Cell-type-specific gene delivery into neuronal cells in vitro and in vivo

Generation of retroviral particles for the spleen necrosis virus (SNV)-based vector system and their use in transduction of various cell types

Genetically engineered retroviruses are widely used for gene delivery into human cells. A number of investigators have studied spleen necrosis virus (SNV) as a vehicle for gene delivery. Vectors developed from SNV and its closely associated avian reticuloendotheliosis virus strain A (REV-A) can be used for gene transfer into a variety of cells, including primary hematopoietic cells and human brain and post-mitotic neuronal cells that are difficult to transduce with other vector systems. SNV-based vector systems have the advantage of being quite safe, because wild-type SNV is unable to infect human cells and has less preference for integration into transcriptionally active sites or genes. However, the generation of retroviral vectors requires cotransfection of more than one plasmid into a packaging cell line, which is a tedious process. The development of stable packaging cell lines expressing envelope (Env) proteins and the structural proteins Gag-Pol will enhance mass production of retroviral vectors for future gene therapy experiments both in vitro and in vivo. This protocol describes the generation of retroviral particles for the SNV-based vector system. These particles can then be used for transduction of various cell types; as an example, a technique for transduction of post-mitotic neurons is also presented.

The cell biology of rabies virus: using stealth to reach the brain

Rabies virus, the prototypical neurotropic virus, causes one of the most lethal zoonotic diseases. According to official estimates, over 55,000 people die of the disease annually, but this is probably a severe underestimation. A combination of virulence factors enables the virus to enter neurons at peripheral sites and travel through the spinal cord to the brain of the infected host, where it often induces aggression that facilitates the transfer of the virus to a new host. This Review summarizes the current knowledge of the replication cycle of rabies virus and virus- host cell interactions, both of which are fundamental elements in our quest to understand the life cycle of rabies virus and the pathogenesis of rabies.

Combined effects of hyperglycemic conditions and HIV-1 Nef: a potential model for induced HIV neuropathogenesis

Hyperglycemic conditions associated with diabetes mellitus (DM) or with the use of antiretroviral therapy may increase the risk of central nervous system (CNS) disorders in HIV-1 infected patients. In support of this hypothesis, we investigated the combined effects of hyperglycemic conditions and HIV-1 accessory protein Nef on the CNS using both in vitro and in vivo models. Astrocytes, the most abundant glial cell type required for normal synaptic transmission and other functions were selected for our in vitro study. The results show that in vitro hyperglycemic conditions enhance the expression of proinflammatory cytokines including caspase-3, complement factor 3 (C3), and the production of total nitrate and 8-iso-PGF2 α as reactive oxygen species (ROS) in human astrocytes leading to cell death in a dose-dependent manner. Delivery of purified recombinant HIV-1 Nef protein, or Nef expressed via HIV-1-based vectors in astrocytes showed similar results. The expression of Nef protein delivered via HIV-1 vectors in combination with hyperglycemia further augmented the production of ROS, C3, activation of caspase-3, modulation of filamentous protein (F-protein), depolarization of the mitochondria, and loss of astrocytes. To further verify the effects of hyperglycemia and HIV-1 Nef protein on CNS individually or in combination, in vivo studies were performed in streptozotocin (STZ) induced diabetic mice, by injecting HIV-1 Nef expressing viral particles into the sub-cortical region of the brain. Our in vivo results were similar to in vitro findings indicating an enhanced production of caspases-3, ROS (lipid oxidation and total nitrate), and C3 in the brain tissues of these animals. Interestingly, the delivery of HIV-1 Nef protein alone caused similar damage to CNS as augmented by hyperglycemia conditions. Taken together, the data suggests that HIV-1 infected individuals with hyperglycemia could potentially be at a higher risk of developing CNS related complications.

Lyssaviruses: current trends

Various technological developments have revitalized the approaches employed to study the disease of rabies. In particular, reverse genetics has facilitated the generation of novel viruses used to improve our understanding of the fundamental aspects of rabies virus (RABV) biology and pathogenicity and yielded novel constructs potentially useful as vaccines against rabies and other diseases. Other techniques such as high throughput methods to examine the impact of rabies virus infection on host cell gene expression and two hybrid systems to explore detailed protein-protein interactions also contribute substantially to our understanding of virus-host interactions. This review summarizes much of the increased knowledge about rabies that has resulted from such studies but acknowledges that this is still insufficient to allow rational attempts at curing those who present with clinical disease.

HIV-1 Vpr potently induces programmed cell death in the CNS in vivo

The human immunodeficiency virus type I (HIV-1) accessory protein Vpr has been associated with the induction of programmed cell death (apoptosis) and cell-cycle arrest. Studies have shown the apoptotic effect of Vpr on primary and established cell lines and on diverse tissues including the central nervous system (CNS) in vitro. However, the relevance of the effect of Vpr observed in vitro to HIV-1 neuropathogenesis in vivo, remains unknown. Due to the narrow host range of HIV-1 infection, no animal model is currently available. This has prompted us to consider a small animal model to evaluate the effects of Vpr on CNS in vivo through surrogate viruses expressing HIV-1Vpr. A single round of replication competent viral vectors, expressing Vpr, were used to investigate the apoptosis-inducing capabilities of HIV-1Vpr in vivo. Viral particles pseudotyped with VSV-G or N2c envelopes were generated from spleen necrosis virus (SNV) and HIV-1-based vectors to transduce CNS cells. The in vitro studies have demonstrated that Vpr generated by SNV vectors had less apoptotic effects on CNS cells compared with Vpr expressed by HIV-1 vectors. The in vivo study has suggested that viral particles, expressing Vpr generated by HIV-1-based vectors, when delivered through the ventricle, caused loss of neurons and dendritic processes in the cortical region. The apoptotic effect was extended beyond the cortical region and affected the hippocampus neurons, the lining of the choroids plexus, and the cerebellum. However, the effect of Vpr, when delivered through the cortex, showed neuronal damage only around the site of injection. Interestingly, the number of apoptotic neurons were significantly higher with HIV-1 vectors expressing Vpr than by the SNV vectors. This may be due to the differences in the proteins expressed by these viral vectors. These results suggest that Vpr induces apoptosis in CNS cells in vitro and in vivo. To our knowledge, this is the first study to investigate the apoptosis-inducing capabilities of HIV-1Vpr in vivo in neonatal mice. We propose that this, in expensive animal model, may be of value to design-targeted neuroprotective therapeutics.

High-throughput, library-based selection of a murine leukemia virus variant to infect nondividing cells

Gammaretroviruses, such as murine leukemia virus (MLV), are functionally distinguished from lentiviruses, such as human immunodeficiency virus, by their inability to infect nondividing cells. Attempts to engineer this property into MLV have been hindered by an incomplete understanding of early events in the viral life cycle. We utilized a transposon-based method to generate saturated peptide insertion libraries of MLV gag-pol variants with nuclear localization signals randomly incorporated throughout these overlapping genes. High-throughput selection of the libraries via iterative retroviral infection of nondividing cells led to the identification of a novel variant that successfully transduced growth-arrested cells. Vector packaging by cotransfection of the gag-pol.NLS variant with wild-type gag-pol produced high-titer virions capable of infecting neurons in vitro and in vivo. The capacity of mutant virions to transduce nondividing cells could help to elucidate incompletely understood mechanisms of the viral life cycle and greatly broaden the gene therapy applications of retroviral vectors. Furthermore, the ability to engineer key intracellular viral infection steps has potential implications for the understanding, design, and control of other post-entry events. Finally, this method of library generation and selection for a desired phenotype directly in a mammalian system can be readily expanded to address other challenges in protein engineering.

Transduction of motor neurons and muscle fibers by intramuscular injection of HIV-1-based vectors pseudotyped with select rabies virus glycoproteins

For studies of motor neuron function or for therapeutic purposes, novel pseudotype HIV-1-based vectors were developed that are capable of expressing transgenes in motor neurons following injection into mouse hind limb muscles. To specifically target motor neurons, glycoproteins from two rabies virus (RV) isolates, the mouse-brain adapted challenge virus 24 (CVS-24) variants, CVS-N2c and CVS-B2c were evaluated for pseudotype formation with an HIV-1-based vector. Both RV glycoproteins incorporated into vector envelopes, and both pseudotypes yielded high titers with Hek293T and cortical plate neuron cultures. Increased neuronotropism by the CVS-N2c pseudotype was not observed, suggesting that vector tropism is not solely determined by the fusogenic viral glycoprotein. Vector injection into hind limb muscles resulted in EYFP reporter gene expression in the injected muscle fibers and in spinal cord motor neurons innervating the same muscle, indicating retrograde vector transport. Intramuscular vector injections into the soleus and tibialis anterior muscles transduced 26% and 16% of all motor neurons in each motor nucleus, respectively. These transduction efficiencies may allow novel approaches to functional studies of the motor system and the treatment of neuromuscular disease.

A nuclear localization signal in the matrix of spleen necrosis virus (SNV) does not allow efficient gene transfer into quiescent cells with SNV-derived vectors

A major limitation in gene therapy for vectors derived from Moloney murine leukemia virus (MLV) is that they only deliver genes into dividing cells. In this study, a careful comparison of spleen necrosis virus (SNV)-derived vectors with MLV and human immunodeficiency virus (HIV)-1 retroviral vectors indicated that SNV vectors can deliver genes 4-fold more efficiently than MLV vectors into aphidicolin-arrested cells, although at a 25-fold lower efficiency than HIV-1-derived vectors. Furthermore, the addition of a NLS in the SNV matrix (MA) that mimics the one located in HIV-1 MA did not increase the ability of SNV vectors to transfer genes into arrested cells. Also, we found that the RD114 envelope was able to pseudotype SNV viral particles in a very efficient manner.

Human Immunodeficiency virus type 1 Nef potently induces apoptosis in primary human brain microvascular endothelial cells via the activation of caspases

The lentiviral protein Nef plays a major role in the pathogenesis of human immunodeficiency virus type I (HIV-1) infection. Although the exact mechanisms of its actions are not fully understood, Nef has been shown to be essential for the maintenance of high-titer viral replication and disease pathogenesis in in vivo models of simian immunodeficiency virus infection of monkeys. Nef has also been suggested to play a pivotal role in the depletion of T cells by promoting apoptosis in bystander cells. In this context, we investigated the ability of extracellular and endogenously expressed HIV-1 Nef to induce apoptosis in primary human brain microvascular endothelial cells (MVECs). Human brain MVECs were exposed to baculovirus-expressed HIV-1 Nef protein, an HIV-1-based vector expressing Nef, spleen necrosis virus (SNV)-Nef virus (i.e., SNV vector expressing HIV-1 Nef as a transgene), and the HIV-1 strain ADA and its Nef deletion mutant, ADADeltaNef. We observed that ADA Nef, the HIV-1 vector expressing Nef, and SNV-Nef were able to induce apoptosis in a dose-dependent manner. The mutant virus with a deletion in Nef was able to induce apoptosis in MVECs to modest levels, but the effects were not as pronounced as with the wild-type HIV-1 strain, ADA, the HIV-1-based vector expressing Nef, or SNV-Nef viruses. We also demonstrated that relatively high concentrations of exogenous HIV-1 Nef protein were able to induce apoptosis in MVECs. Gene microarray analyses showed increases in the expression of several specific proapoptotic genes. Western blot analyses revealed that the various caspases involved with Nef-induced apoptosis are processed into cleavage products, which occur only during programmed cell death. The results of this study demonstrate that Nef likely contributes to the neuroinvasion and neuropathogenesis of HIV-1, through its effects on select cellular processes, including various apoptotic cascades.

Spleen necrosis virus-based vector delivery of anti-HIV-1 genes potently protects human hematopoietic cells from HIV-1 infection

In this study, we report on the efficacy of using a spleen necrosis virus (SNV)-based vector delivery system to block human immunodeficiency virus type I (HIV-1) replication in human hematopoietic cells. These efforts were directed towards the development of human immune system cell resistance to HIV-1 infection, based on the strategy of "intracellular immunization" via generation of a series of anti-HIV-1 therapeutic constructs carrying scFvs, single-chain variable fragments, against HIV-1 integrase and reverse transcriptase in combination with the trans-dominant mutant of HIV-1 Rev, RevM10. The efficiency of the anti-HIV-1 constructs were tested in viral challenge assays with different doses of HIV-1 NL4-3, Bal, 89.6 and R7-GFP strains. These experiments demonstrated the reduction of HIV-1 replication by these retroviral vector constructs in a range of 4- to 10-fold in CD4+ T-lymphocytes, human peripheral blood mononuclear cells (PBMCs), and primary human macrophages. We observed selective efficiency of SNV-based therapeutics in H9, C8166 and Jurkat T-lymphocytic cell lines, demonstrating the most efficient inhibition of HIV-1 replication in Jurkat T-cells. Thus, these data are the first demonstration of the ability of SNV-based retroviral vectors with select transgenes, which may have certain molecular advantages over other retroviral vector systems, to combat HIV-1 replication in human hematopoietic cells and support the potential for using SNV-expressed constructs in anti-HIV-1 molecular therapeutics.

Spliced spleen necrosis virus vector RNA is not encapsidated: implications for retroviral replication and vector design

RNA splicing is a complex event in the retroviral life cycle and can involve multiple steps, as well as cis-acting sequences, to maintain a proper balance of spliced and unspliced viral RNA for translation and encapsidation. The retroviral RNA can be processed by cellular machinery and enables the removal of intronic sequences. We aimed to utilize the removal of a synthetic intron for targeted gene expression. To analyze intron removal and gene expression, we have constructed a novel self-inactivating gene-activating (SIGA) vector for potential universal gene therapy. New vectors for gene therapy are necessary for safe and effective gene delivery in humans. The SIGA vector is derived from spleen necrosis virus (SNV), which is an avian reticuloendotheliosis virus. The vector was designed so that expression of a therapeutic gene is blocked in helper cell lines due to an intervening sequence containing various blocks in transcription and translation. However, after one round of retroviral replication, the intervening sequence should be removed by the cellular machinery and the therapeutic gene will be selectively expressed in target cells. Our studies show that the intervening sequence in SIGA vector RNA is partially spliced. However, spliced vector RNA was not transduced to target cells. Previous studies showed that an infectious SNV vector enabled transduction of spliced RNA. However, yet-undefined differences in infectious and replication-deficient retroviral replication may have an effect on the transduction of spliced RNA. The results of this study present key information on spliced RNA and its encapsidation, as well as data for the construction of a new generation of SNV-derived retroviral vectors.

Cross-packaging of human immunodeficiency virus type 1 vector RNA by spleen necrosis virus proteins: construction of a new generation of spleen necrosis virus-derived retroviral vectors

The ability of the nonlentiviral retrovirus spleen necrosis virus (SNV) to cross-package the genomic RNA of the distantly related human immunodeficiency virus type 1 (HIV-1) and vice versa was analyzed. Such a model may allow us to further study HIV-1 replication and pathogenesis, as well as to develop safe gene therapy vectors. Our results suggest that SNV can cross-package HIV-1 genomic RNA but with lower efficiency than HIV-1 proteins. However, HIV-1-specific proteins were unable to cross-package SNV RNA. We also constructed SNV-based gag-pol chimeric variants by replacing the SNV integrase with the HIV-1 integrase, based on multiple sequence alignments and domain analyses. These analyses revealed that there are conserved domains in all retroviral integrase open reading frames (orf), despite the divergence in the primary sequences. The transcomplementation assays suggested that SNV proteins recognized one of the chimeric variants. This demonstrated that HIV-1 integrase is functional in the SNV gag-pol orf with a lower transduction efficiency, utilizing homologous (SNV) RNA, as well as the heterologous vector RNA of HIV-1. These findings suggest that homology in the conserved sequences of the integrase protein may not be fully competent in the replacement of protein(s) from one retrovirus to another, and there are likely several other factors involved in each of the steps related to replication, integration, and infection. However, further studies to dissect the gag-pol region will be critical for understanding the mechanisms involved in the cleavage of reverse transcriptase, RNase H, and integrase. These studies should provide further insight into the design and development of novel molecular approaches to block HIV-1 replication and to construct a new generation of SNV-based vectors.