Asparagine endopeptidase is an innovative therapeutic target for neurodegenerative diseases

STAT3 ameliorates truncated tau-induced cognitive deficits

Proteolytic cleavage of tau by asparagine endopeptidase (AEP) creates tau-N368 fragments, which may drive the pathophysiology associated with synaptic dysfunction and memory deterioration in the brain of Alzheimer's disease patients. Nonetheless, the molecular mechanisms of truncated tau-induced cognitive deficits remain unclear. Evidence suggests that signal transduction and activator of transcription-3 (STAT3) is associated with modulating synaptic plasticity, cell apoptosis, and cognitive function. Using luciferase reporter assays, electrophoretic mobility shift assays, western blotting, and immunofluorescence, we found that human tau-N368 accumulation inhibited STAT3 activity by suppressing STAT3 translocation into the nucleus. Overexpression of STAT3 improved tau-N368-induced synaptic deficits and reduced neuronal loss, thereby improving the cognitive deficits in tau-N368 mice. Moreover, in tau-N368 mice, activation of STAT3 increased N-methyl-D-aspartic acid receptor levels, decreased Bcl-2 levels, reversed synaptic damage and neuronal loss, and thereby alleviated cognitive deficits caused by tau-N368. Taken together, STAT3 plays a critical role in truncated tau-related neuropathological changes. This indicates a new mechanism behind the effect of tau-N368 on synapses and memory deficits. STAT3 can be used as a new molecular target to treat tau-N368-induced protein pathology.

AEP-cleaved DDX3X induces alternative RNA splicing events to mediate cancer cell adaptation in harsh microenvironments

Oxygen and nutrient deprivation are common features of solid tumors. Although abnormal alternative splicing (AS) has been found to be an important driving force in tumor pathogenesis and progression, the regulatory mechanisms of AS that underly the adaptation of cancer cells to harsh microenvironments remain unclear. Here, we found that hypoxia- and nutrient deprivation-induced asparagine endopeptidase (AEP) specifically cleaved DDX3X in a HIF1A-dependent manner. This cleavage yields truncated carboxyl-terminal DDX3X (tDDX3X-C), which translocates and aggregates in the nucleus. Unlike intact DDX3X, nuclear tDDX3X-C complexes with an array of splicing factors and induces AS events of many pre-mRNAs; for example, enhanced exon skipping (ES) in exon 2 of the classic tumor suppressor PRDM2 leads to a frameshift mutation of PRDM2. Intriguingly, the isoform ARRB1-Δexon 13 binds to glycolytic enzymes and regulates glycolysis. By utilizing in vitro assays, glioblastoma organoids, and animal models, we revealed that AEP/tDDX3X-C promoted tumor malignancy via these isoforms. More importantly, high AEP/tDDX3X-C/ARRB1-Δexon 13 in cancerous tissues was tightly associated with poor patient prognosis. Overall, our discovery of the effect of AEP-cleaved DDX3X switching on alternative RNA splicing events identifies a mechanism in which cancer cells adapt to oxygen and nutrient shortages and provides potential diagnostic and/or therapeutic targets.

Integrative metabolomics science in Alzheimer's disease: Relevance and future perspectives

Alzheimer's disease (AD) is determined by various pathophysiological mechanisms starting 10-25 years before the onset of clinical symptoms. As multiple functionally interconnected molecular/cellular pathways appear disrupted in AD, the exploitation of high-throughput unbiased omics sciences is critical to elucidating the precise pathogenesis of AD. Among different omics, metabolomics is a fast-growing discipline allowing for the simultaneous detection and quantification of hundreds/thousands of perturbed metabolites in tissues or biofluids, reproducing the fluctuations of multiple networks affected by a disease. Here, we seek to critically depict the main metabolomics methodologies with the aim of identifying new potential AD biomarkers and further elucidating AD pathophysiological mechanisms. From a systems biology perspective, as metabolic alterations can occur before the development of clinical signs, metabolomics - coupled with existing accessible biomarkers used for AD screening and diagnosis - can support early disease diagnosis and help develop individualized treatment plans. Presently, the majority of metabolomic analyses emphasized that lipid metabolism is the most consistently altered pathway in AD pathogenesis. The possibility that metabolomics may reveal crucial steps in AD pathogenesis is undermined by the difficulty in discriminating between the causal or epiphenomenal or compensatory nature of metabolic findings.

Mechanistic impacts of bacterial diet on dopaminergic neurodegeneration in a Caenorhabditis elegans α-synuclein model of Parkinson's disease

Failure of inherently protective cellular processes and misfolded protein-associated stress contribute to the progressive loss of dopamine (DA) neurons characteristic of Parkinson's disease (PD). A disease-modifying role for the microbiome has recently emerged in PD, representing an impetus to employ the soil-dwelling nematode, Caenorhabditis elegans, as a preclinical model to correlate changes in gene expression with neurodegeneration in transgenic animals grown on distinct bacterial food sources. Even under tightly controlled conditions, hundreds of differentially expressed genes and a robust neuroprotective response were discerned between clonal C. elegans strains overexpressing human alpha-synuclein in the DA neurons fed either one of only two subspecies of Escherichia coli. Moreover, this neuroprotection persisted in a transgenerational manner. Genetic analysis revealed a requirement for the double-stranded RNA (dsRNA)-mediated gene silencing machinery in conferring neuroprotection. In delineating the contribution of individual genes, evidence emerged for endopeptidase activity and heme-associated pathway(s) as mechanistic components for modulating dopaminergic neuroprotection.

Mutations in α-synuclein, TDP-43 and tau prolong protein half-life through diminished degradation by lysosomal proteases

BACKGROUND

Autosomal dominant mutations in α-synuclein, TDP-43 and tau are thought to predispose to neurodegeneration by enhancing protein aggregation. While a subset of α-synuclein, TDP-43 and tau mutations has been shown to increase the structural propensity of these proteins toward self-association, rates of aggregation are also highly dependent on protein steady state concentrations, which are in large part regulated by their rates of lysosomal degradation. Previous studies have shown that lysosomal proteases operate precisely and not indiscriminately, cleaving their substrates at very specific linear amino acid sequences. With this knowledge, we hypothesized that certain coding mutations in α-synuclein, TDP-43 and tau may lead to increased protein steady state concentrations and eventual aggregation by an alternative mechanism, that is, through disrupting lysosomal protease cleavage recognition motifs and subsequently conferring protease resistance to these proteins.

RESULTS

To test this possibility, we first generated comprehensive proteolysis maps containing all of the potential lysosomal protease cleavage sites for α-synuclein, TDP-43 and tau. In silico analyses of these maps indicated that certain mutations would diminish cathepsin cleavage, a prediction we confirmed utilizing in vitro protease assays. We then validated these findings in cell models and induced neurons, demonstrating that mutant forms of α-synuclein, TDP-43 and tau are degraded less efficiently than wild type despite being imported into lysosomes at similar rates.

CONCLUSIONS

Together, this study provides evidence that pathogenic mutations in the N-terminal domain of α-synuclein (G51D, A53T), low complexity domain of TDP-43 (A315T, Q331K, M337V) and R1 and R2 domains of tau (K257T, N279K, S305N) directly impair their own lysosomal degradation, altering protein homeostasis and increasing cellular protein concentrations by extending the degradation half-lives of these proteins. These results also point to novel, shared, alternative mechanism by which different forms of neurodegeneration, including synucleinopathies, TDP-43 proteinopathies and tauopathies, may arise. Importantly, they also provide a roadmap for how the upregulation of particular lysosomal proteases could be targeted as potential therapeutics for human neurodegenerative disease.

Pharmacological Inhibition of the Asparaginyl Endopeptidase (AEP) in an Alzheimer's Disease Model Improves the Survival and Efficacy of Transplanted Neural Stem Cells

Stem-cell-based therapy is very promising for Alzheimer's disease (AD), yet has not become a reality. A critical challenge is the transplantation microenvironment, which impacts the therapeutic effect of stem cells. In AD brains, amyloid-beta (Aβ) peptides and inflammatory cytokines continuously poison the tissue microenvironment, leading to low survival of grafted cells and restricted efficacy. It is necessary to create a growth-supporting microenvironment for transplanted cells. Recent advances in AD studies suggest that the asparaginyl endopeptidase (AEP) is a potential intervention target for modifying pathological changes. We here chose APP/PS1 mice as an AD model and employed pharmacological inhibition of the AEP for one month to improve the brain microenvironment. Thereafter, we transplanted neural stem cells (NSCs) into the hippocampus and maintained therapy for one more month. We found that inhibition of AEPs resulted in a significant decrease of Aβ, TNF-α, IL-6 and IL-1β in their brains. In AD mice receiving NSC transplantation alone, the survival of NSCs was at a low level, while in combination with AEP inhibition pre-treatment the survival rate of engrafted cells was doubled. Within the 2-month treatment period, implantation of NSCs plus pre-inhibition of the AEP significantly enhanced neural plasticity of the hippocampus and rescued cognitive impairment. Neither NSC transplantation alone nor AEP inhibition alone achieved significant efficacy. In conclusion, pharmacological inhibition of the AEP ameliorated brain microenvironment of AD mice, and thus improved the survival and therapeutic efficacy of transplanted stem cells.

Legumain Functions as a Transient TrkB Sheddase

While primarily found in endo-lysosomal compartments, the cysteine protease legumain can also translocate to the cell surface if stabilized by the interaction with the RGD-dependent integrin receptor αVβ3. Previously, it has been shown that legumain expression is inversely related to BDNF-TrkB activity. Here we show that legumain can conversely act on TrkB-BDNF by processing the C-terminal linker region of the TrkB ectodomain in vitro. Importantly, when in complex with BDNF, TrkB was not cleaved by legumain. Legumain-processed TrkB was still able to bind BDNF, suggesting a potential scavenger function of soluble TrkB towards BDNF. The work thus presents another mechanistic link explaining the reciprocal TrkB signaling and δ-secretase activity of legumain, with relevance for neurodegeneration.

Concomitant protein pathogenesis in Parkinson's disease and perspective mechanisms

Comorbidity is a common phenotype in Parkinson's disease (PD). Patients with PD not only have motor deficit symptoms, but also have heterogeneous non-motor symptoms, including cognitive impairment and emotional changes, which are the featured symptoms observed in patients with Alzheimer's disease (AD), frontotemporal dementia (FTD) and cerebrovascular disease. Moreover, autopsy studies have also confirmed the concomitant protein pathogenesis, such as the co-existences of α-synuclein, amyloid-β and tau pathologies in PD and AD patients' brains. Here, we briefly summarize the recent reports regarding the comorbidity issues in PD from both clinical observations and neuropathological evidences. Furthermore, we provide some discussion about the perspective potential mechanisms underlying such comorbidity phenomenon, with a focus on PD and related neurodegenerative diseases.

From Lysosomal Storage Disorders to Parkinson's Disease - Challenges and Opportunities

Lysosomes are specialized organelles with an acidic pH that act as recycling hubs for intracellular and extracellular components. They harbour numerous different hydrolytic enzymes to degrade substrates like proteins, peptides, and glycolipids. Reduced catalytic activity of lysosomal enzymes can cause the accumulation of these substrates and loss of lysosomal integrity, resulting in lysosomal dysfunction and lysosomal storage disorders (LSDs). Post-mitotic cells, such as neurons, seem to be highly sensitive to damages induced by lysosomal dysfunction, thus LSDs often manifest with neurological symptoms. Interestingly, some LSDs and Parkinson's disease (PD) share common cellular pathomechanisms, suggesting convergence of aetiology of the two disease types. This is further underlined by genetic associations of several lysosomal genes involved in LSDs with PD. The increasing number of lysosome-associated genetic risk factors for PD makes it necessary to understand functions and interactions of lysosomal proteins/enzymes both in health and disease, thereby holding the potential to identify new therapeutic targets. In this review, we highlight genetic and mechanistic interactions between the complex lysosomal network, LSDs and PD, and elaborate on methodical challenges in lysosomal research.

The Mammalian Cysteine Protease Legumain in Health and Disease

The cysteine protease legumain (also known as asparaginyl endopeptidase or δ-secretase) is the only known mammalian asparaginyl endopeptidase and is primarily localized to the endolysosomal system, although it is also found extracellularly as a secreted protein. Legumain is involved in the regulation of diverse biological processes and tissue homeostasis, and in the pathogenesis of various malignant and nonmalignant diseases. In addition to its proteolytic activity that leads to the degradation or activation of different substrates, legumain has also been shown to have a nonproteolytic ligase function. This review summarizes the current knowledge about legumain functions in health and disease, including kidney homeostasis, hematopoietic homeostasis, bone remodeling, cardiovascular and cerebrovascular diseases, fibrosis, aging and senescence, neurodegenerative diseases and cancer. In addition, this review addresses the effects of some marketed drugs on legumain. Expanding our knowledge on legumain will delineate the importance of this enzyme in regulating physiological processes and disease conditions.

The emerging role of autophagy and mitophagy in tauopathies: From pathogenesis to translational implications in Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease, affecting more than 55 million individuals worldwide in 2021. In addition to the "amyloid hypothesis," an increasing number of studies have demonstrated that phosphorylated tau plays an important role in AD pathogenesis. Both soluble tau oligomers and insoluble tau aggregates in the brain can induce structural and functional neuronal damage through multiple pathways, eventually leading to memory deficits and neurodegeneration. Autophagy is an important cellular response to various stress stimuli and can generally be categorized into non-selective and selective autophagy. Recent studies have indicated that both types of autophagy are involved in AD pathology. Among the several subtypes of selective autophagy, mitophagy, which mediates the selective removal of mitochondria, has attracted increasing attention because dysfunctional mitochondria have been suggested to contribute to tauopathies. In this review, we summarize the latest findings on the bidirectional association between abnormal tau proteins and defective autophagy, as well as mitophagy, which might constitute a vicious cycle in the induction of neurodegeneration. Neuroinflammation, another important feature in the pathogenesis and progression of AD, has been shown to crosstalk with autophagy and mitophagy. Additionally, we comprehensively discuss the relationship between neuroinflammation, autophagy, and mitophagy. By elucidating the underlying molecular mechanisms governing these pathologies, we highlight novel therapeutic strategies targeting autophagy, mitophagy and neuroinflammation, such as those using rapamycin, urolithin, spermidine, curcumin, nicotinamide, and actinonin, for the prevention and treatment of AD.

Neuroprotective Function of Rasagiline and Selegiline, Inhibitors of Type B Monoamine Oxidase, and Role of Monoamine Oxidases in Synucleinopathies

Synucleinopathies are a group of neurodegenerative disorders caused by the accumulation of toxic species of α-synuclein. The common clinical features are chronic progressive decline of motor, cognitive, behavioral, and autonomic functions. They include Parkinson’s disease, dementia with Lewy body, and multiple system atrophy. Their etiology has not been clarified and multiple pathogenic factors include oxidative stress, mitochondrial dysfunction, impaired protein degradation systems, and neuroinflammation. Current available therapy cannot prevent progressive neurodegeneration and “disease-modifying or neuroprotective” therapy has been proposed. This paper presents the molecular mechanisms of neuroprotection by the inhibitors of type B monoamine oxidase, rasagiline and selegiline. They prevent mitochondrial apoptosis, induce anti-apoptotic Bcl-2 protein family, and pro-survival brain- and glial cell line-derived neurotrophic factors. They also prevent toxic oligomerization and aggregation of α-synuclein. Monoamine oxidase is involved in neurodegeneration and neuroprotection, independently of the catalytic activity. Type A monoamine oxidases mediates rasagiline-activated signaling pathways to induce neuroprotective genes in neuronal cells. Multi-targeting propargylamine derivatives have been developed for therapy in various neurodegenerative diseases. Preclinical studies have presented neuroprotection of rasagiline and selegiline, but beneficial effects have been scarcely presented. Strategy to improve clinical trials is discussed to achieve disease-modification in synucleinopathies.

The Asparaginyl Endopeptidase Legumain: An Emerging Therapeutic Target and Potential Biomarker for Alzheimer’s Disease

Alzheimer’s disease (AD) is incurable dementia closely associated with aging. Most cases of AD are sporadic, and very few are inherited; the pathogenesis of sporadic AD is complex and remains to be elucidated. The asparaginyl endopeptidase (AEP) or legumain is the only recognized cysteine protease that specifically hydrolyzes peptide bonds after asparagine residues in mammals. The expression level of AEPs in healthy brains is far lower than that of peripheral organs. Recently, growing evidence has indicated that aging may upregulate and overactivate brain AEPs. The overactivation of AEPs drives the onset of AD through cleaving tau and amyloid precursor proteins (APP), and SET, an inhibitor of protein phosphatase 2A (PP2A). The AEP-mediated cleavage of these peptides enhances amyloidosis, promotes tau hyperphosphorylation, and ultimately induces neurodegeneration and cognitive impairment. Upregulated AEPs and related deleterious reactions constitute upstream events of amyloid/tau toxicity in the brain, and represent early pathological changes in AD. Thus, upregulated AEPs are an emerging drug target for disease modification and a potential biomarker for predicting preclinical AD. However, the presence of the blood–brain barrier greatly hinders establishing body-fluid-based methods to measure brain AEPs. Research on AEP-activity-based imaging probes and our recent work suggest that the live brain imaging of AEPs could be used to evaluate its predictive efficacy as an AD biomarker. To advance translational research in this area, AEP imaging probes applicable to human brain and AEP inhibitors with good druggability are urgently needed.

Modular synthesis of 2,4-diaminoanilines as CNS drug-like non-covalent inhibitors of asparagine endopeptidase

Asparagine endopeptidase (AEP), also called legumain, is a pH-dependent endolysosomal cysteine protease that cleaves its substrates after asparagine residues. Recent studies showed that it possesses δ-secretase activity and that it is implicated in numerous neurological diseases such as Alzheimer's disease (AD). Following evidence of aryl-morpholines as useful asparagine endopeptidase inhibitors, a series of morpholinoanilines with diverse substituents at ortho position were synthesized in view of improving the potency and scope of this molecular scaffold, allowing to identify ethyl 2-isonipecotate-4-morpholinoaniline possessing inhibition potency in the nanomolar range. CNS MPO (CNS MultiParameter Optimization) calculations revealed that most of the compounds developed in this work show physicochemical parameters in the desirable range for CNS drug candidates.

Asparagine endopeptidase-targeted Ultrasound-responsive Nanobubbles Alleviate Tau Cleavage and Amyloid-β Deposition in an Alzheimer's Disease Model

Inhibition of asparagine endopeptidase (AEP) has been implied to be effective for treating tau- and amyloid-beta-mediated neurodegenerative diseases, although a method for targeted intracerebral delivery of AEP inhibitors has not yet been achieved. Here, we fabricated ultrasound-responsive nanobubbles (NBs) to load AEP inhibitor RR-11a, and modified the NB surface with either AEP recognizable peptide AAN or pro-transendothelial transversal motif RGD, i.e. NB(11a)-A and NB(11a)-R, for AEP-targeted treatment of Alzheimer's disease (AD). The developed NBs were uniform, small in size (50.1 ± 1.5 nm), with strong echogenicity and high drug loading efficiency (∼91.97%). When intravenously co-injected in the APP/PS1 mouse model, NB(11a)-R could adhere to endothelial cells and enhance transient opening of the blood-brain barrier (BBB) upon focused ultrasound oscillations, allowing the rest NBs/localized released RR-11a molecules to enter the brain, and then NB(11a)-A could selectively bind with the impaired neurons and deposit RR-11a molecules at the AD lesion. As a result, co-administration of NB(11a)-A and NB(11a)-R significantly promoted accumulation of RR-11a in the mouse brain, and substantially alleviated both tau cleavage and amyloid plaques deposition in the hippocampus. Most strikingly, the cognitive ability of the AD model mice was dramatically improved, achieving a level close to the normal mice. Overall, this unique AEP-targeted nanobubble design provides an efficient intracerebral drug delivery strategy and significantly enhances treatment efficacy of AD. STATEMENT OF SIGNIFICANCE: Asparagine endopeptidase (AEP) is an innovative therapeutic target simultaneously involved in Aβ and tau-mediated Alzheimer's disease (AD) pathology, but targeted delivery of AEP inhibitors has not been achieved yet. Here we developed an efficient strategy to deliver AEP inhibitor RR-11a towards impaired neurons. We fabricated RR-11a-loaded ultrasound-responsive nanobubbles (NBs) and modified the NB surface with RGD peptide to promote BBB crossing upon focused ultrasound oscillations, or with AAN peptide to increase binding of NBs on the neurons. Our results indicated that, co-administration of the NB(11a)-A and NB(11a)-R significantly enhanced accumulation of RR-11a molecules at the AD lesion, alleviated both tau cleavage and amyloid plaques deposition in the hippocampus, and consequently restored cognitive function of the AD model mice.

Asparaginyl endopeptidase protects against podocyte injury in diabetic nephropathy through cleaving cofilin-1

Podocyte injury and loss are critical events in diabetic nephropathy (DN); however, the underlying molecular mechanisms remain unclear. Here, we demonstrate that asparaginyl endopeptidase (AEP) protects against podocyte injury through modulating the dynamics of the cytoskeleton. AEP was highly upregulated in diabetic glomeruli and hyperglycemic stimuli treated-podocytes; however, AEP gene knockout and its compound inhibitor treatment accelerated DN in streptozotocin-induced diabetic mice, whereas specific induction of AEP in glomerular cells attenuated podocyte injury and renal function deterioration. In vitro, elevated AEP was involved in actin cytoskeleton maintenance and anti-apoptosis effects. Mechanistically, we found that AEP directly cleaved the actin-binding protein cofilin-1 after the asparagine 138 (N138) site. The protein levels of endogenous cofilin-1 1-138 fragments were upregulated in diabetic podocytes, consistent with the changes in AEP levels. Importantly, we found that cofilin-1 1-138 fragments were remarkably unphosphorylated than full-length cofilin-1, indicating the enhanced cytoskeleton maintenance activity of cofilin-1 1-138. Then we validated cofilin-1 1-138 could rescue podocytes from cytoskeleton disarrangement and injury in diabetic conditions. Taken together, our data suggest a protective role of elevated AEP in podocyte injury during DN progression through cleaving cofilin-1 to maintain podocyte cytoskeleton dynamics and defend damage.

Legumain knockout improves repeated corticosterone injection-induced depression-like emotional and cognitive deficits

Emotional and cognitive impairment has been recognized as a central feature of depression, which is closely related to hyperfunction of the hypothalamic-pituitary-adrenal (HPA) axis caused by down-regulation of glucocorticoid receptor (GR) expression in patients. A decrease in GR expression can cause pathological changes and lead to the impairment of synaptic plasticity. Legumain, a lysosomal cysteine protease, plays an important role in neurological diseases. It is reported that legumain activates the MAPK signaling pathway, which modifies the GR. Therefore, we hypothesize that regulation of the GR by legumain plays a crucial role in the pathological process of depression. The relationships between legumain, GR, synaptic plasticity and emotional and cognitive deficits were explored in this study. The results demonstrated that repeated corticosterone (CORT) injections (3 weeks) induced emotional and cognitive deficits in mice, based on behavioral experiments and the detection of synaptic plasticity. Furthermore, CORT injections decreased the expression of hippocampal synapse-related proteins, cell density and dendritic spine density in the hippocampus, accompanied by increased protein expression in the MAPK signaling pathway and decreased expression of the GR. In conclusion, our results demonstrated that legumain knockout up-regulated expression of the GR by reducing protein expression in the MAPK signaling pathway, thereby improving hippocampal synaptic plasticity as well as the emotional and cognitive impairment of model mice. This suggests that legumain may be an effective therapeutic target for emotional and cognitive deficits.

Legumain Knockout Protects Against Aβ1-42-Induced AD-like Cognitive Deficits and Synaptic Plasticity Dysfunction Via Inhibiting Neuroinflammation Without Cleaving APP

Neuroinflammation is the important pathological feature of Alzheimer's disease (AD). Legumain, a lysosomal cysteine protease, plays an important role in neuroinflammation during ischemic stroke and depressive disorder. Legumain is involved in AD process through cleaving APP; however, it is unclear if legumain can possibly modulate neuroinflammation without cleaving APP in AD. Thus, we established a mouse model of AD by single intracerebroventricular injections of Aβ1-42 in legumain knockout (KO) mice. The behavioral tests showed that legumain-KO effectively ameliorated cognitive impairment induced by Aβ1-42. Moreover, legumain deprivation significantly improves the synaptic plasticity damages in Aβ1-42-treated mice. Moreover, legumain-KO considerably inhibited the activation of microglia and reduced the expression of inflammatory cytokines in the hippocampus of Aβ1-42-treated mice. Interestingly, we found that legumain-KO inhibited TLR4/MyD88/NF-κB pathway, which was activated by Aβ1-42 in the hippocampus. In conclusion, our results suggested that legumain-KO reduced the level of neuroinflammation that was associated with inhibiting TLR4/MyD88/NF-κB pathways, thereby improving the hippocampal synaptic plasticity and reducing the cognitive impairments in Aβ1-42-treated mice. Legumain knockout blocked microglia activation by inhibiting TLR4/MyD88/NF-κB signaling pathways, and further reduced inflammatory cytokine expression. As a result, legumain knockout alleviated synaptic damage and cognitive impairment induced by Aβ1--42.

Pharmacological inhibition of asparaginyl endopeptidase by δ-secretase inhibitor 11 mitigates Alzheimer's disease-related pathologies in a senescence-accelerated mouse model

BACKGROUND

Currently, there is no cure for Alzheimer's disease (AD). Therapeutics that can modify the early stage of AD are urgently needed. Recent studies have shown that the pathogenesis of AD is closely regulated by an endo/lysosomal asparaginyl endopeptidase (AEP). Inhibition of AEP has been reported to prevent neural degeneration in transgenic mouse models of AD. However, more than 90% of AD cases are age-related sporadic AD rather than hereditary AD. The therapeutic efficacy of AEP inhibition in ageing-associated sporadic AD remains unknown.

METHODS

The senescence-accelerated mouse prone 8 (SAMP8) was chosen as an approximate model of sporadic AD and treated with a selective AEP inhibitor,: δ-secretase inhibitor 11. Activation of AEP was determined by enzymatic activity assay. Concentration of soluble amyloid β (Aβ) in the brain was determined by ELISA. Morris water maze test was performed to assess the learning and memory-related cognitive ability. Pathological changes in the brain were explored by morphological and western blot analyses.

RESULTS

The enzymatic activity of AEP in the SAMP8 mouse brain was significantly higher than that in the age-matched SAMR1 mice. The half maximal inhibitory concentration (IC50) for δ-secretase inhibitor 11 to inhibit AEP in vitro is was around 150 nM. Chronic treatment with δ-secretase inhibitor 11 markedly decreased the brain AEP activity, reduced the generation of Aβ1-40/42 and ameliorated memory loss. The inhibition of AEP with this reagent not only reduced the AEP-cleaved tau fragments and tau hyperphosphorylation, but also attenuated neuroinflammation in the form of microglial activation. Moreover, treatment with δ-secretase inhibitor 11 prevented the synaptic loss and alleviated dendritic disruption in SAMP8 mouse brain.

CONCLUSIONS

Pharmacological inhibition of AEP can intervene and prevent AD-like pathological progress in the model of sporadic AD. The up-regulated AEP in the brain could be a promising target for early treatment of AD. The δ-secretase inhibitor 11 can be used as a lead compound for translational development of AD treatment.

Norepinephrine metabolite DOPEGAL activates AEP and pathological Tau aggregation in locus coeruleus

Aberrant Tau inclusions in the locus coeruleus (LC) are the earliest detectable Alzheimer's disease-like (AD-like) neuropathology in the human brain. However, why LC neurons are selectively vulnerable to developing early Tau pathology and degenerating later in disease and whether the LC might seed the stereotypical spread of Tau pathology to the rest of the brain remain unclear. Here, we show that 3,4-dihydroxyphenylglycolaldehyde, which is produced exclusively in noradrenergic neurons by monoamine oxidase A metabolism of norepinephrine, activated asparagine endopeptidase that cleaved Tau at residue N368 into aggregation- and propagation-prone forms, thus leading to LC degeneration and the spread of Tau pathology. Activation of asparagine endopeptidase-cleaved Tau aggregation in vitro and in intact cells was triggered by 3,4-dihydroxyphenylglycolaldehyde, resulting in LC neurotoxicity and propagation of pathology to the forebrain. Thus, our findings reveal that norepinephrine metabolism and Tau cleavage represent the specific molecular mechanism underlying the selective vulnerability of LC neurons in AD.

A novel mutation in the cleavage site N291 of TDP-43 protein in a familial case of amyotrophic lateral sclerosis

Cytoplasmic aggregation of TAR-DNA binding protein (TDP-43) in Amyotrophic Lateral Sclerosis (ALS) and fronto-temporal lobar dementia (FTLD) is associated with post-translational modifications (PTM) and delocalization. Studies on postmortem brains of ALS and FTLD patients showed the existence of TDP-43 fragments that end at position N291. We report a new heterozygous mutation p.N291H in a familial case of ALS. Expression of the mutant protein in cell lines and primary motor neurons induces aggregate formation in the cytoplasm and reduces cell viability. The discovery of mutations at cleavage sites in TDP-43 in patients, which we reviewed here, is valuable for understanding the true role of the various TDP-43 fragments identified in patients and thus, for developing effective targeted therapies for ALS and FTLD treatment.

Brain and blood metabolome for Alzheimer's dementia: findings from a targeted metabolomics analysis

The development of Alzheimer's dementia (AD) accompanies both central and peripheral metabolic disturbance, but the metabolic basis underlying AD and metabolic markers predictive of AD risk remain to be determined. It is also unclear whether the metabolic changes in the peripheral blood and brain are overlapping in relation to AD. The present study addresses these questions by targeted metabolomics in both antemortem blood and postmortem brain samples in 2 community-based longitudinal cohorts of aging and dementia. We found that higher serum levels of 3 acylcarnitines, including decanoylcarnitine (C10), pimelylcarnitine (C7-DC), and tetradecadienylcarnitine (C14:2), significantly predict a lower risk of incident AD (composite hazard ratio = 0.368, 95% CI [0.207, 0.653]) after an average of 4.5-year follow-up, independent of age, sex, and education. In addition, baseline serum levels of ten glycerophospholipids, one amino acid, and 5 acylcarnitines predict the longitudinal change in cognitive functions. Moreover, 28 brain metabolites were associated with AD phenotypes. Of the putative metabolites identified in the serum and brain, 4 metabolites (3 glycerophospholipids [PC aa C30:0, PC ae C34:0, PC ae C36:1] and 1 acylcarnitine [C14:2]) were present in both the postmortem brain and antemortem blood, but only one metabolite (C14:2) was associated with AD in the same direction (i.e., protective). Partial correlation and network analyses suggest a potential tissue-specific regulation of metabolism, although other alternatives exist. Together, we identified significant associations of both central and peripheral metabolites with AD phenotypes, but there seems to be little overlap between the 2 tissues.

What is strain in neurodegenerative diseases?

Neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease, are characterized by the aggregation of misfolded proteins, including Aβ, tau and α-synuclein. It is well recognized that these misfolded proteins are able to self-propagate and spread throughout the nervous system and cause neuronal injury in a way that resembles prion disease. These disease-specific misfolded proteins demonstrate unique features, including the seeding barrier, the conformational memory effect, strain selection and strain evolution, based on the presence of various strains. However, the accurate definition of the term strain remains to be clarified. Here, a clear interpretation is proposed by a retrospective of its history in prion research and the recent progress in neurodegeneration research. Furthermore, the causes contributing to the genesis of various strains are also summarized. Deeper insight into strains helps us to understand the phenomena we observe in this field and it also enlightens us on the elusive mechanisms and management of neurodegeneration.

Legumain acts on neuroinflammatory to affect CUS-induced cognitive impairment

Cognitive impairment has been widely recognized as a central feature of depression. Legumain, a lysosomal cysteine protease, plays an important role in cancer, atherosclerosis, inflammation and other pathological conditions. Meanwhile, it has been reported that the activation of legumain aggravates the cognitive impairment in neurodegenerative diseases. In this study, we explored the role of legumain in cognitive impairment of stressed mice. Legumain knockout (legumain KO) and wildtype (WT) mice were divided into four groups: control group, chronic mild unpredictable stressed (CUS) group, legumain KO group and legumain KO + CUS group. Our results demonstrated that CUS (4 weeks) induced cognitive impairment in mice effectively based on Morris water maze (MWM) test and novel object recognition (NOR) test and decreased the synaptic plasticity. Additionally, CUS exposure significantly decreased the expression of hippocampal synapse related proteins and the cell density in the DG region, accompanied by increasing the expression of hippocampal inflammatory cytokines and promoting the activation of microglia in the hippocampus. Legumain KO distinctly restored the CUS-induced negative effects on the indicators mentioned above. In conclusion, our results suggested that legumain may be an effective therapeutic target for cognitive impairment as was seen within the CUS model and legumain KO reduced the level of neuroinflammation, thereby improving the hippocampal synaptic plasticity and cognitive impairment of stressed mice.

Asparagine endopeptidase cleaves tau at N167 after uptake into microglia

Tau cleavage by different proteolytic enzymes generates short, aggregation-prone fragments that have been implicated in the pathogenesis of Alzheimer's disease (AD). Asparagine endopeptidase (AEP) activity in particular has been associated with tau dysfunction and aggregation, and the activity of the protease is increased in both aging and AD. Using a mass spectrometry approach, we identified a novel tau cleavage site at N167 and confirmed its processing by AEP. In combination with the previously known site at N368, we show that AEP cleavage yields a tau fragment that is present in both control and AD brains at similar levels. AEP is a lysosomal enzyme, and our data suggest that it is expressed in microglia rather than in neurons. Accordingly, we observe tau cleavage at N167 and N368 after endocytotic uptake into microglia, but not neurons. However, tau_168-368 does not accumulate in microglia and we thus conclude that the fragment is part of a proteolytic cascade leading to tau degradation. While we confirm previous studies showing increased overall AEP activity in AD brains, our data suggests that AEP-mediated cleavage of tau is a physiological event occurring during microglial degradation of the secreted neuronal protein. As a consequence, we caution against preventing AEP-mediated tau cleavage as a therapeutic approach in AD.

Lansoprazole inhibits the cysteine protease legumain by binding to the active site

Proton pump inhibitors (PPIs) are prodrugs used in the treatment of peptic ulcer diseases. Once activated by acidic pH, the PPIs subsequently inhibit the secretion of gastric acid by covalently forming disulphide bonds with the SH groups of the parietal proton pump, that is the H⁺ /K⁺ -ATPase. Long-term use of PPIs has been associated with numerous adverse effects, including bone fractures. Considering the mechanism of activation, PPIs could also be active in acidic micro-environments such as in lysosomes, tumours and bone resorption sites. We suggested that the SH group in the active site of cysteine proteases could be susceptible for inhibition by PPIs. In this study, the inhibition by lansoprazole was shown on the cysteine proteases legumain and cathepsin B by incubating purified proteases or cell lysates with lansoprazole at different concentrations and pH conditions. The mechanism of legumain inhibition was shown to be a direct interaction of lansoprazole with the SH group in the active site, and thus blocking binding of the legumain-selective activity-based probe MP-L01. Lansoprazole was also shown to inhibit both legumain and cathepsin B in various cell models like HEK293, monoclonal legumain over-expressing HEK293 cells (M38L) and RAW264.7 macrophages, but not in human bone marrow-derived skeletal (mesenchymal) stem cells (hBMSC-TERT). During hBMSC-TERT differentiation to osteoblasts, lansoprazole inhibited legumain secretion, alkaline phosphatase activity, but had no effects on in vitro mineralization capacity. In conclusion, lansoprazole acts as a direct covalent inhibitor of cysteine proteases via disulphide bonds with the SH group in the protease active site. Such inhibition of cysteine proteases could explain some of the off-target effects of PPIs.

TRH Analog, Taltirelin Protects Dopaminergic Neurons From Neurotoxicity of MPTP and Rotenone

Dopaminergic neurons loss is one of the main pathological characters of Parkinson’s disease (PD), while no suitable neuroprotective agents have been in clinical use. Thyrotropin-releasing hormone (TRH) and its analogs protect neurons from ischemia and various cytotoxins, but whether the effect also applies in PD models remain unclear. Here, we showed that Taltirelin, a long-acting TRH analog, exhibited the neuroprotective effect in both cellular and animal models of PD. The in vitro study demonstrated that Taltirelin (5 μM) reduced the generation of reactive oxygen species (ROS) induced by MPP+ or rotenone, alleviated apoptosis and rescued the viability of SH-SY5Y cells and rat primary midbrain neurons. Interestingly, SH-SY5Y cells treated with Taltirelin also displayed lower level of p-tau (S396) and asparagine endopeptidase (AEP) cleavage products, tau N368 and α-synuclein N103 fragments, accompanied by a lower intracellular monoamine oxidase-B (MAO-B) activity. In the subacute MPTP-induced and chronic rotenone-induced PD mice models, we found Taltirelin (1 mg/kg) significantly improved the locomotor function and preserved dopaminergic neurons in the substantia nigra (SN). In accordance with the in vitro study, Taltirelin down-regulated the levels of p-tau (S396), p-α-synuclein (S129) tau N368 and α-synuclein N103 fragments in SN and striatum. Together, this study demonstrates that Taltirelin may exert neuroprotective effect via inhibiting MAO-B and reducing the oxidative stress and apoptosis, preventing AEP activation and its subsequent pathological cleavage of tau and α-synuclein, thus provides evidence for Taltirelin in protective treatment of PD.

The Potential Use of Plant Natural Products and Plant Extracts with Antioxidant Properties for the Prevention/Treatment of Neurodegenerative Diseases: In Vitro, In Vivo and Clinical Trials

Neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease and Huntington's disease, present a major health issue and financial burden for health care systems around the world. The impact of these diseases will further increase over the next decades due to increasing life expectancies. No cure is currently available for the treatment of these conditions; only drugs, which merely alleviate the symptoms. Oxidative stress has long been associated with neurodegeneration, whether as a cause or as part of the downstream results caused by other factors. Thus, the use of antioxidants to counter cellular oxidative stress within the nervous system has been suggested as a potential treatment option for neurological disorders. Over the last decade, significant research has focused on the potential use of natural antioxidants to target oxidative stress. However, clinical trial results have lacked success for the treatment of patients with neurological disorders. The knowledge that natural extracts show other positive molecular activities in addition to antioxidant activity, however, has led to further research of natural extracts for their potential use as prevention or treatment/management of neurodegenerative diseases. This review will cover several in vitro and in vivo research studies, as well as clinical trials, and highlight the potential of natural antioxidants.

Delta-secretase (AEP) mediates tau-splicing imbalance and accelerates cognitive decline in tauopathies

Wang et al. demonstrate that AEP cleaves SRPK2 in tauopathies and plays a functional role in mediating tau-splicing imbalance and accelerating cognitive decline in mouse models of tauopathy.

SRPK2 is abnormally activated in tauopathies including Alzheimer’s disease (AD). SRPK2 is known to play an important role in pre–mRNA splicing by phosphorylating SR-splicing factors. Dysregulation of tau exon 10 pre–mRNA splicing causes pathological imbalances in 3R- and 4R-tau, leading to neurodegeneration; however, the role of SRPK2 in these processes remains unclear. Here we show that delta-secretase (also known as asparagine endopeptidase; AEP), which is activated in AD, cleaves SRPK2 and increases its nuclear translocation as well as kinase activity, augmenting exon 10 inclusion. Conversely, AEP-uncleavable SRPK2 N342A mutant increases exon 10 exclusion. Lentiviral expression of truncated SRPK2 increases 4R-tau isoforms and accelerates cognitive decline in htau mice. Uncleavable SRPK2 N342A expression improves synaptic functions and prevents spatial memory deficits in tau intronic mutant FTDP-17 transgenic mice. Hence, AEP mediates tau-splicing imbalance in tauopathies via cleaving SRPK2.

Promoting the clearance of neurotoxic proteins in neurodegenerative disorders of ageing

Neurodegenerative disorders of ageing (NDAs) such as Alzheimer disease, Parkinson disease, frontotemporal dementia, Huntington disease and amyotrophic lateral sclerosis represent a major socio-economic challenge in view of their high prevalence yet poor treatment. They are often called 'proteinopathies' owing to the presence of misfolded and aggregated proteins that lose their physiological roles and acquire neurotoxic properties. One reason underlying the accumulation and spread of oligomeric forms of neurotoxic proteins is insufficient clearance by the autophagic-lysosomal network. Several other clearance pathways are also compromised in NDAs: chaperone-mediated autophagy, the ubiquitin-proteasome system, extracellular clearance by proteases and extrusion into the circulation via the blood-brain barrier and glymphatic system. This article focuses on emerging mechanisms for promoting the clearance of neurotoxic proteins, a strategy that may curtail the onset and slow the progression of NDAs.

Proteolytic ectodomain shedding of membrane proteins in mammals-hardware, concepts, and recent developments

Proteolytic removal of membrane protein ectodomains (ectodomain shedding) is a post-translational modification that controls levels and function of hundreds of membrane proteins. The contributing proteases, referred to as sheddases, act as important molecular switches in processes ranging from signaling to cell adhesion. When deregulated, ectodomain shedding is linked to pathologies such as inflammation and Alzheimer's disease. While proteases of the "a disintegrin and metalloprotease" (ADAM) and "beta-site APP cleaving enzyme" (BACE) families are widely considered as sheddases, in recent years a much broader range of proteases, including intramembrane and soluble proteases, were shown to catalyze similar cleavage reactions. This review demonstrates that shedding is a fundamental process in cell biology and discusses the current understanding of sheddases and their substrates, molecular mechanisms and cellular localizations, as well as physiological functions of protein ectodomain shedding. Moreover, we provide an operational definition of shedding and highlight recent conceptual advances in the field. While new developments in proteomics facilitate substrate discovery, we expect that shedding is not a rare exception, but rather the rule for many membrane proteins, and that many more interesting shedding functions await discovery.

Tau Proteolysis in the Pathogenesis of Tauopathies: Neurotoxic Fragments and Novel Biomarkers

With predictions showing that 131.5 million people worldwide will be living with dementia by 2050, an understanding of the molecular mechanisms underpinning disease is crucial in the hunt for novel therapeutics and for biomarkers to detect disease early and/or monitor disease progression. The metabolism of the microtubule-associated protein tau is altered in different dementias, the so-called tauopathies. Tau detaches from microtubules, aggregates into oligomers and neurofibrillary tangles, which can be secreted from neurons, and spreads through the brain during disease progression. Post-translational modifications exacerbate the production of both oligomeric and soluble forms of tau, with proteolysis by a range of different proteases being a crucial driver. However, the impact of tau proteolysis on disease progression has been overlooked until recently. Studies have highlighted that proteolytic fragments of tau can drive neurodegeneration in a fragment-dependent manner as a result of aggregation and/or transcellular propagation. Proteolytic fragments of tau have been found in the cerebrospinal fluid and plasma of patients with different tauopathies, providing an opportunity to develop these fragments as novel disease progression biomarkers. A range of therapeutic strategies have been proposed to halt the toxicity associated with proteolysis, including reducing protease expression and/or activity, selectively inhibiting protease-substrate interactions, and blocking the action of the resulting fragments. This review highlights the importance of tau proteolysis in the pathogenesis of tauopathies, identifies putative sites during tau fragment-mediated neurodegeneration that could be targeted therapeutically, and discusses the potential use of proteolytic fragments of tau as biomarkers for different tauopathies.

Delta-Secretase Phosphorylation by SRPK2 Enhances Its Enzymatic Activity, Provoking Pathogenesis in Alzheimer's Disease

Delta-secretase, a lysosomal asparagine endopeptidase (AEP), simultaneously cleaves both APP and tau, controlling the onset of pathogenesis of Alzheimer's disease (AD). However, how this protease is post-translationally regulated remains unclear. Here we report that serine-arginine protein kinase 2 (SRPK2) phosphorylates delta-secretase and enhances its enzymatic activity. SRPK2 phosphorylates serine 226 on delta-secretase and accelerates its autocatalytic cleavage, leading to its cytoplasmic translocation and escalated enzymatic activities. Delta-secretase is highly phosphorylated in human AD brains, tightly correlated with SRPK2 activity. Overexpression of a phosphorylation mimetic (S226D) in young 3xTg mice strongly promotes APP and tau fragmentation and facilitates amyloid plaque deposits and neurofibrillary tangle (NFT) formation, resulting in cognitive impairment. Conversely, viral injection of the non-phosphorylatable mutant (S226A) into 5XFAD mice decreases APP and tau proteolytic cleavage, attenuates AD pathologies, and reverses cognitive defects. Our findings support that delta-secretase phosphorylation by SRPK2 plays a critical role in aggravating AD pathogenesis.

Roles of tau protein in health and disease

Tau is well established as a microtubule-associated protein in neurons. However, under pathological conditions, aberrant assembly of tau into insoluble aggregates is accompanied by synaptic dysfunction and neural cell death in a range of neurodegenerative disorders, collectively referred to as tauopathies. Recent advances in our understanding of the multiple functions and different locations of tau inside and outside neurons have revealed novel insights into its importance in a diverse range of molecular pathways including cell signalling, synaptic plasticity, and regulation of genomic stability. The present review describes the physiological and pathophysiological properties of tau and how these relate to its distribution and functions in neurons. We highlight the post-translational modifications of tau, which are pivotal in defining and modulating tau localisation and its roles in health and disease. We include discussion of other pathologically relevant changes in tau, including mutation and aggregation, and how these aspects impinge on the propensity of tau to propagate, and potentially drive neuronal loss, in diseased brain. Finally, we describe the cascade of pathological events that may be driven by tau dysfunction, including impaired axonal transport, alterations in synapse and mitochondrial function, activation of the unfolded protein response and defective protein degradation. It is important to fully understand the range of neuronal functions attributed to tau, since this will provide vital information on its involvement in the development and pathogenesis of disease. Such knowledge will enable determination of which critical molecular pathways should be targeted by potential therapeutic agents developed for the treatment of tauopathies.

Suppression of asparaginyl endopeptidase attenuates breast cancer-induced bone pain through inhibition of neurotrophin receptors

Objective Cancer-induced bone pain is a common clinical problem in breast cancer patients with bone metastasis. However, the mechanisms driving cancer-induced bone pain are poorly known. Recent studies show that a novel protease, asparaginyl endopeptidase (AEP) plays crucial roles in breast cancer metastasis and progression. We aim to determine the functions and targeted suppress of AEP in a mouse model of breast cancer-induced bone pain. Methods Breast cancer cells with AEP knocked-down or overexpression were constructed and implanted into the intramedullary space of the femur to induce pain-like behavior in mice. AEP-specific inhibitors or purified AEP proteins were further used in animal model. The histological characters of femur and pain ethological changes were measured. The expressions of AEP and neurotrophin receptors (p75NTR and TrkA) in dorsal root ganglion and spinal cord were examined. Results Femur radiographs and histological analysis revealed that cells with AEP knocked-down reduced bone destruction and pain behaviors. However, cells with AEP overexpression elevated bone damage and pain behaviors. Further, Western blot results found that the expressions of p75NTR and TrkA in dorsal root ganglions and spinal cords were reduced in mice inoculated with AEP knocked-down cells. Targeted suppression of AEP with specific small compounds significantly reduced the bone pain while purified recombinant AEP proteins increased bone pain. Conclusions AEP aggravate the development of breast cancer bone metastasis and bone pain by increasing the expression of neurotrophin receptors. AEP might be an effective target for treatment of breast cancerinduced bone pain.