Improved high-capacity adenoviral vectors for high-level neuron-restricted gene transfer to the CNS

High-Capacity Adenoviral Vectors: Expanding the Scope of Gene Therapy

The adaptation of adenoviruses as gene delivery tools has resulted in the development of high-capacity adenoviral vectors (HC-AdVs), also known, helper-dependent or “gutless”. Compared with earlier generations (E1/E3-deleted vectors), HC-AdVs retain relevant features such as genetic stability, remarkable efficacy of in vivo transduction, and production at high titers. More importantly, the lack of viral coding sequences in the genomes of HC-AdVs extends the cloning capacity up to 37 Kb, and allows long-term episomal persistence of transgenes in non-dividing cells. These properties open a wide repertoire of therapeutic opportunities in the fields of gene supplementation and gene correction, which have been explored at the preclinical level over the past two decades. During this time, production methods have been optimized to obtain the yield, purity, and reliability required for clinical implementation. Better understanding of inflammatory responses and the implementation of methods to control them have increased the safety of these vectors. We will review the most significant achievements that are turning an interesting research tool into a sound vector platform, which could contribute to overcome current limitations in the gene therapy field.

A Single Vector Platform for High-Level Gene Transduction of Central Neurons: Adeno-Associated Virus Vector Equipped with the Tet-Off System

Visualization of neurons is indispensable for the investigation of neuronal circuits in the central nervous system. Virus vectors have been widely used for labeling particular subsets of neurons, and the adeno-associated virus (AAV) vector has gained popularity as a tool for gene transfer. Here, we developed a single AAV vector Tet-Off platform, AAV-SynTetOff, to improve the gene-transduction efficiency, specifically in neurons. The platform is composed of regulator and response elements in a single AAV genome. After infection of Neuro-2a cells with the AAV-SynTetOff vector, the transduction efficiency of green fluorescent protein (GFP) was increased by approximately 2- and 15-fold relative to the conventional AAV vector with the human cytomegalovirus (CMV) or human synapsin I (SYN) promoter, respectively. We then injected the AAV vectors into the mouse neostriatum. GFP expression in the neostriatal neurons infected with the AAV-SynTetOff vector was approximately 40-times higher than that with the CMV or SYN promoter. By adding a membrane-targeting signal to GFP, the axon fibers of neostriatal neurons were clearly visualized. In contrast, by attaching somatodendritic membrane-targeting signals to GFP, axon fiber labeling was mostly suppressed. Furthermore, we prepared the AAV-SynTetOff vector, which simultaneously expressed somatodendritic membrane-targeted GFP and membrane-targeted red fluorescent protein (RFP). After injection of the vector into the neostriatum, the cell bodies and dendrites of neostriatal neurons were labeled with both GFP and RFP, whereas the axons in the projection sites were labeled only with RFP. Finally, we applied this vector to vasoactive intestinal polypeptide-positive (VIP+) neocortical neurons, one of the subclasses of inhibitory neurons in the neocortex, in layer 2/3 of the mouse primary somatosensory cortex. The results revealed the differential distribution of the somatodendritic and axonal structures at the population level. The AAV-SynTetOff vector developed in the present study exhibits strong fluorescence labeling and has promising applications in neuronal imaging.

Systemic gene delivery to the central nervous system using Adeno-associated virus

Adeno-associated virus (AAV)-mediated gene delivery has emerged as an effective and safe tool for both preclinical and clinical studies of neurological disorders. The recent discovery that several serotypes are able to cross the blood–brain barrier when administered systemically has been a real breakthrough in the field of neurodegenerative diseases. Widespread transgene expression after systemic injection could spark interest as a therapeutic approach. Such strategy will avoid invasive brain surgery and allow non-focal gene therapy promising for CNS diseases affecting large portion of the brain. Here, we will review the recent results achieved through different systemic routes of injection generated in the last decade using systemic AAV-mediated delivery and propose a brief assessment of their values. In particular, we emphasize how the methods used for virus engineering could improve brain transduction after peripheral delivery.

Progress in gene therapy for neurological disorders

Diseases of the nervous system have devastating effects and are widely distributed among the population, being especially prevalent in the elderly. These diseases are often caused by inherited genetic mutations that result in abnormal nervous system development, neurodegeneration, or impaired neuronal function. Other causes of neurological diseases include genetic and epigenetic changes induced by environmental insults, injury, disease-related events or inflammatory processes. Standard medical and surgical practice has not proved effective in curing or treating these diseases, and appropriate pharmaceuticals do not exist or are insufficient to slow disease progression. Gene therapy is emerging as a powerful approach with potential to treat and even cure some of the most common diseases of the nervous system. Gene therapy for neurological diseases has been made possible through progress in understanding the underlying disease mechanisms, particularly those involving sensory neurons, and also by improvement of gene vector design, therapeutic gene selection, and methods of delivery. Progress in the field has renewed our optimism for gene therapy as a treatment modality that can be used by neurologists, ophthalmologists and neurosurgeons. In this Review, we describe the promising gene therapy strategies that have the potential to treat patients with neurological diseases and discuss prospects for future development of gene therapy.

Post-injury delivery of rAAV2-CNTF combined with short-term pharmacotherapy is neuroprotective and promotes extensive axonal regeneration after optic nerve trauma

Recombinant adeno-associated viral (rAAV) vectors expressing neurotrophic genes reduce neuronal death and promote axonal regeneration in central nervous system (CNS) injury models. Currently, however, use of rAAV to treat clinical neurotrauma is problematic because there is a delay in the onset of transgene expression. Using the adult rat retina and optic nerve (ON), we have tested whether rAAV gene therapy administered at the time of injury combined with short-term pharmacotherapy has synergistic effects that enhance neuronal survival and regeneration. The ON was transected and a 1.5 cm segment of autologous peripheral nerve (PN) was grafted onto the cut end. At this time, bicistronic rAAV2 encoding ciliary neurotrophic factor (CNTF) and green fluorescent protein (rAAV2-CNTF-GFP) was injected into the injured eye. To provide interim support for axotomized retinal ganglion cells (RGCs) during vector integration and therapeutic transgene expression, rCNTF protein and a cyclic adenosine monophosphate (cAMP) analogue (CPT-cAMP) were injected intravitreally 3 and 10 days postoperatively. For comparison, another rAAV2-CNTF-GFP group received two intravitreal saline injections 3 and 10 days after the PN-ON surgery. A further PN graft group received only postoperative intravitreal injections of rCNTF plus CPT-cAMP. After 4 weeks, regenerating RGCs were retrogradely labelled by applying fluorogold to the distal end of each PN graft. Compared to saline-injected animals, both RGC survival and axonal regrowth were significantly higher in the rCNTF and CPT-cAMP injected rAAV2-CNTF-GFP group; approximately one third of the RGC population survived axotomy, and 27% of these regrew an axon. These values were also higher than those obtained in rats that received only rCNTF plus CPT-cAMP injections. Therefore, we show for the first time that rAAV-mediated gene delivery at the time of, or just after, neurotrauma is most successful when combined with temporary post-injury trophic support, and is potentially a viable treatment strategy for patients after acute CNS injury.

Gene therapy and neurodevelopmental disorders

With a number of recent clinical successes, gene therapy is quickly becoming a realistic treatment option for neurological disorders. Advancements in global central nervous system (CNS) gene delivery, in particular, have accelerated to the point that treatments for neurological disorders such as lysosomal storage diseases seem within reach. Other neurodevelopmental disorders, such as Rett Syndrome, Fragile X, and autism still face significant obstacles to overcome before a viable human gene therapy can be considered. This review focuses on the most common CNS gene delivery vehicle, adeno-associated virus (AAV), and the current state of AAV vector design and delivery for CNS gene therapy. Relevant examples of gene therapy studies for neurodevelopmental disorders, as well as outstanding challenges, are discussed. This article is part of the Special Issue entitled 'Neurodevelopmental Disorders'.

Methods of cell purification: a critical juncture for laboratory research and translational science

Research in cell biology and the development of translational technologies are driven by competition, public expectations, and regulatory oversight, putting these fields at a critical juncture. Success in these fields is quickly becoming dependent on the ability of researchers to identify and isolate specific cell populations from heterogeneous mixtures accurately and efficiently. Many methods for cell purification have been developed, and each has advantages and disadvantages that must be considered in light of the intended application. Current cell separation strategies make use of surface proteins, genetic expression, and physics to isolate specific cells by phenotypic traits. Cell purification is also dependent on the cellular reagents available for use and the intended application, as these factors may preclude certain mechanisms used in the processes of labeling and sorting cells.

pUNISHER: a high-level expression cassette for use with recombinant viral vectors for rapid and long term in vivo neuronal expression in the CNS

Fast onset and high-level neurospecific transgene expression in vivo is of importance for many areas in neuroscience, from basic to translational, and can significantly reduce the amount of vector load required to maintain transgene expression in vivo. In this study, we tested various cis elements to optimize transgene expression at transcriptional, posttranscriptional, and posttranslational levels and combined them together to create the high-level neuronal transgene expression cassette pUNISHER. Using a second-generation adenoviral vector system in combination with the pUNISHER cassette, we characterized its rate of onset of detectable expression and levels of expression compared with a neurospecific expression cassette driven by the 470-bp human synapsin promoter in vitro and in vivo. Our results demonstrate in primary neurons that the pUNISHER cassette, in a recombinant adenovirus type 5 background, led to a faster rate of onset of detectable transgene expression and higher level of transgene expression. More importantly, this cassette led to highly correlated neuronal expression in vivo and to stable transgene expression up to 30 days in the auditory brain stem with no toxicity on the characteristics of synaptic transmission and plasticity at the calyx of Held synapse. Thus the pUNISHER cassette is an ideal high-level neuronal expression cassette for use in vivo for neuroscience applications.

Herpes simplex virus type 1 thymidine kinase sequence fused to the lacz gene increases levels of {beta}-galactosidase activity per genome of high-capacity but not first-generation adenoviral vectors in vitro and in vivo

Increased transgene expression per vector genome is an important goal in the optimization of viral vectors for gene therapy. Herein we demonstrate that herpes simplex virus type 1 (HSV1) thymidine kinase (TK) gene sequences (1,131 bp) fused to the 3' end of lacZ increase transgene expression from high-capacity adenoviral vectors (HCAd), but not from first-generation (Ad) vectors. The woodchuck hepatitis virus posttranscriptional regulatory element (WPRE), in contrast, increased transgene expression levels from Ad but not HCAd vectors. The differential activity of the HSV1 TK gene and WPRE sequences was detected both in vitro and in vivo and suggests potentially different mechanisms of action or the interaction of these elements with vector genomic sequences.

A novel adenoviral vector-mediated neuronal selective gene expression in neonatal mouse brain in response to hypoxia

Selective gene expression targeting neurons is a challenge, which, if successfully overcome, carries an enormous potential for clinical applications in therapeutics against neurodegenerative diseases. We have reported previously the construction of a series of adenoviral vectors capable of selectively expressing a reporter gene luciferase in cultured neurons [D. Huang, A. Desbois, S.T. Hou, A novel adenoviral vector which mediates hypoxia-inducible gene expression selectively in neurons, Gene Ther. 12 (2005) 1369-1376]. A combination of neuron restrictive silencer elements (NRSEs), hypoxia responsive elements (HREs) and CMV minimal promoter (CMVmp) was packaged into replication defective adenovirus to target gene expression selectively in neurons in a hypoxia-regulated manner. In the present study, we injected the adenoviral vectors into the neonatal mouse brain followed by treatment with hypoxia. The expression of the reporter luciferase gene was examined by luciferase assay and fluorescent immunostaining. Neurons and glial cells were identified by staining with antibodies against NeuN and GFAP, respectively. Remarkably, in response to hypoxia, Ad/5HRE-3NRSE showed strong hypoxia-inducible gene expression of the reporter luciferase selectively in neurons but not in glial cells. In contrast, brains infected with the control vector Ad/5HRE showed no selectivity in luciferase expression (in both neurons and glial cells) under the hypoxic condition. Taken together, these studies demonstrated that this vector (Ad/5HRE-3NRSE) can mediate gene expression selectively in neurons both in vitro and in vivo, supporting the suggestion that further refinement of this vector may lead to the development of a useful tool to investigate mechanisms of neuronal damage following cerebral ischemia and a possible effective gene therapy vector to stroke.

A primary culture technique of adult retina for regeneration studies on adult CNS neurons

This protocol details a tissue culture technique that allows for quantified regeneration studies on adult retinal ganglion cells (RGCs), that is, CNS neurons. The method may also allow for elucidation of molecular cues, for example of signals relevant in neuronal survival and axon regeneration. The procedure relies on fractioned stripe culture of previously injured retina in defined culture media. Naive dendritic cell contacts of RGCs are preserved, and the system is independent of growth factors. In contrast to other techniques, the protocol is based on tissue grown from adult animals; it dispenses immature co-cultures and evaluates the outgrowth of unmyelinated neurites in a milieu lacking CNS myelin. The technique is suitable for rodent retina from mouse or rat. A growth-conditioning injury of the optic nerve is set 10 days before retinal explantation. Explants are cultured for 5-7 days. Mere preparation of a single retina should be completed within 20 min.