Transplantation to the rat brain of human neural progenitors that were genetically modified using adenoviruses

Stem cell therapy and gene transfer for regeneration

The committed stem and progenitor cells have been recently isolated from various adult tissues, including hematopoietic stem cell, neural stem cell, mesenchymal stem cell and endothelial progenitor cell. These adult stem cells have several advantages as compared with embryonic stem cells as their practical therapeutic application for tissue regeneration. In this review, we discuss the promising gene therapy application of adult stem and progenitor cells in terms of modifying stem cell potency, altering organ property, accelerating regeneration and forming expressional organization.

Survival, neuronal differentiation, and fiber outgrowth of propagated human neural precursor grafts in an animal model of Huntington's disease

Expanded neural precursor cells provide an attractive alternative to primary fetal tissue for cell replacement therapies in neurodegenerative diseases. In this study we transplanted epigenetically propagated human neural precursor cells into a rat model of Huntington's disease. Neural precursors survived transplantation and large numbers differentiated to express neuronal antigens, including some that expressed DARPP-32, indicating a mature striatal phenotype had been adopted. Neuronal fibers from the grafts projected diffusely throughout the host brain, although there was no evidence that outgrowth was specifically target directed. This study supports the contention that propagated human neural precursors may ultimately be of use in therapeutic neural transplantation paradigms for diseases such as Huntington's disease.

Mitogen and substrate differentially affect the lineage restriction of adult rat subventricular zone neural precursor cell populations

The effects of specific mitogens and substrates on the proliferative capacity and the differentiated phenotypic plasticity of neural precursor cell populations isolated from the adult rat subventricular zone (SVZ) were examined. SVZ cells were grown on uncoated tissue culture plastic, extracellular matrix, or poly-D-ornithine with either laminin or fibronectin. SVZ neural precursor cells could not be generated with platelet-derived growth factor (PDGF), granulocyte macrophage colony stimulating factor, stem cell factor, heparin-binding epidermal growth factor (HB-EGF), granulocyte colony stimulating factor, or ciliary neurotrophic factor (CNTF), but could be with EGF, fibroblast growth factor 2 (FGF2), and FGF2 plus heparin. Varying combinations of substrate and mitogen resulted in very different expansion rates and/or lineage potential. Neurons, oligodendrocytes, and astrocytes differentiated from all cultures, but EGF-generated neural precursor cells were more restricted to an astrocytic lineage and FGF2-generated neural precursor cells had a greater capacity for neuronal differentiation. In both EGF- and FGF2-generated cell populations, CNTF increased the number of differentiated astrocytes, triiodothyronine oligodendrocytes, PDGF neurons, and brain-derived neurotrophic factor neurons only from EGF cells. Electrophysiological analysis of differentiated cells showed three distinct phenotypes, glial, neuronal, and presumed precursor cells, although the neuronal properties were immature. Collectively, these data indicate that CNS neural precursor cell populations isolated with different mitogens and substrates are intrinsically different and their characteristics cannot be directly compared.

Restorative neurosurgery: opportunities for restoration of function in acquired, degenerative, and idiopathic neurological diseases

Historically, neurosurgery has improved the environment of the nervous system to promote maximal spontaneous recovery of function. The population of patients whom we treat at present is a small portion of those who suffer from disabling neurological illnesses. Based on a combination of new technology, and advances in neuroscience, restorative neurosurgery is advancing the frontiers of our specialty, and providing the potential to restore lost function. Significant advancements in gene therapy, the discovery and delivery of neurotrophic factors, and cell transplantation now require neurosurgeons to broaden the scope of our practice so that it includes the restoration of function in an enormous number of patients with acquired, degenerative and idiopathic neurological diseases. In order to meet the present challenge, neurosurgeons must broaden our vision, our role, and our future educational goals. In this review, we summarize the landmark advances in the basic and clinical neurosciences and the results of clinical trials that are driving our evolution from passive reaction to disease to active attempts to restore lost central nervous system function.

Polysialylated neural cell adhesion molecule-positive CNS precursors generate both oligodendrocytes and Schwann cells to remyelinate the CNS after transplantation

Transplantation offers a means of identifying the differentiation and myelination potential of early neural precursors, features relevant to myelin regeneration in demyelinating diseases. In the postnatal rat brain, precursor cells expressing the polysialylated (PSA) form of the neural cell adhesion molecule NCAM have been shown to generate mostly oligodendrocytes and astrocytes in vitro (Ben-Hur et al., 1998). Immunoselected PSA-NCAM+ newborn rat CNS precursors were expanded as clusters with FGF2 and grafted into a focal demyelinating lesion in adult rat spinal cord. We show that these neural precursors can completely remyelinate such CNS lesions. While PSA-NCAM+ precursor clusters contain rare P75+ putative neural crest precursors, they do not generate Schwann cells in vitro even in the presence of glial growth factor. Yet they generate oligodendrocytes, astrocytes, and Schwann cells in vivo when confronted with demyelinated axons in a glia-free area. We confirmed the transplant origin of these Schwann cells using Y chromosome in situ hybridization and immunostaining for the peripheral myelin protein P0 of tissue from female rats that had been grafted with male cell clusters. The number and distribution of Schwann cells within remyelinated tissue, and the absence of P0 mRNAs in donor cells, indicated that Schwann cells were generated by expansion and differentiation of transplanted PSA-NCAM+ neural precursors and were not derived from contaminating Schwann cells. Thus, transplantation into demyelinated CNS tissue reveals an unexpected differentiation potential of a neural precursor, resulting in remyelination of CNS axons by PNS and CNS myelin-forming cells.

New prospects for human stem-cell therapy in the nervous system

It would be of enormous benefit if human neural tissue could be generated in vitro as this would allow screening for neuroactive compounds, and provide a source of tissue for testing cellular and gene therapies for CNS disorders. It is now well established that pluripotent embryonic stem cells (ES cells) from the mouse can be propagated in culture and differentiated into a range of tissues, including neuronal and glial cells. In other studies, more-restricted neural stem cells have been isolated from both the developing and adult rodent brain. Current reports now describe similar pluripotent and neural stem cells cultured from human embryos. While the exact nature of these cells continues to be explored, they can be grown for extended periods of time while retaining the capacity for neuronal and glial differentiation. In some cases, they have been shown to integrate into the developing or damaged adult brain. This article reviews their biology, with a focus on the possible links between ES-cell and neural stem-cell technologies, and the strategies used to isolate and expand defined cell populations.

Building brains: neural chimeras in the study of nervous system development and repair

The ability to isolate multipotential neuroepithelial precursor cells from the mammalian nervous system provides exciting perspectives for the in vitro analysis of early nervous system development and the generation of donor cells for neural repair. New models are needed to study the properties of these cells in vivo. Neural chimeras have revealed a remarkable degree of plasticity in the developmental potential of neuroepithelial precursor cells. Following transplantation into the cerebral ventricle of embryonic hosts, precursors derived from various brain regions and developmental stages participate in host brain development and undergo region-specific differentiation into neurons and glia. These findings indicate that in the developing nervous system, migration and differentiation of neural precursors cells are regulated to a large extent by extrinsic signals. Neural chimeras composed of genetically modified cells will permit the study of the molecular mechanisms underlying these guidance cues, which may eventually be exploited for cell replacement strategies in the adult brain. A key problem in neural transplantation is the availability of suitable donor tissue. Neural chimeras composed of embryonic stem (ES) cell-derived neurons and glia depict ES cells as a versatile and virtually unlimited donor source for neural repair. Generation of interspecies neural chimeras composed of human and rodent cells facilitates the translation of these advances into clinical strategies for human nervous system repair.

Human neural stem cells: isolation, expansion and transplantation

Neural stem cells, with the capacity to self renew and produce the major cell types of the brain, exist in the developing and adult rodent central nervous system (CNS). Their exact function and distribution is currently being assessed, but they represent an interesting cell population, which may be used to study factors important for the differentiation of neurons, astrocytes and oligodendrocytes. Recent evidence suggests that neural stem cells may also exist in both the developing and adult human CNS. These cells can be grown in vitro for long periods of time while retaining the potential to differentiate into nervous tissue. Significantly, many neurons can be produced from a limited number of starting cells, raising the possibility of cell replacement therapy for a wide range of neurological disorders. This review summarises this fascinating and growing field of neurobiology, with a particular focus on human tissues.

Establishment and properties of neural stem cell clones: plasticity in vitro and in vivo

The study of the basic physiology of the neural precursors generated during brain development is driven by two inextricably linked goals. First, such knowledge is instrumental to our understanding of how the high degree of cellular complexity of the mature central nervous system (CNS) is generated, and how to dissect the steps of proliferation, fate commitment, and differentiation that lead early pluripotent neural progenitors to give rise to mature CNS cells. Second, it is hoped that the isolation, propagation, and manipulation of brain precursors and, particularly, of multipotent neural stem cells (NSCs), will lead to therapeutic applications in neurological disorders. The debate is still open concerning the most appropriate definition of a stem cell and on how it is best identified, characterized, and manipulated. By adopting an operational definition of NSCs, we review some of the basic findings in this area and elaborate on their potential therapeutic applications. Further, we discuss recent evidence from our two groups that describe, based on that rigorous definition, the isolation and propagation of clones of NSCs from the human fetal brain and illustrate how they have begun to show promise for neural cell replacement and molecular support therapy in models of degenerative CNS diseases. The extensive propagation and engraftment potential of human CNS stem cells may, in the not-too-distant-future, be directed towards genuine clinical therapeutic ends, and may open novel and multifaceted strategies for redressing a variety of heretofore untreatable CNS dysfunctions.

Lithium chloride induces the expression of tyrosine hydroxylase in hNT neurons

In the present study, several doses of lithium chloride were tested for their ability to induce the expression of tyrosine hydroxylase (TH) in neurons derived from a human teratocarcinoma cell line (hNT) after 5 and 10 days in vitro (DIV). Following immunocytochemical staining for tyrosine hydroxylase, the percentage of TH-positive neurons was determined and morphometric analysis, including mean soma profile area and neuritic length, was performed. hNT neurons responded to lithium treatment in a dose-dependent manner. In 5 DIV, the most effective dose of lithium chloride (1.0 mM) increased the number of TH-positive neurons approximately sixfold. In addition, both TH-positive hNT neuron mean soma profile area and neurite length were significantly larger than controls by 60 and 70%, respectively. Moreover, even after withdrawal of lithium chloride on day 5, the number of TH-positive neurons in 10 DIV cultures remained significantly increased. These data suggest that hNT cells are indeed responsive to lithium exposure and may serve as a continual source of TH-expressing neurons in new therapeutic approaches to degenerative brain disease.

Somatic gene therapy in animal models of Parkinson's disease

Gene therapy in Parkinson's disease (PD) emerged about 10 years ago but until now, no clinical trials are under way, because most approaches have failed to show long-term therapeutic effects in PD animal models and because safety concerns precluded the use in humans so far. This review tries to give an overview on the development of different strategies in gene therapy in PD animal models and point out new and possibly more successful directions, including the transplantation of neural precursor cells and pig tissue.

Histochemical discrimination of endogenous mammalian beta-galactosidase activity from that resulting from lac-Z gene expression

Minces of several organs from the transgenic mouse ROSAbeta-gal 26 (ROSA-26), which robustly expresses bacterial lac-Z in most tissues, were exposed to 4-bromo-5-chloro-3-indoyl-beta-D-galactopyrosanide (X-gal) at pH ranging from 7.5 to 9.5 to determine the optimal pH for in situ demonstration of bacterial beta-galactosidase activity (neutral pH optimum) while minimizing detection of potentially confounding endogenous mammalian beta-galactosidase (acidic pH optimum). Similar studies were performed with organ minces from C57BL/6 mice, Sprague-Dawley rats, New Zealand white rabbits, and macaques to confirm the effect of pH on minimizing detection of endogenous mammalian beta-galactosidase. In all organs evaluated; heart, liver, spleen, kidney, brain, and skeletal muscle, endogenous beta-galactosidase activity was rarely detected following incubation at pH greater than 7.5. In contrast, bacterial beta-galactosidase activity in the ROSA-26 mice was strongly detected in organ minces following incubation at pH 8.0-9.0. These findings are similar to previous observations we have made in lung minces and confirm that a simple alteration of a commonly used histochemical technique for detecting in situ beta-galactosidase activity, raising the reaction buffer pH to weakly alkaline range, can reliably distinguish between endogenous activity and that resulting from exogenous bacterial gene expression.

A single adenovirus vector mediates doxycycline-controlled expression of tyrosine hydroxylase in brain grafts of human neural progenitors

Ex vivo gene transfer is emerging as a promising therapeutic approach to human neurodegenerative diseases. By combining efficient methodologies for cell amplification and gene delivery, large numbers of cells can be generated with the capacity to synthesize therapeutic molecules. These cells can then be transplanted into the degenerating central nervous system (CNS). Applying this approach to human diseases will require the development of suitable cellular vehicles, as well as safe gene delivery systems capable of tightly controlled transgene expression. For such brain repair technologies, human neural progenitors may be extremely valuable, because of their human CNS origin and developmental potential. We have used these cells to develop a system for the regulated expression of a gene of therapeutic potential. We report the construction of a single adenovirus encoding human tyrosine hydroxylase 1 (hTH-1) under the negative control of the tetracycline-based gene regulatory system. Human neural progenitors infected with this vector produced large amounts of hTH-1. Most importantly, doxycycline allowed a reversible switch of transgene transcription both in vitro and in vivo. This system may be applied to the development of therapies for human neurodegenerative diseases.

Isolation and cloning of multipotential stem cells from the embryonic human CNS and establishment of transplantable human neural stem cell lines by epigenetic stimulation

Stem cells that can give rise to neurons, astroglia, and oligodendroglia have been found in the developing and adult central nervous system (CNS) of rodents. Yet, their existence within the human brain has not been documented, and the isolation and characterization of multipotent embryonic human neural stem cells have proven difficult to accomplish. We show that the developing human CNS embodies multipotent precursors that differ from their murine counterpart in that they require simultaneous, synergistic stimulation by both epidermal and fibroblast growth factor-2 to exhibit critical stem cell characteristics. Clonal analysis demonstrates that human C NS stem cells are multipotent and differentiate spontaneously into neurons, astrocytes, and oligodendrocytes when growth factors are removed. Subcloning and population analysis show their extensive self-renewal capacity and functional stability, their ability to maintain a steady growth profile, their multipotency, and a constant potential for neuronal differentiation for more than 2 years. The neurons generated by human stem cells over this period of time are electrophysiologically active. These cells are also cryopreservable. Finally, we demonstrate that the neuronal and glial progeny of long-term cultured human CNS stem cells can effectively survive transplantation into the lesioned striatum of adult rats. Tumor formation is not observed, even in immunodeficient hosts. Hence, as a consequence of their inherent biology, human CNS stem cells can establish stable, transplantable cell lines by epigenetic stimulation. These lines represent a renewable source of neurons and glia and may significantly facilitate research on human neurogenesis and the development of clinical neural transplantation.

Toward autologous ex vivo gene therapy for the central nervous system with human adult astrocytes

The combination of gene transfer techniques and cell transplantation is a promising approach to deliver therapeutic molecules into the CNS. To optimize gene transfer systems, several neural and nonneural cell types are currently under investigation. Among these cells, astrocytes are particularly well suited because of their CNS origin, their efficient secretory mechanisms, and their role as neuronal support. Most importantly, the use of human adult astrocytes as cellular vehicles for ex vivo gene transfer may open the way to autologous transplantation, thus obviating immunological rejection and the side effects of immunosuppressors. In the present study, we report the ability of these cells to be expanded and genetically modified in vitro. Astrocytes derived from human adult cerebral cortex were grown and maintained in vitro as pure primary cultures for at least 10 months. In addition, cells were efficiently transduced by an adenoviral vector encoding human tyrosine hydroxylase (hTH) under the negative control of the tetracycline-based regulatory system (tet-off). The infected cells synthesized large amounts of active hTH and released L-dopa. In addition, doxycycline, a potent analog of tetracycline, efficiently regulated transgene expression. This work is a first step toward the development of therapeutic strategies based on the use of genetically engineered human adult astrocytes for autologous transplantation in human neurodegenerative diseases and CNS trauma.

Survival of genetically engineered, adult-derived rat astrocytes grafted into the 6-hydroxydopamine lesioned adult rat striatum

Astrocytes are potentially useful as vehicles for gene transfer into the CNS. As endogenous CNS cells, they possess secretory mechanisms and can be grown in vitro. We have developed an animal model of this system using autologous astrocyte grafts in Fischer 344 rats. Cultured cells were infected with an adenoviral vector containing the reporter gene lacZ in vitro and then grafted into the striatum of adult Fischer 344 rats previously lesioned with 6-OHDA. Survival of the cells and activity of the beta-galactosidase protein were followed for up to 21 days after injection. The grafted cells were shown to survive throughout the experimental period although the expression of transgene was reduced with time. If long-term expression of therapeutically active substances can be achieved, grafts of adult-derived astrocytes genetically engineered using recombinant adenoviral vectors could be employed in the treatment of Parkinson's disease and other neurological disorders.

A new method for the rapid and long term growth of human neural precursor cells

A reliable source of human neural tissue would be of immense practical value to both neuroscientists and clinical neural transplantation trials. In this study, human precursor cells were isolated from the developing human cortex and, in the presence of both epidermal and fibroblast growth factor-2, grew in culture as sphere shaped clusters. Using traditional passaging techniques and culture mediums the rate of growth was extremely slow, and only a 12-fold expansion in total cell number could be achieved. However, when intact spheres were sectioned into quarters, rather than mechanically dissociated, cell cell contacts were maintained and cellular trauma minimised which permitted the rapid and continual growth of each individual quarter. Using this method we have achieved a 1.5 million-fold increase in precursor cell number over a period of less than 200 days. Upon differentiation by exposure to a substrate, cells migrated out from the spheres and formed a monolayer of astrocytes and neurons. No oligodendrocytes were found to develop from these human neural precursor cells at late passages when whole spheres were differentiated. This simple and novel culture method allows the rapid expansion of large numbers of non-transformed human neural precursor cells which may be of use in drug discovery, ex vivo gene therapy and clinical neural transplantation.

Chimeric brains generated by intraventricular transplantation of fetal human brain cells into embryonic rats

Limited experimental access to the central nervous system (CNS) is a key problem in the study of human neural development, disease, and regeneration. We have addressed this problem by generating neural chimeras composed of human and rodent cells. Fetal human brain cells implanted into the cerebral ventricles of embryonic rats incorporate individually into all major compartments of the brain, generating widespread CNS chimerism. The human cells differentiate into neurons, astrocytes, and oligodendrocytes, which populate the host fore-, mid-, and hindbrain. These chimeras provide a unique model to study human neural cell migration and differentiation in a functional nervous system.

Engraftable human neural stem cells respond to developmental cues, replace neurons, and express foreign genes

Stable clones of neural stem cells (NSCs) have been isolated from the human fetal telencephalon. These self-renewing clones give rise to all fundamental neural lineages in vitro. Following transplantation into germinal zones of the newborn mouse brain they participate in aspects of normal development, including migration along established migratory pathways to disseminated central nervous system regions, differentiation into multiple developmentally and regionally appropriate cell types, and nondisruptive interspersion with host progenitors and their progeny. These human NSCs can be genetically engineered and are capable of expressing foreign transgenes in vivo. Supporting their gene therapy potential, secretory products from NSCs can correct a prototypical genetic metabolic defect in neurons and glia in vitro. The human NSCs can also replace specific deficient neuronal populations. Cryopreservable human NSCs may be propagated by both epigenetic and genetic means that are comparably safe and effective. By analogy to rodent NSCs, these observations may allow the development of NSC transplantation for a range of disorders.

Suppression of inflammation by dexamethasone prolongs adenoviral vector-mediated transgene expression in the facial nucleus of the rat

Adenoviral vector directed gene transfer to rat facial motoneurons occurs efficiently following intra-parenchymal injection of relatively high dosages (> or =10(7) pfu per injection) of a prototype first generation adenoviral vector. However, high level of transgene expression, as observed during the first week following administration of the adenoviral vector, declines during the second and third weeks and is associated with a local inflammatory response. In the present study rats were treated with dexamethasone to suppress the inflammatory response that occurs following administration of a first generation adenoviral vector and the effect of dexamethasone treatment on the duration of transgene expression was studied. Dexamethasone-treated rats displayed a reduced infiltration of macrophages and intra-parenchymal T-cells were not detectable at 14 days post-infusion of the viral vector. Transduced astrocytes persisted following treatment with dexamethasone, whereas only a few transduced motoneurons survived. In conclusion, suppression of inflammation by dexamethasone has a beneficial effect on the survival of transduced astroglial cells but exerts only moderate protection on transduced facial motoneurons.

Remyelination: cellular and gene therapy

Dysfunctional myelination or oligodendroglial abnormalities play a prominent role in a vast array of pediatric neurological diseases of genetic, inflammatory, immunological, traumatic, ischemic, developmental, metabolic, and infectious causes. Recent advances in glial cell biology have suggested that effective remyelination strategies may, indeed, be feasible. Evidence for myelin repair is accumulating in various experimental models of dysmyelinating and demyelinating disease. Attempts at remyelination have either been directed towards creating myelin de novo from exogenous sources of myelin-elaborating cells or promoting an intrinsic spontaneous remyelinating process. Ultimately, some disorders of myelin may require multiple repair strategies, not only the replacement of dysfunctional cells (oligodendroglia) but also the delivery or supplementation of gene products (i.e., growth factors, immune modulators, metabolic enzymes). Although primary oligodendrocytes or oligodendroglial precursors may be effective for glial cell replacement in certain discrete regions and circumstances and although various genetic vectors may be effective for the delivery of therapeutic molecules, multipotent neural stem cells may be most ideally suited for both gene transfer and cell replacement on transplantation into multiple regions of the central nervous system under a wide range of pathological conditions. We propose that, by virtue of their inherent biological properties, neural stem cells possess the multifaceted therapeutic capabilities that many diseases characterized by myelin dysfunction in the pediatric population may demand.

Heparin, but not other proteoglycans potentiates the mitogenic effects of FGF-2 on mesencephalic precursor cells

There is increasing evidence that the proteoglycan heparin plays a critical role in the regulation of the activity of FGF-2 by either interacting with its receptor or modifying its stability and functioning. In this study precursor cells were isolated from the rat E14 ventral mesencephalon and cultured as free floating spheres in FGF-2 alone or in combination with heparin or other related proteoglycans, including chondroitin sulfate, keratin sulfate, dermatan sulfate, or hyaluronic acid. Our results show the mitogenic effects of FGF-2 could be potentiated by heparin but not the other four proteoglycans. Sodium chlorate, which blocks the cells ability to sulfate its proteoglycans, was shown to reduce the mitogenic effects of FGF-2 alone to below that of control levels, suggesting that endogenous sulfated molecules are required for the FGF-2 effects on mesencephalic precursors. Cells expanded for 7 days with either FGF-2 or FGF-2 + heparin were plated onto a substrate and allowed to differentiate for a further 7 days in the absence of growth factors. Approximately 6% of the precursors developed into neurons whether grown with or without heparin and none were positive for TH, a marker for dopamine neurons. However, there was a significant decrease in the number of astrocytes developing from cultures grown in FGF-2 + heparin when compared to FGF-2 alone. Interestingly we could not find an EGF responsive cell in the mesencephalon at this embryonic age in the absence or presence of heparin. However, there was a synergistic effect of combining EGF + FGF-2, which could be potentiated by heparin. We conclude that heparin, but not other closely related proteoglycans, is vital for the growth of FGF-2-responsive mesencephalic neural precursors.

Antibodies for targeted gene therapy: extracellular gene targeting and intracellular expression

Antibody genes of human origin and human antibodies directed against human proteins have become widely available in recent years. These are valuable reagents for gene therapy applications, in which the use of human proteins and genes allows for increased therapeutic benefit. Engineered human antibodies can be used in gene therapy both as a component of a gene delivery system and as a therapeutic gene. As the targeting moiety of a gene delivery system, the antibody should meet certain criteria that have been previously determined from other clinical applications of antibodies. These include bioavailability, specificity for the target cell, and rapid clearance. In addition, if repeat delivery of therapeutic genes is going to be needed, then gene delivery vectors should be non-immunogenic to allow repeated administration. The use of human antibodies in this application should therefore be superior to approaches which use rodent-derived antibodies. Another application of antibodies in gene therapy is the use of antibodies expressed inside the cell (intrabodies) as therapeutic agents. The power of the immune system to rearrange a limited set of genes to create recognition sites for any known molecule is well documented. The ability to harness this information and use these highly specific binding molecules as medicines to inhibit an unwanted cellular function is a promising advance in the field of molecular medicine, and in particular, in the field of intracellular immunization.

Biology and potential strategies for the treatment of GM2 gangliosidoses

The GM2 gangliosidoses are a group of heritable neurodegenerative disorders caused by excessive accumulation of the ganglioside GM2 owing to deficiency in beta-hexosaminidase activity. Tay-Sachs and Sandhoff diseases have similar clinical phenotypes resulting from a deficiency in human hexosaminidase alpha and beta subunits, respectively. The lack of treatment for GM2 gangliosidoses stimulated interest in developing animal models to understand the molecular mechanisms underlying the various forms of this disease and to test new potential therapies. In this review, we discuss the molecular biology of GM2 gangliosidoses and the different strategies that have been tested in animal models for the treatment of this genetic disorder, including gene transfer and cell engraftment of neural stem cells engineered to express the hexosaminidase isoenzymes.

Neural stem and progenitor cells: a strategy for gene therapy and brain repair

The damaged adult mammalian brain is incapable of significant structural self-repair. Although varying degrees of recovery from injury are possible, this is largely because of synaptic and functional plasticity rather than the frank regeneration of neural tissues. The lack of structural plasticity of the adult brain is partly because of its inability to generate new neurons, a limitation that has severely hindered the development of therapies for neurological injury or degeneration. However, a variety of experimental studies, as well as moderately successful clinical engraftment of fetal tissue into the adult parkinsonian brain, suggests that cell replacement is evolving as a valuable treatment modality. Neural stem cells, which are the self-renewing precursors of neurons and glia, have been isolated from both the embryonic and adult mammalian central nervous system. In the adult human brain, both neuronal and oligodendroglial precursors have been identified, and methods for their harvest and enrichment have been established. Neural precursors have several characteristics that make them ideal vectors for brain repair. They may be clonally expanded in tissue culture, providing a renewable supply of material for transplantation. Moreover, progenitors are ideal for genetic manipulation and may be engineered to express exogenous genes for neurotransmitters, neurotrophic factors, and metabolic enzymes. Thus, the persistence of neuronal precursors in the adult mammalian brain may permit us to design novel and effective strategies for central nervous system repair, by which we may yet challenge the irreparability of the structurally damaged adult nervous system.

Adenoviral-mediated transfection of the Lac-Z gene into rat dissociated embryonic central nervous system cells before and after seeding

The adenovirus carrying a reporter gene--the Lac Z gene--is known to infect central nervous system (CNS) cells in primary cell cultures. The percentage of infected neurons with respect to the total number of neurons was studied in primary dissociated cultures as a function of the day of inoculation and the age of three rat CNS cultures: spinal cord, mesencephalon and cortex. Two methods of viral inoculation were compared: the first inoculation was performed on the cultured cell at 2, 3 or 6 days in vitro (DIV) whereas the second inoculation was performed on the cell suspensions before seeding. All the infected CNS cells has the same aspect as the control cultures. In the spinal cord and the mesencephalic cultures, the glial cells were preferentially infected, especially when the cells were inoculated at 6 DIV. In the cortical cultures, there were more infected neurons than infected glial cells. The number of CNS cells was lower when inoculation was performed at 6 DIV as compared with 3 DIV. Very few infected GABA cells were found in the cultures. A high percentage of infected neuronal cells relative to the total number of neuronal cells was found when infection of the three types of cultures was performed on the dissociated embryonic cell suspension before seeding.

Transgene expression in rat fetal brain grafts is maintained for 7 months after ex vivo adenoviral vector-mediated gene transfer

Fetal brain tissue fragments containing the suprachiasmatic nucleus were infected with an adenoviral vector containing the marker gene LacZ encoding for beta-galactosidase, and subsequently cultured or transplanted in the third ventricle of SCN-lesioned adult Wistar rats. In previous studies we optimized the infection procedure and characterized the immunological response directed against the viral vector in this model. The present study reports on beta-gal expression for at least 7 months in neuronal and glial cells. Maturation of the transplanted fetal SCN with respect to immunoreactivity for vasoactive intestinal polypeptide and C-terminal propressophysin was not hampered by the viral infection.

Long-term survival of human central nervous system progenitor cells transplanted into a rat model of Parkinson's disease

Progenitor cells were isolated from the developing human central nervous system (CNS), induced to divide using a combination of epidermal growth factor and fibroblast growth factor-2, and then transplanted into the striatum of adult rats with unilateral dopaminergic lesions. Large grafts were found at 2 weeks survival which contained many undifferentiated cells, some of which were migrating into the host striatum. However, by 20 weeks survival, only a thin strip of cells remained at the graft core while a large number of migrating astrocytes labeled with a human-specific antibody could be seen throughout the striatum. Fully differentiated graft-derived neurons, also labeled with a human-specific antibody, were seen close to the transplant site in some animals. A number of these neurons expressed tyrosine hydroxylase and were sufficient to partially ameliorate lesion-induced behavioral deficits in two animals. These results show that expanded populations of human CNS progenitor cells maintained in a proliferative state in culture can migrate and differentiate into both neurons and astrocytes following intracerebral grafting. As such these cells may have potential for development as an alternative source of tissue for neural transplantation in degenerative diseases.

Gene therapy for Parkinson's disease

Gene therapy is a potentially powerful approach to the treatment of neurological diseases. The discovery of neurotrophic factors inhibiting neurodegenerative processes and neurotransmitter-synthesizing enzymes provides the basis for current gene therapy strategies for Parkinson's disease. Genes can be transferred by viral or nonviral vectors. Of the various possible vectors, recombinant retroviruses are the most efficient for genetic modification of cells in vitro that can thereafter be used for transplantation (ex vivo gene therapy approach). Recently, in vivo gene transfer to the brain has been developed using adenovirus vectors. One of the advantages of recombinant adenovirus is that it can transduce both quiescent and actively dividing cells, thereby allowing both direct in vivo gene transfer and ex vivo gene transfer to neural cells. Probably because the brain is partially protected from the immune system, the expression of adenoviral vectors persists for several months with little inflammation. Novel therapeutic tools, such as vectors for gene therapy have to be evaluated in terms of efficacy and safety for future clinical trials. These vectors still need to be improved to allow long-term and possibly regulatable expression of the transgene.

In situ histochemical detection of beta-galactosidase activity in lung: assessment of X-Gal reagent in distinguishing lacZ gene expression and endogenous beta-galactosidase activity

Bacterial lacZ is one of the most commonly used reporter genes for assessing gene transfer to lung. However, lung contains endogenous beta-galactosidase (beta-Gal), which can confound estimation of exogenous lacZ expression by histochemical techniques (i.e., X-Gal) for in situ demonstration of enzyme activity. We investigated several parameters of the X-Gal reaction, including time and temperature of X-Gal exposure as well as lung tissue processing and fixation techniques, and found that none of these could be used to distinguish between endogenous and exogenous beta-Gal activities. The mammalian and bacterial beta-Gal enzymes, however, have pH optima in the acidic and neutral ranges, respectively. Exposing whole lung, lung minces, or mounted frozen sections of lung to X-Gal at mildly alkaline pH (pH 8.0-8.5), minimized detection of endogenous activity in lungs from a variety of species while preserving that resulting from bacterial enzyme activity in a transgenic mouse expressing lacZ. This technique was also useful in distinguishing endogenous activity from that resulting from adenovirus-mediated lacZ gene transfer to diploid lung fibroblasts in primary culture. An appropriate buffer that maintains the desired pH throughout the duration of X-Gal exposure must be used.

Intrastriatal injection of an adenoviral vector expressing glial-cell-line-derived neurotrophic factor prevents dopaminergic neuron degeneration and behavioral impairment in a rat model of Parkinson disease

Glial-cell-line-derived neurotrophic factor (GDNF) is a potent neurotrophic factor for adult nigral dopamine neurons in vivo. GDNF has both protective and restorative effects on the nigro-striatal dopaminergic (DA) system in animal models of Parkinson disease. Appropriate administration of this factor is essential for the success of its clinical application. Since it cannot cross the blood-brain barrier, a gene transfer method may be appropriate for delivery of the trophic factor to DA cells. We have constructed a recombinant adenovirus (Ad) encoding GDNF and injected it into rat striatum to make use of its ability to infect neurons and to be retrogradely transported by DA neurons. Ad-GDNF was found to drive production of large amounts of GDNF, as quantified by ELISA. The GDNF produced after gene transfer was biologically active: it increased the survival and differentiation of DA neurons in vitro. To test the efficacy of the Ad-mediated GDNF gene transfer in vivo, we used a progressive lesion model of Parkinson disease. Rats received injections unilaterally into their striatum first of Ad and then 6 days later of 6-hydroxydopamine. We found that mesencephalic nigral dopamine neurons of animals treated with the Ad-GDNF were protected, whereas those of animals treated with the Ad-beta-galactosidase were not. This protection was associated with a difference in motor function: amphetamine-induced turning was much lower in animals that received the Ad-GDNF than in the animals that received Ad-beta-galactosidase. This finding may have implications for the development of a treatment for Parkinson disease based on the use of neurotrophic factors.

Adenoviral vector-directed expression of neurotrophin-3 in rat dorsal root ganglion explants results in a robust neurite outgrowth response

The neurotrophins are a family of proteins that promote neuronal survival and neurite outgrowth during development and can also enhance the regeneration of injured adult neurons. The local and continuous delivery of these proteins at the site of injury is problematic, since this requires repeated intraparenchymal injections or the use of invasive canula-micropump devices. In the present study we report the generation and characterization of an adenoviral vector for a member of the neurotrophins, neurotrophin-3 (Ad-NT-3). Using Ad-NT-3, we examined the expression and biological activity of NT-3 in dorsal root ganglia (DRG) explant cultures. Gene transfer with Ad-NT-3 results in the synthesis of genuine NT-3 and in a dosage-dependent neurite outgrowth response in DRG explants. Transduction of DRG explants with a viral vector dosage of 5 x 10(5) to 5 x 10(6) plaque-forming units induced the formation of a dense halo of neurites comparable to outgrowth observed following the addition of 100 ng/mL exogenous NT-3. In addition, a single infection with Ad-NT-3 produced biologically active NT-3 for at least 20 days in culture, as evidenced by continued neurite extension. This indicates that adenoviral vector-mediated expression of NT-3 results in high-level production of biologically active NT-3 and could therefore be used as a strategy to promote the regeneration of injured peripheral and central nerve projections.

Transplant therapy: recovery of function after spinal cord injury

Spinal cord injuries (SCI) result in devastating loss of function and altered sensation. Presently, victims of SCI have few remedies for the loss of motor function and the altered sensation often experienced subsequent to the injury. A goal in SCI research is to improve function in both acute and chronic injuries. Among the most successful interventions is the utilization of transplanted tissues toward improved recovery. The theory is that the transplanted tissue could (1) bridge the spinal lesion and provide chemical and/or mechanical guidance for host neurons to grow across the lesion, (2) bridge the spinal lesion and provide additional cellular elements to repair the damaged circuitry, (3) provide factors that would rescue neurons that would otherwise die and/or modulate neural circuits to improve function. A variety of tissues and cells have been added to the adult mammalian spinal cord to encourage restoration of function. These include Schwann cells, motor neurons, dorsal root ganglia, adrenal tissue, hybridomas, peripheral nerves, and fetal spinal cord (FSC) tissue en bloc or as disassociated cells. It is postulated that these tissues would rescue or replace injured adult neurons, which would then integrate or promote the regeneration of the spinal cord circuitry and restore function. In some instances, host-appropriate circuitry is supplied by the transplant and functional improvement is demonstrated. In this presentation, specific examples of recent work with transplanted tissue and cells that demonstrate improved behavioral outcome are presented. New recent work describing the in vitro propagation and characterization of human fetal spinal cord multipotential progenitor cells are also described in the context of a potential resource for transplantable cells. Additionally, data from transplantation experiments of human FSC cells into nonimmunosuppressed rat spinal cord are described, and the resultant improvements in behavioral outcome reported. Lastly, directions for future SCI research are proposed.

In vivo adenovirus-mediated gene transfer to newborn rat retinal pigment epithelial cells

A successful surgical access to the subretinal space is critical for achieving adenovirus-mediated gene transfer to the retinal pigment epithelial (RPE) cells or photoreceptor cells. We report a novel surgical approach allowing an efficient delivery of recombinant replication-deficient adenoviral vectors into the subretinal space of newborn rats. Our data suggest that this method may be useful for infecting reproducibly large area of the RPE cell layer of normal newborn rats and should be applicable to RCS pups. We also show the feasibility of infecting ex vivo RPE cells in culture using the same recombinant adenoviral vector.

Long-term transgene expression in fetal rat suprachiasmatic nucleus neurografts following ex vivo adenoviral vector-mediated gene transfer

Ex vivo gene transfer to fetal suprachiasmatic nucleus (SCN)-containing solid piece neurografts was explored using a first-generation prototype adenoviral vector containing the reporter gene LacZ (Ad-LacZ). Transgene expression was examined at different intervals following grafting in the IIIrd ventricle of rat brain and was compared to that of explant cultures. Large numbers of beta-galactosidase-positive cells were observed 8 days postgrafting. The number of stained cells had decreased considerably at 21 days but transduced cells were still present at 70 days. In vitro culturing of infected SCN tissue revealed high expression up to 21 days, indicating that the in vivo and in vitro fates of Ad-LacZ-infected cells were different. The main reason for this difference appeared to be cell loss by necrosis in the initial phase after transplantation, a phenomenon not related to the infection with Ad-LacZ since it similarly occurred in control grafts. In vivo inflammatory responses, observed after immunostaining for macrophages and T-lymphocytes, were also comparable in control and Ad-LacZ-treated transplants, except that cytotoxic T-cells were observed in the Ad-LacZ-treated transplants and not in controls. The recruitment of these cells was, however, minor and primarily observed at 8 days postgrafting, indicating that a major immunological rejection of the transduced graft did not occur. In both control and Ad-LacZ-infected transplants similar survival and intraimplant neuritic growth of SCN cells were visible. Ex vivo gene transfer of solid piece fetal SCN grafts with adenoviral vectors therefore appeared to be a nontoxic long-term gene-introducing procedure. This would in principle enable the local production of neurotrophic factors within the transplant and has the potential to improve functional SCN neurografting.

Intrastriatal grafts of embryonic mesencephalic rat neurons genetically modified using an adenovirus encoding human Cu/Zn superoxide dismutase

Intrastriatal grafting of embryonic dopamine-containing neurons is a promising approach for treating clinical and experimental Parkinson's disease. However, neuropathological analyses of grafted patients and transplanted rats have demonstrated that the survival of grafted dopamine neurons is relatively poor. In the present study, we pursued a strategy of transferring a potentially neuroprotective gene into rat embryonic mesencephalic rat cells in vitro, before grafting them into the denervated striatum of 6-hydroxydopamine-lesioned rats. We performed intrastriatal grafts of embryonic day 14 mesencephalic cells infected with replication-defective adenoviruses bearing either the human copper-zinc superoxide dismutase gene or, as a control, the E. coli lac Z marker gene. The transgenes were expressed in the grafts four days after transplantation and the expression persisted for at least five weeks thereafter. After five weeks postgrafting, there was more extensive functional recovery in the superoxide dismutase group as compared to the control (uninfected cells) and beta-galactosidase groups. The functional recovery was significantly correlated with the number of tyrosine hydroxylase-positive cells in the grafts, although the clear trend to increased survival of the dopamine neurons in the superoxide dismutase grafts did not reach statistical significance. Only a moderate inflammatory reaction was revealed by OX-42 immunostaining in all groups, suggesting that ex vivo gene transfer using adenoviral vectors is a promising method for delivering functional proteins into brain grafts.

Morphological and morphometric analysis of serotonin-containing neurons in primary dissociated cultures of human rhombencephalon: a study of development

Primary dissociated cultures of rhombencephalon were prepared from 5-9-week-old human fetuses. Half of some cultures were treated by two non-competitive N-methyl-D-aspartate antagonists, namely 1-(2-thienyl)cyclohexylpiperidine (TCP) and cis-Pip/Mel-[1-(2-thienyl)-2-methyl-cyclohexyl]piperidine (GK11) in negative enantiomeric form, which enhance the survival of human fetal central nervous system cells in culture. At different days in vitro, the treated and the control cultures were processed for immunocytochemical detection of serotonin-containing neurons which were studied by morphological and morphometric analysis. Statistical analysis showed that the surface of the stained neurons increased as a function of two parameters of time, the gestational age of the cells and the duration of the cultures. The complexity of the shape of the serotonin neurons characterized by the shape factor, the number of bifurcations and the morphological feature (bipolar or multipolar) was found to increase with the gestational age. It appears that the in vitro development of the embryonic cells which represents stages of maturation and differentiation can be specifically evaluated. Such an analysis of fetal central nervous system cells improves the knowledge of factors important in grafting experiments. We verified that the two drugs do not appreciably alter the in vitro development of the treated cells; thus they may be considered as promising drugs for human neuroprotection.

Cellular and molecular neurosurgery: pathways from concept to reality--part I: target disorders and concept approaches to gene therapy of the central nervous system

Recent advances in cellular and molecular biology and better understanding of genetic and biochemical bases of different central nervous system (CNS) disorders have made gene therapy of the CNS a realistic goal. Concept approaches for gene therapy of CNS disorders are reviewed and include the following: 1) gene replacement with a single normal allele to correct the inherited global neurodegenerative disorders, such as enzyme deficiencies; 2) brain repair to restore the function of a particular subset of cells that were lost because of a neurodegenerative process; 3) gene therapy of brain tumors; and 4) gene therapy of stroke. Techniques of viral vector-mediated CNS transfer of a therapeutic gene, transplantation of genetically modified cells, fetal embryonic implantation and/or implantation of genetically engineered neural progenitor cells, and production of a specific enzyme, neurotransmitter, and/or growth factor are discussed with respect to the therapeutic potential for global and localized CNS neurodegenerative disorders and stroke. Transfection of the CNS tumor cells with the drug susceptibility ("suicide") gene and/or "toxic" gene and antisense strategies and a concept of adoptive immunotherapy of brain tumors are also discussed. Other approaches, such as transfer of drug-resistant genes and monoclonal antibody gene transfer, are briefly discussed. In addition to summarizing current principles of gene therapy for several groups of CNS disorders, the issues that remain to be resolved in clinical reality, such as delivery of the genetic material and regulation of the cellular expression of the transgene, and the negatives associated with the concepts of gene therapy, such as transient gene expression, toxicity of viral proteins, drawbacks of antisense therapy, and the problem of immune response to the transfected protein, have been also identified.

Adenovirus-mediated gene transfer to the central nervous system for Parkinson's disease

Gene therapy is a potentially powerful approach to the treatment of neurological diseases. The discovery of neurotrophic factors inhibiting neurodegenerative processes and the isolation of genes encoding neurotransmitter synthesizing enzymes provide the basis for current gene therapy strategies for Parkinson's disease. Adenovirus vectors have been shown recently to allow efficient gene transfer to the brain. One of the advantages of recombinant adenovirus is that it can transduce both quiescent and actively dividing cells. Thus expression of transgenes in neurons using adenoviruses is possible after either direct in vivo gene transfer or ex vivo gene transfer. In vivo gene transfer, consisting of the direct intracerebral injection of genetic material, is a novel method that is particularly efficient with the adenoviral vector. Ex vivo gene transfer, combining gene transduction with intracerebral transplantation, is a way to improve the classical grafts which are limited by poor cell survival in Parkinson's disease. Probably because the brain is a partially immunologically privileged site, the expression of adenoviral vectors persists for several months with little inflammation. Recombinant adenoviruses are currently being improved, particularly by inactivating viral genes controlling the expression of immunodominant viral proteins.

Application of ex vivo gene therapy in the treatment of Parkinson's disease

Ex vivo gene therapy approaches hold great promise for the treatment of neurodegenerative diseases where there is currently no cure or adequate treatment for affected individuals. In this review we have focused on the use of ex vivo gene transfer techniques in Parkinson's disease models; however, the issues and approaches outlined are applicable to other neurodegenerative disorders. In utilizing the ex vivo strategy two considerations are critical for delivery of therapeutic levels of transgene product to the target: (i) the vector system and (ii) the cell type for grafting. We describe herein different vector systems that are currently available and briefly review the various cell types that have been transduced and grafted into the striatum of animals with experimental Parkinson's disease. The strategies for application of gene therapy techniques to a treatment for Parkinson's disease have expanded beyond the classical dopamine replacement toward the use of neurotrophic factors in enhancing cell function or preventing cell death. In addition, we explore the utility of CNS-derived neural progenitors as alternative cell types for ex vivo gene therapy in an animal model of Parkinson's disease.

Recombinant adenovirus: a gene transfer vector for study and treatment of CNS diseases

Gene transfer to the CNS with recombinant adenoviral vectors is a relatively recent event. In initial reports it was clearly demonstrated that adenoviral vectors can transfer genetic material to multiple cell types within the CNS. The relative ease in generating recombinant adenovirus (Ad) led to feasibility studies in the CNS with application to animal models of inherited disease, neurodegenerative diseases (e.g., Parkinson's and amyotrophic lateral sclerosis), and cerebrovascular disease. In combination with Ad gene transfer to peripheral tissues, these experiments have identified specific limitations and directed further research to improve vector design, formulation, and delivery.

Adenovirus-mediated gene transfer to the brain: methodological assessment

The purpose of this short review is to analyse major advantages and limitations of the adenovirus (Ad), specifically with relevance to its use as a vector for gene transfer to the brain. The characteristics of Ad transduction include: the relative absence of cell type specificity; the limited spatial spread of the virus; and the long-term expression of the transgene. In the central nervous system, in contrast to that which occurs in other organs, Ad transduction in the adult does not systematically provoke cell death. Nevertheless, a proportion of the transduced cells do die, and this represents a conspicuous problem. Mechanisms leading to cell death in the brain may include immune rejection and inflammation-related toxicity, although this would not explain all of the results, and direct toxicity related to either inappropriate preparation or the transduction itself. Taking into account uncertainties concerning the innocuousness of Ad transduction, it may seem unwise to envisage Ad gene therapy for diseases that are not life-threatening and/or benefit from adequate drug or surgical treatments (e.g. Parkinson's disease or epilepsy). Ad vectors may not be easily used either in diseases displaying major immune dysfunction (e.g. multiple sclerosis). In contrast, malignant brain tumors and numerous neurodegenerative diseases (such as Huntington's, Alzheimer's diseases or amyotrophic lateral sclerosis) are directly life-threatening and deprived of any adequate treatment. They may be appropriate targets for Ad-mediated gene therapy, once both the vector and the gene of interest have been defined and optimized.

Transient gene transfer to neurons and glia: analysis of adenoviral vector performance in the CNS and PNS

In this paper a detailed protocol is presented for neuroscientists planning to start work on first generation recombinant adenoviral vectors as gene transfer agents for the nervous system. The performance of a prototype adenoviral vector encoding the bacterial lacZ gene as a reporter was studied, following direct injection in several regions of the central and peripheral nervous system. The distribution of the cells expressing the transgene appears to be determined by natural anatomical boundaries and possibly by the degree of myelinization of a particular brain region. In highly myelinated areas with a compact cellular structure (e.g. the cortex and olfactory bulb) the spread of the viral vector is limited to the region close to the injection needle, while in areas with a laminar structure (e.g. the hippocampus and the eye) more widespread transgene expression is observed. Retrograde transport of the viral vector may serve as an attractive alternative route of transgene delivery. A time course of expression of beta-galactosidase in neural cells in the facial nucleus revealed high expression during the first week after AdLacZ injection. However, a significant decline in transgene expression during the second and third week was observed. This may be caused by an immune response against the transduced cells or by silencing of the cytomegalovirus promoter used to drive transgene expression. Taken together, the data underscore that for each application of adenoviral vectors as gene transfer agents in the nervous system it is important to examine vector spread in and infectability of the neural structure that is subject to genetic modification.