Neurobiol Aging. 2015 Feb;36(2):762-75. doi: 10.1016/j.neurobiolaging.2014.09.030.

A cross-talk between Aβ and endothelial SSAO/VAP-1 accelerates vascular damage and Aβ aggregation related to CAA-AD.

 

Solé M1, Miñano-Molina AJ2, Unzeta M3.
  • 1Institut de Neurociències i Departament de Bioquímica i Biologia Molecular, Facultat de Medicina, Universitat Autònoma de Barcelona, Barcelona, Spain. Electronic address: montserrat.sole@uab.cat.
  • 2Institut de Neurociències i Departament de Bioquímica i Biologia Molecular, Facultat de Medicina, Universitat Autònoma de Barcelona, Barcelona, Spain; Centro de Investigación Biomédica en Red sobre Enfermedades Neurodegenerativas (CIBERNED), Madrid, Spain.
  • 3Institut de Neurociències i Departament de Bioquímica i Biologia Molecular, Facultat de Medicina, Universitat Autònoma de Barcelona, Barcelona, Spain.

 

Abstract 

An association between semicarbazide-sensitive amine oxidase (SSAO) and cerebral amyloid angiopathy (CAA) related to Alzheimer’s disease (AD) has been largely postulated. Increased SSAO activity and expression have been detected in cerebrovascular tissue and plasma of AD patients, colocalizing with cerebrovascular beta amyloid (Aβ) deposits. As enzyme, SSAO metabolizes primary amines generating hydrogen peroxide, ammonia and aldehydes. The ability of these products to generate oxidative stress, to enhance the advanced glycation end-products generation, to promote the Aβ aggregation in vitro and to induce apoptosis, supports its role in CAA-related vascular pathology. However, whether the SSAO increase constitutes a cause or it is a consequence of the pathologic process has not been elucidated so far. In order to set up the nature of this relationship, vascular cell models expressing SSAO were treated with different Aβ forms, simulating the CAA conditions in vitro. It was found that the presence of the vasculotropic Dutch-mutated Aβ1-40 increases the SSAO-dependent toxicity, which is accompanied by an increase of SSAO protein availability in endothelial cell membranes. In addition, SSAO enhances Aβ1-40 D and Aβ1-42 deposition on vascular cells by both activity-dependent and -independent mechanisms. Thus, we provide evidences indicating that Aβ itself could be one of the factors inducing SSAO increase in AD, enhancing its toxic effect and inducing the vascular dysfunction and, in turn, that SSAO stimulates Aβ deposition on the vascular walls, thereby contributing to the CAA-AD progression. Therefore, molecules inhibiting SSAO could provide an alternative treatment for preventing/delaying the progress of CAA-AD-associated vasculopathy.

KEYWORDS: Alzheimer’s disease; Amyloid-beta aggregation; Cerebral amyloid angiopathy (CAA); Semicarbazide-sensitive amine oxidase/vascular adhesion protein-1 (SSAO/VAP-1); Vascular damage

PMID: 25457560

 

Supplement

Alzheimer’s disease (AD) is a complex multifactorial disorder in which several systems are affected, and all contribute to the final brain dysfunction. In the normal brain, glial and vascular cells work together with neurons constituting the neurovascular unit. This means that any function disturbance of one of its components will affect to the others and thus, alter the brain homeostasis. For a long time, the study of AD has been focused only in neuronal dysfunction, evidenced by the fact that FDA-approved treatments for AD therapy are targeted exclusively to the neuronal activity improvement. These treatments show limited therapeutic interest because neuronal failure occurs at late stages of the pathological process.

 

Nowadays there are strong evidences that cerebrovascular system dysfunctions constitute early events in AD neuropathology. In fact, beta amyloid (Aβ) plaques are not only accumulated in brain parenchyma but also in and around cerebral vessels (known as cerebral amyloid angiopathy – CAA), inducing vascular degeneration, thereby hindering the delivery of nutrients to neurons and contributing to their cell death. To identify proteins involved in these vascular pathological mechanisms is thus a key factor to find new therapeutic targets for AD. In this regard, our group has been studying the involvement in AD of a vascular protein named SSAO/VAP-1 (semicarbazide-sensitive amine oxidase/vascular adhesion protein 1). This enzyme is expressed in the vascular system and shows catalytic activity besides an adhesion protein behavior by mediating the migration of inflammatory blood cells through the vascular wall into the injured tissues. By the analysis of human AD samples we previously found that SSAO/VAP-1 is overexpressed in cerebral vessels in AD, colocalizing with vascular Aβ deposits (1, 2), and that it is also increased in blood plasma (3). However, the molecular link existing between SSAO/VAP-1, Aβ deposits and vascular degeneration was not yet established.

 

Besides the analysis with human samples or in silico experiments, SSAO/VAP-1 has been poorly studied because its expression is lost in cell cultures. Therefore, our group generated vascular cell lines expressing this protein in a stable form (4, 5), allowing molecular and functional experiments to be performed in a living system. Using these cell lines we observed that an increased SSAO/VAP-1, similar to that observed in AD patients, generates enough toxic products through its enzymatic activity able to damage vascular cells (6, 7). Other authors demonstrated that some of these products, aldehydes, could induce Aβ aggregation in vitro (8).

 

Based on the previous findings, we hypothesized that SSAO/VAP-1 might contribute to the onset and progression of AD, and thus constitute a therapeutic target for AD delay or prevention. In order to confirm this hypothesis we aimed to elucidate the link existing between Aβ and SSAO/VAP-1 in the AD context.

 

In our study, we used our vascular cell lines (HUVEC, human umbilical vein endothelial cells; A7r5, smooth muscle cells) expressing or not expressing the SSAO/VAP-1 protein (Figure 1). To simulate the AD vascular pathology in these cells, they were treated with two different forms of Aβ peptide: Aβ1-42, mainly found in capillaries and in brain, and Aβ1-40D found in large vessels and containing a mutation (Dutch-D), which induces an aggressive form of CAA underlying AD pathology in high percentage. In order to evaluate the contribution of SSAO/VAP-1 to the analyzed processes, we compared our results obtained in cells expressing and not expressing the protein. In addition, to evaluate the effect of its enzymatic activity products, cells were also cultured in presence of an SSAO/VAP-1 physiological substrate (methylamine – MA) and/or inhibitor (semicarbazide – SC).

 

 

Fig 1Figure 1. Micrographs of the different vascular cells used in the study, and the western blot showing the SSAO/VAP-1 protein in the transfected cells and the corresponding wild type cells. GAPDH was used as loading control.

 

 

We first analyzed the effects of Aβ on vascular cells, and results showed that treatment with both Aβ forms produced a time and dose-dependent increase of the SSAO/VAP-1 protein and activity in the membrane of endothelial cells but not in smooth muscle cells. In addition, the presence of its substrate MA increased its toxic effect on endothelial cells treated with Aβ, because of its catalytic activity (see Figure 2). Under inflammatory conditions this protein is inducible in endothelial cells, while smooth muscle cells do not change its expression. These results revealed that the presence of Aβ itself could be one mechanism by which SSAO/VAP-1 is found abnormally increased in AD vessels, contributing through its catalytic activity to the vascular degeneration and hence, SSAO/VAP-1 could be involved in the AD progression.

 

Fig 2

Figure 2. Diagram showing the different consequences of Aβ and MA treatment in non-expressing (WT, wild type) and SSAO/VAP-1-expressing cells. In this latest case, Aβ induces an increase of SSAO/VAP-1 that causes additional Aβ aggregation and vascular degeneration through its enzymatic activity. These effects can be prevented by SSAO inhibitors like SC.

 

But, is SSAO/VAP-1 also participating in the AD onset? To answer this question it was analyzed the Aβ aggregation in vascular cells expressing or not the SSAO/VAP-1, treated with MA. Results revealed that Aβ aggregation was higher in cells expressing SSAO/VAP-1 (both endothelial and smooth muscle) than in non-expressing cells, and even higher in SSAO/VAP-1-expressing cells in presence of MA (see Figure 2). These results suggested us that SSAO/VAP-1 is able to promote Aβ aggregation by two different mechanisms: one independent from its enzymatic activity, and the other dependent on its catalytic action. To find an activity-independent mechanism suggested us that SSAO/VAP-1 could directly bind Aβ, acting as a receptor. Thus, SSAO/VAP-1 may act as a seed for Aβ aggregation in both, endothelial cells when any inflammatory condition would induce its expression, and smooth muscle cells when any endothelial injury would give plasma access to the vascular smooth muscle layer. In consequence, SSAO/VAP-1 can also participate in the initiation of the AD pathology, especially after inflammatory and traumatic brain processes. We resumed the actions of SSAO/VAP-1 in the AD context in Figure 3.

Fig 3

Figure 3. Outline of the processes linked to AD onset and progression in which SSAO/VAP-1 participates. When overexpressed, SSAO enzymatic activity generates toxic products (aldehydes, ammonia and hydrogen peroxide) able to induce vascular cytotoxicity through p53 activation, inflammation and protein crosslinking, including Aβ aggregation. Both inflammation and Aβ contribute to a higher increase of SSAO/VAP-1 presence and activity, generating a positive feedback loop. SSAO also mediates leukocyte infiltration through its VAP-1 activity contributing to inflammation. SSAO/VAP-1 inhibition could be beneficial in AD but also in stroke and other inflammatory conditions.

 

Importance of the study: our data confirm that SSAO/VAP-1 can be involved in both the onset and progression of AD, and therefore it may be useful to inhibit its enzymatic activity as a therapeutic target to prevent or delay the AD progression.

 

References

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  2. Valente T, Gella A, Solé M, Durany N, Unzeta M (2012) Immunohistochemical study of semicarbazide-sensitive amine oxidase/vascular adhesion protein-1 in the hippocampal vasculature: pathological synergy of Alzheimer’s disease and diabetes mellitus. J Neurosci Res 90: 1989-1996.
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  4. Solé M, Hernandez M, Boada M, Unzeta M (2007) Characterization of A7r5 cell line transfected in a stable form by hSSAO/VAP-1 gene (A7r5 hSSAO/VAP-1 cell line). J Neural Transm 114: 763-767.
  5. Solé M and Unzeta M (2011) Vascular cell lines expressing SSAO/VAP-1: a new experimental tool to study its involvement in vascular diseases. Biol Cell 103: 543-557.
  6. Hernandez M, Solé M, Boada M, Unzeta M (2006) Soluble semicarbazide-sensitive amine oxidase (SSAO) catalysis induces apoptosis in vascular smooth muscle cells. Biochim Biophys Acta 1763: 164-173.
  7. Solé M, Hernandez-Guillamon M, Boada M, Unzeta M (2008) p53 phosphorylation is involved in vascular cell death induced by the catalytic activity of membrane-bound SSAO/VAP-1. Biochim Biophys Acta 1783: 1085-1094.
  8. Chen K, Maley J, Yu PH (2006) Potential implications of endogenous aldehydes in beta-amyloid misfolding, oligomerization and fibrillogenesis. J Neurochem 99: 1413-1424.

 

 

 

 

 

 

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