potential application in regenerative medicine and disease modelling.

METHODS

In this study, we examined the functional significance of the variety of morphologies and positioning that cell nuclei of human bone marrow-derived mesenchymal stem cells (hBM-MSCs) can adopt during neural-like differentiation using live-cell nuclear fluorescence labelling, time-lapse microscopy, and confocal microscopy analysis.

RESULTS

Here, we showed that after neural induction, hBM-MSCs enter an intermediate cellular state in which the nuclei are able to move within the cells, switching shapes and positioning and even generating cellular protrusions as they attempt to contact the cells around them. These findings suggest that changes in nuclear positioning occur because human cell nuclei somehow sense their environment. In addition, we showed the process of direct interactions between cell nuclei, which opens the possibility of a new level of intercellular interaction.

CONCLUSIONS

The present study advances the understanding of the intermediate stage through which hBM-MSCs pass during neural transdifferentiation, which may be crucial to understanding the mechanisms of these cell conversion processes and eventually harness them for use in regenerative medicine. Importantly, our study provides for the first time evidence that the nuclei of hBM-MSC-derived intermediate cells somehow sense their environment, generating cellular protrusions to contact other cells. In summary, human mesenchymal stromal cells could not only help to increase our understanding of the mechanisms underlying cellular plasticity but also facilitate the exact significance of nuclear positioning in cellular function and in tissue physiology.

hBcl2 overexpression in BMSCs enhances resistance to myelin debris-induced apoptosis and facilitates neuroprotection after spinal cord injury in rats

After spinal cord injury (SCI), the accumulation of myelin debris at the lesion exacerbates cell death and hinders axonal regeneration. Transplanted bone marrow mesenchymal stem cells (BMSCs) have been proven to be beneficial for SCI repair, but they are susceptible to apoptosis. It remains unclear whether this apoptotic process is influenced by myelin debris. Here, we constructed rat BMSCs overexpressing human B-cell lymphoma 2 (hBcl2) alone (hBcl2 group), BMSCs overexpressing hBcl2 with an endoplasmic reticulum-anchored segment (hBcl2-cb) (cb group), and a negative control group (NC group) for transplantation in this study. Immunocytochemistry staining validated the successful expression of hBcl2 in BMSCs within the hBcl2 group and cb group. All BMSCs from each group exhibited the ability to phagocytize myelin debris. Nevertheless, only BMSCs derived from the hBcl2 group exhibited heightened resistance to apoptosis and maintained prolonged viability for up to 5 days when exposed to myelin debris. Notably, overexpression of hBcl2 protein, rather than its endoplasmic reticulum-anchored counterpart, significantly enhanced the resistance of BMSCs against myelin debris-induced apoptosis. This process appeared to be associated with the efficient degradation of myelin debris through the Lamp1+ lysosomal pathway in the hBcl2 group. In vivo, the hBcl2 group exhibited significantly higher numbers of surviving cells and fewer apoptotic BMSCs compared to the cb and NC groups following transplantation. Furthermore, the hBcl2 group displayed reduced GFAP+ glial scarring and greater preservation of NF200+ axons in the lesions of SCI rats. Our results suggest that myelin debris triggers apoptosis in transplanted BMSCs, potentially elucidating the low survival rate of these cells after SCI. Consequently, the survival rate of transplanted BMSCs is improved by hBcl2 overexpression, leading to enhanced preservation of axons within the injured spinal cord.

Donor bone marrow-derived macrophage engraftment into the central nervous system of patients following allogeneic transplantation

Hematopoietic stem cell transplantation is a well-known treatment for hematologic malignancies, wherein nascent stem cells provide regenerating marrow and immunotherapy against the tumor. The progeny of hematopoietic stem cells also populate a wide spectrum of tissues, including the brain, as bone marrow-derived macrophages similar to microglial cells. We developed a sensitive and novel combined immunohistochemistry (IHC) and XY fluorescence in situ hybridization assay to detect, quantify, and characterize donor cells in the cerebral cortices of 19 female patients who underwent allogeneic stem cell transplantation. We showed that the number of male donor cells ranged from 0.14% to 3.0% of the total cells or from 1.2% to 25% of microglial cells. Using tyramide-based fluorescent IHC, we found that at least 80% of the donor cells expressed the microglial marker ionized calcium-binding adapter molecule-1, consistent with bone marrow-derived macrophages. The percentage of donor cells was related to pretransplantation conditioning; donor cells from radiation-based myeloablative cases averaged 8.1% of microglial cells, whereas those from nonmyeloablative cases averaged only 1.3%. The number of donor cells in patients conditioned with busulfan- or treosulfan-based myeloablation was similar to that in total body irradiation-based conditioning; donor cells averaged 6.8% of the microglial cells. Notably, patients who received multiple transplantations and those with the longest posttransplantation survival had the highest level of donor engraftment, with donor cells averaging 16.3% of the microglial cells. Our work represents the largest study characterizing bone marrow-derived macrophages in patients after transplantation. The efficiency of engraftment observed in our study warrants future research on microglial replacement as a therapeutic option for disorders of the central nervous system.

Long COVID as a Tauopathy: Of "Brain Fog" and "Fusogen Storms"

Long COVID, also called post-acute sequelae of SARS-CoV-2, is characterized by a multitude of lingering symptoms, including impaired cognition, that can last for many months. This symptom, often called "brain fog", affects the life quality of numerous individuals, increasing medical complications as well as healthcare expenditures. The etiopathogenesis of SARS-CoV-2-induced cognitive deficit is unclear, but the most likely cause is chronic inflammation maintained by a viral remnant thriving in select body reservoirs. These viral sanctuaries are likely comprised of fused, senescent cells, including microglia and astrocytes, that the pathogen can convert into neurotoxic phenotypes. Moreover, as the enteric nervous system contains neurons and glia, the virus likely lingers in the gastrointestinal tract as well, accounting for the intestinal symptoms of long COVID. Fusogens are proteins that can overcome the repulsive forces between cell membranes, allowing the virus to coalesce with host cells and enter the cytoplasm. In the intracellular compartment, the pathogen hijacks the actin cytoskeleton, fusing host cells with each other and engendering pathological syncytia. Cell-cell fusion enables the virus to infect the healthy neighboring cells. We surmise that syncytia formation drives cognitive impairment by facilitating the "seeding" of hyperphosphorylated Tau, documented in COVID-19. In our previous work, we hypothesized that the SARS-CoV-2 virus induces premature endothelial senescence, increasing the permeability of the intestinal and blood-brain barrier. This enables the migration of gastrointestinal tract microbes and/or their components into the host circulation, eventually reaching the brain where they may induce cognitive dysfunction. For example, translocated lipopolysaccharides or microbial DNA can induce Tau hyperphosphorylation, likely accounting for memory problems. In this perspective article, we examine the pathogenetic mechanisms and potential biomarkers of long COVID, including microbial cell-free DNA, interleukin 22, and phosphorylated Tau, as well as the beneficial effect of transcutaneous vagal nerve stimulation.

Binucleated human bone marrow-derived mesenchymal cells can be formed during neural-like differentiation with independence of any cell fusion events

Although it has been reported that bone marrow-derived cells (BMDCs) can transdifferentiate into neural cells, the findings are considered unlikely. It has been argued that the rapid neural transdifferentiation of BMDCs reported in culture studies is actually due to cytotoxic changes induced by the media. While transplantation studies indicated that BMDCs can form new neurons, it remains unclear whether the underlying mechanism is transdifferentiation or BMDCs-derived cell fusion with the existing neuronal cells. Cell fusion has been put forward to explain the presence of gene-marked binucleated neurons after gene-marked BMDCs transplantation. In the present study, we demostrated that human BMDCs can rapidly adopt a neural-like morphology through active neurite extension and binucleated human BMDCs can form with independence of any cell fusion events. We also showed that BMDCs neural-like differentiation involves the formation of intermediate cells which can then redifferentiate into neural-like cells, redifferentiate back to the mesenchymal fate or even repeatedly switch lineages without cell division. Furthermore, we have discovered that nuclei from intermediate cells rapidly move within the cell, adopting different morphologies and even forming binucleated cells. Therefore, our results provide a stronger basis for rejecting the idea that BMDCs neural transdifferentiation is merely an artefact.

Long COVID and the Neuroendocrinology of Microbial Translocation Outside the GI Tract: Some Treatment Strategies

Similar to previous pandemics, COVID-19 has been succeeded by well-documented post-infectious sequelae, including chronic fatigue, cough, shortness of breath, myalgia, and concentration difficulties, which may last 5 to 12 weeks or longer after the acute phase of illness. Both the psychological stress of SARS-CoV-2 infection and being diagnosed with COVID-19 can upregulate cortisol, a stress hormone that disrupts the efferocytosis effectors, macrophages, and natural killer cells, leading to the excessive accumulation of senescent cells and disruption of biological barriers. This has been well-established in cancer patients who often experience unrelenting fatigue as well as gut and blood–brain barrier dysfunction upon treatment with senescence-inducing radiation or chemotherapy. In our previous research from 2020 and 2021, we linked COVID-19 to myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) via angiotensin II upregulation, premature endothelial senescence, intestinal barrier dysfunction, and microbial translocation from the gastrointestinal tract into the systemic circulation. In 2021 and 2022, these hypotheses were validated and SARS-CoV-2-induced cellular senescence as well as microbial translocation were documented in both acute SARS-CoV-2 infection, long COVID, and ME/CFS, connecting intestinal barrier dysfunction to disabling fatigue and specific infectious events. The purpose of this narrative review is to summarize what is currently known about host immune responses to translocated gut microbes and how these responses relate to fatiguing illnesses, including long COVID. To accomplish this goal, we examine the role of intestinal and blood–brain barriers in long COVID and other illnesses typified by chronic fatigue, with a special emphasis on commensal microbes functioning as viral reservoirs. Furthermore, we discuss the role of SARS-CoV-2/Mycoplasma coinfection in dysfunctional efferocytosis, emphasizing some potential novel treatment strategies, including the use of senotherapeutic drugs, HMGB1 inhibitors, Toll-like receptor 4 (TLR4) blockers, and membrane lipid replacement.

Neuronal and Non-Neuronal GABA in COVID-19: Relevance for Psychiatry

Infection with SARS-CoV-2, the causative agent of the COVID-19 pandemic, originated in China and quickly spread across the globe. Despite tremendous economic and healthcare devastation, research on this virus has contributed to a better understanding of numerous molecular pathways, including those involving γ-aminobutyric acid (GABA), that will positively impact medical science, including neuropsychiatry, in the post-pandemic era. SARS-CoV-2 primarily enters the host cells through the renin–angiotensin system’s component named angiotensin-converting enzyme-2 (ACE-2). Among its many functions, this protein upregulates GABA, protecting not only the central nervous system but also the endothelia, the pancreas, and the gut microbiota. SARS-CoV-2 binding to ACE-2 usurps the neuronal and non-neuronal GABAergic systems, contributing to the high comorbidity of neuropsychiatric illness with gut dysbiosis and endothelial and metabolic dysfunctions. In this perspective article, we take a closer look at the pathology emerging from the viral hijacking of non-neuronal GABA and summarize potential interventions for restoring these systems.

Virus-Induced Membrane Fusion in Neurodegenerative Disorders

A growing body of epidemiological and research data has associated neurotropic viruses with accelerated brain aging and increased risk of neurodegenerative disorders. Many viruses replicate optimally in senescent cells, as they offer a hospitable microenvironment with persistently elevated cytosolic calcium, abundant intracellular iron, and low interferon type I. As cell-cell fusion is a major driver of cellular senescence, many viruses have developed the ability to promote this phenotype by forming syncytia. Cell-cell fusion is associated with immunosuppression mediated by phosphatidylserine externalization that enable viruses to evade host defenses. In hosts, virus-induced immune dysfunction and premature cellular senescence may predispose to neurodegenerative disorders. This concept is supported by novel studies that found postinfectious cognitive dysfunction in several viral illnesses, including human immunodeficiency virus-1, herpes simplex virus-1, and SARS-CoV-2. Virus-induced pathological syncytia may provide a unified framework for conceptualizing neuronal cell cycle reentry, aneuploidy, somatic mosaicism, viral spreading of pathological Tau and elimination of viable synapses and neurons by neurotoxic astrocytes and microglia. In this narrative review, we take a closer look at cell-cell fusion and vesicular merger in the pathogenesis of neurodegenerative disorders. We present a "decentralized" information processing model that conceptualizes neurodegeneration as a systemic illness, triggered by cytoskeletal pathology. We also discuss strategies for reversing cell-cell fusion, including, TMEM16F inhibitors, calcium channel blockers, senolytics, and tubulin stabilizing agents. Finally, going beyond neurodegeneration, we examine the potential benefit of harnessing fusion as a therapeutic strategy in regenerative medicine.

Obesity-linked circular RNA circTshz2-2 regulates the neuronal cell cycle and spatial memory in the brain

Metabolic syndromes, including obesity, cause neuropathophysiological changes in the brain, resulting in cognitive deficits. Only a few studies explored the contribution of non-coding genes in these pathophysiologies. Recently, we identified obesity-linked circular RNAs (circRNA) by analyzing the brain cortices of high-fat-fed obese mice. In this study, we scrutinized a conserved and neuron-specific circRNA, circTshz2-2, which affects neuronal cell cycle and spatial memory in the brain. Transcriptomic and cellular analysis indicated that circTshz2-2 dysregulation altered the expression of cell division-related genes and induced cell cycle arrest at the G2/M phase of the neuron. We found that circTshz2-2 bound to the YY1 transcriptional complex and suppressed Bdnf transcription. Suppression of circTshz2-2 increased BDNF expression and reduced G2/M checkpoint proteins such as Cyclin B2 and CDK1 through BDNF/TrkB signaling pathway, resulting in cell cycle arrest and neurite elongation. Inversely, overexpression of circTshz2-2 decreased BDNF expression, induced cell cycle proteins, and shortened the neurite length, indicating that circTshz2-2 regulates neuronal cell cycle and structure. Finally, we showed that circTshz2-2 affects spatial memory in wild-type and obese mice. Our data have revealed potential regulatory roles of obesity-related circTshz2-2 on the neuronal cell cycle and memory function providing a novel link between metabolic syndromes and cognitive deficits.

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.

Genome Editing and the Problem of Tetraploidy in Cell Modeling of the Genetic Form of Parkinsonism

The prevalent form of familial parkinsonism is caused by mutations in the LRRK2 gene encoding for the mitochondrial protein kinase. In the review, we discuss possible causes of appearance of tetraploid cells in neuronal precursors obtained from induced pluripotent stem cells from patients with the LRRK2-associated form of parkinsonism after genome editing procedure. As LRRK2 protein participates in cell proliferation and maintenance of the nuclear envelope, spindle fibers, and cytoskeleton, mutations in the LRRK2 gene can affect protein functions and lead, via various mechanisms, to the mitotic machinery disintegration and chromosomal aberration. These abnormalities can appear at different stages of fibroblast reprogramming; therefore, editing of the LRRK2 nucleotide sequence should be done during or before the reprogramming stage.

Breast tumor cell hybrids form spontaneously in vivo and contribute to breast tumor metastases

Cancer cell fusion was suggested as a mechanism of metastasis about a century ago. Since then, many additional modes of material transfer (i.e., tunneling nanotubes, and exosomes) to generate cell hybrids have been identified. However, studies documenting spontaneous tumor hybrid formation in vivo as a mechanism that enables metastasis are still lacking. Here, we tested whether spontaneous hybrid formation in vivo contributes to bona fide metastatic tumors. We first used single cell RNASeq to analyze the gene expression profile of spontaneously formed cancer cell-stromal hybrids, and results revealed that hybrids exhibit a clustering pattern that is distinct from either parental cell and suggestive of substantial diversity of individual hybrids. Despite the newly gained diversity, hybrids can retain expression of critical genes of each parental cell. To assess the biological impact of cancer cell hybrids in vivo, we transfected murine mammary tumor cells, isolated from FVB/N-Tg(MMTV-PyVT)634Mul/J mice (PyVT) with Cre recombinase prior to injection to the murine fat pad of FVB.129S6(B6)-Gt(ROSA)26Sor^tm1(Luc)Kael/J mice such that luciferase expression is induced with hybrid formation; luciferase expression was tracked for up to four months. We observed that hybrid formation occurs spontaneously in vivo and that a significantly higher number of hybrids reside in metastases compared to the primary tumor, supporting the possibility that hybrids can emerge from the primary tumor and proliferate to help create a new tumor at a distant site. Additional studies are now warranted to delineate the mechanisms of cancer cell hybrid transit to metastases since drugs to inhibit hybrid formation might prevent metastatic spread.

Aberrant cerebellar Purkinje cell function repaired in vivo by fusion with infiltrating bone marrow-derived cells

Bone marrow-derived cells are known to infiltrate the adult brain and fuse with cerebellar Purkinje cells. Histological observations that such heterotypic cell fusion events are substantially more frequent following cerebellar injury suggest they could have a role in the protection of mature brain neurons. To date, the possibility that cell fusion can preserve or restore the structure and function of adult brain neurons has not been directly addressed; indeed, though frequently suggested, the possibility of benefit has always been rather speculative. Here we report, for the first time, that fusion of a bone marrow-derived cell with a neuron in vivo, in the mature brain, results in the formation of a spontaneously firing neuron. Notably, we also provide evidence supporting the concept that heterotypic cell fusion acts as a biological mechanism to repair pathological changes in Purkinje cell structure and electrophysiology. We induced chronic central nervous system inflammation in chimeric mice expressing bone marrow cells tagged with enhanced green fluorescent protein. Subsequent in-depth histological analysis revealed significant Purkinje cell injury. In addition, there was an increased incidence of cell fusion between bone marrow-derived cells and Purkinje cells, revealed as enhanced green fluorescent protein-expressing binucleate heterokaryons. These fused cells resembled healthy Purkinje cells in their morphology, soma size, ability to synthesize the neurotransmitter gamma-aminobutyric acid, and synaptic innervation from neighbouring cells. Extracellular recording of spontaneous firing ex vivo revealed a shift in the predominant mode of firing of non-fused Purkinje cells in the context of cerebellar inflammation. By contrast, the firing patterns of fused Purkinje cells were the same as in healthy control cerebellum, indicating that fusion of bone marrow-derived cells with Purkinje cells mitigated the effects of cell injury on electrical activity. Together, our histological and electrophysiological results provide novel fundamental insights into physiological processes by which nerve cells are protected in adult life.

Bone marrow transplantation stimulates neural repair in Friedreich's ataxia mice

OBJECTIVE

Friedreich's ataxia is an incurable inherited neurological disease caused by frataxin deficiency. Here, we report the neuroreparative effects of myeloablative allogeneic bone marrow transplantation in a humanized murine model of the disease.

METHODS

Mice received a transplant of fluorescently tagged sex-mismatched bone marrow cells expressing wild-type frataxin and were assessed at monthly intervals using a range of behavioral motor performance tests. At 6 months post-transplant, mice were euthanized for protein and histological analysis. In an attempt to augment numbers of bone marrow-derived cells integrating within the nervous system and improve therapeutic efficacy, a subgroup of transplanted mice also received monthly subcutaneous infusions of the cytokines granulocyte-colony stimulating factor and stem cell factor.

RESULTS

Transplantation caused improvements in several indicators of motor coordination and locomotor activity. Elevations in frataxin levels and antioxidant defenses were detected. Abrogation of disease pathology throughout the nervous system was apparent, together with extensive integration of bone marrow-derived cells in areas of nervous tissue injury that contributed genetic material to mature neurons, satellite-like cells, and myelinating Schwann cells by processes including cell fusion. Elevations in circulating bone marrow-derived cell numbers were detected after cytokine administration and were associated with increased frequencies of Purkinje cell fusion and bone marrow-derived dorsal root ganglion satellite-like cells. Further improvements in motor coordination and activity were evident.

INTERPRETATION

Our data provide proof of concept of gene replacement therapy, via allogeneic bone marrow transplantation, that reverses neurological features of Friedreich's ataxia with the potential for rapid clinical translation. Ann Neurol 2018;83:779-793.

Injury Leads to the Appearance of Cells with Characteristics of Both Microglia and Astrocytes in Mouse and Human Brain

Microglia and astrocytes have been considered until now as cells with very distinct identities. Here, we assessed the heterogeneity within microglia/monocyte cell population in mouse hippocampus and determined their response to injury, by using single-cell gene expression profiling of cells isolated from uninjured and deafferented hippocampus. We found that in individual cells, microglial markers Cx3cr1, Aif1, Itgam, and Cd68 were co-expressed. Interestingly, injury led to the co-expression of the astrocyte marker Gfap in a subpopulation of Cx3cr1-expressing cells from both the injured and contralesional hippocampus. Cells co-expressing astrocyte and microglia markers were also detected in the in vitro LPS activation/injury model and in sections from human brain affected by stroke, Alzheimer's disease, and Lewy body dementia. Our findings indicate that injury and chronic neurodegeneration lead to the appearance of cells that share molecular characteristics of both microglia and astrocytes, 2 cell types with distinct embryologic origin and function.

Hypoxia Enhances Fusion of Oral Squamous Carcinoma Cells and Epithelial Cells Partly via the Epithelial-Mesenchymal Transition of Epithelial Cells

Increasing evidence and indications showed that cell fusion is crucial in tumor development and metastasis, and hypoxia, a closely linked factor to tumor microenvironment, which can lead to EMT, induces angiogenesis and metastasis in tumor growth. However, the relationship between hypoxia and fusion has not been reported yet. EMT will change some proteins in the epithelial cell surface and the changes of proteins in cell surface may increase cell fusion. This study found that hypoxia promotes the spontaneous cell fusion between Oral Squamous Carcinoma Cells (OSCCs) and Human Immortalized Oral Epithelial Cells (HIOECs). At the same time, Hypoxia can lead to EMT, and hypoxia-pretreated HIOECs increased fusion rate with OSCC, while the fusion rate was significantly reduced by DAPT, a kind of EMT blocker. Therefore, epithelial cells can increase spontaneously cell fusion with OSCC by EMT. Our study may provide a new insight to link among tumor microenvironment, cell fusion, and cancer.

Cell-based therapeutic strategies for multiple sclerosis

The availability of multiple disease-modifying medications with regulatory approval to treat multiple sclerosis illustrates the substantial progress made in therapy of the disease. However, all are only partially effective in preventing inflammatory tissue damage in the central nervous system and none directly promotes repair. Cell-based therapies, including immunoablation followed by autologous haematopoietic stem cell transplantation, mesenchymal and related stem cell transplantation, pharmacologic manipulation of endogenous stem cells to enhance their reparative capabilities, and transplantation of oligodendrocyte progenitor cells, have generated substantial interest as novel therapeutic strategies for immune modulation, neuroprotection, or repair of the damaged central nervous system in multiple sclerosis. Each approach has potential advantages but also safety concerns and unresolved questions. Moreover, clinical trials of cell-based therapies present several unique methodological and ethical issues. We summarize here the status of cell-based therapies to treat multiple sclerosis and make consensus recommendations for future research and clinical trials.

Cerebellar Dysfunction in Multiple Sclerosis

Multiple sclerosis (MS) commonly affects the cerebellum causing acute and chronic symptoms. Cerebellar signs contribute significantly to clinical disability, and symptoms such as tremor, ataxia, and dysarthria are particularly difficult to treat. Increasing knowledge concerning the pathophysiology of cerebellar disease in MS from human postmortem studies, experimental models, and clinical trials has raised the hope that cerebellar symptoms will be better treated in the future.

Up-regulation of syncytin-1 contributes to TNF-α-enhanced fusion between OSCC and HUVECs partly via Wnt/β-catenin-dependent pathway

Accumulating evidence implies that cell fusion is one of the driving forces of cancer invasion and metastasis. However, considerably less is still known about the triggering factors and underlying mechanisms associated with cancer-host cell fusion, particularly in inflammatory tumor microenvironment. In this study, we confirmed that inflammatory factor TNF-α could enhance fusion between squamous cell carcinoma cells 9 (SCC-9) and human umbilical vein endothelial cells (HUVEC). Further study revealed that TNF-α could promote up-regulation of syncytin-1 in SCC-9 and its receptor neutral amino acid transporter type 2 (ASCT-2) in HUVEC. Syncytin-1 acted as an important downstream effector in TNF-α-enhanced cancer-endothelial cell fusion. TNF-α treatment also led to the activation of Wnt/β-catenin signal pathway in SCC-9. The activation of Wnt/β-catenin signal pathway was closely associated with the up-regulation of syncytin-1 in SCC-9 and increased fusion between SCC-9 and HUVEC while blocking of Wnt/β-catenin signal pathway resulted in the corresponding down-regulation of syncytin-1 accompanied by sharp decrease of cancer-endothelial cell fusion. Taking together, our results suggest that Wnt/β-catenin signal pathway activation-dependent up-regulation of syncytin-1 contributes to the pro-inflammatory factor TNF-α-enhanced fusion between oral squamous cell carcinoma cells and endothelial cells.

Mechanisms and consequences of aneuploidy and chromosome instability in the aging brain

Aneuploidy and polyploidy are a form of Genomic Instability (GIN) known as Chromosomal Instability (CIN) characterized by sporadic abnormalities in chromosome copy numbers. Aneuploidy is commonly linked to pathological states. It is a hallmark of spontaneous abortions and birth defects and it is observed virtually in every human tumor, therefore being generally regarded as detrimental for the development or the maturation of tissues under physiological conditions. Polyploidy however, occurs as part of normal physiological processes during maturation and differentiation of some mammalian cell types. Surprisingly, high levels of aneuploidy are present in the brain, and their frequency increases with age suggesting that the brain is able to maintain its functionality in the presence of high levels of mosaic aneuploidy. Because somatic aneuploidy with age can reach exceptionally high levels, it is likely to have long-term adverse effects in this organ. We describe the mechanisms accountable for an abnormal DNA content with a particular emphasis on the CNS where cell division is limited. Next, we briefly summarize the types of GIN known to date and discuss how they interconnect with CIN. Lastly we highlight how several forms of CIN may contribute to genetic variation, tissue degeneration and disease in the CNS.

Functional Mitochondria in Health and Disease

The ability to rapidly adapt cellular bioenergetic capabilities to meet rapidly changing environmental conditions is mandatory for normal cellular function and for cancer progression. Any loss of this adaptive response has the potential to compromise cellular function and render the cell more susceptible to external stressors such as oxidative stress, radiation, chemotherapeutic drugs, and hypoxia. Mitochondria play a vital role in bioenergetic and biosynthetic pathways and can rapidly adjust to meet the metabolic needs of the cell. Increased demand is met by mitochondrial biogenesis and fusion of individual mitochondria into dynamic networks, whereas a decrease in demand results in the removal of superfluous mitochondria through fission and mitophagy. Effective communication between nucleus and mitochondria (mito-nuclear cross talk), involving the generation of different mitochondrial stress signals as well as the nuclear stress response pathways to deal with these stressors, maintains bioenergetic homeostasis under most conditions. However, when mitochondrial DNA (mtDNA) mutations accumulate and mito-nuclear cross talk falters, mitochondria fail to deliver critical functional outputs. Mutations in mtDNA have been implicated in neuromuscular and neurodegenerative mitochondriopathies and complex diseases such as diabetes, cardiovascular diseases, gastrointestinal disorders, skin disorders, aging, and cancer. In some cases, drastic measures such as acquisition of new mitochondria from donor cells occurs to ensure cell survival. This review starts with a brief discussion of the evolutionary origin of mitochondria and summarizes how mutations in mtDNA lead to mitochondriopathies and other degenerative diseases. Mito-nuclear cross talk, including various stress signals generated by mitochondria and corresponding stress response pathways activated by the nucleus are summarized. We also introduce and discuss a small family of recently discovered hormone-like mitopeptides that modulate body metabolism. Under conditions of severe mitochondrial stress, mitochondria have been shown to traffic between cells, replacing mitochondria in cells with damaged and malfunctional mtDNA. Understanding the processes involved in cellular bioenergetics and metabolic adaptation has the potential to generate new knowledge that will lead to improved treatment of many of the metabolic, degenerative, and age-related inflammatory diseases that characterize modern societies.

Retinal transplantation of photoreceptors results in donor-host cytoplasmic exchange

Pre-clinical studies provided evidence for successful photoreceptor cell replacement therapy. Migration and integration of donor photoreceptors into the retina has been proposed as the underlying mechanism for restored visual function. Here we reveal that donor photoreceptors do not structurally integrate into the retinal tissue but instead reside between the photoreceptor layer and the retinal pigment epithelium, the so-called sub-retinal space, and exchange intracellular material with host photoreceptors. By combining single-cell analysis, Cre/lox technology and independent labelling of the cytoplasm and nucleus, we reliably track allogeneic transplants demonstrating cellular content transfer between graft and host photoreceptors without nuclear translocation. Our results contradict the common view that transplanted photoreceptors migrate and integrate into the photoreceptor layer of recipients and therefore imply a re-interpretation of previous photoreceptor transplantation studies. Furthermore, the observed interaction of donor with host photoreceptors may represent an unexpected mechanism for the treatment of blinding diseases in future cell therapy approaches.

Donor and host photoreceptors engage in material transfer following transplantation of post-mitotic photoreceptor precursors

Photoreceptor replacement by transplantation is proposed as a treatment for blindness. Transplantation of healthy photoreceptor precursor cells into diseased murine eyes leads to the presence of functional photoreceptors within host retinae that express an array of donor-specific proteins. The resulting improvement in visual function was understood to be due to donor cells integrating within host retinae. Here, however, we show that while integration occurs the majority of donor-reporter-labelled cells in the host arises as a result of material transfer between donor and host photoreceptors. Material transfer does not involve permanent donor–host nuclear or cell–cell fusion, or the uptake of free protein or nucleic acid from the extracellular environment. Instead, RNA and/or protein are exchanged between donor and host cells in vivo. These data require a re-evaluation of the mechanisms underlying rescue by photoreceptor transplantation and raise the possibility of material transfer as a strategy for the treatment of retinal disorders.

Transplantation of healthy photoreceptor cells has been shown to rescue blindness. Here, the authors show that rather than donor cells integrating into the host retina, the predominant mechanism underlying this rescue involves exchange of cytoplasmic material between donor and host cells in vivo.

Cell-cell fusion in the nervous system: Alternative mechanisms of development, injury, and repair

Over a century ago, the seminal work of Ramón y Cajal revealed that the nervous system is made of individual units, the neurons, which are related to each other by contiguity rather than continuity. This view overturned the idea that the nervous system was a reticulum of fibers, a rete diffusa nervosa, as proposed and defined by Camillo Golgi. Although the neuron theory has been widely confirmed in every model system studied and constitutes the basis of modern neuroscience, evidence accumulated over the years suggests that neurons, similar to other types of cells, have the potential to fuse their membranes and undergo cell-cell fusion under certain conditions. This concept adds a substantial layer to our view of the nervous system and how it functions. Here, we bring together past and more recent discoveries on multiple aspects of neuronal fusion, discussing how this cellular event is generated, and what consequences it has for our understanding of nervous system development, disease, injury, and repair.

Emerging therapies for mitochondrial disorders

Mitochondrial disorders are a diverse group of debilitating conditions resulting from nuclear and mitochondrial DNA mutations that affect multiple organs, often including the central and peripheral nervous system. Despite major advances in our understanding of the molecular mechanisms, effective treatments have not been forthcoming. For over five decades patients have been treated with different vitamins, co-factors and nutritional supplements, but with no proven benefit. There is therefore a clear need for a new approach. Several new strategies have been proposed acting at the molecular or cellular level. Whilst many show promise in vitro, the clinical potential of some is questionable. Here we critically appraise the most promising preclinical developments, placing the greatest emphasis on diseases caused by mitochondrial DNA mutations. With new animal and cellular models, longitudinal deep phenotyping in large patient cohorts, and growing interest from the pharmaceutical industry, the field is poised to make a breakthrough.

Long-Term Outcome of Administration of c-kit(POS) Cardiac Progenitor Cells After Acute Myocardial Infarction: Transplanted Cells Do not Become Cardiomyocytes, but Structural and Functional Improvement and Proliferation of Endogenous Cells Persist for at Least One Year

RATIONALE

Cardiac progenitor cells (CPCs) improve left ventricular remodeling and function after acute or chronic myocardial infarction. However, the long-term (>5 weeks) effects, potential tumorigenicity, and fate of transplanted CPCs are unknown.

OBJECTIVE

To assess the outcome of CPC therapy at 1 year.

METHODS AND RESULTS

Female rats underwent a 90-minute coronary occlusion; 4 hours after reperfusion, they received intracoronarily vehicle or 1 million male, syngeneic CPCs. One year later, CPC-treated rats exhibited smaller scars and more viable myocardium in the risk region, along with improved left ventricular remodeling and regional and global left ventricular function. No tumors were observed. Some transplanted (Y-chromosome(POS)) CPCs (or their progeny) persisted and continued to proliferate, but they failed to acquire a mature cardiomyocyte phenotype and were too few (4-8% of nuclei) to account for the benefits of CPC therapy. Surprisingly, CPC transplantation triggered a prolonged proliferative response of endogenous cells, resulting in increased formation of endothelial cells and Y-chromosome(NEG) CPCs for 12 months and increased formation, for at least 7 months, of small cells that expressed cardiomyocytic proteins (α-sarcomeric actin) but did not have a mature cardiomyocyte phenotype.

CONCLUSIONS

The beneficial effects of CPCs on left ventricular remodeling and dysfunction are sustained for at least 1 year and thus are likely to be permanent. Because transplanted CPCs do not differentiate into mature myocytes, their major mechanism of action must involve paracrine actions. These paracrine mechanisms could be very prolonged because some CPCs engraft, proliferate, and persist at 1 year. This is the first report that transplantation of any cell type in the heart induces a proliferative response that lasts at least 1 year. The results strongly support the safety and clinical utility of CPC therapy.

Apoptosis-induced cancer cell fusion: a mechanism of breast cancer metastasis

Although cancer cell fusion has been suggested as a mechanism of cancer metastasis, the underlying mechanisms defining this process are poorly understood. In a recent study, apoptotic cells were newly identified as a type of cue that induces signaling via phosphatidylserine receptors to promote fusion of myoblasts. The microenvironment of breast tumors is often hypoxic, and because apoptosis is greatly increased in hypoxic conditions, we decided to investigate whether the mechanism of breast cancer cell fusion with mesenchymal stem/multipotent stromal cells (MSCs) involves apoptosis. We used a powerful tool for identification and tracking of hybrids based on bimolecular fluorescence complementation (BiFC) and found that breast cancer cells fused spontaneously with MSCs. This fusion was significantly enhanced with hypoxia and signaling associated with apoptotic cells, especially between nonmetastatic breast cancer cells and MSCs. In addition, the hybrids showed a significantly higher migratory capacity than did the parent cells. Taken together, these findings describe a mechanism by which hypoxia-induced apoptosis stimulates fusion between MSCs and breast tumor cells resulting in hybrids with an enhanced migratory capacity that may enable their dissemination to distant sites or metastases. In the long run, this study may provide new strategies for developing novel drugs for preventing cancer metastasis.

Purkinje Cell Pathology and Loss in Multiple Sclerosis Cerebellum

Cerebellar ataxia commonly occurs in multiple sclerosis, particularly in chronic progressive disease. Previous reports have highlighted both white matter and grey matter pathological changes within the cerebellum; and demyelination and inflammatory cell infiltrates appear commonly. As Purkinje cell axons are the sole output of the cerebellar cortex, understanding pathologic processes within these cells is crucial to develop strategies to prevent their loss and thus reduce ataxia. We studied pathologic changes occurring within Purkinje cells of the cerebellum. Using immunohistochemic techniques, we found changes in neurofilament phosphorylation states within Purkinje cells, including loss of dephosphorylated neurofilament and increased phosphorylated and hyperphosphorylated neurofilament. We also found Purkinje axonal spheroids and Purkinje cell loss, both of which occurred predominantly within areas of leucocortical demyelination within the cerebellar cortex. These changes have important implications for the study of cerebellar involvement in multiple sclerosis and may help design therapies to reduce the burden of ataxia in the condition.