Herpesvirus-mediated gene delivery into the rat brain: specificity and efficiency of the neuron-specific enolase promoter

Atomic structure of the human cytomegalovirus capsid with its securing tegument layer of pp150

Herpesviruses possess a genome-pressurized capsid. The 235-kilobase genome of human cytomegalovirus (HCMV) is by far the largest of any herpesvirus, yet it has been unclear how its capsid, which is similar in size to those of other herpesviruses, is stabilized. Here we report a HCMV atomic structure consisting of the herpesvirus-conserved capsid proteins MCP, Tri1, Tri2, and SCP and the HCMV-specific tegument protein pp150-totaling ~4000 molecules and 62 different conformers. MCPs manifest as a complex of insertions around a bacteriophage HK97 gp5-like domain, which gives rise to three classes of capsid floor-defining interactions; triplexes, composed of two "embracing" Tri2 conformers and a "third-wheeling" Tri1, fasten the capsid floor. HCMV-specific strategies include using hexon channels to accommodate the genome and pp150 helix bundles to secure the capsid via cysteine tetrad-to-SCP interactions. Our structure should inform rational design of countermeasures against HCMV, other herpesviruses, and even HIV/AIDS.

Characterization of neural stem cells modified with hypoxia/neuron-specific VEGF expression system for spinal cord injury

Spinal cord injury (SCI) is an incurable disease causing an ischemic environment and functional defect, thus a new therapeutic approach is needed for SCI treatment. Vascular endothelial growth factor (VEGF) is a potent therapeutic gene to treat SCI via angiogenesis and neuroprotection, and both tissue-specific gene expression and high gene delivery efficiency are important for successful gene therapy. Here we design the hypoxia/neuron dual-specific gene expression system (pEpo-NSE) and efficient gene delivery platform can be achieved by the combination ex vivo gene therapy with erythropoietin (Epo) enhancer, neuron-specific enolase (NSE) promoter and neural stem cells (NSCs). An in vitro model, NSCs transfected with pEpo-NSE were consistently and selectively overexpressing therapeutic genes in response to neural differentiation and hypoxic conditions. Also, in SCI model, ex vivo gene therapy using pEpo-NSE system with NSCs significantly enhanced gene delivery efficiency compared with pEpo-NSE system gene therapy alone. However, microarray analysis reveals that introducing exogenous pEpo-NSE and VEGF triggers biological pathways in NSCs such as glycolysis and signaling pathways such as Ras and mitogen-activated protein kinase, leading to cell proliferation, differentiation and apoptosis. Collectively, it indicates that the pEpo-NSE gene expression system works stably in NSCs and ex vivo gene therapy using pEpo-NSE system with NSCs improves gene expression efficiency. However, exogenously introduced pEpo-NSE system has an influence on gene expression profiles in NSCs. Therefore, when we consider ex vivo gene therapy for SCI, the effects of changes in gene expression profiles in NSCs on safety should be investigated.

Diffusion and clearance of superparamagnetic iron oxide nanoparticles infused into the rat striatum studied by MRI and histochemical techniques

The purpose of the present study was to investigate, by MRI and histochemical techniques, the diffusion and clearance abilities of superparamagnetic iron oxide nanoparticles (SPION) coated with dextran (Dextran-SPION) and gold (Au-SPION) following their local infusions into the rat brain. In separate groups of anesthetized rats, the Dextran-SPION and Au-SPION were infused at concentrations of 0.01, 0.1, 1 and 5 µg Fe/0.5 µl and at the flow rate of 0.5 µl min(-1) into the left and right striata, respectively. Repetitive T2-weighted spin-echo MRI scans were performed at time intervals of 1, 6, 12, 24, 48, 72 h, and one, two and eight weeks after inoculation. Following infusion of Dextran-SPION (0.1 µg and 1 µg Fe), the maximal distribution volume was observed at about 12-24 h after inoculation and two weeks later the Fe signals were undetectable for the lower dose. On the other hand, Au-SPION remained tightly localized in the closest vicinity of the infusion site as revealed by unchanged MRI signal intensities and strong histochemical staining of Fe(2+) and Fe(3+) ions in the corresponding brain slices. Immunohistochemical staining of astrocytic and microglial reactions revealed that there were no marked differences in GFAP, VIM or OX-42 labeling observed between the nanoparticle types, however the astrocytic reaction was more pronounced in rats receiving nanoparticles compared to the control (aCSF-infused) rats. In conclusion, the present data demonstrate that the viral-sized Dextran-SPION were able to diffuse freely through the interstitial space of the brain being progressively cleared out from the infusion site within two weeks. Thus, Dextran-SPION could be beneficially used in MRI-guided diagnostic applications such as in experimental oncology or as labels and carriers for targeted drug delivery, whereas Au-SPION could be used for labeling and tracking the transplanted stem cells in experimental MRI.

Physiological and tissue-specific vectors for treatment of inherited diseases

After more than 1500 gene therapy clinical trials in the past two decades, the overall conclusion is that for gene therapy (GT) to be successful, the vector systems must still be improved in terms of delivery, expression and safety. The recent development of more efficient and stable vector systems has created great expectations for the future of GT. Impressive results were obtained in three primary immunodeficiencies and other inherited diseases such as congenital blindness, adrenoleukodystrophy or junctional epidermolysis bullosa. However, the development of leukemia in five children included in the GT clinical trials for X-linked severe combined immunodeficiency and the silencing of the therapeutic gene in the chronic granulomatous disease clearly showed the importance of improving safety and efficiency. In this review, we focus on the main strategies available to achieve physiological or tissue-specific expression of therapeutic transgenes and discuss the importance of controlling transgene expression to improve safety. We propose that tissue-specific and/or physiological viral vectors offer the best balance between efficiency and safety and will be the tools of choice for future clinical trials in GT of inherited diseases.

Lack of humoral immune response to the tetracycline (Tet) activator in rats injected intracranially with Tet-off rAAV vectors

The ability to safely control transgene expression from viral vectors is a long-term goal in the gene therapy field. We have previously reported tight regulation of GFP expression in rat brain using a self-regulating tet-off rAAV vector. The immune responses against tet regulatory elements observed by other groups in nonhuman primates after intramuscular injection of tet-on encoding vectors raise concerns about the clinical value of tet-regulated vectors. However, previous studies have not examined immune responses following injection of AAV vectors into brain. Therefore, rat striatum was injected with tet-off rAAV harboring a therapeutic gene for Parkinson's disease, either hAADC or hGDNF. The expression of each gene was tightly controlled by the tet-off regulatory system. Using an ELISA developed with purified GST-tTA protein, no detectable immunogenicity against tTA was observed in sera of rats that received an intrastriatal injection of either vector. In contrast, sera from rats intradermally injected with an adenovirus containing either tTA or rtTA, as positive controls, had readily detectable antibodies. These observations suggest that tet-off rAAV vectors do not elicit an immune response when injected into rat brain and that these may offer safer vectors for Parkinson's disease than vectors with constitutive expression.

Thinking about repairing thinking

The work of Sinden et al. suggests that it may be possible to produce improvement in the “highest” areas of brain function by transplanting brain tissue. What appears to be the limiting factor is not the complexity of the mental process under consideration but the discreteness of the lesion which causes the impairment and the appropriateness and accuracy of placement of the grafted tissue.

Therapeutic uses for neural grafts: Progress slowed but not abandoned

In spite of Stein and Glasier's justifiable conclusion that initial optimism concerning the immediate clinical applicability of neural transplantation was premature, there exists much experimental evidence to support the potential for incorporating this procedure into a therapeutic arsenal in the future. To realize this potential will require continued evolution of our knowledge at multiple levels of the clinical and basic neurosciences.

The structure, operation, and functionality of intracerebral grafts

The concept of structure, operation, and functionality, as they may be understood by clinicians or researchers using neural transplantation techniques, are briefly defined. Following Stein & Glasier, we emphasize that the question of whether an intracerebral graft is really functional should be addressed not only in terms of what such a graft does in a given brain structure, but also in terms of what it does at the level of the organism.

The NGF superfamily of neurotrophins: Potential treatment for Alzheimer's and Parkinson's disease

Stein & Glasier suggest embryonic neural tissue grafts as a potential treatment strategy for Alzheimer's and Parkinson's disease. As an alternative, we suggest that the family of nerve growth factor-related neurotrophins and their trk (tyrosine kinase) receptors underlie cholinergic basal forebrain (CBF) and dopaminergic substantia nigra neuron degeneration in these diseases, respectively. Therefore, treatment approaches for these disorders could utilize neurotrophins.

Some practical and theoretical issues concerning fetal brain tissue grafts as therapy for brain dysfunctions

Grafts of embryonic neural tissue into the brains of adult patients are currently being used to treat Parkinson's disease and are under serious consideration as therapy for a variety of other degenerative and traumatic disorders. This target article evaluates the use of transplants to promote recovery from brain injury and highlights the kinds of questions and problems that must be addressed before this form of therapy is routinely applied. It has been argued that neural transplantation can promote functional recovery through the replacement of damaged nerve cells, the reestablishment of specific nerve pathways lost as a result of injury, the release of specific neurotransmitters, or the production of factors that promote neuronal growth. The latter two mechanisms, which need not rely on anatomical connections to the host brain, are open to examination for nonsurgical, less intrusive therapeutic use. Certain subjective judgments used to select patients who will receive grafts and in assessment of the outcome of graft therapy make it difficult to evaluate the procedure. In addition, little long-term assessment of transplant efficacy and effect has been done in nonhuman primates. Carefully controlled human studies, with multiple testing paradigms, are also needed to establish the efficacy of transplant therapy.

Models of neurological defects and defects in neurological models

The transition from research to patient following advances in transplantation research is likely to be disappointing unless it includes a better understanding of critically relevant characteristics of the neurological disorder and improvements in the animal models, particularly the behavioral features. The appropriateness of the model has less to do with the species than with how the species is used.

Noninvasive in vivo monitoring of neuronal differentiation using reporter driven by a neuronal promoter

PURPOSE

We imaged neuronal differentiation in vivo using dual reporters (sodium iodide symporter [NIS] and luciferase) coupled to a neuron-specific enolase (NSE) promoter.

METHODS

PC12 (NSE positive) and F11 cells were transfected with a bicistronic (NIS and luciferase; pNSE-NF) or a luciferase (pNSE-Fluc) reporter coupled to the NSE promoter. Weak NSE promoter activity was overcome by a two-step transcriptional amplification (TSTA) system (pNSE-TSTA-Fluc). In vivo, NIS and luciferase expression were examined using a (99m)Tc-pertechnetate gamma camera and bioluminescence imaging, respectively.

RESULTS

pNSE-NF-transfected PC12 cells showed 3-fold higher radioiodine uptakes and >100-fold higher luciferase activity than parental cells. NIS or luciferase activity was not detected in pNSE-NF-transfected HeLa cells. When F11 cells were differentiated into neurons by db-cAMP, NIS and luciferase activities increased 4-fold compared to those without treatment, which was confirmed by Western blot and RT-PCR of NSE. In vivo in pNSE-NF-transfected F11 cells, db-cAMP treatment increased the luciferase activity but not the scintigraphic activity. In vitro, pNSE-TSTA-Fluc produced 130-fold higher luciferase activity than pNSE-Fluc and neuronal differentiation showed 4-fold higher activity from both pNSE-TSTA-Fluc and pNSE-Fluc than before differentiation. In vivo, in pNSE-TSTA-Fluc-transfected F11 cells, luciferase activity increased after neuronal differentiation. In vivo luciferase activity persisted up to 2 days after db-cAMP-induced neuronal differentiation.

CONCLUSION

NSE promoter-driven dual reporter transgenes revealed the possibility of in vivo imaging of neuronal differentiation, which was further enabled by high amplification using a TSTA system. We propose that this strategy be used to follow the transplanted stem cells during differentiation in live animals.

Quantitative comparison of expression with adeno-associated virus (AAV-2) brain-specific gene cassettes

This study compared a range of mammalian CNS expression cassettes in recombinant adeno-associated virus (AAV-2) vectors using strong endogenous promoter sequences, with or without a strong post-regulatory element and polyadenylation signal. Changes in these elements led to transgene expression varying by over three orders of magnitude. In experiments conducted in primary cell culture and in >100 stereotactically injected rats, we observed highly efficient and stable (>15 months) gene expression in neurons and limited expression in glia; the highest expression occurred with endogenous, nonviral promoters such as neuron-specific enolase and beta-actin. The packaging size of AAV-2 was maximized at 5.7 kb without impairing gene expression, as judged by direct comparison with a number of smaller AAV-2 constructs. The genomic insert size and titer were confirmed by Southern blot and quantitative PCR, and infectivity was tested by particle titer using ELISA with a conformation-dependent epitope that requires the full intact capsid. A packaging and purification protocol we describe allows for high-titer, high-capacity AAV-2 vectors that can transduce over 2 x 10(5) neurons in vivo per microliter of vector, using the strongest expression cassette.

Cell-type-specific and regulatable transgenesis in the adult brain: adenovirus-encoded combined transcriptional targeting and inducible transgene expression

To achieve transient transgenesis within specific areas or cell populations in the adult central nervous system (CNS), we have developed a dual adenoviral vector system encoding for cell-type-specific and regulatable transcription units. To achieve combined cell-type-specific transcriptional targeting and inducible expression, we have engineered the expression of the tetracycline-dependent transcriptional elements (1) to be under the transcriptional control of either the astrocyte-specific, glial fibrillary acidic protein (GFAP) (2) or the neuronal specific enolase (NSE) promoter (3) within a dual adenoviral vector system. Cell-type specificity, inducibility, and levels of transgene expression were characterized in vitro in cell lines, and primary neocortical cultures and in the central nervous system (CNS) in vivo, and compared to a powerful pancellular beta-actin/CMV promoter. We demonstrate that the GFAP promoter is able to restrict tetracycline-dependent transgene expression to glial cells in cell lines, primary cultures, and in the CNS in vivo. However, although the NSE promoter did not show neuronal restricted transgene expression in vitro, it did so in the CNS in vivo. Our dual viral system also has provided evidence that an excess of transactivator is needed to achieve maximal transgene expression. Administration of doxycycline completely abrogated transgene expression both in vitro and in vivo. Consequently, our strategy demonstrates that combined cell-type specificity and simultaneous regulation of transgene expression can be obtained in the brain using adenoviral vectors.

The X-gal caution in neural transplantation studies

Cell transplantation into host brain requires a reliable cell marker to trace lineage and location of grafted cells in host tissue. The lacZ gene encodes the bacterial (E. coli) enzyme beta-galactosidase (beta-gal) and is commonly visualized as a blue intracellular precipitate following its incubation with a substrate, "X gal," in an oxidation reaction. LacZ is the "reporter gene" most commonly employed to follow gene expression in neural tissue or to track the fate of transplanted exogenous cells. If the reaction is not performed carefully-with adequate optimization and individualization of various parameters (e.g.. pH, concentration of reagents, addition of chelators, composition of fixatives) and the establishment of various controls--then misleading nonspecific background X-gal positivity can result, leading to the misidentification of cells. Some of this background results from endogenous nonbacterial beta-gal activity in discrete populations of neurons in the mammalian brain; some results from an excessive oxidation reaction. Surprisingly, few articles have empha sized how to recognize and to eliminate these potential confounding artifacts in order to maximize the utility and credibility of this histochemical technique as a cell marker. We briefly review the phenomenon in general, discuss a specific case that illustrates how an insufficiently scrutinized X-gal positivity can be a pitfall in cell transplantation studies, and then provide recommendations for optimizing the specificity and reliability of this histochemical reaction for discerning E. coli beta-gal activity.

Adeno-associated virus vectors: activity and applications in the CNS

Transgenic strategies are useful for functional studies and they may also lead to novel therapies. Controlling transgene expression in defined cell populations over time is increasingly important for both functional and gene therapy experiments. The adeno-associated virus (AAV) vector may provide sufficient spatio-temporal control of gene expression for these purposes. This paper reviews in vivo somatic gene transfer methodology using AAV. Advantageous features of this system include neuronal gene expression that is: (1) efficient; (2) long-lived; and (3) non-toxic. Thus, AAV-mediated gene transfer is a good method for functional genomic research. From characterizing vector activity in the brain using different combinations of promoters and transgenes in the mid to late 1990s, researchers continue to discover novel uses of AAV for both basic and clinical neuroscience.

Gene transfer into neurones for the molecular analysis of behaviour: focus on herpes simplex vectors

The use of viral vectors to transfect genes into specific brain-cell populations is a novel approach that can be used to investigate the molecular and cellular basis of brain function. Ideal vectors should be targetable and capable of regulated transgene expression. From the viral vectors developed so far, this article focuses on herpes simplex virus 1 (HSV-1)-based vectors. HSV-1 vectors can be engineered for gene transfer to the brain, which makes them suitable for neuroscience research applications. In particular, genetic manipulations of the virus can almost eliminate toxicity and allow expression of multiple transgenes simultaneously. In some instances, transfection of selected neuronal populations is also possible. Specific alterations in behaviour and in disease models have been described after the viral-vector-mediated expression of specific genes within highly localized brain regions.

Efficient gene transfer and long-term expression in neurons using a recombinant adenovirus with a neuron-specific promoter

Adenoviruses are highly efficient vectors for gene transfer into brain cells. Restricting transgene expression to specific cell types and maintaining long-term expression are major goals for gene therapy in the central nervous system. We targeted gene expression to neurons by constructing an adenoviral vector that expressed the E. coli LacZ reporter gene under the control of the rat neuron-specific enolase promoter (Ad-NSE). Expression from Ad-NSE was compared with that from an adenoviral vector encoding the same reporter gene under the control of the Rous sarcoma virus LTR promoter (Ad-RSV). Both recombinant adenoviruses were injected stereotactically into rat hippocampus, cerebellum and striatum. Anatomical and immunohistochemical analyses of the Ad-NSE-stained cells showed that neurons were preferentially transduced. More neurons were stained in the hippocampus following infection with Ad-NSE than with Ad-RSV. Cytotoxicity from Ad-NSE was lower than from Ad-RSV. beta-Galactosidase gene expression after Ad-NSE infection remained stable for 3(1/2) months, and was detectable for 6 months. Thus, the NSE-adenoviral vector can be used to transfer potentially therapeutic genes into neuronal cells. The use of a cell-specific promoter also resulted in high in vivo efficiency and long-term transgene expression.

HSV-1-based vectors for gene therapy of neurological diseases and brain tumors: part II. Vector systems and applications

Many properties of HSV-1 are especially suitable for using this virus as a vector to treat diseases affecting the central nervous system (CNS), such as Parkinson's disease or malignant gliomas. These advantageous properties include natural neurotropism, high transduction efficiency, large transgene capacity, and the ability of entering a latent state in neurons. Selective oncolysis in combination with modulation of the immune response mediated by replication-conditional HSV-1 vectors appears to be a highly promising approach in the battle against malignant glioma. Helper virus-free HSV/AAV hybrid amplicon vectors have great promise in mediating long-term gene expression in the PNS and CNS for the treatment of various neurodegenerative disorders or chronic pain. Current research focuses on the design of HSV-1-derived vectors which are targeted to certain cell types and support transcriptionally regulatable transgene expression. Here, we review the recent developments on HSV-1-based vector systems and their applications in experimental and clinical gene therapy protocols.

Gene transfer to the nigrostriatal system by hybrid herpes simplex virus/adeno-associated virus amplicon vectors

To improve gene transfer to CNS neurons, critical elements of herpes simplex virus 1 (HSV-1) amplicons and recombinant adeno-associated virus (AAV) vectors were combined to construct a hybrid amplicon vector, and then packaged via a helper virus-free system. We tested the HSV/AAV hybrid amplicon vectors for transduction efficiency and stability of transgene expression (green fluorescent protein) in primary neuronal cultures from rat fetal ventral mesencephalon, in comparison with traditional HSV amplicon, AAV, or adenovirus (Ad) vectors at the same multiplicity of infection. The HSA/AAV hybrid vectors transduced the highest number of primary neurons in culture 2 days after infection. As compared with all other vectors tested, only hybrid vectors containing the AAV rep gene maintained the 2-day level of transgene expression over 12 days in culture. This rep-containing hybrid vector was then tested for efficiency and safety in the brain. One month after injection into adult rat striatum (1 x 10(6) transducing units injected), transgene expression was observed within the striatum (ranging from 564 to 8610 cells) and the substantia nigra (via retrograde transport, ranging from 130 to 809 neurons). The HSV/AAV hybrid amplicon vectors transduced predominantly neurons within the striatum, and showed transduction efficacy similar to and in many cases higher than that of HSV amplicon vectors. No immune response was observed in the HSA/AAV hybrid vector-injected brains, as determined by immune markers specific for helper T lymphocytes, cytotoxic T lymphocytes, and microglia. This HSV/AAV hybrid system shows high transduction efficiency and stability in culture. The effective and safe transgene delivery into the nigrostriatal system illustrates its potential for therapeutic application for neurologic disorders, such as Parkinson and Huntington disease.

Diverse stabilities of expression in the rat brain from different cellular promoters in a helper virus-free herpes simplex virus type 1 vector system

Many neural gene transfer studies require both long-term and cell type-specific expression. We have reported a helper virus-free HSV-1 plasmid vector system (Fraefel et al., 1996), and this system supports at least some long-term expression from herpesvirus immediate-early promoters. In this study, we constructed vectors that placed the lacZ reporter gene under the regulation of five different cellular promoters. Vector stocks were microinjected into the midbrain, striatum, or hippocampus; the rats were sacrificed at 4 days to 2 months after gene transfer; and the numbers of X-Gal-positive cells were determined. A 6.8-kb fragment of the rat tyrosine hydroxylase (TH) promoter supported relatively stable expression for up to 2 months and targeted expression to TH-immunoreactive neurons in the substantia nigra pars compacta. The other promoters that were examined were chosen with the goal of obtaining long-term, neuronal-specific expression. At 4 days after gene transfer, a 766-bp fragment of the TH promoter supported expression in cells with neuronal morphology in the midbrain and striatum, consistent with results in transgenic mice. However, expression was absent by 2 weeks. Similarly, at 4 days after gene transfer, a mouse neurofilament heavy subunit promoter supported expression in cells with neuronal morphology in the midbrain, striatum, and hippocampus, but expression was absent by 2 weeks. A rat neuron-specific enolase promoter supported only a low level of expression in cultured neuronal cells, and expression was not detected in the brain. A rat voltage-gated sodium channel promoter supported only a low level of expression in PC12 cells and expression could not be detected in cultured cortical cells. These results demonstrate that different promoters support a wide range of levels and stabilities of expression in this vector system, and the results suggest approaches to improving the stability of long-term expression.

Molecular analysis of behavior by gene transfer into neurons with herpes simplex vectors

One goal of neuroscience is to define the molecular and cellular basis for behavior and neurological diseases. A novel approach to this problem is based on the use of viral vectors to transfect specific genes into specific brain cell populations. This review focuses on herpes simplex-based vectors. Major advances have recently been made to improve the characteristics of these vectors, in particular to reduce their toxicity, to modulate the greatness and the time-course of transgene expression, to precisely target specific cell populations, and to transfer multiple genes. Much remains to be done to obtain robust and prolonged transgene expression. However, specific alterations in the behavior and in disease models have already been described following the herpes simplex vector-mediated expression of specific genes within highly localized brain areas. Therefore, this research strategy is likely to provide new clues on the cellular and molecular basis of behavior and of neurological diseases.

A neuron-specific gene transfer by a recombinant defective Sindbis virus

We examined the possibility that Sindbis virus, an alpha virus with a single-stranded RNA genome, would be applied for neuronal gene transfer. The recombinant defective Sindbis viruses were constructed by replacing the structural genes of Sindbis virus with genes encoding beta-galactosidase (rdSind-lacZ) or enhanced green fluorescent protein (rdSind-EGFP). In neuron-glia cocultures prepared from the neocortex, hippocampus, and striatum, EGFP or beta-galactosidase was expressed selectively in neurons 24 h after infection with rdSind-EGFP or rdSind-lacZ. Most cortical neurons were infected with rdSind-lacZ at a multiplicity of infection (M.O.I.) of 5 while glial cells were little infected. In addition, transient neuron-specific expression of beta-galactosidase was observed near injection sites over the next 3 d following administration of rdSind-lacZ in adult rat. In the cortical neurons infected with rdSind-EGFP, treatment with NMDA induced neuritic blebs and cell body swelling in a Na+-dependent manner. Therefore, recombinant defective Sindbis viruses can be used as an efficient and selective vector for gene transfer into neurons and applied to investigate biological role of target genes delivered into neurons in vitro and in vivo.

Cationic lipid-mediated delivery and expression of prepro-neuropeptide Y cDNA after intraventricular administration in rat: feasibility and limitations

The utility of in vivo lipofection for delivery and expression of a neuropeptide gene in the adult rat brain was explored. Prepro-neuropeptide Y (NPY) cDNA was cloned into the episomal eucaryotic expression vector pCEP4. This construct was complexed to lipofectamine or lipofectin. Complexed DNA was injected into the lateral ventricles of adult rats. Brains were removed for analysis following various time intervals. Polymerase chain reaction (PCR) reactions were designed for specific detection of endogenous and vector derived NPY sequence, respectively. PCR of DNA preparations from 5 major brain regions (frontal and parietal cortex, striatum, hypothalamus, brain stem) demonstrated presence of vector DNA up to 1 month (longest interval studied) in all brain regions. Reverse-transcription (RT-) PCR of DNase treated RNA-preparations from brain tissue demonstrated presence of both vector-derived and endogenous NPY mRNA in treated animals, while only endogenous mRNA was detected in controls. In situ hybridization histochemistry indicated scattered patches of vector uptake into tissue in the vicinity of the CSF compartment, but not into deeper located structures. Weight gain was not affected, indicating that the expression levels achieved may not be sufficient to play a functional role, and/or may need to be targeted to specific brain areas. These findings suggest a potential for cationic lipid mediated gene transfer in the brain as an experimental tool and as a possible future therapeutic principle, but also indicate the need for optimization of delivery strategies in order to achieve functionally relevant expression levels.

The principles of gene therapy for the nervous system

Research pertaining to gene transfer into cells of the nervous system is one of the fastest growing fields in neuroscience. An important application of gene transfer is gene therapy, which is based on introducing therapeutic genes into cells of the nervous system by ex vivo or in vivo techniques. With the eventual development of efficient and safe vectors, therapeutic genes, under the control of a suitable promoter, can be targeted to the appropriate neurons or glial cells. Gene therapy is not only applicable to the treatment of genetic diseases of the nervous system and the control of malignant neoplasia, but it also has therapeutic potential for acquired degenerative encephalopathies (Alzheimer's disease, Parkinson's disease), as well as for promoting neuronal survival and regeneration in various pathological states.

A herpes simplex virus-1 vector containing the rat tyrosine hydroxylase promoter directs cell type-specific expression of beta-galactosidase in cultured rat peripheral neurons

A defective herpes simplex virus-1 (HSV-1) vector system was used to study cell type-specific expression of the tyrosine hydroxylase (TH) gene. HSV-1 particles containing 663 bp (pTHlac 663), 278 bp (pTHlac 278), or 181 bp (pTHlac 181) of the rat TH promoter driving E. coli LacZ were used to infect superior cervical ganglia (SCG: TH-expressing tissue) and dorsal root ganglia (DRG:non-TH-expressing tissue) cultures. One day after infection, expression of beta-galactosidase was visualized by X-gal cytochemistry. Following viral transduction with pTHlac 663 at a multiplicity of infection of 0.2, 14.4% of the SCG neurons were X-gal positive whereas only about 0.9% of DRG neurons were X-gal positive. Infection with either pTHlac278 or 181 resulted in 3-fold more X-gal-positive DRG neurons. These results suggest that (i) the defective HSV-1 vector system may be useful in defining regulatory promoter motifs; (ii) 663 bp of the rat TH promoter contains sufficient information for cell type-specific expression in peripheral nervous system neurons; and (iii) sequences between -278 and -663 contain an element(s) that represses gene expression in non-catecholamingeric neurons.

The spinal cord as an alternative model for nerve tissue graft

The spinal cord provides an alternative model for nerve tissue grafting experiments. Anatomo-functional correlations are easier to make here than in any other region of the CNS because of a direct implication of spinal cord neurons in sensorimotor activities. Lesions can be easily performed to isolate spinal cord neurons from descending inputs. The anatomy of descending monoaminergic systems is well defined and these systems offer a favourable paradigm for lesion-graft experiments.

Multiple obstacles to gene therapy in the brain

Neuwelt et al. have proposed gene-transfer experiments utilizing an animal model that offers many important advantages for investigating the feasibility of gene therapy in the human brain. A variety of tissues concerning the viral vector and mode of delivery of the corrective genes need to be resolved, however, before such therapy is scientifically supportable.

Principles of brain tissue engineering

It is often presumed that effects of neural tissue transplants are due to release of neurotransmitter. In many cases, however, effects attributed to transplants may be related to phenomena such as trophic effects mediated by glial cells or even tissue reactions to injury. Any conclusion regarding causation of graft effects must be based on the control groups or other comparisons used. In human clinical studies, for example, comparing the same subject before and after transplantation allows for many interpretations of the causes of clinical changes.

Lessons on transplant survival from a successful model system

Studies on the snailMelampusreveal that connectivity is crucial to the survival of transplanted ganglia. Transplanted CNS ganglia can innervate targets or induce supernumerary structures. Neuron survival is optimized by the neural incorporation that occurs when a transplanted ganglion is substituted for an excised ganglion. Better provision for the trophic requirements of neurons will improve the success of mammalian fetal transplants.

Repairing the brain: Trophic factor or transplant?

Three experiments on neural grafting with adult rat hosts are described. Working memory impairments were produced by lesioning the hippocampus or severing its connections with the septum by ablating the fimbria-fornix. The results suggest that the survival and growth of a neural graft, whether an autograft or a xenograft, is not a necessary condition for functional recovery on a task tapping working memory.

Will brain tissue grafts become an important therapy to restore visual function in cerebrally blind patients?

Grafting embryonic brain tissue into the brain of patients with visual field loss due to cerebral lesions may become a method to restore visual function. This method is not without risk, however, and will only be considered in cases of complete blindness after bilateral occipital lesions, when other, risk-free neuropsychological methods fail.

Difficulties inherent in the restoration of dynamically reactive brain systems

The responses displayed by an injured or diseased nervous system are complex. Some of the responses may effect a functional reorganization of the affected neural circuitry. Strategies aimed at the restoration of function, whether or not these involve transplantation, need to recognize the innate reactive capacity of the nervous system to damage. More successful strategies will probably incorporate, rather than ignore, the adaptive responses of the compromised neural systems.

Elegant studies of transplant-derived repair of cognitive performance

Cholinergic-rich grafts have been shown to be effective in restoring maze-learning deficits in rats with lesions of the forebrain cholinergic projection system. However, the relevance of those studies to developing novel therapies for Alzheimer's disease is questioned.

Neural transplants are grey matters

The lesion and transplantation data cited by Sinden et al., when considered in tandem, seem to harbor an internal inconsistency, raising questions of false localization of function. The extrapolation of such data to cognitive impairment and potential treatment strategies in Alzheimer's disease is problematic. Patients with focal basal forebrain lesions (e.g., anterior communicating artery aneurysm rupture) might be a more appropriate target population.

Immunobiology of neural transplants and functional incorporation of grafted dopamine neurons

In contrast to the views put forth by Stein & Glasier, we support the use of inbred strains of rodents in studies of the immunobiology of neural transplants. Inbred strains demonstrate homology of the major histocompatibility complex (MHC). Virtually all experimental work in transplantation immunology is performed using inbred strains, yet very few published studies of immune rejection in intracerebral grafts have used inbred animals.

Local and global gene therapy in the central nervous system

For focal neurodegenerative diseases or brain tumors, localized delivery of protein or genetic vectors may be sufficient to alleviate symptoms, halt disease progression, or even cure the disease. One may circumvent the limitation imposed by the blood-brain barrier by transplantation of genetically altered cell grafts or focal inoculation of virus or protein. However, permanent gene replacement therapy for diseases affecting the entire brain will require global delivery of genetic vectors. The neurotoxicity of currently available viral vectors and the transient nature of transgene expression invivomust be overcome before their use in human gene therapy becomes clinically applicable.

Neural grafting in human disease versus animal models: Cautionary notes

Over the past two decades, research on neural transplantation in animal models of neurodegeneration has provided provocative in sights into the therapeutic use of grafted tissue for various neurological diseases. Although great strides have been made and functional benefits gained in these animal models, much information is still needed with regard to transplantation in human patients. Several factors are unique to human disease, for example, age of the recipient, duration of disease, and drug interaction with grafted cells; these need to be explored before grafting can be considered a safe and effective therapeutic tool.

Building a rational foundation for neural transplantation

The neural transplantation research described by Sinden and colleagues provides part of the rationale for the clinical application of neural transplantation. The authors are asked to clarify their view of the role of the cholinergic system in cognition, to address extrahippocampal damage caused by transient forebrain ischemia, and to consider the effects of delayed neural degeneration in their structure-function analysis.

Intraretrosplenial grafts of cholinergic neurons and spatial memory function

The transplantation of cholinergic neurons into the hippocampal formation has been well characterized. We describe our studies on the effects of cholinergic transplants in the retrosplenial cortex. These transplants were capable of ameliorating spatial navigation deficits in rats with septohippocampal lesions. In addition, we provide evidence for the modulation of transplanted neurons by the host brain.

Gene therapy and neural grafting: Keeping the message switched on

A major problem in developing an effective gene therapy for the nervous system lies in understanding the principles that maintain or turn off the expression of genes following their transfer into the CNS.

Therapeutic neural transplantation: Boon or boondoggle?

Despite reports of recovery of function after neural transplantation, the biological interactions between transplanted neurons and the host brain that are necessary to mediate recovery are unclear at present. One source of confusion is in the variety of models and protocols used in these studies. It is suggested that multisite experimentation using standard protocols, models, and recovery criteria would be helpful in moving neural transplantation from the laboratory to the clinic.

The ethics of fetal tissue grafting should be considered along with the science

In addition to the scientific and medical issues surrounding the use of fetal tissue transplants, the ethical implications should be considered. Two major ethical issues are relevant. The first of these is whether this experimental procedure can be justified on the basis of potential benefit to the patient. The second is whether the use of tissue obtained from intentionally aborted fetuses can be justified in the context of historical and existing guidelines for the protection of human subjects. The separation of ethical decisions from medical practice and scientific research is necessary to prevent the exploitation of innocent human life.

Gene therapy for neurodegenerative disorders and malignant brain tumors

Gene therapy approaches have great promise in the treatment of neurodegenerative disorders and malignant brain tumors. Neuwelt et al. review available viral-mediated gene therapy methods and their blood-brain-barrier (BBB) disruption delivery technique, briefly mentioning nonviral mediated gene therapy methods. This commentary discussed the BBB disruption delivery technique, viral and nonviral mediated gene therapy approaches to Parkinson's disease, and the potential use of antisense oligo to suppress malignant brain tumors.

Behavioral effects of neural grafts: Action still in search of a mechanism

This commentary reviews data supporting circuitry reconstruction, replacement neurotransmitters, and trophic action as mechanisms whereby transplants promote recovery of function. Issue is taken with the thesis of Sinden et al. that adequate data exist to indicate that reconstruction of hippocampal circuitry damaged by hypoxia with CA1 transplants is a confirmed mechanism whereby these transplants produce recovery. Sinden et al.'s and Stein & Glasier's proposal that there is definitive evidence showing that all transplants produce trophic effects is also questioned.