Novel strategies for Alzheimer's disease treatment

Blood-brain barrier penetrating neprilysin degrades monomeric amyloid-beta in a mouse model of Alzheimer's disease

BACKGROUND

Aggregation of the amyloid-β (Aβ) peptide in the brain is one of the key pathological events in Alzheimer's disease (AD). Reducing Aβ levels in the brain by enhancing its degradation is one possible strategy to develop new therapies for AD. Neprilysin (NEP) is a membrane-bound metallopeptidase and one of the major Aβ-degrading enzymes. The secreted soluble form of NEP (sNEP) has been previously suggested as a potential protein-therapy degrading Aβ in AD. However, similar to other large molecules, peripherally administered sNEP is unable to reach the brain due to the presence of the blood-brain barrier (BBB).

METHODS

To provide transcytosis across the BBB, we recombinantly fused the TfR binding moiety (scFv8D3) to either sNEP or a previously described variant of NEP (muNEP) suggested to have higher degradation efficiency of Aβ compared to other NEP substrates, but not per se to degrade Aβ more efficiently. To provide long blood half-life, an Fc-based antibody fragment (scFc) was added to the designs, forming sNEP-scFc-scFv8D3 and muNEP-scFc-scFv8D3. The ability of the mentioned recombinant proteins to degrade Aβ was first evaluated in vitro using synthetic Aβ peptides followed by sandwich ELISA. For the in vivo studies, a single injection of 125-iodine-labelled sNEP-scFc-scFv8D3 and muNEP-scFc-scFv8D3 was intravenously administered to a tg-ArcSwe mouse model of AD, using scFc-scFv8D3 protein that lacks NEP as a negative control. Different ELISA setups were applied to quantify Aβ concentration of different conformations, both in brain tissues and blood samples.

RESULTS

When tested in vitro, sNEP-scFc-scFv8D3 retained sNEP enzymatic activity in degrading Aβ and both constructs efficiently degraded arctic Aβ. When intravenously injected, sNEP-scFc-scFv8D3 demonstrated 20 times higher brain uptake compared to sNEP. Both scFv8D3-fused NEP proteins significantly reduced aggregated Aβ levels in the blood of tg-ArcSwe mice, a transgenic mouse model of AD, following a single intravenous injection. In the brain, monomeric and oligomeric Aβ were significantly reduced. Both scFv8D3-fused NEP proteins displayed a fast clearance from the brain.

CONCLUSION

A one-time injection of a BBB-penetrating NEP shows the potential to reduce, the likely most toxic, Aβ oligomers in the brain in addition to monomers. Also, Aβ aggregates in the blood were reduced.

Exploring the Role of Gene Therapy for Neurological Disorders

Gene therapy is one of the frontier fields of medical breakthroughs that poses as an effective solution to previously incurable diseases. The delivery of the corrective genetic material or a therapeutic gene into the cell restores the missing gene function and cures a plethora of diseases, incurable by the conventional medical approaches. This discovery holds the potential to treat many neurodegenerative disorders such as muscular atrophy, multiple sclerosis, Parkinson's disease (PD) and Alzheimer's disease (AD), among others. Gene therapy proves as a humane, cost-effective alternative to the exhaustive often arduous and timely impossible process of finding matched donors and extensive surgery. It also overcomes the shortcoming of conventional methods to cross the blood-brain barrier. However, the use of gene therapy is only possible after procuring the in-depth knowledge of the immuno-pathogenesis and molecular mechanism of the disease. The process of gene therapy can be broadly categorized into three main steps: elucidating the target gene, culling the appropriate vector, and determining the best mode of transfer; each step mandating pervasive research. This review aims to dissertate and summarize the role, various vectors and methods of delivery employed in gene therapy with special emphasis on therapy directed at the central nervous system (CNS) associated with neurodegenerative diseases.

Protective effect of recombinant soluble neprilysin against β-amyloid induced neurotoxicity

A few decades ago, researchers found emerging evidence showing that a number of sequential events lead to the pathological cascade of Alzheimer's disease (AD) which is caused by the accumulation of amyloid beta (Aβ), a physiological peptide, in the brain. Therefore, regulation of Aβ represents a crucial treatment approach for AD. Neprilysin (NEP), a membrane metallo-endopeptidase, is a rate-limiting peptidase which is known to degrade the amyloid beta peptide. This study investigated soluble NEP (sNEP) produced by recombinant mammalian cells stably transfected with a non-viral NEP expression vector to demonstrate its protective effect against Aβ peptides in neuronal cells in vitro. Stably transfected HEK 293 cells were used to purify the soluble protein. sNEP and Aβ peptide co-treated hippocampal cells had a decreased level of Aβ peptides shown by an increase in cell viability and decrease in apoptosis measured by the CCK-8 and relative caspase-3 activity ratio assays, respectively. This study shows that stably transfected mammalian cells can produce soluble NEP proteins which could be used to protect against Aβ accumulation in AD and subsequently neuronal toxicity. Additionally, approaches using protein therapy for potential targets could change the pathological cascade of Alzheimer's disease.

A brain-targeted, modified neurosin (kallikrein-6) reduces α-synuclein accumulation in a mouse model of multiple system atrophy

Background

Multiple system atrophy (MSA) is a progressive, neurodegenerative disease characterized by parkinsonism, resistance to dopamine therapy, ataxia, autonomic dysfunction, and pathological accumulation of α-synuclein (α-syn) in oligodendrocytes. Neurosin (kallikrein-6) is a serine protease capable of cleaving α-syn in the CNS, and we have previously shown that lentiviral (LV) vector delivery of neurosin into the brain of a mouse model of dementia with Lewy body/ Parkinson’s disease reduces the accumulation of α-syn and improves neuronal synaptic integrity.

Results

In this study, we investigated the ability of a modified, systemically delivered neurosin to reduce the levels of α-syn in oligodendrocytes and reduce the cell-to-cell spread of α-syn to glial cells in a mouse model of MSA (MBP-α-syn). We engineered a viral vector that expresses a neurosin genetically modified for increased half-life (R80Q mutation) that also contains a brain-targeting sequence (apoB) for delivery into the CNS. Peripheral administration of the LV-neurosin-apoB to the MBP-α-syn tg model resulted in accumulation of neurosin-apoB in the CNS, reduced accumulation of α-syn in oligodendrocytes and astrocytes, improved myelin sheath formation in the corpus callosum and behavioral improvements.

Conclusion

Thus, the modified, brain-targeted neurosin may warrant further investigation as potential therapy for MSA.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-015-0043-6) contains supplementary material, which is available to authorized users.

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.

Protein damage, repair and proteolysis

Proteins are continuously affected by various intrinsic and extrinsic factors. Damaged proteins influence several intracellular pathways and result in different disorders and diseases. Aggregation of damaged proteins depends on the balance between their generation and their reversal or elimination by protein repair systems and degradation, respectively. With regard to protein repair, only few repair mechanisms have been evidenced including the reduction of methionine sulfoxide residues by the methionine sulfoxide reductases, the conversion of isoaspartyl residues to L-aspartate by L-isoaspartate methyl transferase and deglycation by phosphorylation of protein-bound fructosamine by fructosamine-3-kinase. Protein degradation is orchestrated by two major proteolytic systems, namely the lysosome and the proteasome. Alteration of the function for both systems has been involved in all aspects of cellular metabolic networks linked to either normal or pathological processes. Given the importance of protein repair and degradation, great effort has recently been made regarding the modulation of these systems in various physiological conditions such as aging, as well as in diseases. Genetic modulation has produced promising results in the area of protein repair enzymes but there are not yet any identified potent inhibitors, and, to our knowledge, only one activating compound has been reported so far. In contrast, different drugs as well as natural compounds that interfere with proteolysis have been identified and/or developed resulting in homeostatic maintenance and/or the delay of disease progression.

Lentivirus mediated delivery of neurosin promotes clearance of wild-type α-synuclein and reduces the pathology in an α-synuclein model of LBD

Neurosin is a predominant serine protease in the central nervous system (CNS) and has been shown to play a role in the clearance of α-synuclein (α-syn) which is centrally involved in the pathogenesis of Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Although it has been previously shown that neurosin and α-syn colocalize and that neurosin degrades α-syn aggregates in vitro, it is not clear if neurosin is dysregulated in the brains of patients with PD/DLB and to what extent delivery of neurosin into the CNS might ameliorate the deficits associated with α-syn accumulation in vivo. We analyzed the levels of neurosin in the brains of patients with PD/DLB and in α-syn transgenic (tg) models. With increased accumulation of α-syn, we observed decreased neurosin expression. Lentiviral vector (LV) driven expression of neurosin in neuronal cell cultures reduced the accumulation of wild type but not A53T α-syn and prevented α-syn associated toxicity. Neuropathological analysis following delivery of LV-Neurosin to α-syn tg mice resulted in reduced accumulation of α-syn and reversal of neurodegenerative alterations in wild type but not A53T α-syn tg mice. Therefore, viral vector driven expression of neurosin may warrant further investigation as a potential therapeutic tool for DLB.

Amelioration of amyloid β-induced cognitive deficits by Zataria multiflora Boiss. essential oil in a rat model of Alzheimer's disease

INTRODUCTION

The limitations of current Alzheimer's disease (AD) therapeutics have prompted investigation into innovative therapeutics focused on antiinflammatory, antioxidant, and neuroprotective agents including those from medicinal plants. Numerous plants have been tested for their potential for alleviating symptoms of AD.

AIMS

Zataria multiflora Boiss. (ZM) a member of Lamiaceae family has been used in Iranian traditional medicine for its beneficial effects on mental abilities. Therefore, the effect of its essential oil was evaluated in a rat model of AD.

METHODS

Amyloid β-protein (Aβ) fragment 25-35 was injected bilaterally in the CA1 region of rats hippocampus and the effect of different doses of ZM essential oil (50, 100, or 200 μL/kg) on cognitive function was investigated in the Morris water maze. Acute toxicity of the essential oil was also studied.

RESULTS

The results showed increases in escape latency, traveled distance, heading angle, and decreases in target quadrant entries in Aβ-received groups as compared to the control group. This impairment was reversed by ZM essential oil. The results of acute toxicity testing revealed that the calculated LD50 (1264.9 μL/kg) is much higher than the therapeutic dose (100 μL/kg).

CONCLUSIONS

It seems that antioxidant, antiinflammatory, and anticholinesterase activities of ZM or its main constituents might contribute to its beneficial effects in this model. Our findings suggest that ZM may be a potentially valuable source of natural therapeutic agents for the treatment of AD. However, further investigations are necessary to establish its clinical efficacy and potential toxicity, before any recommendations concerning its use as a medication in the treatment of AD.

Transsynaptic progression of amyloid-β-induced neuronal dysfunction within the entorhinal-hippocampal network

The entorhinal cortex (EC) is one of the earliest affected, most vulnerable brain regions in Alzheimer's disease (AD), which is associated with amyloid-β (Aβ) accumulation in many brain areas. Selective overexpression of mutant amyloid precursor protein (APP) predominantly in layer II/III neurons of the EC caused cognitive and behavioral abnormalities characteristic of mouse models with widespread neuronal APP overexpression, including hyperactivity, disinhibition, and spatial learning and memory deficits. APP/Aβ overexpression in the EC elicited abnormalities in synaptic functions and activity-related molecules in the dentate gyrus and CA1 and epileptiform activity in parietal cortex. Soluble Aβ was observed in the dentate gyrus, and Aβ deposits in the hippocampus were localized to perforant pathway terminal fields. Thus, APP/Aβ expression in EC neurons causes transsynaptic deficits that could initiate the cortical-hippocampal network dysfunction in mouse models and human patients with AD.

Countering amyloid polymorphism and drug resistance with minimal drug cocktails

Several fatal, progressive neurodegenerative diseases, including various prion and prion-like disorders, are connected with the misfolding of specific proteins. These proteins misfold into toxic oligomeric species and a spectrum of distinct self-templating amyloid structures, termed strains. Hence, small molecules that prevent or reverse these protein-misfolding events might have therapeutic utility. Yet it is unclear whether a single small molecule can antagonize the complete repertoire of misfolded forms encompassing diverse amyloid polymorphs and soluble oligomers. We have begun to investigate this issue using the yeast prion protein Sup35 as an experimental paradigm. We have discovered that a polyphenol, (-)epigallocatechin-3-gallate (EGCG), effectively inhibited the formation of infectious amyloid forms (prions) of Sup35 and even remodeled preassembled prions. Surprisingly, EGCG selectively modulated specific prion strains and even selected for EGCG-resistant prion strains with novel structural and biological characteristics. Thus, treatment with a single small molecule antagonist of amyloidogenesis can select for novel, drug-resistant amyloid polymorphs. Importantly, combining EGCG with another small molecule, 4,5-bis-(4-methoxyanilino)phthalimide, synergistically antagonized and remodeled a wide array of Sup35 prion strains without producing any drug-resistant prions. We suggest that minimal drug cocktails, small collections of drugs that collectively antagonize all amyloid polymorphs, should be identified to besiege various neurodegenerative disorders.

Allele-specific RNAi mitigates phenotypic progression in a transgenic model of Alzheimer's disease

Despite recent advances suggesting new therapeutic targets, Alzheimer's disease (AD) remains incurable. Aberrant production and accumulation of the Abeta peptide resulting from altered processing of the amyloid precursor protein (APP) is central to the pathogenesis of disease, particularly in dominantly inherited forms of AD. Thus, modulating the production of APP is a potential route to effective AD therapy. Here, we describe the successful use of an allele-specific RNA interference (RNAi) approach targeting the Swedish variant of APP (APPsw) in a transgenic mouse model of AD. Using recombinant adeno-associated virus (rAAV), we delivered an anti-APPsw short-hairpin RNA (shRNA) to the hippocampus of AD transgenic mice (APP/PS1). In short- and long-term transduction experiments, reduced levels of APPsw transprotein were observed throughout targeted regions of the hippocampus while levels of wild-type murine APP remained unaltered. Moreover, intracellular production of transfer RNA (tRNA)-valine promoter-driven shRNAs did not lead to detectable neuronal toxicity. Finally, long-term bilateral hippocampal expression of anti-APPsw shRNA mitigated abnormal behaviors in this mouse model of AD. The difference in phenotype progression was associated with reduced levels of soluble Abeta but not with a reduced number of amyloid plaques. Our results support the development of allele-specific RNAi strategies to treat familial AD and other dominantly inherited neurodegenerative diseases.

Neurofibrillary and neurodegenerative pathology in APP-transgenic mice injected with AAV2-mutant TAU: neuroprotective effects of Cerebrolysin

Alzheimer's disease (AD) continues to be the most common cause of cognitive and motor alterations in the aging population. Accumulation of amyloid beta (Abeta)-protein oligomers and the microtubule associated protein-TAU might be responsible for the neurological damage. We have previously shown that Cerebrolysin (CBL) reduces the synaptic and behavioral deficits in amyloid precursor protein (APP) transgenic (tg) mice by decreasing APP phosphorylation via modulation of glycogen synthase kinase-3beta (GSK3beta) and cyclin-dependent kinase-5 (CDK5) activity. These kinases also regulate TAU phosphorylation and are involved in promoting neurofibrillary pathology. In order to investigate the neuroprotective effects of CBL on TAU pathology, a new model for neurofibrillary alterations was developed using somatic gene transfer with adeno-associated virus (AAV2)-mutant (mut) TAU (P301L). The Thy1-APP tg mice (3 m/o) received bilateral injections of AAV2-mutTAU or AAV2-GFP, into the hippocampus. After 3 months, compared to non-tg controls, in APP tg mice intra-hippocampal injections with AAV2-mutTAU resulted in localized increased accumulation of phosphorylated TAU and neurodegeneration. Compared with vehicle controls, treatment with CBL in APP tg injected with AAV2-mutTAU resulted in a significant decrease in the levels of TAU phosphorylation at critical sites dependent on GSK3beta and CDK5 activity. This was accompanied by amelioration of the neurodegenerative alterations in the hippocampus. This study supports the concept that the neuroprotective effects of CBL may involve the reduction of TAU phosphorylation by regulating kinase activity.

Modulation of aggregate size- and shape-distributions of the amyloid-beta peptide by a designed beta-sheet breaker

A peptide with 42 amino acid residues (Abeta42) plays a key role in the pathogenesis of the Alzheimer's disease. It is highly prone to self aggregation leading to the formation of fibrils which are deposited in amyloid plaques in the brain of diseased individuals. In our study we established a method to analyze the aggregation behavior of the Abeta peptide with a combination of sedimentation velocity centrifugation and enhanced data evaluation software as implemented in the software package UltraScan. Important information which becomes accessible by this methodology is the s-value distribution and concomitantly also the shape-distribution of the Abeta peptide aggregates generated by self-association. With this method we characterized the aggregation modifying effect of a designed beta-sheet breaker molecule. This compound is built from three head-to-tail connected aminopyrazole moieties and represents a derivative of the already described Tripyrazole. By addition of this compound to a solution of the Abeta42 peptide the maximum of the s-value distribution was clearly shifted to smaller s-values as compared to solutions where only the vehicle DMSO was added. This shift to smaller s-values was stable for at least 7 days. The information about size- and shape-distributions present in aggregated Abeta42 solutions was confirmed by transmission electron microscopy and by measurement of amyloid formation by thioflavin T fluorescence.

Analysis of heterogeneity in molecular weight and shape by analytical ultracentrifugation using parallel distributed computing

A computational approach for fitting sedimentation velocity experiments from an analytical ultracentrifuge in a model-independent fashion is presented. This chapter offers a recipe for obtaining high-resolution information for both the shape and the molecular weight distributions of complex mixtures that are heterogeneous in shape and molecular weight and provides suggestions for experimental design to optimize information content. A combination of three methods is used to find the solution most parsimonious in parameters and to verify the statistical confidence intervals of the determined parameters. A supercomputer implementation with a MySQL database back end is integrated into the UltraScan analysis software. The UltraScan LIMS Web portal is used to perform the calculations through a Web interface. The performance and limitations of the method when employed for the analysis of complex mixtures are demonstrated using both simulated data and experimental data characterizing amyloid aggregation.

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

Background

Proteolytic degradation has emerged as a key pathway involved in controlling levels of the Alzheimer's 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.

Results

We 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.

Conclusion

These data support the possibility that long-term neprilysin gene therapy improves behavioral and neurodegenerative pathology by reducing intracellular Aβ.

Emerging amyloid beta (Ab) peptide modulators for the treatment of Alzheimer's disease (AD)

BACKGROUND

According to the 'amyloid cascade hypothesis' of Alzheimer's disease (AD), abnormal processing of beta-amyloid precursor protein (betaAPP) into toxic amyloid beta (Abeta)-peptides is central to the etiopathology of this uniquely human brain disorder.

OBJECTIVE

To review current AD drugs, pharmacological approaches and strategies aimed at modulating Abeta-peptide generation and/or aggregation in the treatment of AD.

METHODS

Data searches at various websites: Alzheimer Research Forum; individual drug company databases; Medline; Pharmaprojects database; unpublished research; inter-University research communications.

RESULTS/CONCLUSION

Considerable research effort has focused on secretase-mediated mechanisms of betaAPP processing, and the latest pharmacological strategies have used selective Abeta-peptide-lowering agents (SALA) to provide therapeutic benefit against Abeta-initiated neurodegenerative pathology. Currently, dedicated anticholinesterase, glutamatergic agonist and Abeta-peptide immunization have had little impact in the clinical treatment of AD. One unexpected benefit of statins (HMG-CoA inhibitors), besides their cholesterol lowering abilities, has been their ancillary effects in potentiating the enzymatic mechanisms that generate Abeta-peptides. The long-term benefits or complications of statin-based therapies for use in the clinical management of AD are not known.

Neuroprotective and neurotoxic properties of glial cells in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) affects more than 18 million people worldwide and is characterized by progressive memory deficits, cognitive impairment and personality changes. The main cause of AD is generally attributed to the increased production and accumulation of amyloid-β (Aβ), in association with neurofibrillary tangle (NFT) formation. Increased levels of pro-inflammatory factors such as cytokines and chemokines, and the activation of the complement cascade occurs in the brains of AD patients and contributes to the local inflammatory response triggered by senile plaque. The existence of an inflammatory component in AD is now well known on the basis of epidemiological findings showing a reduced prevalence of the disease upon long-term medication with anti-inflammatory drugs, and evidence from studies of clinical materials that shows an accumulation of activated glial cells, particularly microglia and astrocytes, in the same areas as amyloid plaques. Glial cells maintain brain plasticity and protect the brain for functional recovery from injuries. Dysfunction of glial cells may promote neurodegeneration and, eventually, the retraction of neuronal synapses, which leads to cognitive deficits. The focus of this review is on glial cells and their diversity properties in AD.