Long-term prevention of Alzheimer's disease-like behavioral deficits in PDAPP mice carrying a mutation in Asp664.

The deficits of Alzheimer's disease (AD) are believed to result, at least in part, from neurotoxicity of beta-amyloid (Abeta), a set of 38-43 amino acid fragments derived from the beta-amyloid precursor protein (APP). In addition, APP generates the APP-C31 and Jcasp toxic fragments intracellularly by cleavage at Asp664. We reported that mutation of Asp664 to Ala in a FAD-human APP transgene prevented AD-like deficits but did not affect Abeta production or deposition in PDAPP mice, arguing that D664A plays a crucial role in the generation of AD-like deficits. Whether D664A simply delays or completely prevents AD-like deficits, however, remained undefined. To address this question, we performed behavioral studies longitudinally on a pretrained mouse cohort at 9 and 13 months (mo) of age. While behavioral deficits were present in PDAPP mice, performance of Tg PDAPP(D664A) mice was not significantly different from non-Tg littermates' across all ages tested. Moreover, aberrant patterns in non-cognitive components of behavior in PDAPP mice were ameliorated in PDAPP(D664A) animals as well. A trend towards poorer retention at 9 mo and poorer learning at 13 mo that did not reach statistical significance was observed in PDAPP(D664A) mice. These results support and extend recent studies showing that cleavage of APP at Asp664 (or protein-protein interactions dependent on Asp664) is a crucial event in the generation of AD-like deficits in PDAPP mice. Our results thus further demonstrate that the D664A mutation either completely precludes, or markedly delays (beyond 13 mo) the appearance of AD-like deficits in this mouse model of AD.

C-terminal cleavage of the amyloid-beta protein precursor at Asp664: a switch associated with Alzheimer's disease.

In addition to the proteolytic cleavages that give rise to amyloid-beta (Abeta), the amyloid-beta protein precursor (AbetaPP) is cleaved at Asp664 intracytoplasmically. This cleavage releases a cytotoxic peptide, APP-C31, removes AbetaPP-interaction motifs required for signaling and internalization, and is required for the generation of AD-like deficits in a mouse model of the disease. Although we and others had previously shown that Asp664 cleavage of AbetaPP is increased in AD brains, the distribution of the Asp664-cleaved forms of AbetaPP in non-diseased and AD brains at different ages had not been determined. Confirming previous reports, we found that Asp664-cleaved forms of AbetaPP were increased in neuronal cytoplasm and nuclei in early-stage AD brains but were absent in age-matched, non-diseased control brains and in late-stage AD brains. Remarkably, however, Asp664-cleaved AbetaPP was prominent in neuronal somata and in processes in entorhinal cortex and hippocampus of non-diseased human brains at ages <45 years. Our observations suggest that Asp664 cleavage of AbetaPP may be part of the normal proteolytic processing of AbetaPP in young (<45 years) human brain and that this cleavage is down-regulated with normal aging, but is aberrantly increased and altered in location in early AD.

Interaction of ASK1 and the beta-amyloid precursor protein in a stress-signaling complex.

The amyloid precursor protein (APP) is a type I transmembrane protein translocated to neuronal terminals, whose function is still unknown. The C-terminus of APP mediates its interaction with cellular adaptor and signaling proteins, some of which signal to the stress-activated protein kinase (SAPK) pathway. Here we show that ASK1, a MAPKKK that activates two SAPKs, c-Jun N-terminal-kinase (JNK) and p38, is present in a complex containing APP, phospho-MKK6, JIP1 and JNK1. In primary neurons deprived of growth factors, as well as in brains of (FAD)APP-transgenic mice, ASK1 was upregulated in neuronal projections, where it interacted with APP. In non-transgenic brains, ASK1 and APP associated mainly in the ER. Our results indicate that recruitment of ASK1 to stress-signaling complexes assembled with APP may be triggered and enhanced by cellular stress. Thus, ASK1 may be the apical MAPKKK in a signaling complex assembled with APP as a response to stress.

Vascular endothelial growth factor improves recovery of sensorimotor and cognitive deficits after focal cerebral ischemia in the rat.

Vascular endothelial growth factor (VEGF) is an angiogenesis factor with neurotrophic, neuroprotective and neuroproliferative effects. Depending on the dose, route and time of administration in relation to focal cerebral ischemia, VEGF can improve histological outcome and sensorimotor function in rodents. However, VEGF also increases vascular permeability, which can lead to brain edema and exacerbate ischemic brain injury. Thus, although VEGF is a candidate therapeutic for stroke and other ischemic disorders, its benefit relative to risk is uncertain. Considering that functional rather than histological measures of outcome are probably most relevant to therapeutic prospects for human stroke, we investigated the effects of VEGF after middle cerebral artery occlusion in rats using a series of behavioral tests. We report that VEGF improves functional outcome in ischemic rats, including both sensorimotor and cognitive deficiencies.

Reversal of Alzheimer's-like pathology and behavior in human APP transgenic mice by mutation of Asp664.

The deficits characteristic of Alzheimer's disease (AD) are believed to result, at least in part, from the neurotoxic effects of beta-amyloid peptides, a set of 39-43 amino acid fragments derived proteolytically from beta-amyloid precursor protein (APP). APP also is cleaved intracytoplasmically at Asp-664 to generate a second cytotoxic peptide, APP-C31, but whether this C-terminal processing of APP plays a role in the pathogenesis of AD is unknown. Therefore, we compared elements of the Alzheimer's phenotype in transgenic mice modeling AD with vs. without a functional Asp-664 caspase cleavage site. Surprisingly, whereas beta-amyloid production and plaque formation were unaltered, synaptic loss, astrogliosis, dentate gyral atrophy, increased neuronal precursor proliferation, and behavioral abnormalities were completely prevented by a mutation at Asp-664. These results suggest that Asp-664 plays a critical role in the generation of Alzheimer-related pathophysiological and behavioral changes in human APP transgenic mice, possibly as a cleavage site or via protein-protein interactions.