Cysteine protease inhibitors reduce brain beta-amyloid and beta-secretase activity in vivo and are potential Alzheimer's disease therapeutics

Aging and brain free cholesterol concentration on amyloid-β peptide accumulation in guinea pigs

Alzheimer's disease is a complex multifactorial neurodegenerative disorder wherein age is a major risk factor. The appropriateness of the Hartley guinea pig (GP), which displays high sequence homologies of its amyloid-β (Aβ40 and Aβ42) peptides, Mdr1 and APP (amyloid precursor protein) and similarity in lipid handling to humans, was appraised among 9-40 weeks old guinea pigs. Protein expression levels of P-gp (Abcb1) and Cyp46a1 (24(S)-hydroxylase) for Aβ40, and Aβ42 efflux and cholesterol metabolism, respectively, were decreased with age, whereas those for Lrp1 (low-density lipoprotein receptor related protein 1), Rage (receptor for advanced glycation endproducts) for Aβ efflux and influx, respectively, and Abca1 (the ATP binding cassette subfamily A member 1) for cholesterol efflux, were unchanged among the ages examined. There was a strong, negative correlation of the brain Aβ peptide concentrations and Abca1 protein expression levels with free cholesterol. The correlation of Aβ peptide concentrations with Cyp46a1 was, however, not significant, and concentrations of the 24(S)-hydroxycholesterol metabolite revealed a decreasing trend from 20 weeks old toward 40 weeks old guinea pigs. The composite data suggest a role for free cholesterol on brain Aβ accumulation. The decreases in P-gp and Lrp1 protein levels should further exacerbate the accumulation of Aβ peptides in guinea pig brain.

Identification of Natural Compounds of the Apple as Inhibitors against Cholinesterase for the Treatment of Alzheimer's Disease: An In Silico Molecular Docking Simulation and ADMET Study

Alzheimer's disease (AD), the most common type of dementia in older people, causes neurological problems associated with memory and thinking. The key enzymes involved in Alzheimer's disease pathways are acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE). Because of this, there is a lot of interest in finding new AChE inhibitors. Among compounds that are not alkaloids, flavonoids have stood out as good candidates. The apple fruit, Malus domestica (Rosaceae), is second only to cranberries regarding total phenolic compound concentration. Computational tools and biological databases were used to investigate enzymes and natural compounds. Molecular docking techniques were used to analyze the interactions of natural compounds of the apple with enzymes involved in the central nervous system (CNS), acetylcholinesterase, and butyrylcholinesterase, followed by binding affinity calculations using the AutoDock tool. The molecular docking results revealed that CID: 107905 exhibited the best interactions with AChE, with a binding affinity of -12.2 kcal/mol, and CID: 163103561 showed the highest binding affinity with BuChE, i.e., -11.2 kcal/mol. Importantly, it was observed that amino acid residue Trp286 of AChE was involved in hydrogen bond formation, Van Der Walls interactions, and Pi-Sigma/Pi-Pi interactions in the studied complexes. Moreover, the results of the Molecular Dynamics Simulation (MDS) analysis indicated interaction stability. This study shows that CID: 12000657 could be used as an AChE inhibitor and CID: 135398658 as a BuChE inhibitor to treat Alzheimer's disease and other neurological disorders.

Intestinal Flora Balance Therapy Based on Probiotic Support Improves Cognitive Function and Symptoms in Patients with Alzheimer's Disease: A Systematic Review and Meta-analysis

Objective

The clinical value of intestinal flora balance therapy based on probiotic support in improving cognitive function and symptoms of patients with Alzheimer's disease was to systematically evaluate, so as to provide evidence-based medicine basis for the promotion and use of this therapy.

Methods

The randomized controlled trials (RCTs) were searched for the improvement of cognitive function and symptoms of patients with Alzheimer's disease by intestinal flora balance therapy supported mainly by probiotics in PubMed, EMBASE, ScienceDirect, Cochrane Library, China Knowledge Network Database (CNKI), China VIP database, Wanfang database, and China Biomedical Literature Database (CBM) online database (RCT). Data were extracted independently by two researchers, and the literature was assessed for risk of bias according to the Cochrane Handbook 5.1.0 criteria. The data were meta-analyzed using RevMan 5.4 statistical software.

Results

Finally, 5 randomized controlled trials were included, with a total sample size of 386 cases. The results of meta-analysis showed that Chi2 = 13.14, df = 2, P = 0.001, and I 2 = 85% showed significant heterogeneity in the inclusion of the study data. Probiotic-supported intestinal microflora balance therapy improves cognitive function in patients with Alzheimer's disease. Through meta-analysis of transient memory scores, it is concluded that intestinal flora balance therapy based on probiotic support can improve transient memory in patients with Alzheimer's disease. Meta-analysis of ADAS-COG score showed that intestinal flora balance therapy supported by probiotics could improve the cognitive function of patients with Alzheimer's disease. The ADL score was analyzed by meta, and the heterogeneity test result was Chi2 = 0.79, df = 1, P = 0.37 > 0.05, and I 2 = 0%, indicating that the intestinal flora balance therapy supported by probiotics can improve the ability of daily living of patients with Alzheimer's disease.

Conclusion

Intestinal flora balance therapy based on probiotic support can effectively improve cognitive function, instantaneous memory, and ability of daily life in patients with Alzheimer's disease. However, more studies and long-term follow-up studies with higher methodological quality are needed to further verify.

Cathepsin B - A Neuronal Death Mediator in Alzheimer’s Disease Leads to Neurodegeneration

Abstract:

The lysosomal cysteine protease enzyme, named Cathepsin B, mainly degrades the protein and manages its average turnover in our body. The Cathepsin B active form is mostly present inside the lysosomal part at a cellular level, providing the slightly acidic medium for its activation. Multiple findings on Cathepsin B reveal its involvement in neurons' degeneration and a possible role as a neuronal death mediator in several neurodegenerative diseases. In this review article, we highlight the participation of Cathepsin B in the etiology/progress of AD, along with various other factors. The enzyme is involved in producing neurotoxic Aβ amyloid in the AD brain by acting as the β-secretase enzyme in the regulated secretory pathways responsible for APP processing. Aβ amyloid accumulation and amyloid plaque formation lead to neuronal degeneration, one of the prominent pathological hallmarks of AD. Cathepsin B is also involved in the production of PGlu-Aβ, which is a truncated and highly neurotoxic form of Aβ. Some of the findings also revealed that Cathepsin B specific gene deletion decreases the level of PGlu-Aβ inside the brain of experimental mice. Therefore, neurotoxicity might be considered a new pathological indication of AD due to the involvement of Cathepsin B. It also damages neurons present in the CNS region by producing inflammatory responses and generating mitochondrial ROS. However, Cathepsin B inhibitors, i.e., CA-074, can prevent neuronal death in AD patients. The other natural inhibitors are also equally effective against neuronal damage with higher selectivity. Its synthetic inhibitors are specific for their target; however, they lose their selectivity in the presence of quite a few reducing agents. Therefore, a humanized monoclonal antibody is used as a selective Cathepsin B inhibitor to overcome the problem experienced. The use of Cathepsin B for the treatment of AD and other neurodegenerative diseases could be considered a rational therapeutic target.

Lysosomal peptidases – Intriguing roles in cancer progression and neurodegeneration

Lysosomal peptidases are hydrolytic enzymes capable of digesting waste proteins that are targeted to lysosomes via endocytosis and autophagy. Besides intracellular protein catabolism, they play more specific roles in several other cellular processes and pathologies, either within lysosomes, upon secretion into the cell cytoplasm or extracellular space, or bound to the plasma membrane. In cancer, lysosomal peptidases are generally associated with disease progression, as they participate in crucial processes leading to changes in cell morphology, signaling, migration, and invasion, and finally metastasis. However, they can also enhance the mechanisms resulting in cancer regression, such as apoptosis of tumor cells or antitumor immune responses. Lysosomal peptidases have also been identified as hallmarks of aging and neurodegeneration, playing roles in oxidative stress, mitochondrial dysfunction, abnormal intercellular communication, dysregulated trafficking, and the deposition of protein aggregates in neuronal cells. Furthermore, deficiencies in lysosomal peptidases may result in other pathological states, such as lysosomal storage disease. The aim of this review was to highlight the role of lysosomal peptidases in particular pathological processes of cancer and neurodegeneration and to address the potential of lysosomal peptidases in diagnosing and treating patients.

New Insights into the Role of Cysteine Cathepsins in Neuroinflammation

Neuroinflammation, which is mediated by microglia and astrocytes, is associated with the progression of neurodegenerative diseases. Increasing evidence shows that activated microglia induce the expression and secretion of various lysosomal cathepsins, particularly during the early stage of neuroinflammation. This trigger signaling cascade that aggravate neurodegeneration. To date, most research on neuroinflammation has focused on the role of cysteine cathepsins, the largest cathepsin family. Cysteine cathepsins are primarily responsible for protein degradation in lysosomes; however, they also play a role in regulating a number of other important physiological and pathological processes. This review focuses on the functional roles of cysteine cathepsins in the central nervous system during neuroinflammation, with an emphasis on their roles in the polarization of microglia and neuroinflammation signaling, which in turn causes neuronal death and thus neurodegeneration.

Neuroinflammation-Induced Upregulation of Glial Cathepsin X Expression and Activity in vivo

Neuroinflammation is an important factor in the pathogenesis of neurodegenerative diseases. Microglia-derived lysosomal cathepsins have been increasingly recognized as important inflammatory mediators that trigger signaling pathways that aggravate neuroinflammation. In vitro, a contribution to neuroinflammation processes has been shown for cathepsin X: however, the expression patterns and functional role of cathepsin X in neuroinflammatory brain pathology remain elusive. In this study we analyzed the expression, activity, regional distribution and cellular localization of cathepsin X in the rat brain with neuroinflammation-induced neurodegeneration. The unilateral injection of lipopolysaccharide (LPS) induced a strong upregulation of cathepsin X expression and its activity in the ipsilateral striatum. In addition to the striatum, cathepsin X overexpression was detected in other brain areas such as the cerebral cortex, corpus callosum, subventricular zone and external globus pallidus, whereas the upregulation was mainly restricted to activated microglia and reactive astrocytes. Continuous administration of the cathepsin X inhibitor AMS36 indicated protective effects against LPS-induced striatal degeneration, as seen by the attenuated LPS-mediated dilation of the lateral ventricles and partial decreased extent of striatal lesion. Taken together, our results indicate that cathepsin X plays a role as a pathogenic factor in neuroinflammation-induced neurodegeneration and represents a potential therapeutic target for neurodegenerative diseases associated with neuroinflammation.

Structure-based Approaches Targeting Parasite Cysteine Proteases

Cysteine proteases are essential hydrolytic enzymes present in the majority of organisms, including viruses and unicellular parasites. Despite the high sequence identity displayed among these proteins, specific structural features across different species grant distinct functions to these biomolecules, frequently related to pathological conditions. Consequently, their relevance as promising targets for potential specific inhibitors has been highlighted and occasionally validated in recent decades. In this review, we discuss the recent outcomes of structure-based campaigns aiming the discovery of new inhibitor prototypes against cruzain and falcipain, as alternative therapeutic tools for Chagas disease and malaria treatments, respectively. Computational and synthetic approaches have been combined on hit optimization strategies and are also discussed herein. These rationales are extended to additional tropical infectious and neglected pathologies, such as schistosomiasis, leishmaniasis and babesiosis, and also to Alzheimer's Disease, a widespread neurodegenerative disease poorly managed by currently available drugs and recently linked to particular physiopathological roles of human cysteine proteases.

Are the neuroprotective effects of exercise training systemically mediated?

To date there is no cure available for dementia, and the field calls for novel therapeutic targets. A rapidly growing body of literature suggests that regular endurance training and high cardiorespiratory fitness attenuate cognitive impairment and reduce dementia risk. Such benefits have recently been linked to systemic neurotrophic factors induced by exercise. These circulating biomolecules may cross the blood-brain barrier and potentially protect against neurodegenerative disorders such as Alzheimer's disease. Identifying exercise-induced systemic neurotrophic factors with beneficial effects on the brain may lead to novel molecular targets for maintaining cognitive function and preventing neurodegeneration. Here we review the recent literature on potential systemic mediators of neuroprotection induced by exercise. We focus on the body of translational research in the field, integrating knowledge from the molecular level, animal models, clinical and epidemiological studies. Taken together, the current literature provides initial evidence that exercise-induced, blood-borne biomolecules, such as BDNF and FNDC5/irisin, may be powerful agents mediating the benefits of exercise on cognitive function and may form the basis for new therapeutic strategies to better prevent and treat dementia.

Loss of Cathepsin B and L Leads to Lysosomal Dysfunction, NPC-Like Cholesterol Sequestration and Accumulation of the Key Alzheimer's Proteins

Proper function of lysosomes is particularly important in neurons, as they cannot dilute accumulated toxic molecules and aggregates by cell division. Thus, impairment of lysosomal function plays an important role in neuronal degeneration and in the pathogenesis of numerous neurodegenerative diseases. In this work we analyzed how inhibition and/or loss of the major lysosomal proteases, the cysteine cathepsins B and L (CtsB/L), affects lysosomal function, cholesterol metabolism and degradation of the key Alzheimer's disease (AD) proteins. Here, we show that cysteine CtsB/L, and not the aspartyl cathepsin D (CtsD), represent a major lysosomal protease(s) that control lysosomal function, intracellular cholesterol trafficking and AD-like amyloidogenic features. Intriguingly, accumulation of free cholesterol in late endosomes/lysosomes upon CtsB/L inhibition resembled a phenotype characteristic for the rare neurodegenerative disorder Niemann-Pick type C (NPC). CtsB/L inhibition and not the inhibition of CtsD led to lysosomal impairment assessed by decreased degradation of EGF receptor, enhanced LysoTracker staining and accumulation of several lysosomal proteins LC3II, NPC1 and NPC2. By measuring the levels of NPC1 and ABCA1, the two major cholesterol efflux proteins, we showed that CtsB/L inhibition or genetic depletion caused accumulation of the NPC1 in lysosomes and downregulation of ABCA1 protein levels and its expression. Furthermore, we revealed that CtsB/L are involved in degradation of the key Alzheimer's proteins: amyloid-β peptides (Aβ) and C-terminal fragments of the amyloid precursor protein (APP) and in degradation of β-secretase (BACE1). Our results imply CtsB/L as major regulators of lysosomal function and demonstrate that CtsB/L may play an important role in intracellular cholesterol trafficking and in degradation of the key AD proteins. Our findings implicate that enhancing the activity or levels of CtsB/L could provide a promising and a common strategy for maintaining lysosomal function and for preventing and/or treating neurodegenerative diseases.

Why calpain inhibitors are interesting leading compounds to search for new therapeutic options to treat leishmaniasis?

Leishmaniasis is a neglected disease, which needs improvements in drug development, mainly due to the toxicity, parasite resistance and low compliance of patients to treatment. Therefore, the development of new chemotherapeutic compounds is an urgent need. This opinion article will briefly highlight the feasible use of calpain inhibitors as leading compounds to search for new therapeutic options to treat leishmaniasis. The milestone of this approach is to take advantage on the myriad of inhibitors developed against calpains, some of which are in advanced clinical trials. The deregulated activity of these enzymes is associated with several pathologies, such as strokes, diabetes and Parkinson's disease, to name a few. In Leishmania, calpain upregulation has been associated to drug resistance and virulence. Whereas the difficulties in developing new drugs for neglected diseases are more economical than biotechnological, repurposing approach with compounds already approved for clinical use by the regulatory agencies can be an interesting shortcut to a successful chemotherapeutic treatment for leishmaniasis.

Cathepsin B is a New Drug Target for Traumatic Brain Injury Therapeutics: Evidence for E64d as a Promising Lead Drug Candidate

There is currently no therapeutic drug treatment for traumatic brain injury (TBI) despite decades of experimental clinical trials. This may be because the mechanistic pathways for improving TBI outcomes have yet to be identified and exploited. As such, there remains a need to seek out new molecular targets and their drug candidates to find new treatments for TBI. This review presents supporting evidence for cathepsin B, a cysteine protease, as a potentially important drug target for TBI. Cathepsin B expression is greatly up-regulated in TBI animal models, as well as in trauma patients. Importantly, knockout of the cathepsin B gene in TBI mice results in substantial improvements of TBI-caused deficits in behavior, pathology, and biomarkers, as well as improvements in related injury models. During the process of TBI-induced injury, cathepsin B likely escapes the lysosome, its normal subcellular location, into the cytoplasm or extracellular matrix (ECM) where the unleashed proteolytic power causes destruction via necrotic, apoptotic, autophagic, and activated glia-induced cell death, together with ECM breakdown and inflammation. Significantly, chemical inhibitors of cathepsin B are effective for improving deficits in TBI and related injuries including ischemia, cerebral bleeding, cerebral aneurysm, edema, pain, infection, rheumatoid arthritis, epilepsy, Huntington's disease, multiple sclerosis, and Alzheimer's disease. The inhibitor E64d is unique among cathepsin B inhibitors in being the only compound to have demonstrated oral efficacy in a TBI model and prior safe use in man and as such it is an excellent tool compound for preclinical testing and clinical compound development. These data support the conclusion that drug development of cathepsin B inhibitors for TBI treatment should be accelerated.

Cysteine cathepsins in neurological disorders

Increased proteolytic activity is a hallmark of several pathological processes, including neurodegeneration. Increased expression and activity of cathepsins, lysosomal cysteine proteases, during degeneration of the central nervous system is frequently reported. Recent studies reveal that a disturbed balance of their enzymatic activities is the first insult in brain aging and age-related diseases. Leakage of cathepsins from lysosomes, due to their membrane permeability, and activation of pro-apoptotic factors additionally contribute to neurodegeneration. Furthermore, in inflammation-induced neurodegeneration the cathepsins expressed in activated microglia play a pivotal role in neuronal death. The proteolytic activity of cysteine cathepsins is controlled by endogenous protein inhibitors-the cystatins-which evidently fail to perform their function in neurodegenerative processes. Exogenous synthetic inhibitors, which may augment their inhibitory potential, are considered as possible therapeutic tools for the treatment of neurological disorders.

Design, synthesis, and optimization of novel epoxide incorporating peptidomimetics as selective calpain inhibitors

Hyperactivation of the calcium-dependent cysteine protease calpain 1 (Cal1) is implicated as a primary or secondary pathological event in a wide range of illnesses and in neurodegenerative states, including Alzheimer's disease (AD). E-64 is an epoxide-containing natural product identified as a potent nonselective, calpain inhibitor, with demonstrated efficacy in animal models of AD. By use of E-64 as a lead, three successive generations of calpain inhibitors were developed using computationally assisted design to increase selectivity for Cal1. First generation analogues were potent inhibitors, effecting covalent modification of recombinant Cal1 catalytic domain (Cal1cat), demonstrated using LC-MS/MS. Refinement yielded second generation inhibitors with improved selectivity. Further library expansion and ligand refinement gave three Cal1 inhibitors, one of which was designed as an activity-based protein profiling probe. These were determined to be irreversible and selective inhibitors by kinetics studies comparing full length Cal1 with the general cysteine protease papain.

Deletion of the cathepsin B gene improves memory deficits in a transgenic ALZHeimer's disease mouse model expressing AβPP containing the wild-type β-secretase site sequence

Therapeutic agents that improve the memory loss of Alzheimer's disease (AD) may eventually be developed if drug targets are identified that improve memory deficits in appropriate AD animal models. One such target is β-secretase which, in most AD patients, cleaves the wild-type (WT) β-secretase site sequence of the amyloid-β protein precursor (AβPP) to produce neurotoxic amyloid-β (Aβ). Thus, an animal model representing most AD patients for evaluating β-secretase effects on memory deficits is one that expresses human AβPP containing the WT β-secretase site sequence. BACE1 and cathepsin B (CatB) proteases have β-secretase activity, but gene knockout studies have not yet validated that the absence of these proteases improves memory deficits in such an animal model. This study assessed the effects of deleting these protease genes on memory deficits in the AD mouse model expressing human AβPP containing the WT β-secretase site sequence and the London γ-secretase site (AβPPWT/Lon mice). Knockout of the CatB gene in the AβPPWT/Lon mice improved memory deficits and altered the pattern of Aβ-related biomarkers in a manner consistent with CatB having WT β-secretase activity. But deletion of the BACE1 gene had no effect on these parameters in the AβPPWT/Lon mice. These data are the first to show that knockout of a putative β-secretase gene results in improved memory in an AD animal model expressing the WT β-secretase site sequence of AβPP, present in the majority of AD patients. CatB may be an effective drug target for improving memory deficits in most AD patients.

The cysteine protease inhibitor, E64d, reduces brain amyloid-β and improves memory deficits in Alzheimer's disease animal models by inhibiting cathepsin B, but not BACE1, β-secretase activity

The cysteine protease cathepsin B is a potential drug target for reducing brain amyloid-β (Aβ) and improving memory in Alzheimer's disease (AD), as reduction of cathepsin B in transgenic mice expressing human wild-type amyloid-β protein precursor (AβPP) results in significantly decreased brain Aβ. Cathepsin B cleaves the wild-type β-secretase site sequence in AβPP to produce Aβ, and cathepsin B inhibitors administered to animal models expressing AβPP containing the wild-type β-secretase site sequence reduce brain Aβ in a manner consistent with β-secretase inhibition. But such inhibitors could act either by direct inhibition of cathepsin B β-secretase activity or by off-target inhibition of the other β-secretase, the aspartyl protease BACE1. To evaluate that issue, we orally administered a cysteine protease inhibitor, E64d, to normal guinea pigs or transgenic mice expressing human AβPP, both of which express the human wild-type β-secretase site sequence. In guinea pigs, oral E64d administration caused a dose-dependent reduction of up to 92% in brain, CSF, and plasma of Aβ40 and Aβ42, a reduction of up to 50% in the C-terminal β-secretase fragment (CTFβ), and a 91% reduction in brain cathepsin B activity, but increased brain BACE1 activity by 20%. In transgenic AD mice, oral E64d administration improved memory deficits and reduced brain Aβ40 and Aβ42, amyloid plaque, brain CTFβ, and brain cathepsin B activity, but increased brain BACE1 activity. We conclude that E64d likely reduces brain Aβ by inhibiting cathepsin B and not BACE1 β-secretase activity and that E64d therefore may have potential for treating AD patients.

Cathepsins S, B and L with aminopeptidases display β-secretase activity associated with the pathogenesis of Alzheimer's disease

β-site APP-cleaving enzyme (BACE1) cleaves the wild type (WT) β-site very slowly (k(cat)/K(m): 46.6 m(-1) s(-1)). Therefore we searched for additional β-secretases and identified three cathepsins that split the WT β-site much faster. Human cathepsin S cleaves the WT β-site (k(cat)/K(m): 54 700 m(-1) s(-1)) 1170-fold faster than BACE1 and cathepsins B and L are 440- and 74-fold faster than BACE1, respectively. These cathepsins split two bonds flanking the WT β-site (K-MD-A), where the K-M bond (85%) is cleaved more efficiently than the D-A bond (15%). Cleavage at the major K-M bond yields Aβ (amyloid β-peptide) extended by N-terminal Met that should be removed to generate Aβ initiated by Asp1. The activity of cytosol and microsomal aminopeptidases on relevant peptides revealed rapid removal of N-terminal Met but not N-terminal Asp. Brain aminopeptidases showed similar specificity. Thus, aminopeptidases would convert Aβ extended by Met into regular Aβ (Asp1) found in amyloid plaques. Earlier studies indicate that Aβ is likely produced in the endosome and lysosome system where cathepsins S, B and L are localized and cysteine cathepsin inhibitors reduce the level of Aβ in cells and animals. Taken together, cathepsins S, B and L deserve further evaluation as therapeutic targets to develop disease modifying drugs to treat Alzheimer's disease.

Cysteine Cathepsins in the secretory vesicle produce active peptides: Cathepsin L generates peptide neurotransmitters and cathepsin B produces beta-amyloid of Alzheimer's disease

Recent new findings indicate significant biological roles of cysteine cathepsin proteases in secretory vesicles for production of biologically active peptides. Notably, cathepsin L in secretory vesicles functions as a key protease for proteolytic processing of proneuropeptides (and prohormones) into active neuropeptides that are released to mediate cell-cell communication in the nervous system for neurotransmission. Moreover, cathepsin B in secretory vesicles has been recently identified as a β-secretase for production of neurotoxic β- amyloid (Aβ) peptides that accumulate in Alzheimer's disease (AD), participating as a notable factor in the severe memory loss in AD. These secretory vesicle functions of cathepsins L and B for production of biologically active peptides contrast with the well-known role of cathepsin proteases in lysosomes for the degradation of proteins to result in their inactivation. The unique secretory vesicle proteome indicates proteins of distinct functional categories that provide the intravesicular environment for support of cysteine cathepsin functions. Features of the secretory vesicle protein systems insure optimized intravesicular conditions that support the proteolytic activity of cathepsins. These new findings of recently discovered biological roles of cathepsins L and B indicate their significance in human health and disease. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Protective effects of positive lysosomal modulation in Alzheimer's disease transgenic mouse models

Alzheimer's disease (AD) is an age-related neurodegenerative pathology in which defects in proteolytic clearance of amyloid β peptide (Aβ) likely contribute to the progressive nature of the disorder. Lysosomal proteases of the cathepsin family exhibit up-regulation in response to accumulating proteins including Aβ_1–42. Here, the lysosomal modulator Z-Phe-Ala-diazomethylketone (PADK) was used to test whether proteolytic activity can be enhanced to reduce the accumulation events in AD mouse models expressing different levels of Aβ pathology. Systemic PADK injections in APP_SwInd and APPswe/PS1ΔE9 mice caused 3- to 8-fold increases in cathepsin B protein levels and 3- to 10-fold increases in the enzyme's activity in lysosomal fractions, while neprilysin and insulin-degrading enzyme remained unchanged. Biochemical analyses indicated the modulation predominantly targeted the active mature forms of cathepsin B and markedly changed Rab proteins but not LAMP1, suggesting the involvement of enhanced trafficking. The modulated lysosomal system led to reductions in both Aβ immunostaining as well as Aβ_x-42 sandwich ELISA measures in APP_SwInd mice of 10–11 months. More extensive Aβ deposition in 20-22-month APPswe/PS1ΔE9 mice was also reduced by PADK. Selective ELISAs found that a corresponding production of the less pathogenic Aβ_1–38 occurs as Aβ_1–42 levels decrease in the mouse models, indicating that PADK treatment leads to Aβ truncation. Associated with Aβ clearance was the elimination of behavioral and synaptic protein deficits evident in the two transgenic models. These findings indicate that pharmacologically-controlled lysosomal modulation reduces Aβ_1–42 accumulation, possibly through intracellular truncation that also influences extracellular deposition, and in turn offsets the defects in synaptic composition and cognitive functions. The selective modulation promotes clearance at different levels of Aβ pathology and provides proof-of-principle for small molecule therapeutic development for AD and possibly other protein accumulation disorders.

Pharmacogenetic features of cathepsin B inhibitors that improve memory deficit and reduce beta-amyloid related to Alzheimer's disease

Beta-amyloid (Abeta) in the brain is a major factor involved in Alzheimer's disease (AD) that results in severe memory deficit. Our recent studies demonstrate pharmacogenetic differences in the effects of inhibitors of cathepsin B to improve memory and reduce Abeta in different mouse models of AD. The inhibitors improve memory and reduce brain Abeta in mice expressing the wild-type (WT) beta-secretase site of human APP, expressed in most AD patients. However, these inhibitors have no effect in mice expressing the rare Swedish (Swe) mutant amyloid precursor protein (APP). Knockout of the cathepsin B decreased brain Abeta in mice expressing WT APP, validating cathepsin B as the target. The specificity of cathepsin B to cleave the WT beta-secretase site, but not the Swe mutant site, of APP for Abeta production explains the distinct inhibitor responses in the different AD mouse models. In contrast to cathepsin B, the BACE1 beta-secretase prefers to cleave the Swe mutant site. Discussion of BACE1 data in the field indicate that they do not preclude cathepsin B as also being a beta-secretase. Cathepsin B and BACE1 could participate jointly as beta-secretases. Significantly, the majority of AD patients express WT APP and, therefore, inhibitors of cathepsin B represent candidate drugs for AD.

An overview of APP processing enzymes and products

The generation of amyloid beta-peptide (A beta) by enzymatic cleavages of the beta-amyloid precursor protein (APP) has been at the center of Alzheimer's disease (AD) research. While the basic process of beta- and gamma-secretase-mediated generation of A beta is text book knowledge, new aspects of A beta and other cleavage products have emerged in recent years. Also our understanding of the enzymes involved in APP proteolysis has increased dramatically. All of these discoveries contribute to a more complete understanding of APP processing and the physiologic and pathologic roles of its secreted and intracellular protein products. Understanding APP processing is important for any therapeutic strategy aimed at reducing A beta levels in AD. In this review, we provide a concise description of the current state of understanding the enzymes involved in APP processing, the cleavage products generated by different processing patterns, and the potential functions of those cleavage products.

Polycystins play a key role in the modulation of cholangiocyte proliferation

BACKGROUND

Polycystin-1 and -2 (PC-1 and PC-2) are critical components of primary cilia, which act as mechanosensors and drive cell response to injury. PC-1 activation involves the cleavage/processing of PC-1 cytoplasmic tail, driven by regulated intramembrane proteolysis or ubiquitine/proteasome, translocation in the nucleus and activation of transcription factors. Mutations of PC-1 or PC-2 occur in polycystic liver where cholangiocyte proliferation is enhanced.

AIM

We evaluated the involvement of PC-1 and PC-2 in modulating cholangiocyte proliferation.

METHODS

We investigated rat cholangiocytes induced to proliferate by 17beta-oestradiol. Proliferation was evaluated by PCNA immunoblotting or [(3)H]-thymidine incorporation into DNA. PC-1 silencing was performed by siRNA, while inhibition of regulated intramembrane proteolysis or proteasome by gamma-secretase inhibitor, leupeptin or MG115.

RESULTS

Cholangiocyte proliferation was associated with decreased PC-1 and PC-2 expression, which was inversely correlated with enhanced PCNA. The selective silencing of PC-1 induced activation of cholangiocyte proliferation in association with decreased PC-1 expression. Two different regulated intramembrane proteolysis inhibitors, gamma-secretase-inhibitor and leupeptin, and the proteasome inhibitor, MG115, abolished the 17beta-oestradiol proliferative effect.

CONCLUSIONS

PC-1 and PC-2 play a major role as modulators of cholangiocyte proliferation suggesting that primary cilia may act as sensors of cell injury driving, when activated, a proliferative cholangiocyte response to trigger the reparative processes.

Genetic cathepsin B deficiency reduces beta-amyloid in transgenic mice expressing human wild-type amyloid precursor protein

Neurotoxic beta-amyloid (Abeta) peptides participate in Alzheimer's disease (AD); therefore, reduction of Abeta generated from APP may provide a therapeutic approach for AD. Gene knockout studies in transgenic mice producing human Abeta may identify targets for reducing Abeta. This study shows that knockout of the cathepsin B gene in mice expressing human wild-type APP (hAPPwt) results in substantial decreases in brain Abeta40 and Abeta42 by 67% and decreases in levels of the C-terminal beta-secretase fragment (CTFbeta) derived from APP. In contrast, knockout of cathepsin B in mice expressing hAPP with the rare Swedish (Swe) and Indiana (Ind) mutations had no effect on Abeta. The difference in reduction of Abeta in hAPPwt mice, but not in hAPPSwe/Ind mice, shows that the transgenic model can affect cathepsin B gene knockout results. Since most AD patients express hAPPwt, these data validate cathepsin B as a target for development of inhibitors to lower Abeta in AD.

Inhibition of trypsin by heparin and dalteparin, a low molecular weight heparin

The interaction between trypsin, a prototype S1 serine protease, with heparin and its low molecular weight derivative dalteparin were investigated. Direct inhibition of the proteolytic activity of trypsin by heparin and dalteparin, used in concentrations typical for their clinical application, was detected. The half-maximum inhibition of the trypsin activity was achieved at 15.25?1.22 ?g/mL for heparin and was estimated to be at 58.47?15.20 ?g/mL for dalteparin. Kinetic analyses showed that heparin and its low molecular weight derivative dalteparin inhibited trypsin by occupation of an exosite, producing noncompetitive and mixed inhibition, respectively. Heparin as a noncompetitive inhibitor with constant of inhibition Ki1,2 = 0.151?0.019 ?M and dalteparin with Ki1 = 0.202?0.030 ?M and Ki2 = 0.463?0.069 ?M in mixed inhibition both represent moderate inhibitors of serine protease trypsin. The obtained constants of inhibition indicate that under the clinically applied concentrations of heparin and dalteparin, trypsins and their homolog S1 serine proteases could be directly inhibited, influencing the delicate control of proteolytic reactions in homeostasis.

Alternative pathways for production of beta-amyloid peptides of Alzheimer's disease

This highlight article describes three Alzheimer's disease (AD) studies presented at the 5th General Meeting of the International Proteolysis Society that address enzymatic mechanisms for producing neurotoxic beta-amyloid (Abeta) peptides. One group described the poor kinetics of BACE 1 for cleaving the wild-type (WT) beta-secretase site of APP found in most AD patients. They showed that cathepsin D displays BACE 1-like specificity and cathepsin D is 280-fold more abundant in human brain than BACE 1. Nevertheless, as BACE 1 and cathepsin D show poor activity towards the WT beta-secretase site, they suggested continuing the search for additional beta-secretase(s). The second group reported cathepsin B as an alternative beta-secretase possessing excellent kinetic efficiency and specificity for the WT beta-secretase site. Significantly, inhibitors of cathepsin B improved memory, with reduced amyloid plaques and decreased Abeta(40/42) in brains of AD animal models expressing amyloid precursor protein containing the WT beta-secretase site. The third group addressed isoaspartate and pyroglutamate (pGlu) posttranslational modifications of Abeta. Results showed that cathepsin B, but not BACE 1, efficiently cleaves the WT beta-secretase isoaspartate site. Furthermore, cyclization of N-terminal Glu by glutaminyl cyclase generates highly amyloidogenic pGluAbeta(3-40/42). These presentations suggest cathepsin B and glutaminyl cyclase as potential new AD therapeutic targets.