Robert A. Marr

Targeting BACE1 with siRNAs ameliorates Alzheimer disease neuropathology in a transgenic model

In Alzheimer disease, increased β-secretase (BACE1) activity has been associated with neurodegeneration and accumulation of amyloid precursor protein (APP) products. Thus, inactivation of BACE1 could be important in the treatment of Alzheimer disease. In this study, we found that lowering BACE1 levels using lentiviral vectors expressing siRNAs targeting BACE1 reduced amyloid production and the neurodegenerative and behavioral deficits in APP transgenic mice, a model of Alzheimer disease. Our results suggest that lentiviral vector delivery of BACE1 siRNA can specifically reduce the cleavage of APP and neurodegeneration in vivo and indicate that this approach could have potential therapeutic value for treatment of Alzheimer disease.

Neprilysin Gene Transfer Reduces Human Amyloid Pathology in Transgenic Mice

The degenerative process of Alzheimers disease is linked to a shift in the balance between amyloid-β (Aβ) production, clearance, and degradation. Neprilysin has recently been implicated as a major extracellular Aβ degrading enzyme in the brain. However, there has been no direct demonstration that neprilysin antagonizes the deposition of amyloid-β in vivo. To address this issue, a lentiviral vector expressing human neprilysin (Lenti-Nep) was tested in transgenic mouse models of amyloidosis. We show that unilateral intracerebral injection of Lenti-Nep reduced amyloid-β deposits by half relative to the untreated side. Furthermore, Lenti-Nep ameliorated neurodegenerative alterations in the frontal cortex and hippocampus of these transgenic mice. These data further support a role for neprilysin in regulating cerebral amyloid deposition and suggest that gene transfer approaches might have potential for the development of alternative therapies for Alzheimers disease.

Neurogenesis and Alzheimer's disease: At the crossroads

While a massive and progressive neuronal loss in specific areas such as the hippocampus and cortex unequivocally underlies cognitive deterioration and memory loss in Alzheimers disease, noteworthy alterations take place in the neurogenic microenvironments, namely, the subgranule layer of the dentate gyrus and the subventricular zone. Compromised neurogenesis presumably takes place earlier than onset of hallmark lesions or neuronal loss, and may play a role in the initiation and progression of neuropathology in Alzheimers disease. Neurogenesis in the adult brain is thought to play a role in numerous forms and aspects of learning and memory and contribute to the plasticity of the hippocampus and olfactory system. Misregulated or impaired neurogenesis on the other hand, may compromise plasticity and neuronal function in these areas and exacerbate neuronal vulnerability. Interestingly, increasing evidence suggests that molecular players in Alzheimers disease, including PS1, APP and its metabolites, play a role in adult neurogenesis. In addition, recent studies suggest that alterations in tau phosphorylation are pronounced in neurogenic areas, and may interfere with the potential central role of tau proteins in neuronal maturation and differentiation. On the other hand, numerous neurogenic players, such as Notch-1, ErbB4 and L1 are substrates of alpha- beta- and gamma- secretase that play a major role in Alzheimers disease. This review will discuss current knowledge concerning alterations of neurogenesis in Alzheimers disease with specific emphasis on the cross-talk between signaling molecules involved in both processes, and the ways by which familial Alzheimers disease-linked dysfunction of these signaling molecules affect neurogenesis in the adult brain.

Neprilysin regulates amyloid Beta peptide levels.

That neprilysin (NEP) is a major Aβ peptide-degrading enzyme in vivo is shown by higher Aβ peptide levels in the brain of an NEP knockout mouse. In addition, we show that infusion of an NEP inhibitor, but not inhibitors of other peptidases, into the brains of an APP transgenic mouse elevates Aβ levels. We have investigated the use of NEP as a potential therapeutic agent to prevent the accumulation of Aβ peptides in the brain. Lentivirus expressing NEP was initially used to demonstrate the ability of the enzyme to reduce Aβ levels in a model CHO cell line and to make primary hippocampal neurons resistant to Aβ-mediated neurotoxicity. Injection of NEP-expressing lentivirus, but not inactive NEP-expressing lentivirus, GFP-expressing lentivirus, or vehicle, into the hippocampus of 12–20-mo-old hAPP transgenic mice led to an approx 50% reduction in the number of amyloid plaques. These studies provide the impetus for further investigating of the use of NEP in a gene transfer therapy paradigm to prevent the accumulation of Aβ and prevent or delay the onset of Alzheimer’s disease.

Neprilysin: An Enzyme Candidate to Slow the Progression of Alzheimer’s Disease

It is well established that the extracellular deposition of amyloid beta (Abeta) peptide plays a central role in the development of Alzheimers disease (AD). Therefore, either preventing the accumulation of Abeta peptide in the brain or accelerating its clearance may slow the rate of AD onset. Neprilysin (NEP) is the dominant Abeta peptide-degrading enzyme in the brain; NEP becomes inactivated and down-regulated during both the early stages of AD and aging. In this study, we investigated the effect of human (h)NEP gene transfer to the brain in a mouse model of AD before the development of amyloid plaques, and assessed how this treatment modality affected the accumulation of Abeta peptide and associated pathogenetic changes (eg, inflammation, oxidative stress, and memory impairment). Overexpression of hNEP for 4 months in young APP/DeltaPS1 double-transgenic mice resulted in reduction in Abeta peptide levels, attenuation of amyloid load, oxidative stress, and inflammation, and improved spatial orientation. Moreover, the overall reduction in amyloidosis and associated pathogenetic changes in the brain resulted in decreased memory impairment by approximately 50%. These data suggest that restoring NEP levels in the brain at the early stages of AD is an effective strategy to prevent or attenuate disease progression.

An antiaggregation gene therapy strategy for Lewy body disease utilizing beta-synuclein lentivirus in a transgenic model.

Current experimental gene therapy approaches for Parkinsons disease (PD) and dementia with Lewy bodies (DLB) include the use of viral vectors expressing antiapoptosis genes, neurotrophic factors and dopaminergic system enzymes. However, since increasing evidence favors a role for α-synuclein accumulation in the pathogenesis of these disorders, an alternative therapy might require the transfer of genes that might block α-synuclein accumulation. β-Synuclein, the nonamyloidogenic homologue of α-synuclein, has recently been identified as a potential candidate. Thus, in vivo transfer of genes encoding β-synuclein might provide a novel approach to the development of experimental treatments for PD and DLB. To assess this possibility and to better understand the mechanisms involved, a lentiviral vector expressing human (h) β-synuclein (lenti-β-synuclein) was tested in a transgenic (tg) mouse model of hα-synuclein aggregation. This study showed that unilateral intracerebral injection of lenti-β-synuclein reduced the formation of hα-synuclein inclusions and the accumulation of hα-synuclein in synapses and ameliorated the neurodegenerative alterations in the tg mice. Both in vivo and in vitro coimmunoprecipitation and immunoblot experiments show that the mechanisms of β-synuclein neuroprotection involve binding of this molecule to hα-synuclein and Akt, resulting in the decreased aggregation and accumulation of hα-synuclein in the synaptic membrane. Together, these data further support a role for β-synuclein in regulating the conformational state of α-synuclein and suggest that this gene transfer approach might have potential for the development of alternative therapies for PD and DLB.

Disruption of antigen-induced inflammatory responses in CD40 ligand knockout mice.

The objective of this study was to investigate the contribution of the interaction between CD40 and its ligand (CD40L) to antigen-induced airways inflammatory responses. To this end, we used a model involving ovalbumin (OVA) sensitization followed by OVA aerosol challenge in CD40L knockout (KO) mice. OVA-specific IgE and IgG1 were detected in the serum of the sensitized control, but not in CD40L-KO mice. After antigen challenge, sensitized control mice developed airway inflammation that was primarily eosinophilic. This inflammatory response was dramatically reduced in CD40L-KO mice. In contrast, similar numbers of eosinophils were observed in both the bone marrow and the peripheral blood in the sensitized controls and mutant strains after antigen challenge. To investigate the mechanisms underlying these findings, we examined levels of the cytokines IL-5, IL-4, and TNFalpha in both bronchoalveolar lavage (BAL) and serum. Similar levels of IL-5 were detected in BAL and serum of control and CD40L-KO mice; however, negligible levels of IL-4 in BAL and serum and of TNFalpha in BAL were detected in CD40L-KO mice when compared with control mice. Furthermore, we demonstrated that endothelial cell expression of vascular cell adhesion molecule 1 in OVA-sensitized and -challenged CD40L-KO mice was, as detected by immunohistochemistry, markedly decreased compared with that observed in similarly treated control mice. In addition, we locally overexpressed IL-4 and TNFalpha by using an adenoviral (Ad)-mediated gene transfer approach. Intranasal administration of either Ad/TNFalpha or Ad/IL-4 into OVA-sensitized and -challenged CD40L-KO mice did not reconstitute airway eosinophilia. However, concurrent administration of Ad/TNFalpha and Ad/IL-4 upregulated endothelial expression of vascular cell adhesion molecule 1, and resulted in full reconstitution of the inflammatory response in the airways. Together, these findings demonstrate the importance of the CD40-CD40L costimulatory pathway in the full expression of the inflammatory response in the airways.

Long-term neprilysin gene transfer is associated with reduced levels of intracellular Abeta and behavioral improvement in APP transgenic mice

BackgroundProteolytic degradation has emerged as a key pathway involved in controlling levels of the Alzheimers disease (AD)-associated amyloid-β (Aβ) peptide in the brain. The endopeptidase, neprilysin, has been implicated as a major Aβ degrading enzyme in mice and humans. Previous short and intermediate term studies have shown the potential therapeutic application of neprilysin by delivering this enzyme into the brain of APP transgenic mice using gene transfer with viral vectors. However the effects of long-term neprilysin gene transfer on other aspects of Aβ associated pathology have not been explored yet in APP transgenic mice.ResultsWe show that the sustained expression of neprilysin for up to 6 months lowered not only the amyloid plaque load but also reduced the levels of intracellular Aβ immunoreactivity. This was associated with improved behavioral performance in the water maze and ameliorated the dendritic and synaptic pathology in the APP transgenic mice.ConclusionThese data support the possibility that long-term neprilysin gene therapy improves behavioral and neurodegenerative pathology by reducing intracellular Aβ.

Altered ryanodine receptor expression in mild cognitive impairment and Alzheimer's disease

Intracellular Ca(2+) dysregulation is an underlying component of Alzheimers disease (AD) pathophysiology, and recent evidence implicates the ryanodine receptor (RyR) in the disease pathway. Three genes code for different RyR isoforms and each gene transcript gives rise to several alternatively spliced messenger RNAs (mRNAs). These variants confer distinct functionality to the RyR channel, such as altering Ca(2+) release properties or subcellular localization. Changes in RyR isoform expression and alternative splicing have not been examined for potential roles in AD pathogenesis. Here, we compare mRNA levels of the RyR2 and RyR3 isoforms as well as specific alternatively spliced variants across vulnerable brain regions from postmortem samples of individuals with no cognitive impairment (NCI), mild cognitive impairment (MCI), and AD. We find an increase in RyR2 transcripts in MCI brains compared with no cognitive impairment. In addition, there is a reduction in a RyR2 splice variant, associated with an antiapoptotic function, in MCI and AD brains. These alterations in RyR expression at early disease stages may reflect the onset of pathologic mechanisms leading to later neurodegeneration.

Presenilin-1 regulates neural progenitor cell differentiation in the adult brain.

Presenilin-1 (PS1) is the catalytic core of the aspartyl protease γ-secretase. Previous genetic studies using germ-line deletion of PS1 and conditional knock-out mice demonstrated that PS1 plays an essential role in neuronal differentiation during neural development, but it remained unclear whether PS1 plays a similar role in neurogenesis in the adult brain. Here we show that neural progenitor cells infected with lentiviral vectors-expressing short interfering RNA (siRNA) for the exclusive knockdown of PS1 in the neurogenic microenvironments, exhibit a dramatic enhancement of cell differentiation. Infected cells differentiated into neurons, astrocytes and oligodendrocytes, suggesting that multipotentiality of neural progenitor cells is not affected by reduced levels of PS1. Neurosphere cultures treated with γ-secretase inhibitors exhibit a similar phenotype of enhanced cell differentiation, suggesting that PS1 function in neural progenitor cells is γ-secretase-dependent. Neurospheres infected with lentiviral vectors expressing siRNA for the targeting of PS1 differentiated even in the presence of the proliferation factors epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF), suggesting that PS1 dominates EFG and bFGF signaling pathways. Reduction of PS1 expression in neural progenitor cells was accompanied by a decrease in EGF receptor and β-catenin expression level, suggesting that they are downstream essential transducers of PS1 signaling in adult neural progenitor cells. These findings suggest a physiological role for PS1 in adult neurogenesis, and a potential target for the manipulation of neural progenitor cell differentiation.