Herpes simplex virus RNAi and neprilysin gene transfer vectors reduce accumulation of Alzheimer's disease-related amyloid-β peptide in vivo

Exploring the Potential of Cytomegalovirus-Based Vectors: A Review

Viral vectors have emerged as powerful tools for delivering and expressing foreign genes, playing a pivotal role in gene therapy. Among these vectors, cytomegalovirus (CMV) stands out as a promising viral vector due to its distinctive attributes including large packaging capacity, ability to achieve superinfection, broad host range, capacity to induce CD8+ T cell responses, lack of integration into the host genome, and other qualities that make it an appealing vector candidate. Engineered attenuated CMV strains such as Towne and AD169 that have a ~15 kb genomic DNA deletion caused by virus passage guarantee human safety. CMV's large genome enables the efficient incorporation of substantial foreign genes as demonstrated by CMV vector-based therapies for SIV, tuberculosis, cancer, malaria, aging, COVID-19, and more. CMV is capable of reinfecting hosts regardless of prior infection or immunity, making it highly suitable for multiple vector administrations. In addition to its broad cellular tropism and sustained high-level gene expression, CMV triggers robust, virus-specific CD8+ T cell responses, offering a significant advantage as a vaccine vector. To date, successful development and testing of murine CMV (MCMV) and rhesus CMV (RhCMV) vectors in animal models have demonstrated the efficacy of CMV-based vectors. These investigations have explored the potential of CMV vectors for vaccines against HIV, cancer, tuberculosis, malaria, and other infectious pathogens, as well as for other gene therapy applications. Moreover, the generation of single-cycle replication CMV vectors, produced by deleting essential genes, ensures robust safety in an immunocompromised population. The results of these studies emphasize CMV's effectiveness as a gene delivery vehicle and shed light on the future applications of a CMV vector. While challenges such as production complexities and storage limitations need to be addressed, ongoing efforts to bridge the gap between animal models and human translation continue to fuel the optimism surrounding CMV-based vectors. This review will outline the properties of CMV vectors and discuss their future applications as well as possible limitations.

Advances in Amyloid-β Clearance in the Brain and Periphery: Implications for Neurodegenerative Diseases

This review examines the role of impaired amyloid-β clearance in the accumulation of amyloid-β in the brain and the periphery, which is closely associated with Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA). The molecular mechanism underlying amyloid-β accumulation is largely unknown, but recent evidence suggests that impaired amyloid-β clearance plays a critical role in its accumulation. The review provides an overview of recent research and proposes strategies for efficient amyloid-β clearance in both the brain and periphery. The clearance of amyloid-β can occur through enzymatic or non-enzymatic pathways in the brain, including neuronal and glial cells, blood-brain barrier, interstitial fluid bulk flow, perivascular drainage, and cerebrospinal fluid absorption-mediated pathways. In the periphery, various mechanisms, including peripheral organs, immunomodulation/immune cells, enzymes, amyloid-β-binding proteins, and amyloid-β-binding cells, are involved in amyloid-β clearance. Although recent findings have shed light on amyloid-β clearance in both regions, opportunities remain in areas where limited data is available. Therefore, future strategies that enhance amyloid-β clearance in the brain and/or periphery, either through central or peripheral clearance approaches or in combination, are highly encouraged. These strategies will provide new insight into the disease pathogenesis at the molecular level and explore new targets for inhibiting amyloid-β deposition, which is central to the pathogenesis of sporadic AD (amyloid-β in parenchyma) and CAA (amyloid-β in blood vessels).

Genetic Therapies for Alzheimer's Disease: A Scoping Review

Effective, disease modifying therapies for Alzheimer's disease (AD) remain a quandary, following a panoply of expensive failures in human clinical trials. Given the stagnation in therapeutics, alternative approaches are needed. Recent successes of genetic therapies in other neurodegenerative diseases may highlight the way forward. This scoping review explores suggested targets of genetic therapy in AD, with a focus on vector-based approaches in pre-clinical and clinical trials. Putative targets of genetic therapies tested in pre-clinical trials include amyloid pathway intermediates and enzymes modulation, tau protein downregulation, APOE4 downregulation and APOE2 upregulation, neurotrophin expression (nerve growth factor (NGF) and brain-derived neurotrophic factor), and inflammatory cytokine alteration, among several other approaches. There have been three completed human clinical trials for genetic therapy in AD patients, all of which upregulated NGF in AD patients, showing some mixed evidence of benefit. Several impediments remain to be surpassed before genetic therapies can be successfully applied to AD, including the challenge of delivering monogenic genetic therapies for complex polygenic disorders, risks in the dominant delivery method (intracranial injection), stability of genetic therapies in vivo, poor translatability of pre-clinical AD models, and the expense of genetic therapy production. Genetic therapies represent an exciting opportunity within the world of AD therapeutics, but clinical applications likely remain a long term, rather than short term, possibility.

Biomaterial-based delivery of nucleic acids for tissue regeneration

Gene therapy is a promising novel method of tissue regeneration by stimulating or inhibiting key signaling pathways. However, their therapeutic applications in vivo are largely limited by several physiological obstacles, such as degradation of nucleases, impermeability of cell membranes, and transport to the desired intracellular compartments. Biomaterial-based gene delivery systems can overcome the problems of stability and local drug delivery, and can temporarily control the overexpression of therapeutic genes, leading to the local production of physiologically relevant levels of regulatory factors. But the gene delivery of biomaterials for tissue regeneration relies on multi-factor design. This review aims to outline the impact of gene delivery methods, therapeutic genes and biomaterials selection on this strategy, emphatically introduce the latest developments in the design of gene delivery vehicles based on biomaterials, summarize the mechanism of nucleic acid for tissue regeneration, and explore the strategies of nucleic acid delivery vehicles for various tissue regeneration.

Targeted Delivery for Neurodegenerative Disorders Using Gene Therapy Vectors: Gene Next Therapeutic Goals

The technique of gene therapy, ever since its advent nearly fifty years ago, has been utilized by scientists as a potential treatment option for various disorders. This review discusses some of the major neurodegenerative diseases (NDDs) like Alzheimer's disease (AD), Parkinson's Disease (PD), Motor neuron diseases (MND), Spinal Muscular Atrophy (SMA), Huntington's Disease (HD), Multiple Sclerosis (MS), etc. and their underlying genetic mechanisms along with the role that gene therapy can play in combating them. The pathogenesis and the molecular mechanisms specifying the altered gene expression of each of these NDDs have also been discussed in elaboration. The use of gene therapy vectors can prove to be an effective tool in the field of curative modern medicine for the generations to come. Therefore, consistent efforts and progressive research towards its implementation can provide us with powerful treatment options for disease conditions that have so far been considered as incurable.

Gene therapy-mediated enhancement of protective protein expression for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is the leading form of dementia but lacks curative treatments. Current understanding of AD aetiology attributes the development of the disease to the misfolding of two proteins; amyloid-β (Aβ) and hyperphosphorylated tau, with their pathological accumulation leading to concomitant oxidative stress, neuroinflammation, and neuronal death. These processes are regulated at multiple levels to maintain homeostasis and avert disease. However, many of the relevant regulatory proteins appear to be downregulated in the AD-afflicted brain. Enhancement/restoration of these 'protective' proteins, therefore, represents an attractive therapeutic avenue. Gene therapy is a desirable means of achieving this because it is not associated with the side-effects linked to systemic protein administration, and sustained protein expression virtually eliminates compliance issues. The current article represents a focused and succinct review of the better established 'protective' protein targets for gene therapy enhancement/restoration rather than being designed as an exhaustive review incorporating less validated protein subjects. In addition, we will discuss how the risks associated with uncontrolled or irreversible gene expression might be mitigated through combining neuronal-specific promoters, inducible expression systems and localised injections. Whilst many of the gene therapy targets reviewed herein are yet to enter clinical trials, preclinical testing has thus far demonstrated encouraging potential for the gene therapy-based treatment of AD.

Gene Therapy, A Novel Therapeutic Tool for Neurological Disorders: Current Progress, Challenges and Future Prospective

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Neurological disorders are one of the major threat for health care system as they put enormous socioeconomic burden. All aged populations are susceptible to one or other neurological problems with symptoms of neuroinflammation, neurodegeneration and cognitive dysfunction. At present, available pharmacotherapeutics are insufficient to treat these diseased conditions and in most cases, they provide only palliative effect. It was also found that the molecular etiology of neurological disorders is directly linked with the alteration in genetic makeup, which can be inherited or triggered by the injury, environmental toxins and by some existing disease. Therefore, to take care of this situation, gene therapy has emerged as an advanced modality that claims to permanently cure the disease by deletion, silencing or edition of faulty genes and by insertion of healthier genes. In this modality, vectors (viral and non-viral) are used to deliver targeted gene into a specific region of the brain via various routes. At present, gene therapy has shown positive outcomes in complex neurological disorders, such as Parkinson's disease, Alzheimer's disease, Huntington disease, Multiple sclerosis, Amyotrophic lateral sclerosis and in lysosomal storage disease. However, there are some limitations such as immunogenic reactions non-specificity of viral vectors and a lack of effective biomarkers to understand the efficacy of therapy. Considerable progress has been made to improve vector design, gene selection and targeted delivery. This review article deals with the current status of gene therapy in neurological disorders along with its clinical relevance, challenges and future prospective.

Molecular Pathogenesis and Interventional Strategies for Alzheimer's Disease: Promises and Pitfalls

Alzheimer's disease (AD) is a debilitating disorder characterized by age-related dementia, which has no effective treatment to date. β-Amyloid depositions and hyperphosphorylated tau proteins are the main pathological hallmarks, along with oxidative stress, N-methyl-d-aspartate (NMDA) receptor-mediated excitotoxicity, and low levels of acetylcholine. Current pharmacotherapy for AD only provides symptomatic relief and limited improvement in cognitive functions. Many molecules have been explored that show promising outcomes in AD therapy and can regulate cellular survival through different pathways. To have a vivid approach to strategize the treatment regimen, AD physiopathology should be better explained considering diverse etiological factors in conjunction with biochemical disturbances. This Review attempts to discuss different disease modification approaches and address the novel therapeutic targets of AD that might pave the way for new drug discovery using the well-defined targets for therapy of the disease.

Neutral Endopeptidase (Neprilysin) in Therapy and Diagnostics: Yin and Yang

Neprilysin (NEP) is a zinc-dependent metalloproteinase that exists in organisms in both transmembrane and soluble forms. NEP substrates are involved in regulating the cardiovascular and nervous systems. In this review, we discuss some of the biochemical characteristics and physiological functions of this enzyme with special emphasis on the use of NEP as a therapeutic target. The history and various physiological aspects of applying NEP inhibitors for treating heart failure and attempts to increase NEP activity when treating Alzheimer's disease using gene and cell therapies are described. Another important issue discussed is the role of NEP as a potential marker for predicting the risk of cardiovascular disease complications. The diagnostic and prognostic performance of soluble NEP in various types of heart failure is analyzed and presented. We also discuss the methods and approaches for measuring NEP activity for prognosis and diagnosis, as well as a possible new role of natriuretic peptides (NEP substrates) in cardiovascular diagnostics.

Localised non-viral delivery of nucleic acids for nerve regeneration in injured nervous systems

Axons damaged by traumatic injuries are often unable to spontaneously regenerate in the adult central nervous system (CNS). Although the peripheral nervous system (PNS) has some regenerative capacity, its ability to regrow remains limited across large lesion gaps due to scar tissue formation. Nucleic acid therapy holds the potential of improving regeneration by enhancing the intrinsic growth ability of neurons and overcoming the inhibitory environment that prevents neurite outgrowth. Nucleic acids modulate gene expression by over-expression of neuronal growth factor or silencing growth-inhibitory molecules. Although in vitro outcomes appear promising, the lack of efficient non-viral nucleic acid delivery methods to the nervous system has limited the application of nucleic acid therapeutics to patients. Here, we review the recent development of efficient non-viral nucleic acid delivery platforms, as applied to the nervous system, including the transfection vectors and carriers used, as well as matrices and scaffolds that are currently used. Additionally, we will discuss possible improvements for localised nucleic acid delivery.

Stem cell- and gene-based therapies as potential candidates in Alzheimer's therapy

Alzheimer's disease (AD) is a progressive neurodegenerative disorder, which is associated with impairments of memory, thinking, language, and reasoning. Despite extensive research aiming at the treatment of AD, durable and complete remissions are rare. Hence, new therapeutic approaches are required. Among various therapeutic approaches, stem cells (ie, neural stem cells, mesenchymal stem cells, and embryonic stem cells) and delivery of protective genes such as encoding nerve growth factor, APOE, and glial cell-derived neurotrophic factor have generated promise in AD therapy. Here, we summarized a variety of effective therapeutic approaches (ie, stem cells, and genes) in AD therapy.

Targeting Alzheimer's disease with gene and cell therapies

Alzheimer's disease (AD) causes dementia in both young and old people affecting more than 40 million people worldwide. The two neuropathological hallmarks of the disease, amyloid beta (Aβ) plaques and neurofibrillary tangles consisting of protein tau are considered the major contributors to the disease. However, a more complete picture reveals significant neurodegeneration and decreased cell survival, neuroinflammation, changes in protein and energy homeostasis and alterations in lipid and cholesterol metabolism. In addition, gene and cell therapies for severe neurodegenerative disorders have recently improved technically in terms of safety and efficiency and have translated to the clinic showing encouraging results. Here, we review broadly current data within the field for potential targets that could modify AD through gene and cell therapy strategies. We envision that not only Aβ will be targeted in a disease-modifying treatment strategy but rather that a combination of treatments, possibly at different intervention times may prove beneficial in curing this devastating disease. These include decreased tau pathology, neuronal growth factors to support neurons and modulation of neuroinflammation for an appropriate immune response. Furthermore, cell based therapies may represent potential strategies in the future.

Degradation of amyloid β peptide by neprilysin expressed from Borna disease virus vector

Accumulation of amyloid β (Aβ40 and Aβ42) in the brain is a characteristic of Alzheimer's disease (AD). Because neprilysin (NEP) is a major Aβ-degrading enzyme, NEP delivery in the brain is a promising gene therapy for AD. Borna disease virus (BoDV) vector enables long-term transduction of foreign genes in the central nerve system. Here, we evaluated the proteolytic ability of NEP transduced by the BoDV vector and found that the amounts of Aβ40 and Aβ42 significantly decreased, which suggests that NEP expressed from the BoDV vector is functional to degrade Aβ.

Use of Adeno-Associated and Herpes Simplex Viral Vectors for In Vivo Neuronal Expression in Mice

Adeno-associated viruses and the herpes simplex virus are the two most widely used vectors for the in vivo expression of exogenous genes. Advances in the development of these vectors have enabled remarkable temporal and spatial control of gene expression. This unit provides methods for storing, delivering, and verifying expression of adeno-associated and herpes simplex viruses in the adult mouse brain. It also describes important considerations for experiments using in vivo expression of these viral vectors, including serotype and promoter selection, as well as timing of expression. Additional protocols are provided that describe methods for preliminary experiments to determine the appropriate conditions for in vivo delivery.

RNAi and miRNA in viral infections and cancers

Since the first report of RNA interference (RNAi) less than a decade ago, this type of molecular intervention has been introduced to repress gene expression in vitro and also for in vivo studies in mammals. Understanding the mechanisms of action of synthetic small interfering RNAs (siRNAs) underlies use as therapeutic agents in the areas of cancer and viral infection. Recent studies have also promoted different theories about cell-specific targeting of siRNAs. Design and delivery strategies for successful treatment of human diseases are becomingmore established and relationships between miRNA and RNAi pathways have been revealed as virus-host cell interactions. Although both are well conserved in plants, invertebrates and mammals, there is also variabilityand a more complete understanding of differences will be needed for optimal application. RNA interference (RNAi) is rapid, cheap and selective in complex biological systems and has created new insight sin fields of cancer research, genetic disorders, virology and drug design. Our knowledge about the role of miRNAs and siRNAs pathways in virus-host cell interactions in virus infected cells is incomplete. There are different viral diseases but few antiviral drugs are available. For example, acyclovir for herpes viruses, alpha-interferon for hepatitis C and B viruses and anti-retroviral for HIV are accessible. Also cancer is obviously an important target for siRNA-based therapies, but the main problem in cancer therapy is targeting metastatic cells which spread from the original tumor. There are also other possible reservations and problems that might delay or even hinder siRNA-based therapies for the treatment of certain conditions; however, this remains the most promising approach for a wide range of diseases. Clearly, more studies must be done to allow efficient delivery and better understanding of unwanted side effects of siRNA-based therapies. In this review miRNA and RNAi biology, experimental design, anti-viral and anti-cancer effects are discussed.

A viral vector expressing hypoxia-inducible factor 1 alpha inhibits hippocampal neuronal apoptosis

Hypoxia-inducible factor 1 (HIF-1) attenuates amyloid-beta protein neurotoxicity and decreases apoptosis induced by oxidative stress or hypoxia in cortical neurons. In this study, we constructed a recombinant adeno-associated virus (rAAV) vector expressing the human HIF-1α gene (rAAV-HIF-1α), and tested the assumption that rAAV-HIF-1α represses hippocampal neuronal apoptosis induced by amyloid-beta protein. Our results confirmed that rAAV-HIF-1α significantly reduces apoptosis induced by amyloid-beta protein in primary cultured hippocampal neurons. Direct intracerebral rAAV-HIF-1α administration also induced robust and prolonged HIF-1α production in rat hippocampus. Single rAAV-HIF-1α administration resulted in decreased apoptosis of hippocampal neurons in an Alzheimer's disease rat model established by intracerebroventricular injection of aggregated amyloid-beta protein (25-35). Our in vitro and in vivo findings demonstrate that HIF-1 has potential for attenuating hippocampal neuronal apoptosis induced by amyloid-beta protein, and provides experimental support for treatment of neurodegenerative diseases using gene therapy.

Clinical genetics of Alzheimer's disease

Alzheimer's disease (AD) is the most common progressive neurodegenerative disease and the most common form of dementia in the elderly. It is a complex disorder with environmental and genetic components. There are two major types of AD, early onset and the more common late onset. The genetics of early-onset AD are largely understood with mutations in three different genes leading to the disease. In contrast, while susceptibility loci and alleles associated with late-onset AD have been identified using genetic association studies, the genetics of late-onset Alzheimer's disease are not fully understood. Here we review the known genetics of early- and late-onset AD, the clinical features of EOAD according to genotypes, and the clinical implications of the genetics of AD.

A neuroprotective brain-penetrating endopeptidase fusion protein ameliorates Alzheimer disease pathology and restores neurogenesis

Alzheimer disease (AD) is characterized by widespread neurodegeneration throughout the association cortex and limbic system, deposition of amyloid-β peptide (Aβ) in the neuropil and around the blood vessels, and formation of neurofibrillary tangles. The endopeptidase neprilysin has been successfully used to reduce the accumulation of Aβ following intracranial viral vector delivery or ex vivo manipulated intracranial delivery. These therapies have relied on direct injections into the brain, whereas a clinically desirable therapy would involve i.v. infusion of a recombinant enzyme. We previously characterized a recombinant neprilysin that contained a 38-amino acid brain-targeting domain. Recombinant cell lines have been generated expressing this brain-targeted enzyme (ASN12). In this report, we characterize the ASN12 recombinant protein for pharmacology in a mouse as well as efficacy in two APPtg mouse models of AD. The recombinant ASN12 transited to the brain with a t½ of 24 h and accumulated to 1.7% of injected dose at 24 h following i.v. delivery. We examined pharmacodynamics in the tg2576 APPtg mouse with the prion promoter APP695 SWE mutation and in the Line41 mThy1 APP751 mutation mouse. Treatment of either APPtg mouse resulted in reduced Aβ, increased neuronal synapses, and improved learning and memory. In addition, the Line41 APPtg mice showed increased levels of C-terminal neuropeptide Y fragments and increased neurogenesis. These results suggest that the recombinant brain-targeted neprilysin, ASN12, may be an effective treatment for AD and warrant further investigation in clinical trials.

Methods for gene transfer to the central nervous system

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

Sustained peripheral depletion of amyloid-β with a novel form of neprilysin does not affect central levels of amyloid-β

Lowering levels of peripheral amyloid-β has been proposed as a strategy to reduce plaques in patients with Alzheimer’s disease. Henderson et al. test a modified version of the amyloid-degrading enzyme neprilysin in rats, monkeys and Tg2576 mice. Levels of amyloid-β were reduced in the bloodstream, but not in the CNS.

Alzheimer’s disease is characterized by the accumulation of amyloid deposits in the brain and the progressive loss of cognitive functions. Although the precise role of amyloid-β in disease progression remains somewhat controversial, many efforts to halt or reverse disease progression have focussed on reducing its synthesis or enhancing its removal. It is believed that brain and peripheral soluble amyloid-β are in equilibrium and it has previously been hypothesized that a reduction in peripheral amyloid-β can lower brain amyloid-β, thereby reducing formation of plaques predominantly composed of insoluble amyloid-β; the so-called peripheral sink hypothesis. Here we describe the use of an amyloid-β degrading enzyme, the endogenous metallopeptidase neprilysin, which is fused to albumin to extend plasma half-life and has been engineered to confer increased amyloid-β degradation activity. We used this molecule to investigate the effect of degradation of peripheral amyloid-β on amyloid-β levels in the brain and cerebrospinal fluid after repeated intravenous dosing for up to 4 months in Tg2576 transgenic mice, and 1 month in rats and monkeys. This molecule proved highly effective at degradation of amyloid-β in the periphery but did not alter brain or cerebrospinal fluid amyloid-β levels, suggesting that the peripheral sink hypothesis is not valid and is the first time that this has been demonstrated in non-human primates.

Antisense gene silencing: therapy for neurodegenerative disorders?

Since the first reports that double-stranded RNAs can efficiently silence gene expression in C. elegans, the technology of RNA interference (RNAi) has been intensively exploited as an experimental tool to study gene function. With the subsequent discovery that RNAi could also be applied to mammalian cells, the technology of RNAi expanded from being a valuable experimental tool to being an applicable method for gene-specific therapeutic regulation, and much effort has been put into further refinement of the technique. This review will focus on how RNAi has developed over the years and how the technique is exploited in a pre-clinical and clinical perspective in relation to neurodegenerative disorders.

Recombinant soluble neprilysin reduces amyloid-beta accumulation and improves memory impairment in Alzheimer's disease mice

Accumulation of amyloid-β (Aβ) is thought to be a central pathology in the brain of patients with Alzheimer's disease (AD). Neprilysin (NEP), a plasma membrane glycoprotein of the neutral zinc metalloendopeptidase family, is known as a major Aβ-degrading enzyme in the brain. The level of NEP is reduced in the brains of patients with AD; therefore, NEP is under intense investigation as a potential therapeutic source for degradation of deposited Aβ in AD. Previous studies have utilized viral vectors expressing NEP for reduction of Aβ deposition in the brain. However, viral vectors have disadvantages regarding difficulty in control of insert size, expression desired (short- or long-term), and target cell type. Here, in order to overcome these disadvantages, we produced recombinant soluble NEP from insect cells using an NEP expression vector, which was administered by intracerebral injection into AD mice, resulting in significantly reduced accumulation of Aβ. In addition, AD mice treated with NEP showed improved behavioral performance on the water maze test. These data support a role of recombinant soluble NEP in improving memory impairment by regulation of Aβ deposition and suggest the possibility that approaches using protein therapy might have potential for development of alternative therapies for treatment of AD.

Intracranial injection of AAV expressing NEP but not IDE reduces amyloid pathology in APP+PS1 transgenic mice

The accumulation of β-amyloid peptides in the brain has been recognized as an essential factor in Alzheimer's disease pathology. Several proteases, including Neprilysin (NEP), endothelin converting enzyme (ECE), and insulin degrading enzyme (IDE), have been shown to cleave β-amyloid peptides (Aβ). We have previously reported reductions in amyloid in APP+PS1 mice with increased expression of ECE. In this study we compared the vector-induced increased expression of NEP and IDE. We used recombinant adeno-associated viral vectors expressing either native forms of NEP (NEP-n) or IDE (IDE-n), or engineered secreted forms of NEP (NEP-s) or IDE (IDE-s). In a six-week study, immunohistochemistry staining for total Aβ was significantly decreased in animals receiving the NEP-n and NEP-s but not for IDE-n or IDE-s in either the hippocampus or cortex. Congo red staining followed a similar trend revealing significant decreases in the hippocampus and the cortex for NEP-n and NEP-s treatment groups. Our results indicate that while rAAV-IDE does not have the same therapeutic potential as rAAV-NEP, rAAV-NEP-s and NEP-n are effective at reducing amyloid loads, and both of these vectors continue to have significant effects nine months post-injection. As such, they may be considered reasonable candidates for gene therapy trials in AD.

Short hairpin RNA-mediated gene silencing

Since the first application of RNA interference (RNAi) in mammalian cells, the expression of short hairpin RNAs (shRNAs) for targeted gene silencing has become a benchmark technology. Using plasmid and viral vectoring systems, the transcription of shRNA precursors that are effectively processed by the RNAi pathway can lead to potent gene knockdown. The past decade has seen continual advancement and improvement to the various strategies that can be used for shRNA delivery, and the use of shRNAs for clinical applications is well underway. Driving these developments has been the many benefits afforded by shRNA technologies, including the stable integration of expression constructs for long-term expression, infection of difficult-to-target cell lines and tissues using viral vectors, and the temporal control of shRNA transcription by inducible promoters. The use of different effector molecule formats, promoters, and vector types, has meant that experiments can be tailored to target specific cell types and minimize cellular toxicities. Through the application of combinatorial RNAi (co-RNAi), multiple shRNA delivery strategies can improve gene knockdown, permit multiple transcripts to be targeted simultaneously, and curtail the emergence of viral escape mutants. This chapter reviews the history, cellular processing, and various applications of shRNAs in mammalian systems, including options for effector molecule design, vector and promoter types, and methods for multiple shRNA delivery.

Lentivirus-mediated APP695-RNAi reduces apoptosis in APP transgenic mouse neurons

Amyloid-β peptide (Aβ) is a cleavage product of the amyloid precursor protein (APP), which is thought to be important in the pathogenesis of Alzheimer's disease (AD). Recent evidence suggests that Aβ induces neuronal apoptosis in the brain and in primary neuronal cultures. If decreased Aβ whether could reduce the neuronal apoptosis? In this study, APP695-siRNA was delivered to hippocampal and cortical neurons of APP695 transgenic mice (AD model) in vitro using a recombinant lentivirus vector. The results show that lentivirus-mediated RNA interference of the APP695 gene could reduce neuronal apoptosis, possibly through the reduction of caspase-3 activity and the neuronal apoptosis pathway. These results suggest that lentivirus-mediated RNA interference may be a potential therapeutic for AD.

Directing neuronal differentiation of primary neural progenitor cells by gene knockdown approach

Directing differentiation of neural stem/progenitor cells (NPCs) to produce functional neurons is a promising remedy for neural pathological conditions. The major challenge, however, lies in the effective and efficient generation of a sizable population of neurons. A potential strategy is to incorporate RNA interference (RNAi) during directed stem cell differentiation to recapitulate the complex cell-signaling cascades that often occurs during the process. In this study, in vitro silencing of RE1-silencing transcription factor (REST) was carried out using small-interfering RNAs (siRNAs) to evaluate the efficacy of combining REST knockdown with conventional differentiation approaches to enhance neurogenesis. While earlier studies have demonstrated enhanced neuronal lineage commitment from embryonic stem cells and mesenchymal stem cells upon REST knockdown, the effects of REST silencing during other stages of neural development have not been extensively evaluated. We hypothesize that REST knockdown would enhance NPC development to mature neurons and that induced REST silencing can serve as a potential biochemical approach to direct cell fate. Under nonspecific induction conditions, REST knockdown induced eightfold higher Tuj1 mRNA expression at day 14 compared with untransfected cells and cells subjected to scrambled-siRNA treatment (controls). Immunostaining also revealed greater percentage of Tuj1 positive cells with REST knockdown. Combined with neuronal induction, REST silencing enhanced the kinetics of neuronal differentiation and the rate of maturation of committed neuronal cells. Specifically, upregulation of MAP2 occurred as early as 3 days after induction with REST silencing and the expression was comparable to the controls at day 14. Likewise, downregulation of REST generated more than twice the percentage of Tuj1 and MAP2 positive cells compared with controls at day 5 (p<0.05). Morphologically, REST-silencing enhanced the number and length of neurite extensions from Tuj1 positive cells (p<0.05), which was not evaluated in previous differentiation studies with REST knockdown. Taken together, these results demonstrate the efficacy of combining REST silencing during directed NPC differentiation to enhance the rate of differentiation and subsequent maturation of NPCs. This study also highlights the potential of RNAi as a biomedical strategy for guided stem cell differentiation.

Illuminating the gateway of gene silencing: perspective of RNA interference technology in clinical therapeutics

A novel laboratory revolution for disease therapy, the RNA interference (RNAi) technology, has adopted a new era of molecular research as the next generation "Gene-targeted prophylaxis." In this review, we have focused on the chief technological challenges associated with the efforts to develop RNAi-based therapeutics that may guide the biomedical researchers. Many non-curable maladies, like neurodegenerative diseases and cancers have effectively been cured using this technology. Rapid advances are still in progress for the development of RNAi-based technologies that will be having a major impact on medical research. We have highlighted the recent discoveries associated with the phenomenon of RNAi, expression of silencing molecules in mammals along with the vector systems used for disease therapeutics.

Animal models in the drug discovery pipeline for Alzheimer's disease

With increasing feasibility of predicting conversion of mild cognitive impairment to dementia based on biomarker profiling, the urgent need for efficacious disease-modifying compounds has become even more critical. Despite intensive research, underlying pathophysiological mechanisms remain insufficiently documented for purposeful target discovery. Translational research based on valid animal models may aid in alleviating some of the unmet needs in the current Alzheimer's disease pharmaceutical market, which includes disease-modification, increased efficacy and safety, reduction of the number of treatment unresponsive patients and patient compliance. The development and phenotyping of animal models is indeed essential in Alzheimer's disease-related research as valid models enable the appraisal of early pathological processes - which are often not accessible in patients, and subsequent target discovery and evaluation. This review paper summarizes and critically evaluates currently available animal models, and discusses their value to the Alzheimer drug discovery pipeline. Models dealt with include spontaneous models in various species, including senescence-accelerated mice, chemical and lesion-induced rodent models, and genetically modified models developed in Drosophila melanogaster, Caenorhabditis elegans, Danio rerio and rodents. Although highly valid animal models exist, none of the currently available models recapitulates all aspects of human Alzheimer's disease, and one should always be aware of the potential dangers of uncritical extrapolating from model organisms to a human condition that takes decades to develop and mainly involves higher cognitive functions.

Small interfering RNA-mediated knockdown of NF-κBp65 attenuates neuropathic pain following peripheral nerve injury in rats. Eur J Pharmacol. 2012;682:79-85. [PMID: 22381070 DOI: 10.1016/j.ejphar.2012.02.017]

Recent reports show that the nuclear factor-κB (NF-κB) can control numerous genes encoding inflammatory and nociceptive mediators and play an important role in the development of central pain sensitization. The aim of the present study is to assess the role of NF-κB signal pathway and its downstream pro-inflammatory cytokines in the modulation of neuropathic pain, by using small interfering RNAs (siRNAs) technique, which has been shown to result in potent, long-lasting post-transcriptional silencing of specific genes. We developed a highly efficient method of lentivirus-mediated delivery of short-hairpin RNA (shRNA) targeting NF-κBp65 for gene silencing. This method successfully transduced LV-shNF-κBp65 into cultured spinal cord neurons in vitro and spinal cord cells in vivo, inhibited the expression of NF-κBp65 and pro-inflammatory factors (TNF-α, IL-1β and IL-6) and alleviated mechanical allodynia and thermal hyperalgesia for more than 4weeks in chronic constriction injury (CCI) model of rats. Taken together, our results suggest that siRNA against NF-κBp65 is a potential strategy for analgesia. Furthermore, the lentiviral vector derived shRNA approach shows a great promise for the management of neuropathic pain and the study of functional NF-κBp65 gene expression.

Utilization of APPswe/PS1dE9 Transgenic Mice in Research of Alzheimer's Disease: Focus on Gene Therapy and Cell-Based Therapy Applications

One of the most extensively used transgenic mouse model of Alzheimer's disease (AD) is APPswe/PS1dE9 mice, which over express the Swedish mutation of APP together with PS1 deleted in exon 9. These mice show increase in parenchymal Aβ load with Aβ plaques starting from the age of four months, glial activation, and deficits in cognitive functions at the age of 6 months demonstrated by radial arm water maze and 12-13 months seen with Morris Water Maze test. As gene transfer technology allows the delivery of DNA into target cells to achieve the expression of a protective or therapeutic protein, and stem cell transplantation may create an environment supporting neuronal functions and clearing Aβ plaques, these therapeutic approaches alone or in combination represent potential therapeutic strategies that need to be tested in relevant animal models before testing in clinics. Here we review the current utilization of APPswe/PS1dE9 mice in testing gene transfer and cell transplantation aimed at improving the protection of the neurons against Aβ toxicity and also reducing the brain levels of Aβ. Both gene therapy and cell based therapy may be feasible therapeutic approaches for human AD.

Selective targeting of ASIC3 using artificial miRNAs inhibits primary and secondary hyperalgesia after muscle inflammation

Acid-sensing ion channels (ASICs) are activated by acidic pH and may play a significant role in the development of hyperalgesia. Earlier studies show ASIC3 is important for induction of hyperalgesia after muscle insult using ASIC3-/- mice. ASIC3-/- mice lack ASIC3 throughout the body, and the distribution and composition of ASICs could be different from wild-type mice. We therefore tested whether knockdown of ASIC3 in primary afferents innervating muscle of adult wild-type mice prevented development of hyperalgesia to muscle inflammation. We cloned and characterized artificial miRNAs (miR-ASIC3) directed against mouse ASIC3 (mASIC3) to downregulate ASIC3 expression in vitro and in vivo. In CHO-K1 cells transfected with mASIC3 cDNA in culture, the miR-ASIC3 constructs inhibited protein expression of mASIC3 and acidic pH-evoked currents and had no effect on protein expression or acidic pH-evoked currents of ASIC1a. When miR-ASIC3 was used in vivo, delivered into the muscle of mice using a herpes simplex viral vector, both muscle and paw mechanical hyperalgesia were reduced after carrageenan-induced muscle inflammation. ASIC3 mRNA in DRG and protein levels in muscle were decreased in vivo by miR-ASIC3. In CHO-K1 cells co-transfected with ASIC1a and ASIC3, miR-ASIC3 reduced the amplitude of acidic pH-evoked currents, suggesting an overall inhibition in the surface expression of heteromeric ASIC3-containing channels. Our results show, for the first time, that reducing ASIC3 in vivo in primary afferent fibers innervating muscle prevents the development of inflammatory hyperalgesia in wild-type mice, and thus, may have applications in the treatment of musculoskeletal pain in humans.

Strategies for the Development of Cell Lines for Ex Vivo Gene Therapy in the Central Nervous System

Disorders of the central nervous system (CNS) as a result of trauma or ischemic or neurodegenerative processes still pose a challenge for modern medicine. Due to the complexity of the CNS, and in spite of the advances in the knowledge of its anatomy, pharmacology, and molecular and cellular biology, treatments for these diseases are still limited. The development of cell lines as a source for transplantation into the damaged CNS (cell therapy), and more recently their genetic modification to favor the expression and delivery of molecules with therapeutic potential (ex vivo gene therapy), are some of the techniques used in search of novel restorative strategies. This article reviews the different approaches that have been used and perfected during the last decade to generate cell lines and their use in experimental models of neuronal damage, and evaluates the prospects of applying these methods to treat CNS disorders.

The miRNA pathway in neurological and skeletal muscle disease: implications for pathogenesis and therapy

RNA interference (RNAi) represents a powerful post-transcriptional gene silencing network which fine-tunes gene expression in all eukaryotic cells. The endogenous triggers of RNAi, microRNAs (miRNAs), are proposed to regulate expression of up to a third of all protein-coding genes, and have been shown to have critical roles in developmental processes including in the central nervous system and skeletal muscle. Further, many have been reported to display differential expression in various disease states. Here we describe present understanding of the biogenesis and function of miRNAs, review current knowledge of miRNA abnormalities in both human neurological and skeletal muscle disease and discuss their potential as novel disease biomarkers. Finally, we highlight the many ways in which the miRNA pathway may be targeted for therapeutic benefit.

Current prospects for RNA interference-based therapies

RNA interference (RNAi) is a powerful approach for reducing expression of endogenously expressed proteins. It is widely used for biological applications and is being harnessed to silence mRNAs encoding pathogenic proteins for therapy. Various methods - including delivering RNA oligonucleotides and expressing RNAi triggers from viral vectors - have been developed for successful RNAi in cell culture and in vivo. Recently, RNAi-based gene silencing approaches have been demonstrated in humans, and ongoing clinical trials hold promise for treating fatal disorders or providing alternatives to traditional small molecule therapies. Here we describe the broad range of approaches to achieve targeted gene silencing for therapy, discuss important considerations when developing RNAi triggers for use in humans, and review the current status of clinical trials.

Genetic therapy for the nervous system

Genetic therapy is undergoing a renaissance with expansion of viral and synthetic vectors, use of oligonucleotides (RNA and DNA) and sequence-targeted regulatory molecules, as well as genetically modified cells, including induced pluripotent stem cells from the patients themselves. Several clinical trials for neurologic syndromes appear quite promising. This review covers genetic strategies to ameliorate neurologic syndromes of different etiologies, including lysosomal storage diseases, Alzheimer's disease and other amyloidopathies, Parkinson's disease, spinal muscular atrophy, amyotrophic lateral sclerosis and brain tumors. This field has been propelled by genetic technologies, including identifying disease genes and disruptive mutations, design of genomic interacting elements to regulate transcription and splicing of specific precursor mRNAs and use of novel non-coding regulatory RNAs. These versatile new tools for manipulation of genetic elements provide the ability to tailor the mode of genetic intervention to specific aspects of a disease state.

Peripheral delivery of a CNS targeted, metalo-protease reduces aβ toxicity in a mouse model of Alzheimer's disease

Alzheimer's disease (AD), an incurable, progressive neurodegenerative disorder, is the most common form of dementia. Therapeutic options have been elusive due to the inability to deliver proteins across the blood-brain barrier (BBB). In order to improve the therapeutic potential for AD, we utilized a promising new approach for delivery of proteins across the BBB. We generated a lentivirus vector expressing the amyloid β-degrading enzyme, neprilysin, fused to the ApoB transport domain and delivered this by intra-peritoneal injection to amyloid protein precursor (APP) transgenic model of AD. Treated mice had reduced levels of Aβ, reduced plaques and increased synaptic density in the CNS. Furthermore, mice treated with the neprilysin targeting the CNS had a reversal of memory deficits. Thus, the addition of the ApoB transport domain to the secreted neprilysin generated a non-invasive therapeutic approach that may be a potential treatment in patients with AD.

Androgens regulate neprilysin expression: role in reducing beta-amyloid levels

Age-related testosterone depletion in men is a risk factor for Alzheimer's disease. Prior studies suggest that androgens affect Alzheimer's disease risk by regulating accumulation of beta-amyloid protein (Abeta) by an undefined mechanism. In this study, we investigated the role of the Abeta-catabolizing enzyme neprilysin (NEP) in this process. First, we observed that androgens positively regulate neural expression of NEP in adult male rats. Next, we investigated androgen regulatory effects on both NEP expression and Abeta levels using cultured hippocampal neurons and neuronally differentiated rat pheochromocytoma cell 12 with or without androgen receptor (AR). Dihydrotestosterone (DHT) induced a time-dependent increase in NEP expression. DHT also significantly decreased levels of Abeta in AR-expressing cells transfected with amyloid precursor protein, but did not affect levels of either full-length or non-amyloidogenic, soluble amyloid precursor protein. Importantly, the DHT induced decrease of Abeta was blocked by pharmacological inhibition of NEP. The DHT-mediated increase in NEP expression and decrease in Abeta levels were (i) not observed in rat pheochromocytoma cell 12 lacking AR and (ii) blocked in AR-expressing cells by the antagonists, cyproterone acetate and flutamide. Together, these findings suggest that androgen regulation of Abeta involves an AR-dependent mechanism requiring up-regulation of the Abeta catabolizing enzyme NEP.

HSV as a vector in vaccine development and gene therapy

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

HSV Recombinant Vectors for Gene Therapy

The very deep knowledge acquired on the genetics and molecular biology of herpes simplex virus (HSV), has allowed the development of potential replication-competent and replication-defective vectors for several applications in human healthcare. These include delivery and expression of human genes to cells of the nervous systems, selective destruction of cancer cells, prophylaxis against infection with HSV or other infectious diseases, and targeted infection to specific tissues or organs. Replication-defective recombinant vectors are non-toxic gene transfer tools that preserve most of the neurotropic features of wild type HSV-1, particularly the ability to express genes after having established latent infections, and are thus proficient candidates for therapeutic gene transfer settings in neurons. A replication-defective HSV vector for the treatment of pain has recently entered in phase 1 clinical trial. Replication-competent (oncolytic) vectors are becoming a suitable and powerful tool to eradicate brain tumours due to their ability to replicate and spread only within the tumour mass, and have reached phase II/III clinical trials in some cases. The progress in understanding the host immune response induced by the vector is also improving the use of HSV as a vaccine vector against both HSV infection and other pathogens. This review briefly summarizes the obstacle encountered in the delivery of HSV vectors and examines the various strategies developed or proposed to overcome such challenges.

HSV Recombinant Vectors for Gene Therapy

The very deep knowledge acquired on the genetics and molecular biology of herpes simplex virus (HSV), has allowed the development of potential replication-competent and replication-defective vectors for several applications in human healthcare. These include delivery and expression of human genes to cells of the nervous systems, selective destruction of cancer cells, prophylaxis against infection with HSV or other infectious diseases, and targeted infection to specific tissues or organs. Replication-defective recombinant vectors are non-toxic gene transfer tools that preserve most of the neurotropic features of wild type HSV-1, particularly the ability to express genes after having established latent infections, and are thus proficient candidates for therapeutic gene transfer settings in neurons. A replication-defective HSV vector for the treatment of pain has recently entered in phase 1 clinical trial. Replication-competent (oncolytic) vectors are becoming a suitable and powerful tool to eradicate brain tumours due to their ability to replicate and spread only within the tumour mass, and have reached phase II/III clinical trials in some cases. The progress in understanding the host immune response induced by the vector is also improving the use of HSV as a vaccine vector against both HSV infection and other pathogens. This review briefly summarizes the obstacle encountered in the delivery of HSV vectors and examines the various strategies developed or proposed to overcome such challenges.

Gene therapy in Alzheimer's disease - potential for disease modification

Alzheimer's disease (AD) is the major cause of dementia in the elderly, leading to memory loss and cognitive decline. The mechanism underlying onset of the disease has not been fully elucidated. However, characteristic pathological manifestations include extracellular accumulation and aggregation of the amyloid beta-peptide (Abeta) into plaques and intracellular accumulation and aggregation of hyperphosphorylated tau, forming neurofibrillary tangles. Despite extensive research worldwide, no disease modifying treatment is yet available. In this review, we focus on gene therapy as a potential treatment for AD, and summarize recent work in the field, ranging from proof-of-concept studies in animal models to clinical trials. The multifactorial causes of AD offer a variety of possible targets for gene therapy, including two neurotrophic growth factors, nerve growth factor and brain-derived neurotrophic factor, Abeta-degrading enzymes, such as neprilysin, endothelin-converting enzyme and cathepsin B, and AD associated apolipoprotein E. This review also discusses advantages and drawbacks of various rapidly developing virus-mediated gene delivery techniques for gene therapy. Finally, approaches aiming at down-regulating amyloid precursor protein (APP) and beta-site APP cleaving enzyme 1 levels by means of siRNA-mediated knockdown are briefly summarized. Overall, the prospects appear hopeful that gene therapy has the potential to be a disease modifying treatment for AD.

Recent advances in the manipulation of murine gene expression and its utility for the study of human neurological disease

Transgenic mouse models have vastly contributed to our knowledge of the genetic and molecular pathways underlying the pathogenesis of neurological disorders that affect millions of people worldwide. Not only have they allowed the generation of disease models mimicking the human pathological state but they have also permitted the exploration of the pathological role of specific genes through the generation of knock-out and knock-in models. Classical constitutive transgenic mice have several limitations however, due to behavioral adaptation process occurring and conditional mouse models are time-consuming and often lack extensive spatial or temporal control of gene manipulation. These limitations could be overcome by means of innovative methods that are now available such as RNAi, viral vectors and large cloning DNA vectors. These tools have been extensively used for the generation of mouse models and are characterized by the superior control of transgene expression that has been proven invaluable in the assessment of novel treatments for neurological diseases and to further investigate the molecular processes underlying the etiopathology of neurological disorders. Furthermore, in association with classical transgenic mouse models, they have allowed the validation of innovative therapeutic strategies for the treatment of human neurological disorders. This review describes how these tools have overcome the limitations of classical transgenic mouse models and how they have been of value for the study of human neurological diseases.