A primary histopathological feature of Alzheimer's disease is the accumulation of β‐amyloid (Aβ) in the brain of afflicted individuals. However, Aβ is produced continuously as a soluble protein in healthy individuals where it is detected in serum and CSF, suggesting the existence of cellular clearance mechanisms that normally prevent its accumulation and aggregation. Here, we demonstrate that Aβ forms stable complexes with activated α2‐macroglobulin (α2M⋆), a physiological ligand for the low‐density lipoprotein receptor‐related protein (LRP) that is abundantly expressed in the CNS. These α2M⋆/125I‐Aβ complexes are immunoreactive with both anti‐Aβ and anti‐α2M IgG and are stable under various pH conditions, sodium dodecyl sulfate, reducing agents, and boiling. We demonstrate that α2M⋆/125I‐Aβ complexes can be degraded by glioblastoma cells and fibroblasts via LRP, because degradation is partially inhibited by receptor‐associated protein (RAP), an antagonist of ligand interactions with LRP. In contrast, the degradation of free 125I‐Aβ is not inhibited by RAP and thus must be mediated via an LRP‐independent pathway. These results suggest that LRP can function as a clearance receptor for Aβ via a physiological ligand.

Tissue factor pathway inhibitor (TFPI) is a potent inhibitor of the blood coagulation factor VIIa-tissue factor complex, as well as a direct inhibitor of factor Xa. Intravenously administered TFPI is rapidly cleared from circulation predominantly via liver. We previously reported that the low density lipoprotein receptor-related protein (LRP), a multifunctional endocytic receptor, mediates the uptake and degradation of TFPI in hepatoma cells. This process is inhibited by a 39-kDa receptor-associated protein which binds to LRP and regulates its ligand binding activity. However, a distinct, low affinity binding site (perhaps heparin sulfate proteoglycans, HSPGs) on the endothelium and liver is thought to be responsible for the majority of TFPI cell surface binding. In the current study, we investigated the role of LRP and this second binding site in the clearance of Tissue factor pathway inhibitor (TFPI) 1 is a serine protease inhibitor that plays a key role in regulating tissue factorinitiated blood coagulation. Human TFPI is a trace 42-kDa plasma glycoprotein consisting of three tandem Kunitz-type domains, followed by a positively charged carboxyl terminus (1). The first Kunitz domain binds to and inhibits factor VIIa, and the second Kunitz domain binds to and inhibits factor Xa (2). Inhibition of tissue factor-induced blood coagulation by TFPI has been postulated to involve the quaternary factor Xa-TFPI-factor VIIa-tissue factor complex (3).Intravenously administered 125 I-TFPI is cleared rapidly from the circulation with a plasma half-life of 2 min in rabbits (4) and Ͻ1 min in rats (5). However, the biology underlying this clearance mechanism has not been elucidated to date. Previously, we demonstrated that the low density lipoprotein receptor-related protein (LRP) mediates the cellular degradation of TFPI in hepatoma cells (6) and that a 39-kDa protein, an inhibitor of all the ligand interactions with LRP (7), inhibits this process. In addition, cell surface heparin sulfate proteoglycans (HSPGs) associated with endothelial cells and liver have been proposed to play a role in the clearance of 125 I-TFPI (8).However, the precise roles of LRP and HSPGs in the plasma clearance of TFPI have yet to be defined.The purpose of the present study was to elucidate the roles of LRP and HSPGs in the catabolism of TFPI both in vivo and in vitro. We took advantage of viral-mediated gene transfer to express the 39-kDa protein in liver in vivo as such an approach has allowed us to define the role of LRP in the clearance of tissue-type plasminogen activator (t-PA) in vivo (9). The current results demonstrate a direct role for LRP as well as HSPGs in the plasma clearance of TFPI and thus suggest strategies for regulation of its catabolism. MATERIALS AND METHODS Reagents-Carrier-free sodium [125 I]iodide was purchased from DuPont NEN. Bovine serum albumin was purchased from Calbiochem Co (La Jolla, CA). Protamine sulfate was purchased from Sigma. Porcine intestinal heparin for intravenous injection was from Elgins-Sinn Inc (Che...

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The ε4 allele of apolipoprotein E (apoE) is an important risk factor for Alzheimer's disease. A major neuronal receptor for apoE within the brain is the low‐density lipoprotein receptor‐related protein (LRP). Using primary cultured hippocampal neurons, we examined the role of LRP in early neuronal development. LRP, as well as a 39‐kDa protein that regulates its activity, is localized abundantly in developing neurons. Both the 39‐kDa protein and an anti‐LRP antibody inhibited neurite outgrowth of primary hippocampal neurons cultured in either serum‐containing medium or on cortical astrocyte monolayers in serum‐free medium. It is noteworthy that microtubule‐associated protein‐2 immunoreactive process outgrowth was decreased significantly in hippocampal neurons cultured on cortical astrocytes derived from apoE‐deficient mice and was not diminished further following incubation with LRP inhibitors. Thus, these results suggest that LRP can influence aspects of neuronal process development and that apoE‐containing lipoproteins may be one of the major LRP ligands that can contribute to this process.

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