A tyrosine hydroxylase-neurofilament chimeric promoter enhances long-term expression in rat forebrain neurons from helper virus-free HSV-1 vectors

Connected neurons in multiple neocortical areas, comprising parallel circuits, encode essential information for visual shape learning

Neocortical areas comprised of multiple neuronal circuits which are encoded with innumerable advanced cognitive tasks. Studies focused on neuronal network and synaptic plasticity has hypothesized that every specific neuron and the circuit process the explicit essential information for the specific tasks. However, the structure of these circuits and the involved critical neurons remain to be elucidated. Considering our previous studies, showing the specificity of rat postrhinal cortex comprising specific neuronal circuit for encoding both the learning and recall of shape discrimination through a fast neurotransmitter release from the transduced neurons, here we have demonstrated that postsynaptic neurons in two distinct areas, perirhinal cortex and the ventral temporal association areas are required for the specific visual shape discriminations learning. The constitutively active PKC was delivered into neuronal cells in postrhinal cortex, and the animals were allowed to learn the new shape discriminations, and then the silencing siRNA was delivered into postsynaptic neurons in either perirhinal cortex or ventral temporal association areas, using a novel technology for gene transfer into connected neurons. We observed that expression of the siRNA caused the deficits in visual performance, via blocking the activity in the neurons, as displayed by activity-dependent gene imaging, and also subsequently obstructed the activation of specific signaling pathways required for further learning, and dendritic protein synthesis and CREB. Thus, ratifying the conclusion that the two parallel circuits are both required for the visual shape discrimination learning.

Efficient gene transfers into neocortical neurons connected by NMDA NR1-containing synapses

BACKGROUND

Within a circuit, specific neurons and synapses are hypothesized to have essential roles in circuit physiology and learning, and dysfunction in these neurons and synapses causes specific disorders. These critical neurons and synapses are embedded in complex circuits containing many neuron and synapse types.

NEW METHOD

We established technology that can deliver different genes into pre- and post-synaptic neurons connected by a specific synapse type. The first, presynaptic gene transfer employs standard gene transfer technology to express a synthetic peptide neurotransmitter which has three domains, a dense core vesicle sorting domain for processing the protein as a peptide neurotransmitter, a receptor-binding domain, here a small peptide that binds to NMDA NR1 subunits, and the His tag. Upon release, this peptide neurotransmitter binds to its cognate receptor on postsynaptic neurons. Gene transfer selectively into these postsynaptic neurons employs antibody-mediated, targeted gene transfer and anti-His tag antibodies, which recognize the His tag domain in the synthetic peptide neurotransmitter.

RESULTS

For the model system, we studied the connection from projection neurons in postrhinal cortex to specific neurons in perirhinal cortex. In our initial report, gene transfer to connected neurons was 20+1% specific. Here, we optimized the technology; we improved the transfection for packaging by using a modern using a modern lipid, Lipofectamine 3000, and used a modern confocal microscope to collect data. We now report 80+2% specific gene transfer to connected neurons.

COMPARISON WITH EXISTING METHODS

There is no existing method with this capability.

CONCLUSIONS

This technology may enable studies on the roles of specific neurons and synapses in circuit physiology and learning, and support gene therapy treatments for specific disorders.

Separate Gene Transfers into Pre- and Postsynaptic Neocortical Neurons Connected by mGluR5-Containing Synapses

mGluR5-containing synapses have essential roles in synaptic plasticity, circuit physiology, and learning, and dysfunction at these synapses is implicated in specific neurological disorders. As mGluR5-containing synapses are embedded in large and complex distributed circuits containing many neuron and synapse types, it is challenging to elucidate the roles of these synapses and to develop treatments for the associated disorders. Thus, it would be advantageous to deliver different genes into pre- and postsynaptic neurons connected by a mGluR5-containing synapse. Here, we develop this capability: The first gene transfer, into the presynaptic neurons, uses standard techniques to deliver a vector that expresses a synthetic peptide neurotransmitter. This peptide neurotransmitter has three domains: a dense core vesicle sorting domain, a mGluR5-binding domain composed of a single-chain variable fragment anti-mGluR5, and the His tag. Upon release, this peptide neurotransmitter binds to mGluR5, predominately located on the postsynaptic neurons. Selective gene transfer into these neurons uses antibody-mediated, targeted gene transfer and anti-His tag antibodies, as the synthetic peptide neurotransmitter contains the His tag. For the model system, we studied the connection between neurons in two neocortical areas: postrhinal and perirhinal cortices. Targeted gene transfer was over 80% specific for mGluR5-containing synapses, but untargeted gene transfer was only ~ 15% specific for these synapses. This technology may enable studies on the roles of mGluR5-containing neurons and synapses in circuit physiology and learning and support gene therapy treatments for specific disorders that involve dysfunction at these synapses.

Delivery of different genes into pre- and post-synaptic neocortical interneurons connected by GABAergic synapses

Local neocortical circuits play critical roles in information processing, including synaptic plasticity, circuit physiology, and learning, and GABAergic inhibitory interneurons have key roles in these circuits. Moreover, specific neurological disorders, including schizophrenia and autism, are associated with deficits in GABAergic transmission in these circuits. GABAergic synapses represent a small fraction of neocortical synapses, and are embedded in complex local circuits that contain many neuron and synapse types. Thus, it is challenging to study the physiological roles of GABAergic inhibitory interneurons and their synapses, and to develop treatments for the specific disorders caused by dysfunction at these GABAergic synapses. To these ends, we report a novel technology that can deliver different genes into pre- and post-synaptic neocortical interneurons connected by a GABAergic synapse: First, standard gene transfer into the presynaptic neurons delivers a synthetic peptide neurotransmitter, containing three domains, a dense core vesicle sorting domain, a GABAA receptor-binding domain, a single-chain variable fragment anti-GABAA ß2 or ß3, and the His tag. Second, upon release, this synthetic peptide neurotransmitter binds to GABAA receptors on the postsynaptic neurons. Third, as the synthetic peptide neurotransmitter contains the His tag, antibody-mediated, targeted gene transfer using anti-His tag antibodies is selective for these neurons. We established this technology by expressing the synthetic peptide neurotransmitter in GABAergic neurons in the middle layers of postrhinal cortex, and the delivering the postsynaptic vector into connected GABAergic neurons in the upper neocortical layers. Targeted gene transfer was 61% specific for the connected neurons, but untargeted gene transfer was only 21% specific for these neurons. This technology may support studies on the roles of GABAergic inhibitory interneurons in circuit physiology and learning, and support gene therapy treatments for specific disorders associated with deficits at GABAergic synapses.

An identified ensemble within a neocortical circuit encodes essential information for genetically-enhanced visual shape learning

Advanced cognitive tasks are encoded in distributed neocortical circuits that span multiple forebrain areas. Nonetheless, synaptic plasticity and neural network theories hypothesize that essential information for performing these tasks is encoded in specific ensembles within these circuits. Relatively simpler subcortical areas contain specific ensembles that encode learning, suggesting that neocortical circuits contain such ensembles. Previously, using localized gene transfer of a constitutively active protein kinase C (PKC), we established that a genetically-modified circuit in rat postrhinal cortex, part of the hippocampal formation, can encode some essential information for performing specific visual shape discriminations. However, these studies did not identify any specific neurons that encode learning; the entire circuit might be required. Here, we show that both learning and recall require fast neurotransmitter release from an identified ensemble within this circuit, the transduced neurons; we blocked fast release from these neurons by coexpressing a Synaptotagmin I siRNA with the constitutively active PKC. During learning or recall, specific signaling pathways required for learning are activated in this ensemble; during learning, calcium/calmodulin-dependent protein kinase II, MAP kinase, and CREB are activated; and, during recall, dendritic protein synthesis and CREB are activated. Using activity-dependent gene imaging, we showed that during learning, activity in this ensemble is required to recruit and activate the circuit. Further, after learning, during image presentation, blocking activity in this ensemble reduces accuracy, even though most of the rest of the circuit is activated. Thus, an identified ensemble within a neocortical circuit encodes essential information for performing an advanced cognitive task.

Delivery of different genes into presynaptic and postsynaptic neocortical neurons connected by a BDNF-TrkB synapse

Brain-Derived Neurotrophic Factor (BDNF) signaling through TrkB receptors has important roles in synapse formation, synaptic plasticity, learning, and specific diseases. However, it is challenging to relate BDNF-TrkB synapses to circuit physiology or learning, as BDNF-TrkB synapses are embedded in complex circuits that contain numerous neuron and synapse types. Thus, analyzing the physiology of neurons connected by BDNF-TrkB synapses would be advanced by a technology to deliver different genes into presynaptic and postsynaptic neurons, connected by a BDNF-TrkB synapse. Here, we report selective gene transfer across BDNF-TrkB synapses: The model system was the large projection from rat postrhinal to perirhinal cortex. The first gene transfer, into presynaptic neurons in postrhinal cortex, used a virus vector and standard gene transfer procedures. This vector expresses a synthetic peptide neurotransmitter composed of three domains, a dense core vesicle sorting domain, BDNF, and the His tag. Upon release, this peptide neurotransmitter binds to TrkB receptors on postsynaptic neurons. The second gene transfer, into connected postsynaptic neurons in perirhinal cortex, uses antibody-mediated, targeted gene transfer and an anti-His tag antibody, as the synthetic peptide neurotransmitter contains the His tag. Confocal microscope images showed that using untargeted gene transfer, only 10-15% of the transduced presynaptic axons were proximal to a transduced postsynaptic dendrite. But using targeted gene transfer, ∼70% of the transduced presynaptic axons were proximal to a transduced postsynaptic dendrite. This technology may support studies on the roles of neurons connected by BDNF-TrkB synapses in circuit physiology and learning.

And Then There Was Light: Perspectives of Optogenetics for Deep Brain Stimulation and Neuromodulation

Deep Brain Stimulation (DBS) has evolved into a well-accepted add-on treatment for patients with severe Parkinsons disease as well as for other chronic neurological conditions. The focal action of electrical stimulation can yield better responses and it exposes the patient to fewer side effects compared to pharmaceuticals distributed throughout the body toward the brain. On the other hand, the current practice of DBS is hampered by the relatively coarse level of neuromodulation achieved. Optogenetics, in contrast, offers the perspective of much more selective actions on the various physiological structures, provided that the stimulated cells are rendered sensitive to the action of light. Optogenetics has experienced tremendous progress since its first in vivo applications about 10 years ago. Recent advancements of viral vector technology for gene transfer substantially reduce vector-associated cytotoxicity and immune responses. This brings about the possibility to transfer this technology into the clinic as a possible alternative to DBS and neuromodulation. New paths could be opened toward a rich panel of clinical applications. Some technical issues still limit the long term use in humans but realistic perspectives quickly emerge. Despite a rapid accumulation of observations about patho-physiological mechanisms, it is still mostly serendipity and empiric adjustments that dictate clinical practice while more efficient logically designed interventions remain rather exceptional. Interestingly, it is also very much the neuro technology developed around optogenetics that offers the most promising tools to fill in the existing knowledge gaps about brain function in health and disease. The present review examines Parkinson's disease and refractory epilepsy as use cases for possible optogenetic stimulation therapies.

Characteristic and intermingled neocortical circuits encode different visual object discriminations

Synaptic plasticity and neural network theories hypothesize that the essential information for advanced cognitive tasks is encoded in specific circuits and neurons within distributed neocortical networks. However, these circuits are incompletely characterized, and we do not know if a specific discrimination is encoded in characteristic circuits among multiple animals. Here, we determined the spatial distribution of active neurons for a circuit that encodes some of the essential information for a cognitive task. We genetically activated protein kinase C pathways in several hundred spatially-grouped glutamatergic and GABAergic neurons in rat postrhinal cortex, a multimodal associative area that is part of a distributed circuit that encodes visual object discriminations. We previously established that this intervention enhances accuracy for specific discriminations. Moreover, the genetically-modified, local circuit in POR cortex encodes some of the essential information, and this local circuit is preferentially activated during performance, as shown by activity-dependent gene imaging. Here, we mapped the positions of the active neurons, which revealed that two image sets are encoded in characteristic and different circuits. While characteristic circuits are known to process sensory information, in sensory areas, this is the first demonstration that characteristic circuits encode specific discriminations, in a multimodal associative area. Further, the circuits encoding the two image sets are intermingled, and likely overlapping, enabling efficient encoding. Consistent with reconsolidation theories, intermingled and overlapping encoding could facilitate formation of associations between related discriminations, including visually similar discriminations or discriminations learned at the same time or place.

Neurons can be labeled with unique hues by helper virus-free HSV-1 vectors expressing Brainbow

BACKGROUND

A central problem in neuroscience is elucidating synaptic connections, the connectome. Because mammalian forebrains contain many neurons, labeling specific neurons with unique tags is desirable. A novel technology, Brainbow, creates hundreds of hues by combinatorial expression of multiple fluorescent proteins (FPs).

NEW METHOD

We labeled small numbers of neurons, and their axons, with unique hues, by expressing Brainbow from a helper virus-free Herpes Simplex Virus (HSV-1) vector.

RESULTS

The vector expresses a Brainbow cassette containing four FPs from a glutamatergic-specific promoter. Packaging HSV-Brainbow produced arrays of seven to eight Brainbow cassettes, and using Cre, each FP gene was in a position to be expressed, in different cassettes. Delivery into rat postrhinal (POR) cortex or hippocampus labeled small numbers of neurons with different, often unique, hues. An area innervated by POR cortex, perirhinal (PER) cortex, contained axons with different hues. Specific axons in PER cortex were matched to specific cell bodies in POR cortex, using hue.

COMPARISON WITH EXISTING METHODS

HSV-Brainbow is the only technology for labeling small numbers of neurons with unique hues. In Brainbow mice, many neurons contain the same hue. Brainbow-adeno-associated virus vectors require transduction of the same neuron with multiple vector particles, confounding neuroanatomical studies. Replication-competent Brainbow-pseudorabies virus vectors label multiple neurons with the same hue.

CONCLUSIONS

Attractive properties of HSV-Brainbow include each vector particle contains multiple cassettes, representing numerous hues, recombination products are stabile, and experimental control of the number of labeled neurons. Labeling neurons with unique hues will benefit mapping forebrain circuits.

Viral vectors for gene delivery to the central nervous system

The potential benefits of gene therapy for neurological diseases such as Parkinson's, Amyotrophic Lateral Sclerosis (ALS), Epilepsy, and Alzheimer's are enormous. Even a delay in the onset of severe symptoms would be invaluable to patients suffering from these and other diseases. Significant effort has been placed in developing vectors capable of delivering therapeutic genes to the CNS in order to treat neurological disorders. At the forefront of potential vectors, viral systems have evolved to efficiently deliver their genetic material to a cell. The biology of different viruses offers unique solutions to the challenges of gene therapy, such as cell targeting, transgene expression and vector production. It is important to consider the natural biology of a vector when deciding whether it will be the most effective for a specific therapeutic function. In this review, we outline desired features of the ideal vector for gene delivery to the CNS and discuss how well available viral vectors compare to this model. Adeno-associated virus, retrovirus, adenovirus and herpesvirus vectors are covered. Focus is placed on features of the natural biology that have made these viruses effective tools for gene delivery with emphasis on their application in the CNS. Our goal is to provide insight into features of the optimal vector and which viral vectors can provide these features.

Targeted gene transfer of different genes to presynaptic and postsynaptic neocortical neurons connected by a glutamatergic synapse

Genetic approaches to analyzing neuronal circuits and learning would benefit from a technology to first deliver a specific gene into presynaptic neurons, and then deliver a different gene into an identified subset of their postsynaptic neurons, connected by a specific synapse type. Here, we describe targeted gene transfer across a neocortical glutamatergic synapse, using as the model the projection from rat postrhinal to perirhinal cortex. The first gene transfer, into the presynaptic neurons in postrhinal cortex, used a virus vector and standard gene transfer procedures. The vector expresses an artificial peptide neurotransmitter containing a dense core vesicle targeting domain, a NMDA NR1 subunit binding domain (from a monoclonal antibody), and the His tag. Upon release, this peptide neurotransmitter binds to NMDA receptors on the postsynaptic neurons. Antibody-mediated targeted gene transfer to these postsynaptic neurons in perirhinal cortex used a His tag antibody, as the peptide neurotransmitter contains the His tag. Confocal microscopy showed that with untargeted gene transfer, ~3% of the transduced presynaptic axons were proximal to a transduced postsynaptic dendrite. In contrast, with targeted gene transfer, ≥ 20% of the presynaptic axons were proximal to a transduced postsynaptic dendrite. Targeting across other types of synapses might be obtained by modifying the artificial peptide neurotransmitter to contain a binding domain for a different neurotransmitter receptor. This technology may benefit elucidating how specific neurons and subcircuits contribute to circuit physiology, behavior, and learning.

CaMKII, MAPK, and CREB are coactivated in identified neurons in a neocortical circuit required for performing visual shape discriminations

Current theories postulate that the essential information for specific cognitive tasks is widely dispersed in multiple forebrain areas. Nonetheless, synaptic plasticity and neural network theories hypothesize that activation of specific signaling pathways, in specific neurons, modifies synaptic strengths, thereby encoding essential information for performance in localized circuits. Consistent with these latter theories, we have shown that gene transfer of a constitutively active protein kinase C into several hundred glutamatergic and GABAergic neurons in rat postrhinal cortex enhances choice accuracy in visual shape discriminations, and the genetically-modified circuit encodes some of the essential information for performance. However, little is known about the role of specific signaling pathways required for learning, in specific neurons within a critical circuit. Here we show that three learning-associated signaling pathways are coactivated in the transduced neurons during both learning and performance. After gene transfer, but before learning a new discrimination, the calcium/calmodulin-dependent protein kinase (CaMKII), MAP kinase, and CREB pathways were inactive. During learning, these three pathways were coactivated in the transduced neurons. During later performance of the discrimination, CaMKII activity declined, but MAP kinase and CREB activity persisted. Because the transduced neurons are part of a circuit that encodes essential information for performance, activation of these learning-associated signaling pathways, in these identified neurons, is likely important for both learning and performance.

Overexpression of either lysine-specific demethylase-1 or CLOCK, but not Co-Rest, improves long-term expression from a modified neurofilament promoter, in a helper virus-free HSV-1 vector system

Long-term expression from helper virus-free Herpes Simplex Virus (HSV-1) vectors is required for many specific neural gene therapies and studies on neuronal physiology. We previously developed a promoter that supports long-term, neuron-specific expression by fusing the chicken ß-globin insulator (INS), followed by an upstream enhancer from the rat tyrosine hydroxylase (TH) promoter, to a neurofilament heavy gene (NFH) promoter. Here, we examined the capability of specific transcription factors to further improve long-term expression from this promoter. Following a HSV-1 virus infection, the virus genome is localized to promyelocytic leukemia protein (PML) nuclear bodies (NB). At these sites, specific cellular transcription factors interact with HSV-1 encoded transcription factors, and together regulate HSV-1 gene expression. Importantly, lysine-specific demethylase-1 (LSD1), CLOCK, and Co-Rest each activate HSV-1 gene expression. However, gene expression from HSV-1 vectors differs in a number of important aspects from the virus, including no HSV-1 genes are expressed. Nonetheless, these observations raise the possibility that specific transcription factors may improve long-term expression from specific promoters in HSV-1 vectors. Here, we show that overexpression of either LSD1 or CLOCK improves long-term expression from the INS-TH-NFH promoter, but overexpression of Co-Rest supports levels of long-term expression similar to those supported by a control vector. Further, overexpression of LSD1 is compatible with neuron-specific expression. Thus, overexpressing specific transcription factors can improve long-term expression from specific cellular promoters in HSV-1 vectors, and the chromatin structure of the vector has an important role in enabling expression.

Herpes simplex virus type 1-based amplicon vectors for fundamental research in neurosciences and gene therapy of neurological diseases

Somatic manipulation of the nervous system without the involvement of the germinal line appears as a powerful counterpart of the transgenic strategy. The use of viral vectors to produce specific, transient and localized knockout, knockdown, ectopic expression or overexpression of a gene, leads to the possibility of analyzing both in vitro and in vivo molecular basis of neural function. In this approach, viral particles engineered to carry transgenic sequences are delivered into discrete brain regions, to transduce cells that will express the transgenic products. Amplicons are replication-incompetent helper-dependent vectors derived from herpes simplex virus type 1 (HSV-1), with several advantages that potentiate their use in neurosciences: (1) minimal toxicity: amplicons do not encode any virus proteins, are neither toxic for the infected cells nor pathogenic for the inoculated animals and elicit low levels of adaptive immune responses; (2) extensive transgene capacity to carry up to 150-kb of foreign DNA; i.e., entire genes with regulatory sequences could be delivered; (3) widespread cellular tropism: amplicons can experimentally infect several cell types including glial cells, though naturally the virus infects mainly neurons and epithelial cells; (4) since the viral genome does not integrate into cellular chromosomes there is low probability to induce insertional mutagenesis. Recent investigations on gene transfer into the brain using these vectors, have focused on gene therapy of inherited genetic diseases affecting the nervous system, such as ataxias, or on neurodegenerative disorders using experimental models of Parkinson's or Alzheimer's disease. Another group of studies used amplicons to investigate complex neural functions such as neuroplasticity, anxiety, learning and memory. In this short review, we summarize recent data supporting the potential of HSV-1 based amplicon vector model for gene delivery and modulation of gene expression in primary cultures of neuronal cells and into the brain of living animals.

A 16 bp upstream sequence from the rat tyrosine hydroxylase promoter supports long-term expression from a neurofilament promoter, in a helper virus-free HSV-1 vector system

Helper virus-free Herpes Simplex Virus vector-mediated gene transfer has supported studies on neuronal physiology, and may support specific gene therapies. Long-term, neuron-specific expression is required for many of these applications. A neurofilament heavy gene (NFH) promoter does not support long-term expression. We previously developed a promoter that supports long-term expression by fusing 6.3 kb of upstream sequences from the rat tyrosine hydroxylase (TH) promoter to a NFH promoter, and this promoter has supported physiological studies. The TH promoter fragment contains an enhancer, as it has activity in both orientations and at a distance from the basal promoter. Identifying this enhancer may support further improvements in long-term expression. A previous deletion analysis identified two ~100 bp fragments that each support long-term expression, and are contained within an ~320 bp fragment located ~3 kb from the TH promoter transcription start site. As this analysis used overlapping fragments, the two ~100 bp fragments contained 44 or 23 bp of unique sequence. Here, we used mutagenesis to identify a short sequence that supports long-term expression. We studied a 42 bp sequence, centered on the 23 bp unique sequence. Analysis of the wt sequence, and five mutations containing clustered changes that spanned the sequence, identified two adjacent mutations that do not support long-term expression, which together defined a 16 bp maximum essential sequence. This 16 bp sequence contains a putative E2F-1/DP-1 transcription factor binding site, and this transcription factor is expressed in many brain areas.

Antibody-mediated targeted gene transfer of helper virus-free HSV-1 vectors to rat neocortical neurons that contain either NMDA receptor 2B or 2A subunits

Because of the numerous types of neurons in the brain, and particularly the forebrain, neuron type-specific expression will benefit many potential applications of direct gene transfer. The two most promising approaches for achieving neuron type-specific expression are targeted gene transfer to a specific type of neuron and using a neuron type-specific promoter. We previously developed antibody-mediated targeted gene transfer with Herpes Simplex Virus (HSV-1) vectors by modifying glycoprotein C (gC) to replace the heparin binding domain, which mediates the initial binding of HSV-1 particles to many cell types, with the Staphylococcus A protein ZZ domain, which binds immunoglobulin (Ig) G. We showed that a chimeric gC-ZZ protein is incorporated into vector particles and binds IgG. As a proof-of-principle for antibody-mediated targeted gene transfer, we isolated complexes of these vector particles and an anti-NMDA NR1 subunit antibody, and demonstrated targeted gene transfer to neocortical cells that contain NR1 subunits. However, because most forebrain neurons contain NR1, we obtained only a modest increase in the specificity of gene transfer, and this targeting specificity is of limited utility for physiological experiments. Here, we report efficient antibody-mediated targeted gene transfer to NMDA NR2B- or NR2A-containing cells in rat postrhinal cortex, and a neuron-specific promoter further restricted recombinant expression to neurons. Of note, because NR2A-containing neurons are relatively rare, these results show that antibody-mediated targeted gene transfer with HSV-1 vectors containing neuron type-specific promoters can restrict recombinant expression to specific types of forebrain neurons of physiological significance.

The vesicular glutamate transporter-1 upstream promoter and first intron each support glutamatergic-specific expression in rat postrhinal cortex

Multiple applications of direct gene transfer into neurons require restricting expression to glutamatergic neurons, or specific subclasses of glutamatergic neurons. Thus, it is desirable to develop and analyze promoters that support glutamatergic-specific expression. The three vesicular glutamate transporters (VGLUTs) are found in different populations of neurons, and VGLUT1 is the predominant VGLUT in the neocortex, hippocampus, and cerebellar cortex. We previously reported on a plasmid (amplicon) Herpes Simplex Virus vector that contains a VGLUT1 promoter. This vector supports long-term expression in VGLUT1-containing glutamatergic neurons in rat postrhinal (POR) cortex, but does not support expression in VGLUT2-containing glutamatergic neurons in the ventral medial hypothalamus. This VGLUT1 promoter contains both the VGLUT1 upstream promoter and the VGLUT1 first intron. In this study, we begin to isolate and analyze the glutamatergic-specific regulatory elements in this VGLUT1 promoter. We show that the VGLUT1 upstream promoter and first intron each support glutamatergic-specific expression. We isolated a small, basal VGLUT1 promoter that does not support glutamatergic-specific expression. Next, we fused either the VGLUT1 upstream promoter or the first intron to this basal promoter. The VGLUT1 upstream promoter or the first intron, fused to the basal promoter, each supported glutamatergic-specific expression in POR cortex.

Genetic labeling of both the axons of transduced, glutamatergic neurons in rat postrhinal cortex and their postsynaptic neurons in other neocortical areas by herpes simplex virus vectors that coexpress an axon-targeted β-galactosidase and wheat germ agglutinin from a vesicular glutamate transporter-1 promoter

Neuronal circuits comprise the foundation for neuronal physiology and synaptic plasticity, and thus for consequent behaviors and learning, but our knowledge of neocortical circuits is incomplete. Mapping neocortical circuits is a challenging problem because these circuits contain large numbers of neurons, a high density of synapses, and numerous classes and subclasses of neurons that form many different types of synapses. Expression of specific genetic tracers in small numbers of specific subclasses of neocortical neurons has the potential to map neocortical circuits. Suitable genetic tracers have been established in neurons in subcortical areas, but application to neocortical circuits has been limited. Enabling this approach, Herpes Simplex Virus (HSV-1) plasmid (amplicon) vectors can transduce small numbers of neurons in a specific neocortical area. Further, expression of a particular genetic tracer can be restricted to specific subclasses of neurons; in particular, the vesicular glutamate transporter-1 (VGLUT1) promoter supports expression in VGLUT1-containing glutamatergic neurons in rat postrhinal (POR) cortex. Here, we show that expression of an axon-targeted β-galactosidase (β-gal) from such vectors supports mapping specific commissural and associative projections of the transduced neurons in POR cortex. Further, coexpression of wheat germ agglutinin (WGA) and an axon-targeted β-gal supports mapping both specific projections of the transduced neurons and identifying specific postsynaptic neurons for the transduced neurons. The neocortical circuit mapping capabilities developed here may support mapping specific neocortical circuits that have critical roles in cognitive learning.

Antibody-mediated targeted gene transfer to NMDA NR1-containing neurons in rat neocortex by helper virus-free HSV-1 vector particles containing a chimeric HSV-1 glycoprotein C-staphylococcus A protein

Because of the heterogeneous cellular composition of the brain, and especially the forebrain, cell type-specific expression will benefit many potential applications of direct gene transfer. The two prevalent approaches for achieving cell type-specific expression are using a cell type-specific promoter or targeting gene transfer to a specific cell type. Targeted gene transfer with Herpes Simplex Virus (HSV-1) vectors modifies glycoprotein C (gC) to replace the heparin binding domain, which binds to many cell types, with a binding activity for a specific cell surface protein. We previously reported targeted gene transfer to nigrostriatal neurons using chimeric gC-glial cell line-derived neurotrophic factor or gC-brain-derived neurotrophic factor protein. Unfortunately, this approach is limited to cells that express the cognate receptor for either neurotrophic factor. Thus, a general strategy for targeting gene transfer to many different types of neurons is desirable. Antibody-mediated targeted gene transfer has been developed for targeting specific virus vectors to specific peripheral cell types; a specific vector particle protein is modified to contain the Staphylococcus A protein ZZ domain, which binds immunoglobulin (Ig) G. Here, we report antibody-mediated targeted gene transfer of HSV-1 vectors to a specific type of forebrain neuron. We constructed a chimeric gC-ZZ protein, and showed this protein is incorporated into vector particles and binds Ig G. Complexes of these vector particles and an antibody to the NMDA receptor NR1 subunit supported targeted gene transfer to NR1-containing neocortical neurons in the rat brain, with long-term (2 months) expression.

A helper virus-free HSV-1 vector containing the vesicular glutamate transporter-1 promoter supports expression preferentially in VGLUT1-containing glutamatergic neurons

Multiple potential uses of direct gene transfer into neurons require restricting expression to specific classes of glutamatergic neurons. Thus, it is desirable to develop vectors containing glutamatergic class-specific promoters. The three vesicular glutamate transporters (VGLUTs) are expressed in distinct populations of neurons, and VGLUT1 is the predominant VGLUT in the neocortex, hippocampus, and cerebellar cortex. We previously reported a plasmid (amplicon) Herpes Simplex Virus (HSV-1) vector that placed the Lac Z gene under the regulation of the VGLUT1 promoter (pVGLUT1lac). Using helper virus-free vector stocks, we showed that this vector supported approximately 90% glutamatergic neuron-specific expression in postrhinal (POR) cortex, in rats sacrificed at either 4 days or 2 months after gene transfer. We now show that pVGLUT1lac supports expression preferentially in VGLUT1-containing glutamatergic neurons. pVGLUT1lac vector stock was injected into either POR cortex, which contains primarily VGLUT1-containing glutamatergic neurons, or into the ventral medial hypothalamus (VMH), which contains predominantly VGLUT2-containing glutamatergic neurons. Rats were sacrificed at 4 days after gene transfer, and the types of cells expressing ss-galactosidase were determined by immunofluorescent costaining. Cell counts showed that pVGLUT1lac supported expression in approximately 10-fold more cells in POR cortex than in the VMH, whereas a control vector supported expression in similar numbers of cells in these two areas. Further, in POR cortex, pVGLUT1lac supported expression predominately in VGLUT1-containing neurons, and, in the VMH, pVGLUT1lac showed an approximately 10-fold preference for the rare VGLUT1-containing neurons. VGLUT1-specific expression may benefit specific experiments on learning or specific gene therapy approaches, particularly in the neocortex.

Herpes Virus Amplicon Vectors

Since its emergence onto the gene therapy scene nearly 25 years ago, the replication-defective Herpes Simplex Virus Type-1 (HSV-1) amplicon has gained significance as a versatile gene transfer platform due to its extensive transgene capacity, widespread cellular tropism, minimal immunogenicity, and its amenability to genetic manipulation. Herein, we detail the recent advances made with respect to the design of the HSV amplicon, its numerous in vitro and in vivo applications, and the current impediments this virus-based gene transfer platform faces as it navigates a challenging path towards future clinical testing.

Transcriptional targeting to brain cells: Engineering cell type-specific promoter containing cassettes for enhanced transgene expression

Transcriptional targeting using a mammalian cellular promoter to restrict transgene expression to target cells is often desirable for gene therapy. This strategy is, however, hindered by relatively weak activity of some cellular promoters, which may lead to low levels of gene expression, thus declining therapeutic efficacy. Here we outline the advances accomplished in the area of transcriptional targeting to brain cells, with a particular focus on engineering gene cassettes to augment cell type-specific expression. Among the effective approaches that improve gene expression while retaining promoter specificity are promoter engineering to change authentic sequences of a cellular promoter and the combined use of a native cellular promoter and other cis-acting elements. Success in achieving high level and sustained transgene expression only in the cell types of interest would be of importance in allowing gene therapy to have its impact on patient treatment.

Improved long-term expression from helper virus-free HSV-1 vectors packaged using combinations of mutated HSV-1 proteins that include the UL13 protein kinase and specific components of the VP16 transcriptional complex

BACKGROUND

Herpes Simplex Virus (HSV-1) gene expression is thought to shut off recombinant gene expression from HSV-1 vectors; however, in a helper virus-free HSV-1 vector system, a number of promoters support only short-term expression. These results raise the paradox that recombinant gene expression remains short-term even in the absence of almost all (approximately 99%) of the HSV-1 genome, HSV-1 genes, and HSV-1 gene expression. To resolve this paradox, we hypothesized that specific proteins in the HSV-1 virus particle shut off recombinant gene expression. In two earlier studies, we examined the effects on recombinant gene expression of packaging vectors using specific mutated HSV-1 proteins. We found that vectors packaged using mutated UL13 (a protein kinase), or VP16, or UL46 and/or UL47 (components of the VP16 transcriptional complex) supported improved long-term expression, and vectors packaged using mutated UL46 and/or UL47 also supported improved gene transfer (numbers of cells at 4 days). These results suggested the hypothesis that specific proteins in the HSV-1 particle act by multiple pathways to reduce recombinant gene expression. To test this hypothesis, we examined combinations of mutated proteins that included both UL13 and specific components of the VP16 transcriptional complex.

RESULTS

A HSV-1 vector containing a neuronal-specific promoter was packaged using specific combinations of mutated proteins, and the resulting vector stocks were tested in the rat striatum. For supporting long-term expression, the preferred combination of mutated HSV-1 proteins was mutated UL13, UL46, and UL47. Vectors packaged using this combination of mutated proteins supported a higher efficiency of gene transfer and high levels expression for 3 months, the longest time examined.

CONCLUSION

Vector particles containing this combination of mutated HSV-1 proteins improve recombinant gene expression. Implications of these results for strategies to further improve long-term expression are discussed. Moreover, long-term expression will benefit specific gene therapy applications.

Improved spatial learning in aged rats by genetic activation of protein kinase C in small groups of hippocampal neurons

Age-related decline in human cognition is well known, and there are correlative changes in the function of neocortical and hippocampal neurons. Similarly, age-related decline in learning has been observed in rodents, including deficits in a hippocampal-dependent learning paradigm, the Morris water maze. Furthermore, there are correlative deficits in specific signaling pathways, including protein kinase C (PKC) pathways, in cerebellar, hippocampal, or neocortical neurons. PKC pathways are strong candidates for mediating the molecular changes that underlie spatial learning, as they play critical roles in neurotransmitter release and synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD), and deletion of specific PKC genes results in deficits in learning. Conversely, genetic activation of PKC pathways in small groups of hippocampal or cortical neurons enhances learning in specific paradigms. In this study, the authors delivered a constitutively active PKC into small groups of hippocampal dentate granule neurons in aged rats (using a herpes simplex virus-1 vector). Aged 2-year-old rats that received the constitutively active PKC displayed improved performance in the Morris water maze relative to controls in three different measures. These results indicate that PKC pathways play an important role in mediating spatial learning in aged rats. Additionally, these results represent a system for studying the neural mechanisms underlying aging-related learning deficits, and potentially developing gene therapies for cognitive and age-related deficits.

Stable transgene expression from HSV amplicon vectors in the brain: potential involvement of immunoregulatory signals

The herpes simplex virus (HSV) amplicon is a plasmid-based, infectious gene delivery system that carries up to 150 kilobase (kb) of exogenous DNA. We previously characterized early host responses and stability of transgene expression in mice systemically injected with HSV amplicon vectors. Transgene expression was readily detected primarily in the liver but rapidly declined to undetectable levels within 2 weeks. Molecular analyses revealed induction of type I interferons (IFN) as the primary response, and early transcriptional silencing of the vector followed IFN's activation of signal transducers and activators of transcription 1 (STAT1). In this study, we investigate vector administration by stereotactic injection into the striatum. In the brain, induction of type I IFN was rather modest, and transgene expression lasted more than 1 year despite dose-dependent inflammation and infiltration of immune cells around injection sites. Further analyses revealed dose-dependent upregulation of immunosuppressive cytokines and molecular markers specific to regulatory T cells in the injected brain regions, which supported the immune-privileged properties of the brain parenchyma. Overall, our findings indicate that the spectrum of host responses can differ significantly depending on target organs and administrative routes, and that HSV amplicon vectors hold great potential for gene therapy of chronic neurological disorders.

Enhanced nigrostriatal neuron-specific, long-term expression by using neural-specific promoters in combination with targeted gene transfer by modified helper virus-free HSV-1 vector particles

BACKGROUND

Direct gene transfer into neurons has potential for developing gene therapy treatments for specific neurological conditions, and for elucidating neuronal physiology. Due to the complex cellular composition of specific brain areas, neuronal type-specific recombinant gene expression is required for many potential applications of neuronal gene transfer. One approach is to target gene transfer to a specific type of neuron. We developed modified Herpes Simplex Virus (HSV-1) particles that contain chimeric glycoprotein C (gC) - glial cell line-derived neurotrophic factor (GDNF) or brain-derived neurotrophic factor (BDNF) proteins. HSV-1 vector particles containing either gC - GDNF or gC - BDNF target gene transfer to nigrostriatal neurons, which contain specific receptors for GDNF or BDNF. A second approach to achieve neuronal type-specific expression is to use a cell type-specific promoter, and we have used the tyrosine hydroxylase (TH) promoter to restrict expression to catecholaminergic neurons or a modified neurofilament heavy gene promoter to restrict expression to neurons, and both of these promoters support long-term expression from HSV-1 vectors. To both improve nigrostriatal-neuron specific expression, and to establish that targeted gene transfer can be followed by long-term expression, we performed targeted gene transfer with vectors that support long-term, neuronal-specific expression.

RESULTS

Helper virus-free HSV-1 vector packaging was performed using either gC - GDNF or gC - BDNF and vectors that contain either the TH promoter or the modified neurofilament heavy gene promoter. Vector stocks were injected into the midbrain proximal to the substantia nigra, and the rats were sacrificed at either 4 days or 1 month after gene transfer. Immunofluorescent costaining was performed to detect both recombinant gene products and nigrostriatal neurons. The combination of targeted gene transfer with neuronal-specific promoters improved nigrostriatal neuron-specific expression (83 to 93%) compared to either approach alone, and supported long-term (1 month) expression at levels similar to those observed using untargeted gene transfer.

CONCLUSION

Targeted gene transfer can be used in combination with neuronal-specific promoters to achieve a high level of nigrostriatal neuron-specific expression. Targeted gene transfer can be followed by long-term expression. Nigrostriatal neuron-specific expression may be useful for specific gene therapy approaches to Parkinson's disease or for genetic analyses of nigrostriatal neuron physiology.

Glutamatergic or GABAergic neuron-specific, long-term expression in neocortical neurons from helper virus-free HSV-1 vectors containing the phosphate-activated glutaminase, vesicular glutamate transporter-1, or glutamic acid decarboxylase promoter

Many potential uses of direct gene transfer into neurons require restricting expression to one of the two major types of forebrain neurons, glutamatergic or GABAergic neurons. Thus, it is desirable to develop virus vectors that contain either a glutamatergic or GABAergic neuron-specific promoter. The brain/kidney phosphate-activated glutaminase (PAG), the product of the GLS1 gene, produces the majority of the glutamate for release as neurotransmitter, and is a marker for glutamatergic neurons. A PAG promoter was partially characterized using a cultured kidney cell line. The three vesicular glutamate transporters (VGLUTs) are expressed in distinct populations of neurons, and VGLUT1 is the predominant VGLUT in the neocortex, hippocampus, and cerebellar cortex. Glutamic acid decarboxylase (GAD) produces GABA; the two molecular forms of the enzyme, GAD65 and GAD67, are expressed in distinct, but largely overlapping, groups of neurons, and GAD67 is the predominant form in the neocortex. In transgenic mice, an approximately 9 kb fragment of the GAD67 promoter supports expression in most classes of GABAergic neurons. Here, we constructed plasmid (amplicon) Herpes Simplex Virus (HSV-1) vectors that placed the Lac Z gene under the regulation of putative PAG, VGLUT1, or GAD67 promoters. Helper virus-free vector stocks were delivered into postrhinal cortex, and the rats were sacrificed 4 days or 2 months later. The PAG or VGLUT1 promoters supported approximately 90% glutamatergic neuron-specific expression. The GAD67 promoter supported approximately 90% GABAergic neuron-specific expression. Long-term expression was observed using each promoter. Principles for obtaining long-term expression from HSV-1 vectors, based on these and other results, are discussed. Long-term glutamatergic or GABAergic neuron-specific expression may benefit specific experiments on learning or specific gene therapy approaches. Of note, promoter analyses might identify regulatory elements that determine a glutamatergic or GABAergic neuron.

Isolation of an enhancer from the rat tyrosine hydroxylase promoter that supports long-term, neuronal-specific expression from a neurofilament promoter, in a helper virus-free HSV-1 vector system

Direct gene transfer into neurons, using a virus vector, has been used to study neuronal physiology and learning, and has potential for supporting gene therapy treatments for specific neurological diseases. Many of these applications require high-level, long-term recombinant gene expression, in forebrain neurons. We previously showed that addition of upstream sequences from the rat tyrosine hydroxylase (TH) promoter to a neurofilament heavy gene (NF-H) promoter supports long-term expression in forebrain neurons, from helper virus-free Herpes Simplex Virus (HSV-1) vectors. This element in the TH promoter satisfied the definition of an enhancer; it displayed activity at a distance from the basal promoter, and in both orientations. This enhancer supported physiological studies that required long-term expression; a modified neurofilament promoter, containing an insulator upstream of the TH-NFH promoter, supported expression in approximately 11,400 striatal neurons at 6 months after gene transfer, and expression for 7, 8, or 14 months, the longest times tested. In contrast, the NF-H promoter alone does not support long-term expression, indicating that the critical sequences are in the 6.3 kb fragment of the TH promoter. In this study, we performed a deletion analysis to identify the critical sequences in the TH promoter that support long-term expression. We localized these critical sequences to an approximately 320 bp fragment, and two subfragments of approximately 100 bp each. Vectors that contained each of these small fragments supported levels of long-term, neuronal-specific expression that were similar to the levels supported by a vector that contained the initial 6.3 kb fragment of the TH promoter. These small fragments of the TH promoter may benefit construction of vectors for physiological studies, and may support studies on the mechanism by which this enhancer supports long-term expression.

Long-term inducible expression in striatal neurons from helper virus-free HSV-1 vectors that contain the tetracycline-inducible promoter system

Direct gene transfer into neurons in the brain via a virus vector system has potential for both examining neuronal physiology and for developing gene therapy treatments for neurological diseases. Many of these applications require precise control of the levels of recombinant gene expression. The preferred method for controlling the levels of expression is by use of an inducible promoter system, and the tetracycline (tet)-inducible promoter system is the preferred system. Helper virus-free Herpes Simplex Virus (HSV-1) vectors have a number of the advantages, including their large size and efficient gene transfer. Also, we have reported long-term (14 months) expression from HSV-1 vectors that contain a modified neurofilament heavy gene promoter. A number of studies have reported short-term, inducible expression from helper virus-containing HSV-1 vector systems. However, long-term, inducible expression has not been reported using HSV-1 vectors. The goal of this study was to obtain long-term, inducible expression from helper virus-free HSV-1 vectors. We examined two different vector designs for adapting the tet promoter system to HSV-1 vectors. One design was an autoregulatory design; one transcription unit used a tet-regulated promoter to express the tet-regulated transcription factor tet-off, and another transcription unit used a tet-regulated promoter to express the Lac Z gene. In the other vector design, one transcription unit used the modified neurofilament heavy gene promoter to express tet-off, and another transcription unit used a tet-regulated promoter to express the Lac Z gene. The results showed that both vector designs supported inducible expression in cultured fibroblast or neuronal cell lines and for a short time (4 days) in the rat striatum. Of note, only the vector design that used the modified neurofilament promoter to express tet-off supported long-term (2 months) inducible expression in striatal neurons.

Targeted gene transfer to nigrostriatal neurons in the rat brain by helper virus-free HSV-1 vector particles that contain either a chimeric HSV-1 glycoprotein C-GDNF or a gC-BDNF protein

Direct gene transfer into neurons has potential for both studying neuronal physiology and for developing gene therapy treatments for specific neurological conditions. Due to the heterogeneous cellular composition of the brain, cell-type-specific recombinant gene expression is required for many potential applications of neuronal gene transfer. The two prevalent approaches for achieving cell-type-specific expression are to use a cell-type-specific promoter to control recombinant gene expression or to modify a virus vector particle to target gene transfer to a specific cell type. Targeted gene transfer to multiple peripheral cell types has been described, but targeted gene transfer to a specific type of neuron in the brain has yet to be reported. Targeted gene transfer approaches with Herpes Simplex Virus (HSV-1) vectors have focused on modifying glycoprotein C (gC) to remove the heparin binding domain and add a binding activity for a specific protein on the cell surface. This study was designed to develop HSV-1 vectors that target gene transfer to cells that contain receptors for either glial-cell-line-derived neurotrophic factor (GDNF) or brain-derived neurotrophic factor (BDNF), such as nigrostriatal neurons. We isolated chimeric gC-GDNF or chimeric gC-BDNF constructs, and the resulting proteins were incorporated into HSV-1 virus particles. We performed helper virus-free HSV-1 vector packaging in the presence of each chimeric protein. The resulting vector stocks supported 2.2- to 5.0-fold targeted gene transfer to nigrostriatal neurons in the rat brain, compared to vector particles that contained wild-type (wt) gC. Gene transfer to nigrostriatal neurons by vector particles that contained chimeric gC-BDNF was reduced by preincubation with an anti-BDNF antibody. Targeted gene transfer to neurons that contain specific neurotrophic factor receptors may benefit specific physiological and gene therapy studies.

Use of herpes virus amplicon vectors to study brain disorders

There is an enormous initiative to establish the genetic basis for disorders of brain function. Unfortunately, genetic intervention is not accomplished easily in the nervous system. One strategy is to engineer and deliver to neurons specialized viral vectors that carry a gene (or genes) of interest, thereby exploiting the natural ability of viruses to insert genetic material into cells. When delivered to brain cells, these vectors cause infected cells to increase the expression of the genes of interest. The ability to deliver genes into neurons in vitro and in vivo with herpes simplex virus (HSV) amplicon vectors has made it possible to carry out exactly these sorts of experiments. This technology has the potential to offer new insights into the etiology of a wide variety of neuropsychiatric disorders. We describe the use of HSV amplicon vectors to study Alzheimer disease, drug addiction, and depression, and discuss the considerations that enter into the use of these vectors both in vitro and in vivo. The HSV amplicon virus is a user-friendly vector for the delivery of genes into neurons that has come of age for the study of brain function.

Comparison of the capability of GDNF, BDNF, or both, to protect nigrostriatal neurons in a rat model of Parkinson's disease

Both glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) can protect nigrostriatal dopaminergic neurons from neurotoxins in rodent and monkey models of Parkinson's disease (PD). These two neurotrophic factors are usually tested individually. This study was designed to compare GDNF, BDNF, or both, for their capabilities to correct behavioral deficits and protect nigrostriatal dopaminergic neurons in a rat model of PD. Gene transfer used a helper virus-free Herpes Simplex Virus (HSV-1) vector system and a modified neurofilament heavy gene promoter that supports long-term expression in forebrain neurons. Rats received unilateral intrastriatal injections of HSV-1 vectors that express either GDNF or BDNF, or both vectors, followed by intrastriatal injections of 6-hydroxydopamine (6-OHDA). Recombinant GDNF or BDNF was detected in striatal neurons in rats sacrificed at 7 months after gene transfer. Of note, GDNF was significantly more effective than BDNF for both correcting behavioral deficits and protecting nigrostriatal dopaminergic neurons. Expression of both neurotrophic factors was no more effective than expression of only GDNF. These results suggest that GDNF is more effective than BDNF for correcting the rat model of PD, and that there are no detectable benefits from expressing both of these neurotrophic factors.

Genetic enhancement of visual learning by activation of protein kinase C pathways in small groups of rat cortical neurons

Although learning and memory theories hypothesize that memories are encoded by specific circuits, it has proven difficult to localize learning within a cortical area. Neural network theories predict that activation of a small fraction of the neurons in a circuit can activate that circuit. Consequently, altering the physiology of a small group of neurons might potentiate a specific circuit and enhance learning, thereby localizing learning to that circuit. In this study, we activated protein kinase C (PKC) pathways in small groups of neurons in rat postrhinal (POR) cortex. We microinjected helper virus-free herpes simplex virus vectors that expressed a constitutively active PKC into POR cortex. This PKC was expressed predominantly in glutamatergic and GABAergic neurons in POR cortex. This intervention increased phosphorylation of five PKC substrates that play critical roles in neurotransmitter release (GAP-43 and dynamin) or glutamatergic neurotransmission (specific subunits of AMPA or NMDA receptors and myristoylated alanine-rich C kinase substrate). Additionally, activation of PKC pathways in cultured cortical neurons supported activation-dependent increases in release of glutamate and GABA. This intervention enhanced the learning rate and accuracy of visual object discriminations. In individual rats, the numbers of transfected neurons positively correlated with this learning. During learning, neuronal activity was increased in neurons proximal to the transfected neurons. These results demonstrate that potentiating small groups of glutamatergic and GABAergic neurons in POR cortex enhances visual object learning. More generally, these results suggest that learning can be mediated by specific cortical circuits.

Enhanced long-term expression from helper virus-free HSV-1 vectors packaged in the presence of deletions in genes that modulate the function of VP16, UL46 and UL47

Herpes simplex virus (HSV-1) gene expression is hypothesized to shut off recombinant gene expression from HSV-1 vectors, but in a helper virus-free HSV-1 vector system, a number of promoters support only short-term expression. Thus paradoxically, recombinant gene expression remains short-term in the absence of almost all (approximately 99%) of the HSV-1 genome. To resolve this paradox, we hypothesize that specific HSV-1 proteins that affect the virion can shut off recombinant gene expression. In an earlier study, we examined the effects on recombinant gene expression of five different proteins that affect the HSV-1 virion. We found that vectors packaged in the presence of mutated vhs or U S 11 exhibited minimal changes in gene expression, vectors packaged in the presence of a mutated U S 3 supported improved gene transfer (numbers of cells at 4 days), and vectors packaged in the presence of mutated U L 13 or VP16 supported improved long-term expression. The capability of the VP16 transcriptional complex to reduce gene expression deserves additional study because VP16 is a powerful enhancer that interacts with a number of cellular and viral proteins. In particular, U L 46 and U L 47 are known to modulate the effects of VP16 on immediate early promoters. In this study, we examined expression from a HSV-1 vector that contains a neuronal-specific promoter and was packaged in the presence of deletions in U L 46, or U L 47, or both U L 46 and U L 47. In the rat striatum, each of these vector stocks supported both improved gene transfer (numbers of cells at 4 days) and improved long-term expression (2 months). Vectors packaged in the presence of a deletion in both U L 46 and U L 47 supported larger improvements in gene expression compared to vectors packaged in the presence of deletions in either gene alone. The implications of these results for strategies to improve long-term expression are discussed.

Coexpression of tyrosine hydroxylase, GTP cyclohydrolase I, aromatic amino acid decarboxylase, and vesicular monoamine transporter 2 from a helper virus-free herpes simplex virus type 1 vector supports high-level, long-term biochemical and behavioral correction of a rat model of Parkinson's disease

Parkinson's disease is due to the selective loss of nigrostriatal dopaminergic neurons. Consequently, many therapeutic strategies have focused on restoring striatal dopamine levels, including direct gene transfer to striatal cells, using viral vectors that express specific dopamine biosynthetic enzymes. The central hypothesis of this study is that coexpression of four dopamine biosynthetic and transporter genes in striatal neurons can support the efficient production and regulated, vesicular release of dopamine: tyrosine hydroxylase (TH) converts tyrosine to L-3,4-dihydroxyphenylalanine (L-DOPA), GTP cyclohydrolase I (GTP CH I) is the rate-limiting enzyme in the biosynthesis of the cofactor for TH, aromatic amino acid decarboxylase (AADC) converts L-DOPA to dopamine, and a vesicular monoamine transporter (VMAT-2) transports dopamine into synaptic vesicles, thereby supporting regulated, vesicular release of dopamine and relieving feedback inhibition of TH by dopamine. Helper virus-free herpes simplex virus type 1 vectors that coexpress the three dopamine biosynthetic enzymes (TH, GTP CH I, and AADC; 3-gene-vector) or these three dopamine biosynthetic enzymes and the vesicular monoamine transporter (TH, GTP CH I, AADC, and VMAT-2; 4-gene-vector) were compared. Both vectors supported production of dopamine in cultured fibroblasts. These vectors were microinjected into the striatum of 6-hydroxydopamine-lesioned rats. These vectors carry a modified neurofilament gene promoter, and gamma-aminobutyric acid (GABA)-ergic neuron-specific gene expression was maintained for 14 months after gene transfer. The 4-gene-vector supported higher levels of correction of apomorphine-induced rotational behavior than did the 3-gene-vector, and this correction was maintained for 6 months. Proximal to the injection sites, the 4-gene-vector, but not the 3-gene-vector, supported extracellular levels of dopamine and dihydroxyphenylacetic acid (DOPAC) that were similar to those observed in normal rats, and only the 4-gene-vector supported significant K(+)-dependent release of dopamine.

Herpes simplex virus-based vectors

Herpes simplex virus (HSV)-based vectors have primarily been developed for neuronal gene delivery, taking advantage of the virus' natural neurotropism. Two types of vector are available: replication defective viruses, whose cytotoxicity has been abolished by deleting viral gene products, and amplicon vectors, which are plasmids packaged into HSV particles with the aid of a helper virus. In this review I discuss how the cytotoxicity of the wild-type virus has been abolished, the progress which has been made toward defining promoter elements capable of directing long-term transgene expression form the latent viral genome and some of the potential clinical uses of these versatile vectors.

A preproenkephalin-neurofilament chimeric promoter in a helper virus-free herpes simplex virus vector enhances long-term expression in the rat striatum

Helper virus-free herpes simplex virus (HSV-1) plasmid vectors are an attractive system for gene transfer into neurons in the brain, but promoters that support long-term, neuronal-specific expression are required. Elucidation of general principles that govern long-term expression would likely assist efforts to develop improved promoters. Although expression from many promoters in HSV-1 vectors is unstable, two neuronal subtype-specific promoters, the preproenkephalin (ENK) promoter and the tyrosine hydroxylase (TH) promoter, support long-term expression. We have previously shown that 5' upstream sequences in the TH promoter are required for long-term expression, and addition of these upstream sequences to a neurofilament heavy gene (NF-H) promoter enhances long-term, neuronal-specific expression. The goal of this study was to determine if the upstream sequences from the TH promoter contain a unique element that enhances expression, or if other neuronal promoters also contain sequences that can enhance expression. To this end, we tested 5' upstream sequences in the ENK promoter. We isolated a vector that fuses upstream sequences from the ENK promoter to the NF-H promoter. This vector supported expression in the striatum for 2 months after gene transfer, the longest time point evaluated. Expression was neuronal specific. As ENK and TH are a peptide neurotransmitter and a classical neurotransmitter biosynthetic enzyme, respectively, these results suggest that a significant number of promoters for neurotransmitter biosynthetic genes may contain elements that can enhance expression from HSV-1 vectors. The strategy of using upstream sequences from neuronal subtype-specific promoters to enhance expression from heterologous promoters is discussed.

Gene transfer of constitutively active protein kinase C into striatal neurons accelerates onset of levodopa-induced motor response alterations in parkinsonian rats

Alterations in motor response that complicate levodopa treatment of Parkinson's disease appear to involve sensitization of striatal ionotropic glutamate receptors. Since protein kinase C (PKC)-mediated phosphorylation regulates glutamatergic receptors of the alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionic acid (AMPA) subtype and has been linked to several forms of behavioral plasticity, activation of PKC signaling in striatal spiny neurons may also contribute to the motor plasticity changes associated with chronic levodopa therapy. To evaluate this possibility, we sought to augment PKC signaling by using Herpes Simplex Virus type 1 vectors (pHSVpkcDelta) to directly transfer the catalytic domain of the PKCbetaII gene into striatal neurons of parkinsonian rats. Microinjection of pHSVpkcDelta vectors lead to the persistent expression of PkcDelta (35% loss over 21 days) in medium spiny neurons together with an increase in serine 831 phosphorylation on AMPA receptor GluR1 subunits and hastened the appearance of the shortened response duration produced by chronic levodopa treatment (P<0.05). In pHSVpkcDelta-infected animals, intrastriatal injection of the PKC inhibitor NPC-15437 (1.0 microg) attenuated both the increased GluR1 phosphorylation (P<0.01) and the accelerated onset of the levodopa-induced response modifications (P<0.01). However, in rats that received levodopa treatment for 21 days without the gene transfer, intrastriatal NPC-15437 had no effect on the response shortening or on GluR1 S831 phosphorylation. The results suggest that an increase in PKC-mediated signaling, including, in part, phosphorylation of AMPA receptors, on striatal spiny neurons may be sufficient to promote the initial appearance, but not necessary the ultimate expression, of the levodopa-induced motor response changes occurring in a rodent model of the human motor complication syndrome.

Correction of a rat model of Parkinson's disease by coexpression of tyrosine hydroxylase and aromatic amino acid decarboxylase from a helper virus-free herpes simplex virus type 1 vector

We previously reported long-term biochemical and behavioral correction of the 6-hydroxydopamine (6-OHDA) rat model of Parkinson's disease (PD) by expression of tyrosine hydroxylase (TH) in the partially denervated striatum, using a herpes simplex virus type 1 (HSV-1) vector. This study had a number of limitations, including the use of a helper virus packaging system, limited long-term expression, and expression of only TH. To address these issues, we developed a helper virus-free packaging system, a modified neurofilament gene promoter that supports long-term expression in forebrain neurons, and a vector that coexpresses TH and aromatic amino acid decarboxylase (AADC). Coexpression of TH and AADC supported high-level (80%), behavioral correction of the 6-OHDA rat model of PD for 5 weeks. Biochemical correction included increases in extracellular dopamine and DOPAC concentrations between 2 and 4 months after gene transfer. Histologic analyses demonstrated neuronal-specific coexpression of TH and AADC at 4 days to 7 months after gene transfer, and cell counts revealed 1000 to 10,000 TH positive cells per rat at 2 months after gene transfer. This improved system efficiently corrects the rat model of PD.

Herpes simplex virus type 1/adeno-associated virus rep(+) hybrid amplicon vector improves the stability of transgene expression in human cells by site-specific integration

Herpes simplex virus type 1 (HSV-1) amplicon vectors are promising gene delivery tools, but their utility in gene therapy has been impeded to some extent by their inability to achieve stable transgene expression. In this study, we examined the possibility of improving transduction stability in cultured human cells via site-specific genomic integration mediated by adeno-associated virus (AAV) Rep and inverted terminal repeats (ITRs). A rep(-) HSV/AAV hybrid amplicon vector was made by inserting a transgene cassette flanked with AAV ITRs into an HSV-1 amplicon backbone, and a rep(+) HSV/AAV hybrid amplicon was made by inserting rep68/78 outside the rep(-) vector 3' AAV ITR sequence. Both vectors also had a pair of loxP sites flanking the ITRs. The resulting hybrid amplicon vectors were successfully packaged and compared to a standard amplicon vector for stable transduction frequency (STF) in human 293 and Gli36 cell lines and primary myoblasts. The rep(+), but not the rep(-), hybrid vector improved STF in all three types of cells; 84% of Gli36 and 40% of 293 stable clones transduced by the rep(+) hybrid vector integrated the transgene into the AAVS1 site. Due to the difficulty in expanding primary myoblasts, we did not assess site-specific integration in these cells. A strategy to attempt further improvement of STF by "deconcatenating" the hybrid amplicon DNA via Cre-loxP recombination was tested, but it did not increase STF. These data demonstrate that introducing the integrating elements of AAV into HSV-1 amplicon vectors can significantly improve their ability to achieve stable gene transduction by conferring the AAV-like capability of site-specific genomic integration in dividing cells.

Highly efficient and specific gene transfer to Purkinje cells in vivo using a herpes simplex virus I amplicon

The transduction of cerebellar neurons in vivo with herpes simplex virus 1 (HSV-1) amplicon carrying the lacZ gene has been investigated after injection of the vector in the cerebellar cortex, ventricles, and inferior olive of adult rats. Injection into the cerebellar cortex resulted in transduction of Purkinje cells near the needle tract and injection into the ventricles yielded no transduced neurons. In contrast, high transduction efficiency was achieved by vector injection into the inferior olive, resulting in one of three positive Purkinje cells all over the ipsilateral and contralateral cerebellar hemispheres. Because neurons in the deep cerebellar nuclei are also transduced, we suggest that the vector is delivered from the inferior olive to the cerebellar nuclei and then to Purkinje cells by retrograde axonal transport. Expression of the lacZ gene within Purkinje cells was surprisingly persistent and was maintained at the same level for at least 40 days. Importantly, no signs of either toxicity or inflammation were observed in the cerebellum after vector injection, except for the borders of the needle tract where some reactive astrocytes were detected. Indeed, motor coordination of treated animals was entirely normal, as assessed by the rota-rod test. These results demonstrate that HSV-1 amplicon vectors can effect safe and stable transgene expression in Purkinje cells in vivo, raising the possibility of using these vectors for long-term gene therapy of human cerebellar disorders.

General strategy for constructing large HSV-1 plasmid vectors that co-express multiple genes

Herpes simplex virus type 1 (HSV-1) plasmid vectors have a number of attractive features for gene transfer into neurons. In particular, the large size of the HSV-1 genome suggests that HSV-1 vectors might be designed to co-express multiple genes. Here, we report a general strategy for constructing large HSV-1 plasmid vectors that co-express multiple genes. Each transcription unit is linked to an antibiotic resistance gene, and genetic selections are used to assemble large vectors. Using this strategy, we constructed large (26 or 31 kb) HSV-1 vectors that contain two transcription units and two or three genes. These vectors were efficiently packaged into HSV-1 particles using a helper virus-free packaging system. The resulting vector stocks supported the expression of two or three genes in both cultured cells and the rat brain. Potential applications of HSV-1 vectors that co-express multiple genes are discussed.

Enhanced reporter gene expression in the rat brain from helper virus-free HSV-1 vectors packaged in the presence of specific mutated HSV-1 proteins that affect the virion

Herpes simplex virus (HSV-1) gene expression is hypothesized to shut off promoters in HSV-1 vectors, but in a helper virus-free HSV-1 vector system, a number of promoters support only short-term expression. Thus, recombinant gene expression remains short-term in the absence of approximately 99% of the HSV-1 genome. To resolve this paradox, we hypothesized that specific HSV-1 proteins that affect the virion can shut off recombinant gene expression. This study evaluated expression from HSV-1 vectors, containing neuronal-specific promoters, that were packaged in the presence of specific mutated HSV-1 proteins that affect the virion. The mutated HSV-1 proteins that were examined included two protein kinases (U(L)13 and U(S)3), the virion host shut-off factor (vhs), the transactivator of immediate early promoters (VP16), and a virion protein that affects RNA metabolism (U(S)11). Helper virus-free packaging could occur in the presence of each mutated protein alone or specific combinations of two or three mutated proteins. In BHK and PC12 cells, vectors packaged in the presence of each mutated protein increased ( approximately 2-fold) the level of expression per cell, and vectors packaged in the presence of specific combinations of mutated proteins supported larger (4-7-fold) increases. In the rat striatum, vectors packaged in the presence of a mutated U(S)3 displayed enhanced gene transfer (13-18-fold increases in the number of cells at 4 days), and vectors packaged in the presence of mutated U(L)13 or VP16 enhanced long-term expression (2 months). Vectors packaged in the presence of mutated vhs or U(S)11 displayed minimal changes in expression.