Neural precursor cells: applications for the study and repair of the central nervous system

Introduction to gene therapy in neurological surgery

Gene therapy is an exciting new discipline in which neurosurgery and neurosurgeons can have a direct impact on both patient care and emerging scientific developments. Unlike other organs, the brain is unique in that it has a blood–brain barrier, often preventing efficient systemic gene delivery to the area of interest. Therefore, not only is gene delivery required, but it will often need to be accomplished in a local and specific manner. Although brain neoplasms have been the most commonly studied application of genetic therapeutics in neurological surgery, there are many other potential applications of this technology to neurosurgical disorders, including spinal instability, neurodegenerative disease, neurogenetic diseases, central nervous system (CNS) injury, aneurysms, trauma, stroke, and epilepsy. As the field of gene therapy for the CNS develops from the preclinical setting to clinical trials to mainstream therapy, the need for safe and specific gene delivery will be increasingly apparent. Neurosurgeons are in an enviable position as there is nobody more qualified to address the issue of how a gene can be delivered to the central nervous system. Not only do we have the training to operate on the nervous system and its coverings, but we have the ability to recognize and take care of complications that may arise from these procedures. However, the neurosurgeon's role in gene therapy for the brain and spine should not be confined to gene delivery only. Instead, we also need to understand and have a role in deciding what genes could have therapeutic utility in our patients. Herein lies our challenge in gene therapy: how can we as neurosurgeons interface with our basic science colleagues to address rapidly all of the problems that are blocking the advancement of nucleic acid therapeutics into the clinical setting.

Effects of heat shock, heat shock protein 40 (HDJ-2), and proteasome inhibition on protein aggregation in cellular models of Huntington's disease

Huntington's disease (HD), spinocerebellar ataxias types 1 and 3 (SCA1, SCA3), and spinobulbar muscular atrophy (SBMA) are caused by CAG/polyglutamine expansion mutations. A feature of these diseases is ubiquitinated intraneuronal inclusions derived from the mutant proteins, which colocalize with heat shock proteins (HSPs) in SCA1 and SBMA and proteasomal components in SCA1, SCA3, and SBMA. Previous studies suggested that HSPs might protect against inclusion formation, because overexpression of HDJ-2/HSDJ (a human HSP40 homologue) reduced ataxin-1 (SCA1) and androgen receptor (SBMA) aggregate formation in HeLa cells. We investigated these phenomena by transiently transfecting part of huntingtin exon 1 in COS-7, PC12, and SH-SY5Y cells. Inclusion formation was not seen with constructs expressing 23 glutamines but was repeat length and time dependent for mutant constructs with 43-74 repeats. HSP70, HSP40, the 20S proteasome and ubiquitin colocalized with inclusions. Treatment with heat shock and lactacystin, a proteasome inhibitor, increased the proportion of mutant huntingtin exon 1-expressing cells with inclusions. Thus, inclusion formation may be enhanced in polyglutamine diseases, if the pathological process results in proteasome inhibition or a heat-shock response. Overexpression of HDJ-2/HSDJ did not modify inclusion formation in PC12 and SH-SY5Y cells but increased inclusion formation in COS-7 cells. To our knowledge, this is the first report of an HSP increasing aggregation of an abnormally folded protein in mammalian cells and expands the current understanding of the roles of HDJ-2/HSDJ in protein folding.

Effects of spinal cord X-irradiation on the recovery of paraplegic rats

Axonal regrowth is limited in the adult CNS, especially in the spinal cord, one of the major sites of traumatic lesions. Pathophysiological changes occurring after spinal cord injury include complex acute, subacute, and late processes. In this study, we assessed whether X-irradiation interferes with the acute/subacute phases, thereby improving the functional recovery of paraplegic animals. Two days after acute compression of adult rat spinal cords, various doses (0, 2, 5, 10, 20 Gy) of X-rays were administered as one single dose to the compression site. The animals were functionally evaluated over the course of 1 month after injury, using the Tarlov scale and the Rivlin and Tator scale. We also designed a "physiological" scale, including an assessment of urinary function and infection, appropriate for the evaluation of spinal-cord-lesioned animals. Behavioral analysis suggested that the high doses, 20 Gy and, to a lesser extent, 5 and 10 Gy, were toxic, as shown by morbidity rate and "physiological" score. The 2-Gy group showed better motor performances than the lesioned nonirradiated (LNI) animals and the 5- and 20-Gy groups. Motor performance in the 5-, 10-, and 20-Gy groups was poorer than that seen in the LNI group. Gliosis was reduced in the 2-Gy group compared to LNI animals, and there was high levels of gliosis in the highly (>/=5 Gy) irradiated animals. There was a 23% less lesion-induced syringomyelia in the 2-Gy group than in the other groups (LNI and 5-20 Gy). Thus, low doses of X-rays may interfere with the formation of syringomyelia and glial scar, thereby facilitating the recovery of paraplegic animals. These findings suggest that low-dose irradiation of the lesion site, in association with other therapies, is a potentially promising treatment for improving recovery after spinal cord injury.

Establishment and properties of a growth factor-dependent, perpetual neural stem cell line from the human CNS

The ready availability of unlimited quantities of neural stem cells derived from the human brain holds great interest for basic and applied neuroscience, including therapeutic cell replacement and gene transfer following transplantation. We report here the combination of epigenetic and genetic procedures for perpetuating human neural stem cell lines. Thus we tested various culture conditions and genes for those that optimally allow for the continuous, rapid expansion and passaging of human neural stem cells. Among them, v-myc (the p110 gag-myc fusion protein derived from the avian retroviral genome) seems to be the most effective gene; we have also identified a strict requirement for the presence of mitogens (FGF-2 and EGF) in the growth medium, in effect constituting a conditional perpetuality or immortalization. A monoclonal, nestin-positive, human neural stem cell line (HNSC.100) perpetuated in this way divides every 40 h and stops dividing upon mitogen removal, undergoing spontaneous morphological differentiation and upregulating markers of the three fundamental lineages in the CNS (neurons, astrocytes, and oligodendrocytes). HNSC.100 cells therefore retain basic features of epigenetically expanded human neural stem cells. Clonal analysis confirmed the stability, multipotency, and self-renewability of the cell line. Finally, HNSC.100 can be transfected and transduced using a variety of procedures and genes encoding proteins for marking purposes and of therapeutic interest (e.g., human tyrosine hydroxylase I).

BDNF gene transfer to the mammalian brain using CNS-derived neural precursors

Neural stem cell lines represent a homogeneous source of cells for genetic, developmental, and gene transfer and repair studies in the nervous system. Since both gene transfer of neurotrophic factors and cell replacement strategies are of immediate interest for therapeutical purposes, we have generated BDNF-secreting neural stem cell lines and investigated to what extent different endogenous levels of BDNF expression affect in vitro survival, proliferation and differentiation of these cells. Also, we have investigated the in vivo effects of such BDNF gene transfer procedure in the rat neostriatum. Hippocampus- and cerebellum-derived cell lines reacted differently to manipulations aimed at varying their levels of BDNF production. Over-expression of BDNF enhanced survival of both cell types, in a serum-deprivation assay. Conversely, and ruling out unspecific effects, expression of an antisense version of BDNF resulted in compromised survival of cerebellum-derived cells, and in a lethal phenotype in hippocampal progenitors. These data indicate that endogenous BDNF level strongly influences the in vitro survival of these cells. These effects are more pronounced for hippocampus- than for cerebellum-derived progenitors. Hippocampus-derived BDNF overproducers showed no major change in their capacity to differentiate towards a neuronal phenotype in vitro. In contrast, cerebellar progenitors overproducing BDNF did not differentiate into neurons, whereas cells expressing the antisense BDNF construct generated cells with morphological features of neurons and expressing immunological neuronal markers. Taken together, these results provide evidence that BDNF controls both the in vitro survival and differentiation of neural stem cells. After in vivo transplantation of BDNF-overproducing cells to the rat neostriatum, these survived better than the control ones, and induced the expected neurotrophic effects on cholinergic neurons. However, long-term (3 months) administration of BDNF resulted in detrimental effects, at this location. These findings may be of importance for the understanding of brain development, for the design of therapeutic neuro-regenerative strategies, and for cell replacement and gene therapy studies.

Molecular Pathology of Huntington's Disease: Animal Models and Nuclear Mechanisms

Huntington's disease (HD) is a late-onset neurodegenerative disorder caused by a polyglutamine expansion in huntingtin, a protein of unknown function. Transgenic models expressing a portion or full-length human huntingtin have been generated to unravel the mechanism through which the mutation causes the symptoms and the selective cell death characteristic of HD. We review here advances in the understanding of HD made possible by transgenic models and the means by which they implicate polyglutamine aggregation in the pathology of triplet repeat disorders.

The search for neural progenitor cells: prospects for the therapy of neurodegenerative disease

The etiology of many neurodegenerative diseases has been identified in recent years. Treatment of central nervous system (CNS) disease could focus on one or more steps that lead to cell loss. In the past decade, cell therapy and/or ex vivo gene therapy have emerged as possible strategies for the treatment of neurodegenerative diseases. The ability to grow CNS-derived neural progenitor cells using growth factors has been extremely useful to study diverse phenomena including lineage choice, commitment and differentiation. By virtue of their biological properties and their presence in the adult CNS, neural progenitors represent good candidates for multiple cell-based therapies for neural diseases. Further identification of the molecules that direct the differentiation of adult neural progenitors may allow their activation in vivo to induce self-repair. This review addresses the nature, distribution and regulation of neural stem cells and the potential for applying these cells to both structural CNS repair and gene therapy.

A molecular investigation of true dominance in Huntington's disease

Huntington's disease (HD) is thought to show true dominance, since subjects with two mutant alleles have been reported to have similar ages at onset of disease compared to heterozygous sibs. We have investigated this phenomenon using a cell culture model. Protein aggregate formation was used as an indicator for pathology, as intraneuronal huntingtin inclusions are associated with pathology in vitro and in vivo. We showed that cytoplasmic and nuclear aggregates are formed by constructs comprising part of exon 1 of huntingtin with 41, 51, 66, or 72 CAG repeats, in a rate that correlates with repeat number. No inclusions were seen with 21 CAG repeat constructs. Mutant and wild type huntingtin fragments can be sequestered into inclusions seeded by a mutant huntingtin. Wild type huntingtin did not enhance or interfere with protein aggregation. The rate of protein aggregation was dose dependent for all mutant constructs tested. These experiments suggested a model for the dominance observed in HD; the decrease in the age at onset of a mutant homozygote may be small compared to the variance in the age at onset for that specific repeat number in heterozygotes. Our experiments also provide a model, which may explain the different repeat size ranges seen in patients and healthy controls for the different polyglutamine diseases.

Focal cortical dysplasia: a neuropathological and developmental perspective

Focal cortical dysplasia (FCD) is a rare, sporadic disorder which is a recognised cause of chronic epilepsy. It is proposed to result from disordered neuronal migration and differentiation and has characteristic histological features which include disturbed cortical lamination, large abnormal neurons and the presence of large balloon cells with glassy eosinophilic cytoplasm and pleomorphic eccentric nuclei. These latter express both glial and neuronal markers indicative of abnormal neuroglial differentiation. In this paper we review the current literature on the neuropathology of FCD and discuss potential mechanisms. We focus on growth factors, signalling pathways and candidate genes with known roles in Drosophila and vertebrate brain development that could be responsible for the developmental brain changes seen in FCD. At issue are the factors that influence cell fate and differentiation and which regulate neural migration. Some of the molecular pathways, such as those involving the Notch and the Wnt pathways have particularly important roles in neuroglial differentiation in vertebrates, and these are proposed as potential candidates.

Genetic regulatory elements introduced into neural stem and progenitor cell populations

The genetic manipulation of neural cells has advantage in both basic biology and medicine. Its utility has provided a clearer understanding of how the survival, connectivity, and chemical phenotype of neurones is regulated during, and after, embryogenesis. Much of this achievement has come from the recent generation by genetic means of reproducible and representative supplies of precursor cells which can then be analyzed in a variety of paradigms. Furthermore, advances made in the clinical use of transplantation for neurodegenerative disease have created a demand for an abundant, efficacious and safe supply of neural cells for grafting. This review describes how genetic methods, in juxtaposition to epigenetic means, have been used advantageously to achieve this goal. In particular, we detail how gene transfer techniques have been developed to enable cell immortalization, manipulation of cell differentiation and commitment, and the controlled selection of cells for purification or safety purposes. In addition, it is now also possible to genetically modify antigen presentation on cell surfaces. Finally, there is detailed the transfer of therapeutic products to discrete parts of the central nervous system (CNS), using neural cells as elegant and sophisticated delivery vehicles. In conclusion, once the epigenetic and genetic controls over neural cell production, differentiation and death have been more fully determined, providing a mixture of hard-wired elements and more flexibly expressed characteristics becomes feasible. Optimization of the contributions and interactions of these two controlling systems should lead to improved cell supplies for neurotransplantation.

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.

Generation of tyrosine hydroxylase-producing neurons from precursors of the embryonic and adult forebrain

We have explored the plastic ability of neuronal precursors to acquire different identities by manipulating their surrounding environment. Specifically, we sought to identify potential signals involved in the specification of forebrain dopaminergic neurons. Here we describe culture conditions under which tyrosine hydroxylase (TH) expression is induced in neuronal precursors, which were derived directly from the embryonic striatum and adult subependyma (SE) of the lateral ventricle or generated from multipotent forebrain stem cells. TH was successfully induced in all of these cell types by 24 hr exposure to basic fibroblast growth factor (FGF2) and glial cell conditioned media (CM). The greatest magnitude of the inductive action was on embryonic striatal precursors. Although FGF2 alone induced limited TH expression in striatal cells (1.1 +/- 0.2% of neurons), these actions were potentiated 17.5-fold (19.6 +/- 1.5% of neurons) when FGF2 was coadministered with B49 glial cell line CM. Of these TH-immunoreactive cells, approximately 15% incorporated bromodeoxyuridine (BrdU), indicating that they were newly generated, and 95% coexpressed the neurotransmitter GABA. To investigate whether precursors of the adult forebrain subependyma were competent to respond to the instructive actions of FGF2+CM, they were first labeled in vivo with a pulse of BrdU. Although none of the cells expressed TH in control, 0.2% of total cells showed TH immunoreactivity in FGF2+CM-treated cultures. Under these same conditions only, in vitro-generated precursors from epidermal growth factor-responsive stem cells exhibited TH expression in 10% of their total neuronal progeny. Regulation of neurotransmitter phenotype in forebrain neuronal precursors, by the synergistic action of FGF2 and glial-derived diffusible factors, may represent a first step in understanding how these cells are generated in the embryonic and adult brain and opens the prospect for their manipulation in vitro and in vivo for therapeutic use.

Self-assembly of polyglutamine-containing huntingtin fragments into amyloid-like fibrils: implications for Huntington's disease pathology

Huntington's disease is a progressive neurodegenerative disorder caused by a polyglutamine [poly(Q)] repeat expansion in the first exon of the huntingtin protein. Previously, we showed that N-terminal huntingtin peptides with poly(Q) tracts in the pathological range (51-122 glutamines), but not with poly(Q) tracts in the normal range (20 and 30 glutamines), form high molecular weight protein aggregates with a fibrillar or ribbon-like morphology, reminiscent of scrapie prion rods and beta-amyloid fibrils in Alzheimer's disease. Here we report that the formation of amyloid-like huntingtin aggregates in vitro not only depends on poly(Q) repeat length but also critically depends on protein concentration and time. Furthermore, the in vitro aggregation of huntingtin can be seeded by preformed fibrils. Together, these results suggest that amyloid fibrillogenesis in Huntington's disease, like in Alzheimer's disease, is a nucleation-dependent polymerization.

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.

Glutamine repeats and neurodegenerative diseases: molecular aspects

Eight severe inherited neurodegenerative diseases are caused by expansion of glutamine repeats in the affected proteins. In every case, proteins with repeats of fewer than 38 glutamine residues are harmless, but those with repeats of more than 41 glutamine residues form toxic neuronal nuclear aggregates in the affected neurons. Similarly, proteins that have repeats of fewer than 37 glutamine residues are soluble in vitro, whereas proteins with repeats of more than 40 glutamine residues precipitate as insoluble fibres, apparently because of a structural transition associated with the increased length.

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.

Neuron-enriched second trimester human cultures: growth factor response and in vivo graft survival

Grafts of first trimester fetal tissue show limited survival and integration in the adult CNS. Alternative grafting strategies have been sought for treatment of neurodegenerative disease. We have developed cultures of human second trimester fetal tissues to study neuronal differentiation. Grafted into the SCID mouse striatum, aggregates of these cultures formed neuron-rich xenografts for at least 8 months. We examined the influence of various neurotrophic factors, including basic fibroblast growth factor (bFGF), brain-derived neurotrophic factor (BDNF), transforming growth factor-beta 1 (TGF-beta1), and hepatocyte growth factor (HGF), on the growth and differentiation of neuronal and glial cell populations. BDNF promoted the survival and differentiation of second trimester neurons whereas bFGF exhibited a strong proliferative effect on precursors and the astroglial population. Our data suggest that second trimester human fetal cultures contain neuroprogenitor cells that can be directed to the neuronal lineage. This process may be amplified by treatment with BDNF, which we hypothesize could improve the long-term in vivo survival of neuron-enriched grafts.

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.

Batten disease: four genes and still counting

The neuronal ceroid lipofuscinoses (NCLs, also known as Batten disease) are the most common childhood neurodegenerative disease. They are a group of inherited neurodegenerative disorders characterized by the accumulation of autofluorescent storage material in many cell types. Clinical features include seizures, psychomotor deterioration, and blindness, the ages and order of onset of which differ for each NCL type. An increasing number of subtypes caused by mutations in different genes are now recognized. With the advent of molecular genetics the basic genetic defect underlying each NCL phenotype is being determined, thus shedding light on the molecular basis of the NCLs and opening the way for the development of effective treatment. Four genes have been identified to date. The function of two of these is known and suggests that the primary defect in the NCLs lies in lysosomal proteolysis, the first example of this type of disease. However, since the function of the other two genes remains elusive, and at least four more genes remain to be identified, the molecular basis underlying the NCLs may be more complex than originally predicted.

Multipotent progenitor cells in the adult dentate gyrus

Neurogenesis persists in the adult dentate gyrus of rodents throughout the life of the organism. The factors regulating proliferation, survival, migration, and differentiation of neuronal progenitors are now being elucidated. Cells from the adult hippocampus can be propagated, cloned in vitro, and induced to differentiate into neurons and glial cells. Cells cultured from the adult rodent hippocampus can be genetically marked and transplanted back to the adult brain, where they survive and differentiate into mature neurons and glial cells. Although multipotent stem cells exist in the adult rodent dentate gyrus, their biological significance remains elusive.

HAP1-huntingtin interactions do not contribute to the molecular pathology in Huntington's disease transgenic mice

HAP1 (huntingtin associated protein) has previously been found to interact with huntingtin (htt) in a glutamine length dependent manner and has been proposed to play a role in the cell specific neurodegeneration observed in Huntington's disease (HD). We have isolated mouse HAP1 (hap1) and have shown that expression is not enriched in areas specifically affected in HD. We have used the yeast two hybrid system to demonstrate that htt amino acids 171-230 are necessary for the hap1-htt binding and that hapl does not interact with the transgene exon 1 protein in a transgenic model of HD.

The use of nonneuronal cells for gene delivery

The implantation of genetically engineered nonneuronal cells can provide an effective method for achieving localized delivery of discrete molecules to the CNS or for providing substrates for regrowth of neural structures. Most primary nonneuronal cells have the advantage of being easily obtainable from the prospective host for ex vivo retrovirus-mediated genetic manipulation (most will be mitotic in culture) and reimplantation as an autologous graft (circumventing the problem of immune rejection). As primary cells, they are unlikely to be tumorigenic. The most vexing problem for such systems remains the apparent loss of transgene expression from viral promoters after prolonged periods of engraftment. Much effort is currently being directed at optimizing sustained transgene expression by varying the promoters, by varying the cell types to be engineered, or by regulating expression by enhancing promoter function or substrate availability. While nonneuronal cells are excellent vehicles for achieving passive delivery of substances to the CNS, they lack the ability to incorporate into the host cytoarchitecture in a functional manner (e.g., make synaptic contacts). For this reason, not only may certain essential circuits not be re-formed, but the regulated release of certain substances through feedback loops may be missing. While apparently unimportant for some substances (e.g., ACh), for others (e.g., NGF), their unregulated, inappropriate, excessive, or ectopic release may actually be inimical to the host. Furthermore, the loss of foreign gene expression (the bane of gene therapy) may leave engineered nonneural cells incapacitated, whereas donor tissue originating from brain may intrinsically produce various CNS factors allowing correction to proceed despite inactivation of the introduced gene. In fact, CNS-derived tissue may provide as-yet-unrecognized endogenous neuralspecific substances which are equally as beneficial to the host as the gene in question. Thus, future developments in gene delivery to the brain for some conditions may emphasize using neurons or neural progenitors for ex vivo genetic manipulation (Fisher, 1997) and refining techniques for the direct injection of therapeutic genes into neurons in vivo (see Snyder and Fisher, 1996). For a wide variety of conditions, however, using nonneuronal cellular vehicles or even nonbiologic synthetic vehicles may be efficient, effective, and safe strategies for the passive delivery of therapeutic molecules to discrete regions of the CNS. In fact, this approach may come closer than any other to immediate human applications.