Effects of choline and quiescence on Drosophila choline acetyltransferase expression and acetylcholine production by transduced rat fibroblasts

Long-term production of choline acetyltransferase in the CNS after transplantation of fibroblasts modified with a regulatable vector

A rat fibroblast cell line was modified to contain the Drosophila choline acetyltransferase (ChAT) cDNA under the control of a tetracycline-regulated system. Several clonal lines were assessed in vitro and in vivo to establish the optimal clone for gene therapy experiments. The influence of in vitro cell density on ChAT expression was compared to biological activity detected after grafting to the rat brain. While each clone had different ChAT activity patterns, all clones had low activity immediately post-grafting which increased over time, reaching a plateau between 1 and 2 months which was maintained for at least 1 year. The clones expressed a high basal ChAT activity level in vitro that was repressed in a dose- and time-dependent manner with doxycycline (DOX) treatment. In the absence of DOX, high levels of ChAT activity were maintained for at least 2 months in vitro. DOX induced a rapid and strong (200-fold) suppression of ChAT activity within 48 h. A dose-response curve indicated that the fibroblasts were very sensitive to low concentrations of DOX (ED50 12 pg/ml). Removal of DOX led to a derepression of ChAT activity within 2 days. These cells will be useful for ex vivo gene therapy of the cholinergic system.

High level transactivation by a modified Bombyx ecdysone receptor in mammalian cells without exogenous retinoid X receptor

Our studies of the Bombyx mori ecdysone receptor (BE) revealed that, unlike the Drosophila melanogaster ecdysone receptor (DE), treatment of BE with the ecdysone agonist tebufenozide stimulated high level transactivation in mammalian cells without adding an exogenous heterodimer partner. Gel mobility shift and transfection assays with both the ultraspiracle gene product (Usp) and retinoid X receptor heterodimer partners indicated that this property of BE stems from significantly augmented heterodimer complex formation and concomitant DNA binding. We have mapped this "gain of function" to determinants within the D and E domains of BE and demonstrated that, although the D domain determinant is sufficient for high affinity heterodimerization with Usp, both determinants are necessary for high affinity interaction with retinoid X receptor. Modified BE receptors alone used as replication-defective retroviruses potently stimulated separate "reporter" viruses in all cell types examined, suggesting that BE has potentially broad utility in the modulation of transgene expression in mammalian cells.

Hippocampal grafts of acetylcholine-producing cells are sufficient to improve behavioural performance following a unilateral fimbria-fornix lesion

Lesions of the septohippocampal pathway produce cognitive deficits that are partially attenuated by grafts of cholinergic-rich tissue into denervated target regions or by systemic administration of cholinomimetic drugs. In the present study, fibroblasts engineered to produce acetylcholine were used to test the hypothesis that restoration of hippocampal acetylcholine in rats with septohippocampal lesions is sufficient to improve cognitive processing post-damage. Rats received unilateral grafts of acetylcholine-producing or control fibroblasts into the hippocampus immediately prior to an aspirative lesion of the ipsilateral fimbria-fornix. Some rats with fimbria-fornix lesions were implanted with acetylcholine-producing or control fibroblasts into the neocortex, another major target of the basal forebrain cholinergic system, to determine if the site of acetylcholine delivery to the damaged brain is critical for functional recovery. Rats were tested in a hidden platform water maze task, a cued water maze task and activity chambers between one and three weeks post-grafting. Compared to unoperated controls, rats with fimbria fornix lesions only were significantly impaired in hidden platform water maze performance. Hippocampal grafts of acetylcholine-producing cells reduced lesion-induced deficits in the water maze, whereas hippocampal control grafts and cortical grafts of either cell type were without effect. Locomotor activity and cued water maze performance were unaffected by the lesion or the implants. Taken together, these data indicate that water maze deficits produced by fimbria fornix lesions, which disrupt a number of hippocampal neurotransmitter systems, can be attenuated by target specific replacement of acetylcholine in the hippocampus and that this recovery occurs in the absence of circuitry repair.

Potential effect of cytokines on transgene expression in primary fibroblasts implanted into the rat brain

Fibroblasts genetically modified with retroviral vectors fail to demonstrate long-term transgene expression upon implantation into the body. Although the mechanisms behind this phenomenon have not been elucidated, one likely cause is the response of the host to the graft. For example, genetically modified fibroblasts grafted into the brain are surrounded by activated microglia and astrocytes. The apparent inflammatory response can last for several weeks. In addition, the center of the graft is typically infiltrated with macrophage-like cells that appear to reside continuously within the graft. This proximity of inflammatory cells to the graft suggests that these cells may somehow influence transgene expression. In the current study, an in vitro model was used to test the effect cytokines [transforming growth factor-beta1 (TGF-beta1), interleukin-1beta, (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha)] that are typically released by peripheral macrophages, activated microglia and/or astrocytes have on long-terminal repeat (LTR)-driven transgene expression in primary fibroblasts. Our data demonstrate that these cytokines can significantly reduce the steady-state level of proviral mRNA. The amount of proviral mRNA returned to control levels within 24 h if the cytokines were removed. In addition, the down-regulation of proviral mRNA levels could be prevented if the cells were incubated with dexamethasone (25 microM) concurrent with the introduction of cytokines. These data demonstrate that cytokines can down-regulate LTR-driven transgene expression in primary fibroblasts maintained in culture. This interaction may be a major reason why transduced cells do not demonstrate long-term transgene expression in vivo.

Astrocytes promote or impair the survival and function of embryonic ventral mesencephalon co-grafts: effects of astrocyte age and expression of recombinant brain-derived neurotrophic factor

Intrastriatal grafting of dopamine-rich embryonic ventral mesencephalon (VM) is a potential therapeutic treatment for Parkinson's disease. However, it has been suggested that the efficacy of this procedure might be improved by enhancing the survival and/or degree of neurite outgrowth by the grafted VM, since these parameters are currently suboptimal. In the present study, we tested the ability of astrocytes retrovirally transduced to produce recombinant brain-derived neurotrophic factor (BDNF) to enhance the survival and/or function of embryonic VM in the unilateral 6-hydroxydopamine (6-OHDA) lesioned rat, a well-characterized rodent model of Parkinson's disease. In culture, primary astrocytes derived from Postnatal Day 0 (P0) rat striatum and transduced with the BDNF vector increased the survival of Embryonic Day 15 (E15) dopaminergic VM neurons by approximately threefold and reduced the loss of dopaminergic neurons following 6-OHDA treatment by approximately 20%. The cultured astrocytes were then mixed 1:1 with freshly dissociated E15 VM and co-grafted into the dopamine-denervated striatum. Unexpectedly, the control nontransduced astrocytes reduced the survival of dopaminergic neurons by 60% and restricted the pattern of neurite outgrowth by the co-grafted VM, compared to grafts of VM alone at 7 weeks postgrafting. These effects were paralleled by an attenuated rate and degree of behavioral recovery. The detrimental effects of the control astrocytes were partially reversed when the astrocytes were transduced to express BDNF, although dopaminergic neuron survival was still reduced by 30% compared to that within VM-only grafts. To begin to assess whether the detrimental effects of the astrocytes were related to the maturational state of the cultured astrocytes, astrocytes were obtained from E18 striatum and maintained in short-term culture (9 days vs several weeks for P0 cultures) prior to co-grafting with VM. Interestingly, the younger astrocytes did not reduce graft survival and allowed for better graft integration. These results suggest that primary astrocytes maintained in long-term culture are detrimental to embryonic neural grafts, an effect that is not completely overcome by expression of recombinant BDNF, and that astrocyte age may be an important consideration in the use of these cells as CNS gene delivery vehicles.

Discrete acetylcholine release from neuroblastoma or hybrid cells overexpressing choline acetyltransferase into the neuromuscular synaptic cleft

Neuroblastoma (clones NS-20Y, N1E-115, and Neuro2A) and neuroblastoma x glioma hybrid (NG108-15) cells were transfected with mouse choline acetyltransferase (ChAT) complementary DNA (cDNA) or vector DNA alone and stably transformed cell lines were established to examine their ability to secrete acetylcholine (ACh). Membrane potentials were recorded from either presynaptic neuroblastoma and hybrid cells or postsynaptic myotubes in co-culture. After transformation with ChAT, synapses were formed and miniature end-plate potentials (MEPPs) were recorded in myotubes co-cultured with Neuro2A and N1E-115 cells, while parental and mock-transfected control cells totally lacked this ability. The rate of synapse formation and/or MEPP frequency was higher in transformed NG108-15 hybrid and NS-20Y cells than that in the control cells. Action potentials of NS-20Y, Neuro2A or NG108-15 cells overexpressing ChAT were able to evoke end-plate potentials in myotubes, though the average quantum content of these cells was 0.04-0.14, which is as low as the control value. The results show that increased concentrations of ACh by ChAT cDNA transfection reveal a masked property in vesicular ACh release from Neuro2A and N1E-115 cells with no endogenous ChAT activity, or modify their secretory capacity upwardly from NG108-15 and NS-20Y cells with endogenous activity.

Overexpression of choline acetyltransferase reconstitutes discrete acetylcholine release in some but not all synapse formation-defective neuroblastoma cells

Secretion of acetylcholine (ACh) in neuroblastoma cells overexpressing choline acetyltransferase (ChAT) was examined. With transient transfection of ChAT cDNA, neuroblastoma cells, which have no endogenous ChAT and either adhere to myotubes or not, failed to form functional synapses, and thus no evidence for release of ACh was detected. Stable neuroblastoma cell lines overexpressing ChAT accumulated ACh inside the cell, and slowly released ACh to the outside of the cell in a calcium-independent fashion. However, after co-culturing them with rat muscle cells, these transformed cells adhered to myotubes and ACh was secreted in a discrete fashion into the synaptic cleft efficiently in some neuroblastoma cell lines but rather inefficiently in another cell line. The results show that the latent secretion machinery of ChAT overexpressing neuroblastoma cells either is competent or possess defect(s) in ACh release.

In vivo and ex vivo gene transfer to the brain

The use of gene transfer techniques to express novel proteins within different cellular populations has provided insights into the function and plasticity of the brain. Recently, this technique has been successfully used to explore physiological processes within the CNS and to intervene in neurodegenerative disease and cancer. Progress in manipulating transgene products in vivo and in achieving cell-specific targeting of genetic material offers promise for enhancing the usefulness of this technique and its therapeutic potential for treating human disorders of the CNS.

Nerve growth factor released by transgenic astrocytes enhances the function of adrenal chromaffin cell grafts in a rat model of Parkinson's disease

Previous studies have demonstrated that astrocytes genetically modified to express recombinant nerve growth factor (NGF) support the survival and neuronal transdifferentiation of intrastriatal adrenal chromaffin cell grafts at 2 weeks post-transplantation [15]. The present study was performed to determine whether these effects would be maintained at longer times post-transplantation and, if so, whether the co-grafts would reduce rotational behavior in the unilateral 6-hydroxydopamine-lesioned rat. In the present study, we have demonstrated that primary type I rat astrocytes infected with a replication-defective retrovirus conferring expression of a mouse beta-NGF cDNA sequence secrete NGF at a rate that is approximately 40-fold higher than that of controls (i.e., 8.0 vs. 0.2 pg NGF/h/10(5) cells, respectively). The genetically modified astrocytes were also found to express recombinant NGF following intrastriatal transplantation, as indicated by a 23% increase in striatal NGF content compared with controls, measured at 4 weeks post-transplantation. When NGF-producing astrocytes and adrenal chromaffin cells were co-grafted into the dopamine-denervated striatum of the unilateral 6-hydroxydopamine-lesioned rat, the chromaffin cells displayed extensive neurite outgrowth and a 5-12-fold increase in survival compared to controls at 10 weeks post-grafting. These effects were paralleled by a 60% reduction of apomorphine-induced rotational behavior, suggesting a partial normalization of striatal function. These results suggest that genetically modified astrocytes promote the prolonged survival and function of adrenal chromaffin cell grafts in a rat model of Parkinson's disease.

Choline acetyltransferase: celebrating its fiftieth year

It is well known that the regulation of choline acetyltransferase (ChAT) activity under physiological and pathological conditions is important for the development and neuronal activities of cholinergic systems involved in many fundamental brain functions. This review focuses on recent progress in understanding the regulation of ChAT at the levels of both the protein and the mRNA. A deficiency in ChAT activity has been reported for neurodegenerative conditions such as Alzheimer's disease, amyotrophic lateral sclerosis, and schizophrenia. Although a major feature of ChAT regulation is likely to involve the spatial and temporal control of transcription, regulation of expression can also be at the level of RNA processing, transport/translocation, turnover, or translation. In addition, there is increasing evidence that ChAT might be regulated at the posttranslational level by compartmentation and/or covalent modification, i.e., phosphorylation, as well as noncovalent modification (protein-protein interaction, etc.). Synaptic activity and the state of neuronal transmission may also involve the regulation of ChAT at different levels via both positive and negative feedback loops, as was demonstrated in the characterization of two ChAT mutant Drosophila strains. Clearly, identification of cholinergic-specific elements and the characterization of the trans-acting factors that bind to them represent an important area of future research. Equally important is research on the mechanisms governing ChAT as an enzymatic entity. The future should be an exciting time during which we look forward to the elucidation of the cholinergic signal and its regulation as well as the determination of the three-dimensional structure of the enzyme.

In vivo production and release of acetylcholine from primary fibroblasts genetically modified to express choline acetyltransferase

Primary rat fibroblasts genetically modified to express Drosophila choline acetyltransferase (dChAT) synthesize and release acetylcholine (ACh) in vitro. The ACh produced from the transduced fibroblasts was found to be enhanced by increasing amounts of choline chloride in the culture media. These dChAT-expressing cells were then implanted into the intact hippocampus of adult rats and in vivo microdialysis was performed 7-10 days after grafting to assess the ability of the cells to produce ACh and respond to exogenous choline in vivo. Samples collected from anesthetized rats revealed fourfold higher levels of ACh around dChAT grafts than from either non-grafted or control-grafted hippocampi. Localized choline infusion (200 microM) through the dialysis probes was found to induce a selective twofold increase in ACh release only from the dChAT-expressing fibroblasts. These results indicate not only that dChAT-expressing fibroblasts continue to synthesize and secrete ACh for at least 10 days after intracerebral grafting, but that the levels of ACh can be manipulated in vivo. The ability to regulate products within genetically modified cells in vivo may provide a powerful avenue for exploring the role of discrete substances within the CNS.

Cells engineered to produce acetylcholine: therapeutic potential for Alzheimer's disease

Alzheimer's disease (AD) is a debilitating disorder of the central nervous system which may affect up to 50% of the population over the age of 85 years. The etiology of AD is unknown and there is currently no cure for the disease. Well-documented losses in cholinergic and other neurotransmitter systems have provided a focal point for attempting pharmacological interventions in AD to ameliorate some of the cognitive deficits that occur. However, current systemic strategies have met with limited success. An alternative strategy, that has been pursued in animal models of neurodegenerative disease, is to augment neurotransmitter function within the brain through tissue transplantation. Such implants have an advantage over conventional drug therapies in that the cells can be precisely placed within compromised areas of the brain. We have pursued a strategy of designing cells, through the use of molecular biology techniques, to produce neurotrophic factors and neurotransmitters. Recently, we developed a primary fibroblast cell line that was genetically modified to express choline acetyltransferase (ChAT). In vitro, these cells produced and released acetylcholine at levels that varied with the amount of choline in the culture media. When implanted into the hippocampus of rats, the in vivo microdialysis technique revealed that the ChAT-expressing fibroblasts continued to produce and release acetylcholine after grafting. Most importantly, the levels of acetylcholine synthesized by the cells could be regulated by the localized infusion of choline in the vicinity of the grafts. These results confirmed previous work which indicated that engineered fibroblasts provide an effective delivery vehicle of different substances to the brain. While the intracerebral implantation of genetically modified cells will not cure AD, the continuing development of this strategy may ultimately provide a powerful approach for ameliorating the devastating cognitive impairments which are a hallmark of this disease.

In vitro labeling strategies for identifying primary neural tissue and a neuronal cell line after transplantation in the CNS

Potential labels for identifying embryonic raphe neurons and a clonal, neuronally differentiating, raphe-derived cell line, RN33B, in CNS transplantation studies were tested by first characterizing the labels in vitro. The labels that were tested included 4',6-diamidino-2-phenylindole hydrochloride, 1,1'-dioctadecyl-3,3,3'-tetramethylindocarbocyanine perchlorate, the Escherichia coli lacZ gene, Fast Blue, Fluoro-Gold, fluorescein-conjugated latex microspheres, fluorescein isothiocyanate-conjugated or nonconjugated Phaseolus vulgaris leucoagglutinin, methyl o-(6-amino-3'-imino-3H-xanthen-9-yl) benzoate monohydrochloride, or tetanus toxin C fragment. Subsequently, the optimal in vitro labels for embryonic raphe neurons and for RN33B cells were characterized in vivo after CNS transplantation. In vitro, 1,1'-dioctadecyl-3,3,3'-tetramethylindocarbocyanine perchlorate (DiI) optimally labeled embryonic neurons. The Escherichia coli lacZ gene optimally labeled RN33B cells. Most labels were rapidly diluted in cultures of embryonic astrocytes and proliferating RN33B cells. Some labels were toxic and were often retained in cellular debris. In vivo, DiI was visualized in transplanted, DiI-labeled raphe neurons, but not in astrocytes up to 1 mo posttransplant. DiI-labeled host cells were seen after transplantation of lysed, DiI-labeled cells. beta-Galactosidase was visualized in transplanted, Escherichia coli lacZ gene-labeled RN33B cells after 15 days in vivo. No beta-galactosidase was seen in host cells after transplantation of lysed, lacZ-labeled RN33B cells. The results demonstrate that labels for use in CNS transplantation studies should be optimized for the specific population of donor cells under study, with the initial step being characterization in vitro followed by in vivo analysis. Appropriate controls for false-positive labeling of host cells should always be assessed.