Efficient adenoviral vector-directed expression of a foreign gene to neurons and sustentacular cells in the mouse olfactory neuroepithelium

Olfactory Loss and Dysfunction in Ciliopathies: Molecular Mechanisms and Potential Therapies

BACKGROUND

Ciliopathies are a class of inherited pleiotropic genetic disorders in which alterations in cilia assembly, maintenance, and/or function exhibit penetrance in the multiple organ systems. Olfactory dysfunction is one such clinical manifestation that has been shown in both patients and model organisms. Existing therapies for ciliopathies are limited to the treatment or management of symptoms. The last decade has seen an increase in potential curative therapeutic options including small molecules and biologics. Recent work in multiciliated olfactory sensory neurons has demonstrated the capacity of targeted gene therapy to restore ciliation in terminally differentiated cells and rescue olfactory function. This review will discuss the current understanding of the penetrance of ciliopathies in the olfactory system. Importantly, it will highlight both pharmacological and biological approaches, and their potential therapeutic value in the olfactory system and other ciliated tissues.

METHODS

We undertook a structured and comprehensive search of peer-reviewed research literature encompassing in vitro, in vivo, model organism, and clinical studies. From these publications, we describe the olfactory system, and discuss the penetrance of ciliopathies and impact of cilia loss on olfactory function. In addition, we outlined the developing therapies for ciliopathies across different organ and cell culture systems, and discussed their potential therapeutic application to the mammalian olfactory system.

RESULTS

One-hundred sixty-one manuscripts were included in the review, centering on the understanding of olfactory penetrance of ciliopathies, and discussing the potential therapeutic options for ciliopathies in the context of the mammalian olfactory system. Forty-four manuscripts were used to generate a table listing the known congenital causes of olfactory dysfunction, with the first ten listed are linked to ciliopathies. Twenty-three manuscripts were used to outline the potential of small molecules for the olfactory system. Emphasis was placed on HDAC6 inhibitors and lithium, both of which were shown to stabilize microtubule structures, contributing to ciliogenesis and cilia lengthening. Seventy-five manuscripts were used to describe gene therapy and gene therapeutic strategies. Included were the implementation of adenoviral, adeno-associated virus (AAV), and lentiviral vectors to treat ciliopathies across different organ systems and application toward the olfactory system. Thus far, adenoviral and AAVmeditated ciliary restoration demonstrated successful proof-of-principle preclinical studies. In addition, gene editing, ex vivo gene therapy, and transplantation could serve as alternative therapeutic and long-term approaches. But for all approaches, additional assessment of vector immunogenicity, specificity, and efficacy need further investigation. Currently, ciliopathy treatments are limited to symptomatic management with no curative options. However, the accessibility and amenability of the olfactory system to treatment would facilitate development and advancement of a viable therapy.

CONCLUSION

The findings of this review highlight the contribution of ciliopathies to a growing list of congenial olfactory dysfunctions. Promising results from other organ systems imply the feasibility of biologics, with results from gene therapies proving to be a viable therapeutic option for ciliopathies and olfactory dysfunction.

Intranasal Delivery of Adenoviral and AAV Vectors for Transduction of the Mammalian Peripheral Olfactory System

Intranasal delivery of solutions is a straightforward methodology for viral vector transduction and gene transfer to the epithelia within the nasal cavity. Beyond the simplicity of the technique, intranasal delivery has demonstrated restricted transduction of the olfactory and respiratory epithelial tissues. Here we outline the procedure of viral vector intranasal delivery in early postnatal and adult mice, as well as adult rats. The procedure allows for robust transduction and ectopic gene delivery that can be used for the visualization of cellular structures, protein distribution, and assessment of viral vector-mediated therapies.

B cell response to herpesvirus infection of the olfactory neuroepithelium

Viruses commonly infect the respiratory tract. Analyses of host defense have focused on the lungs and the respiratory epithelium. Spontaneously inhaled murid herpesvirus 4 (MuHV-4) and herpes simplex virus 1 (HSV-1) instead infect the olfactory epithelium, where neuronal cilia are exposed to environmental antigens and provide a route across the epithelial mucus. We used MuHV-4 to define how B cells respond to virus replication in this less well-characterized site. Olfactory infection elicited generally weaker acute responses than lung infection, particularly in the spleen, reflecting slower viral replication and spread. Few virus-specific antibody-forming cells (AFCs) were found in the nasal-associated lymphoid tissue (NALT), a prominent response site for respiratory epithelial infection. Instead, they appeared first in the superficial cervical lymph nodes. The focus of the AFC response then moved to the spleen, matching the geography of virus dissemination. Little virus-specific IgA response was detected until later in the bone marrow. Neuroepithelial HSV-1 infection also elicited no significant AFC response in the NALT and a weak IgA response. Thus, olfactory herpesvirus infection differed immunologically from an infection of the adjacent respiratory epithelium. Poor IgA induction may help herpesviruses to transmit via long-term mucosal shedding.

IMPORTANCE

Herpesviruses are widespread, persistent pathogens against which vaccines have had limited success. We need to understand better how they interact with host immunity. MuHV-4 and HSV-1 inhaled by alert mice infect the olfactory neuroepithelium, suggesting that this is a natural entry route. Its immunology is almost completely unknown. The antibody response to neuroepithelial herpesvirus infection started in the cervical lymph nodes, and unlike respiratory influenza virus infection, did not significantly involve the nasal-associated lymphoid tissue. MuHV-4 and HSV-1 infections also elicited little virus-specific IgA. Therefore, vaccine-induced IgA might provide a defense that herpesviruses are ill-equipped to meet.

A new approach with less damage: intranasal delivery of tetracycline-inducible replication-defective herpes simplex virus type-1 vector to brain

Gene therapy holds great potential for treating neurological disorders. However, delivering gene vectors to the brain has been either invasive or inefficacious in most studies to date. The aim of this study was to develop a safe and efficacious strategy for delivering gene vectors to the brain. A tetracycline-inducible replication-defective herpes simplex virus type-1 vector, QR9TO-LacZ, was administered to rats intranasally. QR9TO-LacZ could infect primary cortical neurons and express the reporter gene without detectable replication. QR9TO-LacZ was observed in the olfactory bulb, hippocampus, striatum, cortex, medulla, cerebellum, ventricles, and nasal septum after intranasal administration. Expression of the reporter gene could be controlled effectively by tetracycline. In vitro, introduction of QR9TO-LacZ did not change the structure of transfected neurons. In vivo, QR9TO-LacZ did not increase apoptosis in neurons and did not alter levels of interleukin 6 and tumor necrosis factor α in the brain after intranasal delivery. Our data suggest that intranasally applied QR9TO-LacZ has a wide distribution and expresses the reporter gene in the brain under the control of tetracycline with less cytotoxicity than intravenous or stereotactic delivery methods.

Intranasal delivery of therapeutic proteins for neurological diseases

INTRODUCTION

Among the range of therapeutic protein candidates for new generation treatments of neurological diseases, neurotrophic factors and recombinant antibodies hold the greatest potential. However, major difficulties in their safe and effective delivery to the brain severely limit these applications. The BBB restricts the exchange of proteins between the plasma and the CNS. Moreover, therapeutic proteins often need to be selectively targeted to the brain, while minimizing their biodistribution to systemic compartments, to avoid peripheral side effects. The intranasal delivery of proteins has recently emerged as a non-invasive, safe and effective method to target proteins to the CNS, bypassing the BBB and minimizing systemic exposure.

AREAS COVERED

We critically summarize the main experimental and mechanistic facts about the simple and non-invasive nasal delivery approach, which provides a promising strategy and a potential solution for the severe unmet medical need of safely and effectively delivering protein therapeutics to the brain.

EXPERT OPINION

The intranasal route for the effective delivery of recombinant therapeutic proteins represents an emerging and promising non-invasive strategy. Future studies will achieve a detailed understanding of pharmacokinetic and mechanisms of delivery to optimize formulations and fully exploit the nose-to-brain interface in order to deliver proteins for the treatment of neurological diseases. This expanding research area will most likely produce exciting results in the near future towards new therapeutical approaches for the CNS.

Air-assisted intranasal instillation enhances adenoviral delivery to the olfactory epithelium and respiratory tract

Intranasal instillation is used to deliver adenoviral vectors to the olfactory epithelium and respiratory tract. The success of this approach, however, has been tempered by inconsistent infectivity in both the epithelium and lungs. Infection of the epithelium may be hampered in part by the convoluted structure of the cavity, the presence of mucus or poor airflow in the posterior cavity. Delivery of adenovirus to the lungs can be uneven in the various lobes and distal bronchioles may be poorly infected. Current approaches to circumvent these issues rely principally on intubation or intratracheal instillation. Here we describe a technique that significantly improves adenoviral infectivity rates without requiring surgical intervention. We use compressed air to increase circulation of instilled adenovirus, resulting in enhanced infection in both the epithelium and lungs. This procedure is straightforward, simple to perform and requires no specialized equipment. In the epithelium, neurons and sustentacular cells are both labeled. In the lungs, all lobes can be infected, with penetration to the most distal bronchioles. The use of compressed air will likely also be useful for enhancing the distribution of other, desired agents within the epithelium, central nervous system and respiratory tract.

Ion transport across CF and normal murine olfactory and ciliated epithelium

The nasal epithelium of the cystic fibrosis (CF) mouse has been used extensively in CF research because it exhibits ion transport defects similar to those of human CF airways. This tissue is composed of approximately 50% olfactory (OE) and approximately 50% ciliated epithelium (CE), and on the basis of previous observations, we hypothesized that a significant fraction of the bioelectric signals from murine nasal tissue may arise from OE rather than CE, while CE is the target tissue for CF gene therapy. We compared the bioelectric properties of isolated OE from the nasal cavity and CE from the nasopharynx in Ussing chamber studies. Hyperabsorption of Na(+) [amiloride response; CF vs. wild type (WT)] was approximately 7.5-fold greater in the OE compared with the CE. The forskolin response in native tissues did not reliably distinguish genotypes, likely due to a cyclic nucleotide-gated cation conductance in OE and a calcium-mediated Cl(-) conductance in CE. By potential difference assay, hyperabsorption of Na(+) (CF vs. WT) and the difference in response to apical 0 Cl(-) buffer (CF vs. WT) were approximately 2-fold greater in the nasal cavity compared with the nasopharynx. Our studies demonstrate that in the CF mouse, both the hyperabsorption of Na(+) and the Cl(-) transport defect are of larger magnitude in the OE than in the CE. Thus, while the murine CF nasal epithelium is a valuable model for CF studies, the bioelectrics are likely dominated by the signals from the OE, and assays of the nasopharynx may be more specific for studying the ciliated epithelium.

Expression of Coxsackie-Adenovirus receptor (CAR) in the developing mouse olfactory system

Interest in manipulating gene expression in olfactory sensory neurons (OSNs) has led to the use of adenoviruses (AdV) as gene delivery vectors. OSNs are the first order neurons in the olfactory system and the initial site of odor detection. They are highly susceptible to adenovirus infection although the mechanism is poorly understood. The Coxsackie-Adenovirus receptor (CAR) and members of the integrin family have been implicated in the process of AdV infection in various systems. Multiple serotypes of AdV efficiently bind to the CAR, leading to entry and infection of the host cell by a mechanism that can also involve integrins. Cell lines that do not express CAR are relatively resistant, but not completely immune to AdV infection, suggesting that other mechanisms participate in mediating AdV attachment and entry. Using in situ hybridization and western blot analyses, we show that OSNs and olfactory bulbs (OB) of mice express abundant CAR mRNA at embryonic and neonatal stages, with progressive diminution during postnatal development. By contrast to the olfactory epithelium (OE), CAR mRNA is still present in the adult mouse OB. Furthermore, despite a similar postnatal decline, CAR protein expression in the OE and OB of mice continues into adulthood. Our results suggest that the robust AdV infection observed in the postnatal olfactory system is mediated by CAR and that expression of even small amounts of CAR protein as seen in the adult rodent, permits efficient AdV infection and entry. CAR is an immunoglobulin domain-containing protein that bears homology to cell-adhesion molecules suggesting the possibility that it may participate in organization of the developing olfactory system.

Bcl-2 expression mediated by Cre/loxP system in olfactory epithelium

To study the Bcl-2 expression mediated by the Cre/loxP recombination system and its effect on prevention of apoptosis in olfactory epithelium. Adenoviral vectors with cassette for Bcl-2 (AxCALNLBcl-2) and Cre recombinase (AxCANCre) were applied to mouse olfactory epithelium by intranasal instillation. The effect of exogenous Bcl-2 expression on prevention of apoptosis of olfactory receptor neurons was investigated using an apoptosis model induced by bulbectomy. The Bcl-2 product was expressed not only in the olfactory receptor neurons but also in the supporting cells. Although statistical analysis did not show significant difference, the number of apoptotic cells in the infected olfactory epithelium on post-bulbectomy day 2 was lower than that of control and the number of survived mature olfactory receptor neurons in the infected olfactory epithelium on post-bulbectomy day 5 was higher than that of control. Although further studies are required for clinical application, the results of our study suggest that this strategy may be able to deliver exogenous Bcl-2 for the treatment of degeneration of olfactory receptor neurons.

Adenovirus-mediated gene transfer in olfactory epithelium and olfactory bulb: a long-term study

OBJECTIVES

We sought to study the spatiotemporal gene expression mediated by adenoviral vector in the olfactory pathways.

METHODS

The replication-defective adenoviral vector AxCALacZ, which encodes the enzyme Escherichia coli beta-galactosidase, was applied to mouse olfactory epithelium by intranasal instillation.

RESULTS

The LacZ gene product, beta-galactosidase, was expressed not only in the olfactory receptor neurons and their axons, but also in the olfactory bulbs. The first evidence of anterograde labeling was observed at postinfection day (PID) 2. At PID 3, beta-galactosidase was strongly expressed in olfactory nerve axons, as well as their terminal glomeruli, in the olfactory bulbs. beta-Galactosidase expression persisted up to PID 90, and there was a significant decrease in the number of labeled neurons at PID 30.

CONCLUSIONS

These results suggest possible long-term effects of adenovirus-mediated gene transfer on the olfactory neurons, as well as the olfactory bulbs.

Effects of adenoviral vector-mediated BDNF expression on the bulbectomy-induced apoptosis of olfactory receptor neurons

The expression of adenoviral vector (Ad)-mediated lacZ and brain-derived neurotrophic factor (BDNF) in mouse olfactory epithelium (OE) was examined, and the effect of BDNF on the survival of the bulbectomized OE was evaluated. A recombinant adenovirus, Ax1CAlacZ, was administrated into the mouse OE after bulbectomy, and the expression of a transferred E. coli beta-galactosidase (beta-gal) gene was confirmed by X-gal staining. The expression and effects of exogenous BDNF in the OE after bulbectomy were examined using immunohistochemistry and the TUNEL method. The adenoviral vector-mediated expression of beta-gal in the mouse OE was detectable for up to 14 days after bulbectomy in vivo. The Ad-mediated expression of BDNF was also observed in the OE after bulbectomy. Exogenously induced BDNF suppressed the degenerative changes of bulbectomized OE. TUNEL staining indicated that the exogenous BDNF enhanced the survival of the bulbectomized OE by inhibiting apoptosis. Ad-mediated expression of BDNF in the mouse nasal mucosa alleviated degenerative changes in bulbectomized OE. Ad-mediated transfer of neurotrophic factors might be applicable in the treatment of olfactory disorders.

Adenoviral vector-mediated rescue of the OMP-null behavioral phenotype: enhancement of odorant threshold sensitivity

Mice from which the olfactory marker protein (OMP) gene has been deleted demonstrate a number of neurophysiologic and behavioral defects that suggest OMP is an important component in olfactory signal transduction and is critically involved in odor processing. Recently, the potential pleiotropic effects of gene deletion were addressed by adenoviral vector-mediated rescue of the neurophysiologic defects, in vivo. As a complement to this study, the authors used a recombinant adenoviral vector to transiently introduce OMP into olfactory sensory neurons of adult OMP-null mice and, using psychophysical methods, demonstrated the resulting reacquisition of behavioral function subsequent to gene replacement. The rescue of the OMP-null behavioral phenotype further supports the hypothesis that OMP is an important component in olfactory signal amplification and/or transduction processing.

In vivo expression of adenovirus-mediated lacZ gene in murine nasal mucosa

Adenovirus is a good tool for transferring exogenous genes into various organs because the virus has a wide spectrum of infection. In this report, we demonstrate that a recombinant adenovirus, Ax1CAlacZ, can transfer an exogenous lacZ gene into murine nasal mucosa in vivo. The efficiency of the exogenous gene expression varied for different cell types and was improved by optimizing the method of administration. In the olfactory region, the olfactory epithelia, sustentacular cells and olfactory nerve efficiently expressed lacZ gene transferred by Ax1CAlacZ using either of two administration methods, dripping or injecting. In contrast, in the respiratory region, the respiratory epithelia but not the subepithelial tissues expressed lacZ gene transferred by Ax1CAlacZ, and the efficiency of the gene transfer, which was low when the virus was administered by nasal drops, was improved when the virus was administered by injection. Our study demonstrated that gene transfer mediated by adenovirus is more efficient in the olfactory epithelia than in the respiratory epithelia, and may be applicable to nasal or paranasal diseases such as olfactory epithelial disturbances.

Intracellular trafficking of a tagged and functional mammalian olfactory receptor

Tagged G-protein-coupled receptors (GPCRs) have been used to facilitate intracellular visualization of these receptors. We have used a combination of adenoviral vector gene transfer and tagged olfactory receptors to help visualize mammalian olfactory receptor proteins in the normal olfactory epithelium of rats, and in cell culture. Three recombinant adenoviral vectors were generated carrying variously tagged versions of rat olfactory receptor I7. The constructs include an N-terminal Flag epitope tag (Flag:I7), enhanced green fluorescent protein (EGFP) fusion protein (EGFP:I7), and a C-terminal EGFP fusion (I7:EGFP). These receptor constructs were assayed in rat olfactory sensory neurons (OSNs) and in a heterologous system (HEK 293 cell line) for protein localization and functional expression. Functional expression of the tagged receptor proteins was tested by electroolfactogram (EOG) recordings in the infected rat olfactory epithelium, and by calcium imaging in single cells. Our results demonstrate that the I7:EGFP fusion protein and Flag:I7 are functionally expressed in OSNs while the EGFP:I7 fusion is not, probably due to inappropriate processing of the protein in the cells. These data suggest that a small epitope tag (Flag) at the N-terminus, or EGFP located at the C-terminus of the receptor, does not affect ligand binding or downstream signaling. In addition, both functional fusion proteins (Flag:I7 and I7:EGFP) are properly targeted to the plasma membrane of HEK 293 cells.

Preferential transduction of neurons by canine adenovirus vectors and their efficient retrograde transport in vivo

In the central nervous system (CNS), there are innate obstacles to the modification of neurons: their relative low abundance versus glia and oligodendrocytes, the inaccessibility of certain target populations, and the volume one can inject safely. Our aim in this study was to characterize the in vivo efficacy of a novel viral vector derived from a canine adenovirus (CAV-2). Here we show that CAV-2 preferentially transduced i) rat olfactory sensory neurons; ii) rodent CNS neurons in vitro and in vivo; and, more clinically relevant, iii) neurons in organotypic slices of human cortical brain. CAV-2 also showed a high disposition for retrograde axonal transport in vivo. We examined the molecular basis of neuronal targeting by CAV-2 and suggest that due to CAR (coxsackie adenovirus receptor) expression on neuronal cells-and not oligodendrocytes, glia, myofibers, and nasal epithelial cells-CAV-2 vectors transduced neurons preferentially in these diverse tissues.

Adenoviral vector-mediated rescue of the OMP-null phenotype in vivo

The use of gene deletion by homologous recombination to determine gene or protein function has wide application in vertebrate neurobiology. An ideal complement to gene deletion would be subsequent gene replacement to demonstrate re-acquisition of function. Here we used an adenoviral vector to replace the olfactory marker protein (OMP) gene in olfactory receptor neurons of adult OMP-null mice and demonstrated the subsequent re-acquisition of function. Our results show that short-term expression of OMP restores the kinetics of electrophysiological responses of OMP-null mice to those of the control phenotype. This adenoviral-mediated rescue of the OMP-null phenotype is consistent with involvement of OMP in olfactory transduction.

Manipulation of gene expression in the mammalian nervous system: application in the study of neurite outgrowth and neuroregeneration-related proteins

A fundamental issue in neurobiology entails the study of the formation of neuronal connections and their potential to regenerate following injury. In recent years, an expanding number of gene families has been identified involved in different aspects of neurite outgrowth and regeneration. These include neurotrophic factors, cell-adhesion molecules, growth-associated proteins, cytoskeletal proteins and chemorepulsive proteins. Genetic manipulation technology (transgenic mice, knockout mice, viral vectors and antisense oligonucleotides) has been instrumental in defining the function of these neurite outgrowth-related proteins. The aim of this paper is to provide an overview of the above-mentioned four approaches to manipulate gene expression in vivo and to discuss the progress that has been made using this technology in helping to understand the molecular mechanisms that regulate neurite outgrowth. We will show that work with transgenic mice and knockout mice has contributed significantly to the dissection of the function of several proteins with a key role in neurite outgrowth and neuronal survival. Recently developed viral vectors for gene transfer in postmitotic neurons have opened up new avenues to analyze the function of a protein following local expression in naive adult rodents. The initial results with viral vector-based gene transfer provide a conceptual framework for further studies on genetic therapy of neuroregeneration and neurodegenerative diseases.

Functional expression of a mammalian odorant receptor

Candidate mammalian odorant receptors were first cloned some 6 years ago. The physiological function of these receptors in initiating transduction in olfactory receptor neurons remains to be established. Here, a recombinant adenovirus was used to drive expression of a particular receptor gene in an increased number of sensory neurons in the rat olfactory epithelium. Electrophysiological recording showed that increased expression of a single gene led to greater sensitivity to a small subset of odorants.

B-50, the growth associated protein-43: modulation of cell morphology and communication in the nervous system

The growth-associated protein B-50 (GAP-43) is a presynaptic protein. Its expression is largely restricted to the nervous system. B-50 is frequently used as a marker for sprouting, because it is located in growth cones, maximally expressed during nervous system development and re-induced in injured and regenerating neural tissues. The B-50 gene is highly conserved during evolution. The B-50 gene contains two promoters and three exons which specify functional domains of the protein. The first exon encoding the 1-10 sequence, harbors the palmitoylation site for attachment to the axolemma and the minimal domain for interaction with G0 protein. The second exon contains the "GAP module", including the calmodulin binding and the protein kinase C phosphorylation domain which is shared by the family of IQ proteins. Downstream sequences of the second and non-coding sequences in the third exon encode species variability. The third exon also contains a conserved domain for phosphorylation by casein kinase II. Functional interference experiments using antisense oligonucleotides or antibodies, have shown inhibition of neurite outgrowth and neurotransmitter release. Overexpression of B-50 in cells or transgenic mice results in excessive sprouting. The various interactions, specified by the structural domains, are thought to underlie the role of B-50 in synaptic plasticity, participating in membrane extension during neuritogenesis, in neurotransmitter release and long-term potentiation. Apparently, B-50 null-mutant mice do not display gross phenotypic changes of the nervous system, although the B-50 deletion affects neuronal pathfinding and reduces postnatal survival. The experimental evidence suggests that neuronal morphology and communication are critically modulated by, but not absolutely dependent on, (enhanced) B-50 presence.

Adenoviral vector-mediated expression of B-50/GAP-43 induces alterations in the membrane organization of olfactory axon terminals in vivo

B-50/GAP-43 is an intraneuronal membrane-associated growth cone protein with an important role in axonal growth and regeneration. By using adenoviral vector-directed expression of B-50/GAP-43 we studied the morphogenic action of B-50/GAP-43 in mature primary olfactory neurons that have established functional synaptic connections. B-50/GAP-43 induced gradual alterations in the morphology of olfactory synapses. In the first days after overexpression, small protrusions originating from the preterminal axon shaft and from the actual synaptic bouton were formed. With time the progressive formation of multiple ultraterminal branches resulted in axonal labyrinths composed of tightly packed sheaths of neuronal membrane. Thus, B-50/GAP-43 is a protein that can promote neuronal membrane expansion at synaptic boutons. This function of B-50/GAP-43 suggests that this protein may subserve an important role in ongoing structural synaptic plasticity in adult neurons and in neuronal membrane repair after injury to synaptic fields.

Corticospinal tract regrowth

The natural ability of the adult central nervous system of higher vertebrates to recover from injury is highly limited. This limitation is most likely due to an inhospitable environment and/or intrinsic incapacities of the neurons to re-extend their neurites after injury or axotomy. The rat corticospinal tract is the largest tract leading from brain to spinal cord and is often used as a model in developmental and regeneration studies. The extensive know-how of factors involved in the development of the corticospinal tract did provide the foundation for many studies on corticospinal tract regrowth after injury in the adult spinal cord. The results of these experiments, as discussed in this review, have led to important contributions to the further understanding of central nervous system regeneration.

Adenoviral vector-mediated gene expression in the nervous system of immunocompetent Wistar and T cell-deficient nude rats: preferential survival of transduced astroglial cells in nude rats

In the present paper, we examined the effect of the adenoviral vector dosage, the role of T cells, and the influence of the presence of replication-competent adenovirus (RCA) in adenoviral vector stocks, on the efficacy of adenoviral vector-directed transgene expression in the facial nucleus of immunocompetent Wistar and athymic nude rats. A small number of motor neurons and glial cells was transduced at low dosages of viral vector (1 x 10(6) pfu) and in the absence of RCA, and transgene-expressing cells persisted throughout the 3-week period of observation. Intraparenchymal infusion of 2 x 10(7) pfu of a recombinant adenoviral vector free of RCA was required for optimal transduction of facial motor neurons. In Wistar rats, a biphasic immune response occurred at higher dosages of the vector (5 x 10(6) and 2 x 10(7) pfu) that was characterized by early infiltration of macrophages and the occurrence of T cells during the second week after injection of the vector. The immune response was associated with the loss of transduced neural cells. In nude rats, administration of an adenoviral vector free of RCA resulted in a macrophage response comparable to that in the Wistar rat and long-term survival of transduced astroglial cells. However, transduced motor neurons degenerated according to a similar time course as observed in Wistar rats. Small amounts of RCA (2 x 10(5) pfu) injected with 2 x 10(7) pfu recombinant viral vector particles resulted in an accelerated T cell response and a rapid elimination of transduced cells within 1 week in Wistar rats, whereas in nude rats transgene expression continued during this period. Taken together, these observations suggest that at the high viral vector loads necessary to achieve optimal transduction of the facial nucleus, T cells play a role in the degeneration of adenoviral vector-transduced astroglial cells. The adverse effects on neurons appear to be due to the observed inflammatory response or to direct adenoviral vector toxicity.

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

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