Human neprilysin is capable of degrading amyloid beta peptide not only in the monomeric form but also the pathological oligomeric form

Neprilysin-Mediated Amyloid Beta Clearance and Its Therapeutic Implications in Neurodegenerative Disorders

Neprilysin (NEP) is a neutral endopeptidase, important for the degradation of amyloid beta (Aβ) peptides and other neuropeptides, including enkephalins, substance P, and bradykinin, in the brain, that influences various physiological processes such as blood pressure homeostasis, pain perception, and neuroinflammation. NEP breaks down Aβ peptides into smaller fragments, preventing the development of detrimental aggregates such as Aβ plaques. NEP clears Aβ plaques predominantly by enzymatic breakdown in the extracellular space. However, NEP activity may be regulated by a variety of factors, including its expression and activity levels as well as interactions with other proteins or substances present in the brain. The Aβ de novo synthesis results from the amyloidogenic and nonamyloidogenic processing of the amyloid precursor protein (APP). In addition to Aβ synthesis, enzymatic degradation and various clearance pathways also contribute to the degradation of the monomeric form of Aβ peptides in the brain. Higher production, dysfunction of degradation enzymes, defective clearance mechanisms, intracellular accumulation of phosphorylated tau proteins, and extracellular deposition of Aβ are hallmarks of neurodegenerative diseases. Strategies for promoting NEP levels or activity, such as pharmaceutical interventions or gene therapy procedures, are being studied as possible therapies for neurodegenerative diseases including Alzheimer's disease. Therefore, in this perspective, we discuss the recent developments in NEP-mediated amyloidogenic and plausible mechanisms of nonamyloidogenic clearance of Aβ. We further highlight the current therapeutic interventions such as pharmaceutical agents, gene therapy, monoclonal antibodies, and stem-cell-based therapies targeting NEP for the management of neurodegenerative disorders.

Somatostatin: Linking Cognition and Alzheimer Disease to Therapeutic Targeting

Over 4 decades of research support the link between Alzheimer disease (AD) and somatostatin [somatotropin-releasing inhibitory factor (SRIF)]. SRIF and SRIF-expressing neurons play an essential role in brain function, modulating hippocampal activity and memory formation. Loss of SRIF and SRIF-expressing neurons in the brain rests at the center of a series of interdependent pathological events driven by amyloid-β peptide (Aβ), culminating in cognitive decline and dementia. The connection between the SRIF and AD further extends to the neuropsychiatric symptoms, seizure activity, and inflammation, whereas preclinical AD investigations show SRIF or SRIF receptor agonist administration capable of enhancing cognition. SRIF receptor subtype-4 activation in particular presents unique attributes, with the potential to mitigate learning and memory decline, reduce comorbid symptoms, and enhance enzymatic degradation of Aβ in the brain. Here, we review the links between SRIF and AD along with the therapeutic implications. SIGNIFICANCE STATEMENT: Somatostatin and somatostatin-expressing neurons in the brain are extensively involved in cognition. Loss of somatostatin and somatostatin-expressing neurons in Alzheimer disease rests at the center of a series of interdependent pathological events contributing to cognitive decline and dementia. Targeting somatostatin-mediated processes has significant therapeutic potential for the treatment of Alzheimer disease.

Association of MME gene polymorphisms with susceptibility to Alzheimer's disease in an Iranian population

Background

the MME gene encodes a membrane metalloendopeptidase, known as neprilysin (NEP). There are no reports on the potential implications of MME gene polymorphisms on the risk of Alzheimer's disease (AD) in the Iranian population. In this study, we studied the potential association of two single nucleotide polymorphisms (SNPs), rs6797911 and rs3736187, in the MME gene and the risk of developing AD in an Iranian population.

Methods

This case-control study comprised 120 AD-diagnosed patients and 120 healthy individuals without any prior family history of AD. The patient and control groups were matched for major demographic and health characteristics. Genotyping was performed by amplification refractory mutation system-polymerase chain reaction (ARMS-PCR).

Results

All patients included in this study were assessed by an experienced neurologist to exclude cases with other forms of dementia based on a brain computed tomography scan and other clinical findings. There were no significant differences in demographic and health characteristics including sex, diabetes, blood pressure, and cigarette smoking status between case and control groups (p > 0.05). However, the age difference appeared significant. Both SNPs were significantly associated with the risk of AD in our study population. The rs3736187 (T > C, 3:155168489) was strongly associated with AD risk under the log-additive model (OR = 1.67, CI = 1.18-2.37, p-value = 0.003). The rs6797911 (T > A, 3:155144601) also showed a significant association with AD risk under the dominant model (TT vs. TA and AA, OR = 3.37, CI = 1.86-6.1, p-value <0.001).

Conclusion

There is a strong association between MME gene polymorphisms and susceptibility to AD in the Iranian population. Amyloid-β (Aβ) can serve as a substrate for the NEP metalloendopeptidase, the product of the MME gene. However, the mechanistic understanding of how these genetic variations affect NEP expression, function, and consequently susceptibility to AD, is poorly understood. Further research is required to fully understand the exact implication of MME gene variations on AD, particularly in a larger, ethnicity-diverse population.

Molecular basis of selective amyloid-β degrading enzymes in Alzheimer's disease

The accumulation of the small 42-residue long peptide amyloid-β (Aβ) has been proposed as a major trigger for the development of Alzheimer's disease (AD). Within the brain, the concentration of Aβ peptide is tightly controlled through production and clearance mechanisms. Substantial experimental evidence now shows that reduced levels of Aβ clearance are present in individuals living with AD. This accumulation of Aβ can lead to the formation of large aggregated amyloid plaques-one of two detectable hallmarks of the disease. Aβ-degrading enzymes (ADEs) are major players in the clearance of Aβ. Stimulating ADE activity or expression, in order to compensate for the decreased clearance in the AD phenotype, provides a promising therapeutic target. It has been reported in mice that upregulation of ADEs can reduce the levels of Aβ peptide and amyloid plaques-in some cases, this led to improved cognitive function. Among several known ADEs, neprilysin (NEP), endothelin-converting enzyme-1 (ECE-1), insulin degrading enzyme (IDE) and angiotensin-1 converting enzyme (ACE) from the zinc metalloprotease family have been identified as important. These ADEs have the capacity to digest soluble Aβ which, in turn, cannot form the toxic oligomeric species. While they are known for their amyloid degradation, they exhibit complexity through promiscuous nature and a broad range of substrates that they can degrade. This review highlights current structural and functional understanding of these key ADEs, giving some insight into the molecular interactions that leads to the hydrolysis of peptide substrates, the crucial tasks performed by them and the potential for therapeutic use in the future.

Zinc utilization by microglia in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia defined by two key pathological characteristics in the brain, amyloid-β (Aβ) plaques and neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau. Microglia, the primary innate immune cells of the central nervous system (CNS), provide neuroprotection through Aβ and tau clearance but may also be neurotoxic by promoting neuroinflammation to exacerbate Aβ and tau pathogenesis in AD. Recent studies have demonstrated the importance of microglial utilization of nutrients and trace metals in controlling their activation and effector functions. Trace metals, such as zinc, have essential roles in brain health and immunity, and zinc dyshomeostasis has been implicated in AD pathogenesis. As a result of these advances, the mechanisms by which zinc homeostasis influences microglial-mediated neuroinflammation in AD is a topic of continuing interest since new strategies to treat AD are needed. Here, we review the roles of zinc in AD, including zinc activation of microglia, the associated neuroinflammatory response, and the application of these findings in new therapeutic strategies.

Broken but not beaten: Challenge of reducing the amyloids pathogenicity by degradation

BACKGROUND

The accumulation of ordered protein aggregates, amyloid fibrils, accompanies various neurodegenerative diseases (such as Parkinson's, Huntington's, Alzheimer's, etc.) and causes a wide range of systemic and local amyloidoses (such as insulin, hemodialysis amyloidosis, etc.). Such pathologies are usually diagnosed when the disease is already irreversible and a large amount of amyloid plaques have accumulated. In recent years, new drugs aimed at reducing amyloid levels have been actively developed. However, although clinical trials have demonstrated a reduction in amyloid plaque size with these drugs, their effect on disease progression has been controversial and associated with significant side effects, the reasons of which are not fully understood.

AIM OF REVIEW

The purpose of this review is to summarize extensive array of data on the effect of exogenous and endogenous factors (physico-mechanical effects, chemical effects of low molecular weight compounds, macromolecules and their complexes) on the structure and pathogenicity of mature amyloids for proposing future directions of the development of effective and safe anti-amyloid therapeutics.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Our analysis show that destruction of amyloids is in most cases incomplete and degradation products often retain the properties of amyloids (including high and sometimes higher than fibrils, cytotoxicity), accelerate amyloidogenesis and promote the propagation of amyloids between cells. Probably, the appearance of protein aggregates, polymorphic in structure and properties (such as amorphous aggregates, fibril fragments, amyloid oligomers, etc.), formed because of uncontrolled degradation of amyloids, may be one of the reasons for the ambiguous effectiveness and serious side effects of the anti-amyloid drugs. This means that all medications that are supposed to be used both for degradation and slow down the fibrillogenesis must first be tested on mature fibrils: the mechanism of drug action and cytotoxic, seeding, and infectious activity of the degradation products must be analyzed.

Effects of DHA (omega-3 fatty acid) and estradiol on amyloid β-peptide regulation in the brain

In the early stages of sporadic Alzheimer's disease (SAD), there is a strong correlation between memory impairment and cortical levels of soluble amyloid-β peptide oligomers (Aβ). It has become clear that Aβ disrupt glutamatergic synaptic function, which can in turn lead to the characteristic cognitive deficits of SAD, but the actual pathways are still not well understood. This opinion article describes the pathogenic mechanisms underlying cerebral amyloidosis. These mechanisms are dependent on the amyloid precursor protein and concern the synthesis of Aβ peptides with competition between the non-amyloidogenic pathway and the amyloidogenic pathway (i.e. a competition between the ADAM10 and BACE1 enzymes), on the one hand, and the various processes of Aβ residue clearance, on the other hand. This clearance mobilizes both endopeptidases (NEP, and IDE) and removal transporters across the blood-brain barrier (LRP1, ABCB1, and RAGE). Lipidated ApoE also plays a major role in all processes. The disturbance of these pathways induces an accumulation of Aβ. The description of the mechanisms reveals two key molecules in particular: (i) free estradiol, which has genomic and non-genomic action, and (ii) free DHA as a preferential ligand of PPARα-RXRα and PPARɣ-RXRα heterodimers. DHA and free estradiol are also self-regulating, and act in synergy. When a certain level of chronic DHA and free estradiol deficiency is reached, a permanent imbalance is established in the central nervous system. The consequences of these deficits are revealed in particular by the presence of Aβ peptide deposits, as well as other markers of the etiology of SAD.

Modelling Alzheimer's disease in a dish: dissecting amyloid-β metabolism in human neurons

This scientific commentary refers to 'Inhibition of insulin-degrading enzyme in human neurons promotes amyloid-β deposition' by Rowland et al. (https://doi.org/10.1042/NS20230016). Insulin-degrading enzyme (IDE) and neprilysin (NEP) have been proposed as two Aβ-degrading enzymes supported by human genetics and in vivo data. Rowland et al. provide complementary evidence of a key role for IDE in Aβ metabolism in human-induced pluripotent stem cell (iPSC)-derived cortical neurons.

Neprilysin inhibitors and risk of Alzheimer's disease: A future perspective

Alzheimer's disease (AD) is a heterogeneous neurodegenerative disease with multifaceted neuropathological disorders. AD is characterized by intracellular accumulation of phosphorylated tau proteins and extracellular deposition of amyloid beta (Aβ). Various protease enzymes, including neprilysin (NEP), are concerned with the degradation and clearance of Aβ. Indeed, a defective neuronal clearance pathway due to the dysfunction of degradation enzymes might be a possible mechanism for the accumulation of Aβ and subsequent progression of AD neuropathology. NEP is one of the most imperative metalloproteinase enzymes involved in the clearance of Aβ. This review aimed to highlight the possible role of NEP inhibitors in AD. The combination of sacubitril and valsartan which is called angiotensin receptor blocker and NEP inhibitor (ARNI) may produce beneficial and deleterious effects on AD neuropathology. NEP inhibitors might increase the risk of AD by the inhibition of Aβ clearance, and increase brain bradykinin (BK) and natriuretic peptides (NPs), which augment the pathogenesis of AD. These verdicts come from animal model studies, though they may not be applied to humans. However, clinical studies revealed promising safety findings regarding the use of ARNI. Moreover, NEP inhibition increases various neuroprotective peptides involved in inflammation, glucose homeostasis and nerve conduction. Also, NEP inhibitors may inhibit dipeptidyl peptidase 4 (DPP4) expression, ameliorating insulin and glucagon-like peptide 1 (GLP-1) levels. These findings proposed that NEP inhibitors may have a protective effect against AD development by increasing GLP-1, neuropeptide Y (NPY) and substance P, and deleterious effects by increasing brain BK. Preclinical and clinical studies are recommended in this regard.

The effects of enhancing angiotensin converting enzyme in myelomonocytes on ameliorating Alzheimer's-related disease and preserving cognition

This review examines the role of angiotensin-converting enzyme (ACE) in the context of Alzheimer's disease (AD) and its potential therapeutic value. ACE is known to degrade the neurotoxic 42-residue long alloform of amyloid β-protein (Aβ42), a peptide strongly associated with AD. Previous studies in mice, demonstrated that targeted overexpression of ACE in CD115+ myelomonocytic cells (ACE10 models) improved their immune responses to effectively reduce viral and bacterial infection, tumor growth, and atherosclerotic plaque. We further demonstrated that introducing ACE10 myelomonocytes (microglia and peripheral monocytes) into the double transgenic APPSWE/PS1ΔE9 murine model of AD (AD+ mice), diminished neuropathology and enhanced the cognitive functions. These beneficial effects were dependent on ACE catalytic activity and vanished when ACE was pharmacologically blocked. Moreover, we revealed that the therapeutic effects in AD+ mice can be achieved by enhancing ACE expression in bone marrow (BM)-derived CD115+ monocytes alone, without targeting central nervous system (CNS) resident microglia. Following blood enrichment with CD115+ ACE10-monocytes versus wild-type (WT) monocytes, AD+ mice had reduced cerebral vascular and parenchymal Aβ burden, limited microgliosis and astrogliosis, as well as improved synaptic and cognitive preservation. CD115+ ACE10-versus WT-monocyte-derived macrophages (Mo/MΦ) were recruited in higher numbers to the brains of AD+ mice, homing to Aβ plaque lesions and exhibiting a highly Aβ-phagocytic and anti-inflammatory phenotype (reduced TNFα/iNOS and increased MMP-9/IGF-1). Moreover, BM-derived ACE10-Mo/MΦ cultures had enhanced capability to phagocytose Aβ42 fibrils, prion-rod-like, and soluble oligomeric forms that was associated with elongated cell morphology and expression of surface scavenger receptors (i.e., CD36, Scara-1). This review explores the emerging evidence behind the role of ACE in AD, the neuroprotective properties of monocytes overexpressing ACE and the therapeutic potential for exploiting this natural mechanism for ameliorating AD pathogenesis.

Novel developments in non-contrast enhanced MRI of the perivascular clearance system: What are the possibilities for Alzheimer's disease research?

The cerebral waste clearance system (i.e, glymphatic or intramural periarterial drainage) works through a network of perivascular spaces (PVS). Dysfunction of this system likely contributes to aggregation of Amyloid-β and subsequent toxic plaques in Alzheimer's disease (AD). A promising, non-invasive technique to study this system is MRI, though applications in dementia are still scarce. This review focusses on recent non-contrast enhanced (non-CE) MRI techniques which determine and visualise physiological aspects of the clearance system at multiple levels, i.e., cerebrospinal fluid flow, PVS-flow and interstitial fluid movement. Furthermore, various MRI studies focussing on aspects of the clearance system which are relevant to AD are discussed, such as studies on ageing, sleep alterations, and cognitive decline. Additionally, the complementary function of non-CE to CE methods is elaborated upon. We conclude that non-CE studies have great potential to determine which parts of the waste clearance system are affected by AD and in which stages of cognitive impairment dysfunction of this system occurs, which could allow future clinical trials to target these specific mechanisms.

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.

Poly(I:C) promotes neurotoxic amyloid β accumulation through reduced degradation by decreasing neprilysin protein levels in astrocytes

Inflammation associated with viral infection of the nervous system has been involved in the pathogenesis of neurodegenerative diseases, such as Alzheimer's disease (AD) and multiple sclerosis. Polyinosinic:polycytidylic acid (poly[I:C]) is a Toll-like receptor 3 (TLR3) agonist that mimics the inflammatory response to systemic viral infections. Despite growing recognition of the role of glial cells in AD pathology, their involvement in the accumulation and clearance of amyloid β (Aβ) in the brain of patients with AD is poorly understood. Neprilysin (NEP) and insulin-degrading enzyme (IDE) are the main Aβ-degrading enzymes in the brain. This study investigated whether poly(I:C) regulated Aβ degradation and neurotoxicity by modulating NEP and IDE protein levels through TLR3 in astrocytes. To this aim, primary rat primary astrocyte cultures were treated with poly(I:C) and inhibitors of the TLR3 signaling. Protein levels were assessed by Western blot. Aβ toxicity to primary neurons was measured by lactate dehydrogenase release. Poly(I:C) induced a significant decrease in NEP levels on the membrane of astrocytes as well as in the culture medium. The degradation of exogenous Aβ was markedly delayed in poly(I:C)-treated astrocytes. This delay significantly increased the neurotoxicity of exogenous Aβ1-42. Altogether, these results suggest that viral infections induce Aβ neurotoxicity by decreasing NEP levels in astrocytes and consequently preventing Aβ degradation.

Based on molecular structures: Amyloid-β generation, clearance, toxicity and therapeutic strategies

Amyloid-β (Aβ) has long been considered as one of the most important pathogenic factors in Alzheimer’s disease (AD), but the specific pathogenic mechanism of Aβ is still not completely understood. In recent years, the development of structural biology technology has led to new understandings about Aβ molecular structures, Aβ generation and clearance from the brain and peripheral tissues, and its pathological toxicity. The purpose of the review is to discuss Aβ metabolism and toxicity, and the therapeutic strategy of AD based on the latest progress in molecular structures of Aβ. The Aβ structure at the atomic level has been analyzed, which provides a new and refined perspective to comprehend the role of Aβ in AD and to formulate therapeutic strategies of AD.

Comparative studies for amyloid beta degradation: “Neprilysin vs insulysin”, “monomeric vs aggregate”, and “whole Aβ40 vs its peptide fragments”

Amyloid beta (Aβ) proteins are produced from amyloid precursor protein cleaved by β- and γ-secretases, and are the main components of senile plaques pathologically found in Alzheimer's disease (AD) patient brains. Therefore, the relationship between AD and Aβs has been well studied for both therapeutic and diagnostic purposes. Several enzymes have been reported to degrade Aβs in vivo, with neprilysin (NEP) and insulysin (insulin-degrading enzyme, IDE) being the most prominent. In this article, we describe the mass spectrometric characterization of peptide fragments generated using NEP and IDE, and clarify the differences in digestion specificities between these two enzymes for non-aggregated Aβ₄₀, aggregated Aβ₄₀, and Aβ₄₀ peptide fragments, including Aβ₁₆. Our results allowed identification of all the peptide fragments from non-aggregated Aβ₄₀: NEP, 23 peptide fragments consisting of 2–11 amino-acid residues, 17 cleavage sites; IDE, 23 peptide fragments consisting of 6–33 amino-acid residues, 15 cleavage sites. Also, we confirmed that IDE can digest only whole Aβ₄₀, whereas NEP can digest both Aβ₄₀ and partial structures such as Aβ₁₆ and peptide fragments generated by the digestion of Aβ₄₀ by IDE. Furthermore, we confirmed that IDE and NEP are unable to digest aggregated Aβ₄₀.

•

Two Aβ degrading enzymes (neprilysin, NEP and insulysin, IDE) were examined.

•

NEP digested Aβ₄₀ into 23 peptides consisting of 2–11 amino-acid residues.

•

IDE digested Aβ₄₀ into 23 peptides consisting of 6–33 amino-acid residues.

•

Neither IDE nor NEP can digest aggregated Aβ₄₀.

•

NEP can digest Aβ fragment peptides including Aβ₁₆, although IDE cannot.

Prostate cancer treatment and the relationship of androgen deprivation therapy to cognitive function

Prostate cancer is the second most common form of cancer in men. For advanced, high risk prostate cancer, androgen deprivation therapy (ADT) is the preferred treatment and can induce remission, but resistance to ADT brings biochemical recurrence and progression of cancer. ADT brings adverse effects such as erectile dysfunction, decreased libido, and diminished physical strength. It is estimated that between 25 and 50% of men on ADT manifest some form of cognitive dysfunction that may be self-reported or reported by a family member. There is concern that impaired cognitive function with ADT is due to loss of testosterone support. Testosterone and its metabolites are known to possess neuroprotective properties. While a direct causal relationship between ADT and cognitive decline in prostate cancer patients has not been established, this review describes the controversy surrounding the possible connection between ADT and neurocognitive deterioration. The cellular and molecular mechanisms believed to underlie the protection of neuronal integrity by androgens are discussed. Results from animal models and human clinical studies are presented. Finally, we call attention to lifestyle modifications that may minimize cognitive issues in prostate cancer patients.

Toward a New Paradigm for Targeted Natriuretic Peptide Enhancement in Heart Failure

The natriuretic peptide system (NPS) plays a fundamental role in maintaining cardiorenal homeostasis, and its potent filling pressure-regulated diuretic and vasodilatory effects constitute a beneficial compensatory mechanism in heart failure (HF). Leveraging the NPS for therapeutic benefit in HF has been the subject of intense investigation during the last three decades and has ultimately reached widespread clinical use in the form of angiotensin receptor-neprilysin inhibition (ARNi). NPS enhancement via ARNi confers beneficial effects on mortality and hospitalization in HF, but inhibition of neprilysin leads to the accumulation of a number of other vasoactive peptides in the circulation, often resulting in hypotension and raising potential concerns over long-term adverse effects. Moreover, ARNi is less effective in the large group of HF patients with preserved ejection fraction. Alternative approaches for therapeutic augmentation of the NPS with increased specificity and efficacy are therefore warranted, and are now becoming feasible particularly with recent development of RNA therapeutics. In this review, the current state-of-the-art in terms of experimental and clinical strategies for NPS augmentation and their implementation will be reviewed and discussed.

Aβ31-35 Decreases Neprilysin-Mediated Alzheimer's Amyloid-β Peptide Degradation

Alzheimer's disease is associated with the deposition of extracellular senile plaques, made primarily of amyloid-β (Aβ), particularly peptides Aβ_1-42 and Aβ_1-40. Neprilysin, or neutral endopeptidase (NEP), catalyzes proteolysis of the amyloid peptides (Aβ) and is recognized as one of the major regulators of the levels of these peptides in the brain, preventing Aβ accumulation and plaque formation. Here, we used a combination of techniques to elucidate the mechanism of Aβ binding and cleavage by NEP. Our findings indicate that the Aβ_31-X cleavage products remain bound to the neprilysin active site, reducing proteolytic activity. Interestingly, it was already shown that this Aβ_31-35 sequence is also critical for recognition of Aβ peptides by other targets, such as the serpin-enzyme complex receptor in neuronal cells.

Synthesis and structure-activity relationships of 3,4,5-trisubstituted-1,2,4-triazoles: high affinity and selective somatostatin receptor-4 agonists for Alzheimer's disease treatment

Somatostatin receptor-4 (SST₄) is highly expressed in brain regions affiliated with learning and memory. SST₄ agonist treatment may act to mitigate Alzheimer's disease (AD) pathology. An integrated approach to SST₄ agonist lead optimization is presented herein. High affinity and selective agonists with biological efficacy were identified through iterative cycles of a structure-based design strategy encompassing computational methods, chemistry, and preclinical pharmacology. 1,2,4-Triazole derivatives of our previously reported hit (4) showed enhanced SST₄ binding affinity, activity, and selectivity. Thirty-five compounds showed low nanomolar range SST₄ binding affinity, 12 having a K _i < 1 nM. These compounds showed >500-fold affinity for SST₄ as compared to SST_2A. SST₄ activities were consistent with the respective SST₄ binding affinities (EC₅₀ < 10 nM for 34 compounds). Compound 208 (SST₄ K _i = 0.7 nM; EC₅₀ = 2.5 nM; >600-fold selectivity over SST_2A) display a favorable physiochemical profile, and was advanced to learning and memory behavior evaluations in the senescence accelerated mouse-prone 8 model of AD-related cognitive decline. Chronic administration enhanced learning with i.p. dosing (1 mg kg^-1) compared to vehicle. Chronic administration enhanced memory with both i.p. (0.01, 0.1, 1 mg kg^-1) and oral (0.01, 10 mg kg^-1) dosing compared to vehicle. This study identified a novel series of SST₄ agonists with high affinity, selectivity, and biological activity that may be useful in the treatment of AD.

Redox-Active Metal Ions and Amyloid-Degrading Enzymes in Alzheimer's Disease

Redox-active metal ions, Cu(I/II) and Fe(II/III), are essential biological molecules for the normal functioning of the brain, including oxidative metabolism, synaptic plasticity, myelination, and generation of neurotransmitters. Dyshomeostasis of these redox-active metal ions in the brain could cause Alzheimer’s disease (AD). Thus, regulating the levels of Cu(I/II) and Fe(II/III) is necessary for normal brain function. To control the amounts of metal ions in the brain and understand the involvement of Cu(I/II) and Fe(II/III) in the pathogenesis of AD, many chemical agents have been developed. In addition, since toxic aggregates of amyloid-β (Aβ) have been proposed as one of the major causes of the disease, the mechanism of clearing Aβ is also required to be investigated to reveal the etiology of AD clearly. Multiple metalloenzymes (e.g., neprilysin, insulin-degrading enzyme, and ADAM10) have been reported to have an important role in the degradation of Aβ in the brain. These amyloid degrading enzymes (ADE) could interact with redox-active metal ions and affect the pathogenesis of AD. In this review, we introduce and summarize the roles, distributions, and transportations of Cu(I/II) and Fe(II/III), along with previously invented chelators, and the structures and functions of ADE in the brain, as well as their interrelationships.

Hypothesis: Intravenous administration of mesenchymal stem cells is effective in the treatment of Alzheimer's disease

We propose the intravenous administration of autologous adipose-derived stem cells as a new treatment for Alzheimer's disease. We hypothesize that the stem cells will secrete neprilysin in the brain to break down and remove amyloid deposits in the Alzheimer's brain. We have shown a case of skin amyloid deposition that disappeared after stem cell administration and confirmed that the stem cells administered had neprilysin activity. In addition to neprilysin secretion, other mechanisms of action of stem cells include nerve regeneration, nerve repair, growth factor secretion, anti-inflammatory effects, and angiogenesis. The harvesting of adipose-derived stem cells is minimally invasive, and intravenous administration can be safely repeated. We hope that the efficacy of this new treatment will be verified and that it will bring a ray of hope to patients suffering from this incurable disease.

Mechanisms and therapeutic potential of interactions between human amyloids and viruses

The aggregation of specific proteins and their amyloid deposition in affected tissue in disease has been studied for decades assuming a sole pathogenic role of amyloids. It is now clear that amyloids can also encode important cellular functions, one of which involves the interaction potential of amyloids with microbial pathogens, including viruses. Human expressed amyloids have been shown to act both as innate restriction molecules against viruses as well as promoting agents for viral infectivity. The underlying molecular driving forces of such amyloid-virus interactions are not completely understood. Starting from the well-described molecular mechanisms underlying amyloid formation, we here summarize three non-mutually exclusive hypotheses that have been proposed to drive amyloid-virus interactions. Viruses can indirectly drive amyloid depositions by affecting upstream molecular pathways or induce amyloid formation by a direct interaction with the viral surface or specific viral proteins. Finally, we highlight the potential of therapeutic interventions using the sequence specificity of amyloid interactions to drive viral interference.

The Association between Neprilysin gene polymorphisms and Alzheimer's disease in Tibetan population

OBJECTIVES

Alzheimer's disease (AD) is a well-known neurodegenerative disease, of which the hallmark is the disposition of β-amyloid (Aβ) in the form of plaque in the brain. Neprilysin (NEP) is the major enzyme to degrade Aβ and prevent accumulation of Aβ. The present study was undertaken to elucidate the correlation between the NEP gene polymorphisms and AD in Chinese Tibetan population.

METHODS

Ninety-nine sporadic AD Tibetan patients and 113 healthy Tibetan controls were enrolled in this study. The genotype frequencies and allele frequencies of multiple NEP gene loci were analyzed using the case-control association analysis.

RESULTS

No significant correlation was found between polymorphisms of NEP gene loci (rs9829757, rs1816558, rs6776185, rs3736187, rs701109, rs989692) and the occurrence of AD in Tibetan population. However, allele C of NEP gene locus (rs701109) and allele T of gene locus (rs3736187) were possible risk factors of male AD patients in Tibetan population.

CONCLUSIONS

NEP gene loci (rs701109, rs989692, rs9829757, rs3736187, rs1816558, rs6776185) were polymorphic in Tibetan population. No difference was found between these loci but for that male gender combined with allele C of NEP gene locus (rs701109) and T of gene locus (rs3736187) might be risk factors for AD in Tibet.

A Fundamental Role for Oxidants and Intracellular Calcium Signals in Alzheimer's Pathogenesis-And How a Comprehensive Antioxidant Strategy May Aid Prevention of This Disorder

Oxidative stress and increased cytoplasmic calcium are key mediators of the detrimental effects on neuronal function and survival in Alzheimer's disease (AD). Pathways whereby these perturbations arise, and then prevent dendritic spine formation, promote tau hyperphosphorylation, further amplify amyloid β generation, and induce neuronal apoptosis, are described. A comprehensive program of nutraceutical supplementation, comprised of the NADPH oxidase inhibitor phycocyanobilin, phase two inducers, the mitochondrial antioxidant astaxanthin, and the glutathione precursor N-acetylcysteine, may have important potential for antagonizing the toxic effects of amyloid β on neurons and thereby aiding prevention of AD. Moreover, nutraceutical antioxidant strategies may oppose the adverse impact of amyloid β oligomers on astrocyte clearance of glutamate, and on the ability of brain capillaries to export amyloid β monomers/oligomers from the brain. Antioxidants, docosahexaenoic acid (DHA), and vitamin D, have potential for suppressing microglial production of interleukin-1β, which potentiates the neurotoxicity of amyloid β. Epidemiology suggests that a health-promoting lifestyle, incorporating a prudent diet, regular vigorous exercise, and other feasible measures, can cut the high risk for AD among the elderly by up to 60%. Conceivably, complementing such lifestyle measures with long-term adherence to the sort of nutraceutical regimen outlined here may drive down risk for AD even further.

Examining the effects of ovarian hormone loss and diet-induced obesity on Alzheimer's disease markers of amyloid-β production and degradation

After menopause, women experience declines in ovarian sex hormones, an event that has recently been associated with increased amyloid-β peptides, a main feature of Alzheimer's disease. Diet-induced insulin resistance also increases amyloid-β peptides; however, whether this process is exacerbated with ovarian sex hormone loss remains unknown. Female C57BL6/J mice received either bilateral ovariectomy (OVX; n = 20) or remained intact (n = 20) at 24 wk of age and were placed on either a low- or high-fat diet (LFD, n = 10 for OVX and intact; HFD, n = 10 for OVX and intact) for 10 wk. Independently, OVX led to increases in the amyloidogenic marker, soluble amyloid precursor protein β (sAPPβ). The HFD in combination with OVX led to lower insulin degrading enzyme (IDE) protein content and activity in the prefrontal cortex, indicative of decreased amyloid-β degradation; however, no differences in amyloid-β content were observed. Data from this study provide novel evidence of independent effects of peripheral insulin resistance and ovarian sex hormone loss in decreasing brain markers of amyloid-β degradation. Furthermore, findings indicate how the loss of ovarian sex hormones can promote the formation of amyloidogenic APP cleavage products, independent of diet-induced insulin resistance.NEW & NOTEWORTHY This study provides novel insight into the effect of peripheral insulin resistance and ovarian hormone loss in decreasing brain markers of amyloid-β degradation. Results demonstrate that ovarian hormone loss through ovariectomy increased the amyloidogenic marker, sAPPβ, while the high-fat diet in combination with ovariectomy led to lower IDE protein content and activity in the prefrontal cortex, indicative of decreased amyloid-β degradation. These original results provide important information for future targets in early AD pathogenesis.

Modulation of β-Amyloid Fibril Formation in Alzheimer's Disease by Microglia and Infection

Amyloid plaques are a pathological hallmark of Alzheimer's disease. The major component of these plaques are highly ordered amyloid fibrils formed by amyloid-β (Aβ) peptides. However, whilst Aβ amyloid fibril assembly has been subjected to detailed and extensive analysis in vitro, these studies may not reproduce how Aβ fibrils assemble in the brain. This is because the brain represents a highly complex and dynamic environment, and in Alzheimer's disease multiple cofactors may affect the assembly of Aβ fibrils. Moreover, in vivo amyloid plaque formation will reflect the balance between the assembly of Aβ fibrils and their degradation. This review explores the roles of microglia as cofactors in Aβ aggregation and in the clearance of amyloid deposits. In addition, we discuss how infection may be an additional cofactor in Aβ fibril assembly by virtue of the antimicrobial properties of Aβ peptides. Crucially, by understanding the roles of microglia and infection in Aβ amyloid fibril assembly it may be possible to identify new therapeutic targets for Alzheimer's disease.

Target Enzymes Considered for the Treatment of Alzheimer's Disease and Parkinson's Disease

Various amyloidogenic proteins have been suggested to be involved in the onset and progression of neurodegenerative diseases (ND) such as Alzheimer's disease (AD) and Parkinson's disease (PD). Particularly, the aggregation of misfolded amyloid-β and hyperphosphorylated tau and α-synuclein are linked to the pathogenesis of AD and PD, respectively. In order to care the diseases, multiple small molecules have been developed to regulate the aggregation pathways of these amyloid proteins. In addition to controlling the aggregation of amyloidogenic proteins, maintaining the levels of the proteins in the brain by amyloid degrading enzymes (ADE; neprilysin (NEP), insulin-degrading enzyme (IDE), asparagine endopeptidase (AEP), and ADAM10) is also essential to cure AD and PD. Therefore, numerous biological molecules and chemical agents have been investigated as either inducer or inhibitor against the levels and activities of ADE. Although the side effect of enhancing the activity of ADE could occur, the removal of amyloidogenic proteins could result in a relatively good strategy to treat AD and PD. Furthermore, since the causes of ND are diverse, various multifunctional (multitarget) chemical agents have been designed to control the actions of multiple risk factors of ND, including amyloidogenic proteins, metal ions, and reactive oxygen species. Many of them, however, were invented without considerations of regulating ADE levels and actions. Incorporation of previously created molecules with the chemical agents handling ADE could be a promising way to treat AD and PD. This review introduces the ADE and molecules capable of modulating the activity and expression of ADE.

Molecular Level Insight Into the Benefit of Myricetin and Dihydromyricetin Uptake in Patients With Alzheimer's Diseases

Alzheimer's disease (AD) is a neurodegenerative disease with a high incidence rate and complicated pathogenesis. Currently, all anti-AD drugs treat the symptoms of the disease, and with currently no cure for AD. Flavonoid containing natural products, Myricetin (MYR) and Dihydromyricetin (DMY), are abundant in fruits and vegetables, and have been approved as food supplements in some countries. Interestingly, MYR and DMY have been reported to have anti-AD effects. However, the underlying anti-AD mechanism of action of MYR and DMY is complex with many facets being identified. In this review, we explore the benefit of MYR and DMY in AD patients from a molecular level. Their mechanism of action are discussed from various aspects including amyloid β-protein (Aβ) imbalance, neuroinflammation, dyshomeostasis of metal ions, autophagy disorder, and oxidative stress.

Mitochondrial Dysfunction and Alzheimer's Disease: Role of Microglia

In 1907, Alois Alzheimer observed, as he quoted, development of "numerous fibers" and "adipose saccules" in the brain of his diseased patient Auguste Deter. The neurodegenerative disease became known as Alzheimer's disease (AD) and is the most common cause of dementia worldwide. AD normally develops with aging and is mostly initiated because of the imbalance between the formation and clearance of amyloid-β (Aβ). Formation of neurofibrillary tangles (NFTs) of hyperphosphorylated tau is another hallmark of AD. Neuroinflammation plays a significant role in the development and pathology of AD. This chapter explores the role of mitochondrial dysfunction in microglia in case of AD. Mitochondrial oxidative stress in microglia has been linked to the development of AD. Elevated generation of reactive oxygen species (ROS) and loss of mitochondrial membrane potential through various mechanisms have been observed in AD. Aβ interacts with microglial receptors, such as triggering receptor expressed in myeloid cells 2 (TREM2), activating downstream pathways causing mitochondrial damage and aggravating inflammation and cytotoxicity. Fibrillar Aβ activates nicotinamide adenine dinucleotide phosphate (NADPH) oxidase in microglia leading to elevated induction of mitochondrial ROS which further causes neurotoxicity. Elevated ROS in microglia causes activation of inflammatory and cell death pathways. Production of ATP, regulation of mitochondrial health, autophagy, and mitophagy in microglia play significant roles in the AD pathology. Understanding microglial physiology and mitochondrial dysfunction will enable better therapeutic interventions.

The Role of Neurovascular System in Neurodegenerative Diseases

The neurovascular system (NVS), which consisted of neurons, glia, and vascular cells, is a functional and structural unit of the brain. The NVS regulates blood-brain barrier (BBB) permeability and cerebral blood flow (CBF), thereby maintaining the brain's microenvironment for normal functioning, neuronal survival, and information processing. Recent studies have highlighted the role of vascular dysfunction in several neurodegenerative diseases. This is not unexpected since both nervous and vascular systems are functionally interdependent and show close anatomical apposition, as well as similar molecular pathways. However, despite extensive research, the precise mechanism by which neurovascular dysfunction contributes to neurodegeneration remains incomplete. Therefore, understanding the mechanisms of neurovascular dysfunction in disease conditions may allow us to develop potent and effective therapies for prevention and treatment of neurodegenerative diseases. This review article summarizes the current research in the context of neurovascular signaling associated with neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD). We also discuss the potential implication of neurovascular factor as a novel therapeutic target and prognostic marker in patients with neurodegenerative conditions. Graphical Abstract.

Neuroinflammation drives APOE genotype-dependent differential expression of neprilysin

Alzheimer's disease (AD) is a devastating neurodegenerative disorder characterized by the deposition of amyloid-beta (Aβ) plaques and widespread neuroinflammation. While the cause of AD remains unknown, multiple factors likely contribute to the disease, including heart disease, diabetes, previous head injury, as well as a number of genetic determinants. Inheritance of the apolipoprotein (APOE) ε4 allele represents the strongest genetic risk factor for development of AD, driving pathogenesis and increasing overall disease severity. APOE has long been recognized as a key regulator of cholesterol homeostasis, although a greater appreciation now exists for its role in various innate immune system processes. Indeed, APOE modulates inflammatory environments in brain in large part by altering gene expression profiles in glia, important mediators of immunity in the CNS. While the association between APOE and AD was first observed nearly three decades ago, the mechanism by which APOE ε4 influences the etiology and pathophysiology of AD is not well characterized. Overwhelming data supports the hypothesis that APOE ε4 dysregulates central amyloid metabolism by an undetermined molecular mechanism, thus laying the foundation for disease. A host of amyloid-degrading enzymes (ADEs) regulate Aβ accumulation in brain, and therefore represent valuable therapeutic targets. Neprilysin (NEP), a metalloendopeptidase expressed by activated microglia and astrocytes, is a broad-spectrum ADE able to degrade a variety of Aβ species. Here we describe in vivo and in vitro experiments designed to investigate the potential for APOE genotype to differentially regulate glial NEP in brain under neuroinflammatory conditions. Our results provide a novel mechanism by which APOE genotype-dependent differential expression of NEP by glia during neuroinflammation may contribute to AD pathogenesis.

Autophagy Dysfunction in Alzheimer's Disease: Mechanistic Insights and New Therapeutic Opportunities

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive memory loss due to aberrant accumulation of misfolded proteins inside and outside neurons and glial cells, leading to a loss of cellular protein homeostasis. Today, no therapy is available to block or slow down AD progression, and the mechanisms of the disease are not fully understood. Autophagy is an intracellular degradation pathway crucial to maintaining cellular homeostasis by clearing damaged organelles, pathogens, and unwanted protein aggregates. In recent years, autophagy dysfunction has gained considerable attention in AD and other neurodegenerative diseases because it has been linked to the accumulation of misfolded proteins that ultimately causes neuronal death in many of these disorders. Interestingly, autophagy-activating compounds have also shown some promising results in both clinical trials and preclinical studies. This review aims at summarizing the current knowledge on autophagy dysfunction in the context of AD pathophysiology, providing recent mechanistic insights on AD-mediated autophagic flux disruption and highlighting potential and novel therapeutic opportunities that target this system for AD therapy.

Peroxidase activity of heme bound amyloid β peptides associated with Alzheimer's disease

The amyloid cascade hypothesis attributes the neurodegeneration observed in Alzheimer's disease (AD) to the deposition of the amyloid β (Aβ) peptide into plaques and fibrils in the AD brain. The metal ion hypothesis which implicates several metal ions, viz. Zn²⁺, Cu²⁺ and Fe³⁺, in the AD pathology on account of their abnormal accumulation in the Aβ plaques along with an overall dyshomeostasis of these metals in the AD brain was proposed a while back. Metal ion chelators and ionophores, put forward as possible drug candidates for AD, are yet to succeed in clinical trials. Heme, which is widely distributed in the mammalian body as the prosthetic group of several important proteins and enzymes, has been thought to be associated with AD by virtue of its colocalization in the Aβ plaques along with the similarity of several heme deficiency symptoms with those of AD and most importantly, due to its ability to bind Aβ. This feature article illustrates the active site environment of heme-Aβ which resembles those of peroxidases. It also discusses the peroxidase activity of heme-Aβ, its ability to effect oxidative degradation of neurotransmitters like serotonin and also the identification of the highly reactive high-valent intermediate, compound I. The effect of second sphere residues on the formation and peroxidase activity of heme-Aβ along with the generation and decay of compound I is highlighted throughout the article. The reactivities of heme bound Aβ peptides give an alternative theory to understand the possible cause of this disease.

Fish oil protects the blood-brain barrier integrity in a mouse model of Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is ranked as the most prevalent neurodegenerative disease. However, the exact molecular mechanisms underlying pathophysiological alterations in AD remain unclear, especially at the prodromal stage. The decreased proteolytic degradation of Aβ, blood-brain barrier (BBB) disruption, and neuroinflammation are considered to play key roles in the course of AD.

METHODS

Male APPswe/PS1dE9 C57BL/6 J double-transgenic (APP/PS1) mice in the age range from 1 month to 6 months and age-matched wild type mice were used in this study, intending to investigate the expression profiles of Aβ-degrading enzymes for Aβ degradation activities and zonula occludens-1 (zo-1) for BBB integrity at the prodromal stage.

RESULTS

Our results showed that there were no significant genotype-related alterations in mRNA expression levels of 4 well-characterized Aβ-degrading enzymes in APP/PS1 mice within the ages of 6 months. Interestingly, a significant decrease in zo-1 expression was observed in APP/PS1 mice starting from the age of 5 months, suggesting that BBB disrupt occurs at an early stage. Moreover, treatment of fish oil (FO) for 4 weeks remarkably increased zo-1 expression and significantly inhibited the glial activation and NF-κB activation in APP/PS1 mice.

CONCLUSION

The results of our study suggest that FO supplement could be a potential therapeutic early intervention for AD through protecting the BBB integrity and suppressing glial and NF-κB activation.

Copper Transporters? Glutathione Reactivity of Products of Cu-Aβ Digestion by Neprilysin

Aβ_4–42 is the major subspecies of Aβ peptides characterized by avid Cu(II) binding via the ATCUN/NTS motif. It is thought to be produced in vivo proteolytically by neprilysin, but in vitro experiments in the presence of Cu(II) ions indicated preferable formation of C-terminally truncated ATCUN/NTS species including Cu^IIAβ_4–16, Cu^IIAβ_4–9, and also Cu^IIAβ_12–16, all with nearly femtomolar affinities at neutral pH. Such small complexes may serve as shuttles for copper clearance from extracellular brain spaces, on condition they could survive intracellular conditions upon crossing biological barriers. In order to ascertain such possibility, we studied the reactions of Cu^IIAβ_4–16, Cu^IIAβ_4–9, Cu^IIAβ_12–16, and Cu^IIAβ_1–16 with reduced glutathione (GSH) under aerobic and anaerobic conditions using absorption spectroscopy and mass spectrometry. We found Cu^IIAβ_4–16 and Cu^IIAβ_4–9 to be strongly resistant to reduction and concomitant formation of Cu(I)–GSH complexes, with reaction times ∼10 h, while Cu^IIAβ_12–16 was reduced within minutes and Cu^IIAβ_1–16 within seconds of incubation. Upon GSH exhaustion by molecular oxygen, the Cu^IIAβ complexes were reformed with no concomitant oxidative damage to peptides. These finding reinforce the concept of Aβ_4–x peptides as physiological trafficking partners of brain copper.

Aβ₄₋₁₆, Aβ₄₋₉, and Aβ₁₂₋₁₆, oligopeptide products of β-amyloid degradation by neprilysin, bind Cu^II ions very tightly and are considered as possible Cu^II carriers in the brain. We demonstrated that Cu^II(Aβ_4−x) complexes, but not Cu^II(Aβ₁₂₋₁₆), are kinetically resistant to reduction by glutathione. No covalent Aβ peptide modifications were observed during the copper reduction and reoxidation by ambient oxygen, yielding the original complexes. These features suggest that Cu^II(Aβ_4−x) complexes might be able to cross the blood−brain barrier.

Brain Shuttle Neprilysin reduces central Amyloid-β levels

Reducing Amyloid β (Aβ) in the brain is of fundamental importance for advancing the therapeutics for Alzheimer`s disease. The endogenous metallopeptidase neprilysin (NEP) has been identified as one of the key Aβ-degrading enzymes. Delivery of NEP to the brain by utilizing the Brain Shuttle (BS) transport system offers a promising approach for clearing central Aβ. We fused the extracellular catalytic domain of NEP to an active or inactive BS module. The two BS-NEP constructs were used to investigate the pharmacokinetic/pharmacodynamics relationships in the blood and the cerebrospinal fluid (CSF) in dose-response and multiple dosing. As previously shown, NEP was highly effective at degrading Aβ in blood but not in the CSF compartment after systemic administration. In contrast, the NEP with an active BS module led to a significant CSF exposure of BS-NEP, followed by substantial Aβ reduction in CSF and brain parenchyma. Our data show that a BS module against the transferrin receptor facilitates the transport of an Aβ degrading enzyme across the blood-brain barriers to efficiently reduce Aβ levels in both CSF and brain.

The long-term but not short-term use of benzodiazepine impairs motoric function and upregulates amyloid β in part through the suppression of translocator protein

Many elderly American women use CNS depressant benzodiazepine (BZD) to ameliorate anxiety or insomnia. However, the chronic use of BZD (cBZD) is prevalent, causing adverse effects of BZD that include movement deficit. We previously reported that cBZD upregulates neurotoxic amyloid β42 (Aβ42) and downregulates neuroprotective translocator protein (TSPO) in the cerebellum, the brain area of movement and balance. The aim of the current study is two-fold: 1) to determine a direct effect of TSPO (inhibition) on cBZD-induced Aβ42 and Aβ-associated molecules; Aβ-producing-protein presenilin-1 (PS1) and Aβ-degrading-enzyme neprilysin and 2) to determine whether Aβ42 upregulation and motoric deficit occur upon a long-term (cBZD) rather than a short-term BZD (sBZD) treatment. Old female mice received BZD (lorazepam) for 20 days (cBZD) or 3 days (sBZD) with or without prototype TSPO ligand PK11195 and were tested for motoric performance for 3 days using Rotarod. ELISA was conducted to measure Aβ42 level and neprilysin activity in cerebellum. RT-PCR and immunoblot were conducted to measure the mRNA and protein levels of TSPO, PS1, and neprilysin. cBZD treatment decreased TSPO and neprilysin but increased Aβ42 accompanied by motoric deficit. Chronic PK11195 treatment acted as a TSPO inhibitor by suppressing TSPO expression and mimicked or exacerbated the effects of cBZD on all parameters measured except for PS1. None of the molecular and behavioral changes induced by cBZD were reproduced by sBZD treatment. These data suggest that cBZD upregulates Aβ42 and downregulates neprilysin in part through TSPO inhibition, the mechanisms distinct from sBZD, collectively contributing to motoric deficit.

Misfolded proteins as a therapeutic target in Alzheimer's disease

For decades, Alzheimer's Disease (AD) was defined as a disorder of protein misfolding and aggregation. In particular, the extracellular peptide fragment: amyloid-β (Aβ), and the intracellular microtubule-associated protein: tau, were thought to initiate a neurodegenerative cascade which culminated in AD's progressive loss of memory and executive function. As such, both proteins became the focus of intense scrutiny, and served as the principal pathogenic target for hundreds of clinical trials. However, with varying efficacy, none of these investigations produced a disease-modifying therapy - offering patients with AD little recourse aside from transient, symptomatic medications. The near universal failure of clinical trials is unprecedented for a major research discipline. In part, this has motivated an increasing skepticism of the relevance of protein misfolding to AD's etiology. Several recent observations, principally the presence of significant protein pathologies in non-demented seniors, have lent credence to an apparent cursory role for Aβ and tau. Herein, we review both Aβ and tau, examining the processes from their biosynthesis to their pathogenesis and evaluate their vulnerability to medicinal intervention. We further attempt to reconcile the apparent failure of trials with the potential these targets hold. Ultimately, we seek to answer if protein misfolding is a viable platform in the pursuit of a disease-arresting strategy for AD.

Risk of dementia following androgen deprivation therapy for treatment of prostate cancer

BACKGROUND

Evidence for androgen deprivation therapy (ADT) and risk of dementia is both limited and mixed. We aimed to assess the association between ADT and risk of dementia among men with localized and locally advanced prostate cancer (PCa).

METHODS

We conducted a retrospective cohort study using SEER-Medicare-linked data among 100,414 men aged ≥ 66 years and diagnosed with localized and locally advanced PCa (cT1-cT4) between 1992 and 2009. We excluded men with a history of stroke, dementia, or use of psychiatric services. Men were followed until death or administrative end of follow-up at 36 months. Inverse-probability weighted Fine-Gray models were used to estimate hazard ratios (HR) and 95% confidence intervals (CI) for Alzheimer's, all-cause dementia, and use of psychiatric services by duration of pharmacologic ADT (0, 1-6, and ≥ 7 months).

RESULTS

Among 100,414 men with PCa (median age 73 [IQR: 69-77] years; 84% white, 10% black), 38% (n = 37,911) received ADT within 6 months of diagnosis. Receipt of any pharmacologic ADT was associated with a 17% higher risk of all-cause dementia (HR 1.17, 95% CI 1.07-1.27), 23% higher risk of Alzheimer's (HR 1.23, 95% CI 1.11-1.37), and 10% higher risk of psychiatric services use, though the confidence interval included the null (HR 1.10, 95% CI 1.00-1.22). Longer duration of ADT (≥7 months) was associated with a 25% higher risk of all-cause dementia, 34% higher risk of Alzheimer's, and 9% higher risk of psychiatric services, compared with no ADT.

CONCLUSIONS

Our study supports an association between pharmacologic ADT and higher risk of all-cause dementia, Alzheimer's, and use of psychiatric services among men with localized and locally advanced PCa.

Potential microRNA-related targets in clearance pathways of amyloid-β: novel therapeutic approach for the treatment of Alzheimer's disease

Imbalance between amyloid-beta (Aβ) peptide synthesis and clearance results in Aβ deregulation. Failure to clear these peptides appears to cause the development of Alzheimer's disease (AD). In recent years, microRNAs have become established key regulators of biological processes that relate among others to the development and progression of neurodegenerative diseases, such as AD. This review article gives an overview on microRNAs that are involved in the Aβ cascade and discusses their inhibitory impact on their target mRNAs whose products participate in Aβ clearance. Understanding of the mechanism of microRNA in the associated signal pathways could identify novel therapeutic targets for the treatment of AD.

The Rat Prefrontal-Cortex Transcriptome: Effects of Aging and Sporadic Alzheimer's Disease-Like Pathology

Alzheimer's disease (AD) is the most widespread late-life dementia and involves the prefrontal cortex, a vulnerable brain region implicated in memory, emotion, cognition, and decision-making behavior. To understand the molecular differences between the effects of aging and AD on the prefrontal cortex, this study characterized the age-dependent changes in gene expression in Wistar rats (control) and OXYS rats (rodents that simulate key characteristics of sporadic AD) using RNA sequencing. We found that major altered biological processes during aging in Wistar rats were associated with immune processes. Gene expression changes during development of AD-like pathology as well as at the preclinical stage were related to neuronal plasticity, catalytic activity, lipid and immune processes, and mitochondria. A comparison of genes between data sets "OXYS rats" and "human AD" revealed similarity in expression alterations of genes related primarily to mitochondrial function; immune, endocrine, and circulatory systems; signal transduction; neuronal and synaptic processes; hypoxia; and apoptosis. Expression changes in mitochondrial processes identified in OXYS rats by RNA sequencing were confirmed by ultrastructural neuronal organelle alterations and low activity of respiratory chain complexes I, IV, and V in cortical mitochondria, suggesting that mitochondrial dysfunction appears to mediate or possibly even initiate the development of AD.

Receptor-Interacting Protein Kinase 1 (RIPK1) as a Potential Therapeutic Target: An Overview of Its Possible Role in the Pathogenesis of Alzheimer's Disease

Alzheimer's Disease (AD) is an age-dependent neurodegenerative disorder, the most common type of dementia that is clinically characterized by the presence of beta-amyloid (Aβ) extracellularly and intraneuronal tau protein tangles that eventually leads to the onset of memory and cognition impairment, development of psychiatric symptoms and behavioral disorders that affect basic daily activities. Current treatment approved by the U.S Food and Drug Administration (FDA) for AD is mainly focused on the symptoms but not on the pathogenesis of the disease. Recently, receptor-interacting protein kinase 1 (RIPK1) has been identified as a key component in the pathogenesis of AD through necroptosis. Furthermore, genetic and pharmacological suppression of RIPK1 has been shown to revert the phenotype of AD and its mediating pathway is yet to be deciphered. This review is aimed to provide an overview of the pathogenesis and current treatment of AD with the involvement of autophagy as well as providing a novel insight into RIPK1 in reverting the progression of AD, probably through an autophagy machinery.

Somatostatin Receptor Subtype-4 Regulates mRNA Expression of Amyloid-Beta Degrading Enzymes and Microglia Mediators of Phagocytosis in Brains of 3xTg-AD Mice

Alzheimer's disease (AD) is a progressive neurodegenerative disorder resulting in memory and cognitive impairment. The use of somatostatin receptor subtype-4 (SSTR₄) agonists have been proposed for AD treatment. This study investigated the effects of selective SSTR₄ agonist NNC 26-9100 on mRNA expression of key genes associated with AD pathology (microglia mediators of Aβ phagocytosis, amyloid-beta (Aβ)-degrading enzymes, anti-oxidant enzymes and pro-inflammatory cytokines) in 3xTg-AD mice. Mice were administered NNC 26-9100 (0.2 µg, i.c.v.) or vehicle control, with cortical and subcortical brain tissue collected at 6 h and 24 h post-treatment. At 6 h, NNC 26-9100 treatment decreased cortical expression of cluster of differentiation-33 (Cd33) by 25%, while increasing cortical and subcortical macrophage scavenger receptor-1 (Msr1) by 1.8 and 2.0-fold, respectively. The Cd33 downregulation and Msr1 upregulation support a state of microglia associated Aβ phagocytosis. At 24 h, NNC 26-9100 treatment increased the cortical expression of Sstr4 (4.9-fold), Aβ-degrading enzymes neprilysin (9.3-fold) and insulin degrading enzyme (14.8-fold), and the antioxidant catalase (3.6-fold). Similar effects at 24 h were found in subcortical tissue with NNC 26-9100 treatment, but did not reach statistical significance. No changes in pro-inflammatory cytokine expression were found. These data demonstrated NNC 26-9100 facilitates transcriptional changes in brain tissue identified with Aβ phagocytosis and clearance, further supporting SSTR₄ as a treatment target for AD.

Subchronic Pathobiological Response Following Chronic Repetitive Mild Traumatic Brain Injury in an Aged Preclinical Model of Amyloid Pathogenesis

Repetitive mild traumatic brain injury (r-mTBI) is a risk factor for Alzheimer disease (AD). The precise nature of how r-mTBI leads to, or precipitates, AD pathogenesis remains unclear. In this study, we explore subchronic effects of chronic r-mTBI (12-impacts) administered over 1-month in aged-PS1/APP mice and littermate controls. We investigate specific mechanisms that may elucidate the molecular link between AD and r-mTBI, focusing primarily on amyloid and tau pathology, amyloid processing, glial activation states, and associated clearance mechanisms. Herein, we demonstrate r-mTBI in aged PS1/APP mice does not augment, glial activation, amyloid burden, or tau pathology (with exception of pS202-positive Tau) 1 month after exposure to the last-injury. However, we observed a decrease in brain soluble Aβ42 levels without any appreciable change in peripheral soluble Aβ42 levels. This was accompanied by an increase in brain insoluble to soluble Aβ42 ratio in injured PS1/APP mice compared with sham injury. A parallel reduction in phagocytic receptor, triggering receptor expressed on myeloid cells 2, was also observed. This study demonstrates very subtle subchronic effects of r-mTBI on a preexisting amyloid pathology background, which may be on a continuum toward a slow and worsening neurodegenerative outcome compared with sham injury, and therefore, have many implications, especially in the elderly population exposed to TBI.

Oxidative inactivation of amyloid beta-degrading proteases by cholesterol-enhanced mitochondrial stress

Familial early-onset forms of Alzheimer's disease (AD) are linked to overproduction of amyloid beta (Aβ) peptides, while decreased clearance of Aβ is the driving force leading to its toxic accumulation in late-onset (sporadic) AD. Oxidative modifications and defective function have been reported in Aβ-degrading proteases such as neprilysin (NEP) and insulin-degrading enzyme (IDE). However, the exact mechanisms that regulate the proteolytic clearance of Aβ and its deficits are largely unknown. We have previously showed that cellular cholesterol loading, by depleting the mitochondrial GSH (mGSH) content, stimulates Αβ-induced mitochondrial oxidative stress and promotes AD-like pathology in APP-PSEN1-SREBF2 mice. Here, using the same AD mouse model we examined whether cholesterol-enhanced mitochondrial oxidative stress affects NEP and IDE function. We found that brain extracts from APP-PSEN1-SREBF2 mice displayed increased presence of oxidatively modified forms of NEP and IDE, associated with impaired enzymatic activities. Both alterations were substantially recovered after an in vivo treatment with the cholesterol-lowering agent 2-hydroxypropyl-β-cyclodextrin. The recovery of the proteolytic activity after treatment was accompanied with a significant reduction of Aβ levels. Supporting these results, cholesterol-enriched SH-SY5Y cells were more sensitive to Aβ-induced impairment of IDE and NEP function in vitro. The rise of cellular cholesterol also stimulated the extracellular release of IDE by an unconventional autophagy-coordinated mechanism. Recovery of depleted pool of mGSH in these cells not only prevented the detrimental effect of Aβ on intracellular AβDPs activities but also had an impact on extracellular IDE levels and function, stimulating the extracellular Aβ degrading activity. Therefore, changes in brain cholesterol levels by modifying the mGSH content would play a key role in IDE and NEP-mediated proteolytic elimination of Aβ peptides and AD progression.

•

Cholesterol regulates IDE and NEP by enhancing the detrimental effect of Aβ on their proteolytic activities.

•

Cholesterol-mediated mitochondrial GSH depletion is responsible for the oxidative impairment of IDE and NEP.

•

High cholesterol levels induce the release of inactive IDE through secretory autophagy.

•

A rise in cellular cholesterol affects the extracellular Aβ degradation, favoring oligomers formation.

•

Cholesterol lowering compounds and antioxidant therapy restore IDE and NEP activity.

CuII Binding Properties of N-Truncated Aβ Peptides: In Search of Biological Function

As life expectancy increases, the number of people affected by progressive and irreversible dementia, Alzheimer's Disease (AD), is predicted to grow. No drug designs seem to be working in humans, apparently because the origins of AD have not been identified. Invoking amyloid cascade, metal ions, and ROS production hypothesis of AD, herein we share our point of view on Cu(II) binding properties of Aβ_4-x, the most prevalent N-truncated Aβ peptide, currently known as the main constituent of amyloid plaques. The capability of Aβ_4-x to rapidly take over copper from previously tested Aβ_1-x peptides and form highly stable complexes, redox unreactive and resistant to copper exchange reactions, prompted us to propose physiological roles for these peptides. We discuss the new findings on the reactivity of Cu(II)Aβ_4-x with coexisting biomolecules in the context of synaptic cleft; we suggest that the role of Aβ_4-x peptides is to quench Cu(II) toxicity in the brain and maintain neurotransmission.

Alzheimer’s Disease-related Biomarkers in Aging Adults with Down Syndrome: Systematic Review

Background:

Down syndrome (DS) is associated with a high prevalence of cognitive impairment and dementia in middle age and older adults. Given the presence of common neuropathological findings and similar pathogenic mechanisms, dementia in DS is regarded as a form of genetically determined, early-onset AD. The clinical characterization of cognitive decline in persons with DS is a difficult task, due to the presence intellectual disability and pre-existing cognitive impairment. Subtle changes that occur at early stages of the dementing process may not be perceived clinically, given that most cognitive screening tests are not sensitive enough to detect them. Therefore, biological markers will provide support to the diagnosis of DS-related cognitive impairment and dementia, particularly at early stages of this process.

Objective:

To perform a systematic review of the literature on AD-related biomarkers in DS.

Method:

We searched PubMed, Web of Science and Cochrane Library for scientific papers published between 2008 and 2018 using as primary mesh terms ‘Down’, ‘Alzheimer’, ‘biomarker’.

Results:

79 studies were retrieved, and 39 were considered eligible for inclusion in the systematic review: 14 post-mortem studies, 10 neuroimaging, 4 addressing cerebrospinal fluid biomarkers, and 11 on peripheral markers.

Conclusion:

There is consistent growth in the number of publication in this field over the past years. Studies in DS-related dementia tend to incorporate many of the diagnostic technologies that have been more extensively studied and validated in AD. In many instances, the study of CNS and peripheral biomarkers reinforces the presence of AD pathology in DS.

MicroRNAs in Alzheimer's Disease

Alzheimer’s disease (AD) is a progressive and devastating neurodegenerative disorder. It is the leading cause of dementia in the world’s rapidly growing aging population. The characteristics of AD are memory loss and cognitive impairment, meaning patients cannot carry out their daily activities independently. The increase of AD cases poses heavy burdens on families, society and the economy. Despite frequent efforts being made to research the etiology of AD, the causes of AD remain unknown, and no curative treatments are available yet. The pathological hallmarks of AD are amyloid plaques and neurofibrillary tangles in the brain. MicroRNAs are endogenous ∼22 nucleotides non-coding RNAs that could regulate gene expression at a post-transcriptional level by transcript degradation or translation repression. MicroRNAs are involved in many biological processes and diseases, particularly multifactorial diseases, providing an excellent tool with which to research the mechanisms of these diseases. AD is a multifactorial disorder, and accumulating evidence shows that microRNAs play a critical role in the pathogenesis of AD. In this review, we will highlight the effect of microRNAs in different pathological processes throughout AD progression.

Oligopeptides Generated by Neprilysin Degradation of β-Amyloid Have the Highest Cu(II) Affinity in the Whole Aβ Family

The catabolism of β-amyloid (Aβ) is carried out by numerous endopeptidases including neprilysin, which hydrolyzes peptide bonds preceding positions 4, 10, and 12 to yield Aβ_4-9 and a minor Aβ_{12- x} species. Alternative processing of the amyloid precursor protein by β-secretase also generates the Aβ_{11- x} species. All these peptides contain a Xxx-Yyy-His sequence, also known as an ATCUN or NTS motif, making them strong chelators of Cu(II) ions. We synthesized the corresponding peptides, Phe-Arg-His-Asp-Ser-Gly-OH (Aβ_4-9), Glu-Val-His-His-Gln-Lys-am (Aβ_11-16), Val-His-His-Gln-Lys-am (Aβ_12-16), and pGlu-Val-His-His-Gln-Lys-am (pAβ_11-16), and investigated their Cu(II) binding properties using potentiometry, and UV-vis, circular dichroism, and electron paramagnetic resonance spectroscopies. We found that the three peptides with unmodified N-termini formed square-planar Cu(II) complexes at pH 7.4 with analogous geometries but significantly varied K_d values of 6.6 fM (Aβ_4-9), 9.5 fM (Aβ_12-16), and 1.8 pM (Aβ_11-16). Cyclization of the N-terminal Glu11 residue to the pyroglutamate species pAβ_11-16 dramatically reduced the affinity (5.8 nM). The Cu(II) affinities of Aβ_4-9 and Aβ_12-16 are the highest among the Cu(II) complexes of Aβ peptides. Using fluorescence spectroscopy, we demonstrated that the Cu(II) exchange between the Phe-Arg-His and Val-His-His motifs is very slow, on the order of days. These results are discussed in terms of the relevance of Aβ_4-9, a major Cu(II) binding Aβ fragment generated by neprilysin, as a possible Cu(II) carrier in the brain.