Enhanced learning in mice parallels vector-mediated nerve growth factor expression in hippocampus

Regulation of cholinergic basal forebrain development, connectivity, and function by neurotrophin receptors

Cholinergic basal forebrain (cBF) neurons are defined by their expression of the p75 neurotrophin receptor (p75NTR) and tropomyosin-related kinase (Trk) neurotrophin receptors in addition to cholinergic markers. It is known that the neurotrophins, particularly nerve growth factor (NGF), mediate cholinergic neuronal development and maintenance. However, the role of neurotrophin signalling in regulating adult cBF function is less clear, although in dementia, trophic signalling is reduced and p75NTR mediates neurodegeneration of cBF neurons. Here we review the current understanding of how cBF neurons are regulated by neurotrophins which activate p75NTR and TrkA, B or C to influence the critical role that these neurons play in normal cortical function, particularly higher order cognition. Specifically, we describe the current evidence that neurotrophins regulate the development of basal forebrain neurons and their role in maintaining and modifying mature basal forebrain synaptic and cortical microcircuit connectivity. Understanding the role neurotrophin signalling plays in regulating the precision of cholinergic connectivity will contribute to the understanding of normal cognitive processes and will likely provide additional ideas for designing improved therapies for the treatment of neurological disease in which cholinergic dysfunction has been demonstrated.

Methods for gene transfer to the central nervous system

Gene transfer is an increasingly utilized approach for research and clinical applications involving the central nervous system (CNS). Vectors for gene transfer can be as simple as an unmodified plasmid, but more commonly involve complex modifications to viruses to make them suitable gene delivery vehicles. This chapter will explain how tools for CNS gene transfer have been derived from naturally occurring viruses. The current capabilities of plasmid, retroviral, adeno-associated virus, adenovirus, and herpes simplex virus vectors for CNS gene delivery will be described. These include both focal and global CNS gene transfer strategies, with short- or long-term gene expression. As is described in this chapter, an important aspect of any vector is the cis-acting regulatory elements incorporated into the vector genome that control when, where, and how the transgene is expressed.

HSV as a vector in vaccine development and gene therapy

The very deep knowledge acquired on the genetics and molecular biology of herpes simplex virus (HSV), major human pathogen whose lifestyle is based on a long-term dual interaction with the infected host characterized by the existence of lytic and latent infections, has allowed the development of potential vectors for several applications in human healthcare. These include delivery and expression of human genes to cells of the nervous system, selective destruction of cancer cells, prophylaxis against infection with HSV or other infectious diseases and targeted infection of specific tissues or organs. Three different classes of vectors can be derived from HSV-1: replication-competent attenuated vectors, replication-incompetent recombinant vectors and defective helper-dependent vectors known as amplicons. This chapter highlights the current knowledge concerning design, construction and recent applications, as well as the potential and current limitations of the three different classes of HSV-1-based vectors.

Medical application of herpes simplex virus

Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) are important human pathogens that cause a variety of diseases from mild skin diseases such as herpes labialis and herpes genitalis to life-threatening diseases such as herpes encephalitis and neonatal herpes. A number of studies have elucidated the roles of this virus in viral replication and pathogenicity, the regulation of gene expression, interaction with the host cell and immune evasion from the host system. This research has allowed the development of potential therapeutic agents and vectors for human diseases. This review focuses on the basic functions and roles of HSV gene products and reviews the current knowledge of medical applications of genetically engineered HSV mutants using different strategies. These major HSV-derived vectors include: (i) amplicons for gene delivery vectors; (ii) replication-defective HSV recombinants for vaccine vectors; (iii) replication-attenuated HSV recombinants for oncolytic virotherapy.

HSV-1-derived amplicon vectors: recent technological improvements and remaining difficulties--a review

Amplicons are defective and non-integrative vectors derived from herpes simplex virus type 1. As the vector genome carries no virus genes, amplicons are both non-toxic for the infected cells and non-pathogenic for the inoculated organisms. In addition, the large transgenic capacity of amplicons, which allow delivery of up to 150 Kbp of foreign DNA, makes these vectors one of the most powerful, interesting and versatile gene delivery platforms. We present here recent technological developments that have significantly improved and extended the use of amplicons, both in cultured cells and in living organisms. In addition, this review also discusses the many difficulties still pending to be solved, in order to achieve stable and physiologically regulated transgene expression.

HSV-1 amplicon vectors: a promising and versatile tool for gene delivery

Amplicons are defective and non-integrative vectors derived from herpes simplex virus type 1. They carry no virus genes in the vector genome and are, therefore, not toxic to the infected cells or pathogenic for the transduced organisms, making these vectors safe. In addition, the large transgenic capacity of amplicons, which allow delivery of < or = 150 Kbp of foreign DNA, make these vectors one of the most powerful, interesting and versatile gene delivery platforms. Here, the authors present recent technological developments that have significantly improved and extended the use of amplicons, both in cultured cells and in living organisms. In addition, this review illustrates the many possible applications that are presently being developed with amplicons and discuss the many difficulties still pending to be solved in order to achieve stable and physiologically regulated transgenic expression.

THIS ARTICLE HAS BEEN RETRACTED: Functional nerve growth factor and trkA autocrine/paracrine circuits in adult rat cortex are revealed by episodic ethanol exposure and withdrawal

The hypothesis tested is that cortical neurotrophins communicate through an inducible autocrine/paracrine mechanism. As ethanol (Et) can induce cortical nerve growth factor (NGF) expression, adult rats were challenged with Et on three consecutive days per week for 6 weeks. The focus of the study was layer V, the chief repository of receptor-expressing neuronal cell bodies. Brains were collected immediately after the sixth Et exposure or 72 h later [i.e., following withdrawal (WD)]. Double-label in situ hybridization-immunohistochemistry studies showed that many neuronal somata co-expressed NGF mRNA with NGF, trkA, or phosphorylated trk (p-trk), essential components of an inducible autocrine system. The frequencies of co-labeling were affected by neither Et nor WD. On the contrary, Et increased the number of NGF mRNA-expressing neurons and the amount of NGF mRNA expressed per cell. Et also increased total cortical concentration of NGF protein, the number of layer V neurons expressing trkA transcript, the amount of trkA mRNA expressed per neuron, and trkA phosphorylation. Following WD, the frequency of NGF-mRNA-expressing cells increased, although transcript and protein content fell. WD induced an increase in trkA mRNA and protein expression, however, p-trk expression was unaffected. Thus, Et treatment reveals that layer V has inducible autocrine/paracrine and anterograde neurotrophin systems. WD unveils the dynamism and recruitability of these systems.

HSV-1-based amplicon vectors: design and applications

Amplicons are defective, helper-dependent herpes simplex virus type 1 (HSV-1)-based vectors able to convey more than 100 kbp of foreign DNA to the nucleus of mammalian cells. This unique feature make amplicons very appealing for preventive or therapeutic gene transfer requiring the transduction of very large pieces of DNA, as well as for upstream fundamental studies, such as functional genomics. Several recent achievements in amplicon technology have allowed to produce relatively large amounts of essentially helper-free amplicons, as well as to expand the host range of these vectors. In this review, we will update the current know-how concerning design, construction, and recent applications, as well as the potential and current limitations, of this interesting and promising class of vectors.

Improvement of spatial learning and memory after adenovirus-mediated transfer of the nerve growth factor gene to aged rat brain

Adenovirus-mediated transfer of the nerve growth factor gene promotes significant recovery of age-related cholinergic neuronal deficits in aged rats, but the effects of such treatment on cognitive dysfunction remain unclear. Herein we report a beneficial effect of first-generation adenovirus-mediated nerve growth factor gene transfer (AdNGF) on the spatial learning and memory of aged rats. The NGF protein was detected by enzyme-linked immunosorbent assay in cerebrospinal fluid as early as 3 days after gene transfer and was expressed for at least 30 days. Escape latency in the Morris water maze hidden-platform test was significantly improved on day 8 postinoculation in memory-impaired rats treated with AdNGF as well as at later testing intervals. Ultimately, the escape latency values for the AdNGF group become indistinguishable from those for aged rats with normal learning capacity. Immunohistochemical analysis of septal cholinergic neurons for choline acetyltransferase (ChAT) showed significant increases in both the number and somal distribution of ChAT-positive cells after inoculation of memory-impaired rats with AdNGF. Improvement in memory performance was positively correlated with increases in both NGF concentration in cerebrospinal fluid (r = 0.73, p = 0.005) and the number of ChAT-staining cells (r = 0.77, p = 0.0022). We conclude that AdNGF can improve cognitive function in memory-impaired aged rats and, with refinements in vector-driven expression of the transgene, may prove suitable for use in humans.

Prenatal choline supplementation increases NGF levels in the hippocampus and frontal cortex of young and adult rats

Female Sprague-Dawley rats received approximately 300 mg/kg per day of choline chloride through their drinking water on days 11 of pregnancy through birth and the level of nerve growth factor (NGF) in the hippocampus and frontal cortex of their male offspring was measured at 20 and 90 days of age. Prenatal choline supplementation caused significant increases in hippocampal NGF levels at 20 and 90 days of age, while levels of NGF in the frontal cortex were elevated in choline-supplemented rats at 20 days of age, but not 90 days of age. These results suggest that increases in NGF levels during development or adulthood may be one mechanism underlying improvements in spatial and temporal memory of adult rats exposed to elevated levels of choline chloride perinatally.

Functional correction of established central nervous system deficits in an animal model of lysosomal storage disease with feline immunodeficiency virus-based vectors

Gene transfer vectors based on lentiviruses can transduce terminally differentiated cells in the brain; however, their ability to reverse established behavioral deficits in animal models of neurodegeneration has not previously been tested. When recombinant feline immunodeficiency virus (FIV)-based vectors expressing beta-glucuronidase were unilaterally injected into the striatum of adult beta-glucuronidase deficient [mucopolysaccharidosis type VII (MPS VII)] mice, an animal model of lysosomal storage disease, there was bihemispheric correction of the characteristic cellular pathology. Moreover, after the injection of FIV-based vectors expressing beta-glucuronidase into brains of beta-glucuronidase-deficient mice with established impairments in spatial learning and memory, there was dramatic recovery of behavioral function. Cognitive improvement resulting from expression of beta-glucuronidase was associated with alteration in expression of genes associated with neuronal plasticity. These data suggest that enzyme replacement to the MPS VII central nervous system goes beyond restoration of beta-glucuronidase activity in the lysosome, and imparts improvements in plasticity and spatial learning.

Dendritic cells transduced with HSV-1 amplicons expressing prostate-specific antigen generate antitumor immunity in mice

There is currently much interest in generating cytotoxic T lymphocyte (CTL) responses against tumor antigens as a therapy for cancer. This work describes a novel gene transfer technique utilizing dendritic cells (DCs), an extremely potent form of antigen-presenting cell (APC), and herpes simplex virus-1 (HSV-1) amplicons. HSV-1 amplicons are plasmid-based viral vectors that are packaged into HSV-1 capsids, but lack viral coding sequences. Amplicon vectors have been constructed that encode the model tumor antigen ovalbumin (HSV-OVA) and human prostate-specific antigen (HSV-PSA), a protein that is expressed specifically in prostate epithelium and prostate carcinoma cells. These amplicons were packaged using a helper virus-free system that produces vector stocks that are devoid of contaminating cytotoxic helper virus. Transduction of DCs with HSV-OVA or HSV-PSA and co-culture with CTL hybridomas results in specific activation, indicating that transduced DCs express these transgenes and process the tumor antigens for class I MHC presentation to CTL. Mice immunized with HSV-PSA-transduced DCs generate a specific CTL response that can be detected in vitro by a (51)Cr-release assay and are protected from challenge with tumors that express PSA. These results indicate that DCs transduced with HSV-1 amplicon vectors may provide a tool for investigation of the biology of CTL activation by DCs and a new modality for immunotherapy of cancer.

Genetic engineering within the adult brain: implications for molecular approaches to behavioral neuroscience

Currently, the most popular technology used to modify the molecular makeup of the nervous system is through germline modifications of early embryos. This allows to construct gene 'knock-ins' (gene overexpression) or 'knock-outs' (gene deletions). This technology leads to gene additions or deletions from the earliest developmental stages. This can potentially lead to compensatory genetic changes. The technology to achieve inducible and cell-type-specific changes in gene expression in transgenic animals has been established. However, it is not yet possible, to reliably turn a particular gene 'on' or 'off' exclusively in adult animals. Alternatively, the use of gene transfer technology in fully mature animals could overcome many of these shortcomings. Gene therapy is the use of nucleic acids as drugs, and uses gene transfer technology to genetically engineer adult animals. Viral and nonviral vectors have been modified to serve as vectors for nucleic acid sequences of interest. Thus, over the last two decades, methods have been developed to deliver particular nucleic acids directly to target tissues. Further technological advances allow delivery of transgenes or antisense mRNAs directly to predetermined cell types, as well as their delivery under the control of inducible promoter elements. Combined transgenic (i.e., germline modifications) and viral vector technology will also be very powerful in allowing the genetic modification of selected neuronal populations in adult animals. In this review, we discuss the potential of gene delivery to the brain to analyze the effect of genetic engineering of particular neuronal groups on behavior, as well as recent developments and applications of newly engineered vector systems to allow transgenesis within nervous structures of adult animals.