Intracortical glutamate perfusion in vivo induces cellular alterations in specific protein kinase C isoforms and amyloid precursor protein

Axon-glial disruption: the link between vascular disease and Alzheimer's disease?

Vascular risk factors play a critical role in the development of cognitive decline and AD (Alzheimer's disease), during aging, and often result in chronic cerebral hypoperfusion. The neurobiological link between hypoperfusion and cognitive decline is not yet defined, but is proposed to involve damage to the brain's white matter. In a newly developed mouse model, hypoperfusion, in isolation, produces a slowly developing and diffuse damage to myelinated axons, which is widespread in the brain, and is associated with a selective impairment in working memory. Cerebral hypoperfusion, an early event in AD, has also been shown to be associated with white matter damage and notably an accumulation of amyloid. The present review highlights some of the published data linking white matter disruption to aging and AD as a result of vascular dysfunction. A model is proposed by which chronic cerebral hypoperfusion, as a result of vascular factors, results in both the generation and accumulation of amyloid and injury to white matter integrity, resulting in cognitive impairment. The generation of amyloid and accumulation in the vasculature may act to perpetuate further vascular dysfunction and accelerate white matter pathology, and as a consequence grey matter pathology and cognitive decline.

Amyloid precursor protein gene disruption attenuates degeneration of substantia nigra compacta neurons following axotomy

Our past work has shown that the C-terminal fragment of amyloid precursor protein (APP) translocated to the nucleus in neurons destined for delayed excitotoxic degeneration. To test whether nuclear APP fragments also play a role in the progressive loss of dopaminergic (DA) substantia nigra compacta (SNc) neurons, we performed unilateral medial forebrain bundle (MFB) transection on APP wild type (WT) and on mice with disruption of the APP gene (KO). In WT mice immunoreactivity for APP C-terminal, beta-amyloid and Alz90 epitopes appeared in the nuclei of axotomized DA neurons at 3 days post-lesion (dpl), persisted at 7 dpl and was absent in 14 dpl mice. APP N-terminal immunoreactivity was restricted to the cytosol at all time points, precluding the possibility of full length APP in the nucleus. Nuclear localization of APP epitopes was absent in neurons of the contralateral SNc or in neurons of the ipsilateral ventral tegmental area and SN reticulata. The presence of APP C-terminal and Alz90 domains was confirmed by Western blotting performed on the nuclear fraction of the SN ipsilateral to the axotomy. Quantitative morphometric analysis revealed that WT mice demonstrated earlier and more profound loss of tyrosine hydroxylase+SNc neurons than did KO mice. These data showed that a novel nuclear C-terminal fragment appeared coincident with SNc neuron degeneration, and that APP deficiency correlated with significant neuroprotection in vivo.

Neurotoxic APP C-terminal and beta-amyloid domains colocalize in the nuclei of substantia nigra pars reticulata neurons undergoing delayed degeneration

Increased amyloid precursor protein (APP) expression and intracellular accumulation of its toxic fragments have been associated with acute neuronal death processes. However, the role of APP fragments in delayed neurodegeneration remains poorly understood. We have characterized the appearance of APP domains in rat substantia nigra pars reticulata (SNpR) neurons targeted for delayed degeneration following neurotoxic striatal lesion. From 4 to 8 days postlesion (dpl) SNpR neurons ipsilateral to the lesion showed marked cytosolic accumulation of full length APP. Moreover, the nuclei of affected neurons also showed intense immunoreactivity (IR) for APP C-terminal and beta-amyloid domains but not for an N-terminal sequence. These data suggested the presence of APP C-terminal fragment. The absence of nuclear IR for a beta1-40 specific antibody supports this conclusion. Ultrastructural analysis of nigral sections from 6 dpl rats using a beta-amyloid domain antibody showed pronounced accumulation of immunogold-silver reaction product in the nuclei of affected SNpR neurons that was absent in control, contralateral SNpR neurons. These findings suggest that intranuclear APP C-terminal fragment may play a role in genomic events contributing to delayed neuron degeneration in the SNpR.

beta-amyloid (Abeta)42(43), abeta42, abeta40 and apoE immunostaining of plaques in fatal head injury

beta-Amyloid (Abeta) deposits are found in the brains of approximately one-third of patients who die within days after a severe head injury; their presence correlating strongly with possession of an apolipoprotein E (apoE)-epsilon4 allele. The aim of the study was to investigate the relationship between Abeta42, Abeta40 and apoE immunostaining of Abeta plaques in the cerebral cortex and the relevance of apoE genotype in 23 fatally head-injured patients. These cases were known to have Abeta deposits from a previous study in which they were examined and semiquantified and related to apoE genotype. In the present study, the temporal cortex was probed using four different antibodies that recognize Abeta42(43), Abeta40 and an antibody to apoE. Abeta42(43)-positive plaques were observed in all of the 23 cases and Abeta40 immunoreactivity in only 11 of the 23 cases. In addition, semiquantitative analysis showed that relatively fewer plaques were detected with anti-Abeta40 than anti-Abeta42(43). ApoE-immunoreactive plaques were identified in 18 of the 23 cases. The number of plaques stained for apoE was relatively less than for Abeta42(43) but greater than for Abeta40. Furthermore, the density of Abeta plaques detected using either Abeta42(43), Abeta40 or apoE antibodies was associated with possession of apoE-epsilon4 in an allele dose-dependent manner. The results are consistent with Abeta42(43) as the initially deposited species in brain parenchyma and provide evidence that apoE is involved in the early stages of amyloid deposition. Further, the findings may be of relevance to the role of apoE genotype in influencing outcome after acute brain injury.

Protein kinase C anchoring deficit in postmortem brains of Alzheimer's disease patients

Protein kinase C (PKC) has been implicated in the pathophysiology of Alzheimer's disease (AD). The levels of particular isoforms and the activation of PKC are reduced in postmortem brain cortex of AD subjects. Receptors for activated C kinase (RACK) are a family of proteins involved in anchoring activated PKCs to relevant subcellular compartments. Recent evidence has indicated that the impaired activation (translocation) of PKC in the aging brain is associated with a deficit in RACK1, the most well-characterized member of this family. The present study was conducted to determine whether alterations in RACK1 occurred in cortical areas where an impaired translocation of PKC has been demonstrated in AD. Here we report the presence of RACK1 immunoreactivity in human brain frontal cortex for the first time and demonstrate a decrease in RACK1 content in cytosol and membrane extracts in AD when compared with non-AD controls. By comparison, the levels of the RACK1-related PKCbetaII were not modified in the same membrane extracts. These observations add a new perspective in understanding the disease-associated defective PKC signal transduction and indicate that a decrease in an anchoring protein for PKC is an additional determinant of this deficit.

Regulation of amyloid precursor protein secretion by glutamate receptors in human Ntera 2 neurons

The amyloid precursor protein (APP) can be cleaved by a beta-secretase to generate a beta-amyloid peptide, which has been implicated in the pathogenesis of Alzheimer's disease. However, APP can also be cleaved by an alpha-secretase to form a non-amyloidogenic secreted form of APP (APP-S). APP-S secretion can be physiologically regulated. This study examined the glutamatergic regulation of APP in the human neuronal Ntera 2 (NT2N) cell line. Metabotropic glutamate receptor subtypes 1alpha/beta and 5alpha were identified in the NT2N neurons by reverse transcription-polymerase chain reaction. Stimulation of these phosphatidylinositol-linked receptors with glutamate or specific receptor agonists resulted in a dose- and time-dependent increase in the secretion of the amyloid precursor protein (APP-S), measured by the immunoprecipitation of APP-S from the medium of [35S]methionine-labeled NT2N neurons. The glutamate-induced APP-S secretion was maximal at 30 min and at a concentration of 1 mM glutamate. Glutamate-induced APP-S secretion required activation of phospholipase C, which resulted in inositol 1, 4,5-trisphosphate production, as shown by the rapid glutamate-induced accumulation of inositol 1,4,5-trisphosphate. Glutamate also caused an increase in intracellular Ca2+. The protein kinase C activator phorbol 12-myristate 13-acetate, a phorbol ester, as well as 1-oleoyl-2-acetoyl-3-glycerol, a cell-permeable diacylglycerol analog, also stimulated APP-S secretion. These findings suggest that APP-S secretion from NT2N neurons can be regulated by the activation of phosphatidylinositol-linked metabotropic glutamate receptor signaling pathway.