Emerging Concept Of Alzheimer Disease: Interlink Between Angiogenesis and Amyloid Beta Peptide
Angiogenesis has been suggested as one of a contributor to Alzheimer's disease (AD) which is characterized by an increase in micro vascular density, size and length from pre-existing vessels. The amyloid beta (Aβ) peptide appeared as hallmark of AD which induces angiogenesis through dissimilar cellular signaling pathways in hypoxic and inflammatory conditions. In hypoxia, Hypo Inducing factor 1 alpha (HIF-1α) is stimulated and plays a crucial role in accumulating Aβ peptide. In addition, it also causes enhancement in the levels of vascular endothelial growth factor (VEGF). However, in inflammatory signaling pathways, angiogenesis found to be associated with increased level of tumor necrosis factor-α (TNFα), Interleukin 1 Beta (IL-1β) along with VEGF. This review describe the association between Aβ peptide and angiogenesis cell signalling pathways targeting VEGF-A, VEGFR-2, TNF-α, IL-1β, BACE1 and HIF-1α as angiogenic factors, furthermore the effects of clinically available anti-angiogenic agents on Aβ induced angiogenesis and its dependent mechanism(s).
Pathophysiological role of Angiogenesis
It is a pathophysiological phenomenon that involves several mediators and cellular signaling pathways which interact to establish a specific microenvironment appropriate for the development of new capillary from pre-existing vessels.
There are two primary mechanisms of angiogenesis including intussusception and sprouting. Intussusception is a capillary development mechanism in which one capillary divided into two capillaries through the formation of a pillar like structure or longitudinal division of the luminal side of capillary. This phenomenon followed by activation of endothelial cell and then spread intraluminally which facilitates effective blood flow through the vessels. However, sprouting angiogenesis is an another mechanism of capillary formation in which activated endothelial cells branch out from a pre-existing capillary, spreading through the surrounding matrix to form a cord-like structure. Angiogenesis is an important mechanism in developmental growth or in a response of any pathophysiological condition. VEGF is one of the common pathway that connects neurogenesis, angiogenesis and pathogenesis, recognized on the basis of its vascular effects. Initiation of angiogenesis required VEGF as a key factor initially recognized as vascular permeability factor (VPF) in 1983 by Senger et al. Later VEFG is characterized on the basis of its function and types having different length of amino acid in 1989 by Leung et al. (6). In central nervous disorders, vascular supply is acquired via angiogenesis instead of vasculogenesis, here endothelial cells distinguish from angioblasts for the growth of blood vessels. The previous studies described that growth of nervous tissue is another function of VEGF. However, Sondell et al. in 1999 describes axonal outgrowth and cell survival promoted by VEGF via a VEGFR-2 dependent mechanism.
Subsequent studies also proved that VEGF increases in the length and number of neuritis in cortical explants and cultured neurons (8). IAmyloid-β cell signaling mechanism: Amyloid-β is a peptide synthesized via amyloidogenic pathway in the presence of APP in neuronal cells. APP is a transmembrane peptide cleaved through α and β secretase enzyme synthesizing soluble APPα, APPβ and membrane carboxy-terminal fragments. Aβ peptide is dissimilar in size from 38 to 43 amino acids due to auto aggregation ability of Aβ peptide, it can exist as monomer, dimer or oligomer to form fibrils which consist of β-sheet and it can deposit to form extracellular neuritic plaque. These neuritic plaques induce angiogenesis via the activation of endothelial cell due to hypoxic and inflammatory conditions.
The up-regulation of the Aβ peptide in brains of Alzheimer’s patients shows a close relationship with angiogenic VEGF and their receptors. VEGF are the main stimulator of angiogenesis and mediate its action by binding to the particular receptor on the endothelial cell membrane which causes a cascade of reactions. VEGF can bind to three different types of receptors VEGFR1, VEGFR2, and VEGFR3 (13). VEGFR1 shows negative function acting as a false receptor because it has a weak tyrosine phosphorylation. VEGFR2 stimulates growth and permeability and VEGFR3 stimulates lymphatic vessel development.
Hypoxia is a direct consequence of hypo perfusion which shows an important function in the Aβ accumulation. Hypo Inducing factor (HIF) complex is a heterodimer consist of HIF-1β, HIF-1α and oxygen-regulated subunit. HIF-1α regulation occurs at different conditions such as stability of the protein, during phosphorylation and nuclear translocation. However, it is deactivated under normoxic conditions and alternatively, activate under hypoxic conditions. HIF-1α is stabilized and translocate into the nucleus where it dimerizes with HIF-1β and then binds to hypoxic binding sites of target genes which is involved in maintenance of cellular environment, break down of glucose, production of red blood cell, metabolism of iron and above all angiogenesis. Moreover, elevated level of HIF-1α increases BACE1 mRNA expression/activity and maintains angiogenic phenomena. Besides this, HIF-1α also participate in stimulation of inflammatory pathways, hence shows a close relationship between inflammatory and angiogenic pathways.
Pathophysiological role of Amyloid-β: AD is a neuro-degenerative disease categorized by dementia and a higher level of Amyloid-β in the brain parenchyma and cerebral vasculature that are resulting from activation APP. The particular function of APP and Aβ peptide has not been clear for several decades it was assumed that Aβ peptide was by product of APP without showing any physiological function, but recent research demonstrated that APP is required for neuronal growth in neurodevelopment. While, APP also have a physiological role in synaptogenesis, cell adhesion and neuronal communication. It is also interrelated with the calcium sensor of synaptic vesicles possibly take part in the regulation of synaptic vesicle exocytosis and calcium homeostasis. Memory and learning is also regulated by APP and its levels of expression can modulate synaptic spine density and an effect that is mediated by its soluble APPα.
Aβ peptide is important for synaptic plasticity in healthy individuals. Neurotoxic or neurotropic effects of Aβ peptide have been proposed these effects may be different depends on its concentration, age of the individuals and cellular environment. Atwood, C. S. , et al. , report that the levels Aβ peptide in the brain are vigorously influenced by synaptic activity and a higher concentration of Aβ peptide i. e. nanomolar to micromolar may causes neurotoxicity and apoptosis and lower concentrations i. e. picomolar Aβ peptide could act as a modulator of synaptic activity. Furthermore, a lower concentration of Aβ peptide works as antioxidants because of its ability to capture redox metals such as iron, zinc, and copper.
HIF-1 is a major transcription factor that activates in low cellular oxygen concentration. HIF-1 consists of two subunits one is HIF-1α and another one is HIF-1β. They bind to form functional HIF-1 heterodimers for the regulation of a series of transcriptional events. Though, the concentration of HIF-1α is low in the normoxic condition and in the hyypoxic condition the concentration of HIF-1α is much higher as compared to the normoxic condition. During hypoxic condition, hypoxia induces angiogenesis apparently is a response target to increasing blood vessels growth via growth factors such as VEGF and increases oxygen supply to deoxygenated tissues.
Role of Inflammation in Angiogenesis
Inflammation is one of crucial phenomena, which also associates with Aβ peptide and angiogenesis. It has also been shown that Aβ stimulates the production of a pro-inflammatory cytokines. Cytokines, chemokines and vasoactive molecules release in response to the hypoxia which regulate the abnormal capillaries formation, endothelial cell proliferation and neuronal cell death. However, these excessive branching causes oxygen exhaustion and acidosis in the extracellular environment. In various studies it has been proven that Aβ peptides can be used to induce angiogenesis in various experiment models. In an in vitro study using normal human cerebral endothelial cells, angiogenesis was introduced by Aβ peptide stimulating the endothelial cell proliferation, chemotaxis and morphogenesis. Moreover, by using the aorta ring assay, Aβ peptide found to be involved in the formation of capillary-like structures. Another confirmation of angiogenic activity of Aβ peptides comes from the study of Zand et al, in 2005 they examined neovascularization following injection of Aβ peptide into the rat hippocampus via stereotaxic surgery, in addition with increased levels of VEGF and FGF-2 revealing the link of Aβ dependent angiogenesis with inflammatory processes. In transgenic mice (Tg2576) Alzheimer’s disease model that overexpress the human amyloid precursor protein promotes neoangiogenesis and hypervascularity leading to pathophysiology of AD.
Chick chorioallantoic membrane assay, angiogenesis and Alzheimer’s disease
Chick chorioallantoic membrane assay is an in vivo model, rapidly generating a rich vascular network and commonly employed as screening tool for angiogenic and anti-angiogenic molecules. Aβ peptides (Aβ1-40 or Aβ1-42) angiogenic activity has been well defined in the CAM assay and peptides were found to be surrounded by numerous newly developed allantoic blood vessels following the developmental phases of both sprouting and intussuseptive micro vascular growth. In this model system, angiogenesis was demonstrated effectively via stimulation of various angiogenic growth factors including FGF-2 and VEGF with their respective receptors and signaling pathways. Different angiogenic stimulators (chemicals/stress) other than Aβ peptide have also shown development of new blood vessels in CAM model activating the growth factors PDGF, FGF-2 and TGFβ and over expressing VEGFR1, VEGFR2, VEGFR3. Gene expression profiling of VEGF and other angiogenic regulator has been defined to CAM development physiologically and tumor progression with angiogenesi. It is interesting to share, environmental alterations in CAM assay leads to vascularization via up-regulating VEGF-A, VEGFR2, HIF-2A expressions at transcriptional and translational levels.
As mentioned above Aβ induced-angiogenesis is dependent on multiple pathways i. e. growth factors, hypoxia and inflammatory responses, CAM assay can be an ideal model for evaluating the principal regulators of Aβ dependent angiogenesis and mechanism(s) of anti-angiogenic drugs with the potential therapeutic intervention of AD. Anti-angiogenic agents: Anti-angiogenic agent exerts their effects reducing the synthesis of pro angiogenic factors or interfering with their binding to specific receptor on endothelial cells. VEGF and their receptors VEGFR1, VEGFR2 and VEGFR3 are the prime focus for angiogenesis inhibition. The most widely used VEGF tyrosine kinase inhibitors are sorafen, sunitinib, pazopanib and everolimus. Thalidomide is an immunosuppresive and anti-angiogenic drug however, this agent withdrawn from the market due to teratogenic effects. It has been reintroduced and approved by FDA for the treatment of inflammation associated with leprosy (Hansen’s disease) and as a chemotherapeutic agent for patients with multiple myeloma. Moreover, thalidomide has therapeutic effects on neurodegenerative disorders like AD, recent research have shown that inhibitory effect of thalidomide on TNFα, down regulates the activity of BACE1 which ultimately reduce the Aβ synthesis might interfering with inflammation, hypoxia and angiogenesis. Thereby, it is suggested chronic administration of thalidomide is another approach to prevent neuron loss and to improve cognition and therapeutics of AD. Pazopanib is a prominent tyrosine kinase inhibitor, it is a second generation multi-targeted drug against VEGF receptors and it shows effective anti-angiogenic and antitumor activity in vitro and in vivo models. Oral administration of this drug was well tolerated and showed activity against metastatic renal cell carcinoma other solid tumors. It also has a potential for the treatment of taupathies, causing significantly decreases the levels of Tau protein in (Tg4510) transgenic mice with AD.
Pazopanib is an anti angiogenic drug which is used as an anticancer drug which show great efficacy against VEGFRs and thoroughly related tyrosine receptor kinases in vitro and also shows antitumor activity in several human tumor xenografts, including renal cell carcinoma lung and breast cancer. Thalidomide is another drug initially was introduced as an effective tranquillizer and painkiller. However the role of thalidomide against the TNFα it decreases the synthesis of TNFα. Researcher report that genetic inhibition of TNFα/TNF receptor signal transduction down-regulates BACE1 activity which may reduce the synthesis of Aβ peptide. This drug also has an effective role in the regulation of VEGF but the detailed mechanism is still unknown.
Conclusion
Considering the aforementioned facts regarding Aβ peptide induced angiogenesis and its mechanism and targeted therapy for the intervention of AD, through which we can explore combination of multiple signaling molecules which interlink with Aβ peptide and angiogenesis targeting growth factor, inflammation and hypoxia. In addition anti-angiogenic drug Pazopanib and its mechanism will also be evaluated against Aβ peptide induced angiogenesis for introducing a new approach to treat Alzheimer’s diseases using Thalidomide as standard drug.
