Reductions in soluble brain proteins in older subjects with Down's syndrome

Neuropathology of Down syndrome and Alzheimer disease

Patients with Down syndrome (DS) over 40 years of age, prematurely and consistently develop neurofibrillary tangles (NFT), intracytoplasmic inclusions of highly insoluble straight or paired helical 12-16 nm filaments, and senile plaques (SP) composed of abnormal neurites surrounding a core of beta amyloid. These two lesions occur in distributions similar to those seen in Alzheimer disease (AD). SP and NFT are detected also in some younger individuals with DS (10-30+ years) when immunocytochemical and/or silver staining techniques are used. Retrospective and prospective attempts to relate neuropathological lesions and clinical dementia in DS have produced conflicting results. Clinical evidence of dementia and large numbers of SP and NFT were not always concordant. The predictable and consistent appearance of the AD-like neuropathologic changes in DS provides an unusual opportunity to examine the sequential development of SP and NFT. By combining morphological, immunocytochemical, and morphometric techniques with molecular biological approaches, the evolution of the structural and chemical changes in DS and AD can be examined and their relationship to clinical deficits can be evaluated.

Distribution and expression of the alpha-tubulin mRNA in the hippocampus and the temporal cortex in Alzheimer's disease

The distribution of the messenger RNA for alpha-tubulin has been investigated by in situ hybridization in the human hippocampus and temporal cortex in normal subjects and in Alzheimer's disease. The alpha-tubulin mRNA was strongly expressed in neurons in the gyrus dentatus, in the Ammon's horn and in cortical layers of the temporal cortex. The same distribution was observed in Alzheimer's disease. An important reduction of the hybridization signal was apparent, however, in areas rich in neurofibrillary lesions, e.g. as in layer II of entorhinal cortex. Neurons containing neurofibrillary tangles exhibited a weaker hybridization signal than adjacent neurons devoid of neurofibrillary tangles. The immunoreactivity for alpha-tubulin was drastically reduced in tangles-bearing neurons. These results indicate that tubulin transcription is reduced in tangles-bearing neurons, a reduction which might play a role in the reported decrease in the number of microtubules in neurons containing neurofibrillary tangles.

Taxol protects against calcium-mediated death of differentiated rat pheochromocytoma cells

Elevated levels of intraneuronal calcium may contribute to neuronal death in both Alzheimer's disease and stroke. In part, this neuronal death may be due to calcium-induced disruption of microtubules and inhibition of axonal transport. Taxol stabilizes microtubules to disaggregation. To determine whether taxol could protect against calcium-mediated neuron cell death, a test system was established using a nerve growth factor-differentiated rat pheochromocytoma cell line (PC12 cells). PC12 cells were cultured with nerve growth factor to induce a neuronal phenotype. After 15 days, the cells were exposed to taxol, the calcium ionophore, A23187, or taxol plus ionophore for up to 24 h. Taxol alone reduced cell survival in a concentration dependent manner. At a concentration of 50 nM survival was reduced to between 63% and 84% of control after 4 h of exposure. The ionophore (1 microM) variably reduced cell survival to between 10 and 55% at 4h. However, when taxol was added to the ionophore the cell survival was significantly increased by 1.5 to 4-fold. The protective effect of taxol lasted up to 24h. We conclude that taxol has a protective effect on calcium-mediated neurotoxicity. Drugs targeting underlying cellular mechanisms involved in calcium-mediated neuronal death may lead to successful therapy for Alzheimer's disease and stroke.

Effects of microtubule stabilization and destabilization on tau immunoreactivity in cultured hippocampal neurons

Tau immunoreactivity is altered in neurofibrillary tangles (NFT) and degenerating neurites in Alzheimer's disease (AD). In addition, cytoskeletal proteins including tau are excessively phosphorylated in AD. Previous data indicated that calcium influx can cause antigenic changes in tau in cultured rat hippocampal and human cortical neurons similar to those seen in NFT. The present study used cultured hippocampal neurons to test the hypothesis that disruption of microtubules is a key event leading to altered antigenic properties of tau that result from calcium influx. As previously reported, we found that glutamate (100-500 microM) and calcium ionophore A23187 (0.5-1 microM) elevated intraneuronal calcium levels and caused a reduction in microtubules, a marked increase in staining with Alz-50 and 5E2, and a decrease in tau-1 immunoreactivity. The microtubule-disrupting agent colchicine (1 microM) caused increased immunoreactivity of neurons towards tau antibodies Alz-50 and 5E2, and these effects of colchicine occurred in the absence of an increase in intracellular calcium levels. The microtubule-stabilizing drug taxol (100 nM) reduced neuronal immunoreactivity towards Alz-50 and 5E2 in untreated cultures and in cultures exposed to glutamate or A23187. Western blot analysis indicated that A23187 caused a reduction in tau levels which was partially prevented by taxol, suggesting that tau associated with microtubules is less susceptible to calcium-mediated degradation. Acid phosphatase treatment increased neuronal immunoreactivity towards tau-1 and reduced immunoreactivity towards Alz-50. The calcium-induced alterations in tau immunoreactivity were, and the colchicine-induced alterations were not, affected by acid phosphatase treatment. Taken together, the data indicate that microtubule depolymerization can cause antigenic changes in tau similar to those seen in NFT independently of an increase in intraneuronal calcium levels. Stabilization of microtubules prevented the antigenic changes in tau suggesting that microtubules affect the availability and/or properties of epitopes on tau that are recognized by antibodies that stain NFT.

Hypothesis: microtubules, a key to Alzheimer disease

Alzheimer disease (AD) is a clinicopathologic syndrome of unknown etiology with numerous abnormalities in neuronal and nonneuronal cells. A review of the literature suggests that a common basic intracellular defect may underlie many of the reported abnormalities. We hypothesize impairment of the microtubule (MT) system as one explanation for the pathogenesis of AD. Evidence in support of the hypothesis includes the following: MTs are ubiquitous and vital cell components, unequally distributed, with the highest concentration in the brain; various abnormalities, including the key neuropathologic lesions, can be explained by impairments of the MT system; and experiments utilizing pharmacologic agents known to disrupt MTs have reproduced certain abnormalities observed in AD. The hypothesis provides a framework for systematic investigations of MTs at the cellular and molecular levels as well as the basis for in vivo diagnostic tests for AD.