. Amyloid precursor protein mediates a tyrosine kinase-dependent activation response in endothelial cells. J Neurosci. 2009 Nov 18;29(46):14451-62. PubMed.

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  1. In a paper in 2008, Austin and Combs demonstrated increased immunoreactivity of amyloid precursor protein (APP), tyrosine 682 phosphorylated APP (pAPP), and Aβ within the cerebral vasculature and particularly in endothelial cells of both atherosclerotic and Alzheimer disease tissue (1). They also showed that adhesion of monocytes was partially dependent on endothelial cell APP expression. In the present study (2), the authors used cultured endothelial cells from atherosclerotic mouse aorta and from human umbilical vein to demonstrate that APP-mediated adhesion and tyrosine kinase-based endothelial activation may occur as a general feature of vasculature and are not restricted to cerebral vasculature. Furthermore, their results suggest that endothelial APP mediates cell adhesion of monocytes as well as acting as a proinflammatory receptor. They go on to discuss the possible contribution of endothelial and intimal APP to the Aβ load in the cerebral vasculature in cerebral amyloid angiopathy (CAA).

    One of the interesting points raised in this paper is the possibility of therapeutic intervention to impede or reduce the accumulation of Aβ in capillary and artery walls in Alzheimer disease. Although this suggestion is worthy of consideration, the relationship between endothelial cells and the origin of amyloid deposits in capillary and artery walls needs to be carefully considered.

    A number of mechanisms for the elimination of APP-derived Aβ from the brain have been identified from studies in mice and humans. They include perivascular drainage of Aβ along basement membranes of capillaries and arteries (the lymphatic drainage pathway of the brain) (3-6); trans-endothelial absorption of Aβ into the blood and uptake by vascular smooth muscle cells by low-density lipoprotein receptor related protein-1 (LRP-1) mediated pathways (7,8); degradation by neprilysin, angiotensin-converting enzyme and insulin degrading enzyme as Aβ diffuses through extracellular spaces in the brain and along the perivascular drainage pathways (9); and uptake by perivascular macrophages closely applied to cerebral vessel walls (10). Most of these mechanisms appear to fail in the aged and Alzheimer brain (6). Failure of perivascular drainage of Aβ, derived from the brain, along aged arteries appears to be a major factor in the generation of CAA (6).

    Therapeutic reduction of Aβ production by vascular endothelial cells may be a possibility in the future due to the proximity of endothelial cells to the blood. However, the effects on vascular Aβ in CAA may be insignificant in relation to the load of Aβ in vessel walls that is derived from the brain.

    References:

    . Amyloid precursor protein mediates monocyte adhesion in AD tissue and apoE(-)/(-) mice. Neurobiol Aging. 2010 Nov;31(11):1854-66. PubMed.

    . Amyloid precursor protein mediates a tyrosine kinase-dependent activation response in endothelial cells. J Neurosci. 2009 Nov 18;29(46):14451-62. PubMed.

    . Solutes, but not cells, drain from the brain parenchyma along basement membranes of capillaries and arteries: significance for cerebral amyloid angiopathy and neuroimmunology. Neuropathol Appl Neurobiol. 2008 Apr;34(2):131-44. Epub 2008 Jan 16 PubMed.

    . Transgenic mouse models of cerebral amyloid angiopathy. Adv Exp Med Biol. 2001;487:123-8. PubMed.

    . Cerebral amyloid angiopathy: amyloid beta accumulates in putative interstitial fluid drainage pathways in Alzheimer's disease. Am J Pathol. 1998 Sep;153(3):725-33. PubMed.

    . Cerebral amyloid angiopathy in the aetiology and immunotherapy of Alzheimer disease. Alzheimers Res Ther. 2009;1(2):6. PubMed.

    . Neurovascular mechanisms and blood-brain barrier disorder in Alzheimer's disease. Acta Neuropathol. 2009 Jul;118(1):103-13. PubMed.

    . Clearance of Alzheimer's amyloid-ss(1-40) peptide from brain by LDL receptor-related protein-1 at the blood-brain barrier. J Clin Invest. 2000 Dec;106(12):1489-99. PubMed.

    . Insights into the pathogenesis and pathogenicity of cerebral amyloid angiopathy. Front Biosci. 2009;14:4778-92. PubMed.

    . Selective targeting of perivascular macrophages for clearance of beta-amyloid in cerebral amyloid angiopathy. Proc Natl Acad Sci U S A. 2009 Jan 27;106(4):1261-6. PubMed.

  2. AβPP, as its name indicates, is overwhelmingly associated with being the precursor to the Aβ peptides that accumulate in the brains of patients with AD and related disorders. Despite the extensive amount of work on understanding its proteolytic processing to liberate Aβ peptides, comparatively little has been elucidated regarding its physiological functions in neural and non-neural tissues. The recent manuscript by Austin et al. provides new insight in potential functions of AβPP in general vascular endothelial cell biology. Their present work, which extends from earlier studies by this same group and others, offers convincing in vitro data supporting a role for AβPP, and more specifically phosphorylated AβPP, in promoting a reactive vascular endothelial phenotype. In summary, their study suggests that vascular endothelial cells from a variety of sources utilize AβPP to mediate monocytic cell adhesion and a tyrosine kinase-based activation. The findings have important implications for AβPP function in the vasculature both in physiological and pathophysiological situations.

    The findings also solicit a series of questions for future experiments to better understand this potential function of AβPP and its family members in vascular endothelial cell biology. For example, the present study focused on AβPP, but APLP proteins, particularly APLP2, are expressed in many of the same tissues (1). It appears that certain functions of AβPP and APLP2 are redundant, yet required, since AβPP/APLP2 double knockout mice are postnatally lethal (2). AβPP and APLP2 can form heterodimers to promote cell adhesion (3). It would be interesting to compare endothelial cells from APLP2-/- mice to see if they similarly exhibit decreased monocytic cell adhesion and cell activation.

    The results presented in the authors’ present and past articles provide a possible mechanism to account for entrance of circulating monocytic cells into the brain during states of vascular endothelial activation, including AD and CAA. Can this process be thwarted in amyloid depositing mouse models that lack endothelial AβPP expression? This issue could be addressed in vivo by crossing transgenic mice expressing neuronal-specific human AβPP, which develop parenchymal and cerebral vascular amyloid deposits, onto an AβPP-/- background that would lack endothelial expression of endogenous mouse AβPP. This could also be extended to APLP2-/- mice if this protein is also found to participate in this process.

    In their study, the authors show that activation of the cultured endothelial cells requires ligand-induced multimerization—in this case they used an anti-N-terminal AβPP antibody to facilitate this process. Clearly, this would not be a physiological ligand and begs the question, What is? Although it was beyond the scope of their present study, the authors cite previous reports, including ours, demonstrating fibrillar and soluble Aβ peptides as potential N-terminal ligands for AβPP (4,5). This could have important implications in AD, and particularly CAA, where high levels of Aβ would occur. Further study in CAA-specific diseases and animal models will be of interest. Other potential vascular ligands for the N-terminus of AβPP should be considered. For example, circulating coagulation enzymes are specific ligands for the Kunitz proteinase inhibitor domain-containing forms of AβPP that are expressed by vascular endothelial cells (6,7). Circulating soluble forms of AβPP, such as those released by activated platelets, or full-length forms of AβPP/APLP2 expressed on the surface of circulating vascular cells could mediate homo/heterodimeric interactions (3). Also, vascular basement membrane proteins such as collagen or laminin could interact with the N-terminal region of AβPP (8,9).

    Finally, the results presented by these authors appear to be common to cerebral and peripheral vascular endothelial cells and could reflect a systemic process to maintain homeostasis during times of vascular activation. In the case of the ApoE-/- mice presented in their study, this is a model of atherosclerotic disease. This condition results in vascular endothelial activation and damage, inflammation, thrombus formation, vessel occlusion, and eventually cessation of blood flow. These events are not restricted to cerebral vessels but can affect all vessels in the body. In this regard, it is of interest that we have shown that both AβPP-/- and APLP2-/- mice exhibit a pro-thrombotic phenotype (10,11). On the other hand, overexpression of AβPP in platelets or in brain provides an anti-thrombotic environment (10,12). It is interesting to speculate that increased expression of endothelial cell AβPP could be a protective response to this condition, providing an anti-thrombotic environment to mitigate some of these deleterious processes. This hypothesis could be further explored in transgenic mice that specifically express AβPP in vascular endothelial cells. In any case, the manuscript of Austin et al. provides the rationale for continued investigation into the function of AβPP in cerebral and peripheral vascular endothelium.

    References:

    . Immunohistochemical and in situ analysis of amyloid precursor-like protein-1 and amyloid precursor-like protein-2 expression in Alzheimer disease and aged control brains. Brain Res. 1998 Aug 31;804(1):45-51. PubMed.

    . Generation of APLP2 KO mice and early postnatal lethality in APLP2/APP double KO mice. Neurobiol Aging. 1997 Nov-Dec;18(6):661-9. PubMed.

    . Homo- and heterodimerization of APP family members promotes intercellular adhesion. EMBO J. 2005 Oct 19;24(20):3624-34. PubMed.

    . Amyloid beta interacts with the amyloid precursor protein: a potential toxic mechanism in Alzheimer's disease. Nat Neurosci. 2000 May;3(5):460-4. PubMed.

    . Localization of a fibrillar amyloid beta-protein binding domain on its precursor. J Biol Chem. 2002 Sep 27;277(39):36392-8. PubMed.

    . Platelet coagulation factor XIa-inhibitor, a form of Alzheimer amyloid precursor protein. Science. 1990 Jun 1;248(4959):1126-8. PubMed.

    . Immunopurification and protease inhibitory properties of protease nexin-2/amyloid beta-protein precursor. J Biol Chem. 1990 Jun 15;265(17):9591-4. PubMed.

    . Regulation of amyloid protein precursor (APP) binding to collagen and mapping of the binding sites on APP and collagen type I. J Biol Chem. 1996 Jan 19;271(3):1613-20. PubMed.

    . beta-Amyloid precursor protein binds to the neurite-promoting IKVAV site of laminin. Proc Natl Acad Sci U S A. 1993 Nov 1;90(21):10150-3. PubMed.

    . Protease nexin-2/amyloid beta-protein precursor limits cerebral thrombosis. Proc Natl Acad Sci U S A. 2005 Dec 13;102(50):18135-40. PubMed.

    . AbetaPP/APLP2 family of Kunitz serine proteinase inhibitors regulate cerebral thrombosis. J Neurosci. 2009 Apr 29;29(17):5666-70. PubMed.

    . Increased severity of hemorrhage in transgenic mice expressing cerebral protease nexin-2/amyloid beta-protein precursor. Stroke. 2007 Sep;38(9):2598-601. PubMed.

  3. The role of amyloid precursor protein (APP) in normal and pathological contexts remains elusive. Although its degradation products are the focus of intense research in the Alzheimer disease field, APP itself has also earned the nickname “the All-Purpose Protein” for its role in multiple cellular pathways. This interesting paper offers a novel role for APP in mediating endothelial cell activation, which may contribute to the endothelial dysfunction manifested in AD, cerebral amyloid angiopathy (CAA), atherosclerosis, and other cardiovascular diseases.

    In a previous publication, Combs and colleagues demonstrated that cerebrovasculature of atherosclerotic and AD tissues are characterized by increased levels of APP, phosphorylated APP (pAPP), and Aβ, and that the increased APP levels are associated with activation of the tyrosine kinase, Src. Notably, endothelial APP was shown to be involved in monocyte adhesion to brain endothelium, suggesting an APP-dependent mechanism by which inflammatory processes are initiated in the cerebral endothelial tissue (Austin et al., 2008). In their current paper, the authors extended their previous findings to peripheral vasculature using aortic tissues collected from human and murine atherosclerosis cases as well as APP overexpressing and KO mice.

    To define the role of APP signaling in mediating processes leading to endothelial dysfunction, the authors used endothelial cells extracted from aortas and umbilical tissues. Antibody raised against the amino terminus of APP was used in these cellular systems to induce APP signaling, which resulted in a Src-dependent alteration in endothelial phenotype, including increases in pAPP and the proinflammatory mediators COX-2 and VCAM-1, and enhanced secretion of IL-1β and Aβ40. Although the authors suggest that that antibody treatment simulates ligand binding by inducing APP multimerization, the actual mechanism is not well established. It would thus be interesting to evaluate the effect of a monovalent antibody such as Fab or ScFv. The resultant increase in APP phosphorylation may, in turn, be the cause of elevated Aβ40, as pAPP was recently shown to be favored for β-secretase cleavage relative over non-phosphorylated APP. Since Aβ has been shown to affect endothelial activation and alter gene expression, it would be interesting to see whether removal of Aβ-containing conditioned media from these cells would affect the expression of the inflammatory genes COX-2, VCAM-1, and IL-1β.

    The data presented here highlight endothelial APP as a potential key player in mediating early events in the amyloid cascade that might account for altered gene expression, disruption of the blood-brain barrier, diapedesis of immune cells, and accumulation of Aβ that eventually deposits on the vessels walls as CAA. As Aβ40 secretion is elevated, the secreted peptides originating from the endothelium could have a deleterious effect on endothelial cells themselves as well as a paracrine effect on other cell types, including smooth muscle cells, known to degenerate and eventually cause the vessel’s collapse. This paradigm of endothelial generation of Aβ differs from models proposing primarily a neuronal origin and will require further study. It should be noted that enhanced APP processing would also give rise to secreted APPs and the APP intracellular fragments, either of which might mediate the reactive endothelial phenotype shown here.

    It would be highly interesting and worthwhile to address a few of the following questions: Is APP multimerization indeed the key event? And if so, is it an intermembrane or intramembrane interaction? Which are the molecules that actually facilitate the APP-dependent adhesion of monocytes? What role does Aβ play in the APP-dependent endothelial proinflammatory phenotype? And lastly, if this mechanism can occur in both peripheral and cerebral vessels, why is vascular degeneration limited to the brain?

    References:

    . Amyloid precursor protein mediates monocyte adhesion in AD tissue and apoE(-)/(-) mice. Neurobiol Aging. 2010 Nov;31(11):1854-66. PubMed.