Post-mortem correlates of in vivo PiB-PET amyloid imaging in a typical case of Alzheimer's disease.

The positron emission tomography (PET) radiotracer Pittsburgh Compound-B (PiB) binds with high affinity to beta-pleated sheet aggregates of the amyloid-beta (Abeta) peptide in vitro. The in vivo retention of PiB in brains of people with Alzheimer's disease shows a regional distribution that is very similar to distribution of Abeta deposits observed post-mortem. However, the basis for regional variations in PiB binding in vivo, and the extent to which it binds to different types of Abeta-containing plaques and tau-containing neurofibrillary tangles (NFT), has not been thoroughly investigated. The present study examined 28 clinically diagnosed and autopsy-confirmed Alzheimer's disease subjects, including one Alzheimer's disease subject who had undergone PiB-PET imaging 10 months prior to death, to evaluate region- and substrate-specific binding of the highly fluorescent PiB derivative 6-CN-PiB. These data were then correlated with region-matched Abeta plaque load and peptide levels, [(3)H]PiB binding in vitro, and in vivo PET retention levels. We found that in Alzheimer's disease brain tissue sections, the preponderance of 6-CN-PiB binding is in plaques immunoreactive to either Abeta42 or Abeta40, and to vascular Abeta deposits. 6-CN-PiB labelling was most robust in compact/cored plaques in the prefrontal and temporal cortices. While diffuse plaques, including those in caudate nucleus and presubiculum, were less prominently labelled, amorphous Abeta plaques in the cerebellum were not detectable with 6-CN-PiB. Only a small subset of NFT were 6-CN-PiB positive; these resembled extracellular 'ghost' NFT. In Alzheimer's disease brain tissue homogenates, there was a direct correlation between [(3)H]PiB binding and insoluble Abeta peptide levels. In the Alzheimer's disease subject who underwent PiB-PET prior to death, in vivo PiB retention levels correlated directly with region-matched post-mortem measures of [(3)H]PiB binding, insoluble Abeta peptide levels, 6-CN-PiB- and Abeta plaque load, but not with measures of NFT. These results demonstrate, in a typical Alzheimer's disease brain, that PiB binding is highly selective for insoluble (fibrillar) Abeta deposits, and not for neurofibrillary pathology. The strong direct correlation of in vivo PiB retention with region-matched quantitative analyses of Abeta plaques in the same subject supports the validity of PiB-PET imaging as a method for in vivo evaluation of Abeta plaque burden.

Superior frontal cortex cholinergic axon density in mild cognitive impairment and early Alzheimer disease.

BACKGROUND: Loss of cortical choline acetyltransferase (ChAT) activity contributes to end-stage Alzheimer disease (AD) dementia. In general, ChAT activity levels are stable in the neocortex in mild to moderate AD (mAD) and there is a selective up-regulation in the superior frontal cortex (SFC) in mild cognitive impairment (MCI), indicating a transient, region-specific cholinergic neuroplastic response. OBJECTIVE: To assess whether a proliferation of cholinergic axons underlies increased ChAT activity levels in the SFC in subjects with MCI. DESIGN: Stereologic principles were applied to assess the density of ChAT-immunoreactive fibers and axon varicosities in SFC tissue obtained postmortem from subjects with no cognitive impairment, MCI, and mAD. SUBJECTS: Thirty-six subjects enrolled in the Religious Orders Study, with records of annual clinical evaluation for frontal lobe specific and global cognitive functions. RESULTS: Compared with the group with no cognitive impairment, SFC ChAT-immunoreactive fiber and axon varicosity densities were not altered in the MCI group but were significantly reduced in the group with mAD and correlated with impaired frontal lobe and global cognitive function. CONCLUSIONS: The lack of an increase in cholinergic axonal innervation of the SFC in MCI suggests that structural reorganization of cholinergic profiles is not the mechanism underlying the transient cholinergic plasticity reported in this region. Furthermore, the stability of cholinergic enzyme activity in mAD is likely the result of a biochemical up-regulation of ChAT protein or enzyme activity levels in the SFC, compensating for decreased regional cholinergic fibers and axon varicosities.

X-34 labeling of abnormal protein aggregates during the progression of Alzheimer's disease.

Postmortem pathological diagnosis and basic research investigations of neurodegenerative disorders rely on histochemical staining procedures developed specifically to visualize abnormal protein conformation. In Alzheimer's disease (AD), two major pathological hallmarks are required to confirm the clinical diagnosis. Both consist of abnormally aggregated proteins that share the structural and histological properties common to all amyloid deposits. Amyloid-beta peptide (Abeta) of extracellular senile plaques (SP) and hyperphosphorylated tau of intracellular neurofibrillary tangles (NFT) are assembled in the abnormal beta-pleated sheet (amyloid-like) structural conformation that can be visualized with histological staining procedures using Congo red or its derivatives. These histochemical dyes bind amyloid with high affinity and allow easy detection of amyloid structure in postmortem brain samples. This chapter focuses on the development and application of a histological protocol using the compound X-34, a highly fluorescent derivative of Congo red, for sensitive detection of pathological amyloid structures in histopathological investigations of postmortem brain tissue. This procedure provides a simple and effective method for detailed fluorescent visualization of the localization and distribution of the majority of currently known major histopathological structures in AD, including compact cored, neuritic, and diffuse-appearing SP, NFT, dystrophic neurites, neuropil threads, and cerebrovascular amyloidosis.

Caspase inhibition therapy abolishes brain trauma-induced increases in Abeta peptide: implications for clinical outcome.

The detrimental effects of traumatic brain injury (TBI) on brain tissue integrity involve progressive axonal damage, necrotic cell loss, and both acute and delayed apoptotic neuronal death due to activation of caspases. Post-injury accumulation of amyloid precursor protein (APP) and its toxic metabolite amyloid-beta peptide (Abeta) has been implicated in apoptosis as well as in increasing the risk for developing Alzheimer's disease (AD) after TBI. Activated caspases proteolyze APP and are associated with increased Abeta production after neuronal injury. Conversely, Abeta and related APP/Abeta fragments stimulate caspase activation, creating a potential vicious cycle of secondary injury after TBI. Blockade of caspase activation after brain injury suppresses apoptosis and improves neurological outcome, but it is not known whether such intervention also prevents increases in Abeta levels in vivo. The present study examined the effect of caspase inhibition on post-injury levels of soluble Abeta, APP, activated caspase-3, and caspase-cleaved APP in the hippocampus of nontransgenic mice expressing human Abeta, subjected to controlled cortical injury (CCI). CCI produced brain tissue damage with cell loss and elevated levels of activated caspase-3, Abeta(1-42) and Abeta(1-40), APP, and caspase-cleaved APP fragments in hippocampal neurons and axons. Post-CCI intervention with intracerebroventricular injection of 100 nM Boc-Asp(OMe)-CH(2)F (BAF, a pan-caspase inhibitor) significantly reduced caspase-3 activation and improved histological outcome, suppressed increases in Abeta and caspase-cleaved APP, but showed no significant effect on overall APP levels in the hippocampus after CCI. These data demonstrate that after TBI, caspase inhibition can suppress elevations in Abeta. The extent to which Abeta suppression contributes to improved outcome following inhibition of caspases after TBI is unclear, but such intervention may be a valuable therapeutic strategy for preventing the long-term evolution of Abeta-mediated pathology in TBI patients who are at risk for developing AD later in life.

Apolipoprotein D is a component of compact but not diffuse amyloid-beta plaques in Alzheimer's disease temporal cortex.

Apolipoprotein D (apoD) is elevated in Alzheimer's disease (AD) cortex, localizing to cells, blood vessels, and neuropil deposits (plaques). The role of apoD in AD pathology and the extent of its co-distribution with diffuse (amorphous) and compact (dense fibrillar) amyloid-beta (Abeta) plaques are currently unclear. To address this issue, we combined apoD and Abeta immunohistochemistry with ThioS/X-34 staining of the beta-pleated sheet protein conformation in temporal cortex from 36 AD patients and 12 non-demented controls. ApoD-immunoreactive, Abeta-immunoreactive, and ThioS/X-34-stained plaques were detected exclusively in AD tissue. Dual-immunolabeling showed that 63% of Abeta plaques co-localized apoD. All apoD plaques contained Abeta protein and ThioS/X-34 fluorescence. Compared to controls, AD cases showed elevated vascular and intracellular apoD immunostaining which localized primarily to cells clustered within plaques and around large blood vessels. ApoD-immunoreactive cells within plaques morphologically matched MHC-II- and CD-68-immunoreactive microglia, and did not contain the astrocytic marker GFAP, which labeled a subset of apoD-immunoreactive cells surrounding plaques. These data suggest that neuropil deposits of apoD localize only to a subset of Abeta plaques, which contain compact aggregates of fibrillar Abeta. Elevated apoD in AD brain may influence Abeta aggregation, or facilitate phagocytosis and transport of Abeta fibrils from plaques to cerebral vasculature.

22R-hydroxycholesterol and 9-cis-retinoic acid induce ATP-binding cassette transporter A1 expression and cholesterol efflux in brain cells and decrease amyloid beta secretion.

The ATP-binding cassette transporter A1 (ABCA1) is a major regulator of peripheral cholesterol efflux and plasma high density lipoprotein metabolism. In adult rat brain we found high expression of ABCA1 in neurons in the hypothalamus, thalamus, amygdala, cholinergic basal forebrain, and hippocampus. Large neurons of the cholinergic nucleus basalis together with CA1 and CA3 pyramidal neurons were among the most abundantly immunolabeled neurons. Glia cells were largely negative. Because cholesterol homeostasis may have an essential role in central nervous system function and neurodegeneration, we examined ABCA1 expression and function in different brain cell types using cultures of primary neurons, astrocytes, and microglia isolated from embryonic rat brain. The basal ABCA1 mRNA and protein levels detected in these cell types were increased markedly after exposure to oxysterols and 9-cis-retinoic acid, which are ligands for the nuclear hormone liver X receptors and retinoic X receptors, respectively. Functionally, the increased ABCA1 expression caused by these ligands was followed by elevated apoA-I- and apoE-specific cholesterol efflux in neurons and glia. In non-neuronal and neuronal cells overexpressing a human Swedish variant of amyloid precursor protein, 22R-hydroxycholesterol and 9-cis-retinoic acid induced ABCA1 expression and increased apoA-I-mediated cholesterol efflux consequently decreasing cellular cholesterol content. More importantly, we demonstrated that these ligands alone or in combination with apoA-I caused a substantial reduction in the stability of amyloid precursor protein C-terminal fragments and decreased amyloid beta production. These effects of 22R-hydroxycholesterol may provide a novel strategy to decrease amyloid beta secretion and consequently reduce the amyloid burden in the brain.