Glycosaminoglycan-induced activation of the β-secretase (BACE1) of Alzheimer’s disease

Heparan sulfate proteoglycan in Alzheimer's disease: aberrant expression and functions in molecular pathways related to amyloid-β metabolism

Alzheimer's disease (AD) is the most common form of dementia. Currently, there is no effective treatment for AD, as its etiology remains poorly understood. Mounting evidence suggests that the accumulation and aggregation of amyloid-β peptides (Aβ), which constitute amyloid plaques in the brain, is critical for initiating and accelerating AD pathogenesis. Considerable efforts have been dedicated to shedding light on the molecular basis and fundamental origins of the impaired Aβ metabolism in AD. Heparan sulfate (HS), a linear polysaccharide of the glycosaminoglycan family, co-deposits with Aβ in plaques in the AD brain, directly binds and accelerates Aβ aggregation, and mediates Aβ internalization and cytotoxicity. Mouse model studies demonstrate that HS regulates Aβ clearance and neuroinflammation in vivo. Previous reviews have extensively explored these discoveries. Here, this review focuses on the recent advancements in understanding abnormal HS expression in the AD brain, the structural aspects of HS-Aβ interaction, and the molecules involved in modulating Aβ metabolism through HS interaction. Furthermore, this review presents a perspective on the potential effects of abnormal HS expression on Aβ metabolism and AD pathogenesis. In addition, the review highlights the importance of conducting further research to differentiate the spatiotemporal components of HS structure and function in the brain and AD pathogenesis.

Imaging Keratan Sulfate in Ocular Tissue Sections by Immunofluorescence Microscopy and LA-ICP-MS

Carbohydrate-specific antibodies can serve as valuable tools to monitor alterations in the extracellular matrix resulting from pathologies. Here, the keratan sulfate-specific monoclonal antibody MZ15 was characterized in more detail by immunofluorescence microscopy as well as laser ablation ICP-MS using tissue cryosections and paraffin-embedded samples. Pretreatment with keratanase II prevented staining of samples and therefore demonstrated efficient enzymatic keratan sulfate degradation. Random fluorescent labeling and site-directed introduction of a metal cage into MZ15 were successful and allowed for a highly sensitive detection of the keratan sulfate landscape in the corneal stroma from rats and human tissue.

Glycosaminoglycans from Litopenaeus vannamei Inhibit the Alzheimer's Disease β Secretase, BACE1

Only palliative therapeutic options exist for the treatment of Alzheimer's Disease; no new successful drug candidates have been developed in over 15 years. The widely used clinical anticoagulant heparin has been reported to exert beneficial effects through multiple pathophysiological pathways involved in the aetiology of Alzheimer's Disease, for example, amyloid peptide production and clearance, tau phosphorylation, inflammation and oxidative stress. Despite the therapeutic potential of heparin as a multi-target drug for Alzheimer's disease, the repurposing of pharmaceutical heparin is proscribed owing to the potent anticoagulant activity of this drug. Here, a heterogenous non-anticoagulant glycosaminoglycan extract, obtained from the shrimp Litopenaeus vannamei, was found to inhibit the key neuronal β-secretase, BACE1, displaying a more favorable therapeutic ratio compared to pharmaceutical heparin when anticoagulant activity is considered.

Peptidomimetic Modulators of BACE1

The β-site APP Cleaving enzyme 1 (BACE1) is a membrane-associated aspartyl protease which mediates the production of amyloid-β (Aβ), a major component of amyloid plaques in the Alzheimer’s disease brain. We have synthesised and characterised a series of peptidomimetic analogues of BACE substrates that incorporate two distinct stabilising structures. To demonstrate the potential activity of these compounds, a variety of assaying strategies were used to investigate cleavage susceptibility and inhibition potency under competitive and non-competitive conditions. β-Amino acids and scissile site N-methylation were incorporated into peptide substrate templates as transition state isostere (TSI) substitutes by positional scanning to generate series of non-TSI β-peptidomimetics. The amino acid sequences flanking the β-cleavage site within APP carrying the Swedish double mutation (APPSW), Neuregulin, the synthetic hydroxyethylene-based TSI peptide inhibitor OM99-2, and the high affinity peptide sequence SEISYEVEFR, served as the four substrate templates from which over 60 peptides were designed and synthesised by solid phase peptide synthesis. A quenched fluorescent substrate BACE1 assay in conjunction with liquid chromatography–mass spectrometry (LC-MS) analysis was established to investigate cleavage susceptibility and inhibition potency under competitive and non-competitive conditions. It was determined that β-amino acids substituted at the P1 scissile site position within known peptide substrates were resistant to proteolysis, and particular substitutions induced a concentration-dependent stimulation of BACE1, indicating a possible modulatory role of native BACE1 substrates.

A Glycosaminoglycan Extract from Portunus pelagicus Inhibits BACE1, the β Secretase Implicated in Alzheimer's Disease

Therapeutic options for Alzheimer’s disease, the most common form of dementia, are currently restricted to palliative treatments. The glycosaminoglycan heparin, widely used as a clinical anticoagulant, has previously been shown to inhibit the Alzheimer’s disease-relevant β-secretase 1 (BACE1). Despite this, the deployment of pharmaceutical heparin for the treatment of Alzheimer’s disease is largely precluded by its potent anticoagulant activity. Furthermore, ongoing concerns regarding the use of mammalian-sourced heparins, primarily due to prion diseases and religious beliefs hinder the deployment of alternative heparin-based therapeutics. A marine-derived, heparan sulphate-containing glycosaminoglycan extract, isolated from the crab Portunus pelagicus, was identified to inhibit human BACE1 with comparable bioactivity to that of mammalian heparin (IC₅₀ = 1.85 μg mL⁻¹ (R² = 0.94) and 2.43 μg mL⁻¹ (R² = 0.93), respectively), while possessing highly attenuated anticoagulant activities. The results from several structural techniques suggest that the interactions between BACE1 and the extract from P. pelagicus are complex and distinct from those of heparin.

Proteoglycans in brain development and pathogenesis

Proteoglycans are diverse, complex extracellular/cell surface macromolecules composed of a central core protein with covalently linked glycosaminoglycan (GAG) chains; both of these components contribute to the growing list of important bio-active functions attributed to proteoglycans. Increasingly, attention has been paid to the roles of proteoglycans in nervous tissue development due to their highly regulated spatio/temporal expression patterns, whereby they promote/inhibit neurite outgrowth, participate in specification and maturation of various precursor cell types, and regulate cell behaviors like migration, axonal pathfinding, synaptogenesis and plasticity. These functions emanate from both the environments proteoglycans create around cells by retaining ions and water or serving as scaffolds for cell shaping or motility, and from dynamic interactions that modulate signaling fields for cytokines, growth factors and morphogens, which may bind to either the protein or GAG portions. Also, genetic abnormalities impacting proteoglycan synthesis during critical steps of brain development and response to environmental insults and injuries, as well as changes in microenvironment interactions leading to tumors in the central nervous system, all suggest roles for proteoglycans in behavioral and intellectual disorders and malignancies.

Pharmacology of Heparin and Related Drugs

Heparin has been recognized as a valuable anticoagulant and antithrombotic for several decades and is still widely used in clinical practice for a variety of indications. The anticoagulant activity of heparin is mainly attributable to the action of a specific pentasaccharide sequence that acts in concert with antithrombin, a plasma coagulation factor inhibitor. This observation has led to the development of synthetic heparin mimetics for clinical use. However, it is increasingly recognized that heparin has many other pharmacological properties, including but not limited to antiviral, anti-inflammatory, and antimetastatic actions. Many of these activities are independent of its anticoagulant activity, although the mechanisms of these other activities are currently less well defined. Nonetheless, heparin is being exploited for clinical uses beyond anticoagulation and developed for a wide range of clinical disorders. This article provides a "state of the art" review of our current understanding of the pharmacology of heparin and related drugs and an overview of the status of development of such drugs.

Proteoglycans in the central nervous system: role in development, neural repair, and Alzheimer's disease

Proteoglycans (PGs) are major components of the cell surface and extracellular matrix and play critical roles in development and maintenance of the central nervous system (CNS). PGs are a family of proteins, all of which contain a core protein to which glycosaminoglycan side chains are covalently attached. PGs possess diverse physiological roles, particularly in neural development, and are also implicated in the pathogenesis of neurodegenerative diseases such as Alzheimer's disease (AD). The main functions of PGs in the CNS are reviewed as are the roles of PGs in brain injury and in the development or treatment of AD.

Β-site APP-cleaving enzyme 1 trafficking and Alzheimer's disease pathogenesis

β-Site APP-cleaving enzyme (BACE1) cleaves the amyloid precursor protein (APP) at the β-secretase site to initiate the production of Aβ peptides. These accumulate to form toxic oligomers and the amyloid plaques associated with Alzheimer's disease (AD). An increase of BACE1 levels in the brain of AD patients has been mostly attributed to alterations of its intracellular trafficking. Golgi-associated adaptor proteins, GGA sort BACE1 for export to the endosomal compartment, which is the major cellular site of BACE1 activity. BACE1 undergoes recycling between endosome, trans-Golgi network (TGN), and the plasma membrane, from where it is endocytosed and either further recycled or retrieved to the endosome. Phosphorylation of Ser498 facilitates BACE1 recognition by GGA1 for retrieval to the endosome. Ubiquitination of BACE1 C-terminal Lys501 signals GGA3 for exporting BACE1 to the lysosome for degradation. In addition, the retromer mediates the retrograde transport of BACE1 from endosome to TGN. Decreased levels of GGA proteins and increased levels of retromer-associated sortilin have been associated with AD. Both would promote the co-localization of BACE1 and the amyloid precursor protein in the TGN and endosomes. Decreased levels of GGA3 also impair BACE1 degradation. Further understanding of BACE1 trafficking and its regulation may offer new therapeutic approaches for the treatment of Alzheimer's disease.

Modulatory effects of proteoglycans on proteinase activities

Proteoglycans (PGs), composed of a core protein and one or more covalently attached sulfated glycosaminoglycan (GAG) chains, interact with a wide range of bioactive molecules, such as growth factors and chemokines, to regulate cell behaviors in normal and pathological processes. Additionally, PGs, through their compositional diversity, play a broad variety of roles as modulators of proteinase activities. Interactions of proteinases with other molecules on the plasma membrane anchor and activate them at a specific location on the cell surface. These interactions with macromolecules other than their own protein substrates or inhibitors result in changes in their activity and/or may have important biological effects. Thus, GAG chains induce conformational changes upon their binding to peptides or proteins. This behavior may be related to the ability of GAGs to act as modulators for some proteins (1) by acting as crucial structural elements by the control of proteinase activities, (2) by increasing the protein stability, (3) by permitting some binding to occur, exposing binding regions on the target protein, or (4) by acting as coreceptors for some inhibitors, playing important roles for the acceleration of proteinase inhibition. Understanding the modulatory effects exerted by PGs on proteinase activities is expected to lead to new insights in the understanding of some molecular systems present in pathological states, providing new targets for drug therapy.

Effects of heparin and enoxaparin on APP processing and Aβ production in primary cortical neurons from Tg2576 mice

BACKGROUND

Alzheimer's disease (AD) is caused by accumulation of Aβ, which is produced through sequential cleavage of β-amyloid precursor protein (APP) by the β-site APP cleaving enzyme (BACE1) and γ-secretase. Enoxaparin, a low molecular weight form of the glycosaminoglycan (GAG) heparin, has been reported to lower Aβ plaque deposition and improve cognitive function in AD transgenic mice.

METHODOLOGY/PRINCIPAL FINDINGS

We examined whether heparin and enoxaparin influence APP processing and inhibit Aβ production in primary cortical cell cultures. Heparin and enoxaparin were incubated with primary cortical cells derived from Tg2576 mice, and the level of APP and proteolytic products of APP (sAPPα, C99, C83 and Aβ) was measured by western blotting. Treatment of the cells with heparin or enoxaparin had no significant effect on the level of total APP. However, both GAGs decreased the level of C99 and C83, and inhibited sAPPα and Aβ secretion. Heparin also decreased the level of β-secretase (BACE1) and α-secretase (ADAM10). In contrast, heparin had no effect on the level of ADAM17.

CONCLUSIONS/SIGNIFICANCE

The data indicate that heparin and enoxaparin decrease APP processing via both α- and β-secretase pathways. The possibility that GAGs may be beneficial for the treatment of AD needs further study.

Is BACE1 a suitable therapeutic target for the treatment of Alzheimer's disease? Current strategies and future directions

Alzheimer's disease (AD) is characterized by the extracellular deposition of the β-amyloid protein (Aβ). Aβ is a fragment of a much larger precursor protein, the amyloid precursor protein (APP). Sequential proteolytic cleavage of APP by β-secretase and γ-secretase liberates Aβ from APP. The aspartyl protease BACE1 (β-site APP-cleaving enzyme 1) catalyses the rate-limiting step in the production of Aβ, and as such it is considered to be a major target for drug development in Alzheimer's disease. However, the development of a BACE1 inhibitor therapy is problematic for two reasons. First, BACE1 has been found to have important physiological roles. Therefore, inhibition of the enzyme could have toxic consequences. Second, the active site of BACE1 is relatively large, and many of the bulky compounds that are needed to inhibit BACE1 activity are unlikely to cross the blood-brain barrier. This review focuses on the structure BACE1, current therapeutic strategies based on developing active-site inhibitors, and new approaches to therapy involving targeting the expression or post-translational regulation of BACE1.