The low-density lipoprotein (LDL) receptor-related protein (LRP) is a multifunctional endocytic receptor that is expressed abundantly in neurons of the CNS. Both LRP and several of its ligands, including tissue plasminogen activator (tPA), apolipoprotein E/lipoproteins, alpha(2)-macroglobulin, and the beta-amyloid precursor protein, have been implicated in various neuronal functions and in the pathogenesis of Alzheimer's disease. It has been reported that induction of tPA expression may contribute to activity-dependent synaptic plasticity in the hippocampus and cerebellum. In addition, long-term potentiation (LTP) is significantly decreased in mice lacking tPA. Here we demonstrate that tPA receptor LRP is abundantly expressed in hippocampal neurons and participates in hippocampal LTP. Perfusion of hippocampal slices with receptor-associated protein (RAP), an antagonist for ligand interactions with LRP, significantly reduced late-phase LTP (L-LTP). In addition, RAP also blocked the enhancing effect of synaptic potentiation by exogenous tPA in hippocampal slices prepared from tPA knock-out mice. Metabolic labeling and ligand binding analyses showed that both tPA and LRP are synthesized by hippocampal neurons and that LRP is the major cell surface receptor that binds tPA. Finally, we found that tPA binding to LRP in hippocampal neurons enhances the activity of cyclic AMP-dependent protein kinase, a key molecule that is known to be involved in L-LTP. Taken together, our results demonstrate that interactions between tPA and cell surface LRP are important for hippocampal L-LTP.

Amyloid-␤ peptide (A␤) is central to the pathogenesis of Alzheimer's disease, and the low-density lipoprotein receptor-related protein (LRP) has been shown to alter A␤ metabolism in vitro. Here, we show that overexpression of a functional LRP minireceptor in the brain of PDAPP mice results in age-dependent increase of soluble brain A␤, with no changes in A␤ plaque burden. Importantly, soluble brain A␤ was found to be primarily in the form of monomers͞dimers and to be highly correlated with deficits in spatial learning and memory. These results provide in vivo evidence that LRP may contribute to memory deficits typical of Alzheimer's disease by modulating the pool of small soluble forms of A␤.A lzheimer's disease (AD) is characterized by cognitive impairment and neuronal loss that have been primarily linked to the accumulation of extracellular neuritic plaques and intracellular neurofibrillary tangles in the brain (1). The major component of neuritic plaques is amyloid-␤ peptide (A␤), which is derived from the cleavage of amyloid precursor protein (APP). Accumulation of fibrillar aggregates of A␤ in the brain parenchyma, caused by A␤ overproduction, impaired clearance, or both, is the basis for the amyloid cascade hypothesis long proposed to explain the etiology of AD (2).The low-density lipoprotein (LDL) receptor-related protein (LRP) has been genetically linked to AD (3, 4) and has been shown to influence A␤ metabolism in vitro (5-12). LRP is an Ϸ600-kDa cell-surface endocytic receptor member of the LDL receptor family (13). LRP is highly expressed in the brain and is considered the major neuronal receptor for apolipoprotein E (apoE) and ␣ 2 -macroglobulin (␣ 2 M), also implicated in the pathogenesis of AD by both biochemical and genetic evidence (14).A putative role for LRP in AD is supported by in vitro studies showing that apoE and ␣ 2 M can form stable complexes with A␤ and promote its clearance via cell-surface LRP (5-10). Furthermore, LRP appears to influence APP endocytic trafficking and cellular distribution such that processing to A␤ and its extracellular release are enhanced (11,12). To assess the effect of LRP on A␤ deposition in vivo, we generated transgenic (TG) mice that overexpress a functional minireceptor of LRP in the brain. We bred LRP TG mice to PDAPP TG mice, an animal model that develops amyloid plaques similar to those seen in AD (15). Although brain A␤ plaque burden was not significantly altered by the overexpression of LRP, double TG mice showed an age-dependent increase of soluble brain A␤ levels when compared to littermate mice overexpressing APP alone. The A␤ levels in this soluble brain extracts, which we found to be mostly A␤ monomers and dimers, were highly correlated with deficits in spatial learning and memory. These data provide strong evidence that, in addition to A␤ plaques, small soluble forms of A␤ appear to impair neuronal function and contribute to memory deficits in vivo. Materials and MethodsAnimals and Tissue Preparation. mLRP2 TG mice were generated in a B6͞C3H backgr...

Previous work from this laboratory indicates a role for the complement component C5 in neuroprotection against excitotoxicity. In the present study, we tested the hypothesis that the C5-derived anaphylatoxin C5a protects against kainic acid (KA)-induced neurodegeneration and investigated the mechanism of C5a neuronal activity in vitro. Brain intraventricular infusion of KA into adult mice caused neuronal morphological features of apoptosis in the pyramidal layer of the hippocampal formation as indicated by counts of neurons with pyknotic/condensed nuclei associated with cytoplasmic eosinophilia. Co-intraventricular infusion of human recombinant C5a with KA resulted in a marked reduction of morphological features of apoptotic neuronal death. In vitro studies confirmed C5a neuroprotection: treatment of primary murine corticohippocampal neurons with human or mouse recombinant C5a reduced glutamate neurotoxicity, as measured by trypan blue exclusion assay. This protection concurred with inhibition of glutamate-mediated induction of the caspase-3-related cysteine protease and coincided with marked reduction of neurons with morphological features of apoptosis, as found in vivo. Our studies indicate that C5a may inhibit glutamate-mediated neuronal death through partial inhibition of caspase-3 activity. These findings suggest a novel noninflammatory role for C5a in modulating neuronal responses to excitotoxins.

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Previous work from this laboratory indicates a role for the complement component C5 in neuroprotection against excitotoxicity. In the present study, we tested the hypothesis that the C5-derived anaphylatoxin C5a protects against kainic acid (KA)-induced neurodegeneration and investigated the mechanism of C5a neuronal activity in vitro. Brain intraventricular infusion of KA into adult mice caused neuronal morphological features of apoptosis in the pyramidal layer of the hippocampal formation as indicated by counts of neurons with pyknotic/condensed nuclei associated with cytoplasmic eosinophilia. Co-intraventricular infusion of human recombinant C5a with KA resulted in a marked reduction of morphological features of apoptotic neuronal death. In vitro studies confirmed C5a neuroprotection: treatment of primary murine corticohippocampal neurons with human or mouse recombinant C5a reduced glutamate neurotoxicity, as measured by trypan blue exclusion assay. This protection concurred with inhibition of glutamate-mediated induction of the caspase-3-related cysteine protease and coincided with marked reduction of neurons with morphological features of apoptosis, as found in vivo. Our studies indicate that C5a may inhibit glutamate-mediated neuronal death through partial inhibition of caspase-3 activity. These findings suggest a novel noninflammatory role for C5a in modulating neuronal responses to excitotoxins.