Limited expression of heparan sulphate proteoglycans associated with Aβ deposits in the APPswe/PS1dE9 mouse model for Alzheimer's disease

Basal lamina changes in neurodegenerative disorders

BACKGROUND

Neurodegenerative disorders are a group of age-associated diseases characterized by progressive degeneration of the structure and function of the CNS. Two key pathological features of these disorders are blood-brain barrier (BBB) breakdown and protein aggregation.

MAIN BODY

The BBB is composed of various cell types and a non-cellular component---the basal lamina (BL). Although how different cells affect the BBB is well studied, the roles of the BL in BBB maintenance and function remain largely unknown. In addition, located in the perivascular space, the BL is also speculated to regulate protein clearance via the meningeal lymphatic/glymphatic system. Recent studies from our laboratory and others have shown that the BL actively regulates BBB integrity and meningeal lymphatic/glymphatic function in both physiological and pathological conditions, suggesting that it may play an important role in the pathogenesis and/or progression of neurodegenerative disorders. In this review, we focus on changes of the BL and its major components during aging and in neurodegenerative disorders, including Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). First, we introduce the vascular and lymphatic systems in the CNS. Next, we discuss the BL and its major components under homeostatic conditions, and summarize their changes during aging and in AD, PD, and ALS in both rodents and humans. The functional significance of these alterations and potential therapeutic targets are also reviewed. Finally, key challenges in the field and future directions are discussed.

CONCLUSIONS

Understanding BL changes and the functional significance of these changes in neurodegenerative disorders will fill the gap of knowledge in the field. Our goal is to provide a clear and concise review of the complex relationship between the BL and neurodegenerative disorders to stimulate new hypotheses and further research in this field.

Glypican 4 Regulates Aβ Internalization in Neural Stem Cells Partly via Low-Density Lipoprotein Receptor-Related Protein 1

Neural stem cell (NSC) damage has been reported in patients with Alzheimer’s disease. Intracellular Aβ plays a vital role in NSC damage. Heparan sulfate proteoglycans are potent mediators of Aβ enrichment in the brain. We hypothesized the heparan sulfate proteoglycan glypican 4 (Gpc4) regulates Aβ internalization by NSCs. We evaluated Gpc4 expression in NSCs from P0–P2 generations using immunofluorescence. Adenovirus and lentivirus were used to regulate Gpc4 expression in NSCs and APP/PS1 mice, respectively. Co-immunoprecipitation was used to determine the relationship between Gpc4, Aβ, and low-density lipoprotein receptor-related protein 1 (LRP1). Intracellular Aβ concentrations were detected using enzyme-linked immunosorbent assay and immunofluorescence. The role of Gpc4/LRP1 on toxic/physical Aβ-induced effects was evaluated using the JC-1 kit, terminal deoxynucleotidyl transferase dUPT nick end labeling, and western blotting. Gpc4 was stably expressed in NSCs, neurons, and astrocytes. Gpc4 was upregulated by Aβ in NSCs and regulated Aβ internalization. Gpc4 attenuation reduced Aβ uptake; Gpc4 overexpression increased Aβ uptake. Gpc4 regulated Aβ internalization through LRP1 and contributed to Aβ internalization and toxic/physical concentrations of Aβ-induced mitochondrial membrane potential and cell apoptosis, partly via LRP1. Therefore, Gpc4 is a key regulator of Aβ enrichment in NSCs. Inhibiting Gpc4 rescued the Aβ-induced toxic effect and attenuated the nontoxic Aβ enrichment into intracellular toxic concentrations. Gpc4 contributed to Aβ internalization and toxic/physical concentrations of Aβ-induced mitochondrial membrane potential damage and cell apoptosis, partly via LRP1. These findings suggest a potential role of Gpc4 in treating Alzheimer’s disease at an early stage, by targeting NSCs.

Review of Alterations in Perlecan-Associated Vascular Risk Factors in Dementia

Perlecan is a heparan sulfate proteoglycan protein in the extracellular matrix that structurally and biochemically supports the cerebrovasculature by dynamically responding to changes in cerebral blood flow. These changes in perlecan expression seem to be contradictory, ranging from neuroprotective and angiogenic to thrombotic and linked to lipid retention. This review investigates perlecan's influence on risk factors such as diabetes, hypertension, and amyloid that effect Vascular contributions to Cognitive Impairment and Dementia (VCID). VCID, a comorbidity with diverse etiology in sporadic Alzheimer's disease (AD), is thought to be a major factor that drives the overall clinical burden of dementia. Accordingly, changes in perlecan expression and distribution in response to VCID appears to be injury, risk factor, location, sex, age, and perlecan domain dependent. While great effort has been made to understand the role of perlecan in VCID, additional studies are needed to increase our understanding of perlecan's role in health and in cerebrovascular disease.

Neuronal heparan sulfates promote amyloid pathology by modulating brain amyloid-β clearance and aggregation in Alzheimer's disease

Accumulation of amyloid-β (Aβ) peptide in the brain is the first critical step in the pathogenesis of Alzheimer's disease (AD). Studies in humans suggest that Aβ clearance from the brain is frequently impaired in late-onset AD. Aβ accumulation leads to the formation of Aβ aggregates, which injure synapses and contribute to eventual neurodegeneration. Cell surface heparan sulfates (HSs), expressed on all cell types including neurons, have been implicated in several features in the pathogenesis of AD including its colocalization with amyloid plaques and modulatory role in Aβ aggregation. We show that removal of neuronal HS by conditional deletion of the Ext1 gene, which encodes an essential glycosyltransferase for HS biosynthesis, in postnatal neurons of amyloid model APP/PS1 mice led to a reduction in both Aβ oligomerization and the deposition of amyloid plaques. In vivo microdialysis experiments also detected an accelerated rate of Aβ clearance in the brain interstitial fluid, suggesting that neuronal HS either inhibited or represented an inefficient pathway for Aβ clearance. We found that the amounts of various HS proteoglycans (HSPGs) were increased in postmortem human brain tissues from AD patients, suggesting that this pathway may contribute directly to amyloid pathogenesis. Our findings have implications for AD pathogenesis and provide insight into therapeutic interventions targeting Aβ-HSPG interactions.

Amyloid-β oligomer detection by ELISA in cerebrospinal fluid and brain tissue

Amyloid-β (Aβ) deposits are important pathological hallmarks of Alzheimer's disease (AD). Aβ aggregates into fibrils; however, the intermediate oligomers are believed to be the most neurotoxic species and, therefore, are of great interest as potential biomarkers. Here, we have developed an enzyme-linked immunosorbent assay (ELISA) specific for Aβ oligomers by using the same capture and (labeled) detection antibody. The ELISA predominantly recognizes relatively small oligomers (10-25 kDa) and not monomers. In brain tissue of APP/PS1 transgenic mice, we found that Aβ oligomer levels increase with age. However, for measurements in human samples, pretreatment to remove human anti-mouse antibodies (HAMAs) was required. In HAMA-depleted human hippocampal extracts, the Aβ oligomer concentration was significantly increased in AD compared with nondemented controls. Aβ oligomer levels could also be quantified in pretreated cerebrospinal fluid (CSF) samples; however, no difference was detected between AD and control groups. Our data suggest that levels of small oligomers might not be suitable as biomarkers for AD. In addition, we demonstrate the importance of avoiding HAMA interference in assays to quantify Aβ oligomers in human body fluids.

Butyrylcholinesterase is associated with β-amyloid plaques in the transgenic APPSWE/PSEN1dE9 mouse model of Alzheimer disease

Histochemical analysis of Alzheimer disease (AD) brain tissues indicates that butyrylcholinesterase (BuChE) is present in β-amyloid (Aβ) plaques. The role of BuChE in AD pathology is unknown, but an animal model developing similar BuChE-associated Aβ plaques could provide insights. The APPSWE/PSEN1dE9 transgenic mouse (ADTg), which develops Aβ plaques, was examined to determine if BuChE associates with these plaques, as in AD. We found that in mature ADTg mice, BuChE activity associated with Aβ plaques. The Aβ-, thioflavin-S- and BuChE-positive plaques mainly accumulated in the olfactory structures, cerebral cortex, hippocampal formation, amygdala, and cerebellum. No plaques were stained for acetylcholinesterase activity. The distribution and abundance of plaque staining in ADTg closely resembled many aspects of plaque staining in AD. Butyrylcholinesterase staining consistently showed fewer plaques than were detected with Aβ immunostaining but a greater number of plaques than were visualized with thioflavin-S. Double-labeling experiments demonstrated that all BuChE-positive plaques were Aβ positive, whereas only some BuChE-positive plaques were thioflavin-S positive. These observations suggest that BuChE is associated with a subpopulation of Aβ plaques and may play a role in AD plaque maturation. A further study of this animal model could clarify the role of BuChE in AD pathology.

Changes in brain β-amyloid deposition and aquaporin 4 levels in response to altered agrin expression in mice

Conditions that compromise the blood-brain barrier (BBB) have been increasingly implicated in the pathogenesis of Alzheimer disease (AD). AGRIN is a heparan sulfate proteoglycan found abundantly in basement membranes of the cerebral vasculature, where it has been proposed to serve a functional role in the BBB. Furthermore, AGRIN is the major heparan sulfate proteoglycan associated with amyloid plaques in AD brains. To examine the relationship of AGRIN, the BBB, and AD-related pathologies, we generated mice in which the Agrn gene was deleted from either endothelial cells or neurons using gene targeting or was overexpressed using a genomic transgene construct. These mice were combined with a transgenic model of AD that over expresses disease-associated forms of amyloid precursor protein and presenilin 1. In mice lacking endothelial cell expression of Agrn, the BBB remained intact but aquaporin 4 levels were reduced, indicating that the loss of AGRIN affects BBB-associated components. This change in Agrn resulted in an increase in β-amyloid (Aβ) in the brain. Conversely, overexpression of Agrn decreased Aβ deposition, whereas elimination of Agrn from neurons did not change Aβ levels. These results indicate that AGRIN is important for maintaining BBB composition and that changes in Agrn expression (particularly vessel-associated AGRIN) influence Aβ homeostasis in mouse models of AD.

Colocalisation of plasma derived apo B lipoproteins with cerebral proteoglycans in a transgenic-amyloid model of Alzheimer's disease

Alzheimer's disease (AD) is characterized by cerebral proteinaceous deposits comprised of amyloid beta (Aβ). Evidence suggests that enhanced blood-to-brain delivery of Aβ occurs when plasma concentration is increased, exacerbating amyloidosis. In blood, significant Aβ is associated with apolipoprotein (apo) B lipoproteins. In this study, immunofluorescent microscopy was utilised to explore if there is an association between apo B lipoproteins and proteoglycan expression within Aβ-rich plaques in transgenic-amyloid mice. Focal accumulation of apo B was found with Aβ-plaque in APP/PS1 mice. There was enrichment in the proteoglycans, agrin, perlecan, biglycan and decorin within the core of dense Aβ-plaque. Perlecan, biglycan and decorin were positively associated with apo B lipoprotein abundance within amyloid plaque consistent with a cause-for-retention effect. These findings show that proteoglycans are an integral component of Aβ deposits in APP/PS1 mice. This study suggests that some proteoglycans contribute to Aβ retention, whilst other proteoglycans have different functions in the aetiology of AD.