Fusion of hematopoietic cells with Purkinje neurons does not lead to stable heterokaryon formation under noninvasive conditions

Cellular component transfer between photoreceptor cells of the retina

Photoreceptor transplantation is a potential therapeutic strategy for degenerative retinal diseases. Studies on mechanisms contributing to retinal regeneration and vision repair identified cellular components transfer (CCT) as playing a role, in addition to somatic augmentation (referred to as "cell replacement" in this paper). In CCT, donor photoreceptors shuttle proteins, RNA, and mitochondria to host photoreceptors through intercellular connections. The discovery of CCT in the transplantation context triggered a re-interpretation of prior transplantation studies that generally did not include specific CCT assays and thereby broadly emphasized the cell replacement model, reflecting the prevailing understanding of retinal transplantation biology at that time. In addition to clarifying our understanding of photoreceptor biology, CCT has raised the possibility of developing treatments to replenish molecular deficiencies in diseased photoreceptor cells. As the CCT field evolves, investigators have used diverse terminology, and implemented different CCT assays following transplantation in animal models. The non-standardized terminology of CCT and absent minimal assay standards for detection can hinder communication between investigators and comparison between studies. In this review, we discuss the current understanding of CCT, provide an overview of transplantation and regeneration studies in small and large animals, and propose terminology and a minimal assay standard for CCT. Further research on CCT may eventually provide new avenues to treat a range of hereditary and acquired retinopathies while illuminating mechanisms of cell-cell interaction in the retina.

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.

Differentiation Capacity of Bone Marrow-Derived Rat Mesenchymal Stem Cells from DsRed and Cre Transgenic Cre/loxP Models

Cre/loxP recombination is a well-established technique increasingly used for modifying DNA both in vitro and in vivo. Nucleotide alterations can be edited in the genomes of mammalian cells, and genetic switches can be designed to target the expression or excision of a gene in any tissue at any time in animal models. In this study, we propose a system which worked via the Cre/loxP switch gene and DsRed/emGFP dual-color fluorescence imaging. Mesenchymal stem cells (MSCs) can be used to regenerate damaged tissue because of their differentiation capacity. Although previous studies have presented evidence of fusion of transplanted MSCs with recipient cells, the possibility of fusion in such cases remains debated. Moreover, the effects and biological implications of the fusion of MSCs at the tissue and organ level have not yet been elucidated. Thus, the method for determining this issue is significant and the models we proposed can illustrate the question. However, the transgenic rats exhibited growth slower than that of wild-type rats over several weeks. The effects on the stemness, proliferation, cell cycle, and differentiation ability of bone marrow–derived rat MSCs (BM-rMSCs) from the models were examined to ensure our design was appropriate for the in vivo application. We demonstrated that MSC surface markers were maintained in DsRed and Cre transgenic rMSCs (DsRed-rMSCs and Cre-rMSCs, respectively). A WST-8 assay revealed decreased proliferative activity in these DsRed-rMSCs and Cre-rMSCs; this result was validated through cell counting. Furthermore, cell cycle analysis indicated a decrease in the proportion of G1-phase cells and a concomitant increase in the proportion of S-phase cells. The levels of cell cycle–related proteins also decreased in the DsRed-rMSCs and Cre-rMSCs, implying decelerated phase transition. However, the BM-rMSCs collected from the transgenic rats did not exhibit altered adipogenesis, osteogenesis, or chondrogenesis. The specific markers of these types of differentiation were upregulated after induction. Therefore, BM-rMSCs from DsRed and Cre transgenic models can be used to investigate the behavior of MSCs and related mechanisms. Such application may further the development of stem cell therapy for tissue damage and other diseases.

Binucleated and Multinucleated Neurons are Formed by Fusion

In the era of molecular biology and atomic force microscopy, some important macroscopic issues such as simultaneous bidirectional axonal flow or neuronal multinucleosis remain unaddressed. However, these issues have to be addressed, because they distort the results of our current achievements. Using videorecording technique, we studied adhesive contacts between neurons and their processes and kinetics of anastomosis retraction between the cell bodies up to their complete fusion with introduction of neurites into the cell cytoplasm and formation of binuclear cells. Three proofs refuting the mechanism of binuclearity formation by amitosis are presented. Live trinuclear neurons without signs of amitotic division were identified. Electron microscopy showed that fusion of many living neurons into one simplest during centrifugation of isolated cells.

Neuronal activity triggers uptake of hematopoietic extracellular vesicles in vivo

Communication with the hematopoietic system is a vital component of regulating brain function in health and disease. Traditionally, the major routes considered for this neuroimmune communication are by individual molecules such as cytokines carried by blood, by neural transmission, or, in more severe pathologies, by the entry of peripheral immune cells into the brain. In addition, functional mRNA from peripheral blood can be directly transferred to neurons via extracellular vesicles (EVs), but the parameters that determine their uptake are unknown. Using varied animal models that stimulate neuronal activity by peripheral inflammation, optogenetics, and selective proteasome inhibition of dopaminergic (DA) neurons, we show that the transfer of EVs from blood is triggered by neuronal activity in vivo. Importantly, this transfer occurs not only in pathological stimulation but also by neuronal activation caused by the physiological stimulus of novel object placement. This discovery suggests a continuous role of EVs under pathological conditions as well as during routine cognitive tasks in the healthy brain.

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 improves motor activity in a mouse model of ataxia

Ataxias are locomotor disorders that can have an origin both neural and muscular, although both impairments are related. Unfortunately, ataxia has no cure, and the current therapies are aimed at motor re-education or muscular reinforcement. Nevertheless, cell therapy is becoming a promising approach to deal with incurable neural diseases, including neuromuscular ataxias. Here, we have used a model of ataxia, the Purkinje Cell Degeneration (PCD) mutant mouse, to study the effect of healthy (wild-type) bone marrow transplantation on the restoration of defective mobility. Bone marrow transplants (from both mutant and healthy donors) were performed in wild-type and PCD mice. Then, a wide battery of behavioural tests was employed to determine possible motor amelioration in mutants. Finally, cerebellum, spinal cord, and muscle were analysed to study the integration of the transplant-derived cells and the origin of the behavioural changes. Our results demonstrated that the transplant of wild-type bone marrow restores the mobility of PCD mice, increasing their capabilities of movement (52-100% of recovery), exploration (20-71% of recovery), speed (35% of recovery), and motor coordination (25% of recovery). Surprisingly, our results showed that bone marrow transplant notably improves the skeletal muscle structure, which is severely damaged in the mutants, rather than ameliorating the central nervous system. Although a multimodal effect of the transplant is not discarded, muscular improvements appear to be the basis of this motor recovery. Furthermore, the results from our study indicate that bone marrow stem cell therapy can be a safe and effective alternative for dealing with movement disorders such as ataxias.

Neurotransplantation therapy

Neurotransplantation may be a promising approach for therapy of cerebellar diseases characterized by a substantial loss of neurons. Neurotransplantation could rescue neurons from degeneration and maintain cerebellar reserve, facilitate cerebellar compensation, or help reconstruct damaged neural circuits by cell substitution. These mechanisms of action can be of varying importance according to the type of cerebellar disease. Neurotransplantation therapy in cerebellar ataxias is still at the stage of experimental studies. There is currently little knowledge regarding cerebellar patients. Nevertheless, data provided by experiments in animal models of cerebellar degeneration and both clinical studies and experiences in patients with other neurologic diseases enable us to suggest basic principles, expectations, limitations, and future directions of neurotransplantation therapy for cerebellar diseases.

The contribution of bone marrow-derived cells to the human adipocyte pool

White adipose tissue is a remarkably expandable organ with results in the last decade showing that human white adipocytes are continuously turned over during the entire life-span. Data primarily in murine models have demonstrated that adipocytes are derived from precursors present mainly in the perivascular areas of adipose tissue but their precise origin remains unclear. Subsets of cells present in bone marrow display a multipotent differentiation capacity which has prompted the hypothesis that bone marrow-derived cells (BMDCs) may also contribute to the adipocyte pool present in peripheral fat depots. This notion was initially demonstrated in a murine transplantation model, however, subsequent animal studies have been conflicting resulting in a debate of whether BMDCs actually differentiate into adipocytes or just fuse with resident fat cells. This controversy was recently resolved in 2 studies of human subjects undergoing bone marrow transplantation. Using a combination of different assays these data suggest that bone marrow contributes to at least 10% of the adipocyte pool. This proportion is doubled in obesity, suggesting that BMDCs may constitute a reserve pool for adipogenesis, particularly upon weight gain. This review discusses the possible mechanisms and relevance of these findings for human pathophysiology.

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.

Mesenchymal stem cells generate distinct functional hybrids in vitro via cell fusion or entosis

Homotypic and heterotypic cell-to-cell fusion are key processes during development and tissue regeneration. Nevertheless, aberrant cell fusion can contribute to tumour initiation and metastasis. Additionally, a form of cell-in-cell structure called entosis has been observed in several human tumours. Here we investigate cell-to-cell interaction between mouse mesenchymal stem cells (MSCs) and embryonic stem cells (ESCs). MSCs represent an important source of adult stem cells since they have great potential for regenerative medicine, even though they are also involved in cancer progression. We report that MSCs can either fuse forming heterokaryons, or be invaded by ESCs through entosis. While entosis-derived hybrids never share their genomes and induce degradation of the target cell, fusion-derived hybrids can convert into synkaryons. Importantly we show that hetero-to-synkaryon transition occurs through cell division and not by nuclear membrane fusion. Additionally, we also observe that the ROCK-actin/myosin pathway is required for both fusion and entosis in ESCs but only for entosis in MSCs. Overall, we show that MSCs can undergo fusion or entosis in culture by generating distinct functional cellular entities. These two processes are profoundly different and their outcomes should be considered given the beneficial or possible detrimental effects of MSC-based therapeutic applications.

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.

On the origin of human adipocytes and the contribution of bone marrow-derived cells

In the last decade, results in both animal models and humans have demonstrated that white adipocytes are generated over the entire life-span. This adds to the plasticity of adipose tissue and alterations in adipocyte turnover are linked to metabolic dysfunction. Adipocytes are derived from precursors present primarily in the perivascular areas of adipose tissue but their precise origin remains unclear. The multipotent differentiation capacity of bone marrow-derived cells (BMDC) has prompted the suggestion that BMDC may contribute to different cell tissue pools, including adipocytes. However, data in murine transplantation models have been conflicting and it has been a matter of debate whether BMDC actually differentiate into adipocytes or just fuse with resident fat cells. To resolve this controversy in humans, we recently performed a study in 65 subjects that had undergone bone marrow transplantation. Using a set of newly developed assays including single cell genome-wide analyses of mature adipocytes, we demonstrated that bone marrow contributes with approximately 10 % to the adipocyte pool. This proportion was more than doubled in obesity, suggesting that BMDC may constitute a reserve pool for adipogenesis, particularly upon weight gain. This commentary discusses the possible relevance of these and other recent findings for human pathophysiology.

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.

Experimental neurotransplantation treatment for hereditary cerebellar ataxias

Hereditary cerebellar degenerations are a heterogeneous group of diseases often having a detrimental impact on patients’ quality of life. Unfortunately, no sufficiently effective causal therapy is available for human patients at present. There are several therapies that have been shown to affect the pathogenetic process and thereby to delay the progress of the disease in mouse models of cerebellar ataxias. The second experimental therapeutic approach for hereditary cerebellar ataxias is neurotransplantation. Grafted cells might provide an effect via delivery of a scarce neurotransmitter, substitution of lost cells if functionally integrated and rescue or trophic support of degenerating cells. The results of cerebellar transplantation research over the past 30 years are reviewed here and potential benefits and limitations of neurotransplantation therapy are discussed.

Cell Fusion along the Anterior-Posterior Neuroaxis in Mice with Experimental Autoimmune Encephalomyelitis

Background

It is well documented that bone marrow-derived cells can fuse with a diverse range of cells, including brain cells, under normal or pathological conditions. Inflammation leads to robust fusion of bone marrow-derived cells with Purkinje cells and the formation of binucleate heterokaryons in the cerebellum. Heterokaryons form through the fusion of two developmentally differential cells and as a result contain two distinct nuclei without subsequent nuclear or chromosome loss.

Aim

In the brain, fusion of bone marrow-derived cells appears to be restricted to the complex and large Purkinje cells, raising the question whether the size of the recipient cell is important for cell fusion in the central nervous system. Purkinje cells are among the largest neurons in the central nervous system and accordingly can harbor two nuclei.

Results

Using a well-characterized model for heterokaryon formation in the cerebellum (experimental autoimmune encephalomyelitis - a mouse model of multiple sclerosis), we report for the first time that green fluorescent protein-labeled bone marrow-derived cells can fuse and form heterokaryons with spinal cord motor neurons. These spinal cord heterokaryons are predominantly located in or adjacent to an active or previously active inflammation site, demonstrating that inflammation and infiltration of immune cells are key for cell fusion in the central nervous system. While some motor neurons were found to contain two nuclei, co-expressing green fluorescent protein and the neuronal marker, neuron-specific nuclear protein, a number of small interneurons also co-expressed green fluorescent protein and the neuronal marker, neuron-specific nuclear protein. These small heterokaryons were scattered in the gray matter of the spinal cord.

Conclusion

This novel finding expands the repertoire of neurons that can form heterokaryons with bone marrow-derived cells in the central nervous system, albeit in low numbers, possibly leading to a novel therapy for spinal cord motor neurons or other neurons that are compromised in the central nervous system.

Analyzing cell fusion events within the central nervous system using bone marrow chimerism

It has emerged that cells which typically reside in the bone marrow have the capacity to cross the blood brain barrier and contribute genetic material to a range of neuronal cell types within the central nervous system. One such mechanism to account for this phenomenon is cellular fusion, occurring between migrating bone marrow-derived stem cells and neuronal cells in-situ. Biologically, the significance as to why cells from distinct lineages fuse with cells of the central nervous system is, as yet, unclear. Growing evidence however suggests that these cell fusion events could provide an efficient means of rescuing the highly complex and differentiated neuronal cell types that cannot be replaced in adulthood. To facilitate further understanding of cell fusion within the central nervous system, we describe here a technique to establish chimeric mice that are stably reconstituted with green fluorescent protein expressing sex-mismatched bone marrow. These chimeric mice are known to represent an excellent model for studying bone marrow cell migration and infiltration throughout the body, while in parallel, as will be described here, also provide a means to neatly analyze both bone marrow-derived cell fusion and trans-differentiation events within the central nervous system.

Bone marrow transplantation for research and regenerative therapies in the central nervous system

Bone marrow stem cells are probably the best known stem cell type and have been employed for more than 50 years, especially in pathologies related to the hematopoietic and immune systems. However, their potential for therapeutic application is much broader (because these cells can differentiate into hepatocytes, myocytes, cardiomyocytes, pneumocytes or neural cells, among others), and they can also presumably be employed to palliate neural diseases. Current research addressing the integration of bone marrow -derived cells in the neural circuits of the central nervous system together with their features and applications are hotspots in current Neurobiology. Nevertheless, as in other leading research lines the efficacy and possibilities of their therapeutic application depend on the technical procedures employed, which are still far from being standardized. In this chapter we shall explain one of these procedures in depth, namely the transplantation of whole bone marrow from harvested bone marrow stem cells for subsequent integration into the encephalon.

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

Adult stem cell populations, notably those which reside in the bone marrow, have been shown to contribute to several neuronal cell types in the rodent and human brain. The observation that circulating bone marrow cells can migrate into the central nervous system and fuse with, in particular, cerebellar Purkinje cells has suggested, at least in part, a potential mechanism behind this process. Experimentally, the incidence of cell fusion in the brain is enhanced with age, radiation exposure, inflammation, chemotherapeutic drugs and even selective damage to the neurons themselves. The presence of cell fusion, shown by detection of increased bi-nucleated neurons, has also been described in a variety of human central nervous system diseases, including both multiple sclerosis and Alzheimer’s disease. Accumulating evidence is therefore raising new questions into the biological significance of cell fusion, with the possibility that it represents an important means of cell-mediated neuroprotection or rescue of highly complex neurons that cannot be replaced in adult life. Here, we discuss the evidence behind this phenomenon in the rodent and human brain, with a focus on the subsequent research investigating the physiological mechanisms of cell fusion underlying this process. We also highlight how these studies offer new insights into endogenous neuronal repair, opening new exciting avenues for potential therapeutic interventions against neurodegeneration and brain injury.

Intranasal delivery of central nervous system-retargeted human mesenchymal stromal cells prolongs treatment efficacy of experimental autoimmune encephalomyelitis

Treatment with mesenchymal stromal cells (MSCs) is currently of interest for a number of diseases including multiple sclerosis. MSCs are known to target inflamed tissues, but in a therapeutic setting their systemic administration will lead to few cells reaching the brain. We hypothesized that MSCs may target the brain upon intranasal administration and persist in central nervous system (CNS) tissue if expressing a CNS-targeting receptor. To demonstrate proof of concept, MSCs were genetically engineered to express a myelin oligodendrocyte glycoprotein-specific receptor. Engineered MSCs retained their immunosuppressive capacity, infiltrated into the brain upon intranasal cell administration, and were able to significantly reduce disease symptoms of experimental autoimmune encephalomyelitis (EAE). Mice treated with CNS-targeting MSCs were resistant to further EAE induction whereas non-targeted MSCs did not give such persistent effects. Histological analysis revealed increased brain restoration in engineered MSC-treated mice. In conclusion, MSCs can be genetically engineered to target the brain and prolong therapeutic efficacy in an EAE model.

Bone Marrow-Derived Stem Cells and Strategies for Treatment of Nervous System Disorders: Many Protocols, and Many Results

Bone marrow stem cells are the best known stem cell type and have been employed for more than 50 years, especially in pathologies of the hematopoietic and immune systems. However, their therapeutic potential is much broader, and they can also be employed to palliate neural diseases. Apart from their plastic properties, these cells lack the legal or ethical constraints of other stem cell populations, that is, embryonic stem cells. Current research addressing the integration of bone marrow-derived cells into the neural circuits of the central nervous system, their features, and applications is a hotspot in neurobiology. Nevertheless, as in other leading research lines the efficacy and possibilities of their application depend on technical procedures, which are still far from being standardized. Accordingly, for efficient research this large range of variants should be taken into account as they could lead to unexpected results. Rather than focusing on clinical aspects, this review offers a compendium of the methodologies aimed at providing a guide for researchers who are working in the field of bone marrow transplantation in the central nervous system. It seeks to be useful for both introductory and trouble-shooting purposes, and in particular for dealing with the large array of bone marrow transplantation protocols available.

Extracellular vesicle-mediated transfer of genetic information between the hematopoietic system and the brain in response to inflammation

When stimulated by inflammation, peripheral blood cells signal directly to neurons in the brain via the transfer of functional RNA enclosed in extracellular vesicles.

Mechanisms behind how the immune system signals to the brain in response to systemic inflammation are not fully understood. Transgenic mice expressing Cre recombinase specifically in the hematopoietic lineage in a Cre reporter background display recombination and marker gene expression in Purkinje neurons. Here we show that reportergene expression in neurons is caused by intercellular transfer of functional Cre recombinase messenger RNA from immune cells into neurons in the absence of cell fusion. In vitro purified secreted extracellular vesicles (EVs) from blood cells contain Cre mRNA, which induces recombination in neurons when injected into the brain. Although Cre-mediated recombination events in the brain occur very rarely in healthy animals, their number increases considerably in different injury models, particularly under inflammatory conditions, and extend beyond Purkinje neurons to other neuronal populations in cortex, hippocampus, and substantia nigra. Recombined Purkinje neurons differ in their miRNA profile from their nonrecombined counterparts, indicating physiological significance. These observations reveal the existence of a previously unrecognized mechanism to communicate RNA-based signals between the hematopoietic system and various organs, including the brain, in response to inflammation.

Peripheral infections leading to an inflammatory response can initiate signaling from the hematopoietic system to various organs including the brain. The traditional view of this communication between blood and brain is that individual factors are released by immune cells that in turn bind to neuronal or nonneuronal target cells in the brain where they exert their effects. By using a genetic tracing system, we now show that extracellular vesicles, small membrane structures that can contain a multitude of different molecules, can transfer functional RNA directly from blood cells to neurons. Although this type of signaling is highly restricted in the healthy animal, inflammatory injuries increase both the frequency of transfer and the range of the neuronal target populations in the brain. By showing altered miRNA profiles in neurons receiving extracellular vesicle cargo, we predict a complex regulation of gene expression in neural cells in response to peripheral inflammation.

Mild cerebellar neurodegeneration of aged heterozygous PCD mice increases cell fusion of Purkinje and bone marrow-derived cells

Bone marrow-derived cells have different plastic properties, especially regarding cell fusion, which increases with time and is prompted by tissue injury. Several recessive mutations, including Purkinje Cell Degeneration, affect the number of Purkinje cells in homozygosis; heterozygous young animals have an apparently normal phenotype but they undergo Purkinje cell loss as they age. Our findings demonstrate that heterozygous pcd mice undergo Purkinje cell loss at postnatal day 300, this slow but steadily progressing cell death starting sooner than has been reported previously and without massive reactive gliosis or inflammation. Here, transplantation of bone marrow stem cells was performed to assess the arrival of bone marrow-derived cells in the cerebellum in these heterozygous mice. Our results reveal that a higher number of cell fusion events occurs in heterozygous animals than in the controls, on days 150 and 300 postnatally. In sum, this study indicates that mild cell death promotes the fusion of bone marrow-derived cells with surviving Purkinje neurons. This phenomenon suggests new therapies for long-lasting neurodegenerative disorders.

Purkinje cell fusion and binucleate heterokaryon formation in multiple sclerosis cerebellum

A major conceptual consideration in both endogenous and therapeutic central nervous system repair is how damaged (or senescent) neurons, given their often enormously complex and extensive network of connections, can possibly be replaced. The recent observation of fusion of circulating bone marrow cells with, in particular, cerebellar Purkinje cells, as well as the subsequent formation of stable heterokaryons, offers a tantalizing potential solution to this difficulty. Here, we have explored Purkinje cell fusion and heterokaryon formation in the human brain and the influence of central nervous system inflammation. We analysed post-mortem cerebellum tissue from patients who had multiple sclerosis and from appropriate controls. Purkinje cells were analysed for heterokaryon formation using immunohistochemistry techniques and chromosome composition using fluorescence in situ hybridization. For the first time in humans we show a disease-related increase in Purkinje cell fusion and heterokaryon formation. We have shown that heterokaryon formation takes place in control subjects, and that the frequency of this event is considerably increased in patients with multiple sclerosis, the prototypical inflammatory brain disease, with ~0.4% of Purkinje cells being binucleate heterokaryons. No mononucleate polyploid Purkinje cell heterokaryons were found. The observation that heterokaryon formation in the cerebellum occurs as part of the central nervous system inflammatory reaction suggests a potential mechanism of neural repair. It also suggests an exciting new avenue for therapeutic intervention, as enhancement or manipulation of fusion events may have a therapeutic role in cellular protection in multiple sclerosis.

High-resolution in-vivo analysis of normal brain response to cranial irradiation

Radiation therapy (RT) is a widely accepted treatment strategy for many central nervous system (CNS) pathologies. However, despite recognized therapeutic success, significant negative consequences are associated with cranial irradiation (CR), which manifests months to years post-RT. The pathophysiology and molecular alterations that culminate in the long-term detrimental effects of CR are poorly understood, though it is thought that endothelial injury plays a pivotal role in triggering cranial injury. We therefore explored the contribution of bone marrow derived cells (BMDCs) in their capacity to repair and contribute to neo-vascularization following CR. Using high-resolution in vivo optical imaging we have studied, at single-cell resolution, the spatio-temporal response of BMDCs in normal brain following CR. We demonstrate that BMDCs are recruited specifically to the site of CR, in a radiation dose and temporal-spatial manner. We establish that BMDCs do not form endothelial cells but rather they differentiate predominantly into inflammatory cells and microglia. Most notably we provide evidence that more than 50% of the microglia in the irradiated region of the brain are not resident microglia but recruited from the bone marrow following CR. These results have invaluable therapeutic implications as BMDCs may be a primary therapeutic target to block acute and long-term inflammatory response following CR. Identifying the critical steps involved in the sustained recruitment and differentiation of BMDCs into microglia at the site of CR can provide new insights into the mechanisms of injury following CR offering potential therapeutic strategies to counteract the long-term adverse effects of CR.

Peripheral nervous system progenitors can be reprogrammed to produce myelinating oligodendrocytes and repair brain lesions

Neural crest stem cells (NCSCs) give rise to the neurons and glia of the peripheral nervous system (PNS). NCSC-like cells can be isolated from multiple peripheral organs and maintained in neurosphere culture. Combining in vitro culture and transplantation, we show that expanded embryonic NCSC-like cells lose PNS traits and are reprogrammed to generate CNS cell types. When transplanted into the embryonic or adult mouse CNS, they differentiate predominantly into cells of the oligodendrocyte lineage without any signs of tumor formation. NCSC-derived oligodendrocytes generate CNS myelin and contribute to the repair of the myelin deficiency in shiverer mice. These results demonstrate a reprogramming of PNS progenitors to CNS fates without genetic modification and imply that PNS cells could be a potential source for cell-based CNS therapy.

Cell-fusion-mediated somatic-cell reprogramming: a mechanism for tissue regeneration

Spontaneous cell fusion between two cells of different lineages will originate new hybrid cells that have different features from the original parent cells. It has been shown that injury to a tissue can enhance spontaneous cell-cell fusion events. If one of the parent cells of a cell-cell fusion is highly plastic, such as a stem cell, and the other is a somatic cell, their fusion can be followed by reprogramming events that can generate new hybrid pluripotent cells. These, in turn, have the potential to differentiate and regenerate the damaged tissue. However, if this process is deregulated, this would provide a mechanism for cancer development.

Inflammation and proliferation act together to mediate intestinal cell fusion

Cell fusion between circulating bone marrow-derived cells (BMDCs) and non-hematopoietic cells is well documented in various tissues and has recently been suggested to occur in response to injury. Here we illustrate that inflammation within the intestine enhanced the level of BMDC fusion with intestinal progenitors. To identify important microenvironmental factors mediating intestinal epithelial cell fusion, we performed bone marrow transplantation into mouse models of inflammation and stimulated epithelial proliferation. Interestingly, in a non-injury model or in instances where inflammation was suppressed, an appreciable baseline level of fusion persisted. This suggests that additional mediators of cell fusion exist. A rigorous temporal analysis of early post-transplantation cellular dynamics revealed that GFP-expressing donor cells first trafficked to the intestine coincident with a striking increase in epithelial proliferation, advocating for a required fusogenic state of the host partner. Directly supporting this hypothesis, induction of augmented epithelial proliferation resulted in a significant increase in intestinal cell fusion. Here we report that intestinal inflammation and epithelial proliferation act together to promote cell fusion. While the physiologic impact of cell fusion is not yet known, the increased incidence in an inflammatory and proliferative microenvironment suggests a potential role for cell fusion in mediating the progression of intestinal inflammatory diseases and cancer.