Gene therapy in the CNS: intracerebral grafting of genetically modified cells

Nerve Growth Factor Pathobiology During the Progression of Alzheimer's Disease

The current review summarizes the pathobiology of nerve growth factor (NGF) and its cognate receptors during the progression of Alzheimer's disease (AD). Both transcript and protein data indicate that cholinotrophic neuronal dysfunction is related to an imbalance between TrkA-mediated survival signaling and the NGF precursor (proNGF)/p75NTR-mediated pro-apoptotic signaling, which may be related to alteration in the metabolism of NGF. Data indicate a spatiotemporal pattern of degeneration related to the evolution of tau pathology within cholinotrophic neuronal subgroups located within the nucleus basalis of Meynert (nbM). Despite these degenerative events the cholinotrophic system is capable of cellular resilience and/or plasticity during the prodromal and later stages of the disease. In addition to neurotrophin dysfunction, studies indicate alterations in epigenetically regulated proteins occur within cholinotrophic nbM neurons during the progression of AD, suggesting a mechanism that may underlie changes in transcript expression. Findings that increased cerebrospinal fluid levels of proNGF mark the onset of MCI and the transition to AD suggests that this proneurotrophin is a potential disease biomarker. Novel therapeutics to treat NGF dysfunction include NGF gene therapy and the development of small molecule agonists for the cognate prosurvival NGF receptor TrkA and antagonists against the pan-neurotrophin p75NTR death receptor for the treatment of AD.

Potential therapeutic strategies for Alzheimer's disease targeting or beyond β-amyloid: insights from clinical trials

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with two hallmarks: β-amyloid plagues and neurofibrillary tangles. It is one of the most alarming illnesses to elderly people. No effective drugs and therapies have been developed, while mechanism-based explorations of therapeutic approaches have been intensively investigated. Outcomes of clinical trials suggested several pitfalls in the choice of biomarkers, development of drug candidates, and interaction of drug-targeted molecules; however, they also aroused concerns on the potential deficiency in our understanding of pathogenesis of AD, and ultimately stimulated the advent of novel drug targets tests. The anticipated increase of AD patients in next few decades makes development of better therapy an urgent issue. Here we attempt to summarize and compare putative therapeutic strategies that have completed clinical trials or are currently being tested from various perspectives to provide insights for treatments of Alzheimer's disease.

Cholinergic system during the progression of Alzheimer's disease: therapeutic implications

Alzheimer's disease (AD) is characterized by a progressive phenotypic downregulation of markers within cholinergic basal forebrain (CBF) neurons, frank CBF cell loss and reduced cortical choline acetyltransferase activity associated with cognitive decline. Delaying CBF neurodegeneration or minimizing its consequences is the mechanism of action for most currently available drug treatments for cognitive dysfunction in AD. Growing evidence suggests that imbalances in the expression of NGF, its precursor proNGF and the high (TrkA) and low (p75(NTR)) affinity NGF receptors are crucial factors underlying CBF dysfunction in AD. Drugs that maintain a homeostatic balance between TrkA and p75(NTR) may slow the onset of AD. A NGF gene therapy trial reduced cognitive decline and stimulated cholinergic fiber growth in humans with mild AD. Drugs treating the multiple pathologies and clinical symptoms in AD (e.g., M1 cholinoceptor and/or galaninergic drugs) should be considered for a more comprehensive treatment approach for cholinergic dysfunction.

Induction of neurotrophic factors GDNF and BDNF associated with the mechanism of neurorescue action of rasagiline and ladostigil: new insights and implications for therapy

Parkinson's disease (PD) and Alzheimer's disease (AD) are the most common neurodegenerative disorders, although there is no drug or therapeutic treatment to demonstrate disease-modifying effects. Previous work has proposed that neurodegeneration is linked to a lack of trophic support in those neurons and brain areas associated with PD and AD. Indeed, previous studies have found that neurotrophic factors (NTFs) support neuronal survival in various cellular and animal models of PD and AD. Thus, attention has begun to turn to the possibility of NTF neuroprotective-neurorescue therapies for these diseases, indicating that NTFs may be of significant clinical importance as exogenously supplied or endogenously induced elements that obliterate neuronal deficits and degeneration. We have recently reported that the anti-PD drug rasagiline, the anti-AD drug ladostigil, and their propargyl moiety, propargylamine, enhanced the expression levels of brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor, endogenous NTFs associated with activation of phosphatidylinositol 3-kinase, protein kinase, and mitogen-activated protein kinase cell signaling/survival pathways. These studies indicate that the induction of NTFs by rasagiline and ladostigil might suppress apoptosis and induce neurorescue in neurodegenerative disorders and may support the drugs' possible disease-modifying mechanism of action.

Nerve growth factor in treatment and pathogenesis of Alzheimer's disease

The etiology of Alzheimer's disease (AD) is still unknown. In addition, this terrible neurodegenerative disease will increase exponentially over the next two decades due to longer lifespan and an aging "baby-boomer" generation. All treatments currently approved for AD have moderate efficacy in slowing the rate of cognitive decline in patients, and no efficacy in halting progression of the disease. Hence, there is an urgent need for new drug targets and delivery methods to slow or reverse the progression of AD. One molecule that has received much attention in its potential therapeutic role in AD is nerve growth factor (NGF). This review will demonstrate data from humans and animals which promote NGF as a potential therapeutic target by (1) outlining the hypothesis behind using NGF for the treatment of AD, (2) reviewing both the normal and AD altered signaling pathways and effects of NGF in the central nervous system (CNS), and (3) examining the results of NGF treatment obtained from animal models of AD and AD patients.

Direct Gene Therapy for Repair of the Spinal Cord

For regrowth of injured nerve fibers following spinal cord injury (SCI), the environment must be favorable for axonal growth. The delivery of a therapeutic gene, beneficial for axonal growth, into the central nervous system for repair can be accomplished in many ways. Perhaps the most simple and elegant strategy is the so-called direct gene therapy approach that uses a single injection for delivery of a gene therapy vehicle. Among the vectors that have been used to transduce neural tissue in vivo are non-viral, herpes simplex viral, adeno-associated viral, adenoviral, and lentiviral vectors, each with their own merits and limitations. Many studies have been undertaken using direct gene therapy, ranging from strategies for neuroprotection to axonal growth promotion at the injury site, dorsal root injury repair, and initiation of a growth-supporting genetic program. The limitations and successes of direct gene transfer for spinal cord repair are discussed in this review.

Biologic Transplantation and Neurotrophin-Induced Neuroplasticity After Traumatic Brain Injury

OBJECTIVE

In this review, we analyze progress in the treatment of traumatic brain injury with neurotrophins, growth factors and cell and tissue neurotransplantation. The primary objective of these therapies is to reduce neurologic deficits associated with the trauma by inducing neuroplasticity. These therapies are restorative and not necessarily neuroprotective.

MAIN OUTCOME MEASURES

An extensive literature on administration of neurotrophics factors and cell and tissue cerebral transplantation is reviewed. The effects of these therapeutic approaches on brain biochemical, molecular, cellular, and tissue responses are summarized.

CONCLUSION

The cumulative data indicate that cell therapy shows substantial promise in the treatment of neural injury.

Growth factor gene therapy for Alzheimer disease

The capacity to prevent neuronal degeneration and death during the course of progressive neurological disorders such as Alzheimer disease (AD) would represent a significant advance in therapy. Nervous system growth factors are families of naturally produced proteins that, in animal models, exhibit extensive potency in preventing neuronal death due to a variety of causes, reversing age-related atrophy of neurons, and ameliorating functional deficits. The main challenge in translating growth factor therapy to the clinic has been delivery of growth factors to the brain in sufficient concentrations to influence neuronal function. One means of achieving growth factor delivery to the central nervous system in a highly targeted, effective manner may be gene therapy. In this article the authors summarize the development and implementation of nerve growth factor gene delivery as a potential means of reducing cell loss in AD.

Transplants of NGF-secreting fibroblasts restore stimulus-evoked activity in barrel cortex of basal-forebrain-lesioned rats

Cholinergic nuclei in the basal forebrain supply the cerebral cortex with acetylcholine (ACh). Depletion of cholinergic fibers following basal forebrain lesion results in reduced stimulus-evoked functional activity in rat barrel cortex in response to whisker stimulation. We showed previously that exogenous delivery of nerve growth factor (NGF) to the lateral ventricle restores reduced functional activity toward normal despite persistent reductions in cortical cholinergic activity. Gene transfer of therapeutic peptides using genetically engineered cells allows for localized and biological delivery of compounds to the CNS, circumventing systemic administration or repetitive invasive surgery. In this study, we grafted genetically engineered fibroblasts that secrete NGF (NGF+) into three CNS loci of rats with unilateral basal forebrain lesions, along with control fibroblasts (NGF-) that did not secrete NGF. Only NGF+ fibroblasts grafted into ACh-depleted somatosensory cortex resulted in improvement of functional activity following cholinergic depletion. NGF+ fibroblast transplants into the lateral ventricle or basal forebrain did not improve functional activity nor did NGF- fibroblasts in any site. Similar to our previous experiments using intraventricular NGF injections, despite improvements in functional activity, the affected barrel cortex remained depleted of acetylcholinesterase-stained fibers following insertion of NGF+ fibroblasts. These data support the idea that NGF can act directly on the cerebral cortex following reductions in cholinergic innervation. The mechanism of NGF action is elusive, however, since the presence of its high-affinity receptor, trkA, in the cerebral cortex is controversial.

Neurite outgrowth can be modulated in vitro using a tetracycline-repressible gene therapy vector expressing human nerve growth factor

The delivery of neurotrophic factors to the adult nervous system has potential applications for the treatment of neurodegenerative diseases and trauma. In vivo and ex vivo gene therapy offer a means of delivering growth factors and other therapeutic substances to the central nervous system (CNS) in an intraparenchymal, accurately targeted, and regionally restricted manner. Ideally, gene therapy delivery systems should also be regulatable, allowing exogenous control of amount of gene product delivery. In the present experiment, a tetracycline-regulatable gene expression system was generated to determine whether controllable release of nerve growth factor (NGF) and green fluorescent protein (GFP) from primary rat fibroblasts could modulate biological responses (neurite outgrowth) in vitro. Using a tetracycline-repressible construct, it was found that NGF mRNA, NGF protein, and NGF-induced neurite outgrowth could be tightly regulated within a 24 hour period, and in a dose-dependent fashion, by exposure to the tetracycline analog doxycycline. Similarly, levels of green fluorescence could be regulated in GFP-transfected cells. These findings in a neurobiological system lay the framework for future studies using regulated neurotrophin delivery in in vivo models of neurodegenerative diseases and CNS injury.

Promoter-activated expression of nerve growth factor for treatment of neurodegenerative diseases

Genetic transfer approaches have received recent consideration as potential treatment modalities for human central and peripheral nervous system (CNS and PNS, respectively) neurodegenerative disorders, including Parkinson's disease, Alzheimer's disease, and amyotrophic lateral sclerosis. Transplantation of genetically modified cells into the brain represents a promising strategy for the delivery and expression of specific neurotrophic factors, neurotransmitter-synthesizing enzymes, and cellular regulatory proteins for intervention in neurodegenerative diseases. The use of specific regulatable promoters may also provide potential control of gene expression required for dose-specific or time-specific therapeutic strategies. In this article, we review the potential use of activated promoters in ex vivo systems for the potential genetic therapy of neurodegenerative disorders, and then describe our own studies using the zinc-inducible metallothionein promoter for the regulated expression of nerve growth factor (NGF) in rodent brain transplants.

Neural stem and progenitor cells: a strategy for gene therapy and brain repair

The damaged adult mammalian brain is incapable of significant structural self-repair. Although varying degrees of recovery from injury are possible, this is largely because of synaptic and functional plasticity rather than the frank regeneration of neural tissues. The lack of structural plasticity of the adult brain is partly because of its inability to generate new neurons, a limitation that has severely hindered the development of therapies for neurological injury or degeneration. However, a variety of experimental studies, as well as moderately successful clinical engraftment of fetal tissue into the adult parkinsonian brain, suggests that cell replacement is evolving as a valuable treatment modality. Neural stem cells, which are the self-renewing precursors of neurons and glia, have been isolated from both the embryonic and adult mammalian central nervous system. In the adult human brain, both neuronal and oligodendroglial precursors have been identified, and methods for their harvest and enrichment have been established. Neural precursors have several characteristics that make them ideal vectors for brain repair. They may be clonally expanded in tissue culture, providing a renewable supply of material for transplantation. Moreover, progenitors are ideal for genetic manipulation and may be engineered to express exogenous genes for neurotransmitters, neurotrophic factors, and metabolic enzymes. Thus, the persistence of neuronal precursors in the adult mammalian brain may permit us to design novel and effective strategies for central nervous system repair, by which we may yet challenge the irreparability of the structurally damaged adult nervous system.

Gene therapy for Parkinson's disease: an approach to the prevention or palliation of levodopa-associated motor complications

Gene therapy holds considerable potential for the treatment of central nervous system disease. The introduction of functional genes into the brain of patients with Parkinson's disease may, for example, prove useful as a means to replace a defective gene, introduce a potentially neuroprotective or neurorestorative protein, or permit the physiological delivery of a deficient neurotransmitter. Recent observations suggest that the oral administration of currently available dopaminomimetics to relatively advanced parkinsonian patients leads to nonphysiologic intermittent stimulation of striatal neurons that express dopamine receptors. Resultant activation of signal transduction pathways from these dopaminergic receptors on medium-sized GABAergic neurons apparently induces long-term potentiation of adjacent glutamatergic receptors of the N-methyl-D-aspartate subtype. The effects of dopaminergic drugs thus become modified in ways that favor the clinical appearance of response fluctuations and peak-dose dyskinesias. In parkinsonian models was well as in patients with Parkinson's disease, continuous dopaminergic replacement tends to prevent or alleviate these adverse effects. By continuously maintaining appropriate cerebral dopamine concentrations, molecular techniques which stimulate an increase in the intrastriatal activity of tyrosine hydroxylase, the rate-limiting enzyme for dopamine synthesis, might be expected to palliate parkinsonian symptoms with less risk of the disabling consequences of current therapy. Clinical study of these approaches could also serve as initial, relatively simple, proof-of-principle evaluations of the safety and efficacy of genetic approaches to the treatment of basic disease processes in Parkinson's disease and related neurodegenerative disorders.

Reporter genes in transgenic mice

Although in vivo models utilizing endogenous reporter genes have been exploited for many years, the use of reporter transgenes to dissect biological issues in transgenic animals has been a relatively recent development. These transgenes are often, but not always, of prokaryotic origin and encode products not normally associated with eukaryotic cells and tissues. Some encode enzymes whose activities are detected in cell and tissue homogenates, whereas others encode products that can be detected in situ at the single cell level. Reporter genes have been used to identify regulatory elements that are important for tissue-specific gene expression or for development; they have been used to produce in vivo models of cancer; they have been employed for the study of in vivo mutagenesis; and they have been used as a tool in lineage analysis and for marking cells in transplantation experiments. The most commonly used in situ reporter gene is lacZ, which encodes a bacterial beta-galactosidase, a sensitive histochemical marker. Although it has been used with striking success in cultured cells and in transgenic mouse embryos, its postnatal in vivo expression has been unreliable and disappointing. Nevertheless, the ability to express reporter genes in transgenic mice has been an invaluable resource, providing insights into in vivo biological mechanisms. The development of new in vivo models, such as those in which expression of transgenes can be activated or repressed, should produce transgenic animal systems that extend our capacity to address heretofore unresolved biological questions.

Herpesvirus-mediated gene delivery into the rat brain: specificity and efficiency of the neuron-specific enolase promoter

1. Herpesvirus infection with genetically engineered vectors is a way to deliver foreign gene products to various cell populations in culture and in vivo. Selective neuronal gene expression can be achieved using the neuron-specific enolase (NSE) promoter regulating expression of a transgene placed in and delivered by a herpesvirus vector. 2. We sought to determine the anatomical specificity and efficiency of herpesvirus-mediated gene transfer into the rat brain following placement of virus particles carrying a transgene (lacZ) under control of the NSE promoter. The virus utilized was thymidine kinase (TK) deficient and therefore replication deficient in the brain. 3. Infusion of 10(6) plaque-forming units of virus into the striatum caused a limited number of striatal neurons to express the lacZ transgene mRNA and protein product 7 days postinfection. In addition, small numbers of neurons expressing the transgene mRNA and protein were found ipsilateral to the viral injection in the frontal cortex, substantia nigra pars compacta, and thalamus. Neurons at these anatomic loci project directly to the striatal injection site. No other cells within the brains of injected animals expressed the lacZ gene. 4. While this herpesvirus NSE vector was capable of introducing novel functional genetic information into postmitotic neurons within defined neuroanatomic constraints, the numbers of neurons expressing detectable levels of beta-galactosidase was minimal. The calculated efficiency of delivery and transgene expression at 7 days postinfection was 1 transgenic neuron per 10(4) virus particles infused. 5. We conclude that NSE probably is not an optimal promoter for use in gene delivery to CNS neurons in herpesvirus vectors and that the efficacy of gene delivery using other neuron-specific promoters placed at various sites in the herpes viral genome needs to be explored.

Striatal degeneration induced by mitochondrial blockade is prevented by biologically delivered NGF

Consistent with the notion that a defect in cellular energy metabolism is a cause of human neurodegenerative disease, systemic treatment with the mitochondrial complex II inhibitor 3-nitropropionic acid (3-NPA) can model the striatal neurodegeneration seen in Huntington's disease. Previously, we have found that nerve growth factor (NGF), delivered biologically by the implantation of a genetically altered fibroblast cell-line, can protect locally against striatal degeneration induced by infusions of high doses of glutamate receptor agonists. We now report that implantation of NGF-secreting fibroblasts reduces the size of adjacent striatal 3-NPA lesions by an average of 64%. We conclude that biologically delivered NGF protects neurons against excitotoxicity and mitochondrial blockade--both energy-depleting processes--implying that appropriate neurotrophic support in the adult brain could protect against neurodegenerative diseases caused in part by energy depletion.

Local protective effects of nerve growth factor-secreting fibroblasts against excitotoxic lesions in the rat striatum

Neurotrophic factors, such as nerve growth factor (NGF), in addition to their role in neuronal development, have protective effects on neuronal survival. Intracerebral implantation of cells genetically altered to secrete high levels of NGF is also found to promote neuronal survival in experimental lesioning models of the brain. The range of activity for such biological delivery systems has not yet been well described either spatially or temporally. Therefore, the authors chose to study the local and distant protective effects of an NGF-secreting rat fibroblast cell line implanted in an excitotoxic lesion model of Huntington's disease. They found that preimplantation of NGF-secreting fibroblasts placed within the corpus callosum reduced the maximum cross-sectional area of a subsequent excitotoxic lesion in the ipsilateral striatum by 80% when compared to the effects of a non-NGF-secreting fibroblast graft, and by 83% when compared to excitotoxic lesions in ungrafted animals (p < 0.003). However, NGF-secreting cells placed in the contralateral corpus callosum failed to affect striatal lesion size significantly when compared to contralateral or ipsilateral non-NGF-secreting cell implants. Of note, fibroblasts were clearly visible within the graft site at 7 and 18 days after implantation; however, few cells within the grafts stained positively for NGF peptide or for the messenger ribonucleic acid (mRNA) encoding the transfected NGF gene-construct at either time point. These results show that biological delivery systems for NGF appear to have a profound but local effect on neuronal excitotoxicity, which will necessitate careful neurosurgical placement for maximum effect. Furthermore, the ability of this genetically altered cell line to synthesize NGF mRNA and peptide appears to decrease spontaneously in vivo, a characteristic that will need to be addressed before this method of biological delivery can be utilized as a treatment for chronic degenerative diseases.

Detrusor function with lesions of the conus medullaris

Conventional urodynamic evaluation is unable to distinguish between a pure conus lesion and one with concomitant cauda equina involvement. Lumbosacral evoked potentials to tibial nerve stimulation assesses the sensory root and dorsal horn interneurons of the L5 to S2 spinal cord segments. This allows for the diagnosis of a pure lesion of the conus medullaris with preservation of the sensory root response (R wave) with absence of the dorsal horn gray matter response (S wave). Urodynamic evaluation in 5 patients with a conus lesion showed a variety of detrusor responses ranging from hyperreflexia through areflexia with decreased compliance to areflexia with normal compliance. The ability to diagnose a pure conus lesion may have prognostic significance as newer modalities of treatment emerge, all of which require intact gray matter of the spinal cord.